ALBERT

Description: Influence of biomass burning on CCN number and hygroscopicity during summertime in the eastern Mediterranean Atmospheric Chemistry and Physics Discussions, 15, 21539-21582, 2015 Author(s): A. Bougiatioti, S. Bezantakos, I. Stavroulas, N. Kalivitis, P. Kokkalis, G. Biskos, N. Mihalopoulos, A. Papayannis, and A. Nenes This study investigates the CCN activity and hygroscopic properties of particles influenced by biomass burning in the eastern Mediterranean. Air masses sampled were subject to a range of atmospheric processing (several hours up to 3 days). Values of the hygroscopicity parameter, κ, were derived from cloud condensation nuclei (CCN) measurements and a Hygroscopic Tandem Differential Mobility Analyzer (HTDMA). An Aerosol Chemical Speciation Monitor (ACSM) was also used to determine the chemical composition and mass concentration of non-refractory components of the submicron aerosol fraction. During fire events, the increased organic content (and lower inorganic fraction) of the aerosol decreases the hygroscopicity parameter, κ, for all particle sizes. The reason, however, for this decrease was not the same for all size modes; smaller particle sizes appeared to be richer in less hygroscopic, less CCN-active components due to coagulation processes while larger particles become less hygroscopic during the biomass burning events due to condensation of less hygroscopic gaseous compounds. In addition, smaller particles exhibited considerable chemical dispersion (where hygroscopicity varied up to 100 % for particles of same size); larger particles, however, exhibited considerably less dispersion owing to the effects of aging and retained high levels of CCN activity. These conclusions are further supported by the observed mixing state determined by the HTDMA measurements. ACSM measurements indicate that the bulk composition reflects the hygroscopicity and chemical nature of the largest particles and a large fraction of the CCN concentrations sampled. Based on Positive Matrix Factorization (PMF) analysis of the organic ACSM spectra, CCN concentrations follow a similar trend with the BBOA component, with enhancements of CCN in biomass burning plumes ranging between 65 and 150 %, for supersaturations ranging between 0.2 and 0.7 %. Using multilinear regression, we determine the hygroscopicity of the prime organic aerosol components (BBOA, OOA-BB and OOA); it is found that the total organic hygroscopicity is very close to the inferred hygroscopicity of the oxygenated organic aerosol components. Finally, the transformation of freshly-emitted biomass burning (BBOA) to more oxidized organic aerosol (OOA-BB) can result in a two-fold increase of the inferred organic hygroscopicity. Almost 10 % of the total aerosol hygroscopicity is related to the two biomass burning components (BBOA and OOA-BB), which in turn contribute almost 35 % to the fine-particle organic water of the aerosol. This is important as organic water can contribute to the atmospheric chemistry and the direct radiative forcing.

Description: Measurement-based direct radiative effect by brown carbon over Indo-Gangetic Plain Atmospheric Chemistry and Physics Discussions, 15, 21583-21606, 2015 Author(s): A. Arola, G. L. Schuster, M. R. A. Pitkänen, O. Dubovik, H. Kokkola, A. V. Lindfors, T. Mielonen, T. Raatikainen, S. Romakkaniemi, S. N. Tripathi, and H. Lihavainen The importance of light absorbing organic aerosols, often called brown carbon (BrC), has become evident in recent years. However, there are relatively few measurement-based estimates for the direct radiative effect of BrC so far. In those earlier studies, the AErosol RObotic NETwork (AERONET) measured Aerosol Absorption Optical Depth (AAOD) and Absorption Angstrom Exponent (AAE) have been exploited. However, these two pieces of information are clearly not sufficient to separate properly carbonaceous aerosols from dust, while imaginary indices of refraction would contain more and better justified information for this purpose. This is first time that the direct radiative effect (DRE) of BrC is estimated by exploiting the AERONET-retrieved imaginary indices. We estimated it for four sites in Indo-Gangetic Plain (IGP), Karachi, Lahore, Kanpur and Gandhi College. We found a distinct seasonality, which was generally similar among all the sites, but with slightly different strengths. The monthly warming effect up to 0.5 W m -2 takes place during spring season. On the other hand, BrC results in overall cooling effect in the winter season, which can reach levels close to −1W m -2 . We then estimated similarly also DRE of black carbon and total aerosol, in order to assess the relative significance of BrC radiative effect in the radiative effects of other components. Even though BrC impact seems minor in this context, we demonstrated that it is not insignificant and moreover that it is crucial to perform spectrally resolved radiative transfer calculations to obtain good estimates for DRE of BrC.

Description: Size distribution and optical properties of mineral dust aerosols transported in the western Mediterranean Atmospheric Chemistry and Physics Discussions, 15, 21607-21669, 2015 Author(s): C. Denjean, F. Cassola, A. Mazzino, S. Triquet, S. Chevaillier, N. Grand, T. Bourrianne, G. Momboisse, K. Sellegri, A. Schwarzenbock, E. Freney, M. Mallet, and P. Formenti This study presents in situ aircraft measurements of Saharan mineral dust transported over the western Mediterranean basin in June–July 2013 during the ChArMEx/ADRIMED (the Chemistry-Aerosol Mediterranean Experiment/Aerosol Direct Radiative Impact on the regional climate in the MEDiterranean region) airborne campaign. Dust events differing in terms of source region (Algeria, Tunisia and Morocco), time of tranport (1–5 days) and height of transport were sampled. Mineral dust were transported above the marine boundary layer, which conversely was dominated by pollution and marine aerosols. The dust vertical structure was extremely variable and characterized by either a single layer or a more complex and stratified structure with layers originating from different source regions. Mixing of mineral dust with pollution particles was observed depending on the height of transport of the dust layers. Dust layers carried higher concentration of pollution particles at intermediate altitude (1–3 km) than at elevated altitude (〉 3 km), resulting in scattering Angstrom exponent up to 2.2 within the intermediate altitude. However, the optical properties of the dust plumes remained practically unchanged with respect to values previously measured over source regions, regardless of the altitude. Moderate light absorption of the dust plumes was observed with values of aerosol single scattering albedo at 530 nm ranging from 0.90 to 1.00 ± 0.04. Concurrent calculations from the aerosol chemical composition revealed a negligible contribution of pollution particles to the absorption properties of the dust plumes that was due to a low contribution of refractory black carbon in regards to the fraction of dust and sulfate particles. This suggests that, even in the presence of moderate pollution, likely a persistent feature in the Mediterranean, the optical properties of the dust plumes could be assimilated to those of native dust in radiative transfer simulations, modeling studies and satellite retrievals over the Mediterranean. Measurements also showed that the coarse mode of mineral dust was conserved even after 5 days of transport in the Mediterranean, which contrasts with the gravitational depletion of large particles observed during the transport of dust plumes over the Atlantic. Simulations with the WRF mesoscale meteorological model highlighted a strong vertical turbulence within the dust layers that could prevent deposition of large particles during their atmospheric transport. This has important implications for the dust radiative effects due to surface dimming, atmospheric heating and cloud formation. The results presented here add to the observational dataset necessary for evaluating the role of mineral dust on the regional climate and rainfall patterns in the western Mediterranean basin.

Description: Identification of particulate organosulfates in three megacities at the middle and lower reaches of the Yangtze River Atmospheric Chemistry and Physics Discussions, 15, 21415-21448, 2015 Author(s): X. K. Wang, S. Rossignol, Y. Ma, L. Yao, M. Y. Wang, J. M. Chen, C. George, and L. Wang PM 2.5 filter samples have been collected in three megacities i.e., Wuhan (WH), Nanjing (NJ), and Shanghai (SH) at the middle and lower reaches of the Yangtze River, respectively. Analysis of those samples using an ultra-high performance liquid chromatography (UHPLC) coupled to an orbitrap mass spectrometer (MS) allowed detection of about two hundred particulate organosulfates (OSs), including dozens of nitrooxy-organosulfates, at each location. While aliphatic OSs represented more than 78 % of the detected OSs at the three locations, aromatic OSs were much less abundant. OSs with two to four isomers accounted for about 50 % of the total OSs on average in these megacities, and the percentage of OSs with six and more isomers in WH was more significant than those in SH and NJ. The average molecular weight, and the degrees of oxidation and saturation of OSs in the WH summer samples were greater than those in WH winter samples. In SH, the average molecular weight and the degree of oxidation of OSs in summer samples were greater than those in winter samples, but the degree of saturation was similar between the two seasons. In summer, the average molecular weight, and the degrees of oxidation and unsaturation of OSs were smallest in WH among the three cities. Between NJ and SH, the average molecular weight and the degree of saturation of OSs were close and the degree of oxidation of OSs in NJ was smaller. Kendrick mass defect diagrams and Van Krevelen diagrams indicated that the characteristics of identified OSs between in NJ and in SH shared better similarity. In addition, the identity and abundance of OSs in SH showed clear seasonal and diurnal variations. OSs in summer were more abundant than they were in winter due to stronger photochemical reactions in summer. The relative abundance of OSs at night was greater than that in the daytime and more nitrooxy-OSs existed at night, probably because of active NO 3 radical chemistry at night. In SH summer samples, OSs with 5 and 10 carbons (C 5 and C 10 ) were the most abundant, indicating the importance of isoprene and monoterpenes as precursors of OSs, whereas the relative abundances of OSs with 8, 9, and more than 14 carbons (C 8 , C 9 , and C 14+ ) were also high in SH winter samples, urging the need to further understand the precursors of OSs.

Description: The tropopause inversion layer in baroclinic life cycles experiments: the role of diabatic and mixing processes Atmospheric Chemistry and Physics Discussions, 15, 21495-21537, 2015 Author(s): D. Kunkel, P. Hoor, and V. Wirth Recent studies on the formation of a quasi-permanent layer of enhanced static stability above the thermal tropopause revealed the contributions of dynamical and radiative processes. Dry dynamics lead to the evolution of a tropopause inversion layer (TIL) which is, however, too weak compared to observations and thus diabatic contributions are required. In this study we aim to assess the importance of diabatic as well as mixing processes in the understanding of TIL formation at midlatitudes. The non-hydrostatic model COSMO is applied in an idealized mid-latitude channel configuration to simulate baroclinic life cycles. The effect of individual diabatic, i.e. related to humidity and radiation, and turbulent processes is studied first to estimate the additional contribution of these processes to dry dynamics. In a second step these processes are stepwise included in the model to increase the complexity and finally estimate the relative importance of each process. The results suggest that including turbulence leads to a weaker TIL than in a dry reference simulation. In contrast, the TIL evolves stronger when radiation is included but the temporal occurrence is still comparable to the reference. Using various cloud schemes in the model shows that latent heat release and consecutive increased vertical motions foster an earlier and stronger appearance of the TIL than in all other life cycles. Furthermore, updrafts moisten the upper troposphere and as such increase the radiative effect from water vapor. Particularly, this process becomes more relevant for maintaining the TIL during later stages of the life cycles. Increased convergence of the vertical wind induced by updrafts and by propagating and potentially dissipating inertia-gravity waves further contributes to the enhanced stability of the lower stratosphere. Furthermore, radiative feedback of ice clouds reaching up to the tropopause is identified to potentially further affect the strength of the TIL in the region of the cloud.

Description: Radiative and climate impacts of a large volcanic eruption during stratospheric sulfur geoengineering Atmospheric Chemistry and Physics Discussions, 15, 21837-21881, 2015 Author(s): A. Laakso, H. Kokkola, A.-I. Partanen, U. Niemeier, C. Timmreck, K. E. J. Lehtinen, H. Hakkarainen, and H. Korhonen Both explosive volcanic eruptions, which emit sulfur dioxide into the stratosphere, and stratospheric geoengineering via sulfur injections can potentially cool the climate by increasing the amount of scattering particles in the atmosphere. Here we employ a global aerosol-climate model and an earth system model to study the radiative and climate impacts of an erupting volcano during solar radiation management (SRM). According to our simulations, the radiative impacts of an eruption and SRM are not additive: in the simulated case of concurrent eruption and SRM, the peak increase in global forcing is about 40 % lower compared to a corresponding eruption into a clean background atmosphere. In addition, the recovery of the stratospheric sulfate burden and forcing was significantly faster in the concurrent case since the sulfate particles grew larger and thus sedimented faster from the stratosphere. In our simulation where we assumed that SRM would be stopped immediately after a volcano eruption, stopping SRM decreased the overall stratospheric aerosol load. For the same reasons, a volcanic eruption during SRM lead to only about 1/3 of the peak global ensemble-mean cooling compared to an eruption under unperturbed atmospheric conditions. Furthermore, the global cooling signal was seen only for 12 months after the eruption in the former scenario compared to over 40 months in the latter. In terms of the global precipitation rate, we obtain a 36 % smaller decrease in the first year after the eruption and again a clearly faster recovery in the concurrent eruption and SRM scenario. We also found that an explosive eruption could lead to significantly different regional climate responses depending on whether it takes place during geoengineering or into an unperturbed background atmosphere. Our results imply that observations from previous large eruptions, such as Mt Pinatubo in 1991, are not directly applicable when estimating the potential consequences of a volcanic eruption during stratospheric geoengineering.

Description: Validation of farm-scale methane emissions using nocturnal boundary layer budgets Atmospheric Chemistry and Physics Discussions, 15, 21765-21802, 2015 Author(s): J. Stieger, I. Bamberger, N. Buchmann, and W. Eugster This study provides the first experimental validation of Swiss agricultural methane emission estimates at the farm scale. We measured CH 4 concentrations at a Swiss farmstead during two intensive field campaigns in August 2011 and July 2012 to (1) quantify the source strength of livestock methane emissions using a tethered balloon system, and (2) to validate inventory emission estimates via nocturnal boundary layer (NBL) budgets. Field measurements were performed at a distance of 150 m from the nearest farm buildings with a tethered balloon system in combination with gradient measurements at eight heights on a 10 m tower to better resolve the near-surface concentrations. Vertical profiles of air temperature, relative humidity, CH 4 concentration, wind speed and wind direction showed that the NBL was strongly influenced by local transport processes and by the valley wind system. Methane concentrations showed a pronounced time course, with highest concentrations in the second half of the night. NBL budget flux estimates were obtained via a time–space kriging approach. Main uncertainties of NBL budget flux estimates were associated with instationary atmospheric conditions and the estimate of the inversion height z i (top of volume integration). The mean NBL budget fluxes of 1.60 ± 0.31 μg CH 4 m -2 s -1 (1.40 ± 0.50 and 1.66 ± 0.20 μg CH 4 m -2 s -1 in 2011 and 2012, respectively) were in good agreement with local inventory estimates based on current livestock number and default emission factors, with 1.29 ± 0.47 and 1.74 ± 0.63 μg CH 4 m -2 s -1 for 2011 and 2012, respectively. This indicates that emission factors used for the national inventory reports are adequate, and we conclude that the NBL budget approach is a useful tool to validate emission inventory estimates.

Description: Seasonality of ultrafine and sub-micron aerosols and the inferences on particle formation processes Atmospheric Chemistry and Physics Discussions, 15, 21803-21835, 2015 Author(s): H. C. Cheung, C. C.-K. Chou, M.-J. Chen, W.-R. Huang, S.-H. Huang, C.-Y. Tsai, and C. S.-L. Lee The aim of this study is to investigate the seasonal variations in the physicochemical properties of atmospheric ultrafine particles (UFPs, d ≤ 100nm) and submicron particles (PM 1 , d ≤ 1 μm) in an East-Asian urban area, which are hypothesized to be affected by the interchange of summer and winter monsoons. An observation experiment was conducted at the TARO, an urban aerosol station in Taipei, Taiwan, from October 2012 to August 2013. The measurements included the mass concentration and chemical composition of UFPs and PM 1 , as well as the particle number concentration (PNC) and size distribution (PSD) with size range of 4–736 nm. The results indicate that the mass concentration of PM 1 was elevated during cold seasons with peak level of 18.5 μg m -3 in spring, whereas the highest UFPs concentration was measured in summertime with a seasonal mean of 1.62 μg m -3 . Moreover, chemical analysis revealed that the UFPs and PM 1 were characterized by distinct composition; UFPs were composed mostly of organics, whereas ammonium and sulfate were the major constituents in PM 1 . The seasonal median of total PNCs ranged from 13.9 × 10 3 cm -3 in autumn to 19.4 × 10 3 cm -3 in spring. The PSD information retrieved from the corresponding PNC measurements indicates that the nucleation mode PNC ( N 4–25 ) peaked at 11.6 × 10 3 cm -3 in winter, whereas the Aitken mode ( N 25–100 ) and accumulation mode ( N 100–736 ) exhibited summer maxima at 6.0 × 10 3 and 3.1 × 10 3 cm -3 , respectively. The shift in PSD during summertime is attributed to the enhancement in the photochemical production of condensable organic matter that, in turn, contributes to the growth of aerosol particles in the atmosphere. In addition, remarkable photochemical production of particles was observed in spring and summer seasons, which was characterized with averaged particle growth and formation rates of 4.3 ± 0.8 nm h -1 and 1.6 ± 0.8 cm -3 s -1 , respectively. The prevalence of new particle formation (NPF) in summer is suggested as a result of seasonally enhanced photochemical oxidation of SO 2 , which contributes to the production of H 2 SO 4 , and low level of PM 10 ( d ≤ 10 μm) that serves as the condensation sink. Regarding the sources of aerosol particles, correlation analysis upon the PNCs against NO x revealed that the local vehicular exhaust was the dominant contributor of the UFPs throughout a year. On the contrary, the Asian pollution outbreaks can have significant influence in the PNC of accumulation mode particles during the seasons of winter monsoons. The results of this study underline the significance of secondary organic aerosols in the seasonal variations of UFPs and the influences of continental pollution outbreaks in the downwind areas of Asian outflows.

Description: Biomass burning emissions and potential air quality impacts of volatile organic compounds and other trace gases from temperate fuels common in the United States Atmospheric Chemistry and Physics Discussions, 15, 21713-21763, 2015 Author(s): J. B. Gilman, B. M. Lerner, W. C. Kuster, P. D. Goldan, C. Warneke, P. R. Veres, J. M. Roberts, J. A. de Gouw, I. R. Burling, and R. J. Yokelson A comprehensive suite of instruments was used to quantify the emissions of over 200 organic gases, including methane and volatile organic compounds (VOCs), and 9 inorganic gases from 56 laboratory burns of 18 different biomass fuel types common in the southeastern, southwestern, or northern United States. A gas chromatograph-mass spectrometer (GC-MS) provided extensive chemical detail of discrete air samples collected during a laboratory burn and was complemented by real-time measurements of organic and inorganic species via an open-path Fourier transform infrared spectrometer (OP-FTIR) and 3 different chemical ionization-mass spectrometers. These measurements were conducted in February 2009 at the U.S. Department of Agriculture's Fire Sciences Laboratory in Missoula, Montana. The relative magnitude and composition of the gases emitted varied by individual fuel type and, more broadly, by the 3 geographic fuel regions being simulated. Emission ratios relative to carbon monoxide (CO) were used to characterize the composition of gases emitted by mass; reactivity with the hydroxyl radical, OH; and potential secondary organic aerosol (SOA) precursors for the 3 different US fuel regions presented here. VOCs contributed less than 0.78 ± 0.12 % of emissions by mole and less than 0.95 ± 0.07 % of emissions by mass (on average) due to the predominance of CO 2 , CO, CH 4 , and NO x emissions; however, VOCs contributed 70–90 (±16) % to OH reactivity and were the only measured gas-phase source of SOA precursors from combustion of biomass. Over 82 % of the VOC emissions by mole were unsaturated compounds including highly reactive alkenes and aromatics and photolabile oxygenated VOCs (OVOCs) such as formaldehyde. OVOCs contributed 57–68 % of the VOC mass emitted, 42–57 % of VOC-OH reactivity, and aromatic-OVOCs such as benzenediols, phenols, and benzaldehyde were the dominant potential SOA precursors. In addition, ambient air measurements of emissions from the Fourmile Canyon Fire that affected Boulder, Colorado in September 2010 allowed us to investigate biomass burning (BB) emissions in the presence of other VOC sources (i.e., urban and biogenic emissions) and identify several promising BB markers including benzofuran, 2-furaldehyde, 2-methylfuran, furan, and benzonitrile.

Description: Real-time measurements of secondary organic aerosol formation and aging from ambient air in an oxidation flow reactor in the Los Angeles area Atmospheric Chemistry and Physics Discussions, 15, 21907-21958, 2015 Author(s): A. M. Ortega, P. L. Hayes, Z. Peng, B. B. Palm, W. Hu, D. A. Day, R. Li, M. J. Cubison, W. H. Brune, M. Graus, C. Warneke, J. B. Gilman, W. C. Kuster, J. A. de Gouw, and J. L. Jimenez Field studies in polluted areas over the last decade have observed large formation of secondary organic aerosol (SOA) that is often poorly captured by models. The study of SOA formation using ambient data is often confounded by the effects of advection, vertical mixing, emissions, and variable degrees of photochemical aging. An Oxidation Flow Reactor (OFR) was deployed to study SOA formation in real-time during the CalNex campaign in Pasadena, CA, in 2010. A high-resolution aerosol mass spectrometer (AMS) and a scanning mobility particle sizer (SMPS) alternated sampling ambient and reactor-aged air. The reactor produced OH concentrations up to 4 orders of magnitude higher than in ambient air, achieving equivalent atmospheric aging from hours up to several weeks in 3 min of processing. OH radical concentration was continuously stepped, obtaining measurements of real-time SOA formation and oxidation at multiple equivalent ages from 0.8 days–6.4 weeks. Enhancement of OA from aging showed a maximum net SOA production between 0.8–6 days of aging with net OA mass loss beyond 2 weeks. Reactor SOA mass peaked at night, in the absence of ambient photochemistry, and correlated with trimethylbenzene concentrations. Reactor SOA formation was inversely correlated with ambient SOA and O x , which along with the short-lived VOC correlation, indicates the importance of relatively reactive (τ OH ∼ 0.3 day) SOA precursors in the LA-Basin. Evolution of the elemental composition in the reactor was similar to trends observed in the atmosphere (O : C vs. H : C slope ∼ -0.65). Oxidation state of carbon (OS C ) in reactor SOA increased steeply with age and remained elevated (OS C ∼ 2) at the highest photochemical ages probed. The ratio of OA in the reactor output to excess CO (ΔCO, ambient CO above regional background) vs. photochemical age is similar to previous studies at low to moderate ages and also extends to higher ages where OA loss dominates. The mass added at low-to-intermediate ages is due primarily to condensation of oxidized species, not heterogeneous oxidation. The OA decrease at high photochemical ages is dominated by heterogeneous oxidation followed by fragmentation/evaporation. A comparison of urban SOA formation in this study with a similar study of vehicle SOA in a tunnel supports the dominance of vehicle emissions in urban SOA. Pre-2007 SOA models underpredict SOA formation by an order of magnitude, while a more recent model performs better but overpredicts at higher ages. These results demonstrate the value of the reactor as a tool for in situ evaluation of the SOA formation potential and OA evolution from ambient air.

Description: An adsorption theory of heterogeneous nucleation of water vapour on nanoparticles Atmospheric Chemistry and Physics Discussions, 15, 21883-21906, 2015 Author(s): A. Laaksonen and J. Malila Heterogeneous nucleation of water vapour on insoluble nuclei is a phenomenon that can induce atmospheric water and ice cloud formation. However, modelling of the phenomenon is hampered by the fact that the predictive capability of the classical heterogeneous nucleation theory is rather poor. A reliable theoretical description of the influence of different types of water-insoluble nuclei in triggering the water condensation or ice deposition would help to decrease uncertainty in large scale model simulations. In this paper we extend a recently formulated adsorption theory of heterogeneous nucleation to be applicable to highly curved surfaces, and test the theory against laboratory data for water vapour nucleation on silica, titanium dioxide and silver oxide nanoparticles. We show that unlike the classical heterogeneous nucleation theory, the new theory is able to quantitatively predict the experimental results.

Description: Inverse modeling of black carbon emissions over China using ensemble data assimilation Atmospheric Chemistry and Physics Discussions, 15, 20851-20879, 2015 Author(s): P. Wang, H. Wang, Y. Q. Wang, X. Y. Zhang, S. L. Gong, M. Xue, C. H. Zhou, H. L. Liu, X. Q. An, T. Niu, and Y. L. Chen Emissions inventories of black carbon (BC), which are traditionally constructed using a "bottom-up" approach based on activity data and emissions factors, are considered to contain a large level of uncertainty. In this paper, an ensemble optimal interpolation (EnOI) data assimilation technique is used to investigate the possibility of optimally recovering the spatially resolved emissions bias of BC. An inverse modeling system for emissions is established for an atmospheric chemistry aerosol model and two key problems related to ensemble data assimilation in the top-down emissions estimation are discussed: (1) how to obtain reasonable ensembles of prior emissions; and (2) establishing a scheme to localize the background-error matrix. An experiment involving a one month simulation cycle with EnOI inversion of BC emissions is performed for January 2008. The bias of the BC emissions intensity in China at each grid point is corrected by this inverse system. The inversed emission over China in January is 240.1 Gg, and annual emission is about 2750 Gg, which is over 1.8 times of bottom-up emission inventory. The results show that, even though only monthly mean BC measurements are employed to inverse the emissions, the accuracy of the daily model simulation improves. Using top-down emissions, the average root-mean-square error of simulated daily BC is decreased by nearly 30 %. These results are valuable and promising for a better understanding of aerosol emissions and distributions, as well as aerosol forecasting.

Description: Conceptual design of a measurement network of the global change Atmospheric Chemistry and Physics Discussions, 15, 21063-21093, 2015 Author(s): P. Hari, T. Petäjä, J. Bäck, V.-M. Kerminen, H. K. Lappalainen, T. Vihma, T. Laurila, Y. Viisanen, T. Vesala, and M. Kulmala The global environment is changing rapidly due to anthropogenic emissions and actions. Such activities modify aerosol and greenhouse gas concentrations in the atmosphere, leading to regional and global climate change and affecting e.g. food and fresh-water security, sustainable use of natural resources and even demography. Here we present a conceptual design of a global, hierarchical observation network that can provide tools and increased understanding to tackle the inter-connected environmental and societal challenges that we will face in the coming decades. The philosophy behind the conceptual design relies on physical conservation laws of mass, energy and momentum, as well as on concentration gradients that act as driving forces for the atmosphere-biosphere exchange. The network is composed of standard, flux/advanced and flagship stations, each of which having specific and identified tasks. Each ecosystem type on the globe has its own characteristic features that have to be taken into consideration. The hierarchical network as a whole is able to tackle problems related to large spatial scales, heterogeneity of ecosystems and their complexity. The most comprehensive observations are envisioned to occur in flag ship stations, with which the process-level understanding can be expanded to continental and global scales together with advanced data analysis, earth system modelling and satellite remote sensing. The denser network of the flux and standard stations allow application and up-scaling of the results obtained from flag ship stations to the global level.

Description: Sensitivities of UK PM 2.5 concentrations to emissions reductions Atmospheric Chemistry and Physics Discussions, 15, 20881-20910, 2015 Author(s): M. Vieno, M. R. Heal, M. L. Williams, E. J. Carnell, J. R. Stedman, and S. Reis The reduction of ambient concentrations of fine particulate matter (PM 2.5 ) is a key objective for air pollution control policies in the UK and elsewhere. Long-term exposure to PM 2.5 has been identified as a major contributor to adverse human health effects in epidemiological studies and underpins ambient PM 2.5 legislation. As a range of emission sources and atmospheric chemistry transport processes contribute to PM 2.5 concentrations, atmospheric chemistry transport models are an essential tool to assess emissions control effectiveness. The EMEP4UK atmospheric chemistry transport model was used to investigate the impact of reductions in UK anthropogenic emissions of primary PM 2.5 , NH 3 , NO x , SO x or non-methane VOC on surface concentrations of PM 2.5 in the UK for a recent year (2010) and for a future current legislation emission scenario (2030). In general, the sensitivity to UK mitigation is rather small. A 30 % reduction in UK emissions of any one of the above components yields (for the 2010 simulation) a maximum reduction in PM 2.5 in any given location of ~ 0.6 μg m −3 (equivalent to ~ 6 % of the modelled PM 2.5 ). On average across the UK, the sensitivity of PM 2.5 concentrations to a 30 % reduction in UK emissions of individual contributing components, for both the 2010 and 2030 CLE baselines, increases in the order NMVOC, NO x , SO x , NH 3 and primary PM 2.5 , but there are strong spatial differences in the PM 2.5 sensitivities across the UK. Consequently, the sensitivity of PM 2.5 to individual component emissions reductions varies between area and population weighting. Reductions in NH 3 have the greatest effect on area-weighted PM 2.5 . A full UK population weighting places greater emphasis on reductions of primary PM 2.5 emissions, which is simulated to be the most effective single-component control on PM 2.5 for the 2030 scenario. An important observation is that weighting corresponding to the Average Exposure Indicator metric (using data from the 45 model grids containing a monitor whose measurements are used to calculate the UK AEI) further increases the emphasis on the effectiveness of primary PM 2.5 emissions reductions (and of NO x emissions reductions) relative to the effectiveness of NH 3 emissions reductions. Reductions in primary PM 2.5 have the largest impact on the AEI in both 2010 and the 2030 CLE scenario. The summation of the modelled reductions to the UK PM 2.5 AEI from 30 % reductions in UK emissions of primary PM 2.5 , NH 3 , SO x , NO x and VOC totals 1.17 and 0.82 μg m −3 for the 2010 and 2030 CLE simulations, respectively.

Description: Upper tropospheric water vapour variability at high latitudes – Part 1: Influence of the annular modes Atmospheric Chemistry and Physics Discussions, 15, 22291-22329, 2015 Author(s): C. E. Sioris, J. Zou, D. A. Plummer, C. D. Boone, C. T. McElroy, P. E. Sheese, O. Moeini, and P. F. Bernath Seasonal and monthly zonal medians of water vapour in the upper troposphere and lower stratosphere (UTLS) are calculated for both Atmospheric Chemistry Experiment (ACE) instruments for the northern and southern high-latitude regions (60–90 and 60–90° S). Chosen for the purpose of observing high-latitude processes, the ACE orbit provides sampling of both regions in eight of 12 months of the year, with coverage in all seasons. The ACE water vapour sensors, namely MAESTRO (Measurements of Aerosol Extinction in the Stratosphere and Troposphere Retrieved by Occultation) and the Fourier Transform Spectrometer (ACE-FTS) are currently the only satellite instruments that can probe from the lower stratosphere down to the mid-troposphere to study the vertical profile of the response of UTLS water vapour to the annular modes. The Arctic oscillation (AO), also known as the northern annular mode (NAM), explains 64 % ( r = −0.80) of the monthly variability in water vapour at northern high-latitudes observed by ACE-MAESTRO between 5 and 7 km using only winter months (January to March 2004–2013). Using a seasonal timestep and all seasons, 45 % of the variability is explained by the AO at 6.5 ± 0.5 km, similar to the 46 % value obtained for southern high latitudes at 7.5 ± 0.5 km explained by the Antarctic oscillation or southern annular mode (SAM). A large negative AO event in March 2013 produced the largest relative water vapour anomaly at 5.5 km (+70 %) over the ACE record. A similarly large event in the 2010 boreal winter, which was the largest negative AO event in the record (1950–2015), led to 〉 50 % increases in water vapour observed by MAESTRO and ACE-FTS at 7.5 km.

Description: Introduction: The Pan-Eurasian Experiment (PEEX) – multi-disciplinary, multi-scale and multi-component research and capacity building initiative Atmospheric Chemistry and Physics Discussions, 15, 22567-22596, 2015 Author(s): M. Kulmala, H. K. Lappalainen, T. Petäjä, T. Kurten, V.-M. Kerminen, Y. Viisanen, P. Hari, V. Bondur, N. Kasimov, V. Kotlyakov, G. Matvienko, A. Baklanov, H. D. Guo, A. Ding, H.-C. Hansson, and S. Zilitinkevich The Pan-Eurasian Experiment (PEEX) is a multi-disciplinary, multi-scale and multi-component research, research infrastructure and capacity building program. PEEX has originated from a bottom-up approach by the science communities, and is aiming at resolving the major uncertainties in Earth System Science and global sustainability issues concerning the Arctic and boreal Pan-Eurasian regions, as well as China. The vision of PEEX is to solve interlinked global grand challenges influencing human well-being and societies in northern Eurasia and China. Such challenges include climate change, air quality, biodiversity loss, urbanization, chemicalization, food and fresh water availability, energy production and use of natural resources by mining, industry, energy production and transport sectors. Our approach is integrative and supra-disciplinary, recognizing the important role of the Arctic and boreal ecosystems in the Earth system. The PEEX vision includes establishing and maintaining long-term, coherent and coordinated research activities as well as continuous, comprehensive research and educational infrastructures and related capacity building across the PEEX domain. In this paper we present the PEEX structure, summarize its motivation, objectives and future outlook.

Description: Possible influence of atmospheric circulations on winter hazy pollution in Beijing-Tianjin-Hebei region, northern China Atmospheric Chemistry and Physics Discussions, 15, 22493-22526, 2015 Author(s): Z. Zhang, X. Zhang, D. Gong, S.-J. Kim, R. Mao, and X. Zhao Using the daily records derived from the synoptic weather stations and the NCEP/NCAR and ERA-Interim reanalysis data, the variability of the winter hazy pollutions (indicated by the mean visibility and number of hazy days) in Beijing-Tianjin-Hebei (BTH) region during the period 1981 to 2015 and its relationship to the atmospheric circulations in middle-high latitude were analyzed in this study. The winter hazy pollution in BTH had distinct inter-annual and inter-decadal variabilities without a significant long-term trend. According to the spatial distribution of correlation coefficients, six atmospheric circulation indices ( I 1 to I 6 ) were defined from the key areas in sea level pressure (SLP), zonal and meridional winds at 850 hPa (U850, V850), geopotential height field at 500 hPa (H500), zonal wind at 200 hPa (U200), and air temperature at 200 hPa (T200), respectively. All of the six indices have significant and stable correlations with the winter visibility and number of hazy days in BTH. Both the visibility and number of hazy days can be estimated well by using the six indices and fitting and the cross-validation with leave- N -out method, respectively. The high level of the prediction statistics and the reasonable mechanism suggested that the winter hazy pollutions in BTH can be forecasted or estimated credibly based on the optimized atmospheric circulation indices. However, we also noted that the statistic estimation models would be largely influenced by the artificial control of a pollutant discharge. Thus it is helpful for government decision-making departments to take actions in advance in dealing with probably severe hazy pollutions in BTH indicated by the atmospheric circulation conditions.

Description: The radiative impact of desert dust on orographic rain in the Cevennes–Vivarais area: a case study from HyMeX Atmospheric Chemistry and Physics Discussions, 15, 22451-22492, 2015 Author(s): C. Flamant, J.-P. Chaboureau, P. Chazette, P. Di Girolamo, T. Bourrianne, J. Totems, and M. Cacciani The study is focused on Intensive Observation Period (IOP) 14 of the Hydrology Cycle in the Mediterranean Experiment first Special Observing Period (HyMeX SOP 1) that took place from 17 to 19 October and was dedicated to the study of orographic rain in the Cevennes Vivarais (CV) target area. During this IOP a dense dust plume originating from North Africa (Maghreb and Sahara) was observed to be transported over the Balearic Islands towards the south of France. The plume was characterized by an aerosol optical depth between 0.2 and 0.8 at 550 nm, highly variable in time and space over the Western Mediterranean basin. The impact of this dust plume, the biggest event observed during the 2 month long HyMeX SOP 1, on the precipitation over the CV area has been analyzed using high resolution simulations from the convection permitting mesoscale model Meso-NH validated against measurements obtained from numerous instruments deployed specifically during SOP 1 (ground-based/airborne water vapor and aerosol lidars, airborne microphysics probes) as well as space-borne aerosol products. The 4 day simulation reproduced realistically the temporal and spatial variability (incl. vertical distribution) of the dust. The dust radiative impact led to an average 0.6 K heating at the altitude of the dust layer in the CV area (and up to +3 K locally) and an average 100 J kg -1 increase of most unstable convective available potential energy (and up to +900 J kg -1 locally) with respect to a simulation without prescribed dust aerosols. The rainfall amounts and location were only marginally affected by the dust radiative effect, even after 4 days of simulation. The transient nature of this radiative effect in dynamical environments such as those found in the vicinity of heavy precipitation events in the Mediterranean is not sufficient to impact 24 h accumulated rainfall in the dusty simulation.

Description: Seasonal variation of ozone and black carbon observed at Paknajol, an urban site in the Kathmandu Valley, Nepal Atmospheric Chemistry and Physics Discussions, 15, 22527-22566, 2015 Author(s): D. Putero, P. Cristofanelli, A. Marinoni, B. Adhikary, R. Duchi, S. D. Shrestha, G. P. Verza, T. C. Landi, F. Calzolari, M. Busetto, G. Agrillo, F. Biancofiore, P. Di Carlo, A. K. Panday, M. Rupakheti, and P. Bonasoni The Kathmandu Valley in South Asia is considered as one of the global "hot spots" in terms of urban air pollution. It is facing severe air quality problems as a result of rapid urbanization and land use change, socioeconomic transformation and high population growth. In this paper, we present the first full year (February 2013–January 2014) analysis of simultaneous measurements of two short-lived climate forcers/pollutants (SLCF/P), i.e. ozone (O 3 ) and equivalent black carbon (hereinafter noted as BC) and aerosol number concentration at Paknajol, in the center of the Kathmandu metropolitan city. The diurnal behavior of equivalent black carbon (BC) and aerosol number concentration indicated that local pollution sources represent the major contributions to air pollution in this city. In addition to photochemistry, the planetary boundary layer (PBL) and wind play important roles in determining O 3 variability, as suggested by the analysis of seasonal diurnal cycle and correlation with meteorological parameters and aerosol properties. Especially during pre-monsoon, high values of O 3 were found during the afternoon/evening; this could be related to mixing and entrainment processes between upper residual layers and the PBL. The high O 3 concentrations, in particular during pre-monsoon, appeared well related to the impact of major open vegetation fires occurring at regional scale. On a synoptic-scale perspective, westerly and regional atmospheric circulations appeared to be especially conducive for the occurrence of the high BC and O 3 values. The very high values of SLCF/P, detected during the whole measurement period, indicated persisting adverse air quality conditions, dangerous for the health of over 3 million residents of the Kathmandu Valley, and the environment. Consequently, all of this information may be useful for implementing control measures to mitigate the occurrence of acute pollution levels in the Kathmandu Valley and surrounding area.

Description: Variability of water vapour in the Arctic stratosphere Atmospheric Chemistry and Physics Discussions, 15, 22013-22045, 2015 Author(s): L. Thölix, L. Backman, R. Kivi, and A. Karpechko This study evaluates the stratospheric water vapour distribution and variability in the Arctic. A FinROSE chemistry climate model simulation covering years 1990–2013 is compared to observations (satellite and frostpoint hygrometer soundings) and the sources of stratospheric water vapour are studied. According to observations and the simulations the water vapour concentration in the Arctic stratosphere started to increase after year 2006, but around 2011 the concentration started to decrease. Model calculations suggest that the increase in water vapour during 2006–2011 (at 56 hPa) is mostly explained by transport related processes, while the photochemically produced water vapour plays a relatively smaller role. The water vapour trend in the stratosphere may have contributed to increased ICE PSC occurrence. The increase of water vapour in the precense of the low winter temperatures in the Arctic stratosphere led to more frequent occurrence of ICE PSCs in the Arctic vortex. The polar vortex was unusually cold in early 2010 and allowed large scale formation of the polar stratospheric clouds. The cold pool in the stratosphere over the Northern polar latitudes was large and stable and a large scale persistent dehydration was observed. Polar stratospheric ice clouds and dehydration were observed at Sodankylä with accurate water vapour soundings in January and February 2010 during the LAPBIAT atmospheric sounding campaign. The observed changes in water vapour were reproduced by the model. Both the observed and simulated decrease of the water vapour in the dehydration layer was up to 1.5 ppm.

Description: Putting the clouds back in aerosol-cloud interactions Atmospheric Chemistry and Physics Discussions, 15, 20775-20810, 2015 Author(s): A. Gettelman Aerosol Cloud Interactions (ACI) are the consequence of perturbed aerosols affecting cloud drop and crystal number, with corresponding microphysical and radiative effects. ACI are sensitive to both cloud microphysical processes (the "C" in ACI) and aerosol emissions and processes (the "A" in ACI). This work highlights the importance of cloud microphysical processes, using idealized and global tests of a cloud microphysics scheme used for global climate prediction. Uncertainties in cloud microphysical processes cause uncertainties of up to −35 to +50 % in ACI, stronger than uncertainties due to natural aerosol emissions (−20 to +30 %). The different dimensions and sensitivities of ACI to microphysical processes are analyzed in detail, showing that precipitation processes are critical for understanding ACI and that uncertain cloud lifetime effects are 1/3 of simulated ACI. Buffering of different processes is important, as is the mixed phase and coupling of the microphysics to the condensation and turbulence schemes in the model.

Description: Evaluation of VIIRS, GOCI, and MODIS Collection 6 AOD retrievals against ground sunphotometer measurements over East Asia Atmospheric Chemistry and Physics Discussions, 15, 20709-20741, 2015 Author(s): Q. Xiao, H. Zhang, M. Choi, S. Li, S. Kondragunta, J. Kim, B. Holben, R. C. Levy, and Y. Liu Persistent high aerosol loadings together with extremely high population density have raised serious air quality and public health concerns in many urban centers in East Asia. However, ground based air quality monitoring is relatively limited in this area. Recently, satellite retrieved Aerosol Optical Depth (AOD) at high resolution has become a powerful tool to characterize aerosol patterns in space and time. Using ground AOD measurements from the Aerosol Robotic Network (AERONET) and the Distributed Regional Aerosol Gridded Observation Networks (DRAGON)-Asia Campaign, as well as from handheld sunphotometers, we evaluated emerging aerosol products from the Visible Infrared Imaging Radiometer Suite (VIIRS) aboard the Suomi National Polar-orbiting Partnership (S-NPP), the Geostationary Ocean Color Imager (GOCI) aboard the Communication, Ocean, and Meteorology Satellite (COMS), and Terra and Aqua Moderate Resolution Imaging Spectroradiometer (MODIS) (Collection 6) in East Asia in 2012 and 2013. In the case study in Beijing, when compared with AOD measured by handheld sunphotometers, 51 % of VIIRS Environmental Data Record (EDR) AOD, 33 % of VIIRS Intermediate Product (IP) AOD, 31 % of GOCI AOD, 26 % of Terra MODIS C6 3 km AOD, and 16 % of Aqua MODIS C6 3 km AOD fell within the reference expected error (EE) envelop (±0.05 ± 0.15 AOD). Comparing against AERONET measurements over the Japan–South Korea region, 64 % of EDR, 37 % of IP, 62 % of GOCI, 39 % of Terra MODIS and 56 % of Aqua MODIS C6 3 km AOD fell within the EE. In general, satellite aerosol products performed better in tracking the day-to-day variability than tracking the spatial variability at high resolutions. The VIIRS EDR and GOCI products provided the most accurate AOD retrievals, while VIIRS IP and MODIS C6 3 km products had positive biases.

Description: Toward enhanced capability for detecting and predicting dust events in the Western United States: the Arizona Case Study Atmospheric Chemistry and Physics Discussions, 15, 20743-20774, 2015 Author(s): M. Huang, D. Tong, P. Lee, L. Pan, Y. Tang, I. Stajner, R. B. Pierce, J. McQueen, and J. Wang Dust aerosols affect human life, ecosystems, atmospheric chemistry and climate in various aspects. Studies have revealed intensified dust activity in the western US during the past decades despite the weaker dust activity in non-US regions. It is important to extend the historical dust records, to better understand their temporal changes, and use such information to improve the daily dust forecasting skill as well as the projection of future dust activity under the changing climate. This study develops dust records in Arizona in 2005–2013 using multiple observation datasets, including in-situ measurements at the surface Air Quality System (AQS) and Interagency Monitoring of Protected Visual Environments (IMPROVE) sites, and level 2 deep blue aerosol product by the Moderate Resolution Imaging Spectroradiometer. The diurnal and inter-annual variability of identified dust events are shown related to observed weather patterns (e.g., wind and soil moisture) and vegetation conditions, suggesting a potential for use of satellite soil moisture and vegetation index products to interpret and predict dust activity. Back-trajectories computed using NOAA's Hybrid Single Particle Lagrangian Integrated Trajectory (HYSPLIT) Model indicate that the Sonoran and Chihuahuan deserts are important dust source regions during identified dust events in Phoenix, Arizona. Finally, we evaluate the performance of the US National Air Quality Forecasting Capability (NAQFC) 12 km CMAQ model during a recent strong dust event in the western US accompanied by stratospheric ozone intrusion. It is shown that the current modeling system well captures the temporal variability and the magnitude of aerosol concentrations during this event, and the usefulness and limitations of different observations in model evaluation are discussed. Directions of integrating observations to further improve dust emission modeling in CMAQ are also suggested.

Description: Size distributions of polycyclic aromatic hydrocarbons in urban atmosphere: sorption mechanism and source contributions to respiratory deposition Atmospheric Chemistry and Physics Discussions, 15, 20811-20850, 2015 Author(s): Y. Lv, X. Li, T. T. Xu, T. T. Cheng, X. Yang, J. M. Chen, Y. Linuma, and H. Herrmann Current knowledge on atmospheric particle-phase polycyclic aromatic hydrocarbons (PAHs) size distribution remains incomplete. Information is missing on sorption mechanisms and the influence of the PAHs' sources on their transport in human respiratory system. Here we present the studies systematically investigating the modal distribution characteristics of the size-fractioned PAHs and calculating the source contribution to adverse health effects through inhalation. Aerosol samples with nine size fractions were collected from Shanghai urban air over one year period 2012–2013. A high correlation coefficient existed between measured and predicted values ( R 2 = 0.87), indicated that the data worked very well in current study. Most PAHs were observed on the small particles followed with seasonality differences. When normalized by PAHs across particle diameters, the size distribution of PAHs exhibited bimodal patterns, with a peak (0.4–2.1 μm) in fine mode and another peak (3.3–9.0 μm) in coarse mode, respectively. Along with the increasing ring number of PAHs, the intensity of the fine mode peak increased, while coarse mode peak decreased. Plotting of log(PAH/PM) against log( D p ) showed that all slope values were above −1 with the increase towards less-ring PAHs, suggesting that multiple mechanisms, i.e. adsorption and absorption controlled the PAHs on particles, but adsorption played a much stronger role for 5- and 6-ring than 3- and 4-ring PAHs. The mode distribution behavior of PAHs showed that fine particles were major carriers for the more-ring PAHs. Further calculations using inhaling PAHs data showed the total deposition fluxes in respiratory tract were 8.8 ± 2.0 ng h -1 . Specifically, fine particles contributed 10–40 % of PAHs deposition fluxes to the alveolar region, while coarse particles contributed 80–95 % of ones to the head region. Estimated lifetime cancer risk (LCR) for people exercised in haze days (1.5 × 10 -6 ) was bigger than the cancer risk guideline value (10 -6 ). The largest PAHs contribution for LCR mainly came from the accumulation particles. Based on source apportionment results generated by positive matrix factorization (PMF), it was found that the cancer risk caused in accumulated mode mainly resulted from biomass burning (24 %), coal combustion (25 %) and vehicular emission (27 %). The present results contribute to a mechanistic understanding of PAHs size distribution causing adverse health effects and will help develop some source control strategies or policies by relying on respiratory assessment data.

Description: Instantaneous longwave radiative impact of ozone: an application on IASI/MetOp observations Atmospheric Chemistry and Physics Discussions, 15, 21177-21218, 2015 Author(s): S. Doniki, D. Hurtmans, L. Clarisse, C. Clerbaux, H. M. Worden, K. W. Bowman, and P.-F. Coheur Ozone is an important greenhouse gas in terms of anthropogenic radiative forcing (RF). RF calculations for ozone were until recently entirely model based and significant discrepancies were reported due to different model characteristics. However, new instantaneous radiative kernels (IRKs) calculated from hyperspectral thermal IR satellites have been able to help adjudicate between different climate model RF calculations. IRKs are defined as the sensitivity of the outgoing longwave radiation (OLR) flux with respect to the ozone vertical distribution in the full 9.6 μm band. Previous methods applied to measurements from the Tropospheric Emission Spectrometer (TES) on Aura, rely on an anisotropy approximation for the angular integration. In this paper, we present a more accurate but more computationally expensive method to calculate these kernels. The method of direct integration is based on similar principles with the anisotropy approximation, but deals more precisely with the integration of the Jacobians. We describe both methods and highlight their differences with respect to the IRKs and the ozone longwave radiative effect (LWRE), i.e. the radiative impact in OLR due to absorption by ozone, for both tropospheric and total columns, from measurements of the Infrared Atmospheric Sounding Interferometer (IASI) onboard MetOp-A. Biases between the two methods vary from −25 to +20 % for the LWRE, depending on the viewing angle. These biases point to the inadequacy of the anisotropy method, especially at nadir, suggesting that the TES derived LWRE are biased low by around 25 % and that chemistry-climate model OLR biases with respect to TES are underestimated. In this paper we also exploit the sampling performance of IASI to obtain first daily global distributions of the LWRE, for 12 days (the 15th of each month) in 2011, calculated with the direct integration method. We show that the temporal variation of global and latitudinal averages of the LWRE shows patterns which are controlled by changes in the surface temperature and ozone variation due to specific processes, such as the ozone hole in the Polar regions and stratospheric intrusions into the troposphere.

Description: Reactive nitrogen partitioning and its relationship to winter ozone events in Utah Atmospheric Chemistry and Physics Discussions, 15, 21383-21413, 2015 Author(s): R. J. Wild, P. M. Edwards, T. S. Bates, R. C. Cohen, J. A. de Gouw, W. P. Dubé, J. B. Gilman, J. Holloway, J. Kercher, A. Koss, L. Lee, B. Lerner, R. McLaren, P. K. Quinn, J. M. Roberts, J. Stutz, J. A. Thornton, P. R. Veres, C. Warneke, E. Williams, C. J. Young, B. Yuan, and S. S. Brown High wintertime ozone levels have been observed in the Uintah Basin, Utah, a sparsely populated rural region with intensive oil and gas operations. The reactive nitrogen budget plays an important role in tropospheric ozone formation. Measurements were taken during three field campaigns in the winters of 2012, 2013, and 2014, which experienced varying climatic conditions. Average concentrations of ozone and total reactive nitrogen were observed to be 2.5 times higher in 2013 than 2012, with 2014 an intermediate year in most respects. However, photochemically active NO x (NO+NO 2 ), remained remarkably similar all three years. Roughly half of the more oxidized forms of nitrogen were composed of nitric acid in 2013, with nighttime nitric acid formation through heterogeneous uptake of N 2 O 5 contributing approximately 6 times more than daytime formation. The nighttime N 2 O 5 lifetime between the high-ozone year 2013 and the low-ozone year 2012 is lower by a factor 2.6, and much of this is due to higher aerosol surface area in the high ozone year of 2013. A box-model simulation supports the importance of nighttime chemistry on the reactive nitrogen budget, showing a large sensitivity of NO x and ozone concentrations to nighttime processes.

Description: Global and regional emissions estimates of 1,1-difluoroethane (HFC-152a, CH 3 CHF 2 ) from in situ and air archive observations Atmospheric Chemistry and Physics Discussions, 15, 21335-21381, 2015 Author(s): P. G. Simmonds, M. Rigby, A. J. Manning, M. F. Lunt, S. O'Doherty, D. Young, A. McCulloch, P. J. Fraser, S. Henne, M. K. Vollmer, S. Reimann, A. Wenger, J. Mühle, C. M. Harth, P. K. Salameh, T. Arnold, R. F. Weiss, P. B. Krummel, L. P. Steele, B. L. Dunse, B. R. Miller, C. R. Lunder, O. Hermansen, N. Schmidbauer, T. Saito, Y. Yokouchi, S. Park, S. Li, B. Yao, L. X. Zhou, J. Arduini, M. Maione, R. H. J. Wang, and R. G. Prinn High frequency, ground-based, in situ measurements from eleven globally-distributed sites covering 1994–2014, combined with measurements of archived air samples dating from 1978 onward and atmospheric transport models, have been used to estimate the growth of 1,1-difluoroethane (HFC-152a, CH 3 CHF 2 ) mole fractions in the atmosphere and the global emissions required to derive the observed growth. HFC-152a is a significant greenhouse gas but since it does not contain chlorine or bromine, HFC-152a makes no direct contribution to the destruction of stratospheric ozone and is therefore used as a substitute for the ozone depleting chlorofluorocarbons (CFCs) and hydrochlorofluorocarbons (HCFCs). HFC-152a has exhibited substantial atmospheric growth since the first measurements reaching a maximum annualised global growth rate of 0.81 ± 0.05 ppt yr −1 in 2006, implying a substantial increase in emissions up to 2006. However, since 2007, the annualised rate of growth has slowed to 0.38 ± 0.04 ppt yr −1 in 2010 with a further decline to an average rate of change in 2013–2014 of −0.06 ± 0.05 ppt yr −1 . The average Northern Hemisphere (NH) mixing ratio in 1994 was 1.2 ppt rising to a mixing ratio of 10.2 ppt in December 2014. Average annual mixing ratios in the Southern Hemisphere (SH) in 1994 and 2014 were 0.34 and 4.4 ppt, respectively. We estimate global emissions of HFC-152a have risen from 7.3 ± 5.6 Gg yr −1 in 1994 to a maximum of 54.4 ± 17.1 Gg yr −1 in 2011, declining to 52.5 ± 20.1 Gg yr −1 in 2014 or 7.2 ± 2.8 Tg-CO 2 eq yr −1 . Analysis of mixing ratio enhancements above regional background atmospheric levels suggests substantial emissions from North America, Asia and Europe. Global HFC emissions (so called "bottom up" emissions) reported by the United Nations Framework Convention on Climate Change (UNFCCC) are based on cumulative national emission data reported to the UNFCCC, which in turn are based on national consumption data. There appears to be a significant underestimate of "bottom-up" global emissions of HFC-152a, possibly arising from largely underestimated USA emissions and undeclared Asian emissions.

Description: Contributions of dust and biomass-burning to aerosols at a Colorado mountain-top site Atmospheric Chemistry and Physics Discussions, 15, 21299-21334, 2015 Author(s): A. G. Hallar, R. Petersen, E. Andrews, J. Michalsky, I. B. McCubbin, and J. A. Ogren Visible Multifilter Rotating Shadowband Radiometer (MFRSR) data were collected at Storm Peak Laboratory (SPL), a mountain top facility in northwest Colorado, from 1999–2011 and in 2013. From 2011–2014, in situ measurements of aerosol light scattering were also obtained. Using these datasets together, the seasonal impact of dust and biomass burning is considered for the western United States. Analysis indicates that the median contributions to spring and summer aerosol optical depth (AOD) from dust and biomass-burning aerosols across the dataset are comparable. The mean AOD is slightly greater in the summer, with significantly more frequent and short duration high AOD measurements due to biomass-burning episodes, than in the spring. The Ångström exponent showed a significant increase in the summer for both the in situ and MFRSR data, indicating an increase in combustion aerosols. Spring dust events are less distinguishable in the in situ data than the column measurement, suggesting that a significant amount of dust may be found above the elevation of SPL, 3220 m a.s.l. Twenty-two known case studies of intercontinental dust, regional dust, and biomass burning events were investigated. These events were found to follow a similar pattern, in both aerosol loading and Ångström exponent, as the seasonal mean signal in both the MFRSR and ground-based nephelometer. This dataset highlights the wide scale implications of a warmer, drier climate on visibility in the western United States.

Description: Neutral atmosphere temperature change at 90 km, 70° N, 19° E, 2003–2014 Atmospheric Chemistry and Physics Discussions, 15, 15289-15317, 2015 Author(s): S. E. Holmen, C. M. Hall, and M. Tsutsumi Neutral temperatures for 90 km height above Tromsø, Norway, have been determined using ambipolar diffusion coefficients calculated from meteor echo fading times using the Nippon/Norway Tromsø Meteor Radar (NTMR). Daily temperature averages have been calculated from November 2003 to October 2014 and calibrated against temperature measurements from the Microwave Limb Sounder (MLS) on board Aura. The long-term trend of temperatures from the NTMR radar is investigated, and winter and summer seasons are looked at separately. Seasonal variation has been accounted for, as well as solar response, using the F10.7 cm flux as a proxy for solar activity. The long-term temperature trend from 2003 to 2014 is −3.6 K ± 1.1 K decade −1 , with summer and winter trends −0.8 K ± 2.9 K decade −1 and −8.1 K ± 2.5 K decade −1 , respectively. How well suited a meteor radar is for estimating neutral temperatures at 90 km using meteor trail echoes is discussed, and physical explanations behind a cooling trend are proposed.

Description: Size-resolved measurements of ice nucleating particles at six locations in North America and one in Europe Atmospheric Chemistry and Physics Discussions, 15, 20521-20559, 2015 Author(s): R. H. Mason, M. Si, C. Chou, V. E. Irish, R. Dickie, P. Elizondo, R. Wong, M. Brintnell, M. Elsasser, W. M. Lassar, K. M. Pierce, W. R. Leaitch, A. M. MacDonald, A. Platt, D. Toom-Sauntry, R. Sarda-Estève, C. L. Schiller, K. J. Suski, T. C. J. Hill, J. P. D. Abbatt, J. A. Huffman, P. J. DeMott, and A. K. Bertram Detailed information on the size of ice nucleating particles (INPs) may be useful in source identification, modeling their transport in the atmosphere to improve climate predictions, and determining how effectively or ineffectively instrumentation used for quantifying INPs in the atmosphere captures the full INP population. In this study we report immersion-mode INP number concentrations as a function of size at six ground sites in North America and one in Europe. The lowest INP number concentrations were observed at Arctic and alpine locations and the highest at suburban and agricultural locations, consistent with previous studies of INP concentrations in similar environments. We found that 91, 79, and 63 % of INPs had an aerodynamic diameter 〉 1 μm at ice activation temperatures of −15, −20, and −25 °C, respectively, when averaging over all sampling locations. In addition, 62, 55, and 42 % of INPs were in the coarse mode (〉 2.5 μm) at ice activation temperatures of −15, −20, and −25 °C, respectively, when averaging over all sampling locations. These results are consistent with six out of the seven studies in the literature that have focused on the size distribution of INPs in the atmosphere. Taken together, these findings strongly suggest that supermicron and coarse mode aerosol particles are a significant component of the ice nuclei population in many different ground-level environments. Further size-resolved studies of INPs as a function of altitude are required.

Description: The impact of residential combustion emissions on atmospheric aerosol, human health and climate Atmospheric Chemistry and Physics Discussions, 15, 20449-20520, 2015 Author(s): E. W. Butt, A. Rap, A. Schmidt, C. E. Scott, K. J. Pringle, C. L. Reddington, N. A. D. Richards, M. T. Woodhouse, J. Ramirez-Villegas, H. Yang, V. Vakkari, E. A. Stone, M. Rupakheti, P. S. Praveen, P. G. van Zyl, J. P. Beukes, M. Josipovic, E. J. S. Mitchell, S. M. Sallu, P. M. Forster, and D. V. Spracklen Combustion of fuels in the residential sector for cooking and heating, results in the emission of aerosol and aerosol precursors impacting air quality, human health and climate. Residential emissions are dominated by the combustion of solid fuels. We use a global aerosol microphysics model to simulate the uncertainties in the impact of residential fuel combustion on atmospheric aerosol. The model underestimates black carbon (BC) and organic carbon (OC) mass concentrations observed over Asia, Eastern Europe and Africa, with better prediction when carbonaceous emissions from the residential sector are doubled. Observed seasonal variability of BC and OC concentrations are better simulated when residential emissions include a seasonal cycle. The largest contributions of residential emissions to annual surface mean particulate matter (PM 2.5 ) concentrations are simulated for East Asia, South Asia and Eastern Europe. We use a concentration response function to estimate the health impact due to long-term exposure to ambient PM 2.5 from residential emissions. We estimate global annual excess adult (〉 30 years of age) premature mortality of 308 000 (113 300–497 000, 5th to 95th percentile uncertainty range) for monthly varying residential emissions and 517 000 (192 000–827 000) when residential carbonaceous emissions are doubled. Mortality due to residential emissions is greatest in Asia, with China and India accounting for 50 % of simulated global excess mortality. Using an offline radiative transfer model we estimate that residential emissions exert a global annual mean direct radiative effect of between −66 and +21 mW m -2 , with sensitivity to the residential emission flux and the assumed ratio of BC, OC and SO 2 emissions. Residential emissions exert a global annual mean first aerosol indirect effect of between −52 and −16 mW m -2 , which is sensitive to the assumed size distribution of carbonaceous emissions. Overall, our results demonstrate that reducing residential combustion emissions would have substantial benefits for human health through reductions in ambient PM 2.5 concentrations.

Description: Remote sensing of soot carbon – Part 2: Understanding the absorption Angstrom exponent Atmospheric Chemistry and Physics Discussions, 15, 20911-20956, 2015 Author(s): G. L. Schuster, O. Dubovik, A. Arola, T. F. Eck, and B. N. Holben Recently, some authors have suggested that the absorption Angstrom exponent (AAE) can be used to deduce the component aerosol absorption optical depths (AAOD) of carbonaceous aerosols in the AERONET database. This "AAE approach" presumes that AAE ≪ 1 for soot carbon, which contrasts the traditional small particle limit of AAE = 1 for soot carbon. Thus, we provide an overview of the AERONET retrieval, and investigate how the microphysics of carbonaceous aerosols can be interpreted in the AERONET AAE product. We find that AAE ≪ 1 in the AERONET database requires large coarse mode fractions and/or imaginary refractive indices that increase with wavelength. Neither of these characteristics are consistent with the current definition of soot carbon, so we explore other possibilities for the cause of AAE ≪ 1. We note that AAE is related to particle size, and that coarse mode particles have a smaller AAE than fine mode particles for a given aerosol mixture of species. We also note that the mineral goethite has an imaginary refractive index that increases with wavelength, is very common in dust regions, and can easily contribute to AAE ≪ 1. We find that AAE ≪ 1 can not be caused by soot carbon, unless soot carbon has an imaginary refractive index that increases with wavelength throughout the visible and near infrared spectrums. Finally, AAE is not a robust parameter for separating carbonaceous absorption from dust aerosol absorption in the AERONET database.

Description: Ozone and carbon monoxide over India during the summer monsoon: regional emissions and transport Atmospheric Chemistry and Physics Discussions, 15, 21133-21176, 2015 Author(s): N. Ojha, A. Pozzer, A. Rauthe-Schöch, A. K. Baker, J. Yoon, C. A. M. Brenninkmeijer, and J. Lelieveld We compare in situ measurements of ozone (O 3 ) and carbon monoxide (CO) profiles from the CARIBIC program with the results from the regional chemistry transport model (WRF-Chem) to investigate the role of local/regional emissions and long-range transport over southern India during the summer monsoon of 2008. WRF-Chem successfully reproduces the general features of O 3 and CO distributions over the South Asian region. However, the absolute CO concentrations in lower troposphere are typically underestimated. Here we investigate the influence of local relative to remote emissions through sensitivity simulations. The influence of 50 % enhanced CO emissions over South Asia is found to be 33 % increase in surface CO during June. The influence of enhanced local emissions is found to be smaller (5 %) in the free troposphere, except during September. Local to regional emissions are therefore suggested to play a minor role in the underestimation of CO by WRF-Chem during June–August. In the lower troposphere, ahigh pollution (O 3 : 146.4 ± 12.8 nmol mol −1 , CO: 136.4 ± 12.2 nmol mol −1 ) event (15 July 2008), not reproduced by the model, is shown to be due to transport of photochemically processed air masses from the boundary layer into southern India. Sensitivity simulation combined with backward trajectories indicates that long-range transport of CO to southern India is significantly underestimated, particularly in air masses from the west, i.e. from Central Africa. This study highlights the need for more aircraft-based measurements over India and adjacent regions and the improvement of emission inventories.

Description: Understanding cirrus ice crystal number variability for different heterogeneous ice nucleation spectra Atmospheric Chemistry and Physics Discussions, 15, 21671-21711, 2015 Author(s): S. C. Sullivan, R. Morales Betancourt, D. Barahona, and A. Nenes Along with minimizing parameter uncertainty, understanding the cause of temporal and spatial variability of nucleated ice crystal number, N i , is key to improving the representation of cirrus clouds in climate models. To this end, sensitivities of N i to input variables like aerosol number and diameter provide valuable information about nucleation regime and efficiency for a given model formulation. Here we use the adjoint model of the Barahona and Nenes cirrus formation parameterization to understand N i variability for various ice-nucleating particle (INP) spectra. Inputs are generated with the Community Atmosphere Model version 5, and simulations are done with a theoretically-derived spectrum, a lab-based empirical spectrum, and two field-based empirical spectra that differ in the nucleation threshold for black carbon aerosol and in the active site density for dust. The magnitude and sign of N i sensitivity to insoluble aerosol number can be directly linked to nucleation regime and efficiency of various INP. The lab-based spectrum calculates much higher INP efficiencies than field-based ones, which reveals a disparity in aerosol surface properties. N i sensitivity to temperature tends to be low, due to the compensating effects of temperature on INP spectrum parameters; this low temperature sensitivity regime has been experimentally reported before but never unraveled as done here.

Description: Viscosity controls humidity dependence of N 2 O 5 uptake to citric acid aerosol Atmospheric Chemistry and Physics Discussions, 15, 21983-22011, 2015 Author(s): G. Gržinić, T. Bartels-Rausch, T. Berkemeier, A. Türler, and M. Ammann The heterogeneous loss of dinitrogen pentoxide (N 2 O 5 ) to aerosol particles has a significant impact on the night time nitrogen oxide cycle and therefore the oxidative capacity in the troposphere. Using a 13 N short lived radioactive tracer method we studied the uptake kinetics of N 2 O 5 on citric acid aerosol particles as a function of relative humidity (RH). The results show that citric acid exhibits lower reactivity than similar di- and polycarboxylic acids, with uptake coefficients between ~ 3 × 10 −4 –~ 3 × 10 −3 depending on humidity (17–70 % RH). This humidity dependence can be explained by a changing viscosity and, hence, diffusivity in the organic matrix. Since the viscosity of highly concentrated citric acid solutions is not well established, we present four different parameterizations of N 2 O 5 diffusivity based on the available literature data or estimates for viscosity and diffusivity. Above 50 % RH, uptake is consistent with the reacto-diffusive kinetic regime whereas below 50 % RH, the uptake coefficient is higher than expected from hydrolysis of N 2 O 5 within the bulk of the particles, and the uptake kinetics may be limited by loss on the surface only. This study demonstrates the impact of viscosity in highly oxidized and highly functionalized secondary organic aerosol material on the heterogeneous chemistry of N 2 O 5 and may explain some of the unexpectedly low loss rates to aerosol derived from field studies.

Description: Distinguishing the drivers of trends in land carbon fluxes and plant volatile emissions over the past three decades Atmospheric Chemistry and Physics Discussions, 15, 21449-21494, 2015 Author(s): X. Yue, N. Unger, and Y. Zheng The terrestrial biosphere has experienced dramatic changes in recent decades. Estimates of historical trends in land carbon fluxes remain uncertain because long-term observations are limited on the global scale. Here, we use the Yale Interactive terrestrial Biosphere (YIBs) model to estimate decadal trends in land carbon fluxes and emissions of biogenic volatile organic compounds (BVOCs) and to identify the key drivers for these changes during 1982–2011. Driven with hourly meteorology from WFDEI (WATCH Forcing Data methodology applied to ERA-Interim data), the model simulates an increasing trend of 297 Tg C a −2 in gross primary productivity (GPP) and 185 Tg C a −2 in the net primary productivity (NPP). CO 2 fertilization is the main driver for the flux changes in forest ecosystems, while meteorology dominates the changes in grasslands and shrublands. Warming boosts summer GPP and NPP at high latitudes, while drought dampens carbon uptake in tropical regions. North of 30° N, increasing temperatures induce a substantial extension of 0.22 day a −1 for the growing season; however, this phenological change alone does not promote regional carbon uptake and BVOC emissions. Nevertheless, increases of LAI at peak season accounts for ~ 25 % of the trends in GPP and isoprene emissions at the northern lands. The net land sink shows statistically insignificant increases of only 3 Tg C a −2 globally because of simultaneous increases in soil respiration. In contrast, driven with alternative meteorology from MERRA (Modern Era-Retrospective Analysis), the model predicts significant increases of 59 Tg C a −2 in the land sink due to strengthened uptake in the Amazon. Global BVOC emissions are calculated using two schemes. With the photosynthesis-dependent scheme, the model predicts increases of 0.4 Tg C a −2 in isoprene emissions, which are mainly attributed to warming trends because CO 2 fertilization and inhibition effects offset each other. Using the MEGAN (Model of Emissions of Gases and Aerosols from Nature) scheme, the YIBs model simulates global reductions of 1.1 Tg C a −2 in isoprene and 0.04 Tg C a −2 in monoterpene emissions in response to the CO 2 inhibition effects. Land use change shows limited impacts on global carbon fluxes and BVOC emissions, but there are regional contrasting impacts over Europe (afforestation) and China (deforestation).

Description: A method to retrieve super-thin cloud optical depth over ocean background with polarized sunlight Atmospheric Chemistry and Physics Discussions, 15, 21959-21982, 2015 Author(s): W. Sun, R. R. Baize, G. Videen, Y. Hu, and Q. Fu In this work, an algorithm that uses the polarization angle of the backscattered solar radiation to detect clouds with optical depth (OD) 〈 ~ 0.3 is further developed. We find that at viewing angles within ± ~ 8° around the backscattering direction, the p -polarized intensity that is parallel to the meridian plane of reflected light from surface is sensitive to and nearly linearly related to the optical depth of super-thin clouds. Moreover, our sensitivity study suggests that the p -polarized intensity at these viewing angles is not sensitive to the ocean surface conditions. Using this property of p -polarized intensity, super-thin clouds' optical depth can be retrieved.

Description: Comparisons of urban and rural PM 10−2.5 and PM 2.5 mass concentrations and semi-volatile fractions in Northeastern Colorado Atmospheric Chemistry and Physics Discussions, 15, 24587-24622, 2015 Author(s): N. Clements, M. P. Hannigan, S. L. Miller, J. L. Peel, and J. B. Milford Coarse (PM 10−2.5 ) and fine (PM 2.5 ) particulate matter in the atmosphere adversely affect human health and influence climate. While PM 2.5 is relatively well studied, less is known about the sources and fate of PM 10−2.5 . The Colorado Coarse Rural-Urban Sources and Health (CCRUSH) study measured PM 10−2.5 and PM 2.5 mass concentrations, as well as the fraction of semi-volatile material (SVM) in each size regime (SVM 2.5 , SVM 10−2.5 ), for three years in Denver and comparatively rural Greeley, Colorado. Agricultural operations east of Greeley appear to have contributed to the peak PM 10−2.5 concentrations there, but concentrations were generally lower in Greeley than in Denver. Traffic-influenced sites in Denver had PM 10−2.5 concentrations that averaged from 14.6 to 19.7 μg m −3 and mean PM 10−2.5 /PM 10 ratios of 0.56 to 0.70, higher than at residential sites in Denver or Greeley. PM 10−2.5 concentrations were more temporally variable than PM 2.5 concentrations. Concentrations of the two pollutants were not correlated. Spatial correlations of daily averaged PM 10−2.5 concentrations ranged from 0.59 to 0.62 for pairs of sites in Denver and from 0.47 to 0.70 between Denver and Greeley. Compared to PM 10−2.5 , concentrations of PM 2.5 were more correlated across sites within Denver and less correlated between Denver and Greeley. PM 10−2.5 concentrations were highest during the summer and early fall, while PM 2.5 and SVM 2.5 concentrations peaked in winter during periodic multi-day inversions. SVM 10−2.5 concentrations were low at all sites. Diurnal peaks in PM 10−2.5 and PM 2.5 concentrations corresponded to morning and afternoon peaks of traffic activity, and were enhanced by boundary layer dynamics. SVM 2.5 concentrations peaked around noon on both weekdays and weekends. PM 10−2.5 concentrations at sites located near highways generally increased with wind speeds above about 3 m s −1 . Little wind speed dependence was observed for the residential sites in Denver and Greeley.

Description: Speciation of 127 I and 129 I in atmospheric aerosols at Risø, Denmark: insight into sources of iodine isotopes and their species transformations Atmospheric Chemistry and Physics Discussions, 15, 25139-25173, 2015 Author(s): L. Y. Zhang, X. L. Hou, and S. Xu Speciation analysis of iodine in aerosols is a very useful approach for understanding geochemical cycling of iodine in the atmosphere. In this study, overall iodine species, including water-soluble iodine species (iodide, iodate and water-soluble organic iodine), NaOH-soluble iodine and insoluble iodine have been determined for 129 I and 127 I in the aerosols collected at Risø, Denmark, between March and May 2011 (shortly after the Fukushima nuclear accident) and in December 2014. The measured concentrations of total iodine are in the range of 1.04–2.48 ng m −3 for 127 I and (11.3–97.0) × 10 5 atoms m −3 for 129 I, and 129 I / 127 I atomic ratios of (17.8–86.8) × 10 −8 . The contribution of Fukushima-derived 129 I (peak value of 6.3 × 10 4 atoms m −3 ) is estimated to be negligible (less than 6 %) compared to the total 129 I concentration in northern Europe. The concentrations and species of 129 I and 127 I in the aerosols are found to be strongly related to their sources and atmospheric pathways. Aerosols that were transported over the contaminated ocean, contained higher amounts of 129 I than aerosols transported over the European continent. The high 129 I concentrations of the marine aerosols are attributed to secondary emission from heavily 129 I-contaminated seawater rather than primary gaseous release from nuclear reprocessing plants. Water-soluble iodine was found to be a minor fraction to total iodine for both 127 I (7.8–13.7 %) and 129 I (6.5–14.1 %) in ocean-derived aerosols, but accounted for 20.2–30.3 % for 127 I and 25.6–29.5 % for 129 I in land-derived aerosols. Iodide was the predominant form of water-soluble iodine, accounting for more than 97 % of the water-soluble iodine. NaOH-soluble iodine seems to be independent of the sources of aerosols. The significant proportion of 129 I and 127 I found in NaOH-soluble fractions is likely bound with organic substances. In contrast to water-soluble iodine however, the sources of air masses exerted distinct influences on insoluble iodine for both 129 I and 127 I, with higher values for marine air masses and lower values for terrestrial air masses.

Description: Measurements of non-volatile aerosols with a VTDMA and their correlations with carbonaceous aerosols in Guangzhou, China Atmospheric Chemistry and Physics Discussions, 15, 25269-25298, 2015 Author(s): H. H. Y. Cheung, H. B. Tan, H. B. Xu, F. Li, C. Wu, J. Z. Yu, and C. K. Chan Simultaneous measurements of aerosols of varying volatilities and carbonaceous matters at an urban site of Guangzhou, China were conducted in February and March 2014 using a Volatility Tandem Differential Mobility Analyzer (VTDMA) and an Organic Carbon/Elemental Carbon (OC/EC) Analyzer. In VTDMA, selected aerosols of 40 to 300 nm in mobility diameter were heated at elevated temperatures up to 300 °C and the size distributions of the residual particles were measured. Size dependent distributions were observed for both non-volatile and volatile materials (VM). The 40 nm particles were dominated by particles that completely vaporized (CV) at 300 °C, with an average number fraction of about 0.4. Particles larger than 80 nm were dominated by the medium and low volatility (MV and LV, respectively) fractions, with average number fractions of 0.5 and 0.15 respectively. VM did not contribute separately to number fraction but contributed to over 50 % of the total volume fraction for all sizes. In both number and volume fractions, diurnal variation was only observed for the high volatility (HV) group, CV and VM in 40 nm particles, likely because these particles were related to fresh emissions. The little diurnal variation of larger particles could be attributed to non-locally aged aerosols. Closure analysis between the residual mass of LV + MV and mass of EC or EC+OC 2–4 (sum of EC, OC 2 , OC 3 , and OC 4 of the OC/EC Analyzer) suggests that non-volatile materials measured by the VTDMA likely contain less volatile OC.

Description: Observations of high droplet number concentrations in Southern Ocean boundary layer clouds Atmospheric Chemistry and Physics Discussions, 15, 25503-25545, 2015 Author(s): T. Chubb, Y. Huang, J. Jensen, T. Campos, S. Siems, and M. Manton Data from the standard cloud physics payload during the NSF/NCAR High-performance Instrumented Airborne Platform for Environmental Research (HIAPER) Pole-to-Pole Observations (HIPPO) campaigns provide a snapshot of unusual wintertime microphysical conditions in the boundary layer over the Southern Ocean. On 29 June 2011, the HIAPER sampled the boundary layer in a region of pre-frontal warm air advection between 58 and 48° S to the south of Tasmania. Cloud droplet number concentrations were consistent with climatological values in the northernmost profiles but were exceptionally high for wintertime in the Southern Ocean at 100–200 cm −3 in the southernmost profiles. Sub-micron (0.06 〈 D 25 m s −1 ) were most likely responsible for production of sea spray aerosol which influenced the microphysical properties of the boundary layer clouds. The smaller size and higher number concentration of cloud droplets is inferred to increase the albedo of these clouds, and these conditions occur regularly, and are expected to increase in frequency, over windy parts of the Southern Ocean.

Description: Simulating the formation of carbonaceous aerosol in a European Megacity (Paris) during the MEGAPOLI summer and winter campaigns Atmospheric Chemistry and Physics Discussions, 15, 25547-25582, 2015 Author(s): C. Fountoukis, A. G. Megaritis, K. Skyllakou, P. E. Charalampidis, H. A. C. Denier van der Gon, M. Crippa, A. S. H. Prévôt, F. Freutel, A. Wiedensohler, C. Pilinis, and S. N. Pandis We use a three dimensional regional chemical transport model (PMCAMx) with high grid resolution and high resolution emissions (4 km × 4 km) over the Paris greater area to simulate the formation of carbonaceous aerosol during a summer (July 2009) and a winter (January/February 2010) period as part of the MEGAPOLI (Megacities: Emissions, urban, regional, and Global Atmospheric POLlution and climate effects, and Integrated tools for assessment and mitigation) campaigns. Model predictions of carbonaceous aerosol are compared against Aerodyne aerosol mass spectrometer and black carbon (BC) high time resolution measurements from three ground sites. PMCAMx predicts BC concentrations reasonably well reproducing the majority (70 %) of the hourly data within a factor of two during both periods. The agreement for the summertime secondary organic aerosol (OA) concentrations is also encouraging (mean bias = 0.1 μg m −3 ) during a photochemically intense period. The model tends to underpredict the summertime primary OA concentrations in the Paris greater area (by approximately 0.8 μg m −3 ) mainly due to missing primary OA emissions from cooking activities. The total cooking emissions are estimated to be approximately 80 mg d −1 per capita and have a distinct diurnal profile in which 50 % of the daily cooking OA is emitted during lunch time (12:00–14:00 LT) and 20 % during dinner time (20:00–22:00 LT). Results also show a large underestimation of secondary OA in the Paris greater area during wintertime (mean bias = −2.3 μg m −3 ) pointing towards a secondary OA formation process during low photochemical activity periods that is not simulated in the model.

Description: Potential sensitivity of photosynthesis and isoprene emission to direct radiative effects of atmospheric aerosol pollution Atmospheric Chemistry and Physics Discussions, 15, 25433-25475, 2015 Author(s): S. Strada and N. Unger A global Earth system model is applied to quantify the impacts of direct anthropogenic aerosol effective radiative forcing on gross primary productivity (GPP) and isoprene emission. The impacts of different pollution aerosol sources (all anthropogenic, biomass burning and non-biomass burning) are investigated by performing sensitivity experiments. On the global scale, our results show that land carbon fluxes (GPP and isoprene emission) are not sensitive to pollution aerosols, even under a global decline in surface solar radiation (direct + diffuse) by ~ 9 %. At the regional scale, plant productivity (GPP) and isoprene emission show a robust but opposite sensitivity to pollution aerosols, in regions where complex canopies dominate. In eastern North America and Europe, anthropogenic pollution aerosols (mainly from non-biomass burning sources) enhance GPP by +8–12 % on an annual average, with a stronger increase during the growing season (〉 12 %). In the Amazon basin and central Africa, biomass burning aerosols increase GPP by +2–5 % on an annual average, with a peak in the Amazon basin during the dry-fire season (+5–8 %). In Europe and China, anthropogenic pollution aerosols drive a decrease in isoprene emission of −2 to −12 % on the annual average. Anthropogenic aerosols affect land carbon fluxes via different mechanisms and we suggest that the dominant mechanism varies across regions: (1) light scattering dominates in the eastern US; (2) cooling in the Amazon basin; and (3) reduction in direct radiation in Europe and China.

Description: Atmospheric constraints on the methane emissions from the East Siberian Shelf Atmospheric Chemistry and Physics Discussions, 15, 25477-25501, 2015 Author(s): A. Berchet, P. Bousquet, I. Pison, R. Locatelli, F. Chevallier, J.-D. Paris, E. J. Dlugokencky, T. Laurila, J. Hatakka, Y. Viisanen, D. E. J. Worthy, E. G. Nisbet, R. E. Fisher, J. L. France, D. Lowry, and V. Ivakhov Sub-sea permafrost and hydrates in the East Siberian Arctic Ocean Continental Shelf (ESAS) constitute a substantial carbon pool, and a potentially large source of methane to the atmosphere. Previous studies based on interpolated oceanographic campaigns estimated atmospheric emissions from this area at 8–17 Tg CH 4 y −1 . Here, we propose insights based on atmospheric observations to evaluate these estimates. Isotopic observations suggest a biogenic origin (either terrestrial or marine) of the methane in air masses originating from ESAS during summer 2010. The comparison of high-resolution simulations of atmospheric methane mole fractions to continuous methane observations during the entire year 2012 confirms the high variability and heterogeneity of the methane releases from ESAS. Simulated mole fractions including a 8 Tg CH 4 y −1 source from ESAS are found largely overestimated compared to the observations in winter, whereas summer signals are more consistent with each other. Based on a comprehensive statistical analysis of the observations and of the simulations, annual methane emissions from ESAS are estimated in a range of 0.5–4.3 Tg CH 4 y −1 .

Description: What do correlations tell us about anthropogenic–biogenic interactions and SOA formation in the Sacramento Plume during CARES? Atmospheric Chemistry and Physics Discussions, 15, 25381-25431, 2015 Author(s): L. Kleinman, C. Kuang, A. Sedlacek, G. Senum, S. Springston, J. Wang, Q. Zhang, J. Jayne, J. Fast, J. Hubbe, J. Shilling, and R. Zaveri During the Carbonaceous Aerosols and Radiative Effects Study (CARES) the DOE G-1 aircraft was used to sample aerosol and gas phase compounds in the Sacramento, CA plume and surrounding region. We present data from 66 plume transects obtained during 13 flights in which southwesterly winds transported the plume towards the foothills of the Sierra Nevada Mountains. Plume transport occurred partly over land with high isoprene emission rates. Our objective is to empirically determine whether organic aerosol (OA) can be attributed to anthropogenic or biogenic sources, and to determine whether there is a synergistic effect whereby OA concentrations are enhanced by the simultaneous presence of high concentrations of CO and either isoprene, MVK+MACR (sum of methyl vinyl ketone and methacrolein) or methanol, which are taken as tracers of anthropogenic and biogenic emissions, respectively. Linear and bilinear correlations between OA, CO, and each of three biogenic tracers, "Bio", for individual plume transects indicate that most of the variance in OA over short time and distance scales can be explained by CO. For each transect and species a plume perturbation, (i.e., ΔOA, defined as the difference between 90th and 10th percentiles) was defined and regressions done amongst Δ values in order to probe day to day and location dependent variability. Species that predicted the largest fraction of the variance in ΔOA were ΔO 3 and ΔCO. Background OA was highly correlated with background methanol and poorly correlated with other tracers. Because background OA was ~ 60 % of peak OA in the urban plume, peak OA should be primarily biogenic and therefore non-fossil. Transects were split into subsets according to the percentile rankings of ΔCO and ΔBio, similar to an approach used by Setyan et al. (2012) and Shilling et al. (2013) to determine if anthropogenic-biogenic interactions enhance OA production. As found earlier, ΔOA in the data subset having high ΔCO and high ΔBio was several-fold greater than in other subsets. Part of this difference is consistent with a synergistic interaction between anthropogenic and biogenic precursors and part to an independent linear dependence of ΔOA on precursors. Highest values of ΔO 3 also occur in the high ΔCO–high ΔBio data set, raising the possibility that the coincidence of high concentrations of anthropogenic and biogenic tracers as well as OA and O 3 may be associated with high temperatures, clear skies, and poor ventilation in addition to specific interaction between anthropogenic and biogenic compounds.

Description: Stratospheric sulfate geoengineering enhances terrestrial gross primary productivity Atmospheric Chemistry and Physics Discussions, 15, 25627-25645, 2015 Author(s): L. Xia, A. Robock, S. Tilmes, and R. R. Neely III Stratospheric sulfate geoengineering could impact the terrestrial carbon cycle by enhancing the carbon sink. With an 8 Tg yr −1 injection of SO 2 to balance a Representative Concentration Pathway 6.0 (RCP6.0) scenario, we conducted climate model simulations with the Community Earth System Model, with the Community Atmospheric Model 4 fully coupled to tropospheric and stratospheric chemistry (CAM4-chem). During the geoengineering period, as compared to RCP6.0, land-averaged downward visible diffuse radiation increased 3.2 W m −2 (11 %). The enhanced diffuse radiation combined with the cooling increased plant photosynthesis by 2.4 %, which could contribute to an additional 3.8 ± 1.1 Gt C yr −1 global gross primary productivity without nutrient limitation. This increase could potentially increase the land carbon sink. Suppressed plant and soil respiration due to the cooling would reduce natural land carbon emission and therefore further enhance the terrestrial carbon sink during the geoengineering period. This beneficial impact of stratospheric sulfate geoengineering would need to be balanced by a large number of potential risks in any future decisions about implementation of geoengineering.

Description: Aerosol optical properties in the southeastern United States in summer – Part 1: Hygroscopic growth Atmospheric Chemistry and Physics Discussions, 15, 25695-25738, 2015 Author(s): C. A. Brock, N. L. Wagner, B. E. Anderson, A. R. Attwood, A. Beyersdorf, P. Campuzano-Jost, A. G. Carlton, D. A. Day, G. S. Diskin, T. D. Gordon, J. L. Jimenez, D. A. Lack, J. Liao, M. Z. Markovic, A. M. Middlebrook, N. L. Ng, A. E. Perring, M. S. Richardson, J. P. Schwarz, R. A. Washenfelder, A. Welti, L. Xu, L. D. Ziemba, and D. M. Murphy Aircraft observations of meteorological, trace gas, and aerosol properties were made during May–September 2013 in the southeastern United States (US) under fair-weather, afternoon conditions with well-defined planetary boundary layer structure. Optical extinction at 532 nm was directly measured at three relative humidities and compared with extinction calculated from measurements of aerosol composition and size distribution using the κ-Köhler approximation for hygroscopic growth. Using this approach, the hygroscopicity parameter κ for the organic fraction of the aerosol must have been 〈 0.10 to be consistent with 75 % of the observations within uncertainties. This subsaturated κ value for the organic aerosol in the southeastern US is consistent with several field studies in rural environments. We present a new parameterization of the change in aerosol extinction as a function of relative humidity that better describes the observations than does the widely used power-law (gamma, γ) parameterization. This new single-parameter κ ext formulation is based upon κ-Köhler and Mie theories and relies upon the well-known approximately linear relationship between particle volume (or mass) and optical extinction (Charlson et al., 1967). The fitted parameter, κ ext , is nonlinearly related to the chemically derived κ parameter used in κ-Köhler theory. The values of κ ext we determined from airborne measurements are consistent with independent observations at a nearby ground site.

Description: Water vapour variability in the high-latitude upper troposphere – Part 2: Impact of volcanic emissions Atmospheric Chemistry and Physics Discussions, 15, 25873-25905, 2015 Author(s): C. E. Sioris, J. Zou, C. T. McElroy, C. D. Boone, P. E. Sheese, and P. F. Bernath The impact of volcanic eruptions on water vapour in the region of the high latitude tropopause is studied using deseasonalized time series based on observations by the Atmospheric Chemistry Experiment (ACE) water vapour sensors, namely MAESTRO (Measurements of Aerosol Extinction in the Stratosphere and Troposphere Retrieved by Occultation) and the Fourier Transform Spectrometer (ACE-FTS). The three eruptions with the greatest impact on the high latitude upper troposphere during the time frame of this satellite-based remote sensing mission are chosen. The Puyehue-Cordón Caulle volcanic eruption in June 2011 was the most explosive eruption in the past 24 years and resulted in an observed (50 ± 12) % increase in water vapour in the southern high-latitude upper troposphere in July 2011 that persisted into September 2011. A pair of Northern Hemisphere volcanoes, namely Eyjafjallajökull and Nabro, erupted in 2010 and 2011 respectively, increasing water vapour in the upper troposphere at northern high latitudes significantly for a period of ~ 3 months following each eruption. Both had a volcanic explosivity index of 4. Nabro led to a statistically significant increase of ~ 1 ppm in lower stratospheric (13.5–15.5 km) water vapour at northern high-latitudes (60–90° N) in September 2011, when the brunt of its plume arrived in the Arctic. These findings imply that steam emitted into the high-latitude, upper troposphere during volcanic eruptions must be taken into account to properly determine the magnitude of the trend in water vapour over the last decade.

Description: Simultaneous Retrievals of Polar Mesospheric Clouds (PMCs) with Ozone from OMI UV measurements Atmospheric Chemistry and Physics Discussions, 15, 25907-25932, 2015 Author(s): J. Bak, X. Liu, J. H. Kim, M. T. Deland, and K. Chance The presence of polar mesospheric clouds (PMCs) at high latitudes could affect the retrieval of ozone profiles using backscattered ultraviolet (BUV) measurements. PMC-induced errors in ozone profile retrievals from Ozone Monitoring Instrument (OMI) BUV measurements are investigated through comparisons with Microwave Limb Sounder (MLS) ozone measurements. This comparison demonstrates that the presence of PMCs leads to systematic biases at altitudes above 6 hPa in summer high latitudes; the biases increase from ~ −2 % at 2 hPa to ~ −20 % at 0.5 hPa on average, and are significantly correlated with brightness of PMCs. Sensitivity studies show that the radiance sensitivity to PMCs strongly depends on wavelengths, increasing by a factor of ~ 4 from 300 to 265 nm. It also strongly depends on the PMC scattering, thus depending on viewing geometry. The optimal estimation-based retrieval sensitivity analysis shows that PMCs located at 80–85 km have the greatest effect on ozone retrievals at ~ 0.2 hPa (~ 60 km), where the retrieval errors range from −2.5 % with PMC optical depth (POD) of 10 −4 to −20 % with 10 −3 at back scattering angles, and the impacts increase by a factor of ~ 5 at forward scattering angles due to stronger PMC sensitivities. To reduce the interference of PMCs on ozone retrievals, we perform simultaneous retrievals of POD and ozone with a loose constraint of 10 −3 for POD, which results in retrieval errors of 1–4 × 10 −4 . It is demonstrated that the negative bias of OMI ozone retrievals relative to MLS could be improved by including the PMC in the forward model calculation and retrieval.

Description: Simulating secondary organic aerosol in a regional air quality model using the statistical oxidation model – Part 1: Assessing the influence of constrained multi-generational ageing Atmospheric Chemistry and Physics Discussions, 15, 25837-25872, 2015 Author(s): S. H. Jathar, C. D. Cappa, A. S. Wexler, J. H. Seinfeld, and M. J. Kleeman Multi-generational oxidation of volatile organic compound (VOC) oxidation products can significantly alter the mass, chemical composition and properties of secondary organic aerosol (SOA) compared to calculations that consider only the first few generations of oxidation reactions. However, the most commonly used state-of-the-science schemes in 3-D regional or global models that account for multi-generational oxidation (1) consider only functionalization reactions but do not consider fragmentation reactions, (2) have not been constrained to experimental data; and (3) are added on top of existing parameterizations. The incomplete description of multi-generational oxidation in these models has the potential to bias source apportionment and control calculations for SOA. In this work, we used the Statistical Oxidation Model (SOM) of Cappa and Wilson (2012), constrained by experimental laboratory chamber data, to evaluate the regional implications of multi-generational oxidation considering both functionalization and fragmentation reactions. SOM was implemented into the regional UCD/CIT air quality model and applied to air quality episodes in California and the eastern US. The mass, composition and properties of SOA predicted using SOM are compared to SOA predictions generated by a traditional "two-product" model to fully investigate the impact of explicit and self-consistent accounting of multi-generational oxidation. Results show that SOA mass concentrations predicted by the UCD/CIT-SOM model are very similar to those predicted by a two-product model when both models use parameters that are derived from the same chamber data. Since the two-product model does not explicitly resolve multi-generational oxidation reactions, this finding suggests that the chamber data used to parameterize the models captures the majority of the SOA mass formation from multi-generational oxidation under the conditions tested. Consequently, the use of low and high NO x yields perturbs SOA concentrations by a factor of two and are probably a much stronger determinant in 3-D models than constrained multi-generational oxidation. While total predicted SOA mass is similar for the SOM and two-product models, the SOM model predicts increased SOA contributions from anthropogenic (alkane, aromatic) and sesquiterpenes and decreased SOA contributions from isoprene and monoterpene relative to the two-product model calculations. The SOA predicted by SOM has a much lower volatility than that predicted by the traditional model resulting in better qualitative agreement with volatility measurements of ambient OA. On account of its lower-volatility, the SOA mass produced by SOM does not appear to be as strongly influenced by the inclusion of oligomerization reactions, whereas the two-product model relies heavily on oligomerization to form low volatility SOA products. Finally, an unconstrained contemporary hybrid scheme to model multi-generational oxidation within the framework of a two-product model in which "ageing" reactions are added on top of the existing two-product parameterization is considered. This hybrid scheme formed at least three times more SOA than the SOM during regional simulations as a result of excessive transformation of semi-volatile vapors into lower volatility material that strongly partitions to the particle phase. This finding suggests that these "hybrid" multi-generational schemes should be used with great caution in regional models.

Description: Transport across the tropical tropopause layer and convection Atmospheric Chemistry and Physics Discussions, 15, 26231-26271, 2015 Author(s): A.-S. Tissier and B. Legras Transit properties across the tropical tropopause layer are studied using extensive forward and backward Lagrangian trajectories between cloud tops and the reference surface 380 K. The tropical domain being subdivided into 11 sub-regions according to the distribution of land and convection, we estimate the contribution of each region to the upward mass flux across the 380 K surface, the vertical distribution of convective sources and of transit times over the period 2005–2008. The good agreement between forward and backward statistics is the basis of the results presented here. It is found that about 85 % of the tropical parcels at 380 K originate from convective sources all along the year. From November to April, the sources are dominated by the warm pool which accounts for up to 70 % of the upward flux. During summer, Asian monsoon region is the largest contributor with similar contributions from oceanic regions and Asian mainland, although the signature in vertical distribution and transit time is very different, Asian mainland displaying higher sources and smaller transit times. The Tibetan plateau, although a minor overall contributor, is found to be the region with the highest impact of convection at 380 K due to its central location beneath the Asian upper level anticyclone. We show the robustness of our main results to uncertainties in data and methods but we also find some sensitivity of the vertical distribution of sources and transit times to the representation of cloud tops and heating rates.

Description: Using beryllium-7 to assess cross-tropopause transport in global models Atmospheric Chemistry and Physics Discussions, 15, 26131-26189, 2015 Author(s): H. Liu, D. B. Considine, L. W. Horowitz, J. H. Crawford, J. M. Rodriguez, S. E. Strahan, M. R. Damon, S. D. Steenrod, X. Xu, J. Kouatchou, C. Carouge, and R. M. Yantosca We use the Global Modeling Initiative (GMI) modeling framework to assess the utility of cosmogenic beryllium-7 ( 7 Be), a natural aerosol tracer, for evaluating cross-tropopause transport in global models. The GMI chemical transport model (CTM) was used to simulate atmospheric 7 Be distributions using four different meteorological data sets (GEOS1-STRAT DAS, GISS II' GCM, fvGCM, and GEOS4-DAS), featuring significantly different stratosphere–troposphere exchange (STE) characteristics. The simulations were compared with the upper troposphere/lower stratosphere (UT/LS) 7 Be climatology constructed from ~ 25 years of aircraft and balloon data, as well as climatological records of surface concentrations and deposition fluxes. Comparison of the fraction of surface air of stratospheric origin estimated from the 7 Be simulations with observationally-derived estimates indicates excessive cross-tropopause transport at middle latitudes in simulations using GEOS1-STRAT and at high latitudes using GISS II' meteorological data. These simulations also overestimate 7 Be deposition fluxes at middle latitudes (GEOS1-STRAT) and at high latitudes (GISS II'), respectively. We show that excessive cross-tropopause transport of 7 Be corresponds to overestimated stratospheric contribution to tropospheric ozone. Our perspectives on STE in these meteorological fields based on 7 Be simulations are consistent with previous modeling studies of tropospheric ozone using the same meteorological fields. We further apply observational constraints to other global models including GFDL AM2 and GEOS-Chem (driven by GEOS3-DAS and GEOS5-DAS). We conclude that the observational constraints for 7 Be and observed 7 Be total deposition fluxes can be used routinely as a first-order assessment of cross-tropopause transport in global models.

Description: The importance of temporal collocation for the evaluation of aerosol models with observations Atmospheric Chemistry and Physics Discussions, 15, 26191-26230, 2015 Author(s): N. A. J. Schutgens, D. G. Partridge, and P. Stier It is often implicitly assumed that over suitably long periods the mean of observations and models should be comparable, even if they have different temporal sampling. We assess the errors incurred due to ignoring temporal sampling and show they are of similar magnitude as (but smaller than) actual model errors (20–60 %). Using temporal sampling from remote sensing datasets (the satellite imager MODIS and the ground-based sun photometer network AERONET) and three different global aerosol models, we compare annual and monthly averages of full model data to sampled model data. Our results show that sampling errors as large as 100 % in AOT (Aerosol Optical Thickness), 0.4 in AE (Ångström Exponent) and 0.05 in SSA (Single Scattering Albedo) are possible. Even in daily averages, sampling errors can be significant. More-over these sampling errors are often correlated over long distances giving rise to artificial contrasts between pristine and polluted events and regions. Additionally, we provide evidence that suggests that models will underestimate these errors. To prevent sampling errors, model data should be temporally collocated to the observations before any analysis is made. We also discuss how this work has consequences for in-situ measurements (e.g. aircraft campaigns or surface measurements) in model evaluation.

Description: Towards understanding the variability in biospheric CO 2 fluxes: using FTIR spectrometry and a chemical transport model to investigate the sources and sinks of carbonyl sulfide and its link to CO 2 Atmospheric Chemistry and Physics Discussions, 15, 26025-26065, 2015 Author(s): Y. Wang, N. M. Deutscher, M. Palm, T. Warneke, J. Notholt, I. Baker, J. Berry, P. Suntharalingam, N. Jones, E. Mahieu, B. Lejeune, J. E. Campbell, A. Wolf, and S. Kremser Understanding carbon dioxide (CO 2 ) biospheric processes is of great importance because the terrestrial exchange drives the seasonal and inter-annual variability of CO 2 in the atmosphere. Atmospheric inversions based on CO 2 concentration measurements alone can only determine net biosphere fluxes, but not differentiate between photosynthesis (uptake) and respiration (production). Carbonyl sulfide (OCS) could provide an important additional constraint: it is also taken up by plants during photosynthesis but not emitted during respiration, and therefore is a potential mean to differentiate between these processes. Solar absorption Fourier Transform InfraRed (FTIR) spectrometry allows for the retrievals of the atmospheric concentrations of both CO 2 and OCS from measured solar absorption spectra. Here, we investigate co-located and quasi-simultaneous FTIR measurements of OCS and CO 2 performed at three selected sites located in the Northern Hemisphere. These measurements are compared to simulations of OCS and CO 2 using a chemical transport model (GEOS-Chem). The OCS simulations are driven by different land biospheric fluxes to reproduce the seasonality of the measurements. Increasing the plant uptake of Kettle et al. (2002a) by a factor of three resulted in the best comparison with FTIR measurements. However, there are still discrepancies in the latitudinal distribution when comparing with HIPPO (HIAPER Pole-to-Pole Observations) data spanning both hemispheres. The coupled biospheric fluxes of OCS and CO 2 from the simple biosphere model (SiB) are used in the study and compared to measurements. The CO 2 simulation with SiB fluxes agrees with the measurements well, while the OCS simulation reproduced a weaker drawdown than FTIR measurements at selected sites, and a smaller latitudinal gradient in the Northern Hemisphere during growing season. An offset in the timing of the seasonal cycle minimum between SiB simulation and measurements is also seen. Using OCS as a photosynthesis proxy can help to understand how the biospheric processes are reproduced in models and to further understand the carbon cycle in the real world.

Description: Ability of the 4-D-Var analysis of the GOSAT BESD XCO 2 retrievals to characterize atmospheric CO 2 at large and synoptic scales Atmospheric Chemistry and Physics Discussions, 15, 26273-26313, 2015 Author(s): S. Massart, A. Agustí-Panareda, J. Heymann, M. Buchwitz, F. Chevallier, M. Reuter, M. Hilker, J. P. Burrows, F. Hase, F. Desmet, D. G. Feist, and R. Kivi This study presents results from the European Centre for Medium-Range Weather Forecasts (ECMWF) carbon dioxide (CO 2 ) analysis system where the atmospheric CO 2 is controlled through the assimilation of column-average dry-air mole fractions of CO 2 (XCO 2 ) from the Greenhouse gases Observing Satellite (GOSAT). The analysis is compared to a free run simulation and they are both evaluated against XCO 2 data from the Total Carbon Column Observing Network (TCCON). We show that the assimilation of the GOSAT XCO 2 product from the Bremen Optimal Estimation DOAS (BESD) algorithm during the year 2013 provides XCO 2 fields with an improved station-to-station bias deviation of 0.7 parts per million (ppm) compared to the free run (1.4 ppm) and an improved estimated precision of ~ 1 ppm compared to the used GOSAT data (3.4 ppm). We also show that the analysis has skill for synoptic situations in the vicinity of frontal systems where the GOSAT retrievals are sparse due to cloud contamination. We finally computed the 10 day forecast from each analysis at 00:00 UTC. Compared to its own analysis the CO 2 forecast shows synoptic skill for the largest scale weather patterns even up to day 5 according to the anomaly correlation coefficient.

Description: Meteorological-gaseous influences on seasonal PM 2.5 variability in the Klang Valley urban-industrial environment Atmospheric Chemistry and Physics Discussions, 15, 26423-26479, 2015 Author(s): N. Amil, M. T. Latif, M. F. Khan, and M. Mohamad This study attempts to investigate the fine particulate matter (PM 2.5 ) variability in the Klang Valley urban-industrial environment. In total, 94 daily PM 2.5 samples were collected during a one-year campaign from August 2011 to July 2012, covering all four seasons. The samples were analysed for various inorganic components and black carbon. The chemical compositions were statistically analysed and the aerosol pattern was characterised using descriptive analysis, correlation matrices, enrichment factors (EF), stoichiometric analysis and chemical mass closure (CMC). For source apportionment purposes, a combination of positive matrix factorisation (PMF) and multi-linear regression (MLR) was employed. Further, meteorological-gaseous parameters were incorporated into each analysis for improved assessment. The results showed that PM 2.5 mass averaged at 28 ± 18 μg m −3 , 2.8 fold higher than the World Health Organisation (WHO) annual guideline. On a daily basis, the PM 2.5 mass ranged between 6 and 118 μg m −3 with 43 % exceedance of the daily WHO guideline. The North-East monsoon (NE) was the only season with 〈 50 % sample exceedance of the daily WHO guideline. On an annual scale, PM 2.5 mass correlated positively with temperature ( T ) and wind speed (WS) but negatively with relative humidity (RH). With the exception of NO x , the gases analysed (CO, NO 2 , NO and SO 2 ) were found to significantly influence the PM 2.5 mass. Seasonal variability unexpectedly showed that rainfall, WS and wind direction (WD) did not significantly correlate with PM 2.5 mass. Further analysis on the PM 2.5 / PM 10 , PM 2.5 / TSP and PM 10 / TSP ratios reveal that meteorological parameters only greatly influenced the coarse particles (PM 〉 2.5μm) and less so the fine particles at the site. Chemical composition showed that both primary and secondary pollutants of PM 2.5 are equally important, albeit with seasonal variability. The CMC components identified were: black carbon (BC) 〉 secondary inorganic aerosols (SIA) 〉 dust 〉 trace elements (TE) 〉 sea salt 〉 K + . The EF analysis distinguished two groups of trace elements: those with anthropogenic sources (Pb, Se, Zn, Cd, As, Bi, Ba, Cu, Rb, V and Ni) and those with a crustal source (Sr, Mn, Co and Li). The five identified factors resulting from PMF 5.0 were: (1) combustion of engine oil; (2) mineral dust; (3) mixed SIA and biomass burning; (4) mixed traffic and industrial; and (5) sea salt. Each of these sources had an annual mean contribution of 17, 14, 42, 10 and 17 %, respectively. The dominance of each identified source largely varied with changing season and a few factors were in agreement with the CMC, EF and stoichiometric analysis, accordingly. In relation to meteorological-gaseous parameters, PM 2.5 sources were influenced by different parameters during different seasons. In addition, two air pollution episodes (HAZE) revealed the influence of local and/or regional sources. Overall, our study clearly suggests that the chemical constituents and sources of PM 2.5 were greatly influenced and characterised by meteorological and gaseous parameters which largely vary with season.

Description: Dicarboxylic acids, oxoacids, benzoic acid, α-dicarbonyls, WSOC, OC, and ions in spring aerosols from Okinawa Island in the western North Pacific Rim: size distributions and formation processes Atmospheric Chemistry and Physics Discussions, 15, 26509-26554, 2015 Author(s): D. K. Deshmukh, K. Kawamura, M. Lazaar, B. Kunwar, and S. K. R. Boreddy Size-segregated aerosols (9-stages from 〈 0.43 to 〉 11.3 μm in diameter) were collected at Cape Hedo, Okinawa in spring 2008 and analyzed for water-soluble diacids (C 2 –\C 12 ), ω-oxoacids (ωC 2 –ωC 9 ), pyruvic acid, benzoic acid and α-dicarbonyls (C 2 –C 3 ) as well as water-soluble organic carbon (WSOC), organic carbon (OC) and major ions. In all the size-segregated aerosols, oxalic acid (C 2 ) was found as the most abundant species followed by malonic and succinic acids whereas glyoxylic acid (ωC 2 ) was the dominant oxoacid and glyoxal (Gly) was more abundant than methylglyoxal. Diacids (C 2 –C 5 ), ωC 2 and Gly as well as WSOC and OC peaked at 0.65–1.1 μm in fine mode whereas azelaic (C 9 ) and 9-oxononanoic (ωC 9 ) acids peaked at 3.3–4.7 μm in coarse mode. Sulfate and ammonium are enriched in fine mode whereas sodium and chloride are in coarse mode. These results imply that water-soluble species in the marine aerosols could act as cloud condensation nuclei (CCN) to develop the cloud cover over the western North Pacific Rim. The organic species are likely produced by a combination of gas-phase photooxidation, and aerosol-phase or in-cloud processing during long-range transport. The coarse mode peaks of malonic and succinic acids were obtained in the samples with marine air masses, suggesting that they may be associated with the reaction on sea salt particles. Bimodal size distributions of longer-chain diacid (C 9 ) and oxoacid (ωC 9 ) with a major peak in the coarse mode suggest their production by photooxidation of biogenic unsaturated fatty acids via heterogeneous reactions on sea salt particles.

Description: Can we use modelling methodologies to assess airborne benzo[ a ]pyrene from biomonitors? A comprehensive evaluation approach Atmospheric Chemistry and Physics Discussions, 15, 26481-26507, 2015 Author(s): N. Ratola and P. Jiménez-Guerrero Biomonitoring data available on levels of atmospheric polycyclic aromatic hydrocarbons (PAHs) in pine needles from the Iberian Peninsula was used to estimate air concentrations of benzo[ a ]pyrene (BaP) and, at the same time, fuelled the comparison with chemistry transport model representations. Simulations with the modelling system WRF + CHIMERE were validated against data from the European Monitoring and Evaluation Programme (EMEP) air sampling network and using modelled atmospheric concentrations as a consistent reference in order to compare the performance of vegetation-to-air estimating methods. A spatial and temporal resolution of 9 km and 1 h was implemented. The field-based database relied on a pine needles sampling scheme comprising 33 sites in Portugal and 37 sites in Spain complemented with the BaP measurements available from the EMEP sites. The ability of pine needles to act as biomonitoring markers for the atmospheric concentrations of BaP was estimated converting the levels obtained in pine needles into air concentrations by six different approaches, one of them presenting realistic concentrations when compared to the modelled atmospheric values. The justification for this study is the gaps still existing in the knowledge of the life cycles of semi-volatile organic compounds (SVOCs), particularly the partition processes between air and vegetation. The strategy followed in this work allows the definition of the transport patterns (e.g. dispersion) established by the model for atmospheric concentrations and the estimated values in vegetation.

Description: Shortwave direct radiative effects of above cloud aerosols over global oceans derived from eight years of CALIOP and MODIS observations Atmospheric Chemistry and Physics Discussions, 15, 26357-26421, 2015 Author(s): Z. Zhang, K. Meyer, H. Yu, S. Platnick, P. Colarco, Z. Liu, and L. Oreopoulos In this paper, we studied the frequency of occurrence and shortwave direct radiative effects (DRE) of above-cloud aerosols (ACAs) over global oceans using eight years of collocated CALIOP and MODIS observations. Similar to previous work, we found high ACA occurrence in four regions: Southeast (SE) Atlantic region where ACAs are mostly light-absorbing aerosols, i.e., smoke and polluted dust according to CALIOP classification, originating from biomass burning over African Savanna; Tropical Northeast Atlantic and Arabian Sea where ACAs are predominantly windblown dust from the Sahara and Arabian desert, respectively; and Northwest Pacific where ACAs are mostly transported smoke and polluted dusts from Asian. From radiative transfer simulations based on CALIOP-MODIS observations and a set of the preselected aerosol optical models, we found the DREs of ACAs at the top of atmosphere (TOA) to be positive (i.e., warming) in the SE Atlantic and NW Pacific regions, but negative (i.e., cooling) in TNE Atlantic and Arabian Sea. The cancellation of positive and negative regional DREs results in a global ocean annual mean diurnally averaged cloudy-sky DRE of 0.015 W m −2 (range of −0.03 to 0.06 W m −2 ) at TOA. The DREs at surface and within atmosphere are −0.15 W m −2 (range of −0.09 to −0.21 W m −2 ), and 0.17 W m −2 (range of 0.11 to 0.24 W m −2 ), respectively. The regional and seasonal mean DREs are much stronger. For example, in the SE Atlantic region the JJA (July ~ August) seasonal mean cloudy-sky DRE is about 0.7 W m −2 (range of 0.2 to 1.2 W m −2 ) at TOA. The uncertainty in our DRE computations is mainly cause by the uncertainties in the aerosol optical properties, in particular aerosol absorption, and uncertainties in the CALIOP operational aerosol optical thickness retrieval. In situ and remotely sensed measurements of ACA from future field campaigns and satellite missions, and improved lidar retrieval algorithm, in particular vertical feature masking, would help reduce the uncertainty.

Description: Fossil and non-fossil source contributions to atmospheric carbonaceous aerosols during extreme spring grassland fires in Eastern Europe Atmospheric Chemistry and Physics Discussions, 15, 26315-26355, 2015 Author(s): V. Ulevicius, S. Byčenkienė, C. Bozzetti, A. Vlachou, K. Plauškaitė, G. Mordas, V. Dudoitis, G. Abbaszade, V. Remeikis, A. Garbaras, A. Masalaite, J. Blees, R. Fröhlich, K. R. Dällenbach, F. Canonaco, J. G. Slowik, J. Dommen, R. Zimmermann, J. Schnelle-Kreis, G. A. Salazar, K. Agrios, S. Szidat, I. El Haddad, and A. S. H. Prévôt In early spring the Baltic region is frequently affected by high pollution events due to biomass burning in that area. Here we present a comprehensive study to investigate the impact of biomass/grass burning (BB) on the evolution and composition of aerosol in Preila, Lithuania, during springtime open fires. Non-refractory submicron particulate matter (NR-PM 1 ) was measured by an Aerodyne aerosol chemical speciation monitor (ACSM) and a source apportionment with the multilinear engine (ME-2) running the positive matrix factorization (PMF) model was applied to the organic aerosol fraction to investigate the impact of biomass/grass burning. Satellite observations over regions of biomass burning activity supported the results and identification of air mass transport to the area of investigation. Sharp increases in biomass burning tracers, such as levoglucosan up to 683 ng m −3 and black carbon (BC) up to 17 μg m −3 were observed during this period. A further separation between fossil and non-fossil primary and secondary contributions was obtained by coupling ACSM PMF results and radiocarbon ( 14 C) measurements of the elemental (EC) and organic (OC) carbon fractions. Non-fossil organic carbon (OC nf ) was the dominant fraction of PM 1 , with the primary (POC nf ) and secondary (SOC nf ) fractions contributing 26–44 % and 13–23 % to the TC, respectively. 5–8 % of the TC had a primary fossil origin (POC f ), whereas the contribution of fossil secondary organic carbon (SOC f ) was 4–13 %. Non-fossil EC (EC nf ) and fossil EC (EC f ) ranged from 13–24 % and 7–12 %, respectively. Isotope ratio of stable carbon and nitrogen isotopes were used to distinguish aerosol particles associated with solid and liquid fossil fuel burning.

Description: Effects of aerosols on solar radiation in the ALADIN-HIRLAM NWP system Atmospheric Chemistry and Physics Discussions, 15, 32519-32560, 2015 Author(s): E. Gleeson, V. Toll, K. P. Nielsen, L. Rontu, and J. Mašek The direct shortwave radiative effect of aerosols in the ALADIN-HIRLAM numerical weather prediction system was investigated using three different shortwave radiation schemes in diagnostic single-column experiments. The aim was to evaluate the strengths and weaknesses of the model in this regard and to prepare the model for eventual use of real-time aerosol information. Experiments were run using observed, climatologically-averaged and zero aerosols, with particular focus on the August 2010 Russian wildfire case. One of these schemes is a revised version of the HLRADIA scheme with improved treatment of aerosols. Each radiation scheme accurately simulates the direct shortwave effect when observed aerosol optical properties are used rather than climatological-averages or no aerosols which result in large errors, particularly for heavy pollution scenarios. The dependencies of the direct radiative effect of aerosols on relative humidity and the vertical profile of the aerosols on the shortwave heating rates were also investigated and shown to be non-negligible.

Description: North Atlantic Oscillation model projections and influence on tracer transport Atmospheric Chemistry and Physics Discussions, 15, 33049-33075, 2015 Author(s): S. Bacer, T. Christoudias, and A. Pozzer The North Atlantic Oscillation (NAO) plays an important role in the climate variability of the Northern Hemisphere with significant consequences on pollutant transport. We study the influence of the NAO on the atmospheric dispersion of pollutants in the near past and in the future by considering simulations performed by the ECHAM/MESSy Atmospheric Chemistry (EMAC) general circulation model. We analyze two model runs: a simulation with circulation dynamics nudged towards ERA-Interim reanalysis data over a period of 35 years (1979–2013) and a simulation with prescribed Sea Surface Temperature (SST) boundary conditions over 150 years (1950–2099). The model is shown to reproduce the NAO spatial and temporal variability and to be comparable with observations. We find that the decadal variability in the NAO, which has been pronounced since 1950s until 1990, will continue to dominate in the future considering decadal periods, although no significant trends are present in the long term projection (100–150 years horizon). We do not find in the model projections any significant temporal trend of the NAO for the future, meaning that neither positive or negative phases will dominate. Tracers with idealised decay and emissions are considered to investigate the NAO effects on transport; it is shown that during the positive phase of the NAO, the transport from North America towards northern Europe is stronger and pollutants are shifted northwards over the Arctic and southwards over the Mediterranean and North Africa, with two distinct areas of removal and stagnation of pollutants.

Description: Effect of varying experimental conditions on the viscosity of α-pinene derived secondary organic material Atmospheric Chemistry and Physics Discussions, 15, 32967-33002, 2015 Author(s): J. W. Grayson, Y. Zhang, A. Mutzel, L. Renbaum-Wolff, O. Böge, S. Kamal, H. Herrmann, S. T. Martin, and A. K. Bertram To predict the role of secondary organic material (SOM) particles in climate, visibility, and health, information on the viscosity of particles containing SOM is required. In this study we investigate the viscosity of SOM particles as a function of relative humidity and SOM particle mass concentration during SOM synthesis. The SOM was generated via the ozonolysis of α-pinene at 〈 5 % relative humidity (RH). Experiments were carried out using the poke-and-flow technique, which measures the experimental flow time (τ exp, flow ) of SOM after poking the material with a needle. In the first set of experiments, we show that τ exp, flow increased by a factor of 3600 as the RH increased from 〈 0.5 to 50 % RH, for SOM with a production mass concentration of 121 μg m −3 . Based on simulations, the viscosities of the particles were between 6 × 10 5 and 5 × 10 7 Pa s at 〈 0.5 % RH and between 3 × 10 2 and 9 × 10 3 Pa s at 50 % RH. In the second set of experiments we show that under dry conditions τ exp, flow decreased by a factor of 45 as the production mass concentration increased from 121 to 14 000 μg m −3 . From simulations of the poke-and-flow experiments, the viscosity of SOM with a production mass concentration of 14 000 μg m −3 was determined to be between 4 × 10 4 and 1.5 × 10 6 Pa s compared to between 6 × 10 5 and 5 × 10 7 Pa s for SOM with a production mass concentration of 121 μg m −3 . The results can be rationalised by a dependence of the chemical composition of SOM on production conditions. These results emphasise the shifting characteristics of SOM, not just with RH and precursor type, but also with the production conditions, and suggest that production mass concentration and the RH at which the viscosity was determined should be considered both when comparing laboratory results and when extrapolating these results to the atmosphere.

Description: Analysis of the potential of near ground measurements of CO 2 and CH 4 in London, UK for the monitoring of city-scale emissions using an atmospheric transport model Atmospheric Chemistry and Physics Discussions, 15, 33003-33048, 2015 Author(s): A. Boon, G. Broquet, D. J. Clifford, F. Chevallier, D. M. Butterfield, I. Pison, M. Ramonet, J. D. Paris, and P. Ciais Carbon dioxide (CO 2 ) and methane (CH 4 ) mole fractions were measured at four near ground sites located in and around London during the summer of 2012 in view to investigate the potential of assimilating such measurements in an atmospheric inversion system for the monitoring of the CO 2 and CH 4 emissions in the London area. These data were analysed and compared with simulations using a modelling framework suited to building an inversion system: a 2 km horizontal resolution South of England configuration of the transport model CHIMERE driven by European Centre for Medium-Range Weather Forecasting (ECMWF) meteorological forcing, coupled to a 1 km horizontal resolution emission inventory (the UK National Atmospheric Emission Inventory). First comparisons reveal that local sources have a large impact on measurements and these local sources cannot be represented in the model at 2 km resolution. We evaluate methods to minimise some of the other critical sources of misfits between the observation data and the model simulation that overlap the signature of the errors in the emission inventory. These methods should make it easier to identify the corrections that should be applied to the inventory. Analysis is supported by observations from meteorological sites around the city and a three-week period of atmospheric mixing layer height estimations from lidar measurements. The difficulties of modelling the mixing layer depth and thus CO 2 and CH 4 concentrations during the night, morning and late afternoon led us to focus on the afternoon period for all further analyses. The misfits between observations and model simulations are high for both CO 2 and CH 4 (i.e., their root mean square (RMS) is between 8 and 12 parts per million (ppm) for CO 2 and between 30 and 55 parts per billion (ppb) for CH 4 at a given site). By analysing the gradients between the urban sites and a suburban or rural reference site, we are able to decrease the impact of uncertainties in the fluxes and transport outside the London area and in the model domain boundary conditions, and to better focus attention on the signature of London urban CO 2 and CH 4 emissions. This considerably improves the statistical agreement between the model and observations for CO 2 (model–data RMS misfit of between 3 and 7 ppm) and to a lesser degree for CH 4 (model–data RMS misfit of between 29 and 38 ppb). Between one of the urban sites and either reference site, selecting the gradients during periods wherein the reference site is upwind of the urban site further decreases the statistics of the misfits in general even though not systematically. In a final attempt to focus on the signature of the city anthropogenic emission in the mole fraction measurements, we use a theoretical ratio of gradients of CO to gradients of CO 2 from fossil fuel emissions in the London area to diagnose observation based fossil fuel CO 2 gradients, and compare them with the modelled ones. This estimate increases the consistency between the model and the measurements when considering one of the urban sites, but not when considering the other. While this study evaluates different approaches for increasing the consistency between the mesoscale model and the near ground data, and manages to decrease the random component of the analysed model data misfits to an extent that should not be prohibitive to extracting the signal from the London urban emissions, large biases remain in the final misfits. These biases are likely to be due to local emissions, to which the urban near ground sites are highly sensitive. This questions our current ability to exploit urban near ground data for the atmospheric inversion of city emissions based on models at spatial resolution coarser than 2 km.

Description: Observations and implications of liquid–liquid phase separation at high relative humidities in secondary organic material produced by α-pinene ozonolysis without inorganic salts Atmospheric Chemistry and Physics Discussions, 15, 33379-33405, 2015 Author(s): L. Renbaum-Wolff, M. Song, C. Marcolli, Y. Zhang, P. F. Liu, J. W. Grayson, F. M. Geiger, S. T. Martin, and A. K. Bertram Particles consisting of secondary organic material (SOM) are abundant in the atmosphere. To predict the role of these particles in climate, visibility, and atmospheric chemistry, information on particle phase state (i.e. single liquid, two liquids, solid and so forth) is needed. This paper focuses on the phase state of SOM particles free of inorganic salts produced by the ozonolysis of α-pinene. Phase transitions were investigated both in the laboratory and with a thermodynamic model over the range of 〈 0.5 % to 100 % relative humidity (RH) at 290 K. In the laboratory studies, a single phase was observed from 0 to 95 % RH while two liquid phases were observed above 95 % RH. For increasing RH, the mechanism of liquid–liquid phase separation (LLPS) was spinodal decomposition. The RH range at which two liquid phases were observed did not depend on the direction of RH change. In the modelling studies at low RH values, the SOM took up hardly any water and was a single organic-rich phase. At high RH values, the SOM underwent LLPS to form an organic-rich phase and an aqueous phase, consistent with the laboratory studies. The presence of LLPS at high RH-values has consequences for the cloud condensation nuclei (CCN) activity of SOM particles. In the simulated Köhler curves for SOM particles, two local maxima are observed. Depending on the composition of the SOM, the first or second maximum can determine the critical supersaturation for activation. The presence of LLPS at high RH-values can explain inconsistencies between measured CCN properties of SOM particles and hygroscopic growth measured below water saturation.

Description: Vertical and horizontal variability of PM 10 source contributions in Barcelona during SAPUSS Atmospheric Chemistry and Physics Discussions, 15, 33331-33378, 2015 Author(s): M. Brines, M. Dall'Osto, F. Amato, M. C. Minguillón, A. Karanasiou, A. Alastuey, and X. Querol During the SAPUSS campaign (Solving Aerosol Problems by Using Synergistic Strategies) PM 10 samples at twelve hours resolution were simultaneously collected at four monitoring sites located in the urban agglomerate of Barcelona (Spain). A total of 221 samples were collected from 20 September to 20 October 2010. The Road Site (RS) site and the Urban Background (UB) site were located at street level, whereas the Torre Mapfre (TM) and the Torre Collserola (TC) sites were located at 150 m a.s.l. by the sea side within the urban area and at 415 m a.s.l. 8 km inland, respectively. For the first time, we are able to report simultaneous PM 10 aerosol measurements allowing us to study aerosol gradients at both horizontal and vertical levels. The complete chemical composition of PM 10 was determined on the 221 samples, and factor analysis (Positive Matrix Factorisation, PMF) was applied. This resulted in eight factors which were attributed to eight main aerosol sources affecting PM 10 concentrations in the studied urban environment: (1) vehicle exhaust and wear (2–9 μg m −3 , 10–27 % of PM 10 mass on average), (2) road dust (2–4 μg m −3 , 8–12 %), (3) mineral dust (5 μg m −3 , 13–26 %), (4) aged marine (3–5 μg m −3 , 13–20 %), (5) heavy oil (0.4–0.6 μg m −3 , 2 %), (6) industrial (1 μg m −3 , 3–5 %), (7) sulphate (3–4 μg m −3 , 11–17 %) and (8) nitrate (4–6 μg m −3 , 17–21 %). Three aerosol sources were found enhanced at the ground levels (confined within the urban ground levels of the city) relative to the upper levels: (1) vehicle exhaust and wear (2.8 higher), (2) road dust (1.8 higher) and (3) local urban industries/crafts workshops (1.6 higher). Surprisingly, the other aerosol sources were relatively homogeneous at both horizontal and vertical levels. However, air mass origin and meteorological parameters also played a key role in influencing the variability of the factors concentrations. The mineral dust and aged marine factors were found to be a mixture of natural and anthropogenic components and were thus further investigated. Overall, three types of dust were identified to affect the urban study area: road dust (35 % of the mineral dust load, 2–4 μg m −3 on average), Saharan dust (28 %, 2.1 μg m −3 ) and background mineral dust (37 %, 2.8 μg m −3 ). Our results evidence that although the city of Barcelona broadly shows a homogeneous distribution of PM 10 pollution sources, non-exhaust traffic, exhaust traffic and local urban industrial activities are major coarse PM 10 aerosol sources.

Description: Comprehensive tool for calculation of radiative fluxes: illustration of shortwave aerosol radiative effect sensitivities to the details in aerosol and underlying surface characteristics Atmospheric Chemistry and Physics Discussions, 15, 33445-33492, 2015 Author(s): Y. Derimian, O. Dubovik, X. Huang, T. Lapyonok, P. Litvinov, A. Kostinski, P. Dubuisson, and F. Ducos The evaluation of aerosol radiative effect on broadband hemispherical solar flux is often performed using simplified spectral and directional scattering characteristics of atmospheric aerosol and underlying surface reflectance. In this study we present a rigorous yet fast computational tool that accurately accounts for detailed variability of both spectral and angular scattering properties of aerosol and surface reflectance in calculation of direct aerosol radiative effect. The tool is developed as part of the GRASP (Generalized Retrieval of Aerosol and Surface Properties) project. We use the tool to evaluate instantaneous and daily average radiative efficiencies of several key atmospheric aerosol models over different surface types. We then examine the differences due to neglect of surface reflectance anisotropy, non-sphericity of aerosol particle shape and accounting only for aerosol angular scattering asymmetry instead of using full phase function. For example, it is shown that neglecting aerosol particle nonsphericity causes mainly overestimation of the aerosol cooling effect and that magnitude of this overestimate changes significantly as a function of solar zenith angle (SZA) if only asymmetry parameter is used instead of detailed phase function. It was also found that the nonspherical–spherical differences in the calculated aerosol radiative effect are not modified significantly if detailed BRDF (Bidirectional Reflectance Distribution Function) is used instead of Lambertian approximation of surface reflectance. Additionally, calculations show that usage of only angular scattering asymmetry, even for case of spherical aerosols, modifies dependence of instantaneous aerosol radiative effect on SZA. This effect can be canceled for daily average values, but only if sun reaches the zenith, otherwise a systematic bias remains. Since the daily average radiative effect is obtained by integration over a range of SZAs, the errors vary with latitude and season. In summary, the present analysis showed that use of simplified assumptions causes systematic biases, rather than random uncertainties, in calculation of both instantaneous and daily average aerosol radiative effect. Finally, we illustrate application of the rigorous aerosol radiative effect calculations performed as part of GRASP aerosol retrieval from real POLDER/PARASOL satellite observations.

Description: A numerical study of back-building process in a quasi-stationary rainband with extreme rainfall over northern Taiwan during 11–12 June 2012 Atmospheric Chemistry and Physics Discussions, 15, 32679-32722, 2015 Author(s): C.-C. Wang, B.-K. Chiou, G. T.-J. Chen, and H.-C. Kuo During 11–12 June 2012, quasi-stationary linear mesoscale convective systems (MCSs) developed near northern Taiwan and produced extreme rainfall up to 510 mm and severe flooding in Taipei. Evident back-building (BB) process in these MCSs contributed to the extreme rainfall, and thus is investigated using a cloud-resolving model. Specifically, we seek answers to the question why the location about 15–30 km upstream from the old cell is often more favorable for new cell initiation without the cold pool mechanism in this subtropical event during the mei-yu season. With a horizontal grid size of 1.5 km, the model successfully reproduced the linear MCS and the BB process, which is found to be influenced by both dynamical and thermodynamical effects. During initiation in a background with convective instability, new cells are associated with positive (negative) buoyancy below (above) due to latent heating (adiabatic cooling), which represent a gradual destabilization. At the beginning, the new development is close to the old convection, which provides stronger warming below and additional cooling at mid-levels from evaporation of condensates, thus yielding a more rapid destabilization. This enhanced upward decrease in buoyancy at a lower height eventually creates an upward perturbation pressure gradient force to drive further development along with the buoyancy itself. After the new cell has gain sufficient strength, a descending branch at the old cell's rear flank acts to separate the new cell to about 20 km upstream. Therefore, the advantages of the spot in the BB process can be explained.

Description: High concentrations of sub-3 nm clusters and frequent new particle formation observed in the Po Valley, Italy, during the PEGASOS 2012 campaign Atmospheric Chemistry and Physics Discussions, 15, 33077-33119, 2015 Author(s): J. Kontkanen, E. Järvinen, H. E. Manninen, K. Lehtipalo, J. Kangasluoma, S. Decesari, G. P. Gobbi, A. Laaksonen, T. Petäjä, and M. Kulmala The concentrations of neutral and charged sub-3 nm clusters and their connection to new particle formation (NPF) were investigated during the PEGASOS campaign (7 June–9 July 2012) at the San Pietro Capofiume measurement station in the Po Valley, Italy. Continuous high concentrations of sub-3 nm clusters were detected during the measurement period, although the condensation sink was relatively high (median value 1.1 × 10 -2 s -1 ). The median cluster concentrations were 2140 and 7980 cm -3 in the size bins of 1.5–1.8 nm and 1.8–3 nm, and the majority of them were electrically neutral. NPF events were observed during the measurement period frequently, on 86 % of the days. The median growth rates of clusters during the events were 4.3, 6.0 and 7.2 nm h -1 in the size ranges of 1.5–3, 3–7 and 7–20 nm. The median formation rate of 1.6 nm clusters was high, 45 cm -3 s -1 , and it exceeded the median formation rate of 2 nm clusters by one order of magnitude. The ion-induced nucleation fraction was low; the median values were 0.7 % at 1.6 nm and 3.0 % at 2 nm. On NPF event days the neutral cluster concentration had a maximum around 9 a.m. (local winter time), which was absent on a non-event day. The increase in the cluster concentrations in the morning coincided with the increase in the boundary layer height. At the same time radiation and temperature increased and RH and condensation sink decreased. The concentration of neutral clusters was observed to have apositive correlation with sulfuric acid proxy, indicating the significance of sulfuric acid for the cluster formation in San Pietro Capofiume. The condensation sink had anegative correlation with the concentration of charged clusters but no clear relation to the neutral cluster concentration. This finding, together with back-trajectory analysis, suggests that the precursor vapors of the clusters and background aerosol particles, acting as their sink, have possibly originated from the same sources, including e.g. power plants and industrial areas in the Po Valley.

Description: Mercury transformation and speciation in flue gases from anthropogenic emission sources: a critical review Atmospheric Chemistry and Physics Discussions, 15, 32889-32929, 2015 Author(s): L. Zhang, S. X. Wang, Q. R. Wu, F. Y. Wang, C.-J. Lin, L. M. Zhang, M. L. Hui, and J. M. Hao Mercury transformation mechanisms and speciation profiles are reviewed for mercury formed in and released from flue gases of coal-fired boilers, non-ferrous metal smelters, cement plants, iron and steel plants, municipal solid waste incinerators, and biomass burning. Mercury in coal, ores and other raw materials is released to flue gases in the form of Hg 0 during combustion or smelting in boilers, kilns or furnaces. Decreasing temperature from over 800 °C to below 300 °C in flue gases leaving boilers, kilns or furnaces promotes homogeneous and heterogeneous oxidation of gaseous elemental mercury (Hg 0 ) to gaseous divalent mercury (Hg 2+ ), with a portion of Hg 2+ adsorbed onto fly ash to form particulate-bound mercury (Hg p ). Halogen is the primary oxidizer for Hg 0 in flue gases, and active components (e.g.,TiO 2 , Fe 2 O 3 , etc.) on fly ash promote heterogeneous oxidation and adsorption processes. In addition to mercury removal, mercury transformation also occurs when passing through air pollution control devices (APCDs), affecting the mercury speciation in flue gases. In coal-fired power plants, selective catalytic reduction (SCR) system promotes mercury oxidation by 34–85 %, electrostatic precipitator (ESP) and fabric filter (FF) remove over 99 % of Hg p , and wet flue gas desulfurization system (WFGD) captures 60–95 % of Hg 2+ . In non-ferrous metal smelters, most Hg 0 is converted to Hg 2+ and removed in acid plants (APs). For cement clinker production, mercury cycling and operational conditions promote heterogeneous mercury oxidation and adsorption. The mercury speciation profiles in flue gases emitted to the atmosphere are determined by transformation mechanisms and mercury removal efficiencies by various APCDs. For all the sectors reviewed in this study, Hg p accounts for less than 5 % in flue gases. In China, mercury emission has a higher fraction (66–82 % of total mercury) in flue gases from coal combustion, in contrast to a greater Hg 2+ fraction (29–90 %) from non-ferrous metal smelting, cement and iron/steel production. The higher Hg 2+ fractions shown here than previous estimates may imply stronger local environmental impacts than previously thought, caused by mercury emissions in East Asia. Future research should focus on determining mercury speciation in flue gases from iron and steel plants, waste incineration and biomass burning, and on elucidating the mechanisms of mercury oxidation and adsorption in flue gases.

Description: Time-resolved characterization of primary and secondary particle emissions of a modern gasoline passenger car Atmospheric Chemistry and Physics Discussions, 15, 33253-33282, 2015 Author(s): P. Karjalainen, H. Timonen, E. Saukko, H. Kuuluvainen, S. Saarikoski, P. Aakko-Saksa, T. Murtonen, M. Dal Maso, E. Ahlberg, B. Svenningsson, W. H. Brune, R. Hillamo, J. Keskinen, and T. Rönkkö Changes in traffic systems and vehicle emission reduction technologies significantly affect traffic-related emissions in urban areas. In many densely populated areas the amount of traffic is increasing, keeping the emission level high or even increasing. To understand the health effects of traffic related emissions, both primary and secondary particles that are formed in the atmosphere from gaseous exhaust emissions need to be characterized. In this study we used a comprehensive set of measurements to characterize both primary and secondary particulate emissions of a modern gasoline passenger car. Our aerosol particle study covers the whole process chain in emission formation, from the engine to the atmosphere, and takes into account also differences in driving patterns. We observed that in mass terms, the amount of secondary particles was 13 times higher than the amount of primary particles. The formation, composition, number, and mass of secondary particles was significantly affected by driving patterns and engine conditions. The highest gaseous and particulate emissions were observed at the beginning of the test cycle when the performance of the engine and the catalyst was below optimal. The key parameter for secondary particle formation was the amount of gaseous hydrocarbons in primary emissions; however, also the primary particle population had an influence. Thus, in order to enhance human health and wellbeing in urban areas, our study strongly indicates that in future legislation, special attention should be directed into the reduction of gaseous hydrocarbons.

Description: Simulating the SOA formation of isoprene from partitioning and aerosol phase reactions in the presence of inorganics Atmospheric Chemistry and Physics Discussions, 15, 33121-33159, 2015 Author(s): R. L. Beardsley and M. Jang The secondary organic aerosol (SOA) produced by the photooxidation of isoprene with and without inorganic seed is simulated using the Unified Partitioning Aerosol Phase Reaction (UNIPAR) model. Recent work has found the SOA formation of isoprene to be sensitive to both aerosol acidity ([H + ]) and aerosol liquid water content (LWC) with the presence of either leading to significant aerosol phase organic mass generation and large growth in SOA yields ( Y SOA ). Classical partitioning models alone are insufficient to predict isoprene SOA formation due to the high volatility of the photooxidation products and the sensitivity of their mass yields to variations in inorganic aerosol composition. UNIPAR utilizes the chemical structures provided by a near-explicit chemical mechanism to estimate the thermodynamic properties of the gas phase products, which are lumped based on their calculated vapor pressure (8 groups) and aerosol phase reactivity (6 groups). UNIPAR then determines the SOA formation of each lumping group from both partitioning and aerosol phase reactions (oligomerization, acid catalyzed reactions, and organosulfate formation) assuming a single homogeneously mixed organic–inorganic phase as a function of inorganic composition and VOC / NO x . The model is validated using isoprene photooxidation experiments performed in the dual, outdoor UF APHOR chambers. UNIPAR is able to predict the experimental SOA formation of isoprene without seed, with H 2 SO 4 seed gradually titrated by ammonia, and with the acidic seed generated by SO 2 oxidation. Oligomeric mass is predicted to account for more than 65 % of the total OM formed in all cases and over 85 % in the presence of strongly acidic seed. The model is run to determine the sensitivity of Y SOA to [H + ], LWC, and VOC / NO x , and it is determined that the SOA formation of isoprene is most strongly related to [H + ] but is dynamically related to all three parameters. For VOC / NO x 〉 10, with increasing NO x both experimental and simulated Y SOA increase and are found to be more sensitive to [H + ] and LWC. For atmospherically relevant conditions, Y SOA is found to be more than 150 % higher in partially titrated acidic seeds (NH 4 HSO 4 ) than in effloresced inorganics or in isoprene only.

Description: Characteristics of aerosol pollution during heavy haze events in Suzhou, China Atmospheric Chemistry and Physics Discussions, 15, 33407-33443, 2015 Author(s): M. Tian, H. B. Wang, Y. Chen, F. M. Yang, X. H. Zhang, Q. Zou, R. Q. Zhang, Y. L. Ma, and K. B. He A comprehensive measurement was carried out to analyze the heavy haze events in Suzhou in January 2013 when extremely severe haze pollution occurred in many cities in China especially in the East. Hourly concentrations of PM 2.5 , chemical composition (including water-soluble inorganic ions, OC, and EC), and gas-phase precursors were obtained via on-line monitoring system. Based on these data, detailed aerosol composition, light extinction and gas-phase precursors were analyzed to understand the characteristics of the haze events, moreover, the formation mechanism of nitrate and sulfate in PM 2.5 and the regional sources deduced from trajectory and PSCF were discussed to explore the origin of the heavy aerosol pollution. The results showed that frequent haze events were occurred on January 2013 and the concentrations of PM 2.5 often exceeded 150 μg m -3 during the haze occurrence, with a maximum concentration of 324 μg m -3 on 14 January 2013. Unfavorable weather conditions (high RH, and low rainfall, wind speed and atmospheric pressure), high concentration of secondary aerosol species (including SO 4 2- , NO 3 - , NH 4 + , and SOC) and precursors were observed during the haze events. Additionally, OM, (NH 4 ) 2 SO 4 , NH 4 NO 3 were demonstrated to be the major contributors to the visibility impairment but the share differed from haze events. This study also found that the high concentration of sulfate might be explained by the heterogeneous reactions in the aqueous surface layer of pre-existing particles or in cloud processes while nitrate might be mainly formed through homogeneous gas-phase reactions. The results of trajectory clustering and the PSCF method manifested that aerosol pollutions in the studied areas were mainly affected by local activities and surrounding sources transported from nearby cities.

Description: Insights into a historic severe haze weather in Shanghai: synoptic situation, boundary layer and pollutants Atmospheric Chemistry and Physics Discussions, 15, 32561-32605, 2015 Author(s): C. Leng, J. Duan, C. Xu, H. Zhang, Q. Zhang, Y. Wang, X. Li, L. Kong, J. Tao, T. Cheng, R. Zhang, and J. Chen A historic winter haze weather, characterized by long duration, large scale and strong pollution intensity, occurred in the Yangtze River Delta (YRD) region of China during the time frame of 1 to 10 December 2013. This severe haze event constituted of several hazy episodes and significantly influenced air quality throughout the region, especially in urban areas. Aerosol physical, chemical and optical properties were measured in Shanghai, where the instantaneous particulate mass burden per volume (e.g. PM 2.5 ) exceeded 600 μg m −3 in some time, breaking the existing historical observation records, and examined to give insights into severe haze weathers. Inorganic water-soluble ions in particles, trace gases and aerosol scattering/absorption coefficients had the same tendency to increase evidently from clear episodes to hazy episodes. A combination of various factors contributed to the formation and evolution of the severe haze, among which meteorological conditions, local anthropogenic emissions and aerosol properties played the major roles. During the haze weather, the YRD region was under the control of a high-pressure system with extremely small surface pressure gradients. The calm surface wind and subsidence airflow were responsible for decreasing planetary boundary layer (PBL) height and constructive to the build-up of air pollutants wandering inside the region, and ultimately induced the haze occurrence. Nonlinear regression analyses indicated that single water-soluble ion did not correlated with the atmospheric visibility degradation so strong, while high ambient relative humidity (RH) indeed exerted a great impact with a correlation coefficient ( R 2 ) of 0.41. Moreover, the close relationship was derived between atmospheric visibility and aerosols in size of 600–1400 nm with R 2 of 0.70, which further improved to 0.73 when combined aerosol hygroscopicity. This study may provide supports for the public and authorities to recognize severe haze weathers in urban environments, and act as a reference for forecasting and eliminating the occurrences of regional atmospheric pollutions in China.

Description: Temporal consistency of lidar observables during aerosol transport events in the framework of the ChArMEx/ADRIMED campaign at Menorca Island in June 2013 Atmospheric Chemistry and Physics Discussions, 15, 32723-32757, 2015 Author(s): P. Chazette, J. Totems, G. Ancellet, J. Pelon, and M. Sicard We performed synergetic daytime and night-time active and passive remote sensing observations at Menorca (Balearic Island, Spain), over more than 3 weeks during the Chemistry–Aerosol Mediterranean Experiment/Aerosol Direct Radiative Effect in the Mediterranean (ChArMEx/ADRIMED) special observation period (SOP 1a, June–July 2013). We characterized the aerosol optical properties and type in the low and middle troposphere using an automated procedure combining Rayleigh–Mie–Raman lidar (355, 387 and 407 nm) with depolarization (355 nm) and AERONET Cimel® sun-photometer data. Results show a high variability due to varying dynamical forcing. The mean column-averaged lidar backscatter-to-extinction ratio (BER) was close to 0.024 sr -1 (lidar ratio of ∼ 41.7 sr), with a large dispersion of ±33 % over the whole observation period due to changing atmospheric transport regimes and aerosol sources. The ground-based remote sensing measurements, coupled with satellite observations, allowed to document (i) dust particles up to 5 km a.s.l. in altitude originating from Morocco and Algeria from 15 to 18 June with a peak in aerosol optical thickness (AOT) of 0.25 ± 0.05 at 355 nm, (ii) a long-range transport of biomass burning aerosol (AOT = 0.18 ± 0.16) related to North American forest fires detected from 26 to 28 June 2013 by the lidar between 2 and 7 km and (iii) mixture of local sources including marine aerosol particles and pollution from Spain. During the biomass burning event, the high value of the particle depolarization ratio (8–14 %) may imply the presence of dust-like particles mixed with the biomass burning aerosols in the mid troposphere. We show also linearity with SEVIRI retrievals of the aerosol optical thickness within 35 % relative bias, which is discussed as a function of aerosol type.

Description: Limitations of passive satellite remote sensing to constrain global cloud condensation nuclei Atmospheric Chemistry and Physics Discussions, 15, 32607-32637, 2015 Author(s): P. Stier Aerosol–cloud interactions are considered a key uncertainty in our understanding of climate change (Boucher et al., 2013). Knowledge of the global abundance of aerosols suitable to act as cloud condensation nuclei (CCN) is fundamental to determine the strength of the anthropogenic climate perturbation. Direct measurements are limited and sample only a very small fraction of the globe so that remote sensing from satellites and ground based instruments is widely used as a proxy for cloud condensation nuclei (Nakajima et al., 2001; Andreae, 2009; Clarke and Kapustin, 2010; Boucher et al., 2013). However, the underlying assumptions cannot be robustly tested with the small number of measurements available so that no reliable global estimate of cloud condensation nuclei exists. This study overcomes this limitation using a fully self-consistent global model (ECHAM-HAM) of aerosol radiative properties and cloud condensation nuclei. An analysis of the correlation of simulated aerosol radiative properties and cloud condensation nuclei reveals that common assumptions about their relationships are violated for a significant fraction of the globe: 71 % of the area of the globe shows correlation coefficients between CCN 0.2% at cloud base and aerosol optical depth (AOD) below 0.5, i.e. AOD variability explains only 25 % of the CCN variance. This has significant implications for satellite based studies of aerosol–cloud interactions. The findings also suggest that vertically resolved remote sensing techniques, such as satellite-based high spectral resolution lidars, have a large potential for global monitoring of cloud condensation nuclei.

Description: Comparison of eddy covariance and modified Bowen ratio methods for measuring gas fluxes and implications for measuring fluxes of persistent organic pollutants Atmospheric Chemistry and Physics Discussions, 15, 32759-32777, 2015 Author(s): D. J. Bolinius, A. Jahnke, and M. MacLeod Semi-volatile persistent organic pollutants (POPs) cycle between the atmosphere and terrestrial surfaces, however measuring fluxes of POPs between the atmosphere and other media is challenging. Sampling times of hours to days are required to accurately measure trace concentrations of POPs in the atmosphere, which rules out the use of eddy covariance techniques that are used to measure gas fluxes of major air pollutants. An alternative, the modified Bowen ratio (MBR) method, has been used instead. In this study we used data from FLUXNET for CO 2 and water vapor (H 2 O) to compare fluxes measured by eddy covariance to fluxes measured with the MBR method using vertical concentration gradients in air derived from averaged data that simulates the long sampling times typically required to measure POPs. When concentration gradients are strong and fluxes are unidirectional, the MBR method and the eddy covariance method agree within a factor of 3 for CO 2 , and within a factor of 10 for H 2 O. To remain within the range of applicability of the MBR method field, studies should be carried out under conditions such that the direction of net flux does not change during the sampling period. If that condition is met then the performance of the MBR method is not strongly affected by the length of sample duration nor the use of a fixed value for the transfer coefficient.

Description: Profiling of aerosol microphysical properties at several EARLINET/AERONET sites during July 2012 ChArMEx/EMEP campaign Atmospheric Chemistry and Physics Discussions, 15, 32831-32887, 2015 Author(s): M. J. Granados-Muñoz, F. Navas-Guzmán, J. L. Guerrero-Rascado, J. A. Bravo-Aranda, I. Binietoglou, S. N. Pereira, S. Basart, J. M. Baldasano, L. Belegante, A. Chaikovsky, A. Comerón, G. D'Amico, O. Dubovik, L. Ilic, P. Kokkalis, C. Muñoz-Porcar, S. Nickovic, D. Nicolae, F. J. Olmo, A. Papayannis, G. Pappalardo, A. Rodríguez, K. Schepanski, M. Sicard, A. Vukovic, U. Wandinger, F. Dulac, and L. Alados-Arboledas The analysis of aerosol microphysical properties profiles at different European stations is made in the framework of the ChArMEx/EMEP 2012 field campaign (9–11 July 2012). During and in support to this campaign, five lidar ground-based stations (Athens, Barcelona, Bucharest, Évora and Granada) performed 72 h of continuous lidar and collocated and coincident sun-photometer measurements. Therefore it was possible to retrieve volume concentration profiles with the Lidar Radiometer Inversion Code (LIRIC). Results indicated the presence of a mineral dust plume affecting the Western Mediterranean region (mainly Granada station) whereas a different aerosol plume was observed over the Balkans area. LIRIC profiles showed a predominance of coarse spheroid particles above Granada, as expected for mineral dust, and an aerosol plume composed mainly of fine and coarse spherical particles above Athens and Bucharest. Due to the exceptional characteristics of the ChArMEx database, the analysis of the microphysical properties profiles temporal evolution was also possible. An in depth analysis was performed mainly at Granada station because of the availability of continuous lidar measurements and frequent AERONET inversion retrievals. The analysis at Granada was of special interest since the station was affected by mineral dust during the complete analyzed period. LIRIC was found to be a very useful tool for performing continuous monitoring of mineral dust, allowing for the analysis of the dynamics of the dust event in the vertical and temporal coordinates. Results obtained here illustrate the importance of having collocated and simultaneous advanced lidar and sun-photometer measurements in order to characterize the aerosol microphysical properties both in the vertical and temporal coordinates at a regional scale. In addition, this study revealed that the use of the depolarization information as input in LIRIC in the stations of Bucharest, Évora and Granada was crucial for the characterization of the aerosol types and their distribution in the vertical column, whereas in stations lacking of depolarization lidar channels ancillary information was needed. Results obtained were also used for the validation of different mineral dust models. In general, the models better forecast the vertical distribution of the mineral dust than the column integrated mass concentration, which was underestimated in most of the cases.

Description: Microphysics-based black carbon aging in a global CTM: constraints from HIPPO observations and implications for global black carbon budget Atmospheric Chemistry and Physics Discussions, 15, 32779-32829, 2015 Author(s): C. He, Q. Li, K. N. Liou, L. Qi, S. Tao, and J. P. Schwarz We develop and examine a microphysics-based black carbon (BC) aerosol aging scheme that accounts for condensation and coagulation processes in a global 3-D chemical transport model (GEOS-Chem) by interpreting the BC measurements from the HIAPER Pole-to-Pole Observations (HIPPO, 2009–2011) using the model. We convert aerosol mass in the model to number concentration by assuming lognormal aerosol size distributions and compute the microphysical BC aging rate explicitly from the condensation of soluble materials onto hydrophobic BC and the coagulation between hydrophobic BC and preexisting soluble particles. The resulting aging rate is ∼ 4 times higher in the lower troposphere over source regions than that from a fixed aging scheme with an e -folding time of 1.2 days. The higher aging rate reflects the large emissions of sulfate-nitrate and secondary organic aerosol precursors hence faster BC aging through condensation and coagulation. In contrast, the microphysical aging is more than fivefold slower than the fixed aging in remote regions, where condensation and coagulation are weak. Globally BC microphysical aging is dominated by condensation, while coagulation contribution is largest over East China, India, and Central Africa. The fixed aging scheme results in an overestimate of HIPPO BC throughout the troposphere by a factor of 6 on average. The microphysical scheme reduces this discrepancy by a factor of ∼ 3, particularly in the middle and upper troposphere. It also leads to a threefold reduction in model bias in the latitudinal BC column burden averaged along the HIPPO flight tracks, with largest improvements in the tropics. The resulting global annual mean BC lifetime is 4.2 days and BC burden is 0.25 mg m -2 , with 7.3 % of the burden at high altitudes (above 5 km). Wet scavenging accounts for 80.3 % of global BC deposition. We find that in source regions the microphysical aging rate is insensitive to aerosol size distribution, condensation threshold, and chemical oxidation aging, while it is the opposite in remote regions, where the aging rate is orders of magnitude smaller. As a result, global BC burden and lifetime show little sensitivity ( 〈 5 % change) to these three factors.

Description: Atmospheric speciated mercury concentrations on an island between China and Korea: sources and transport pathways Atmospheric Chemistry and Physics Discussions, 15, 32931-32966, 2015 Author(s): G.-S. Lee, P.-R. Kim, Y.-J. Han, T. M. Holsen, Y.-S. Seo, and S.-M. Yi As a global pollutant, mercury (Hg) is of particular concern in East Asia where anthropogenic emissions are the largest. In this study, speciated Hg concentrations were measured in the western most island in Korea, located between China and the Korean mainland to identify the importance of local, regional and distant Hg sources. Various tools including correlations with other pollutants, conditional probability function, and back-trajectory based analysis consistently indicated that Korean sources were important for gaseous oxidized mercury (GOM) whereas, for total gaseous mercury (TGM) and particulate bound mercury (PBM), long-range and regional transport were also important. A trajectory cluster based approach considering both Hg concentration and the fraction of time each cluster was impacting the site was developed to quantify the effect of Korean sources and out-of-Korean source. This analysis suggests that Korean sources contributed approximately 55 % of the GOM and PBM while there were approximately equal contributions from Korean and out-of-Korean sources for the TGM measured at the site. The ratio of GOM / PBM decreased when the site was impacted by long-range transport, suggesting that this ratio may be a useful tool for identifying the relative significance of local sources vs. long-range transport. The secondary formation of PBM through gas-particle partitioning with GOM was found to be important at low temperatures and high relative humidity.

Description: Modelled thermal and dynamical responses of the middle atmosphere to EPP-induced ozone changes Atmospheric Chemistry and Physics Discussions, 15, 33283-33329, 2015 Author(s): K. Karami, P. Braesicke, M. Kunze, U. Langematz, M. Sinnhuber, and S. Versick Energetic particles including protons, electrons and heavier ions, enter the Earth's atmosphere over the polar regions of both hemispheres, where they can greatly disturb the chemical composition of the upper and middle atmosphere and contribute to ozone depletion in the stratosphere and mesosphere. The chemistry–climate general circulation model EMAC is used to investigate the impact of changed ozone concentration due to Energetic Particle Precipitation (EPP) on temperature and wind fields. The results of our simulations show that ozone perturbation is a starting point for a chain of processes resulting in temperature and circulation changes over a wide range of latitudes and altitudes. In both hemispheres, as winter progresses the temperature and wind anomalies move downward with time from the mesosphere/upper stratosphere to the lower stratosphere. In the Northern Hemisphere (NH), once anomalies of temperature and zonal wind reach the lower stratosphere, another signal develops in mesospheric heights and moves downward. Analyses of Eliassen and Palm (EP) flux divergence show that accelerating or decelerating of the stratospheric zonal flow is in harmony with positive and negative anomalies of the EP flux divergences, respectively. This results suggest that the oscillatory mode in the downwelling signal of temperature and zonal wind in our simulations are the consequence of interaction between the resolved waves in the model and the mean stratospheric flow. Therefore, any changes in the EP flux divergence lead to anomalies in the zonal mean zonal wind which in turn feed back on the propagation of Rossby waves from the troposphere to higher altitudes. The analyses of Rossby waves refractive index show that the EPP-induced ozone anomalies are capable of altering the propagation condition of the planetary-scale Rossby waves in both hemispheres. It is also found that while ozone depletion was confined to mesospheric and stratospheric heights, but it is capable to alter Rossby wave propagation down to tropospheric heights. In response to an accelerated polar vortex in the Southern Hemisphere (SH) late wintertime, we found almost two weeks delay in the occurrence of mean dates of Stratospheric Final Warming (SFW). These results suggest that the stratosphere is not merely a passive sink of wave activity from below, but it plays an active role in determining its own budget of wave activity.

Description: SOA formation from the photooxidation of α-pinene: systematic exploration of the simulation of chamber data Atmospheric Chemistry and Physics Discussions, 15, 33161-33207, 2015 Author(s): R. C. McVay, X. Zhang, B. Aumont, R. Valorso, M. Camredon, Y. S. La, P. O. Wennberg, and J. H. Seinfeld Chemical mechanisms play an important role in simulating the atmospheric chemistry of volatile organic compound oxidation. Comparison of mechanism simulations with laboratory chamber data tests our level of understanding of the prevailing chemistry as well as the dynamic processes occurring in the chamber itself. α-pinene photooxidation is a well-studied system experimentally, for which detailed chemical mechanisms have been formulated. Here, we present the results of simulating low-NO α-pinene photooxidation experiments conducted in the Caltech chamber with the Generator for Explicit Chemistry and Kinetics of Organics in the Atmosphere (GECKO-A) under varying concentrations of seed particles and OH levels. Unexpectedly, experiments conducted at low and high OH levels yield the same SOA growth, whereas GECKO-A predicts greater SOA growth under high OH levels. SOA formation in the chamber is a result of a competition among the rates of gas-phase oxidation to low volatility products, wall deposition of these products, and condensation into the aerosol phase. Various processes, such as photolysis of condensed-phase products, particle-phase dimerization, and peroxy radical autoxidation, are explored to rationalize the observations. In order to explain the observed similar SOA growth under different OH levels, we conclude that vapor wall loss in the Caltech chamber is likely of order 10 −5 s −1 , consistent with previous experimental measurements in that chamber. We find that GECKO-A tends to overpredict the contribution to SOA of later-generation oxidation products under high OH conditions. The key role of the interplay among oxidation rate, product volatility, and vapor-wall deposition in chamber experiments is illustrated.

Description: Spatial, temporal and vertical distribution of ammonia concentrations over Europe – comparing a static and dynamic approach with WRF-Chem Atmospheric Chemistry and Physics Discussions, 15, 22935-22973, 2015 Author(s): M. Werner, M. Kryza, C. Geels, T. Ellermann, and C. Ambelas Skjøth The study focuses on the application of a dynamic ammonia emission into the Weather Research and Forecasting Chemistry model (WRF-Chem) and the influence on the simulated ammonia concentrations and the overall model performance. We have focused on agricultural ammonia sources and have analysed both hourly and daily patterns of ammonia emissions and concentrations at measurement sites located in agricultural areas or influenced by this activity. For selected episodes, we have also investigated the 3-D patterns of the ammonia concentrations in the atmosphere. The application of the dynamic ammonia emission into the WRF-Chem model (the "DYNAMIC" simulation) results in an improvement of the modelled daily ammonia concentrations in comparison to a static approach (the "BASE" simulation), which is currently widely used in chemical transport models. In the case of hourly resolution, we have observed an improvement for the DYNAMIC approach for the winter and autumn seasons, but for the entire year the modelled hourly ammonia peaks are shifted toward the afternoon hours if compared with measurements. This study indicates that the current description of the diurnal cycle of the ammonia concentration from fields is not accurate and more research is needed in order to improve the processes that describe the emission from fertilised fields. The results suggest that the governing processes in relation to the diurnal cycle are the atmospheric mixing and the emission strength. Therefore, an improved description of the diurnal profile of ammonia concentrations within atmospheric models requires a better description of the planetary boundary layer height and a stronger daily pattern of ammonia emission, e.g. through increased evaporation or increased fluxes from the surface.

Description: Modeling study of the 2010 regional haze event in the North China Plain Atmospheric Chemistry and Physics Discussions, 15, 22781-22822, 2015 Author(s): M. Gao, G. R. Carmichael, Y. Wang, P. E. Saide, M. Yu, J. Xin, Z. Liu, and Z. Wang The online coupled Weather Research and Forecasting-Chemistry (WRF-Chem) model was applied to simulate a haze event that happened in January 2010 in the North China Plain (NCP), and was validated against various types of measurements. The evaluations indicate that WRF-Chem provides reliable simulations for the 2010 haze event in the NCP. This haze event is mainly caused by high emissions of air pollutants in the NCP and stable weather conditions in winter. Secondary inorganic aerosols also played an important role and cloud chemistry had important contributions. Air pollutants outside Beijing contributed about 47.8 % to the PM 2.5 levels in Beijing during this haze event, and most of them are from south Hebei, Shandong and Henan provinces. In addition, aerosol feedback has important impacts on surface temperature, Relative Humidity (RH) and wind speeds, and these meteorological variables affect aerosol distribution and formation in turn. In Shijiazhuang, Planetary Boundary Layer (PBL) decreased about 300 m and PM 2.5 increased more than 20 μg m -3 due to aerosol feedback. Feedbacks associated to Black Carbon (BC) account for about 50 % of the PM 2.5 increases and 50 % of the PBL decreases in Shijiazhuang, indicating more attention should be paid to BC from both air pollution control and climate change perspectives.

Description: Investigation of error sources in regional inverse estimates of greenhouse gas emissions in Canada Atmospheric Chemistry and Physics Discussions, 15, 22715-22779, 2015 Author(s): E. Chan, D. Chan, M. Ishizawa, F. Vogel, J. Brioude, A. Delcloo, Y. Wu, and B. Jin Inversion models can use atmospheric concentration measurements to estimate surface fluxes. This study is an evaluation of the errors in a regional flux inversion model for different provinces of Canada, Alberta (AB), Saskatchewan (SK) and Ontario (ON). Using CarbonTracker model results as the target, the synthetic data experiment analyses examined the impacts of the errors from the Bayesian optimisation method, prior flux distribution and the atmospheric transport model, as well as their interactions. The scaling factors for different sub-regions were estimated by the Markov chain Monte Carlo (MCMC) simulation and cost function minimization (CFM) methods. The CFM method results are sensitive to the relative size of the assumed model-observation mismatch and prior flux error variances. Experiment results show that the estimation error increases with the number of sub-regions using the CFM method. For the region definitions that lead to realistic flux estimates, the numbers of sub-regions for the western region of AB/SK combined and the eastern region of ON are 11 and 4 respectively. The corresponding annual flux estimation errors for the western and eastern regions using the MCMC (CFM) method are -7 and -3 % (0 and 8 %) respectively, when there is only prior flux error. The estimation errors increase to 36 and 94 % (40 and 232 %) resulting from transport model error alone. When prior and transport model errors co-exist in the inversions, the estimation errors become 5 and 85 % (29 and 201 %). This result indicates that estimation errors are dominated by the transport model error and can in fact cancel each other and propagate to the flux estimates non-linearly. In addition, it is possible for the posterior flux estimates having larger differences than the prior compared to the target fluxes, and the posterior uncertainty estimates could be unrealistically small that do not cover the target. The systematic evaluation of the different components of the inversion model can help in the understanding of the posterior estimates and percentage errors. Stable and realistic sub-regional and monthly flux estimates for western region of AB/SK can be obtained, but not for the eastern region of ON. This indicates that it is likely a real observation-based inversion for the annual provincial emissions will work for the western region whereas; improvements are needed with the current inversion setup before real inversion is performed for the eastern region.

Description: Characteristics and sources of submicron aerosols above the urban canopy (260 m) in Beijing, China during 2014 APEC summit Atmospheric Chemistry and Physics Discussions, 15, 22889-22934, 2015 Author(s): C. Chen, Y. L. Sun, W. Q. Xu, W. Du, L. B. Zhou, T. T. Han, Q. Q. Wang, P. Q. Fu, Z. F. Wang, Z. Q. Gao, Q. Zhang, and D. R. Worsnop The megacity of Beijing has experienced frequent severe fine particle pollution during the last decade. Although the sources and formation mechanisms of aerosol particles have been extensively investigated on the basis of ground measurements, real-time characterization of aerosol particle composition and sources above the urban canopy in Beijing is rare. In this study, we conducted real-time measurements of non-refractory submicron aerosol (NR-PM 1 ) composition at 260 m at the 325 m Beijing Meteorological Tower (BMT) from 10 October to 12 November 2014, by using an aerosol chemical speciation monitor (ACSM) along with synchronous measurements of size-resolved NR-PM 1 composition at near ground level using a High-Resolution Time-of-Flight Aerosol Mass Spectrometer (HR–ToF–AMS). The NR-PM 1 composition above the urban canopy was dominated by organics (46 %), followed by nitrate (27 %) and sulfate (13 %). The high contribution of nitrate and high NO 3 − /SO 4 2− mass ratios illustrate an important role of nitrate in particulate matter (PM) pollution during the study period. The organic aerosol (OA) was mainly composed by secondary OA (SOA), accounting for 61 % on an average. Different from that measured at the ground site, primary OA (POA) correlated moderately with SOA, likely suggesting a high contribution from regional transport above the urban canopy. The Asia–Pacific Economic Cooperation (APEC) summit with strict emission controls provides a unique opportunity to study the impacts of emission controls on aerosol chemistry. All aerosol species were shown to have significant decreases of 40–80 % during APEC from those measured before APEC, suggesting that emission controls over regional scales substantially reduced PM levels. However, the bulk aerosol composition was relatively similar before and during APEC as a result of synergetic controls of aerosol precursors such as SO 2 , NO x , and volatile organic compounds (VOCs). In addition to emission controls, the routine circulations of mountain–valley breezes were also found to play an important role in alleviating PM levels and achieving the "APEC blue" effect. The evolution of vertical differences between 260 m and the ground level was also investigated. Our results show complex vertical differences during the formation and evolution of severe haze episodes that are closely related to aerosol sources and boundary layer dynamics.

Description: Aircraft-measured indirect cloud effects from biomass burning smoke in the Arctic and subarctic Atmospheric Chemistry and Physics Discussions, 15, 22823-22887, 2015 Author(s): L. M. Zamora, R. A. Kahn, M. J. Cubison, G. S. Diskin, J. L. Jimenez, Y. Kondo, G. M. McFarquhar, A. Nenes, K. L. Thornhill, A. Wisthaler, A. Zelenyuk, and L. D. Ziemba The incidence of wildfires in the Arctic and subarctic is increasing; in boreal North America, for example, the burned area is expected to increase by 200–300 % over the next 50–100 years, which previous studies suggest could have a large effect on cloud microphysics, lifetime, albedo, and precipitation. However, the interactions between smoke particles and clouds remain poorly quantified due to confounding meteorological influences and remote sensing limitations. Here, we use data from several aircraft campaigns in the Arctic and subarctic to explore cloud microphysics in liquid-phase clouds influenced by biomass burning. Median cloud droplet radii in smoky clouds were ~ 50 % smaller than in background clouds. Based on the relationship between cloud droplet number ( N liq ) and various biomass burning tracers (BB t ) across the multi-campaign dataset, we calculated the magnitude of subarctic and Arctic smoke aerosol-cloud interactions (ACI, where ACI = (1/3) × d ln ( N liq ) / d ln (BB t )) to be ~ 0.12 out of a maximum possible value of 0.33 that would be obtained if all aerosols were to nucleate cloud droplets. Interestingly, in a separate subarctic case study with low liquid water content (~ 0.02 g m −3 ) and very high aerosol concentrations (2000–3000 cm −3 ) in the most polluted clouds, the estimated ACI value was only 0.06. In this case, competition for water vapor by the high concentration of CCN strongly limited the formation of droplets and reduced the cloud albedo effect, which highlights the importance of cloud feedbacks across scales. Using our calculated ACI values, we estimate that the smoke-driven cloud albedo effect may decrease shortwave radiative flux by 2–4 W m −2 or more under some low and homogeneous cloud cover conditions in the subarctic, although the changes should be smaller in high surface albedo regions of the Arctic. We lastly show evidence to suggest that numerous northern latitude background Aitken particles can interact with combustion particles, perhaps impacting their properties as cloud condensation and ice nuclei. However, the influence of background particles on smoke-driven indirect effects is currently unclear.

Description: Delivery of anthropogenic bioavailable iron from mineral dust and combustion aerosols to the ocean Atmospheric Chemistry and Physics Discussions, 15, 23051-23088, 2015 Author(s): A. Ito and Z. Shi Atmospheric deposition of anthropogenic soluble iron (Fe) to the ocean has been suggested to modulate primary ocean productivity and thus indirectly affect the climate. A key process contributing to anthropogenic sources of soluble Fe is associated with air pollution, which acidifies Fe-containing mineral aerosols during their transport and leads to Fe transformation from insoluble to soluble forms. However, there is large uncertainty in our estimate of this anthropogenic soluble Fe. Here, we, for the first time, interactively combined laboratory kinetic experiments with global aerosol modeling to more accurately quantify anthropogenic soluble Fe due to air pollution. We firstly examined Fe dissolution kinetics of African dust samples at acidic pH values with and without ionic species commonly found in aerosol water (i.e., sulfate and oxalate). We then constructed a new empirical scheme for Fe release from mineral dust due to inorganic and organic anions in aerosol water, by using acidity as a master variable. We implemented this new scheme and applied an updated mineralogical emission database in a global atmospheric chemistry transport model to estimate the atmospheric concentration and deposition flux of soluble Fe under preindustrial and modern conditions. Our improved model successfully captured the inverse relationship of Fe solubility and total Fe loading measured over the North Atlantic Ocean (i.e., 1–2 orders of magnitude lower Fe solubility in North African- than combustion-influenced aerosols). The model results show a positive relationship between Fe solubility and water soluble organic carbon (WSOC)/Fe molar ratio, which is consistent with previous field measurements. We estimated that deposition of soluble Fe to the ocean increased from 0.05–0.07 Tg Fe yr −1 in preindustrial era to 0.11–0.12 Tg Fe yr −1 in present days, due to air pollution. Over the High Nitrate Low Chlorophyll (HNLC) regions of the ocean, the modeled Fe solubility remains low for mineral dust ( 〈 1 %) in a base simulation but is substantially enhanced in a sensitivity simulation, which permits the Fe dissolution for mineral aerosols in the presence of excess oxalate under low acidity during daytime. Our model results suggest that human activities contribute to about half of the soluble Fe supply to a significant portion of the oceans in the Northern Hemisphere, while their contribution to oceans in high latitudes remains uncertain due to limited understanding of dust Fe dissolution under pristine conditions.

Description: The impact of atmospheric mineral aerosol deposition on the albedo of snow and sea ice: are snow and sea ice optical properties more important than mineral aerosol optical properties? Atmospheric Chemistry and Physics Discussions, 15, 23131-23172, 2015 Author(s): M. L. Lamare, J. Lee-Taylor, and M. D. King Knowledge of the albedo of polar regions is crucial for understanding a range of climatic processes that have an impact on a global scale. Light absorbing impurities in atmospheric aerosols deposited on snow and sea ice by aeolian transport absorb solar radiation, reducing albedo. Here, the effects of five mineral aerosol deposits reducing the albedo of polar snow and sea ice are considered. Calculations employing a coupled atmospheric and snow/sea ice radiative-transfer model (TUV-snow) show that the effects of mineral aerosol deposits is strongly dependent on the snow or sea ice type rather than the differences between the aerosol optical characteristics. The change in albedo between five different mineral aerosol deposits with refractive indices varying by a factor of 2 reaches a maximum of 0.0788, whereas the difference between cold polar snow and melting sea ice is 0.8893 for the same mineral loading. Surprisingly, the thickness of a surface layer of snow or sea ice loaded with the same mass-ratio of mineral dust has little effect on albedo. On the contrary, multiple layers of mineral aerosols deposited during episodic events evenly distributed play a similar role in the surface albedo of snow as a loading distributed throughout, even when the layers are further apart. The impact of mineral aerosol deposits is much larger on melting sea ice than on other types of snow and sea ice. Therefore, the higher input of shortwave radiation during the summer melt cycle associated with melting sea ice accelerates the melt process.

Description: Limited effect of anthropogenic nitrogen oxides on Secondary Organic Aerosol formation Atmospheric Chemistry and Physics Discussions, 15, 23231-23277, 2015 Author(s): Y. Zheng, N. Unger, A. Hodzic, L. Emmons, C. Knote, S. Tilmes, J.-F. Lamarque, and P. Yu Globally, secondary organic aerosol (SOA) is mostly formed from emissions of biogenic volatile organic compounds (VOCs) by vegetation, but can be modified by human activities as demonstrated in recent research. Specifically, nitrogen oxides (NO x = NO + NO 2 ) have been shown to play a critical role in the chemical formation of low volatility compounds. We have updated the SOA scheme in the global NCAR Community Atmospheric Model version 4 with chemistry (CAM4-chem) by implementing a 4-product Volatility Basis Set (VBS) scheme, including NO x -dependent SOA yields and aging parameterizations. The predicted organic aerosol amounts capture both the magnitude and distribution of US surface annual mean measurements from the Interagency Monitoring of Protected Visual Environments (IMPROVE) network by 50 %, and the simulated vertical profiles are within a factor of two compared to Aerosol Mass Spectrometer (AMS) measurements from 13 aircraft-based field campaigns across different region and seasons. We then perform sensitivity experiments to examine how the SOA loading responds to a 50 % reduction in anthropogenic nitric oxide (NO) emissions in different regions. We find limited SOA reductions of 0.9 to 5.6, 6.4 to 12.0 and 0.9 to 2.8 % for global, the southeast US and the Amazon NO x perturbations, respectively. The fact that SOA formation is almost unaffected by changes in NO x can be largely attributed to buffering in chemical pathways (low- and high-NO x pathways, O 3 versus NO 3 -initiated oxidation) and to offsetting tendencies in the biogenic versus anthropogenic SOA responses.

Description: Wintertime aerosol chemical composition, volatility, and spatial variability in the greater London area Atmospheric Chemistry and Physics Discussions, 15, 23173-23229, 2015 Author(s): L. Xu, L. R. Williams, D. E. Young, J. D. Allan, H. Coe, P. Massoli, E. Fortner, P. Chhabra, S. Herndon, W. A. Brooks, J. T. Jayne, D. R. Worsnop, A. C. Aiken, S. Liu, K. Gorkowski, M. K. Dubey, Z. L. Fleming, S. Visser, A. S. H. Prevot, and N. L. Ng The composition of PM 1 (particulate matter with diameter less than 1 μm) in the greater London area was characterized during the Clean Air for London (ClearfLo) project in winter 2012. Two High-Resolution Time-of-Flight Aerosol Mass Spectrometers (HR-ToF-AMS) were deployed at a rural site (Detling, Kent) and an urban site (North Kensington, London). The simultaneous and high-temporal resolution measurements at the two sites provide a unique opportunity to investigate the spatial distribution of PM 1 . We find that the organic aerosol (OA) concentration is comparable between the rural and urban sites, but the sources of OA are distinctly different. The concentration of solid fuel OA at the urban site is about twice as high as at the rural site, due to elevated domestic heating in the urban area. While the concentrations of oxygenated OA (OOA) are well-correlated between the two sites, the OOA concentration at the rural site is almost twice that of the urban site. At the rural site, more than 70 % of the carbon in OOA is estimated to be non-fossil, which suggests that OOA is likely related to aged biomass burning considering the small amount of biogenic SOA in winter. Thus, it is possible that the biomass burning OA contributes a larger fraction of ambient OA in wintertime than what previous field studies have suggested. A suite of instruments was deployed downstream of a thermal denuder (TD) to investigate the volatility of PM 1 species at the rural Detling site. After heating at 250 °C in the TD, 40 % of the residual mass is OA, indicating the presence of non-volatile organics in the aerosol. Although the OA associated with refractory black carbon (rBC, measured by a soot-particle aerosol mass spectrometer) only accounts for 〈 10 % of the total OA (measured by a HR-ToF-AMS) at 250 °C, the two measurements are well-correlated, suggesting that the non-volatile organics have similar sources or have undergone similar chemical processing as rBC in the atmosphere. Finally, we discuss the relationship between the OA volatility and atomic O : C and find that particles with a wide range of O : C could have similar mass fraction remaining after heating. This analysis emphasizes the importance of understanding the distribution of volatility and O : C in bulk OA.

Description: Aerosol composition and variability in the Baltimore–Washington, DC region Atmospheric Chemistry and Physics Discussions, 15, 23317-23355, 2015 Author(s): A. J. Beyersdorf, L. D. Ziemba, G. Chen, C. A. Corr, J. H. Crawford, G. S. Diskin, R. H. Moore, K. L. Thornhill, E. L. Winstead, and B. E. Anderson In order to utilize satellite-based aerosol measurements for the determination of air quality, the relationship between aerosol optical properties (wavelength-dependent, column-integrated extinction measured by satellites) and mass measurements of aerosol loading (PM 2.5 used for air quality monitoring) must be understood. This connection varies with many factors including those specific to the aerosol type, such as composition, size and hygroscopicity, and to the surrounding atmosphere, such as temperature, relative humidity (RH) and altitude, all of which can vary spatially and temporally. During the DISCOVER-AQ (Deriving Information on Surface conditions from Column and Vertically Resolved Observations Relevant to Air Quality) project, extensive in-situ atmospheric profiling in the Baltimore, MD–Washington, DC region was performed during fourteen flights in July 2011. Identical flight plans and profile locations throughout the project provide meaningful statistics for determining the variability in and correlations between aerosol loading, composition, optical properties and meteorological conditions. Measured water-soluble aerosol mass was composed primarily of ammonium sulfate (campaign average of 32 %) and organics (57 %). A distinct difference in composition was observed with high-loading days having a proportionally larger percentage of ammonium sulfate (up to 49 %) due to transport from the Ohio River Valley. This composition shift caused a change in the aerosol water-uptake potential (hygroscopicity) such that higher relative contributions of ammonium sulfate increased the bulk aerosol hygroscopicity. These days also tended to have higher relative humidity causing an increase in the water content of the aerosol. Conversely, low aerosol loading days had lower ammonium sulfate and higher black carbon contributions causing lower single scattering albedos (SSAs). The average black carbon concentrations were 240 ng m −3 in the lowest 1 km decreasing to 35 ng m −3 in the free troposphere (above 3 km). Routine airborne sampling over six locations was used to evaluate the relative contributions of aerosol loading, composition, and relative humidity (the amount of water available for uptake onto aerosols) to variability in mixed layer aerosol. Aerosol loading was found to be the predominant source accounting for 88 % on average of the measured spatial variability in extinction with lesser contributions from variability in relative humidity (10 %) and aerosol composition (1.3 %). On average, changes in aerosol loading also caused 82 % of the diurnal variability in ambient aerosol extinction. However on days with relative humidity above 60 %, variability in RH was found to cause up to 62 % of the spatial variability and 95 % of the diurnal variability in ambient extinction. This work shows that extinction is driven to first-order by aerosol mass loadings; however, humidity-driven hydration effects play an important secondary role. This motivates combined satellite/modelling assimilation products that are able to capture these components of the AOD-PM 2.5 link. Conversely, aerosol hygroscopicity and SSA play a minor role in driving variations both spatially and throughout the day in aerosol extinction and therefore AOD. However, changes in aerosol hygroscopicity from day-to-day were large and could cause a bias of up to 27 % if not accounted for. Thus it appears that a single daily measurement of aerosol hygroscopicity can be used for AOD-to-PM 2.5 conversions over the study region (on the order of 1400 km 2 ). This is complimentary to the results of Chu et al. (2015) that determined the aerosol vertical distribution from "a single lidar is feasible to cover the range of 100 km" in the same region.

Description: Modeling study on the transport of summer dust and anthropogenic aerosols over the Tibetan Plateau Atmospheric Chemistry and Physics Discussions, 15, 15005-15037, 2015 Author(s): Y. Liu, Y. Sato, R. Jia, Y. Xie, J. Huang, and T. Nakajima The Tibetan Plateau (TP) is located at the juncture of several important natural and anthropogenic aerosol sources. Satellites have observed substantial dust and anthropogenic aerosols in the atmosphere during summer over the TP. These aerosols have distinct effects on the earth's energy balance, microphysical cloud properties, and precipitation rates. To investigate the transport of summer dust and anthropogenic aerosols over the TP, we combined the Spectral Radiation-Transport Model for Aerosol Species (SPRINTARS) with a non-hydrostatic regional model (NHM). The model simulation shows heavily loaded dust aerosols over the northern slope and anthropogenic aerosols over the southern slope and to the east of the TP. The dust aerosols are primarily mobilized around the Taklimakan Desert, where a portion of the aerosols are transported eastward due to the northwesterly current; simultaneously, a portion of the particles are transported northward when a second northwesterly current becomes northeasterly because of the topographic blocking of the northern slope of the TP. Because of the strong upward current, dust plumes can extend upward to approximately 7–8 km a.s.l. over the northern slope of the TP. When a dust event occurs, anthropogenic aerosols that entrain into the southwesterly current via the Indian summer monsoon are transported from India to the southern slope of the TP. Simultaneously, a large amount of anthropogenic aerosols is also transported from eastern China to east of the TP by easterly winds. An investigation on the transport of dust and anthropogenic aerosols over the plateau may provide the basis for determining aerosol impacts on summer monsoons and climate systems.

Description: A synthesis of cloud condensation nuclei counter (CCNC) measurements within the EUCAARI network Atmospheric Chemistry and Physics Discussions, 15, 15039-15086, 2015 Author(s): M. Paramonov, V.-M. Kerminen, M. Gysel, P. P. Aalto, M. O. Andreae, E. Asmi, U. Baltensperger, A. Bougiatioti, D. Brus, G. Frank, N. Good, S. S. Gunthe, L. Hao, M. Irwin, A. Jaatinen, Z. Jurányi, S. M. King, A. Kortelainen, A. Kristensson, H. Lihavainen, M. Kulmala, U. Lohmann, S. T. Martin, G. McFiggans, N. Mihalopoulos, A. Nenes, C. D. O'Dowd, J. Ovadnevaite, T. Petäjä, U. Pöschl, G. C. Roberts, D. Rose, B. Svenningsson, E. Swietlicki, E. Weingartner, J. Whitehead, A. Wiedensohler, C. Wittbom, and B. Sierau Cloud Condensation Nuclei Counter (CCNC) measurements performed at 14 locations around the world within the EUCAARI framework have been analysed and discussed with respect to the cloud condensation nuclei (CCN) activation and hygroscopic properties of the atmospheric aerosol. The annual mean ratio of activated cloud condensation nuclei ( N CCN ) to the total number concentration of particles ( N CN ), known as the activated fraction A , shows a similar functional dependence on supersaturation S at many locations; exceptions to this being certain marine locations, a free troposphere site and background sites in south-west Germany and northern Finland. The use of total number concentration of particles above 50 and 100 nm diameter when calculating the activated fractions ( A 50 and A 100 , respectively) renders a much more stable dependence of A on S ; A 50 and A 100 also reveal the effect of the size distribution on CCN activation. With respect to chemical composition, it was found that the hygroscopicity of aerosol particles as a function of size differs among locations. The hygroscopicity parameter κ decreased with an increasing size at a continental site in south-west Germany and fluctuated without any particular size dependence across the observed size range in the remote tropical North Atlantic and rural central Hungary. At all other locations κ increased with size. In fact, in Hyytiälä, Vavihill, Jungfraujoch and Pallas the difference in hygroscopicity between Aitken and accumulation mode aerosol was statistically significant at the 5% significance level. In a boreal environment the assumption of a size-independent κ can lead to a potentially substantial overestimation of N CCN at S levels above 0.6%; similar is true for other locations where κ was found to increase with size. While detailed information about aerosol hygroscopicity can significantly improve the prediction of N CCN , total aerosol number concentration and aerosol size distribution remain more important parameters. The seasonal and diurnal patterns of CCN activation and hygroscopic properties vary among three long-term locations, highlighting the spatial and temporal variability of potential aerosol-cloud interactions in various environments.

Description: Transport pathways of peroxyacetyl nitrate in the upper troposphere and lower stratosphere from different monsoon systems during the summer monsoon season Atmospheric Chemistry and Physics Discussions, 15, 15087-15135, 2015 Author(s): S. Fadnavis, K. Semeniuk, M. G. Schultz, M. Kiefer, A. Mahajan, L. Pozzoli, and S. Sonbawane The Asian summer monsoon involves complex transport patterns with large scale redistribution of trace gases in the upper troposphere and lower stratosphere (UTLS). We employ the global chemistry-climate model ECHAM5-HAMMOZ in order to evaluate the transport pathways and the contributions of nitrogen oxide species PAN, NO x , and HNO 3 from various monsoon regions, to the UTLS over Southern Asia and vice versa. Simulated long term seasonal mean mixing ratios are compared with trace gas retrievals from the Michelson Interferometer for Passive Atmospheric Sounding aboard ENVISAT(MIPAS-E) and aircraft campaigns during the monsoon season (June–September) in order to evaluate the model's ability to reproduce these transport patterns. The model simulations show that there are three regions which contribute substantial pollution to the South Asian UTLS: the Asian summer monsoon (ASM), the North American Monsoon (NAM) and the West African monsoon (WAM). However, penetration due to ASM convection reaches deeper into the UTLS as compared to NAM and WAM outflow. The circulation in all three monsoon regions distributes PAN into the tropical latitude belt in the upper troposphere. Remote transport also occurs in the extratropical upper troposphere where westerly winds drive North American and European pollutants eastward where they can become part of the ASM convection and be lifted into the lower stratosphere. In the lower stratosphere the injected pollutants are transported westward by easterly winds. The intense convective activity in the monsoon regions is associated with lightning and thereby the formation of additional NO x . This also affects the distribution of PAN in the UTLS. According to sensitivity simulations with and without lightning, increase in concentrations of PAN (~ 40%), HNO 3 (75%), NO x (70%) and ozone (30%) over the regions of convective transport, especially over equatorial Africa and America and comparatively less over the ASM. This indicates that PAN in the UTLS over the ASM region is primarily of anthropogenic origin.

Description: Sources of long-lived atmospheric VOCs at the rural boreal forest site, SMEAR II Atmospheric Chemistry and Physics Discussions, 15, 14593-14641, 2015 Author(s): J. Patokoski, T. M. Ruuskanen, M. K. Kajos, R. Taipale, P. Rantala, J. Aalto, T. Ryyppö, T. Nieminen, H. Hakola, and J. Rinne In this study a long-term volatile organic compounds (VOCs) data set, measured at the SMEAR II (Station for measuring Ecosystem–Atmosphere Relations) boreal forest site at Hyytiälä, Finland during the years 2006–2011, was investigated. VOC mixing ratios were measured using proton transfer reaction mass spectrometry. Four-day backward trajectories and the Unmix 6.0 receptor model were used for source area and source composition analysis. Two major forest fire events, one in Eastern Europe and one in Russia, took place during the measurement period. The effect of these fires was clearly visible in the trajectory analysis, lending confidence to the method employed with this data set. Elevated volume mixing ratios (VMRs) of non-biogenic VOCs, e.g. acetonitrile and aromatic VOCs, related to forest fires were observed. Ten major source areas for long-lived VOCs (methanol, acetonitrile, acetaldehyde, acetone, benzene and toluene) were identified at the SMEAR II site. The main source areas for all the targeted VOCs were Western Russia, Northern Poland, Kaliningrad and Baltic countries. Industrial areas in Northern Continental Europe were also found to be source areas for certain VOCs. Both trajectory and receptor analysis showed that air masses from Northern Fennoscandia were less polluted with both the VOCs studied and with other trace gases (CO, SO 2 and NO x ) than areas of Eastern and Western Continental Europe, Western Russia and Southern Fennoscandia.

Description: Oceanic bromine emissions weighted by their ozone depletion potential Atmospheric Chemistry and Physics Discussions, 15, 14643-14684, 2015 Author(s): S. Tegtmeier, F. Ziska, I. Pisso, B. Quack, G. J. M. Velders, X. Yang, and K. Krüger At present, anthropogenic halogens and oceanic emissions of Very Short-Lived Substances (VSLS) are responsible for stratospheric ozone destruction. Emissions of the, mostly long-lived, anthropogenic halogens have been reduced, and as a consequence, their atmospheric abundance has started to decline since the beginning of the 21st century. Emissions of VSLS are, on the other hand, expected to increase in the future. VSLS are known to have large natural sources; however increasing evidence arises that their oceanic production and emission is enhanced by anthropogenic activities. Here, we introduce a new approach of assessing the overall impact of all oceanic halogen emissions on stratospheric ozone by calculating Ozone Depletion Potential (ODP)-weighted emissions of VSLS. Seasonally and spatially dependent, global distributions are derived exemplary for CHBr 3 for the period 1999–2006. At present, ODP-weighted emissions of CHBr 3 amount up to 50% of ODP-weighted anthropogenic emissions of CFC-11 and to 9% of all long-lived ozone depleting substances. The ODP-weighted emissions are large where strong oceanic emissions coincide with high-reaching convective activity and show pronounced peaks at the equator and the coasts with largest contributions from the Maritime Continent and West Pacific. Variations of tropical convective activity lead to seasonal shifts in the spatial distribution of the ODP with the updraught mass flux explaining 71% of the variance of the ODP distribution. Future climate projections based on RCP8.5 scenario suggest a 31% increase of the ODP-weighted CHBr 3 emissions until 2100 compared to present values. This increase is related to larger convective activity and increasing emissions in a future climate; however, is reduced at the same time by less effective bromine-related ozone depletion. The comparison of the ODP-weighted emissions of short and long-lived halocarbons provides a new concept for assessing the overall impact of oceanic bromine emissions on stratospheric ozone depletion for current conditions and future projections.

Description: Global HCFC-22 measurements with MIPAS: retrieval, validation, climatologies and trends Atmospheric Chemistry and Physics Discussions, 15, 14783-14841, 2015 Author(s): M. Chirkov, G. P. Stiller, A. Laeng, S. Kellmann, T. von Clarmann, C. Boone, J. W. Elkins, A. Engel, N. Glatthor, U. Grabowski, C. M. Harth, M. Kiefer, F. Kolonjari, P. B. Krummel, A. Linden, C. R. Lunder, B. R. Miller, S. A. Montzka, J. Mühle, S. O'Doherty, J. Orphal, R. G. Prinn, G. Toon, M. K. Vollmer, K. A. Walker, R. F. Weiss, A. Wiegele, and D. Young We report on HCFC-22 data acquired by the Michelson Interferometer for Passive Atmospheric Sounding (MIPAS) in reduced spectral resolution nominal mode in the period from January 2005 to April 2012 from version 5.02 level-1b spectral data and covering an altitude range from the upper troposphere (above cloud top altitude) to about 50 km. The profile retrieval was performed by constrained nonlinear least squares fitting of measured limb spectral radiances to modelled spectra. The spectral ν 4 -band at 816.5 ± 13 cm −1 was used for the retrieval. A Tikhonov-type smoothing constraint was applied to stabilise the retrieval. In the lower stratosphere, we find a global volume mixing ratio of HCFC-22 of about 185 pptv in January 2005. The linear growth rate in the lower latitudes lower stratosphere was about 6 to 7 pptv yr −1 in the period 2005–2012. The obtained profiles were compared with ACE-FTS satellite data v3.5, as well as with MkIV balloon profiles and in situ cryosampler balloon measurements. Between 13 and 22 km, average agreement within −3 to +5 pptv (MIPAS–ACE) with ACE-FTS v3.5 profiles is demonstrated. Agreement with MkIV solar occultation balloon-borne measurements is within 10–20 pptv below 30 km and worse above, while in situ cryosampler balloon measurements are systematically lower over their full altitude range by 15–50 pptv below 24 km and less than 10 pptv above 28 km. Obtained MIPAS HCFC-22 time series below 10 km altitude are shown to agree mostly well to corresponding time series of near-surface abundances from NOAA/ESRL and AGAGE networks, although a more pronounced seasonal cycle is obvious in the satellite data, probably due to tropopause altitude fluctuations and subsidence of polar winter stratospheric air into the troposphere. A parametric model consisting of constant, linear, quasi-biennial oscillation (QBO) and several sine and cosine terms with different periods has been fitted to the temporal variation of stratospheric HCFC-22 for all 10° latitude/1 to 2 km altitude bins. The relative linear variation was always positive, with relative increases of 40–70% decade −1 in the tropics and global lower stratosphere, and up to 120% decade −1 in the upper stratosphere of the northern polar region and the southern extratropical hemisphere. In the middle stratosphere between 20 and 30 km, the observed trend is not consistent with the age of stratospheric air-corrected trend at ground, but stronger positive at the Southern Hemisphere and less strong increasing in the Northern Hemisphere, hinting towards changes in the stratospheric circulation over the observation period.

Description: Improving aerosol interaction with clouds and precipitation in a regional chemical weather modeling system Atmospheric Chemistry and Physics Discussions, 15, 15755-15790, 2015 Author(s): C. Zhou, X. Zhang, S. Gong, Y. Wang, and M. Xue A comprehensive aerosol–cloud–precipitation interaction (ACI) scheme has been developed under CMA chemical weather modeling system GRAPES/CUACE. Calculated by a sectional aerosol activation scheme based on the information of size and mass from CUACE and the thermal-dynamic and humid states from the weather model GRAPES at each time step, the cloud condensation nuclei (CCN) is fed online interactively into a two-moment cloud scheme (WDM6) and a convective parameterization to drive the cloud physics and precipitation formation processes. The modeling system has been applied to study the ACI for January 2013 when several persistent haze-fog events and eight precipitation events occurred. The results show that interactive aerosols with the WDM6 in GRAPES/CUACE obviously increase the total cloud water, liquid water content and cloud droplet number concentrations while decrease the mean diameter of cloud droplets with varying magnitudes of the changes in each case and region. These interactive micro-physical properties of clouds improve the calculation of their collection growth rates in some regions and hence the precipitation rate and distributions in the model, showing 24 to 48% enhancements of TS scoring for 6 h precipitation in almost all regions. The interactive aerosols with the WDM6 also reduce the regional mean bias of temperature by 3 °C during certain precipitation events, but the monthly means bias is only reduced by about 0.3 °C.

Description: Investigation of processes controlling summertime gaseous elemental mercury oxidation at midlatitudinal marine, coastal, and inland sites Zhuyun Ye, Huiting Mao, Che-Jen Lin, and Su Youn Kim Atmos. Chem. Phys., 16, 8461-8478, doi:10.5194/acp-16-8461-2016, 2016 In this study, a state-of-the-art chemical mechanism was incorporated into a box model to investigate the atmospheric Hg cycling in different environments. As a result, for each of the three environments, GOM diurnal cycles of over half the selected cases were reasonably represented by the box model. A realistic model can be a powerful tool, providing important information on atmospheric Hg cycling and implications for policy makers.