ALBERT

Beschreibung: Mesoscale modeling study of the interactions between aerosols and PBL meteorology during a haze episode in China Jing–Jin–Ji and its near surrounding region – Part 1: Aerosol distributions and meteorological features Atmospheric Chemistry and Physics Discussions, 14, 31675-31717, 2014 Author(s): H. Wang, M. Xue, X. Y. Zhang, H. L. Liu, C. H. Zhou, S. C. Tan, H. Z. Che, B. Chen, and T. Li The urbanized region of Beijing–Tianjin–Hebei – often shortened to Jing–Jin–Ji and referred to as the 3JNS region in this paper – and its near surrounding region is becoming China's most polluted area by haze, exceeding even the Yangtze and Pearl river deltas. Aside from pollutant emission, the meteorology of the planetary boundary layer (PBL) is the most important factor affecting haze pollution. Focusing on July 2008, the aerosol optical properties and PBL meteorology features closely related with haze formation were simulated in 3JNS region using an online atmospheric chemical transport model. The relationship between regional PBL meteorology, PM 2.5 , and haze is discussed. Model results accurately simulated the aerosol optical depth (AOD), single scattering albedo (SSA) and asymmetry parameter (ASY), validate by comparison with observations from the MODerate Resolution Imaging Spectroradiometer (MODIS), the China Aerosol Remote Sensing NETwork (CARSNET) and the Aerosol Robotic NETwork (AERONET). Modeled PBL wind speeds showed reasonable agreement with those from the National Centers for Environmental Prediction (NCEP) Reanalysis 2. A monthly mean AOD value as high as 1.2 was found from both model and observations, with a daily mean larger than 2.0 during haze episodes in the 3JNS Region. Modeled and observed SSA values of 0.9–0.96 and ASY values of 0.72–0.74 demonstrated the high scattering characteristic of summer aerosols in this region. PBL wind speeds from modeled and NCEP data both showed a reversing trend of PM 2.5 variation, illustrating the importance of the "PBL window shadow" on haze formation. Turbulence diffusion and PBL height showed had opposite phases to surface PM 2.5 , indicating that lower PBL height and weaker PBL turbulence diffusion are essential to haze formation. It is noted that homogeneous air pressure does not occur at the surface but at an 85–950 hPa height during the haze episode. The momentum transmitting downward of the cold air from above the PBL to the low PBL and surface lead to an increase in surface wind speeds and haze dispersal.

Beschreibung: Long term measurements of optical properties and their hygroscopic enhancement Atmospheric Chemistry and Physics Discussions, 14, 27731-27767, 2014 Author(s): M. Hervo, K. Sellegri, J. M. Pichon, J. C. Roger, and P. Laj Optical properties of aerosols were measured from the GAW Puy de Dôme station (1465 m) over a seven year period (2006–2012). The impact of hygroscopicity on aerosol optical properties was calculated over a two year period (2010–2011). The analysis of the spatial and temporal variability of the optical properties showed that while no long term trend was found, a clear seasonal and diurnal variation was observed on the extensive parameters (scattering, absorption). Scattering and absorption coefficients were highest during the warm season and daytime, in concordance with the seasonality and diurnal variation of the PBL height reaching the site. Intensive parameters (single scattering albedo, asymmetry factor, refractive index) did not show such a strong diurnal variability, but still indicated different values depending on the season. Both extensive and intensive optical parameters were sensitive to the air mass origin. A strong impact of hygroscopicity on aerosol optical properties was calculated, mainly on aerosol scattering, with a dependence on the aerosol type. At 90% humidity, the scattering factor enhancement ( f σ sca ) was more than 4.4 for oceanic aerosol that have mixed with a pollution plume. Consequently, the aerosol radiative forcing was estimated to be 2.8 times higher at RH = 90% and 1.75 times higher at ambient RH when hygroscopic growth of the aerosol was considered. The hygroscopicity enhancement factor of the scattering coefficient was parameterized as a function of humidity and air mass type.

Beschreibung: A global model simulation for 3-D radiative transfer impact on surface hydrology over Sierra Nevada and Rocky Mountains Atmospheric Chemistry and Physics Discussions, 14, 31603-31625, 2014 Author(s): W.-L. Lee, Y. Gu, K. N. Liou, L. R. Leung, and H.-H. Hsu We investigate 3-D mountain effects on solar flux distributions and their impact on surface hydrology over the Western United States, specifically the Rocky Mountains and Sierra Nevada using CCSM4 (CAM4/CLM4) global model with a 0.23° × 0.31° resolution for simulations over 6 years. In 3-D radiative transfer parameterization, we have updated surface topography data from a resolution of 1 km to 90 m to improve parameterization accuracy. In addition, we have also modified the upward-flux deviation [3-D − PP (plane-parallel)] adjustment to ensure that energy balance at the surface is conserved in global climate simulations based on 3-D radiation parameterization. We show that deviations of the net surface fluxes are not only affected by 3-D mountains, but also influenced by feedbacks of cloud and snow in association with the long-term simulations. Deviations in sensible heat and surface temperature generally follow the patterns of net surface solar flux. The monthly snow water equivalent (SWE) deviations show an increase in lower elevations due to reduced snowmelt, leading to a reduction in cumulative runoff. Over higher elevation areas, negative SWE deviations are found because of increased solar radiation available at the surface. Simulated precipitation increases for lower elevations, while decreases for higher elevations with a minimum in April. Liquid runoff significantly decreases in higher elevations after April due to reduced SWE and precipitation.

Beschreibung: Spatial and temporal variation of CO over Alberta using measurements from satellite, aircrafts, and ground stations Atmospheric Chemistry and Physics Discussions, 14, 31767-31812, 2014 Author(s): H. S. Marey, Z. Hashisho, L. Fu, and J. Gille Alberta is Canada's largest oil producer and its oil sand deposits comprise 30% of the world's oil reserves. The process of bitumen extraction and upgrading releases trace gases and aerosols to the atmosphere. In this study we present satellite-based analysis to explore, for the first time, various contributing factors that affect tropospheric carbon monoxide (CO) levels over Alberta. The multispectral product that uses both near-infrared (NIR) and the thermal-infrared (TIR) radiances for CO retrieval from the Measurements of Pollution in the Troposphere (MOPITT) are examined for the 12 year period from 2002–2013. Moderate Resolution Imaging Spectroradiometer (MODIS) thermal anomaly product from 2001 to 2013 is employed to investigate the seasonal and temporal variations of forest fires. Additionally, in situ CO measurements at industrial and urban sites are compared to satellite data. Furthermore, the available MOZAIC/IAGOS (Measurement of Ozone, Water Vapor, Carbon Monoxide, Nitrogen Oxide by Airbus In-Service Aircraft/In service Aircraft for Global Observing System) aircraft CO profiles (April 2009–December 2011) are used to validate MOPITT CO data. The climatological time curtain plot and spatial maps for CO over northern Alberta indicate the signatures of transported CO for two distinct biomass burning seasons, summer and spring. Distinct seasonal patterns of CO at the urban site s (Edmonton and Calgary cities) point to the strong influence of traffic. Meteorological parameters play an important role on the CO spatial distribution at various pressure levels. Northern Alberta shows stronger upward lifting motion which leads to larger CO total column values while the poor dispersion in central and south Alberta exacerbate s the surface CO pollution. Inter-annual variations of satellite data depict a slightly decreasing trend for both regions while the decline trend is more evident from ground observations, especially at the urban sites. MOPITT CO vertical averages and MOZAIC/IAGOS aircraft profiles were in good agreement within the standard deviation at all pressure levels. There is consistency between the time evolution of high CO episodes monitored by satellite and ground measurements and the fire frequency peak time which implies that biomass burning has affected the tropospheric CO distribution in northern Alberta. These findings have further demonstrated the potential use of MOPITT V5 multispectral (NIR+TIR) product for assessing a complicated surface process.

Beschreibung: Particulate emissions from residential wood combustion in Europe – revised estimates and an evaluation Atmospheric Chemistry and Physics Discussions, 14, 31719-31765, 2014 Author(s): H. A. C. Denier van der Gon, R. Bergström, C. Fountoukis, C. Johansson, S. N. Pandis, D. Simpson, and A. Visschedijk Currently residential wood combustion (RWC) is increasing in Europe because of rising fossil fuel prices but also due to climate change mitigation policies. However, especially in small-scale applications, RWC may cause high emissions of particulate matter (PM). Recently we have developed a new high-resolution (7 km × 7 km) anthropogenic carbonaceous aerosol emission inventory for Europe. The inventory indicated that about half of the total PM 2.5 emission in Europe is carbonaceous aerosol and identified RWC as the largest organic aerosol (OA) source in Europe. The inventory was partly based on national reported PM emissions. Use of this OA inventory as input for two Chemical Transport Models (CTMs), PMCAMx and EMEP MSC-W, revealed major underestimations of OA in winter time, especially for regions dominated by RWC. Interestingly, this was not universal but appeared to differ by country. In the present study we constructed a new bottom-up emission inventory for RWC accounting for the semi-volatile components of the emissions. The new RWC emissions are higher than those in the previous inventory by a factor of 2–3 but with substantial inter-country variation. The new emission inventory served as input for the CTMs and a substantially improved agreement between measured and predicted organic aerosol was found. The new RWC inventory improves the model calculated OA significantly. Comparisons to Scandinavian source apportionment studies also indicate substantial improvements in the modeled wood-burning component of OA. This suggests that primary organic aerosol emission inventories need to be revised to include the semi-volatile OA that is formed almost instantaneously due to cooling of the flue gas or exhaust. Since RWC is a key source of fine PM in Europe, a major revision of the emission estimates as proposed here is likely to influence source-receptor matrices and modelled source apportionment. Since usage of biofuels, such as wood, in small combustion units is a globally significant source, this insight may also dramatically change global estimates of organic aerosol emissions.

Beschreibung: Impact of planetary boundary layer turbulence on model climate and tracer transport Atmospheric Chemistry and Physics Discussions, 14, 31627-31674, 2014 Author(s): E. L. McGrath-Spangler, A. Molod, L. E. Ott, and S. Pawson Planetary boundary layer (PBL) processes are important for weather, climate, and tracer transport and concentration. One measure of the strength of these processes is the PBL depth. However, no single PBL depth definition exists and several studies have found that the estimated depth can vary substantially based on the definition used. In the Goddard Earth Observing System (GEOS-5) atmospheric general circulation model, the PBL depth is particularly important because it is used to calculate the turbulent length scale that is used in the estimation of turbulent mixing. This study analyzes the impact of using three different PBL depth definitions in this calculation. Two definitions are based on the scalar eddy diffusion coefficient and the third is based on the bulk Richardson number. Over land, the bulk Richardson number definition estimates shallower nocturnal PBLs than the other estimates while over water this definition generally produces deeper PBLs. The near surface wind velocity, temperature, and specific humidity responses to the change in turbulence are spatially and temporally heterogeneous, resulting in changes to tracer transport and concentrations. Near surface wind speed increases in the bulk Richardson number experiment cause Saharan dust increases on the order of 1 × 10 −4 kg m −2 downwind over the Atlantic Ocean. Carbon monoxide (CO) surface concentrations are modified over Africa during boreal summer, producing differences on the order of 20 ppb, due to the model's treatment of emissions from biomass burning. While differences in carbon dioxide (CO 2 ) are small in the time mean, instantaneous differences are on the order of 10 ppm and these are especially prevalent at high latitude during boreal winter. Understanding the sensitivity of trace gas and aerosol concentration estimates to PBL depth is important for studies seeking to calculate surface fluxes based on near-surface concentrations and to studies projecting future concentrations.

Beschreibung: Examining the major contributors and controlling factors of ozone production in a rural area of the Yangtze River Delta region during harvest season Atmospheric Chemistry and Physics Discussions, 14, 30913-30945, 2014 Author(s): X. Pan, Y. Kanaya, H. Tanimoto, S. Inomata, Z. Wang, S. Kudo, and I. Uno Open biomass burning (OBB) has been reported to emit substantial amounts of non-methane hydrocarbons (NMHCs), and the mixing of OBB with urban plumes could exacerbate regional ozone (O 3 ) pollution. In the present study, an observational field campaign was performed in a rural area at the edge of Yangtze River Delta region (YRDR) during harvest season when intensive open burning of wheat residues was observed. The O 3 production rate at the site was calculated using a photochemical box model (Regional Atmospheric Chemical Mechanism, Version 2) constrained by real-time ambient measurements (e.g., O 3 , volatile organic compounds (VOCs), the sum of NO 2 + NO (NO x ), J values). During the period impacted by OBB, the O 3 concentration frequently exceeded 100 ppbv. Analysis showed that the net O 3 production was pronounced, in particular when the site was characterized by a prevailing southerly wind that also brought substantial amounts of NO x emitted from urban areas. At these times, the maximum rate of O 3 production was 20 ppbv h −1 with potential production rate of 102 ppbv on a daily basis. The O 3 production at the site was typically VOC-sensitive in the morning because NO x dominated the plumes. However, in the afternoon, conditions became NO x -sensitive due to the rapid photochemical consumption of NO x in the production of O 3 . A positive matrix factorization analysis indicated that solvent usage and OBB were the primary contributors to the mass fraction of ambient NMHCs observed at the study site, and were responsible for 35 and 23% of the total O 3 production, respectively. The preferential presence of NO x in the morning inhibited net O 3 production; meanwhile O 3 built up in the afternoon due to a decrease in NO x concentrations. These results indicated that a joint effort in the regulation of solvent (aromatics) usage and OBB, as well as NO x from on-road vehicle exhaust may be effective in eliminating high-O 3 pollution risk in the rural areas of the YRDR.

Beschreibung: Formaldehyde (HCHO) in air, snow and interstitial air at Concordia (East Antarctic plateau) in summer Atmospheric Chemistry and Physics Discussions, 14, 32027-32070, 2014 Author(s): S. Preunkert, M. Legrand, M. Frey, A. Kukui, J. Savarino, H. Gallée, M. King, B. Jourdain, W. Vicars, and D. Helmig During the 2011/12 and 2012/13 austral summers HCHO was investigated for the first time in ambient air, snow, and interstitial air at the Concordia site located near Dome C on the East Antarctic plateau by deploying an Aerolaser AL-4021 analyser. Snow emission fluxes were estimated from vertical gradients of mixing ratios observed between 1 cm and 1 m above the snow surface as well as between interstitial air a few cm below the surface and in air just above the snow-pack. Typical flux values range between 1 to 2 × 10 12 molecules m −2 s −1 at night and 3 to 5 × 10 12 molecules m −2 s −1 at noon. Shading experiments suggest that the photochemical HCHO production in the snowpack at Concordia remains negligible compared to temperature-driven air–snow exchanges. At 1 m above the snow surface, the observed mean mixing ratio of 130 pptv and its diurnal cycle characterized by a slight decrease around noon are quite well reproduced by 1-D simulations that include snow emissions and gas phase methane oxidation chemistry.

Beschreibung: Sources and fluxes of organic nitrogen in precipitation over the southern East/Japan Sea: potential impacts on marine productivity Atmospheric Chemistry and Physics Discussions, 14, 31987-32025, 2014 Author(s): G. Yan and G. Kim We measured total dissolved reactive nitrogen in precipitation samples collected at Uljin, a Korean coastal site upwind of the southern East/Japan Sea (EJS), selected as a representative study site of atmospheric deposition over the northwestern Pacific margin. NO 3 − was found to be the most abundant nitrogen species, followed by NH 4 + and dissolved organic nitrogen (DON). Air mass back trajectory analysis revealed that a significant fraction of the inorganic nitrogen (NO 3 − and NH 4 + ) originated from mainland Asia, whereas the DON was primarily derived from Korea. Using varimax-rotated factor analysis in combination with major ions as tracers, agricultural activities in Korea were identified as the primary sources of DON in these samples. In addition, a positive correlation was found at Uljin between the size of organic fraction in total reactive nitrogen and nitrogen to carbon atomic ratio in organic matter. This correlation has also been observed at other locations worldwide, implying the utilization potential of atmospheric organic nitrogen might increase with its proportion in total nitrogen. Combining wet deposition measurements in this study with literature values for dry deposition observed at a remote island in the EJS, the total atmospheric depositional flux of reactive nitrogen was estimated to be 115 mmol N m −2 yr −1 over the southern EJS. Our study sheds new light on the potentially significant contribution to primary productivity of the northwestern Pacific Ocean by atmospheric deposition of nitrogen, especially the organic fraction.

Beschreibung: Investigation of post-depositional processing of nitrate in East Antarctic snow: isotopic constraints on photolytic loss, re-oxidation, and source inputs Atmospheric Chemistry and Physics Discussions, 14, 31943-31986, 2014 Author(s): G. Shi, A. M. Buffen, M. G. Hastings, C. Li, H. Ma, Y. Li, B. Sun, C. An, and S. Jiang Snowpits along a traverse from coastal East Antarctica to the summit of the ice sheet (Dome Argus) are used to investigate the post-depositional processing of nitrate in snow. Seven snowpits from sites with accumulation rates between 24 and 172 kg m −2 a −1 were sampled to depths of 150 to 300 cm. At sites from the continental interior (low accumulation, 〈 55 m −2 a −1 ), nitrate mass fraction is generally 〉 200 ng g −1 in surface snow and decreases quickly with depth to 〈 50 ng g −1 . Considerably increasing values of δ 15 N of nitrate are also observed (16–461 ‰ vs. air N 2 ), particularly in the top 25 cm, which is consistent with predicted fractionation constants for the photolysis of nitrate. The δ 18 O of nitrate (17–84 ‰ vs. VSMOW), on the other hand, decreases with increasing δ 15 N, suggestive of secondary formation of nitrate in situ (following photolysis) with a low δ 18 O source. Previous studies have suggested that δ 15 N and δ 18 O of nitrate at deeper snow depths should be predictable based upon an exponential decrease derived near the surface. At deeper depths sampled in this study, however, the relationship between nitrate mass fraction and δ 18 O changes, with increasing δ 18 O of nitrate observed between 100–200 cm. Predicting the impact of post-depositional loss, and therefore changes in the isotopes with depth, is highly sensitive to the depth interval over which an exponential decrease is assumed. In the snowpits collected closer to the coast (accumulation 〉 91 kg m −2 a −1 ), there are no obvious trends detected with depth and instead seasonality in nitrate mass fraction and its isotopic composition is found. In comparison to the interior sites, the coastal pits are lower in δ 15 N (−15–71 ‰ vs. air N 2 ) and higher in δ 18 O of nitrate (53–111 ‰ vs. VSMOW). The relationships found amongst mass fraction, δ 15 N, δ 18 O and Δ 17 O (Δ 17 O = δ 17 O − 0.52 × δ 18 O) of nitrate cannot be explained by local post-depositional processes, and are instead interpreted in the context of a primary atmospheric signal. Consistent with other Antarctic observational and modeling studies, the isotopic results are suggestive of an important influence of stratospheric chemistry on nitrate formation during the cold season and a mix of tropospheric sources and chemistry during the warm season. Overall, the findings in this study speak to the sensitivity of nitrate isotopic composition to post-depositional processing and highlight the strength of combined use of the nitrogen and oxygen isotopes for a mechanistic understanding of this processing.

Beschreibung: Sources of black carbon aerosols in South Asia and surrounding regions during the Integrated Campaign for Aerosols, Gases and Radiation Budget (ICARB) Atmospheric Chemistry and Physics Discussions, 14, 30727-30759, 2014 Author(s): R. Kumar, M. C. Barth, V. S. Nair, G. G. Pfister, S. S. Babu, S. K. Satheesh, K. K. Moorthy, and G. R. Carmichael The dominant sources of black carbon (BC) in South Asia and surrounding regions are inferred during March–May 2006 (Integrated Campaign for Aerosols, Gases and Radiation Budget, ICARB) period by introducing BC tracers in the Weather Research and Forecasting Model coupled with Chemistry. The model reproduced the magnitude, temporal and spatial variability of BC distribution observed during the ICARB ship-cruise. Average and SD (representing the spatial and temporal variability) in observed and modeled BC mass concentrations along the ship-track are estimated as 755 ± 734 and 732 ± 913 ng m −3 respectively. Average modeled values at most of the inland stations were also found to fall within the range of observed values. Model results show that ICARB measurements were fairly well representative of the Arabian Sea and the Bay of Bengal during the pre-monsoon season. Results show that anthropogenic and biomass burning emissions, respectively, accounted for 70 and 28 % of the average ± SD BC mass concentration (1480 ± 5920 ng m −3 ) in South Asia. BC emissions from residential (49 %) and industrial (37 %) sectors appear to be the major anthropogenic sources, except in the Himalayas where vehicular emissions dominated. We find that, while all parts of continental India contributed to anthropogenic BC over the Bay of Bengal, contribution over the Arabian Sea came mostly from southern Peninsula. We also show that regional-scale transport of anthropogenic emissions contribute up to 30 % of BC mass concentrations in western and eastern India, suggesting that it is important to consider non-local sources along with the local emissions while designing strategies for mitigating BC emissions.

Beschreibung: Rapid transport of East Asian pollution to the deep tropics Atmospheric Chemistry and Physics Discussions, 14, 30705-30726, 2014 Author(s): M. J. Ashfold, J. A. Pyle, A. D. Robinson, M. S. M. Nadzir, S. M. Phang, A. A. Samah, S. Ong, H. E. Ung, L. K. Peng, S. E. Yong, and N. R. P. Harris Anthropogenic emissions from East Asia have increased over recent decades, and under the prevailing westerly winds, these increases have led to changes in atmospheric composition as far afield as North America. Here we show that, during Northern Hemisphere (NH) winter, pollution originating in East Asia also directly affects atmospheric composition in the deep tropics. We present observations of marked intra-seasonal variability in the anthropogenic tracer perchloroethene (C 2 Cl 4 ) collected at two locations in Borneo during the NH winter of 2008/09. We use the NAME trajectory model to show that the observed enhancements in C 2 Cl 4 mixing ratio are caused by rapid meridional transport, in the form of "cold surges", from the relatively polluted East Asian land mass. In these events air masses can move across 〉 30° of latitude in 4 days. We then present data from the Monitoring Atmospheric Composition and Climate reanalysis which suggests that air masses high in C 2 Cl 4 may also contain levels of the pollutants carbon monoxide and ozone that are approximately double the typical "background" levels in Borneo. Convection in Southeast Asia can be enhanced by cold surges, and further trajectory calculations indicate that the polluted air masses can subsequently be lifted to the tropical upper troposphere. This suggests a potentially important connection between mid-latitude pollution sources and the very low stratosphere.

Beschreibung: Evaluation of a regional chemistry transport model using a newly developed regional OMI NO 2 retrieval Atmospheric Chemistry and Physics Discussions, 14, 31039-31090, 2014 Author(s): G. Kuhlmann, Y. F. Lam, H. M. Cheung, A. Hartl, J. C. H. Fung, P. W. Chan, and M. O. Wenig In this paper, we evaluate a high-resolution chemistry transport model (CTM) (3 km x 3 km spatial resolution) with the new Hong Kong (HK) NO 2 retrieval developed for the Ozone Monitoring Instrument (OMI) on-board the Aura satellite. The three-dimensional atmospheric chemistry was modelled in the Pearl River Delta (PRD) region in southern China by the Models-3 Community Multiscale Air Quality (CMAQ) modelling system from October 2006 to January 2007. In the HK NO 2 retrieval, tropospheric air mass factors (AMF) were recalculated using high-resolution ancillary parameters of surface reflectance, NO 2 profile shapes and aerosol profiles of which the latter two were taken from the CMAQ simulation. We also tested four different aerosol parametrizations. Ground level measurements by the PRD Regional Air Quality Monitoring (RAQM) network were used as additional independent measurements. The HK NO 2 retrieval increases the NO 2 vertical column densities (VCD) by (+31 ± 38) %, when compared to NASA's standard product (SP2), and reduces the mean bias (MB) between satellite and ground measurements by 26 percentage points from −41 to −15 %. The correlation coefficient r is low for both satellite datasets ( r = 0.35) due to the high spatial variability of NO 2 concentrations. The correlation between CMAQ and the RAQM network is low ( r ≈ 0.3) and the model underestimates the NO 2 concentrations in the north-western model domain (Foshan and Guangzhou). We compared the CMAQ NO 2 time series of the two main plumes with our regional OMI NO 2 product. The model overestimates the NO 2 VCDs by about 15 % in Hong Kong and Shenzhen, while the correlation coefficient is satisfactory ( r = 0.56). In Foshan and Guangzhou, the correlation is low ( r = 0.37) and the model underestimates the VCDs strongly (MB = −40 %). In addition, we estimated that the OMI VCDs are also underestimated by about 10 to 20 % in Foshan and Guangzhou because of the influence of the model parameters on the AMF. In this study, we demonstrate that the HK OMI NO 2 retrieval reduces the bias of the satellite measurements and thus the dataset can be used to study the magnitude of NO 2 concentrations in a regional model. The low bias can be achieved if AMFs are recalculated with more accurate surface reflectance, aerosol profiles and NO 2 profiles; only NO 2 profiles have been replaced in earlier studies. Since unbiased concentrations are important, for example, in air pollution studies, the results of this paper can be very helpful in future model evaluation studies.

Beschreibung: Aerosols over Continental Portugal (1978–1993): their sources and an impact on the regional climate Atmospheric Chemistry and Physics Discussions, 14, 31009-31038, 2014 Author(s): A. L. Morozova and I. A. Mironova Understanding of aerosol sources which affect climate is an important problem open in front of scientists as well as policymakers. The role of aerosols in local climate variability depends on a~balance between aerosol absorbing and scattering particles as well as on variability of environmental conditions. In this paper we investigate variability of aerosol content (both absorbing and scattering UV radiation) over Continental Portugal in dependence on aerosol sources (volcanic eruptions, dust events, wildfires and anthropogenic pollution). The effect of the aerosol on the climate is studied analyzing their contribution to variations of temperature, sunshine duration and precipitation over Portuguese regions. The present analysis is based on a developed modern multiple regression technique allowing us to build the statistical correlation models to determine both the main local aerosol sources and aerosol's influence on the climate of the Continental Portugal during 1978–1993 time period. The analysis allows us to conclude that the main sources driving the variations of the aerosol content over studied locations are wildfires, mineral dust intrusions and anthropogenic pollution. The relations between the aerosol content variations and the atmospheric parameters depend on the level of urbanization of the studied region, the type of aerosol and the season. The most significant finding is the decrease of the daily temperature (and diurnal temperature range) related to the decrease of sunshine duration observed during the summer periods of increased content of the absorbing aerosols in the atmosphere.

Beschreibung: The role of organic condensation on ultrafine particle growth during nucleation events Atmospheric Chemistry and Physics Discussions, 14, 30761-30798, 2014 Author(s): D. Patoulias, C. Fountoukis, I. Riipinen, and S. N. Pandis A new aerosol dynamics model (DMANx) has been developed that simulates the aerosol size/composition distribution and includes the condensation of organic vapors on nanoparticles through the implementation of the recently developed Volatility Basis Set framework. Simulations were performed for Hyytiala (Finland) and Finokalia (Greece), two locations with different organic sources where detailed measurements were available to constrain the new model. We investigate the effect of condensation of organics and chemical aging reactions of secondary organic aerosol (OA) on ultrafine particle growth and particle number concentration. This work highlights the importance of the pathways of oxidation of biogenic volatile organic compounds and the production of extremely low-volatility organics. At Hyytiala, organic condensation dominates the growth process of new particles. The low-volatility secondary OA contributes to particle growth during the early growth stage, but after a few hours most of the growth is due to semi-volatile secondary OA. At Finokalia, simulations show that organics have a complementary role to new particle growth contributing 45% to the total mass of new particles. Condensation of organics increases the number concentration of particles that can act as CCN ( N 100 ) by 13% at Finokalia and 25% at Hyytiala. The sensitivity of our results to the surface tension used is discussed.

Beschreibung: On the link between hygroscopicity, volatility, and oxidation state of ambient and water-soluble aerosol in the Southeastern United States Atmospheric Chemistry and Physics Discussions, 14, 30835-30877, 2014 Author(s): K. M. Cerully, A. Bougiatioti, J. R. Hite Jr., H. Guo, L. Xu, N. L. Ng, R. Weber, and A. Nenes The formation of secondary organic aerosol (SOA) combined with the partitioning of semi-volatile organic components can impact numerous aerosol properties including cloud condensation nuclei (CCN) activity, hygroscopicity and volatility. During the summer 2013 Southern Oxidant and Aerosol Study (SOAS) field campaign in a rural site in the Southeastern United States, a suite of instruments including a CCN counter, a thermodenuder (TD) and a high resolution time-of-flight aerosol mass spectrometer (AMS) were used to measure CCN activity, aerosol volatility, composition and oxidation state. Particles were either sampled directly from ambient or through a Particle Into Liquid Sampler (PILS), allowing the investigation of the water-soluble aerosol component. Ambient aerosol exhibited size-dependent composition with larger particles being more hygroscopic. The hygroscopicity of thermally-denuded aerosol was similar between ambient and PILS-generated aerosol and showed limited dependence on volatilization. Results of AMS 3-factor Positive Matrix Factorization (PMF) analysis for the PILS-generated aerosol showed that the most hygroscopic components are most likely the most and the least volatile features of the aerosol. No clear relationship was found between organic hygroscopicity and oxygen-to-carbon ratio; in fact, Isoprene organic aerosol (Isoprene-OA) was found to be the most hygroscopic factor, while at the same time being the least oxidized and likely most volatile of all PMF factors. Considering the diurnal variation of each PMF factor and its associated hygroscopicity, Isoprene-OA and More Oxidized – Oxidized Oxygenated Organic Aerosol (MO-OOA) are the prime contributors to hygroscopicity and covary with Less Oxidized – Oxidized Oxygenated Organic Aerosol (LO-OOA) in a way that induces the observed diurnal invariance in total organic hygroscopicity. Biomass Burning Organic Aerosol (BBOA) contributed little to aerosol hygroscopicity, which is expected since there was little biomass burning activity during the sampling period examined.

Beschreibung: Elemental composition and clustering of α-pinene oxidation products for different oxidation conditions Atmospheric Chemistry and Physics Discussions, 14, 30799-30833, 2014 Author(s): A. P. Praplan, S. Schobesberger, F. Bianchi, M. P. Rissanen, M. Ehn, T. Jokinen, H. Junninen, A. Adamov, A. Amorim, J. Dommen, J. Duplissy, J. Hakala, A. Hansel, M. Heinritzi, J. Kangasluoma, J. Kirkby, M. Krapf, A. Kürten, K. Lehtipalo, F. Riccobono, L. Rondo, N. Sarnela, M. Simon, A. Tomé, J. Tröstl, P. M. Winkler, C. Williamson, P. Ye, J. Curtius, U. Baltensperger, N. M. Donahue, M. Kulmala, and D. R. Worsnop This study presents the difference between oxidised organic compounds formed by α-pinene ozonolysis and hydroxyl radical (OH) oxidation in the CLOUD environmental chamber. The results from three Atmospheric Pressure interface Time-Of-Flight (APi-TOF) mass spectrometers measuring simultaneously the composition of naturally charged, as well as neutral species (via chemical ionisation with nitrate) are discussed. Natural chemical ionisation takes place in the CLOUD chamber and organic oxidised compounds form clusters with nitrate, bisulphate, bisulphate/sulphuric acid clusters, ammonium, and dimethylaminium, or get protonated. This process is selective towards various oxidised organic compounds, so that in order to get a comprehensive picture of the elemental composition of oxidation products, several instruments must be used. A comparison between oxidation products containing 10 and 20 carbon atoms is presented. Oxidation products from ozonolysis showed a higher oxidation state than the ones from OH oxidation. Also, highly oxidised organic compounds are shown to be formed in the early stages of the oxidation, for low α-pinene levels.

Beschreibung: Solar cycle in current reanalyses: (non)linear attribution study Atmospheric Chemistry and Physics Discussions, 14, 30879-30912, 2014 Author(s): A. Kuchar, P. Sacha, J. Miksovsky, and P. Pisoft This study focusses on the variability of temperature, ozone and circulation characteristics in the stratosphere and lower mesosphere with regard to the influence of the 11 year solar cycle. It is based on attribution analysis using multiple nonlinear techniques (Support Vector Regression, Neural Networks) besides the traditional linear approach. The analysis was applied to several current reanalysis datasets for the 1979–2013 period, including MERRA, ERA-Interim and JRA-55, with the aim to compare how this type of data resolves especially the double-peaked solar response in temperature and ozone variables and the consequent changes induced by these anomalies. Equatorial temperature signals in the lower and upper stratosphere were found to be sufficiently robust and in qualitative agreement with previous observational studies. The analysis also pointed to the solar signal in the ozone datasets (i.e. MERRA and ERA-Interim) not being consistent with the observed double-peaked ozone anomaly extracted from satellite measurements. Consequently the results obtained by linear regression were confirmed by the nonlinear approach through all datasets, suggesting that linear regression is a relevant tool to sufficiently resolve the solar signal in the middle atmosphere. Furthermore, the seasonal dependence of the solar response was also discussed, mainly as a source of dynamical causalities in the wave propagation characteristics in the zonal wind and the induced meridional circulation in the winter hemispheres. The hypothetical mechanism of a weaker Brewer Dobson circulation was reviewed together with discussion of polar vortex stability.

Beschreibung: The oxidation capacity of the boreal forest: first simulated reactivities of O 3 and NO 3 Atmospheric Chemistry and Physics Discussions, 14, 30947-31007, 2014 Author(s): D. Mogensen, R. Gierens, J. N. Crowley, P. Keronen, S. Smolander, A. Sogachev, A. C. Nölscher, L. Zhou, M. Kulmala, M. J. Tang, J. Williams, and M. Boy Using the 1D atmospheric chemistry–transport model SOSAA, we have investigated the atmospheric reactivity of a boreal forest ecosystem during the HUMPPA-COPEC-10 campaign (summer 2010, at SMEAR II in Southern Finland). For the very first time, we present vertically resolved model simulations of the NO 3 - and O 3 -reactivity ( R ) together with the modelled and measured reactivity of OH. We find that OH is the most reactive oxidant ( R ~3 s −1 ) followed by NO 3 ( R ~0.07 s −1 ) and O 3 ( R ~2 × 10 −5 s −1 ). The missing OH-reactivity was found to be large in accordance with measurements (~65%) as would be expected from the chemical subset described in the model. The accounted OH radical sinks were inorganic compounds (~41%, mainly due to reaction with CO), emitted monoterpenes (~14%) and oxidised biogenic volatile organic compounds (~44%). The missing reactivity is expected to be due to unknown biogenic volatile organic compounds and their photoproducts, indicating that the true main sink of OH is not expected to be inorganic compounds. The NO 3 radical was found to react mainly with primary emitted monoterpenes (~60%) and inorganic compounds (~37%, including NO 2 ). NO 2 is, however, only a temporary sink of NO 3 under the conditions of the campaign and does not affect the NO 3 concentration. We discuss the difference between instantaneous and steady state reactivity and present the first boreal forest steady state lifetime of NO 3 (113 s). O 3 almost exclusively reacts with inorganic compounds (~91%, mainly NO, but also NO 2 during night) and less with primary emitted sesquiterpenes (~6%) and monoterpenes (~3%). When considering the concentration of the oxidants investigated, we find that O 3 is the oxidant that is capable of removing pollutants fastest. As part of this study, we developed a simple empirical parameterisation for conversion of measured spectral irradiance into actinic flux. Further, the meteorological conditions were evaluated using radiosonde observations and ground based measurements. The overall vertical structure of the boundary layer is discussed, together with validation of the surface energy balance and turbulent fluxes. The sensible heat and momentum fluxes above the canopy were on average overestimated, while the latent heat flux was underestimated.

Beschreibung: Large-eddy simulations of contrails in a turbulent atmosphere Atmospheric Chemistry and Physics Discussions, 14, 29499-29546, 2014 Author(s): J. Picot, R. Paoli, O. Thouron, and D. Cariolle In this work, the evolution of contrails in the vortex and dissipation regimes is studied by means of fully three-dimensional large-eddy simulation (LES) coupled to a Lagrangian particle tracking method to treat the ice phase. This is the first paper where fine-scale atmospheric turbulence is generated and sustained by means of a stochastic forcing that mimics the properties of stably stratified turbulent flows as those occurring in the upper troposphere lower stratosphere. The initial flow-field is composed by the turbulent background flow and a wake flow obtained from separate LES of the jet regime. Atmospheric turbulence is the main driver of the wake instability and the structure of the resulting wake is sensitive to the intensity of the perturbations, primarily in the vertical direction. A stronger turbulence accelerates the onset of the instability, which results in shorter contrail decent and more effective mixing in the interior of the plume. However, the self-induced turbulence that is produced in the wake after the vortex break-up dominates over background turbulence at the end of the vortex regime and dominates the mixing with ambient air. This results in global microphysical characteristics such as ice mass and optical depth that are be slightly affected by the intensity of atmospheric turbulence. On the other hand, the background humidity and temperature have a first order effect on the survival of ice crystals and particle size distribution, which is in line with recent and ongoing studies in the literature.

Beschreibung: Impacts of 20th century aerosol emissions on the South Asian monsoon in the CMIP5 models Atmospheric Chemistry and Physics Discussions, 14, 30639-30666, 2014 Author(s): L. Guo, A. G. Turner, and E. J. Highwood Comparison of single-forcing varieties of 20th century historical experiments in a subset of models from the Fifth Coupled Model Intercomparison Project (CMIP5) reveals that South Asian summer monsoon rainfall increases towards the present day in Greenhouse Gas (GHG)-only experiments with respect to pre-industrial levels, while it decreases in anthropogenic aerosol-only experiments. Comparison of these single-forcing experiments with the all-forcings historical experiment suggests aerosol emissions have dominated South Asian monsoon rainfall trends in recent decades, especially during the 1950s to 1970s. The variations in South Asian monsoon rainfall in these experiments follows approximately the time-evolution of inter-hemispheric temperature gradient over the same period, suggesting a contribution from the large-scale background state relating to the asymmetric distribution of aerosol emissions about the equator. By examining the twenty-five available all-forcings historical experiments, we show that models including aerosol indirect effects dominate the negative rainfall trend. Indeed, models including only the direct radiative effect of aerosol show an increase in monsoon rainfall, consistent with the dominance of increasing greenhouse gas emissions and planetary warming on monsoon rainfall in those models. For South Asia, reduced rainfall in the models with indirect effects is related to decreased evaporation at the land surface rather than from anomalies in horizontal moisture flux, suggesting the impact of indirect effects on local aerosol emissions. This is confirmed by examination of aerosol loading and cloud droplet number trends over the South Asia region. Thus while remote aerosols and their asymmetric distribution about the equator play a role in setting the inter-hemispheric temperature distribution on which the South Asian monsoon, as one of the global monsoons, operates, the addition of indirect aerosol effects acting on very local aerosol emissions also plays a role in declining monsoon rainfall. The disparity between the response of monsoon rainfall to increasing aerosol emissions in models containing direct aerosol effects only and those also containing indirect effects needs to be urgently investigated since the suggested future decline in Asian anthropogenic aerosol emissions inherent to the representative concentration pathways (RCPs) used for future climate projection may turn out to be optimistic. In addition, both groups of models show declining rainfall over China, also relating to local aerosol mechanisms. We hypothesize that aerosol emissions over China are large enough, in the CMIP5 models, to cause declining monsoon rainfall even in the absence of indirect aerosol effects. The same is not true for India.

Beschreibung: An evaluation of O 3 dry deposition simulations in East Asia Atmospheric Chemistry and Physics Discussions, 14, 919-951, 2014 Author(s): R. J. Park, S. K. Hong, H.-A. Kwon, S. Kim, A. Guenther, J.-H. Woo, and C. P. Loughner We used a 3-D regional atmospheric chemistry transport model (WRF-Chem) to examine processes that determine O 3 in East Asia; in particular, we focused on O 3 dry deposition, which is an uncertain research area due to insufficient observation and numerical studies in East Asia. Here, we compare two widely used dry deposition parameterization schemes, Wesely and M3DRY, which are used in the WRF-Chem and CMAQ models, respectively. The O 3 dry deposition velocities simulated using the two aforementioned schemes under identical meteorological conditions show considerable differences (a factor of 2) due to surface resistance parameterization discrepancies. The O 3 concentration differed by up to 10 ppbv for the monthly mean. The simulated and observed dry deposition velocities were compared, which showed that the Wesely scheme model is consistent with the observations and successfully reproduces the observed diurnal variation. We conduct several sensitivity simulations by changing the land use data, the surface resistance of the water and the model's spatial resolution to examine the factors that affect O 3 concentrations in East Asia. As shown, the model was considerably sensitive to the input parameters, which indicates a high uncertainty for such O 3 dry deposition simulations. Observations are necessary to constrain the dry deposition parameterization and input data to improve the East Asia air quality models.

Beschreibung: Variations of ground-level O 3 and its precursors in Beijing in summertime between 2005 and 2011 Atmospheric Chemistry and Physics Discussions, 14, 1019-1050, 2014 Author(s): Q. Zhang, B. Yuan, M. Shao, X. Wang, S. Lu, K. Lu, M. Wang, L. Chen, C. Chang, and S. Liu Increased levels of ground-level ozone (O 3 ), reflecting the oxidative capacity of the atmosphere, are of increasing concern. High levels of total oxidants (O x = O 3 + NO 2 ) have been persistently observed as a feature of Beijing's air pollution. Beijing is a typical global mega-city requiring the enforcement of stringent air quality controls as rapid economic growth continues. To evaluate the effect of air quality controls in recent years, ground-based on-line measurements at an urban site were conducted in summer and the variations in O 3 with simultaneous changes in NO x and volatile organic compounds (VOCs) between 2005 and 2011 were analyzed. Both NO x and anthropogenic VOCs in Beijing decreased over the study period, 1.4 ppbv yr −1 and 1.6 ppbv yr −1 respectively, the VOCs reactivity, in term of OH loss rate showed an indistinct statistical trend due to the large contribution from naturally emitted isoprene. Meanwhile, the daytime average O 3 concentrations increased significantly at an annual rate 2.6 ppbv yr −1 , around 5% yr −1 between 2005 and 2011. Considering the influence of NO titration effect and the increasing in regional background in the North China Plain (NCP), the main reason for such an increase in oxidants was local photochemistry. A simplified model was used to evaluate the effect of changes in the levels of ozone precursors on ozone production, we found that between 2001 and 2006, the production rate of total oxidants, P (O x ) increased rapidly due to increased VOC levels and decreasing of NO 2 , while from 2006 to2011, P (O x ) remained high though, decreased slightly as a~consequence of the decrease in both VOC reactivity (5% yr −1 ) and NO x (4% yr −1 ). Measurements have shown that the air pollution control efforts of Beijing city were effective in cutting ozone precursors, but even led to higher ground-level ozone. Therefore, putting ozone as the target for air quality, a faster reduction of VOCs, especially the reactive VOCs, will be needed to go together with NO x emission control programs.

Beschreibung: Concentrations and fluxes of isoprene and oxygenated VOCs at a French Mediterranean oak forest Atmospheric Chemistry and Physics Discussions, 14, 871-917, 2014 Author(s): C. Kalogridis, V. Gros, R. Sarda-Esteve, B. Langford, B. Loubet, B. Bonsang, N. Bonnaire, E. Nemitz, A.-C. Genard, C. Boissard, C. Fernandez, E. Ormeño, D. Baisnée, I. Reiter, and J. Lathière The CANOPEE project aims to better understand the biosphere-atmosphere exchanges of biogenic volatile organic compounds (BVOC) in the case of Mediterranean ecosystems and the impact of in-canopy processes on the atmospheric chemical composition above the canopy. Based on an intensive field campaign, the objective of our work was to determine the chemical composition of the air inside a canopy as well as the net fluxes of reactive species between the canopy and the boundary layer. Measurements were carried out during spring 2012 at the Oak Observatory of the Observatoire de Haute Provence (O 3 HP) located in the southeast of France. The field site presents one dominant tree species, Quercus pubescens L., a typical Mediterranean species which features large isoprene emission rates. Mixing ratios of isoprene, its degradation products methylvinylketone (MVK) and methacrolein (MACR) and several other oxygenated VOC (OxVOC) were measured above the canopy using an online proton transfer reaction mass spectrometer (PTR-MS), and fluxes were calculated by the disjunct eddy covariance approach. The O 3 HP site was found to be a very significant source of isoprene emissions, with daily maximum ambient concentrations ranging between 2–16 ppbv inside and 2–5 ppbv just above the top of the forest canopy. Significant isoprene fluxes were observed only during daytime, following diurnal cycles with midday net emission fluxes from the canopy ranging between 2–8 mg m −2 h 1 . Net isoprene normalised flux (at 30 °C, 1000 μmol m −2 s −1 ) was estimated at 6.6 mg m −2 h −1 . The (MVK+MACR)-to-isoprene ratio was used to assess the degree of isoprene oxidation. In-canopy chemical oxidation of isoprene was found to be weak, as indicated by the low (MVK+MACR)-to-isoprene ratio (~ 0.13) and low MVK+MACR fluxes, and did not seem to have a significant impact on isoprene concentrations and fluxes above the canopy. Evidence of direct emission of methanol was also found exhibiting maximum daytime fluxes ranging between 0.2–0.4 mg m −2 h −1 , whereas flux values for monoterpenes and others OxVOC such as acetone and acetaldehyde were below the detection limit.

Beschreibung: Bromocarbons in the tropical coastal and open ocean atmosphere during the Prime Expedition Scientific Cruise 2009 (PESC 09) Atmospheric Chemistry and Physics Discussions, 14, 953-984, 2014 Author(s): M. S. Mohd Nadzir, S. M. Phang, M. R. Abas, N. Abdul Rahman, A. Abu Samah, W. T. Sturges, D. E. Oram, G. P. Mills, E. C. Leedham, J. A. Pyle, N. R. P. Harris, A. D. Robinson, M. J. Ashfold, M. I. Mead, M. T. Latif, M. Mohd Hanafiah, M. F. Khan, and A. M. Amiruddin Atmospheric concentrations of very short-lived species (VSLS) bromocarbons, including CHBr 3 , CH 2 Br 2 , CHCl 2 Br, CHClBr 2 , CH 2 BrCl, were measured in the Strait of Malacca and the South China and Sulu-Sulawesi Seas during a two month research cruise in June/July 2009. The highest bromocarbon concentrations were found in the Strait of Malacca, with smaller enhancements in coastal regions of Northern Borneo. CHBr 3 was the most abundant bromocarbon, ranging from 5.2 pmol mol −1 in the Strait of Malacca to 0.94 pmol mol −1 over the open ocean. Other bromocarbons showed lower concentrations, in the range of 0.8–1.3 pmol mol −1 for CH 2 Br 2 , 0.1–0.5 pmol mol −1 (CHCl 2 Br) and 0.1–0.4 pmol mol −1 (CHClBr 2 ). There was no significant correlation between bromocarbons and in situ chlorophyll a . Together the short-lived bromocarbons contribute an average of 8.9 pmol mol −1 (range 5.2–21.4 pmol mol −1 ) to tropospheric bromine load, which is similar to that found in previous studies (Montzka et al., 2011). Statistical tests showed strong Spearman correlations amongst brominated compounds suggesting a common source. Log-log plots of CHBr 3 /CH 2 Br 2 vs. CHBr 2 Cl/CH 2 Br 2 show that both chemical reactions and dilution into the background atmosphere contribute to the composition of these halocarbons at each sampling point. We have used the correlation to make a crude estimate of the regional emissions of CHBr 3 and derive a value of 63 Gg yr −1 for the South East (S.E.) Asian region (10° N–20° S, 90–150° E). Finally, we note that satellite-derived chlorophyll a (chl a ) products do not always agree well with in situ measurements, particularly in coastal regions of high turbidity, meaning that satellite chl a may not always be a good proxy for marine productivity.

Beschreibung: Influence of cloud processing on CCN activation behaviour in the Thuringian Forest, Germany during HCCT-2010 Atmospheric Chemistry and Physics Discussions, 14, 1617-1645, 2014 Author(s): S. Henning, K. Dieckmann, K. Ignatius, M. Schäfer, P. Zedler, E. Harris, B. Sinha, D. van Pinxteren, S. Mertes, W. Birmili, M. Merkel, Z. Wu, A. Wiedensohler, H. Wex, H. Herrmann, and F. Stratmann Within the framework of the international cloud experiment "Hill Cap Cloud Thuringia 2010" (HCCT-2010), the influence of cloud processing on the activation properties of ambient aerosol particles was investigated. Particles were probed up- and downwind of an orographic cap cloud on Mt. Schmücke, which is part of a large mountain ridge in Thuringia, Germany. The activation properties of the particles were investigated by means of size-segregated Cloud Condensation Nuclei (CCN) measurements at 3 to 4 different supersaturations. The observed CCN spectra together with the total particle spectra were used to calculate the hygroscopicity parameter κ for the up- and the downwind stations. The up- and downwind critical diameters and κ values were then compared for defined Cloud Events and Non Cloud Events. Cloud processing was found to significantly increase the hygroscopicity of the aerosol particles, with an average increase in κ of 50%. Mass spectrometry analysis and isotopic analysis of the particles show that the observed increase in hygroscopicity of the cloud-processed particles is due to an enrichment of nitrate and sulfate in the particle phase.

Beschreibung: Temporal and spatial variations in rainwater methanol Atmospheric Chemistry and Physics Discussions, 14, 1375-1398, 2014 Author(s): J. D. Felix, S. B. Jones, G. B. Avery, J. D. Willey, R. N. Mead, and R. J. Kieber This work reports the first detailed analysis of methanol concentrations in rainwater. Methanol concentrations measured in 49 rain events collected between 28 August 2007 to 10 July 2008 in Wilmington, NC, USA, ranged from below the detection limit of 6 nM to 9.3 μM with a volume weighted average concentration of 1.2 ± 0.2 μM. Methanol concentrations in rainwater were up to ~200× greater than concentrations observed in marine waters indicating wet deposition as a potential significant source to marine waters. Assuming these methanol concentrations are an appropriate proxy for global methanol rainwater concentrations the global methanol wet deposition sink is estimated as 20 Tg yr −1 which implies previous methanol budgets underestimate removal by precipitation. Methanol concentrations did not correlate with H + , NO 3 − , and NSS, which suggest that the dominant source of the alcohol to rainwater is not anthropogenic. However, methanol concentrations were strongly correlated with acetaldehyde which has a primarily biogenic input. Methanol volume weighted concentration during the growing season (1.5 + 0.3 μM) was more than double that of the non-growing season (0.7 + 0.1 μM), further promoting biogenic emissions as the primary cause of fluctuating methanol concentrations. Methanol concentrations peaked in rainwater collected between the time period 12:00–06:00 p.m. Peaking during this period of optimal sunlight implies a direct relationship to photochemical methanol production but there are also increases in biogenic activity during this time period. Rain events with terrestrial origins had higher concentrations than those of marine origin demonstrating the significance of the continental source of methanol in rainwater.

Beschreibung: Overview of the Manitou Experimental Forest Observatory: site description and selected science results from 2008–2013 Atmospheric Chemistry and Physics Discussions, 14, 1647-1709, 2014 Author(s): J. Ortega, A. Turnipseed, A. B. Guenther, T. G. Karl, D. A. Day, D. Gochis, J. A. Huffman, A. J. Prenni, E. J. T. Levin, S. M. Kreidenweis, P. J. DeMott, Y. Tobo, E. G. Patton, A. Hodzic, Y. Cui, P. C. Harley, R. H. Hornbrook, E. C. Apel, R. K. Monson, A. S. D. Eller, J. P. Greenberg, M. Barth, P. Campuzano-Jost, B. B. Palm, J. L. Jimenez, A. C. Aiken, M. K. Dubey, C. Geron, J. Offenberg, M. G. Ryan, P. J. Fornwalt, S. C. Pryor, F. N. Keutsch, J. P. DiGangi, A. W. H. Chan, A. H. Goldstein, G. M. Wolfe, S. Kim, L. Kaser, R. Schnitzhofer, A. Hansel, C. A. Cantrell, R. L. Mauldin III, and J. N. Smith The Bio-hydro-atmosphere interactions of Energy, Aerosols, Carbon, H 2 O, Organics & Nitrogen (BEACHON) project seeks to understand the feedbacks and inter-relationships between hydrology, biogenic emissions, carbon assimilation, aerosol properties, clouds and associated feedbacks within water-limited ecosystems. The Manitou Experimental Forest Observatory (MEFO) was established in 2008 by the National Center for Atmospheric Research to address many of the BEACHON research objectives, and it now provides a fixed field site with significant infrastructure. MEFO is a mountainous, semi-arid ponderosa pine-dominated forest site that is normally dominated by clean continental air, but is periodically influenced by anthropogenic sources from Colorado Front Range cities. This article summarizes the past and ongoing research activities at the site, and highlights some of the significant findings that have resulted from these measurements. These activities include: – soil property measurements, – hydrological studies, – measurements of high-frequency turbulence parameters, – eddy covariance flux measurements of water, energy, aerosols and carbon dioxide through the canopy, – biogenic and anthropogenic volatile organic compound emissions and their influence on regional atmospheric chemistry, – aerosol number and mass distributions, – chemical speciation of aerosol particles, – characterization of ice and cloud condensation nuclei, – trace gas measurements, and – model simulations using coupled chemistry and meteorology. In addition to various long-term continuous measurement, three focused measurement campaigns with state-of-the-art instrumentation have taken place since the site was established, and two of these are the subjects of this special issue: BEACHON-ROCS (Rocky Mountain Organic Carbon Study, 2010) and BEACHON-RoMBAS (Rocky Mountain Biogenic Aerosol Study, 2011).

Beschreibung: Comparison of surface and column measurements of aerosol scattering properties over the western North Atlantic Ocean at Bermuda Atmospheric Chemistry and Physics Discussions, 14, 1791-1821, 2014 Author(s): R. P. Aryal, K. J. Voss, P. A. Terman, W. C. Keene, J. L. Moody, E. J. Welton, and B. N. Holben Light scattering and spectral absorption by size-resolved aerosols in near-surface air at Tudor Hill, Bermuda were measured continuously between January and June 2009. Vertical distributions of aerosol backscattering and column-averaged aerosol optical properties were characterized in parallel with a Micro-pulse lidar (MPL) and an automated sun-sky radiometer. Aerosol optical properties measured near the surface were often significantly correlated with those averaged over the column. These include scattering by near-surface bulk aerosol at 530 nm vs. column aerosol optical depth (AOD), near-surface sub-μm scattering fraction vs. column averaged sub-μm scattering fraction, the column averaged Angstrom exponent derived using a column integrated size distribution and complex refractive index. The relative contribution of submicron aerosol light scattering to total aerosol light scattering shows a slight enhancement of the column contribution of submicron particles over the surface measurements. Physical factors such as surface level wind speed have a more important affect on bulk aerosol light scattering at the surface.

Beschreibung: Influence of airmass transport events on the variability of surface ozone at Xianggelila Regional Atmosphere Background Station, Southwest China Atmospheric Chemistry and Physics Discussions, 14, 1823-1859, 2014 Author(s): J. Ma, W. L. Lin, X. D. Zheng, X. B. Xu, Z. Li, and L. L. Yang In situ measurements of ozone (O 3 ), carbon monoxide (CO) and meteorological parameters were made from December 2007 to November 2009 at the Xianggelila Regional Atmosphere Background Station (28.006° N, 99.726° E, 3580 m a.s.l.), Southwest China. It is found that both O 3 and CO peaked in spring while the valleys of O 3 and CO occurred in summer and winter, respectively. A normalized indicator (marked as " Y ") of transport events on the basis of the monthly normalized O 3 , CO, and water vapor, is proposed to evaluate the occurrence of O 3 transport events from the upper, O 3 -rich atmosphere. This composite indicator has the advantage of being less influenced by and seasonal or occasional variations of individual factors. It is shown that the most frequent transport events occurred in winter and they can make a significant contribution to surface O 3 at Xianggelila. A case of strong O 3 transport event under the synoptic condition of a~deep westerly trough is studied by the combination of the Y indicator, potential vorticity, total column ozone, and trajectory analysis. A 9.6 ppb increase (21.0%) of surface ozone is estimated based on the impacts of deep transport events in winter. Asian Monsoon plays an important role in suppressing O 3 accumulation in summer and fall.

Beschreibung: The effect of climate change and emission scenarios on ozone concentrations over Belgium: a high resolution model study for policy support Atmospheric Chemistry and Physics Discussions, 14, 1761-1790, 2014 Author(s): D. Lauwaet, P. Viaene, E. Brisson, T. van Noije, A. Strunk, S. Van Looy, N. Veldeman, L. Blyth, K. De Ridder, and S. Janssen Belgium is one of the areas within Europe experiencing the highest levels of air pollution. To provide insight to policy makers about expected changes in the air quality towards the near future (2026–2035), a high resolution (3 km) modelling experiment is set up. The regional air quality model AURORA (Air quality modelling in Urban Regions using an Optimal Resolution Approach), driven by output from a regional climate model, is used to simulate several 10-yr time slices to investigate the impact of climatic changes and different emission scenarios on near-surface O 3 concentrations, one of the key indices for air quality. Model evaluation against measurements from 34 observation stations shows that the AURORA model is capable of reproducing 10-yr mean concentrations, daily cycles and spatial patterns. The results for the RCP4.5 emission scenario indicate that the mean surface O 3 concentrations are expected to increase significantly in the near future due to less O 3 titration by reduced NO x emissions. Applying an alternative emission scenario for Europe is found to have only a minor impact on the overall concentrations, which are dominated by the background changes. Climate change alone has a much smaller effect on the near-surface O 3 concentrations over Belgium than the projected emission changes. The very high horizontal resolution that is used in this study results in much improved spatial correlations and simulated peak concentrations compared to a standard 25 km simulation. This allows to investigate the number of peak episodes during summer, which are found to be reduced with 25% by the emission reductions in RCP4.5.

Beschreibung: Long term halocarbon observations from a~coastal and an inland site in Sabah, Malaysian Borneo Atmospheric Chemistry and Physics Discussions, 14, 1919-1969, 2014 Author(s): A. D. Robinson, N. R. P. Harris, M. J. Ashfold, B. Gostlow, N. J. Warwick, L. M. O'Brien, E. J. Beardmore, M. S. M. Nadzir, S. M. Phang, A. A. Samah, S. Ong, H. E. Ung, L. K. Peng, S. E. Yong, M. Mohamad, and J. A. Pyle Short lived halocarbons are believed to have important sources in the tropics where rapid vertical transport could provide a significant source to the stratosphere. In this study, quasi-continuous measurements of short-lived halocarbons are reported for two tropical sites in Sabah (Malaysian Borneo), one coastal and one inland (rainforest). We present the observations for C 2 Cl 4 , CHBr 3 , CH 2 Br 2 * (actually ~80% CH 2 Br 2 and ~20% CHBrCl 2 ) and CH 3 I from November 2008 to January 2010 made using our μDirac gas chromatographs with electron capture detection (GC-ECD). We focus on the first 15 months of observations, showing over one annual cycle for each compound and therefore adding significantly to the few limited-duration observational studies that have been conducted thus far in southeast Asia. The main feature in the C 2 Cl 4 behaviour at both sites is its annual cycle with the winter months being influenced by northerly flow with higher concentrations, typical of the Northern Hemisphere, with the summer months influenced by southerly flow and lower concentrations representative of the Southern Hemisphere. No such clear annual cycle is seen for CHBr 3 , CH 2 Br 2 Br 2 * or CH 3 I. The baseline values for CHBr 3 and CH 2 Br 2 Br 2 * are similar at the coastal (overall median: CHBr 3 1.7 ppt; CH 2 Br 2 Br 2 * 1.4 ppt) and inland sites (CHBr 3 1.6 ppt, CH 2 Br 2 Br 2 * 1.1 ppt), but periods with elevated values are seen at the coast (overall 95th percentile: CHBr 3 4.4 ppt; CH 2 Br 2 Br 2 * 1.9 ppt) presumably resulting from the stronger influence of coastal emissions. Overall median bromine values from [CHBr 3 ] + [CH 2 Br 2 Br 2 * ] are 8.0 ppt at the coast and 6.8 ppt inland. The median values reported here are largely consistent with other limited tropical data and imply that southeast Asia generally is not, as has been suggested, a hot-spot for emissions of these compounds. These baseline values are consistent with the most recent emissions found for southeast Asia using the p-TOMCAT model. CH 3 I, which is only observed at the coastal site, is the shortest-lived compound measured in this study and the observed atmospheric variations reflect this, with high variability throughout the study period.

Beschreibung: Modeling and sensitivity analysis of transport and deposition of radionuclides from the Fukushima Daiichi accident Atmospheric Chemistry and Physics Discussions, 14, 2113-2173, 2014 Author(s): X. Hu, D. Li, H. Huang, S. Shen, and E. Bou-Zeid The atmospheric transport and ground deposition of radioactive isotopes 131 I and 137 Cs during and after the Fukushima Daiichi Nuclear Power Plant (FDNPP) accident (March 2011) are investigated using the Weather Research and Forecasting/Chemistry (WRF/Chem) model. The aim is to assess the skill of WRF in simulating these processes and the sensitivity of the model's performance to various parameterizations of unresolved physics. The WRF/Chem model is first upgraded by implementing a radioactive decay term into the advection-diffusion solver and adding three parameterizations for dry deposition and two parameterizations for wet deposition. Different microphysics and horizontal turbulent diffusion schemes are then tested for their ability to reproduce observed meteorological conditions. Subsequently, the influence on the simulated transport and deposition of the characteristics of the emission source, including the emission rate, the gas partitioning of 131 I and the size distribution of 137 Cs, is examined. The results show that the model can predict the wind fields and rainfall realistically. The ground deposition of the radionuclides can also potentially be captured well but it is very sensitive to the emission characterization. It is found that the total deposition is most influenced by the emission rate for both 131 I and 137 Cs; while it is less sensitive to the dry deposition parameterizations. Moreover, for 131 I, the deposition is also sensitive to the microphysics schemes, the horizontal diffusion schemes, gas partitioning and wet deposition parameterizations; while for 137 Cs, the deposition is very sensitive to the microphysics schemes and wet deposition parameterizations, and it is also sensitive to the horizontal diffusion schemes and the size distribution.

Beschreibung: On transition-zone water clouds Atmospheric Chemistry and Physics Discussions, 14, 1051-1071, 2014 Author(s): E. Hirsch, I. Koren, Z. Levin, O. Altaratz, and E. Agassi A recent field campaign was conducted to measure the properties of thin, warm, convective clouds forming under conditions of weak updrafts. During the campaign, short-lived clouds (on the order of minutes) with droplets' effective radius of 1–2 μm and low liquid water path (~500 mg m −2 ) were measured. These low values are puzzling, since in most studies an effective radius of 4 μm is reported to serve as the lower bound for clouds. A theoretical cloud model designed to resolve the droplet-activation process suggested conditions that favor the formation of such clouds. Here we show that these clouds, which mark the transition from haze to cloud, are highly sensitive to the magnitude of the initial perturbation that initiated them. We define these clouds as "transition-zone clouds". The existence of such clouds poses a key challenge for the analysis of atmospheric observations and models, since they "further smooth" the transition from dry aerosol through haze pockets to cumulus clouds.

Beschreibung: Missing SO 2 oxidant in the coastal atmosphere? – Evidence from high resolution measurements of OH and atmospheric sulfur compounds Atmospheric Chemistry and Physics Discussions, 14, 1159-1190, 2014 Author(s): H. Berresheim, M. Adam, C. Monahan, C. O'Dowd, J. M. C. Plane, B. Bohn, and F. Rohrer Diurnal and seasonal variations of gaseous sulfuric acid (H 2 SO 4 ) and methane sulfonic acid (MSA) were measured in N.E. Atlantic air at the Mace Head atmospheric research station during the years 2010 and 2011. The measurements utilized selected ion/chemical ionization mass spectrometry (SI/CIMS) with a detection limit for both compounds of 4.3 × 10 4 cm −3 at 5 min signal integration. The H 2 SO 4 and MSA gas-phase concentrations were analysed in conjunction with the condensational sink for both compounds derived from 3 nm–10 μm (diameter) aerosol size distributions. Accommodation coefficients of 1.0 for H 2 SO 4 and 0.12 for MSA were assumed leading to estimated atmospheric lifetimes of the order of 7 min and 25 min, respectively. With the SI/CIMS instrument in OH measurement mode alternating between OH signal and background (non-OH) signal evidence was obtained for the presence of one or more unknown oxidants of SO 2 in addition to OH. Depending on the nature of the oxidant(s) their ambient concentration may be enhanced in the CIMS inlet system by additional production. The apparent unknown SO 2 oxidant was additionally confirmed by direct measurements of SO 2 in conjunction with calculated H 2 SO 4 concentrations. The calculated concentrations were consistently lower than the measured concentrations by a factor 4.8 ± 3.4 when considering the oxidation of SO 2 by OH as the only source of H 2 SO 4 . Both the OH and the background signal were also observed to increase significantly during daytime aerosol nucleation events, independent of the ozone photolysis frequency, J (O 1 D), and were followed by peaks in both H 2 SO 4 and MSA concentrations. This suggests a strong relation between the unknown oxidant(s), OH chemistry, and the atmospheric photo-oxidation of biogenic iodine compounds. As to the identity of the oxidant(s), we have been able to exclude ClO, BrO, IO, and OIO as possible candidates based on ab initio calculations. Stabilized Criegee intermediates (sCI) produced from ozonolysis of alkenes potentially contribute to the oxidation efficiency of the coastal and marine atmosphere. However, analysis of the CIMS background signal in context with recently published kinetic data currently suggests that larger Criegee intermediates produced from ozonolysis play no significant role for SO 2 oxidation in the marine atmosphere. The possibility of H 2 SO 4 formation without SO 2 as precursor or from SO 2 oxidation by small sCI produced photolytically should be explored.

Beschreibung: Extensive spatio-temporal analyses of surface ozone and related meteorological variables in South Korea for 1999–2010 Atmospheric Chemistry and Physics Discussions, 14, 1191-1238, 2014 Author(s): J. Seo, D. Youn, J. Y. Kim, and H. Lee Spatio-temporal characteristics of surface ozone (O 3 ) variations over South Korea are investigated with consideration of meteorological factors and time-scales based on the Kolmogorov–Zurbenko filter (KZ-filter), using measurement data at 124 air quality monitoring sites and 72 weather stations for the 12 yr period of 1999–2010. In general, O 3 levels at coastal cities are high due to dynamic effects of the sea breeze while those at the inland and Seoul Metropolitan Area (SMA) cities are low due to the NO x titration by local precursor emissions. We examine the meteorological influences on the O 3 using a combined analysis of the KZ-filter and linear regressions between O 3 and meteorological variables. We decomposed O 3 time-series at each site into short-term, seasonal, and long-term components by the KZ-filter and regressed them on meteorological variables. Impact of temperature on the O 3 levels is significantly high in the highly populated SMA and inland region while that is low in the coastal region. In particular, the probability of high-O 3 occurrence doubled with 4 °C of temperature increase in the SMA during high-O 3 months (May to October). It implies that those regions will experience frequent high-O 3 events in the future warming climate. In terms of short-term variation, distribution of high-O 3 probability classified by wind direction shows the effect of both local precursor emissions and long-range transport from China. In terms of long-term variation, the O 3 concentrations have increased by +0.26 ppbv yr −1 on nationwide average, but their trends show large spatial variability. Additional statistical analysis of the singular value decomposition further reveals that the long-term temporal evolution of O 3 is similar to that of the nitrogen dioxide measurement although the spatial distributions of their trends are different. This study would be helpful as a reference for diagnostics and evaluation of regional- and local-scale O 3 and climate simulations and a guide to appropriate O 3 control policy in South Korea.

Beschreibung: Where do the air masses between double tropopauses come from? Atmospheric Chemistry and Physics Discussions, 14, 1349-1374, 2014 Author(s): A. C. Parracho, C. A. F. Marques, and J. M. Castanheira An analysis of the origin of air masses that end up between double tropopauses (DT) in the subtropics and midlatitudes is presented. The double tropopauses were diagnosed in the ERA-Interim reanalysis (1979–2010), and the origin of air masses was analysed using the Lagrangian model FLEXPART. Different processes for the formation of double tropopauses (DT) have been suggested in the literature. Some studies have suggested that double tropopauses may occur as a response to the vertical profile of adiabatic heating, due to the residual meridional circulation, while others have put forward contradicting explanations. Whereas some studies have suggested that double tropopauses result from poleward excursions of the tropical tropopause over the extratropical one, others have argued that DTs develop in baroclinic unstable processes involving transport of air from high latitudes. In some regions, the DT structure has a semipermanent character which cannot be explained by excursions of the tropical tropopause alone. However, the results presented in this paper confirm that processes involving excursions of the tropical tropopause over the extratropical tropopause, which are therefore accompanied by intrusions of air from the tropical troposphere into the lower extratropical stratosphere, make a significant contribution for the occurrence of DTs in the subtropics and midlatitudes. Specifically, it is shown that the air between double tropopauses comes from equatorward regions, and has a higher percentage of tropospheric particles and a lower mean potential vorticity.

Beschreibung: Interpretation of freezing nucleation experiments: singular and stochastic; sites and surfaces Atmospheric Chemistry and Physics Discussions, 14, 1711-1760, 2014 Author(s): G. Vali Publications of recent years dealing with laboratory experiments of immersion freezing reveal uncertainties about the fundamentals of heterogeneous freezing nucleation. While it appears well accepted that there are two major factors that determine the process, namely fluctuations in the size and configuration of incipient embryos of the solid phase and the role of the substrate to aid embryo formation, views have been evolving about the relative importance of these two elements. The importance of specific surface sites is being established in a growing number of experiments and a number of approaches have been proposed to incorporate these results into model descriptions. Many of these models share a common conceptual basis yet diverge in the way random and deterministic factors are combined. The divergence can be traced to uncertainty about the permanence of nucleating sites, to the lack of detailed knowledge about what surface features constitute nucleating sites, and to the consequent need to rely on empirical or parametric formulas to define the population of sites of different effectiveness. The goal of this paper is to demonstrate that recent experiments and models, consistent with earlier work, point to the existence and primary role of permanent nucleating sites and to the continued need for empirically based formulations of heterogeneous freezing. The paper focuses on three identifiably separate but interrelated issues: (i) the combination of singular and stochastic factors, (ii) the role of specific surface sites, and (iii) the modeling of heterogeneous ice nucleation.

Beschreibung: Critical assessment of meteorological conditions and airflow connectivity during HCCT-2010 Atmospheric Chemistry and Physics Discussions, 14, 1861-1917, 2014 Author(s): A. Tilgner, L. Schöne, P. Bräuer, D. van Pinxteren, E. Hoffmann, G. Spindler, S. Mertes, W. Birmili, R. Otto, M. Merkel, K. Weinhold, A. Wiedensohler, H. Deneke, W. Haunold, A. Engel, A. Wéber, and H. Herrmann This study presents a comprehensive and critical assessment of the meteorological conditions and atmospheric flow during the Lagrangian-type "Hill Cap Cloud Thuringia 2010" experiment (HCCT-2010). HCCT-2010 was performed in September and October 2010 at Mt. Schmücke in the Thuringian forest, Germany, applying three measurements sites (upwind, in-cloud, downwind) to study physical and chemical aerosol-cloud-interactions. A Lagrangian-type hill cap cloud experiment requires suitable cloud and particularly connected airflow conditions, i.e. representative air masses at the different measurement sites. Therefore, the present study aimed at the identification of time periods during the 6-weeks duration of the campaign, where such conditions were fulfilled and which can be used in further data examinations. The following topics were studied in detail: (i) the general synoptic weather situations including the mesoscale flow conditions by means of a classification of advected air masses and calculation of non-dimensional flow parameters (e.g. Froude number), (ii) local meteorological conditions, including synoptic front passages, the presence of orographic or frontal cloudiness, cloud base heights and vertical stratification, and (iii) local flow conditions by means of statistical analyses using the quasi-inert trace gas ozone and selected size bins of particle number size distributions as well as SF 6 tracer experiments in the campaign area. A comprehensive analyses using statistical measures such as the COD (Coefficient Of Divergence) and cross-correlation have been carried out for the first time in the context of a Lagrangian-type hill cap cloud experiment. Suitable criteria for the aimed statistical analyses were thus developed and applied in the present study to characterise the local flow connectivity in detail. The comprehensive examination resulted in a total of 14 so-called "Full Cloud Events" (FCE), which are shown to conform to the Lagrange-type experimental philosophy of HCCT-2010. In addition, 15 so-called "Non-Cloud Events" (NCEs) could be established, which can be used as reference cases as they provide similarly suitable flow conditions but no cloud at the summit site. Orographic cloudiness was identified for approx. one third of the FCE periods, while about two thirds were associated to synoptic fronts. The statistical flow analyses indicate the existence of a strong link between the sites during the events, particularly under constant south-westerly flow conditions, high wind speeds and slightly stable stratification. The COD analyses using continuously measured concentrations of ozone and the 49 nm diameter particle bin revealed particularly for COD values below 0.1 very consistent time series, i.e. closely linked air masses between the different sites. The cross-correlation analysis revealed under connected flow conditions typical overflow times of about 15 to 30 min between the two valley sites. Additionally, the performed SF 6 tracer experiments during the campaign clearly demonstrate that under appropriate meteorological conditions a Lagrangian-type approach is valid and that the connected flow validation procedure developed in this work is suitable for identifying such conditions. Finally, an overall evaluation of the identified FCEs is presented, which provides the basis for subsequent investigations of the measured chemical and physical data during HCCT-2010.

Beschreibung: Composition of 15–80 nm particles in marine air Atmospheric Chemistry and Physics Discussions, 14, 2087-2111, 2014 Author(s): M. J. Lawler, J. Whitehead, C. O'Dowd, C. Monahan, G. McFiggans, and J. N. Smith The chemical composition of 15–80 nm diameter particles was measured at Mace Head, Ireland, during May 2011 using the TDCIMS (Thermal Desorption Chemical Ionization Mass Spectrometer). Measurable levels of chloride, sodium, and sulfate were present in essentially all collected samples of these particles at this coastal Atlantic site. Organic compounds were rarely detectable, but this was likely an instrumental limitation. Concomitant particle hygroscopicity observations usually showed two main modes, one which contained a large sea salt component and another which was likely dominated by sulfate. There were several occasions lasting from hours to about two days during which 10–60 nm particle number increased dramatically in polar oceanic air. During these events, the sulfate mode increased substantially in number. This observation, along with the presence of very small (

Beschreibung: Characterization of OMI tropospheric NO 2 over the Baltic Sea region Atmospheric Chemistry and Physics Discussions, 14, 2021-2042, 2014 Author(s): I. Ialongo, J. Hakkarainen, N. Hyttinen, J.-P. Jalkanen, L. Johansson, F. Boersma, N. Krotkov, and J. Tamminen Satellite-based data are very important for air quality applications in the Baltic Sea area, because they provide information on air pollution over sea and there where ground-based network and aircraft measurements are not available. Both the emissions from urban sites over land and ships over sea, contribute to the tropospheric NO 2 levels. The tropospheric NO 2 monitoring at high latitudes using satellite data is challenging because of the reduced light hours in winter and the snow-covered surface, which make the retrieval complex, and because of the reduced signal due to low Sun. This work presents a detailed characterization of the tropospheric NO 2 columns focused on part of the Baltic Sea region using the Ozone Monitoring Instrument (OMI) tropospheric NO 2 standard product. Previous works have focused on larger seas and lower latitudes. The results showed that, despite the regional area of interest, it is possible to distinguish the signal from the main coastal cities and from the ships by averaging the data over a seasonal time range. The summertime NO 2 emission and lifetime values ( E = (1.0 ± 0.1) × 10 28 molec. and τ = (3.0 ± 0.5) h, respectively) in Helsinki were estimated from the decay of the signal with distance from the city center. The method developed for megacities was successfully applied to a smaller scale source, in both size and intensity (i.e., the city of Helsinki), which is located at high latitudes (∼60° N). The same methodology could be applied to similar scale cities elsewhere, as far as they are relatively isolated from other sources. The transport by the wind plays an important role in the Baltic Sea area. The NO 2 spatial distribution is mainly determined by the contribution of strong westerly winds, which dominate the wind patterns during summer. The comparison between the emissions from model calculations and OMI NO 2 tropospheric columns confirmed the applicability of satellite data for ship emission monitoring. In particular, both the emission data and the OMI observations showed similar year-to-year variability, with a drop in year 2009, corresponding to the effect of the economical crisis.

Beschreibung: Chemistry of new particle growth in mixed urban and biogenic emissions – insights from CARES Atmospheric Chemistry and Physics Discussions, 14, 2043-2085, 2014 Author(s): A. Setyan, C. Song, M. Merkel, W. B. Knighton, T. B. Onasch, M. R. Canagaratna, D. R. Worsnop, A. Wiedensohler, J. E. Shilling, and Q. Zhang Regional new particle formation and growth events (NPE) were observed on most days over the Sacramento and western Sierra Foothills area of California in June 2010 during the Carbonaceous Aerosols and Radiative Effect Study (CARES). Simultaneous particle measurements at both the T0 (Sacramento, urban site) and the T1 (Cool, rural site located ~40 km northeast of Sacramento) sites of CARES indicate that the NPE usually occurred in the morning with the appearance of an ultrafine mode centered at ~15 nm (in mobility diameter, D m , measured by a scanning mobility particle sizer operating in the range 10–858 nm) followed by the growth of this mode to ~50 nm in the afternoon. These events were generally associated with southwesterly winds bringing urban plumes from Sacramento to the T1 site. The growth rate was on average higher at T0 (7.1 ± 2.7 nm h −1 ) than at T1 (6.2 ± 2.5 nm h −1 ), likely due to stronger anthropogenic influences at T0. Using a high-resolution time-of-flight aerosol mass spectrometer (HR-ToF-AMS), we investigated the evolution of the size-resolved chemical composition of new particles at T1. Our results indicate that the growth of new particles was driven primarily by the condensation of oxygenated organic species and, to a lesser extent, ammonium sulfate. New particles appear to be fully neutralized during growth, consistent with high NH 3 concentration in the region. Nitrogen-containing organic ions (i.e., CHN + , CH 4 N + , C 2 H 3 N + , and C 2 H 4 N + ) that are indicative of the presence of alkyl-amine species in submicrometer particles enhanced significantly during the NPE days, suggesting that amines might have played a role in these events. Our results also indicate that the bulk composition of the ultrafine mode organics during NPE was very similar to that of anthropogenically-influenced secondary organic aerosol (SOA) observed in transported urban plumes. In addition, the concentrations of species representative of urban emissions (e.g., black carbon, CO, NO x , and toluene) were significantly higher whereas the photo-oxidation products of biogenic VOC and the biogenically-influenced SOA also increased moderately during the NPE days compared to the non-event days. These results indicate that the frequently occurring NPE over the Sacramento and Sierra Nevada regions were mainly driven by urban plumes from Sacramento and that the interaction of regional biogenic emissions with the urban plumes has enhanced the new particle growth. This finding has important implication for quantifying the climate impacts of NPE on global scale.

Beschreibung: Online measurements of water-soluble organic acids in the gas and aerosol phase from the photooxidation of 1,3,5-trimethylbenzene Atmospheric Chemistry and Physics Discussions, 14, 985-1018, 2014 Author(s): A. P. Praplan, K. Hegyi-Gaeggeler, P. Barmet, L. Pfaffenberger, J. Dommen, and U. Baltensperger The formation of organic acids during photooxidation of 1,3,5-trimethylbenzene (TMB) was investigated with an online ion chromatography (IC) instrument coupled to a mass spectrometer (MS) at the Paul Scherrer Institute (PSI) smog chamber. Gas and aerosol phase were both sampled. Molecular formulae were attributed to twelve compounds with the help of high resolution MS data from filter extracts (two compounds in the gas phase only, two in the aerosol phase only and eight in both). Seven of those species could be identified unambiguously (each of them present in gas and aerosol phase): formic acid, acetic acid, glycolic acid, butyric acid, pyruvic acid, lactic acid and methylmaleic acid. The influence of the precursor concentration (TMB: 1200 and 600 ppbv) and of the presence of 2 ppbv of sulphur dioxide (SO 2 ) on aerosol and gas phase products were further investigated. While the organic acid fraction present in the aerosol phase does not strongly depend on the precursor concentration (6 to 14%), the presence of SO 2 reduces this amount to less than 3% for both high and low precursor concentration scenarios. The addition of acetic acid during the experiments indicated that the presence of small acids in the particle phase might not be due to partitioning effects.

Beschreibung: Effects of dust aerosols on tropospheric chemistry during a typical pre-monsoon season dust storm in northern India Atmospheric Chemistry and Physics Discussions, 14, 1113-1158, 2014 Author(s): R. Kumar, M. C. Barth, S. Madronich, M. Naja, G. R. Carmichael, G. G. Pfister, C. Knote, G. P. Brasseur, N. Ojha, and T. Sarangi This study examines the effect of a typical pre-monsoon season dust storm on tropospheric chemistry through a case study in northern India. Dust can alter photolysis rates by scattering and absorbing solar radiation, and provide surface area for heterogeneous reactions. We use the Weather Research and Forecasting model coupled with Chemistry (WRF-Chem) to simulate the dust storm that occurred during 17–22 April 2010 and investigate the contribution of different processes on mixing ratios of several key trace gases including ozone, nitrogen oxides, hydrogen oxides, methanol, acetic acid and formaldehyde. We revised the Fast Troposphere Ultraviolet Visible (F-TUV) photolysis scheme to include effects of dust aerosols on photolysis rates in a manner consistent with the calculations of aerosol optical properties for feedback to the meteorology radiation schemes. In addition, we added twelve heterogeneous reactions on the dust surface, for which six reactions have relative humidity dependent reactive uptake coefficients (γ). The inclusion of these processes in WRF-Chem is found to reduce difference between observed and modeled ozone from 16 ± 9 to 2 ± 8 ppbv and that in NO y from 2129 ± 1425 to 372 ± 1225 pptv compared to measurements at the high altitude site Nainital in the central Himalayas, and reduce biases by up to 30% in tropospheric column NO 2 compared to OMI retrievals. The simulated dust storm acted as a sink for all the trace gases examined here and significantly perturbed their spatial and vertical distributions. The reductions in these gases are estimated as 5–100% and more than 80% of this reduction was due to heterogeneous chemistry. The RH dependence of γ is also found to have substantial impact on the distribution of trace gases, with changes of up to 20–25% in ozone and HO 2 , 50% in H 2 O 2 and 100% in HNO 3 . A set of sensitivity analyses revealed that dust aging can reduce the uptake of trace gases (especially of H 2 O 2 and acetic acid) by up to 50% in dust source regions.

Beschreibung: Evidence for an earlier greenhouse cooling effect in the stratosphere before the 1980s over the Northern Hemisphere Atmospheric Chemistry and Physics Discussions, 14, 1073-1112, 2014 Author(s): C. S. Zerefos, K. Tourpali, P. Zanis, K. Eleftheratos, C. Repapis, A. Goodman, D. Wuebbles, I. S. A. Isaksen, and J. Luterbacher This study provides a new look at the observed and calculated long-term temperature changes since 1958 for the region extending from the lower troposphere up to the lower stratosphere of the Northern Hemisphere. The analysis is mainly based on monthly layer mean temperatures derived from geopotential height thicknesses between specific pressure levels. Layer mean temperatures from thickness improve homogeneity in both space and time and reduce uncertainties in the trend analysis. Datasets used include the NCEP/NCAR I reanalysis, the Free University of Berlin (FU-Berlin) and the RICH radiosonde datasets as well as historical simulations with the CESM1-WACCM global model participating in CMIP5. After removing the natural variability with an autoregressive multiple regression model our analysis shows that the time interval of our study 1958–2011 can be divided in two distinct sub-periods of long term temperature variability and trends; before and after 1980s. By calculating trends for the summer time to reduce interannual variability, the two periods are as follows. From 1958 until 1979, non-significant trends or slight cooling trends prevail in the lower troposphere (0.06 ± 0.06 °C decade −1 for NCEP and −0.12 ± 0.06 °C decade −1 for RICH). The second period from 1980 to the end of the records shows significant warming trends (0.25 ± 0.05 °C decade −1 for both NCEP and RICH). Above the tropopause a persistent cooling trend is clearly seen in the lower stratosphere both in the pre-1980s period (−0.58 ± 0.17 °C decade −1 for NCEP, −0.30 ± 0.16 °C decade −1 for RICH and −0.48 ± 0.20 °C decade −1 for FU-Berlin) and the post-1980s period (−0.79 ± 0.18 °C decade −1 for NCEP, −0.66 ± 0.16 °C decade −1 for RICH and −0.82 ± 0.19 °C decade −1 for FU-Berlin). The cooling in the lower stratosphere is a persistent feature from the tropics up to 60 north for all months. At polar latitudes competing dynamical and radiative processes are reducing the statistical significance of these trends. Model results are in line with re-analysis and the observations, indicating a persistent cooling in the lower stratosphere during summer before and after the 1980s by −0.33 °C decade −1 ; a feature that is also seen throughout the year. However, the lower stratosphere modelled trends are generally lower than re-analysis and the observations. The contrasting effects of ozone depletion at polar latitudes in winter/spring and the anticipated strengthening of the Brewer Dobson circulation from man-made global warming at polar latitudes are discussed. Our results provide additional evidence for an early greenhouse cooling signal in the lower stratosphere before the 1980s, which it appears well in advance relative to the tropospheric greenhouse warming signal. Hence it may be postulated that the stratosphere could have provided an early warning of man-made climate change. The suitability for early warning signals in the stratosphere relative to the troposphere is supported by the fact that the stratosphere is less sensitive to changes due to cloudiness, humidity and man-made aerosols. Our analysis also indicates that the relative contribution of the lower stratosphere vs. the upper troposphere low frequency variability is important for understanding the added value of the long term tropopause variability related to human induced global warming.

Beschreibung: Electrical charging changes the composition of sulfuric acid-ammonia/dimethylamine clusters Atmospheric Chemistry and Physics Discussions, 14, 1317-1348, 2014 Author(s): I. K. Ortega, T. Olenius, O. Kupiainen-Määttä, V. Loukonen, T. Kurtén, and H. Vehkamäki Sulfuric acid clusters stabilized by base molecules are likely to have a significant role in atmospheric new particle formation. Recent advances in mass spectrometry techniques have permitted the detection of electrically charged clusters. However, direct measurement of electrically neutral clusters is not possible. Mass spectrometry instruments can be combined with a charger, but the possible effect of charging on the composition of neutral clusters must be addressed before the measured data can be linked to properties of neutral clusters. In the present work we have used formation free energies from quantum chemical methods to calculate the evaporation rates of electrically charged (both positive and negative) sulfuric acid-ammonia/dimethylamine clusters. To understand how charging will affect the composition of these clusters, we have compared the evaporation rates of charged clusters with those of the corresponding neutral clusters. We found that the only cluster studied in this paper which will retain its composition is H 2 SO 4 · NH 3 when charged positively; all other clusters will be altered by both positive and negative charging. In the case of charging clusters negatively, base molecules will completely evaporate from clusters with 1 to 3 sulfuric acid molecules in the case of ammonia, and from clusters with 1 or 2 sulfuric acid molecules in the case of dimethylamine. Larger clusters will maintain some base molecules, but the H 2 SO 4 : base ratio will increase. In the case of positive charging, some of the acid molecules will evaporate, decreasing the H 2 SO 4 : base ratio.

Beschreibung: The changing oxidizing environment in London – trends in ozone precursors and their contribution to ozone production Atmospheric Chemistry and Physics Discussions, 14, 1287-1316, 2014 Author(s): E. von Schneidemesser, M. Vieno, and P. S. Monks Ground-level ozone is recognized to be a threat to human health (WHO, 2003), have a deleterious impact on vegetation (Fowler et al., 2009), is also an important greenhouse gas (IPCC, 2007) and key to the oxidative ability of the atmosphere (Monks et al., 2009). Owing to its harmful effect on health, much policy and mitigation effort has been put into reducing its precursors – the nitrogen oxides (NO x ) and non-methane volatile organic compounds (NMVOCs). The non-linear chemistry of tropospheric ozone formation, dependent mainly on NO x and NMVOC concentrations in the atmosphere, makes controlling tropospheric ozone complex. Furthermore, the concentration of ozone at any given point is a complex superimposition of in-situ produced or destroyed ozone and transported ozone on the regional and hemispheric-scale. In order to effectively address ozone, a more detailed understanding of its origins is needed. Here we show that roughly half (5 μg m −3 ) of the observed increase in urban (London) ozone (10 μg m −3 ) in the UK from 1998 to 2008 is owing to factors of local origin, in particular, the change in NO : NO 2 ratio, NMVOC : NO x balance, NMVOC speciation, and emission reductions (including NO x titration). In areas with previously higher large concentrations of nitrogen oxides, ozone that was previously suppressed by high concentrations of NO has now been "unmasked", as in London and other urban areas of the UK. The remaining half (approximately 5 μg m −3 ) of the observed ozone increase is attributed to non-local factors such as long-term transport of ozone, changes in background ozone, and meteorological variability. These results show that a two-pronged approach, local action and regional-to-hemispheric cooperation, is needed to reduce ozone and thereby population exposure, which is especially important for urban ozone.

Beschreibung: Analysis of the effect of water activity on ice formation using a new thermodynamic framework Atmospheric Chemistry and Physics Discussions, 14, 1525-1557, 2014 Author(s): D. Barahona In this work a new thermodynamic framework is developed and used to investigate the effect of water activity on the formation of ice within supercooled droplets. The new framework is based on a novel concept where the interface is assumed to be made of liquid molecules "trapped" by the solid matrix. Using this concept new expressions are developed for the critical ice germ size and the nucleation work, with explicit dependencies on temperature and water activity. However unlike previous approaches, the new model does not depend on the interfacial tension between liquid and ice. Comparison against experimental results shows that the new theory is able to reproduce the observed effect of water activity on nucleation rate and freezing temperature. It allows for the first time a phenomenological derivation of the constant shift in water activity between melting and nucleation. The new framework offers a consistent thermodynamic view of ice nucleation, simple enough to be applied in atmospheric models of cloud formation.

Beschreibung: Technical Note: Adjoint formulation of the TOMCAT atmospheric transport scheme in the Eulerian backtracking framework (RETRO-TOM) Atmospheric Chemistry and Physics Discussions, 14, 1481-1524, 2014 Author(s): P. E. Haines, J. G. Esler, and G. D. Carver A new methodology for the formulation of an adjoint to the transport component of the chemistry transport model TOMCAT is described and implemented in a new model RETRO-TOM. The Eulerian backtracking method is used, allowing the forward advection scheme (Prather's second-order moments), to be efficiently exploited in the backward adjoint calculations. Prather's scheme is shown to be time-symmetric suggesting the possibility of high accuracy. To attain this accuracy, however, it is necessary to make a careful treatment of the "density inconsistency" problem inherent to offline transport models. The results are verified using a series of test experiments. These demonstrate the high accuracy of RETRO-TOM when compared with direct forward sensitivity calculations, at least for problems in which flux-limiters in the advection scheme are not required. RETRO-TOM therefore combines the flexibility and stability of a "finite difference of adjoint" formulation with the accuracy of an "adjoint of finite difference" formulation.

Beschreibung: One-year observations of size distribution characteristics of major aerosol constituents at a coastal receptor site in Hong Kong – Part 1: Inorganic ions and oxalate Atmospheric Chemistry and Physics Discussions, 14, 1443-1480, 2014 Author(s): Q. Bian, X. H. H. Huang, and J. Z. Yu Size distribution data of major aerosol constituents are essential in source apportioning of visibility degradation, testing and verification of air quality models incorporating aerosols. We report here one-year observations of mass size distributions of major inorganic ions (sulfate, nitrate, chloride, ammonium, sodium, potassium, magnesium and calcium) and oxalate at a coastal suburban receptor site in Hong Kong, China. A total of 43 sets of size segregated samples in the size range of 0.056–18 μm were collected from March 2011 to February 2012. The size distributions of sulfate, ammonium, potassium and oxalate were characterized by a dominant droplet mode with a mass mean aerodynamic diameter (MMAD) in the range of ~0.7–0.9 μm. Oxalate had a slightly larger MMAD than sulfate on days with temperatures above 22 °C as a result of the process of volatilization and repartitioning. Nitrate was mostly dominated by the coarse mode but enhanced presence in fine mode was detected on winter days with lower temperature and lower concentrations of sea salt and soil particles. This data set reveals an inversely proportional relationship between the fraction of nitrate in the fine mode and product of the sum of sodium and calcium in equivalent concentrations and the dissociation constant of ammonium nitrate (i.e., (1/[Na + ] + 2[Ca 2+ ]) × (1/ K e ')). The seasonal variation observed for sea salt aerosol abundance, with lower values in summer and winter, is possibly linked with the lower marine salinities in these two seasons. Positive matrix factorization was applied to estimate the relative contributions of local formation and transport to the observed ambient sulfate level through the use of the combined datasets of size-segregated sulfate and select gaseous air pollutants. On average, the regional/super-regional transport of air pollutants was the dominant source at this receptor site, especially on high sulfate days, while local formation processes contributed approximately 30% of the total sulfate. This work provides field measurement-based evidence for importance of understanding both local photochemistry and regional/super-regional transport in order to properly simulate sulfate aerosols in air quality models.

Beschreibung: Global investigation of the Mg atom and ion layers using SCIAMACHY/Envisat observations between 70 km and 150 km altitude and WACCM-Mg model results Atmospheric Chemistry and Physics Discussions, 14, 1971-2019, 2014 Author(s): M. Langowski, C. von Savigny, J. P. Burrows, W. Feng, J. M. C. Plane, D. R. Marsh, D. Janches, M. Sinnhuber, and A. C. Aikin Mg and Mg + concentration fields in the upper mesosphere/lower thermosphere (UMLT) region are retrieved from SCIAMACHY/Envisat limb measurements of Mg and Mg + dayglow emissions using a 2-D tomographic retrieval approach. The time series of monthly means of Mg and Mg + for number density as well as vertical column density in different latitudinal regions are shown. Data from the limb mesosphere-thermosphere mode of SCIAMACHY/Envisat are used, which covers the 50 km to 150 km altitude region with a vertical sampling of 3.3 km and a highest latitude of 82°. The high latitudes are not covered in the winter months, because there is no dayglow emission during polar night. The measurements were performed every 14 days from mid-2008 until April 2012. Mg profiles show a peak at around 90 km altitude with a density between 750 cm −3 and 2000 cm −3 . Mg does not show strong seasonal variation at mid-latitudes. The Mg + peak occurs 5–15 km above the neutral Mg peak at 95–105 km. Furthermore, the ions show a significant seasonal cycle with a summer maximum in both hemispheres at mid- and high-latitudes. The strongest seasonal variations of the ions are observed at mid-latitudes between 20–40° and densities at the peak altitude range from 500 cm −3 to 6000 cm −3 . The peak altitude of the ions shows a latitudinal dependence with a maximum at mid-latitudes that is up to 10 km higher than the peak altitude at the equator. The SCIAMACHY measurements are compared to other measurements and WACCM model results. In contrast to the SCIAMACHY results, the WACCM results show a strong seasonal variability for Mg with a winter maximum, which is not observable by SCIAMACHY, and globally higher peak densities. Although the peak densities do not agree the vertical column densities agree, since SCIAMACHY results show a wider vertical profile. The agreement of SCIAMACHY and WACCM results is much better for Mg + , showing the same seasonality and similar peak densities. However, there are the following minor differences: there is no latitudinal dependence of the peak altitude for WACCM and the density maximum, passing the equatorial region during equinox conditions, is not reduced as for SCIAMACHY.

Beschreibung: Distinguishing molecular characteristics of aerosol water soluble organic matter from the 2011 trans-North Atlantic US GEOTRACES cruise Atmospheric Chemistry and Physics Discussions, 14, 6427-6470, 2014 Author(s): A. S. Wozniak, A. S. Willoughby, S. C. Gurganus, and P. G. Hatcher The molecular characteristics of aerosol organic matter (OM) determines to a large extent its impacts on the atmospheric radiative budget and ecosystem function in terrestrial and aquatic environments, yet the molecular details of aerosol OM from different sources are not well established. Aerosol particulate samples having North American-influenced, North African-influenced, and marine (minimal recent continental influence) air mass back trajectories were collected as part of the 2011 trans-North Atlantic US GEOTRACES cruise and analyzed for their water soluble OM (WSOM) molecular characteristics using electrospray ionization Fourier transform ion cyclotron resonance mass spectrometry. Principal component analysis (PCA) separated the samples into five groups defined by distinct molecular formula characteristics. An abundance of nitrogen containing compounds with molecular formulas consistent with amino acid derivatives defined the two samples comprising the Primary Marine group suggesting a primary marine biological source to their WSOM in spite of their North American-influenced air mass trajectories. A second group of samples (Aged Marine) having primarily marine air mass trajectories was characterized by an abundance of low O / C (0.15–0.45) sulfur containing compounds consistent with organosulfate compounds formed via secondary aging reactions in the atmosphere. Several samples having North American-influenced air mass trajectories formed another group again characterized by organosulfate and nitrooxyorganosulfate type compounds with higher O / C ratios (0.5–1.0) than the Aged Marine samples reflecting the combustion influence from the North American continent. All the samples having North African-influenced air mass trajectories grouped together in the PCA and were characterized by a lack of heteroatom (N, S, P) containing molecular formulas covering a wide O / C range (0.15–0.90) reflecting the desert source of this WSOM. The two marine groups showed molecular formulas that, on average, had higher O / C ratios and lower O / C ratios and modified aromaticity indices than the two continentally-influenced groups suggesting these properties are characteristic of marine vs. continental aerosol WSOM. The final sample group, the Mixed Source samples, showed intermediate molecular characteristics suggesting no dominant continental or marine source. The source-specific OM details described here will aid efforts to link aerosol OM source with molecular characteristics and impacts in the environment.

Beschreibung: An improved dust emission model with insights into the global dust cycle's climate sensitivity Atmospheric Chemistry and Physics Discussions, 14, 6361-6425, 2014 Author(s): J. F. Kok, N. M. Mahowald, S. Albani, G. Fratini, J. A. Gillies, M. Ishizuka, J. F. Leys, M. Mikami, M.-S. Park, S.-U. Park, R. S. Van Pelt, D. S. Ward, and T. M. Zobeck Simulations of the global dust cycle and its interactions with a changing Earth system are hindered by the empirical nature of dust emission parameterizations in climate models. Here we take a step towards improving global dust cycle simulations by presenting a physically-based dust emission model. The resulting dust flux parameterization depends only on the wind friction speed and the soil's threshold friction speed, and can therefore be readily implemented into climate models. We show that our parameterization's functional form is supported by a compilation of quality-controlled vertical dust flux measurements, and that it better reproduces these measurements than existing parameterizations. Both our theory and measurements indicate that many climate models underestimate the dust flux's sensitivity to soil erodibility. This finding can explain why dust cycle simulations in many models are improved by using an empirical preferential sources function that shifts dust emissions towards the most erodible regions. In fact, implementing our parameterization in a climate model produces even better agreement against aerosol optical depth measurements than simulations that use such a source function. These results indicate that the need to use a source function is at least partially eliminated by the additional physics accounted for by our parameterization. Since soil erodibility is affected by climate changes, our results further suggest that many models have underestimated the climate sensitivity of the global dust cycle.

Beschreibung: Global emissions of HFC-143a (CH 3 CF 3 ) and HFC-32 (CH 2 F 2 ) from in situ and air archive atmospheric observations Atmospheric Chemistry and Physics Discussions, 14, 6471-6500, 2014 Author(s): S. O'Doherty, M. Rigby, J. Mühle, D. J. Ivy, B. R. Miller, D. Young, P. G. Simmonds, S. Reimann, M. K. Vollmer, P. B. Krummel, P. J. Fraser, L. P. Steele, B. Dunse, P. K. Salameh, C. M. Harth, T. Arnold, R. F. Weiss, J. Kim, S. Park, S. Li, C. Lunder, O. Hermansen, N. Schmidbauer, L. X. Zhou, B. Yao, R. H. J. Wang, A. Manning, and R. G. Prinn High frequency, in situ observations from the Advanced Global Atmospheric Gases Experiment (AGAGE), for the period 2003 to 2012, combined with archive flask measurements dating back to 1977, have been used to capture the rapid growth of HFC-143a (CH 3 CF 3 ) and HFC-32 (CH 2 F 2 ) mole fractions and emissions into the atmosphere. Here we report the first in situ global measurements of these two gases. HFC-143a and HFC-32 are the third and sixth most abundant HFCs respectively and they currently make an appreciable contribution to the HFCs in terms of atmospheric radiative forcing (1.7 and 0.7 mW m 2 in 2012, respectively). In 2012 the global average mole fraction of HFC-143a was 13.4 ± 0.3 ppt (1-sigma) in the lower troposphere and its growth rate was 1.4 ± 0.04 ppt yr −1 ; HFC-32 had a global mean mole fraction of 6.2 ± 0.2 ppt and a growth rate of 1.1 ± 0.04 ppt yr −1 in 2012. The extensive observations presented in this work have been combined with an atmospheric transport model to simulate global atmospheric abundances and derive global emission estimates. It is estimated that 23 ± 3 Gg yr −1 of HFC-143a and 21 ± 11 Gg yr −1 of HFC-32 were emitted globally in 2012, and the emission rates are estimated to be increasing 7 ± 5% yr −1 for HFC-143a and 14 ± 11% yr −1 for HFC-32.

Beschreibung: New emission factors for Australian vegetation fires measured using open-path Fourier transform infrared spectroscopy – Part 2: Australian tropical savanna fires Atmospheric Chemistry and Physics Discussions, 14, 6311-6360, 2014 Author(s): T. E. L. Smith, C. Paton-Walsh, C. P. Meyer, G. D. Cook, S. W. Maier, J. Russell-Smith, M. J. Wooster, and C. P. Yates Savanna fires contribute approximately 40–50% of total global annual biomass burning carbon emissions. Recent comparisons of emission factors from different savanna regions have highlighted the need for a regional approach to emission factor development, and better assessment of the drivers of the temporal and spatial variation in emission factors. This paper describes the results of open-path Fourier Transform Infrared (OP-FTIR) spectroscopic field measurements at twenty-one fires occurring in the tropical savannas of the Northern Territory, Australia, within different vegetation assemblages and at different stages of the dry season. Spectra of infrared light passing through a long (22–70 m) open-path through ground-level smoke released from these fires were collected using an infrared lamp and a field-portable FTIR system. The IR spectra were used to retrieve the mole fractions of fourteen different gases present within the smoke, and these measurements used to calculate the emission ratios and emission factors of the various gases emitted by the burning. Only a handful of previous emission factor measures are available specifically for the tropical savannas of Australia and here we present the first reported emission factors for methanol, acetic acid, and formic acid for this biome. Given the relatively large sample size, it was possible to study the potential causes of the within-biome variation of the derived emission factors. We find that the emission factors vary substantially between different savanna vegetation assemblages; with a majority of this variation being mirrored by variations in the modified combustion efficiency (MCE) of different vegetation classes. We conclude that a significant majority of the variation in the emission factor for trace gases can be explained by MCE, irrespective of vegetation class, as illustrated by variations in the calculated methane emission factor for different vegetation classes using data subsetted by different combustion efficiencies. Therefore, the selection of emission factors for emissions modelling purposes need not necessarily require detailed fuel type information, if data on MCE (e.g. from future spaceborne total column measurements) or a correlated variable were available. From measurements at twenty-one fires, we recommend the following emission factors for Australian tropical savanna fires (in grams of gas emitted per kilogram of dry fuel burned) which are our mean measured values: 1674 g kg −1 of carbon dioxide; 87 g kg −1 of carbon monoxide; 2.1 g kg −1 of methane; 0.11 g kg −1 of acetylene; 0.49 g kg −1 of ethylene; 0.08 g kg −1 of ethane; 1.57 g kg −1 of formaldehyde; 1.06 g kg −1 of methanol; 1.54 g kg −1 of acetic acid; 0.16 g kg −1 of formic acid; 0.53 g kg −1 of hydrogen cyanide; and 0.70 g kg −1 of ammonia.

Beschreibung: Evaluation of tropospheric SO 2 retrieved from MAX-DOAS measurements in Xianghe, China Atmospheric Chemistry and Physics Discussions, 14, 6501-6536, 2014 Author(s): T. Wang, F. Hendrick, P. Wang, G. Tang, K. Clémer, H. Yu, C. Fayt, C. Hermans, C. Gielen, G. Pinardi, N. Theys, H. Brenot, and M. Van Roozendael Ground-based Multi-Axis Differential Optical Absorption Spectroscopy (MAX-DOAS) measurements of sulfur dioxide (SO 2 ) have been performed at the Xianghe station (39.75° N, 116.96° E) located at ~50 km southeast of Beijing from March 2010 to February 2013. Tropospheric SO 2 vertical profiles and corresponding vertical column densities (VCDs), retrieved by applying the Optimal Estimation Method to the MAX-DOAS observations, have been used to study the seasonal and diurnal cycles of SO 2 , in combination to correlative measurements from in situ instruments, as well as meteorological data. A marked seasonality was observed in both SO 2 VCD and surface concentration, with a maximum in winter (February) and a minimum in summer (July). This can be explained by the larger emissions in winter due to the domestic heating and more favorable meteorological conditions for the accumulation of SO 2 close to the ground during this period. Wind speed and direction are also found to be two key factors in controlling the level of the SO 2 -related pollution at Xianghe. In the case of east or southwest wind, the SO 2 concentration rises with the increase of the wind speed, since heavy polluting industries are located to the east and southwest of the station. In contrast, when wind comes from other directions, the stronger the wind, the less SO 2 is observed. Regarding the diurnal cycle, the SO 2 amount is larger in the early morning and late evening and lower at noon, in line with the diurnal variation of pollutant emissions and atmospheric stability. The observed diurnal cycles of MAX-DOAS SO 2 surface concentration are also in very good agreement (correlation coefficient close to 0.9) with those from collocated in-situ data, demonstrating the reliability and robustness of our retrieval.

Beschreibung: Comparison of GEOS-5 AGCM planetary boundary layer depths computed with various definitions Atmospheric Chemistry and Physics Discussions, 14, 6589-6617, 2014 Author(s): E. L. McGrath-Spangler and A. Molod Accurate models of planetary boundary layer (PBL) processes are important for forecasting weather and climate. The present study compares seven methods of calculating PBL depth in the GEOS-5 atmospheric general circulation model (AGCM) over land. These methods depend on the eddy diffusion coefficients, bulk and local Richardson numbers, and the turbulent kinetic energy. The computed PBL depths are aggregated to the Köppen climate classes, and some limited comparisons are made using radiosonde profiles. Most methods produce similar midday PBL depths, although in the warm, moist climate classes, the bulk Richardson number method gives midday results that are lower than those given by the eddy diffusion coefficient methods. Additional analysis revealed that methods sensitive to turbulence driven by radiative cooling produce greater PBL depths, this effect being most significant during the evening transition. Nocturnal PBLs based on Richardson number are generally shallower than eddy diffusion coefficient based estimates. The bulk Richardson number estimate is recommended as the PBL height to inform the choice of the turbulent length scale, based on the similarity to other methods during the day, and the improved nighttime behavior.

Beschreibung: TEM analysis of the internal structures and mineralogy of Asian dust particles and the implications for optical modeling Atmospheric Chemistry and Physics Discussions, 14, 6619-6661, 2014 Author(s): G. Y. Jeong and T. Nousiainen Mineral dust interacts with incoming/outgoing electromagnetic radiation in the atmosphere. This interaction depends on the microphysical properties of the dust particles, including size, mineral composition, external morphology, and internal structure. Ideally all these properties should be accounted for in dust remote sensing, the modeling of single-scattering properties, and radiative effect assessment. There have been many reports on the microphysical characterizations of mineral dust, but no investigations of the internal structures or mineral composition of individual dust particles. We explored the interiors of Asian dust particles using the combined application of focused ion beam thin-slice preparation and high-resolution transmission electron microscopy. The results showed that individual dust particles consisted of numerous mineral grains, which were organized into several types of internal structure: single and polycrystalline cores of quartz, feldspars, calcite, and amphibole often with oriented clay coatings; individual clay agglomerates of nano-thin clay platelets showing preferred to random orientations commonly with coarser mineral inclusions; and platy coarse phyllosilicates (muscovite, biotite, and chlorite). Micron to submicron pores were scattered throughout the interior of particles. Clays in the coatings and agglomerates were dominated by nano-thin platelets of the clay minerals of illite-smectite series including illite, smectite, and their mixed layers with subordinate kaolinite and clay-size chlorite. Submicron iron oxide grains, dominantly goethite, were distributed throughout the clay agglomerates and coatings. Unlike the common assumptions and simplifications, we found that the analyzed dust particles were irregularly shaped with birefringent, polycrystalline, and polymineralic heterogeneous compositions. Accounting for this structural and mineralogical makeup may improve the remote sensing retrieval of dust and the evaluation of radiation effects, but will also require sophisticated single-scattering modeling. In particular, the observed internal structures of dust particles such as clay coatings, preferred orientation, embedded grains in clays, and pores, likely have a great impact on the light scattering of dust particles. The distribution and size of structural components with contrasting dielectric properties, such as iron oxides, should also be explicitly accounted for.

Beschreibung: Global and regional modeling of clouds and aerosols in the marine boundary layer during VOCALS: the VOCA Intercomparison Atmospheric Chemistry and Physics Discussions, 14, 6537-6587, 2014 Author(s): M. C. Wyant, C. S. Bretherton, R. Wood, G. R. Carmichael, A. Clarke, J. Fast, R. George, W. I. Gustafson Jr., C. Hannay, A. Lauer, Y. Lin, J.-J. Morcrette, J. Mulcahy, P. E. Saide, S. N. Spak, and Q. Yang A diverse collection of models are used to simulate the marine boundary layer in the Southeast Pacific region during the period of the October–November 2008 VOCALS REx field campaign. Regional models simulate the period continuously in boundary-forced free-running mode, while global forecast models and GCMs are run in forecast mode. The models are compared to extensive observations along a line at 20° S extending westward from the South American coast. Most of the models simulate cloud and aerosol characteristics and gradients across the region that are recognizably similar to observations, despite the complex interaction of processes involved in the problem, many of which are parameterized or poorly resolved. Some models simulate the regional low cloud cover well, though many models underestimate MBL depth near the coast. Most models qualitatively simulate the observed offshore gradients of SO 2 , sulfate aerosol, CCN concentration in the MBL, and the related gradient in cloud droplet concentrations, but there are large quantitative intermodel differences in both means and gradients of these quantities. Most models underestimate large CCN (at 0.1% supersaturation) in the MBL and free troposphere. The GCMs also have difficulty simulating coastal gradients in CCN and cloud droplet number concentration. The overall performance of the models demonstrates their potential utility in simulating aerosol-cloud interactions in the MBL, though quantitative estimation of aerosol-cloud interactions and aerosol indirect effects of MBL clouds with these models remains uncertain.

Beschreibung: Hydroxy nitrate production in the OH-initiated oxidation of alkenes Atmospheric Chemistry and Physics Discussions, 14, 6721-6757, 2014 Author(s): A. P. Teng, J. D. Crounse, L. Lee, J. M. St. Clair, R. C. Cohen, and P. O. Wennberg Alkenes generally react rapidly by addition of OH and subsequently O 2 to form beta hydroxy peroxy radicals. These peroxy radicals react with NO to form beta hydroxy nitrates with a branching ratio α. We quantify α for C 2 –C 8 alkenes at 296 K ±3 and 993 hPa. The branching ratio can be expressed as α = (0.042 ± 0.008) × N − (0.11 ± 0.04) where N is the number of heavy atoms (excluding the peroxy moiety), and listed errors are 2σ. These branching ratios are larger than previously reported and are similar to those for peroxy radicals formed from H abstraction from alkanes. We find the isomer distributions of beta hydroxy nitrates formed under NO-dominated peroxy radical chemistry to be similar to the isomer distribution of hydroxy hydroperoxides produced under HO 2 -dominated peroxy radical chemistry. With the assumption of unity yield for the hydroperoxides, this implies that the branching ratio to form beta hydroxy nitrates from primary, secondary, and tertiary RO 2 are similar. Deuterium substitution enhances the branching ratio to form hydroxy nitrates in both propene and isoprene by a factor of ~1.5. These observations provide further evidence for importance of the ROONO lifetime in determining the branching ratio to form alkyl nitrates. We use these measurements to re-evaluate the role of alkene chemistry in the Houston region. We find that small alkenes play a larger role in oxidant formation than previously recognized.

Beschreibung: Links between satellite retrieved aerosol and precipitation Atmospheric Chemistry and Physics Discussions, 14, 6821-6861, 2014 Author(s): E. Gryspeerdt, P. Stier, and D. G. Partridge Many theories have been proposed detailing how aerosols might impact precipitation, predicting both increases and decreases depending on the prevailing meteorological conditions and aerosol type. In convective clouds, increased aerosol concentrations have been speculated to invigorate convective activity. Previous studies have shown large increases in precipitation with increasing aerosol optical depth, concluding an aerosol effect on precipitation. Our analysis reveals that these studies may have been influenced by cloud effects on the retrieved aerosol, as well as by meteorological covariations. We use a regime-based approach to separate out different cloud regimes, allowing the study of aerosol-cloud interactions in individual cloud regimes. We account for the influence of cloud properties on the aerosol retrieval and make use of the diurnal sampling of the TRMM satellite and the TRMM merged precipitation product to investigate the precipitation development. We find that whilst there is little effect on precipitation at the time of the aerosol retrieval, there is an increase in precipitation from cloud in high aerosol environments in the 6 h after the aerosol retrieval, consistent with the invigoration hypothesis. Increases in lightning flash count with increased aerosol are also observed in this period. The invigoration effect appears to be dependent on the cloud top temperature, with clouds with tops colder than 0 °C showing increases in precipitation at times after the retrieval as well as increases in wet scavenging. Warm clouds show little change in precipitation development with increasing aerosol, suggesting ice processes are important for the invigoration of precipitation.

Beschreibung: Impacts of climate and emission changes on nitrogen deposition in Europe: a multi-model study Atmospheric Chemistry and Physics Discussions, 14, 6663-6720, 2014 Author(s): D. Simpson, C. Andersson, J.H. Christensen, M. Engardt, C. Geels, A. Nyiri, M. Posch, J. Soares, M. Sofiev, P. Wind, and J. Langner The impact of climate and emissions changes on the deposition of reactive nitrogen (Nr) over Europe was studied using four offline regional chemistry transport models (CTMs) driven by the same global projection of future climate over the period 2000–2050. Anthropogenic emissions for the years 2005 and 2050 were used for simulations of both present and future periods in order to isolate the impact of climate change, hemispheric boundary conditions and emissions, and to assess the robustness of the result across the different models. The results from these four CTMs clearly show that the main driver of future N-deposition changes is the specified emission change. Under the specified emission scenario for 2050, emissions of oxidised nitrogen were reduced substantially, whereas emissions of NH 3 increase to some extent, and these changes are largely reflected in the modelled concentrations and depositions. The lack of sulphur and oxidised nitrogen in the future atmosphere results in a much larger fraction of NH x being present in the form of gaseous ammonia. Predictions for wet and total deposition were broadly consistent, although the three fine-scale models resolve European emission areas and changes better than the hemispheric-scale model. The biggest difference in the models is for predictions of individual N-compounds. One model (EMEP) was used to explore changes in critical loads, also in conjunction with speculative climate-induced increases in NH 3 emissions. These calculations suggest that the area of ecosystems which exceed critical loads is reduced from 64% for year 2005 emissions levels to 50% for currently estimated 2050 levels. A possible climate-induced increase in NH 3 emissions could worsen the situation, with areas exceeded increasing again to 57% (for a 30% NH 3 emission increase).

Beschreibung: A global non-hydrostatic model study of a downward coupling through the tropical tropopause layer during a stratospheric sudden warming Atmospheric Chemistry and Physics Discussions, 14, 6803-6820, 2014 Author(s): N. Eguchi, K. Kodera, and T. Nasuno The dynamical coupling process between the stratosphere and troposphere in the tropical tropopause layer (TTL) during a stratospheric sudden warming (SSW) in boreal winter was investigated using simulation data from a global non-hydrostatic model (NICAM) that does not use cumulus parameterization. The model reproduced well the observed tropical tropospheric changes during the SSW including the enhancement of convective activity following the amplification of planetary waves. Deep convective activity was enhanced in the latitude zone 20–10° S, in particular over the southwest Pacific and southwest Indian Ocean. Although the upwelling in the TTL was correlated with that in the stratosphere, the temperature tendency in the TTL was mainly controlled by diabatic heating originating from cloud formation. This result suggests that the stratospheric meridional circulation affects cloud formation in the TTL.

Beschreibung: Hygroscopic properties and mixing state of aerosol measured at the high altitude site Puy de Dôme (1465 m a.s.l.), France Atmospheric Chemistry and Physics Discussions, 14, 6759-6802, 2014 Author(s): H. Holmgren, K. Sellegri, M. Hervo, C. Rose, E. Freney, P. Villani, and P. Laj A Hygroscopicity Tandem Differential Mobility Analyzer (HTDMA) was used to evaluate the hygroscopic properties of aerosol particles measured at the Puy de Dôme research station in central France from September 2008 to December 2012. This high-altitude site is ideally situated to allow for both the upper part of the planetary boundary layer and the lower free troposphere to be sampled. The aim of the study is to investigate both the influence of year-to-year, seasonal, and diurnal cycles, as well as the influence of air mass type on particle hygroscopicity and mixing state. Results show that particle hygroscopicity increases with particle size and depends both on air mass type and on season. Average growth factor values are lowest in winter (1.21 ± 0.13, 1.23 ± 0.18 and 1.38 ± 0.25 for 25, 50 and 165 nm particles, respectively) and highest in autumn (1.27 ± 0.11, 1.32 ± 0.12 and 1.49 ± 0.15 for 25, 50 and 165 nm particles, respectively). Particles are generally more hygroscopic at night than during the day. The seasonal and diurnal variations are likely to be strongly influenced by boundary layer dynamics. Furthermore, particles originating from oceanic and continental regions tend to be more hygroscopic than those measured in African and local air masses. The high hygroscopicity of marine aerosol may be explained by large proportions of inorganic aerosol and sea salts, and it is speculated that continental particles are more hygroscopic than local and African ones due to ageing of fresh combustion aerosol. Aerosol measured at the Puy de Dôme display a high degree of external mixing, and hygroscopic growth spectra can be divided into three different hygroscopic modes: a less hygroscopic mode (GF 〈 1.3), a hygroscopic mode (GF 1.3–1.7) and a more hygroscopic mode (GF 〉 1.7). The majority of particles measured can be classified as being in either the less hygroscopic mode or the hygroscopic mode, and only few of them have more hygroscopic properties. The degree of external mixing, evaluated as the fraction of time when the aerosol is found with two or more populations with different hygroscopic properties, is found to increase with particle size (average yearly values are 22, 33 and 49% for 25, 50, and 165 nm particles, respectively). The degree of external mixing is more sensitive to season than to air mass type, and it is higher in the cold seasons than in the warm seasons. This study gathers the results from one of the longest data sets of hygroscopic growth factor measurements to date, allowing a statistically relevant hygroscopic growth parameterization to be determined as a function of both air mass type and season.

Beschreibung: Long-term trends in aerosol and precipitation composition over the western North Atlantic Ocean at Bermuda Atmospheric Chemistry and Physics Discussions, 14, 7025-7066, 2014 Author(s): W. C. Keene, J. L. Moody, J. N. Galloway, J. M. Prospero, O. R. Cooper, S. Eckhardt, and J. R. Maben Since the 1980s, emissions of SO 2 and NO x (NO + NO 2 ) from anthropogenic sources in the United States (US) and Europe have decreased significantly suggesting that the export of oxidized S and N compounds from surrounding continents to the atmosphere overlying North Atlantic Ocean (NAO) has also decreased. The chemical compositions of aerosols and precipitation sampled daily on Bermuda (32.27° N, 64.87° W) from 1989 to 1997 and from 2006 to 2009 were evaluated to quantify the magnitudes, significance, and implications of associated tends in atmospheric composition. The chemical data were stratified based on FLEXPART retroplumes into four discrete transport regimes: westerly flow from the eastern North America (NEUS/SEUS); easterly trade-wind flow from northern Africa and the subtropical NAO (Africa); long, open-ocean, anticyclonic flow around the Bermuda High (Oceanic); and transitional flow from the relatively clean open ocean to the polluted northeastern US (North). Based on all data, annual average concentrations of non-sea-salt (nss) SO 4 2- associated with aerosols and annual VWA concentrations in precipitation decreased significantly (by 22 and 49%, respectively) whereas annual VWA concentrations of NH 4 + in precipitation increased significantly (by 70%). Corresponding trends in aerosol and precipitation NO 3 - and of aerosol NH 4 + were insignificant. Nss SO 4 2- in precipitation under NEUS/SEUS and Oceanic flow decreased significantly (61% each) whereas corresponding trends in particulate nss SO 4 2- under both flow regimes were insignificant. Trends for precipitation were driven in part by decreasing emissions of SO 2 over upwind continents and associated decreases in anthropogenic contributions to nss SO 4 2- concentrations. Under NEUS/SEUS and Oceanic flow, the ratio of anthropogenic to biogenic contributions to to nss SO 4 2- in the column scavenged by precipitation were relatively greater than those in near surface aerosol, which implies that, for these flow regimes, precipitation is a better indicator of overall anthropogenic impacts on the lower troposphere. Particulate nss SO 4 2- under African flow also decreased significantly (34%) whereas the corresponding decrease in nss SO 4 2- associated with precipitation was marginally insignificant. We infer that these trends were driven in part by reductions in the emissions and transport of oxidized S compounds from Europe. The lack of significant trends in NO 3 - associated with aerosols and precipitation under NEUS/SEUS flow is notable in light of the large decrease (39%) in NO x emissions in the US over the period of record. Rapid chemical processing of oxidized N in marine air contributed to this lack of correspondence. Decreasing ratios of nss SO 4 2- to NH 4 + and the significant decreasing trend in precipitation acidity (37%) indicate that the total amount of acidity in the multiphase gas-aerosol system in the western NAO troposphere decreased over the period of record. Decreasing aerosol acidities would have shifted the phase partitioning of total NH 3 (NH 3 + particulate NH 4 + ) towards the gas phase thereby decreasing the atmospheric lifetime of total NH 3 against wet plus dry deposition. The trend of increasing NH 4 + in precipitation at Bermuda over the period of record suggests that NH 3 emissions from surrounding continents also increased. Decreasing particulate nss SO 4 2- in near-surface air under NEUS/SEUS flow over the period of record suggests a lower limit for net warming in the range of 0.1–0.3 W m -2 resulting from the decreased shortwave scattering and absorption by nss SO 4 2- and associated aerosol constituents.

Beschreibung: Improved model of isoprene emissions in Africa using OMI satellite observations of formaldehyde: implications for oxidants and particulate matter Atmospheric Chemistry and Physics Discussions, 14, 6951-6979, 2014 Author(s): E. A. Marais, D. J. Jacob, A. Guenther, K. Chance, T. P. Kurosu, J. G. Murphy, C. E. Reeves, and H. O. T. Pye We use a 2005–2009 record of isoprene emissions over Africa derived from OMI satellite observations of formaldehyde (HCHO) to better understand the factors controlling isoprene emission on the scale of the continent and evaluate the impact of isoprene emissions on atmospheric composition in Africa. OMI-derived isoprene emissions show large seasonality over savannas driven by temperature and leaf area index (LAI), and much weaker seasonality over equatorial forests driven by temperature. The commonly used MEGAN (version 2.1) global isoprene emission model reproduces this seasonality but is biased high, particularly for equatorial forests, when compared to OMI and relaxed-eddy accumulation measurements. Isoprene emissions in MEGAN are computed as the product of an emission factor E o , LAI, and activity factors dependent on environmental variables. We use the OMI-derived emissions to provide improved estimates of E o that are in good agreement with direct leaf measurements from field campaigns ( r = 0.55, bias = −19%). The largest downward corrections to MEGAN E o values are for equatorial forests and semi-arid environments, and this is consistent with latitudinal transects of isoprene over West Africa from the AMMA aircraft campaign. Total emission of isoprene in Africa is estimated to be 77 Tg C a −1 , compared to 104 Tg C a −1 in MEGAN. Simulations with the GEOS-Chem oxidant-aerosol model suggest that isoprene emissions increase mean surface ozone in West Africa by up to 8 ppbv, and particulate matter by up to 1.5 μg m −3 , due to coupling with anthropogenic influences.

Beschreibung: A global model simulation of present and future nitrate aerosols and their direct radiative forcing of climate Atmospheric Chemistry and Physics Discussions, 14, 6863-6949, 2014 Author(s): D. A. Hauglustaine, Y. Balkanski, and M. Schulz The ammonia cycle and nitrate particle formation have been introduced in the LMDz-INCA global model. Both fine nitrate particles formation in the accumulation mode and coarse nitrate forming on existing dust and sea-salt particles are considered. The model simulates distributions of nitrates and related species in agreement with previous studies and observations. The calculated present-day total nitrate direct radiative forcing since the pre-industrial is −0.056 W m −2 . This forcing has the same magnitude than the forcing associated with organic carbon particles and represents 18% of the sulfate forcing. Fine particles largely dominate the nitrate forcing representing close to 90% of this value. The model has been used to investigate the future changes in nitrates and direct radiative forcing of climate based on snapshot simulations for the four Representative Concentration Pathway (RCP) scenarios and for the 2030, 2050 and 2100 time horizons. Due to a decrease in fossil fuel emissions in the future, the concentrations of most of the species involved in the nitrate-ammonium-sulfate system drop by 2100 except for ammonia which originates from agricultural practices and for which emissions significantly increase in the future. Despite the decrease of nitrate surface levels in Europe and Northern America, the global burden of accumulation mode nitrates increases by up to a factor of 2.6 in 2100. This increase in nitrate in the future arises despite decreasing NO x emissions due to increased availability of ammonia to form ammonium nitrate. The total aerosol direct forcing decreases from its present-day value of −0.234 W m −2 to a range of −0.070 to −0.130 W m −2 in 2100 based on the considered scenario. The direct forcing decreases for all aerosols except for nitrates for which the direct negative forcing increases to a range of −0.060 to −0.115 W m −2 in 2100. Including nitrates in the radiative forcing calculations increases the total direct forcing of aerosols by a factor of 1.3 in 2000, by a factor of 1.7–2.6 in 2030, by 1.9–4.8 in 2050 and by 6.4–8.6 in 2100. These results show that agricultural emissions of ammonia will play a key role in the future mitigation of climate change with nitrates becoming the dominant contributor to the anthropogenic aerosol optical depth during the second half of the XXIst century and significantly increasing the calculated aerosol direct forcing. This significant increase in the influence that nitrate exert on climate in the future will at the same time affect regional air quality and nitrogen deposition to the ecosystems.

Beschreibung: Climatology of stratocumulus cloud morphologies: microphysical properties and radiative effects Atmospheric Chemistry and Physics Discussions, 14, 6981-7023, 2014 Author(s): A. Muhlbauer, I. L. McCoy, and R. Wood An artificial neural network cloud classification scheme is combined with A-Train observations to characterize the physical properties and radiative effects of marine low clouds based on their morphology and type of mesoscale cellular convection (MCC) on a global scale. The cloud morphological categories are (i) organized closed MCC, (ii) organized open MCC and (iii) cellular but disorganized MCC. Global distributions of the frequency of occurrence of MCC types show clear regional signatures. Organized closed and open MCCs are most frequently found in subtropical regions and in mid-latitude storm tracks of both hemispheres. Cellular but disorganized MCC are the predominant type of marine low clouds in regions with warmer sea surface temperature such as in the tropics and trade wind zones. All MCC types exhibit a pronounced seasonal cycle. The physical properties of MCCs such as cloud fraction, radar reflectivity, drizzle rates and cloud top heights as well as the radiative effects of MCCs are found highly variable and a function of the type of MCC. On a global scale, the cloud fraction is largest for closed MCC with mean cloud fractions of about 90% whereas cloud fractions of open and cellular but disorganized MCC are only about 51% and 40%, respectively. Probability density functions (PDFs) of cloud fractions are heavily skewed and exhibit modest regional variability. PDFs of column maximum radar reflectivities and inferred cloud base drizzle rates indicate fundamental differences in the cloud and precipitation characteristics of different MCC types. Similarly, the radiative effects of MCCs differ substantially from each other in terms of shortwave reflectance and transmissivity. These differences highlight the importance of low cloud morphologies and their associated cloudiness on the shortwave cloud forcing.

Beschreibung: Development of an aerosol microphysical module: Aerosol Two-dimensional bin module for foRmation and Aging Simulation (ATRAS) Atmospheric Chemistry and Physics Discussions, 14, 10659-10699, 2014 Author(s): H. Matsui, M. Koike, Y. Kondo, J. D. Fast, and M. Takigawa Number concentrations, size distributions, and mixing states of aerosols are essential parameters for accurate estimation of aerosol direct and indirect effects. In this study, we develop an aerosol module, designated Aerosol Two-dimensional bin module for foRmation and Aging Simulation (ATRAS), that can represent these parameters explicitly by considering new particle formation (NPF), black carbon (BC) aging, and secondary organic aerosol (SOA) processes. A two-dimensional bin representation is used for particles with dry diameters from 40 nm to 10 μm to resolve both aerosol size (12 bins) and BC mixing state (10 bins) for a total of 120 bins. The particles with diameters from 1 to 40 nm are resolved using an additional 8 size bins to calculate NPF. The ATRAS module is implemented in the WRF-chem model and applied to examine the sensitivity of simulated mass, number, size distributions, and optical and radiative parameters of aerosols to NPF, BC aging and SOA processes over East Asia during the spring of 2009. BC absorption enhancement by coating materials is about 50% over East Asia during the spring, and the contribution of SOA processes to the absorption enhancement is estimated to be 10–20% over northern East Asia and 20–35% over southern East Asia. A clear north-south contrast is also found between the impacts of NPF and SOA processes on cloud condensation nuclei (CCN) concentrations: NPF increases CCN concentrations at higher supersaturations (smaller particles) over northern East Asia, whereas SOA increases CCN concentrations at lower supersaturations (larger particles) over southern East Asia. Application of ATRAS to East Asia also shows that the impact of each process on each optical and radiative parameter depends strongly on the process and the parameter in question. The module can be used in the future as a benchmark model to evaluate the accuracy of simpler aerosol models and examine interactions between NPF, BC aging, and SOA processes under different meteorological conditions and emissions.

Beschreibung: Relating hygroscopicity and optical properties to chemical composition and structure of secondary organic aerosol particles generated from the ozonolysis of α-pinene Atmospheric Chemistry and Physics Discussions, 14, 10543-10596, 2014 Author(s): C. Denjean, P. Formenti, B. Picquet-Varrault, E. Pangui, P. Zapf, Y. Katrib, C. Giorio, A. Tapparo, A. Monod, B. Temime-Roussel, P. Decorse, C. Mangeney, and J. F. Doussin Secondary Organic Aerosol (SOA) were generated from the ozonolysis of α-pinene in the CESAM simulation chamber. The formation and ageing of the SOA were studied by following their optical, hygroscopic and chemical properties. The optical properties investigated by determining the particle Complex Refractive Index (CRI). The hygroscopicity was quantified by measuring the effect of RH on particle size (Growth Factor, GF) and scattering coefficient ( f (RH)). The oxygen to carbon (O : C) atomic ratio of the particle surface and bulk were used as a sensitive parameter to correlate the changes in hygroscopic and optical properties of the SOA composition in CESAM. The real CRI at 525 nm wavelength decreased from 1.43–1.60 (±0.02) to 1.32–1.38 (±0.02) during the SOA formation. The decrease in real CRI correlates with a decrease in the O : C ratio of SOA from 0.68 (±0.20) to 0.55 (±0.16). In contrast, the GF stayed roughly constant over the reaction time, with values of 1.02–1.07 (±0.02) at 90% (±4.2) RH. Simultaneous measurements of O : C ratio of the particle surface revealed that the SOA was not composed of a homogeneous mixture, but with less oxidised species at the surface which would limit the water adsorption onto particle. In addition, an apparent change of both mobility diameter and scattering coefficient with increasing RH from 0 to 30% was observed for SOA after 16 h reaction. We postulate that this change could be due to a change in the viscosity of the SOA from a predominantly glassy state to a predominantly liquid state.

Beschreibung: The effects of aerosols on water cloud microphysics and macrophysics based on satellite observations over East Asia and the North Pacific Atmospheric Chemistry and Physics Discussions, 14, 10515-10541, 2014 Author(s): T. Michibata, K. Kawamoto, and T. Takemura This study examines the characteristics of the microphysics and macrophysics of water clouds from East Asia to the North Pacific, using data from satellite observations. Our goals are to clarify differences in microphysics and macrophysics between land and oceanic clouds, seasonal differences unique to the mid-latitudes, characteristics of the drizzling process, and cloud vertical structure. In pristine oceanic areas, fractional occurrences of cloud optical thickness (COT) and cloud droplet effective radius (CDR) increase systematically with an increase in drizzle intensity, but in polluted land areas these characteristics of the COT and CDR transition are not as evident. Additionally, regional and seasonal differences are identified in terms of drizzle intensity as a function of the liquid water path (LWP) and cloud droplet number concentration ( N c ). The correlations between drizzle intensity and LWP, and between drizzle intensity and N c are both more robust over oceanic areas than over land areas. We also demonstrate regional and seasonal characteristics of the cloud vertical structure. As a result, we find aerosol–cloud interaction mainly occurs around the cloud base in polluted land areas during the winter season. In addition, a difference between polluted and pristine areas in the efficiency of cloud droplet growth is confirmed. These results suggest that water clouds over the mid-latitudes exhibit a different drizzle system to those over the tropics.

Beschreibung: Variability of the infrared complex refractive index of African mineral dust: experimental estimation and implications for radiative transfer and satellite remote sensing Atmospheric Chemistry and Physics Discussions, 14, 10597-10657, 2014 Author(s): C. Di Biagio, H. Boucher, S. Caquineau, S. Chevaillier, J. Cuesta, and P. Formenti Experimental estimations of the infrared refractive index of African mineral dust have been retrieved from laboratory measurements of particle transmission spectra in the wavelength range 2.5–25 μm. Five dust samples collected at Banizoumbou (Niger) and Tamanrasset (Algeria) during dust events originated from different Western Saharan and Sahelian areas have been investigated. The obtained real (n) and imaginary ( k ) parts of the refractive index for the different dust cases vary in the range 1.1–2.7 and 0.05–1.0, respectively, and appear to be strongly sensitive to the mineralogical composition of the particles, especially in the 8–12 μm and 17–25 μm spectral intervals. Dust absorption is controlled mainly by clays, and, in minor fraction, by quartz and Ca-rich minerals. Size distribution, and the coarse fraction in particular, plays also a role in determining the refractive index. Significant differences are obtained when comparing our results with existing experimental estimations available in the literature, and with the values of the OPAC (Optical Properties of Aerosols and Clouds) database. The different datasets appear comparable in magnitude, with our values of n and k falling in the range of variability of past studies. However, literature data fail in accurately reproducing the spectral signatures of main minerals, in particular clays, and they significantly overestimate the contribution of quartz. We also found that the real and the imaginary parts of the refractive index from part of literature studies do not verify Kramers–Kronig relations, thus resulting theoretically incorrect. The comparison between our results, from Western Africa, and literature data, from different locations in Europe, Africa, and the Caribbean, nonetheless, confirms the expected large variability of the infrared refractive index of dust, thus highlighting the necessity for an extended systematic investigation. Aerosol intensive optical properties relevant to radiative transfer (mass extinction efficiency, k ext , single scattering albedo, ω, and asymmetry factor, g ), have been calculated, by using the Mie theory, for the five analysed dust samples, based on the estimated refractive index and measured particle size distribution. The optical properties show a large sample-to-sample variability. This variability is expected to significantly impact satellite retrievals of atmospheric and surface parameters and estimates of the dust radiative forcing.

Beschreibung: Observation and analysis of speciated atmospheric mercury in Shangri-la, Tibetan Plateau, China Atmospheric Chemistry and Physics Discussions, 14, 11041-11074, 2014 Author(s): H. Zhang, X. W. Fu, C.-J. Lin, X. Wang, and X. B. Feng This study reports the speciated concentration and the potential sources of atmospheric mercury measured at the Shangri-La Atmosphere Watch Regional Station (SAWRS), a pristine high-altitude site (3580 m a.s.l.) in Tibetan Plateau, China. The total gaseous mercury (TGM, defined as the sum of Gaseous Elemental Mercury, GEM, and gaseous oxidized mercury, GOM), GOM and particulate-bound mercury (PBM) were monitored from November 2009 to November 2010 to investigate the characteristics and atmospheric transport of mercury influenced by the Indian summer monsoon (ISM) and westerlies. The mean concentrations of TGM, PBM and GOM were 2.55 ± 0.73 ng m −3 , 37.78 ± 31.35 pg m −3 and 7.90 ± 7.89 ng m −3 . A notable seasonal pattern was observed with higher TGM concentrations in the beginning and end of the ISM. High TGM concentrations were associated with the transport of dry air that carried regional anthropogenic emissions from both domestic and foreign sources. The low PBM and GOM level was attributed to the deposition and wet scavenging during the ISM period. Backward trajectory analysis of air masses associated with TGM levels suggested that both the ISM and westerlies can carry Hg emitted in Burma, Bengal bay and north India to the SAWRS.

Beschreibung: The relative dispersion of cloud droplets: its robustness with respect to key cloud properties Atmospheric Chemistry and Physics Discussions, 14, 11153-11176, 2014 Author(s): E. Tas, A. Teller, O. Altaratz, D. Axisa, R. Bruintjes, Z. Levin, and I. Koren The relative dispersion (ε) of cloud droplet size distribution, defined as the ratio between cloud droplet size distribution width (σ) and cloud droplet average radius (⟨ r ⟩), is investigated using airborne measurements of warm cumulus clouds. The data is used to study the relation of ε with microphysical and thermodynamic characteristics of the clouds. The results show that ε is constrained with average values in the range of ~0.25–0.35. It is shown that ε is not correlated with the cloud droplet concentration or with the Liquid Water Content (LWC). However, the relative dispersion variance (related to the third moment of the droplets distribution) shows sensitivity to the droplets' concentration and LWC, suggesting smaller ε variability in more adiabatic regions in the clouds. A clear criterion for the usage of the in situ airborne measurements data for statistical moments' calculations is suggested.

Beschreibung: Estimation of direct and indirect impacts of fireworks on the physicochemical characteristics of atmospheric fine and coarse particles Atmospheric Chemistry and Physics Discussions, 14, 11075-11101, 2014 Author(s): Y.-Z. Tian, J. Wang, X. Peng, G.-L. Shi, and Y.-C. Feng To quantify total, direct and indirect impacts of fireworks individually, size-resolved PM samples were collected before, during, and after a Chinese folk-custom festival (Chinese New Year) in a megacity in China. Through chemical analysis and morphology characterization, strong influence of fireworks on physicochemical characteristics of PM 10 and PM 2.5 was observed. Concentrations of many species exhibited an increasing trend during heavy-firework period, especially for K + , Mg 2+ and Cr; and the results of non-sea-salt ions demonstrated anthropogenic influence on them. Then, source apportionment was conducted by receptor models and Peak Analysis. Total influence of fireworks was quantified by PMF, showing that fireworks contributed rather higher fractions (23.40% to PM 10 and 29.66% to PM 2.5 ) during heavy-firework period than those during light-firework period (4.28% to PM 10 and 7.18% to PM 2.5 ). Profiles of total fireworks obtained by two independent methods (PMF and Peak Analysis) were consistent, with higher abundances of K + , Al, Si, Ca and OC. Finally, individual contributions of direct and indirect impacts of fireworks were quantified by CMB. The percentage contributions of resuspended dust, biomass combustion and direct-fireworks were 36.82, 14.08 and 44.44% for PM 10 and 34.89, 16.60 and 52.54% for PM 2.5 , in terms of the total fireworks. The quantification of total, direct and indirect impacts of fireworks to ambient PM gives an original contribution to understand the physicochemical characteristics and mechanisms of such high-intensity anthropogenic activities.

Beschreibung: Air quality simulations of wildfires in the Pacific Northwest evaluated with surface and satellite observations during the summers of 2007 and 2008 Atmospheric Chemistry and Physics Discussions, 14, 11103-11152, 2014 Author(s): F. L. Herron-Thorpe, G. H. Mount, L. K. Emmons, B. K. Lamb, D. A. Jaffe, N. L. Wigder, S. H. Chung, R. Zhang, M. D. Woelfle, and J. K. Vaughan Evaluation of a regional air quality forecasting system for the Pacific Northwest was carried out for the 2007 and 2008 fire seasons using suite of surface and satellite observations. Wildfire events in the Pacific Northwest during the summers of 2007 and 2008 were simulated using the Air Information Report for Public Access and Community Tracking v.3 (AIRPACT-3) framework utilizing the Community Multi-scale Air Quality (CMAQ) model. Fire emissions were simulated using the BlueSky framework with fire locations determined by the Satellite Mapping Automated Reanalysis Tool for Fire Incident Reconciliation (SMARTFIRE). Plume rise was simulated using two different methods: the Fire Emission Production Simulator (FEPS) and the Sparse Matrix Operator Kernel Emissions (SMOKE) model. Predicted plume top heights were compared to the Cloud-Aerosol LIDAR with Orthogonal Polarization (CALIOP) instrument aboard the Cloud Aerosol LIDAR and Infrared Pathfinder Satellite Observation (CALIPSO) satellite. Carbon monoxide predictions were compared to the Atmospheric InfraRed Sounder (AIRS) instrument aboard the Aqua satellite. Horizontal distributions of column aerosol optical depth (AOD) were compared to retrievals by the Moderate Resolution Imaging Spectroradiometer (MODIS) instrument aboard the Aqua satellite. Model tropospheric nitrogen dioxide distributions were compared to retrievals from the Ozone Monitoring Instrument (OMI) aboard the Aura satellite. Surface ozone and PM 2.5 predictions were compared to surface observations. The AIRPACT-3 model captured the location and transport direction of fire events well, but sometimes missed the timing of fire events and overall underestimated the impact of wildfire events at regional surface monitor locations. During the 2007 fire period the fractional biases of AIRPACT-3 for average 24 h PM 2.5 , maximum daily average 8 h Ozone, AOD, total column CO, and tropospheric column NO 2 were found to be −33%, −8%, −61%, −10%, and −39%, respectively; while during the 2008 fire period the fractional biases were −27%, +1%, −53%, −5%, and −28%, respectively. Fractional biases of AIRPACT-3 plume tops were found to be −46% above mean sea level (a.m.s.l.), but only −28% above ground level (a.g.l.), partly due to the under-estimation of AIRPACT-3 elevation in complex terrain that results from the 12 km grid-cell smoothing.

Beschreibung: Meteorological factors controlling low-level continental pollutant outflow across a coast Atmospheric Chemistry and Physics Discussions, 14, 10853-10890, 2014 Author(s): D. L. Peake, H. F. Dacre, J. Methven, and O. Coceal Coastal outflow describes the horizontal advection of pollutants from the continental boundary layer across a coastline into a layer above the marine boundary layer. This process can ventilate polluted continental boundary layers and thus regulate air quality in highly populated coastal regions. This paper investigates the factors controlling coastal outflow and quantifies its importance as a ventilation mechanism. Tracers in the Met Office Unified Model (MetUM) are used to examine the magnitude and variability of coastal outflow over the eastern United States for a 4 week period during summer 2004. Over the 4 week period, ventilation of tracer from the continental boundary layer via coastal outflow occurs with the same magnitude as vertical ventilation via convection and advection. The relative importance of tracer decay rate, cross-coastal advection rate, and a parameter based on the relative continental and marine boundary layer heights, on coastal outflow is assessed by reducing the problem to a time-dependent box-model. The ratio of the advection rate and decay rate is a dimensionless parameter which determines whether tracers are long-lived or short-lived. Long- and short-lived tracers exhibit different behaviours with respect to coastal outflow. For short-lived tracers, increasing the advection rate increases the diurnally averaged magnitude of coastal outflow, but has the opposite effect for very long-lived tracers. Short-lived tracers exhibit large diurnal variability in coastal outflow but long-lived tracers do not. By combining the MetUM and box-model simulations a landwidth is determined which represents the distance inland over which emissions contribute significantly to coastal outflow. A landwidth of between 100 and 400 km is found to be representative for a tracer with a lifetime of 24 h.

Beschreibung: A new model of ragweed pollen release based on the analysis of meteorological conditions Atmospheric Chemistry and Physics Discussions, 14, 10891-10927, 2014 Author(s): L. Menut, R. Vautard, A. Colette, D. Khvorostyanov, A. Potier, L. Hamaoui-Laguel, N. Viovy, and M. Thibaudon In order to propose a new deterministic ragweed pollen emission scheme, the meteorological conditions conducive to common ragweed pollen emission are studied over Europe between 2005 and 2011. Correlations are calculated between daily modelled meteorological variables (wind speed, temperature, humidity, precipitations rates, surface fluxes) and surface concentrations at nine stations in Hungary, Croatia and France. We found that the 2 m temperature is the most correlated parameter, followed by convective velocity and incoming shortwave radiation. On the other hand, the precipitation rate and the 2 m specific humidity act as limiting factors. A new emission scheme is designed. Compared to two existing schemes, we show that it is able to better estimate the daily release of ragweed pollen in 73% of modelled cases.

Beschreibung: Modeling ultrafine particle growth at a pine forest site influenced by anthropogenic pollution during BEACHON-RoMBAS 2011 Atmospheric Chemistry and Physics Discussions, 14, 5611-5651, 2014 Author(s): Y. Y. Cui, A. Hodzic, J. N. Smith, J. Ortega, J. Brioude, H. Matsui, A. Turnipseed, P. Winkler, and B. de Foy Formation and growth of ultrafine particles is crudely represented in chemistry-climate models, which contributes to uncertainties in aerosol composition, size distribution, and aerosol effects on cloud condensation nuclei (CCN) concentrations. Measurements of ultrafine particles, their precursor gases, and meteorological parameters were performed in a ponderosa pine forest in the Colorado Front Range in July–August 2011, and were analyzed to study processes leading to Aitken-mode Particle burst Events (APEs). These measurements suggest that APEs were associated with the arrival at the site of anthropogenic pollution plumes around noon or in the early afternoon. Number concentrations of ultrafine (4 to 30 nm diameter) particles typically exceeded 10 000 cm −3 during APEs, and these elevated concentrations coincided with increased SO 2 and monoterpene concentrations, and led to a factor of two increase in CCN concentrations at 0.5% supersaturation. The APEs were simulated using the regional WRF-Chem model, which was extended to account for ultrafine particle sizes starting at 1 nm in diameter, to include an empirical activation nucleation scheme in the planetary boundary layer, and to explicitly simulate the subsequent growth of Aitken particles by condensation of organic and inorganic vapors. Comparisons with aerosol size distribution measurements showed that simulations using the activation nucleation parameterization reasonably captured aerosol number concentrations and size distribution during APEs, as well as ground level CCN concentrations. Results suggest that sulfuric acid from anthropogenic SO 2 triggers APEs, and that the condensation of monoterpene oxidation products onto freshly nucleated particles drives their growth. The simulated growth rate of 3.4 nm h −1 for small particles (4–30 nm in diameter) was comparable to the measured average value of 2.3 nm h −1 . Model results also suggest that the presence of APEs tends to modify the composition of sub-100 nm diameter particles, leading to generally higher absolute mass concentrations of sulfate as well as organic aerosols with a higher sulfate content. Sensitivity simulations suggest that the representation of nucleation processes in the model largely influences the predicted number concentrations and thus CCN concentrations. We estimate that nucleation contributes to 65% of surface CCN at 0.5% supersaturation in this pine forest environment.

Beschreibung: Comparison of ice particle characteristics simulated by the Community Atmosphere Model (CAM5) with in-situ observations Atmospheric Chemistry and Physics Discussions, 14, 7637-7681, 2014 Author(s): T. Eidhammer, H. Morrison, A. Bansemer, A. Gettelman, and A. J. Heymsfield Detailed measurements of ice crystals in cirrus clouds were used to compare with results from the Community Atmospheric Model Version 5 (CAM5) global climate model. The observations are from two different field campaigns with contrasting conditions: Atmospheric Radiation Measurements Spring Cloud Intensive Operational Period in 2000 (ARM-IOP), which was characterized primarily by midlatitude frontal clouds and cirrus, and Tropical Composition, Cloud and Climate Coupling (TC4), which was dominated by anvil cirrus. Results show that the model typically overestimates the slope parameter of the exponential size distributions of cloud ice and snow, while the variation with temperature (height) is comparable. The model also overestimates the ice/snow number concentration (0th moment of the size distribution) and underestimates higher moments (2nd through 5th), but compares well with observations for the 1st moment. Overall the model shows better agreement with observations for TC4 than for ARM-IOP in regards to the moments. The mass-weighted terminal fallspeed is lower in the model compared to observations for both ARM-IOP and TC4, which is partly due to the overestimation of the size distribution slope parameter. Sensitivity tests with modification of the threshold size for cloud ice to snow autoconversion ( D cs ) do not show noticeable improvement in modeled moments, slope parameter and mass weighed fallspeed compared to observations. Further, there is considerable sensitivity of the cloud radiative forcing to D cs , consistent with previous studies, but no value of D cs improves modeled cloud radiative forcing compared to measurements. Since the autoconversion of cloud ice to snow using the threshold size D cs has little physical basis, future improvement to combine cloud ice and snow into a single category, eliminating the need for autoconversion, is suggested.

Beschreibung: A physically-based framework for modelling the organic fractionation of sea spray aerosol from bubble film Langmuir equilibria Atmospheric Chemistry and Physics Discussions, 14, 5375-5443, 2014 Author(s): S. M. Burrows, O. Ogunro, A. A. Frossard, L. M. Russell, P. J. Rasch, and S. Elliott The presence of a large fraction of organic matter in primary sea spray aerosol (SSA) can strongly affect its cloud condensation nuclei activity and interactions with marine clouds. Global climate models require new parameterizations of the SSA composition in order to improve the representation of these processes. Existing proposals for such a parameterization use remotely-sensed chlorophyll a concentrations as a proxy for the biogenic contribution to the aerosol. However, both observations and theoretical considerations suggest that existing relationships with chlorophyll a , derived from observations at only a few locations, may not be representative for all ocean regions. We introduce a novel framework for parameterizing the fractionation of marine organic matter into SSA based on a competitive Langmuir adsorption equilibrium at bubble surfaces. Marine organic matter is partitioned into classes with differing molecular weights, surface excesses, and Langmuir adsorption parameters. The classes include a lipid-like mixture associated with labile dissolved organic carbon (DOC), a polysaccharide-like mixture associated primarily with semi-labile DOC, a protein-like mixture with concentrations intermediate between lipids and polysaccharides, a processed mixture associated with recalcitrant surface DOC, and a deep abyssal humic-like mixture. Box model calculations have been performed for several cases of organic adsorption to illustrate the underlying concepts. We then apply the framework to output from a global marine biogeochemistry model, by partitioning total dissolved organic carbon into several classes of macromolecules. Each class is represented by model compounds with physical and chemical properties based on existing laboratory data. This allows us to globally map the predicted organic mass fraction of the nascent submicron sea spray aerosol. Predicted relationships between chlorophyll a and organic fraction are similar to existing empirical parameterizations, but can vary between biologically productive and non-productive regions, and seasonally within a given region. Major uncertainties include the bubble film thickness at bursting and the variability of organic surfactant activity in the ocean, which is poorly constrained. In addition, polysaccharides may enter the aerosol more efficiently than Langmuir adsorption would suggest. Potential mechanisms for include the formation of marine colloidal particles that may be more efficiently swept up by rising particles, and cooperative adsorption of polysaccharides with proteins or lipids. These processes may make important contributions to the aerosol, but are not included here. This organic fractionation framework is an initial step towards a closer linking of ocean biogeochemistry and aerosol chemical composition in Earth system models. Future work should focus on improving constraints on model parameters through new laboratory experiments or through empirical fitting to observed relationships in the real ocean and atmosphere, as well as on atmospheric implications of the variable composition of organic matter in sea spray.

Beschreibung: Distributions and radiative forcings of various cloud types based on active and passive satellite datasets – Part 1: Geographical distributions and overlap of cloud types Atmospheric Chemistry and Physics Discussions, 14, 10463-10514, 2014 Author(s): J. Li, J. Huang, K. Stamnes, T. Wang, Y. Yi, X. Ding, Q. Lv, and H. Jin Based on four year' 2B-CLDCLASS-Lidar (Radar-Lidar) cloud classification product from CloudSat, we analyze the geographical distributions of different cloud types and their co-occurrence frequency across different seasons, moreover, utilize the vertical distributions of cloud type to further evaluate the cloud overlap assumptions. The statistical results show that more high clouds, altocumulus, stratocumulus or stratus and cumulus are identified in the Radar-Lidar cloud classification product compared to previous results from Radar-only cloud classification (2B-CLDCLASS product from CloudSat). In particularly, high clouds and cumulus cloud fractions increased by factors 2.5 and 4–7, respectively. The new results are in more reasonable agreement with other datasets (typically the International Satellite Cloud Climatology Project (ISCCP) and surface observer reports). Among the cloud types, altostratus and altocumulus are more popular over the arid/semi-arid land areas of the Northern and Southern Hemispheres, respectively. These features weren't observed by using the ISCCP D1 dataset. For co-occurrence of cloud types, high cloud, altostratus, altocumulus and cumulus are much more likely to co-exist with other cloud types. However, stratus/stratocumulus, nimbostratus and convective clouds are much more likely to exhibit individual features. After considering the co-occurrence of cloud types, the cloud fraction based on the random overlap assumption is underestimated over the vast ocean except in the west-central Pacific Ocean warm pool. Obvious overestimations are mainly occurring over land areas in the tropics and subtropics. The investigation therefore indicates that incorporate co-occurrence information of cloud types based on Radar-Lidar cloud classification into the overlap assumption schemes used in the current GCMs possible be able to provide an better predictions for vertically projected total cloud fraction.

Beschreibung: Comparisons of continuous atmospheric CH 4 , CO 2 and N 2 O measurements – results of InGOS travelling instrument campaign at Mace Head Atmospheric Chemistry and Physics Discussions, 14, 10429-10462, 2014 Author(s): S. N. Vardag, S. Hammer, S. O'Doherty, T. G. Spain, B. Wastine, A. Jordan, and I. Levin A two-month measurement campaign with a Fourier Transform InfraRed (FTIR) analyser as a Travelling Comparison Instrument (TCI) was performed at the AGAGE and WMO GAW station Mace Head, Ireland. The aim was to evaluate the compatibility of atmospheric CH 4 , CO 2 and N 2 O measurements of the routine station instrumentation, consisting of a gas chromatograph (GC-MD) for CH 4 and N 2 O as well as a cavity ring-down spectroscopy (CRDS) system for CH 4 and CO 2 . The advantage of a TCI approach for quality control is that the comparison covers the entire ambient air measurement system, including the sample intake system and the data evaluation process. For initial quality and performance control the TCI was run in parallel to the Heidelberg GC (GC-HEI) before and after the measurement campaign at Mace Head. Median differences between the GC-HEI and the TCI were well within the WMO Inter Laboratory Compatibility (ILC) target for all three greenhouse gases. At Mace Head, the median difference between the GC-MD and the TCI were −0.04 nmol mol −1 for CH 4 and −0.37 nmol mol −1 for N 2 O. For N 2 O a similar difference (−0.40 nmol mol −1 ) was found when measuring surveillance or working gas cylinders with both instruments. This suggests that the difference observed in ambient air originates from a calibration offset that could partly be due to a difference between the WMO N 2 O X2006a scale used for the TCI and the SIO-1998 scale used at Mace Head and in the whole AGAGE network. Median differences between the CRDS G1301 and the TCI at Mace Head were 0.12 nmol mol −1 for CH 4 and 0.14 μmol mol −1 for CO 2 (CRDS G1301 − TCI). The difference between both instruments for CO 2 could not be explained, as direct measurements of calibration gases show no such difference. The CH 4 differences between the TCI, the GC-MD and the CRDS G1301 at Mace Head are smaller than the WMO Inter Laboratory compatibility (ILC) target (WMO, 2009), while this is not the case for CO 2 and N 2 O. During the campaign it was also demonstrated that the new optical instrumentation allows detection of very small vertical CO 2 and CH 4 gradients, here between 10 m a.g.l. and 25 m a.g.l. This provides a new opportunity of evaluating greenhouse gases gradients in terms of regional fluxes.

Beschreibung: Organic aerosol concentration and composition over Europe: insights from comparison of regional model predictions with aerosol mass spectrometer factor analysis Atmospheric Chemistry and Physics Discussions, 14, 7597-7635, 2014 Author(s): C. Fountoukis, A. G. Megaritis, K. Skyllakou, P. E. Charalampidis, C. Pilinis, H. A. C. Denier van der Gon, M. Crippa, F. Canonaco, C. Mohr, A. S. H. Prévôt, J. D. Allan, L. Poulain, T. Petäjä, P. Tiitta, S. Carbone, A. Kiendler-Scharr, E. Nemitz, C. O'Dowd, E. Swietlicki, and S. N. Pandis A detailed three-dimensional regional chemical transport model (PMCAMx) was applied over Europe focusing on the formation and chemical transformation of organic matter. Three periods representative of different seasons were simulated, corresponding to intensive field campaigns. An extensive set of AMS measurements was used to evaluate the model and, using factor analysis results, gain more insight into the sources and transformations of organic aerosol (OA). Overall, the agreement between predictions and measurements for OA concentration is encouraging with the model reproducing two thirds of the data (daily average mass concentrations) within a factor of two. Oxygenated OA (OOA) is predicted to contribute 93% to total OA during May, 87% during winter and 96% during autumn with the rest consisting of fresh primary OA (POA). Predicted OOA concentrations compare well with the observed OOA values for all periods with an average fractional error of 0.53 and a bias equal to −0.07 (mean error = 0.9 μg m −3 , mean bias = −0.2 μg m −3 ). The model systematically underpredicts fresh POA in most sites during late spring and autumn (mean bias up to −0.8 μg m −3 ). Based on results from a source apportionment algorithm running in parallel with PMCAMx, most of the POA originates from biomass burning (fires and residential wood combustion) and therefore biomass burning OA is most likely underestimated in the emission inventory. The model performs well at all sites when the PMF-estimated low volatility OOA is compared against the OA with C * ≤ 0.1 μg m −3 and semivolatile OOA against the OA with C * 〉 0.1 μg m −3 respectively.

Beschreibung: Transport of aerosol to the Arctic: analysis of CALIOP and French aircraft data during the spring 2008 POLARCAT campaign Atmospheric Chemistry and Physics Discussions, 14, 5721-5769, 2014 Author(s): G. Ancellet, J. Pelon, Y. Blanchard, B. Quennehen, A. Bazureau, K. S. Law, and A. Schwarzenboeck Lidar and in situ observations performed during POLARCAT campaign are reported here in terms of statistics to characterize aerosol properties over northern Europe using daily airborne measurements conducted between Svalbard Island and Scandinavia from 30 March to 11 April 2008. It is shown that during this period, a rather large number of aerosol layers was observed in the troposphere, with a backscatter ratio at 532 nm of 1.2 (1.5 below 2 km, 1.2 between 5 and 7 km and a minimum in-between). Their sources were identified using multispectral backscatter and depolarization airborne lidar measurements after careful calibration analysis. Transport analysis and comparisons between in situ and airborne lidar observations are also provided to assess the quality of this identification. Comparison with level 1 backscatter observations of the spaceborne CALIOP lidar were done to adjust CALIOP multispectral observations to airborne observations on a statistical basis. Re-calibration for CALIOP daytime 1064 nm signals led to an increase of their values by about 30% in agreement with previous analyses. No re-calibration is made at 532 nm, but scattering ratios appear to be biased low. Regional analyses in the European Arctic then performed as a test, emphasize the potential of the CALIOP spaceborne lidar to further monitor more in depth properties of the aerosol layers over Arctic using infrared and depolarization observations. The CALIOP April 2008 global distribution of the aerosol backscatter reveal two regions with large backscatter below 2 km: the Northern Atlantic between Greenland and Norway, and Northern Siberia. The aerosol color ratio increase between the sources regions and the observations at latitudes above 70° N is consistent with a growth of the aerosol size once transported to the Arctic. The distribution of the aerosol optical properties in the mid troposphere supports the known main transport pathways between mid-latitudes and the Arctic.

Beschreibung: Reconciling aerosol light extinction measurements from spaceborne lidar observations and in-situ measurements in the Arctic Atmospheric Chemistry and Physics Discussions, 14, 5687-5720, 2014 Author(s): M. Tesche, N. Rastak, R. J. Charlson, P. Glantz, P. Zieger, and H.-C. Hansson In this study we investigate to what degree it is possible to reconcile continuously recorded particle light extinction coefficients derived from dry in-situ measurements at Zeppelin station (78.92° N, 11.85° E, 475 m a.s.l.) at Ny-Ålesund, Svalbard, that are recalculated to ambient relative humidity, and simultaneous ambient observations with the Cloud-Aerosol Lidar with Orthogonal Polarization (CALIOP) aboard the Cloud-Aerosol Lidar and Infrared Pathfinder Satellite Observations (CALIPSO) satellite. To our knowledge, this represents the first study that compares spaceborne lidar measurements to optical aerosol properties from short-term in-situ observations (averaged over 5 h) on a case-by-case basis. Finding suitable comparison cases requires an elaborate screening and matching of the CALIOP data with respect to the location of the Zeppelin station as well as in the selection of temporal and spatial averaging intervals for both the ground-based and spaceborne observations. Trustworthy reconciliation of these data cannot be achieved with the closest approach method that is often used in matching CALIOP observations to those taken at ground sites due to the transport pathways of the air parcels that were sampled. The use of trajectories allowed us to establish a connection between spaceborne and ground-based observations for 57 individual overpasses out of a total of 2018 that occurred in our region of interest around Svalbard (0 to 25° E; 75 to 82° N) in the considered year of 2008. Matches could only be established during winter and spring, since the low aerosol load during summer in connection with the strong solar background and the high occurrence rate of clouds strongly influences the performance and reliability of CALIOP observations. Extinction coefficients in the range from 1 to 100 Mm −1 were found for successful matches with an agreement of a factor of 1.85 (median value for a range from 0.38 to 17.9) between the findings of in-situ and spaceborne observations (the latter being generally larger than the former). The remaining difference is likely to be due to the natural variability in aerosol concentration and ambient relative humidity, an insufficient representation of aerosol particle growth in the used hygroscopicity model, or a misclassification of aerosol type (i.e., choice of lidar ratio) in the CALIPSO retrieval.

Beschreibung: SO 2 noontime peak phenomenon in the North China Plain Atmospheric Chemistry and Physics Discussions, 14, 5653-5685, 2014 Author(s): W. Y. Xu, C. S. Zhao, L. Ran, W. L. Lin, P. Yan, and X. B. Xu Frequent SO 2 noontime peak phenomenon was discovered in a detailed analysis on the SO 2 concentrations in the North China Plain (NCP). The possible causes and their contributions are analysed. The impacts of such a phenomenon on the sulphur cycle were studied and the implications of the phenomenon for atmospheric chemistry, cloud physics and climate were discussed. Different from the common SO 2 diurnal patterns with high nighttime concentrations, NCP witnessed high frequencies of SO 2 noontime peaks, with an occurrence frequency of 50–72% at the four stations. Down-mixing of elevated pollution layers, plume transport processes, mountain-valley-winds and fog/high RH haze events were the possible causes. The contribution of each process varies from each other and from station to station, however, neither of those four processes can be neglected. SO 2 peaks occurring during noontime instead of nighttime will lead to a 13–35% increase in sulphur dry deposition, 9–23% increase in gas phase oxidation and 8–33% increase in aqueous phase conversions, which will increase the hygroscopicity and the light scattering of aerosols, thus having important impacts on atmospheric chemistry, cloud physics and climate.

Beschreibung: African dust outbreaks over the western Mediterranean basin: 11 year characterization of atmospheric circulation patterns and dust source areas Atmospheric Chemistry and Physics Discussions, 14, 5495-5533, 2014 Author(s): P. Salvador, S. Alonso, J. Pey, B. Artíñano, J. J. de Bustos, A. Alastuey, and X. Querol The occurrence of African dust outbreaks over the western Mediterranean basin were identified on an 11 year period (2001–2011). PM 10 daily data from nine regional background air quality monitoring sites across the study area were compiled and the net dust load transported during each event was estimated. Then, the main atmospheric circulation patterns causing the transport of African air masses, were characterized by mean of an objective classification methodology of atmospheric variables fields. Next, the potential source areas of mineral dust, associated to each circulation pattern were identified by trajectory statistical methods. Finally, an impact index was calculated to estimate the incidence of the African dust outbreaks produced during each circulation pattern, on the levels of dust load in PM 10 concentrations recorded in the different regions. Our results indicate that the values of the impact index and the areas affected by African dust, strongly depended on the atmospheric circulation pattern. Four circulation types were obtained by the classification procedure. Two of them (CT-1 and CT-4) occurred predominantly during the warm season, bringing dust from areas of Algeria, Tunisia, Western Sahara, western Libya and Mauritania. African dust outbreaks produced during the CT-4 were the most frequent across the period of study, generating the highest impact index over southern, central and eastern regions of the Iberian Peninsula as well as over the Balearic Islands. Conversely, the events caused by the CT-1 encompassed the highest impact index over the western areas of the Iberian Peninsula. The two remaining circulation types (CT-2 and CT-3) were more frequently observed during the spring season. The prevailing flows generated by these two atmospheric circulation patterns, carried mineral dust from areas of Algeria, Tunisia and Western Sahara, giving rise to higher values of the impact index from eastern to western areas of the western Mediterranean basin.

Beschreibung: Greenhouse gas network design using backward Lagrangian particle dispersion modelling – Part 1: Methodology and Australian test case Atmospheric Chemistry and Physics Discussions, 14, 7557-7595, 2014 Author(s): T. Ziehn, A. Nickless, P. J. Rayner, R. M. Law, G. Roff, and P. Fraser This paper describes the generation of optimal atmospheric measurement networks for determining carbon dioxide fluxes over Australia using inverse methods. A Lagrangian particle dispersion model is used in reverse mode together with a Bayesian inverse modelling framework to calculate the relationship between weekly surface fluxes and hourly concentration observations for the Australian continent. Meteorological driving fields are provided by the regional version of the Australian Community Climate and Earth System Simulator (ACCESS) at 12 km resolution at an hourly time scale. Prior uncertainties are derived on a weekly time scale for biosphere fluxes and fossil fuel emissions from high resolution BIOS2 model runs and from the Fossil Fuel Data Assimilation System (FFDAS), respectively. The influence from outside the modelled domain is investigated, but proves to be negligible for the network design. Existing ground based measurement stations in Australia are assessed in terms of their ability to constrain local flux estimates from the land. We find that the six stations that are currently operational are already able to reduce the uncertainties on surface flux estimates by about 30%. A candidate list of 59 stations is generated based on logistic constraints and an incremental optimization scheme is used to extend the network of existing stations. In order to achieve an uncertainty reduction of about 50% we need to double the number of measurement stations in Australia. Assuming equal data uncertainties for all sites, new stations would be mainly located in the northern and eastern part of the continent.

Beschreibung: Climatic controls on water vapor deuterium excess in the marine boundary layer of the North Atlantic based on 500 days of in situ, continuous measurements Atmospheric Chemistry and Physics Discussions, 14, 2363-2401, 2014 Author(s): H. C. Steen-Larsen, A. E. Sveinbjörnsdottir, A. J. Peters, V. Masson-Delmotte, M. P. Guishard, G. Hsiao, J. Jouzel, D. Noone, J. K. Warren, and J. W. C. White Continuous, in situ measurements of water vapor isotopic composition have been conducted in the North Atlantic, Bermuda Islands (32.26° N, 64.88° W) between November 2011 and June~2013, using a~cavity-ring-down-spectrometer water vapor isotope analyzer and an autonomous self-designed calibration system. Meticulous calibration allows us to reach an accuracy and precision on 10 min average of δ 18 O, δ D , and d -excess of respectively 0.14 ‰, 0.85 ‰, and 1.1 ‰, verified using two parallel instruments with independent calibration. As a result of more than 500 days with 6 hourly data the relationships between deuterium excess, relative humidity (rh), sea surface temperature (SST), wind speed and direction are assessed. From the whole dataset, 84% of d -excess variance is explained by a strong linear relationship with relative humidity. The slope of this relationship (−42.6 ± 0.4 ‰ % (rh)) is similar to the theoretical prediction of Merlivat and Jouzel (1979) for SST between 20 °C and 30 °C. However, in contrast with theory, no effect of wind speed could be detected on the relationship between d -excess and relative humidity. Separating the dataset into winter, spring, summer, and autumn seasons reveals different linear relationships between d -excess and humidity. Changes in wind directions are observed to affect the relationships between d -excess and humidity. The observed seasonal variability in the relationship between d -excess and relative humidity underlines the importance of long-term monitoring to make accurate conclusions.

Beschreibung: CarbonTracker-CH 4 : an assimilation system for estimating emissions of atmospheric methane Atmospheric Chemistry and Physics Discussions, 14, 2175-2233, 2014 Author(s): L. M. Bruhwiler, E. Dlugokencky, K. Masarie, M. Ishizawa, A. Andrews, J. Miller, C. Sweeney, P. Tans, and D. Worthy We describe an assimilation system for atmospheric methane (CH 4 ), CarbonTracker-CH 4 , and demonstrate the diagnostic value of global or zonally averaged CH 4 abundances for evaluating the results. We show that CarbonTracker-CH 4 is able to simulate the observed zonal average mole fractions and capture inter-annual variability in emissions quite well at high northern latitudes (53–90° N). CarbonTracker-CH 4 estimates of total fluxes at high northern latitudes are about 81 Tg CH 4 yr −1 , about 12 Tg CH 4 yr −1 (13%) lower than prior estimates, a result that is consistent with other atmospheric inversions. Emissions from European wetlands are decreased by 30%, a result consistent with previous; however, emissions from wetlands in Boreal Eurasia are increased relative to the prior estimate. Although CarbonTracker-CH 4 does not estimate increases in emissions from high northern latitudes for 2000 through 2010, significant inter-annual variability in high northern latitude fluxes is recovered. During the exceptionally warm Arctic summer of 2007, estimated emissions were greater than the decadal average by 4.4 Tg CH 4 yr −1 . In 2008, temperatures returned to more normal values over Arctic North America while they stayed above normal over Arctic Eurasia. CarbonTracker-CH 4 estimates were 2.4 Tg CH 4 yr −1 higher than the decadal average, and the anomalous emissions occurred over Arctic Eurasia, suggesting that the data allow discrimination between these two source regions. Also, the emission estimates respond to climate variability without having the system constrained by climate parameters. CarbonTracker-CH 4 estimates for temperate latitudes are only slightly increased over prior estimates, but about 10 Tg CH 4 yr −1 is redistributed from Asia to North America. We used time invariant prior flux estimates, so for the period from 2000 to 2006, when the growth rate of global atmospheric CH 4 was very small, the assimilation does not produce increases in natural or anthropogenic emissions in contrast to bottom-up emission datasets. After 2006, when atmospheric CH 4 began its recent increases, CarbonTracker-CH 4 allocates some of the increases to anthropogenic emissions at temperate latitudes, and some to tropical wetland emissions. For temperate North America the prior flux increases by about 4 Tg CH 4 yr −1 during winter when biogenic emissions are small. Examination of the residuals at some North American observation sites suggests that increased gas and oil exploration may play a role since sites near fossil fuel production are particularly hard for the inversion to fit. The tropics are not currently well resolved by CarbonTracker-CH 4 due to sparse observational coverage and a short assimilation window. However, there is a small uncertainty reduction and posterior emissions are about 18% higher than prior estimates. Most of this increase is allocated to tropical South America rather than being distributed among the global tropics.

Beschreibung: Understanding the anthropogenic influence on formation of biogenic secondary organic aerosols via analysis of organosulfates and related oxidation products Atmospheric Chemistry and Physics Discussions, 14, 2449-2498, 2014 Author(s): Q. T. Nguyen, M. K. Christensen, F. Cozzi, A. Zare, A. M. K. Hansen, K. Kristensen, T. E. Tulinius, H. H. Madsen, J. H. Christensen, J. Brandt, A. Massling, J. K. Nøjgaard, and M. Glasius Anthropogenic emissions of sulfur dioxide (SO 2 ) and nitrogen oxides (NO x ) may affect concentration levels and composition of biogenic secondary organic aerosols (BSOA) through photochemical reactions with biogenic organic precursors to form organosulfates and nitrooxy organosulfates. We investigated this influence in a field study from 19 May–22 June 2011 at two sampling sites in Denmark. Within the study, we identified a substantial number of organic acids, organosulfates and nitrooxy organosulfates in the ambient urban curbside and semi-rural background air. A high degree of correlation in concentrations was found among a group of specific organic acids, organosulfates and nitrooxy organosulfates, which may originate from various precursors, suggesting a common mechanism or factor affecting their concentration levels at the sites. It was proposed that the formation of those species most likely occurred on a larger spatial scale with the compounds being long-range transported to the sites on the days with highest concentrations. The origin of the long-range transported aerosols was investigated using the Hybrid Single Particle Lagrangian Integrated Trajectory (HYSPLIT) model in addition to modeled emissions of related precursors including isoprene and monoterpenes using the global Model of Emissions of Gases and Aerosols from Nature (MEGAN) and SO 2 emissions using the European Monitoring and Evaluation Program (EMEP) database. The local impacts were also studied by examining the correlation between selected species which showed significantly enhanced concentrations at the urban curbside site and the local concentrations of various gases including SO 2 , ozone (O 3 ), carbon monoxide (CO), NO x , aerosol acidity and other meteorological conditions. This investigation showed that an inter-play of the local parameters such as the aerosol acidity, NO x , relative humidity (RH), temperature and global radiation seemed to influence the concentration level of those species, via such as wet aerosol chemistry. The local impacts however seemed minor on the concentration levels of the studied compounds. The total concentrations of organosulfates and nitrooxy organosulfates contributed to approximately 0.7% of PM 1 mass.

Beschreibung: Constraining CO 2 emissions from open biomass burning by satellite observations of co-emitted species: a method and its application to wildfires in Siberia Atmospheric Chemistry and Physics Discussions, 14, 3099-3168, 2014 Author(s): I. B. Konovalov, E. V. Berezin, P. Ciais, G. Broquet, M. Beekmann, J. Hadji-Lazaro, C. Clerbaux, M. O. Andreae, J. W. Kaiser, and E.-D. Schulze A method to constrain carbon dioxide (CO 2 ) emissions from open biomass burning by using satellite observations of co-emitted species and a chemistry-transport model (CTM) is proposed and applied to the case of wildfires in Siberia. CO 2 emissions are assessed by means of an emission model assuming a direct relationship between the biomass burning rate (BBR) and the Fire Radiative Power (FRP) derived from the MODIS measurements. The key features of the method are (1) estimating the FRP-to-BBR conversion factors (α) for different vegetative land cover types by assimilating the satellite observations of co-emitted species into the CTM, (2) optimal combination of the estimates of α derived independently from satellite observations of different species (CO and aerosol in this study), and (3) estimation of the diurnal cycle of the fire emissions directly from the FRP measurements. Values of α for forest and grassland fires in Siberia and their uncertainties are estimated by using the IASI carbon monoxide (CO) retrievals and the MODIS aerosol optical depth (AOD) measurements combined with outputs from the CHIMERE mesoscale chemistry transport model. The constrained CO emissions are validated through comparison of the respective simulations with the independent data of ground based CO measurements at the ZOTTO site. Using our optimal regional-scale estimates of the conversion factors (which are found to be in agreement with the earlier published estimates obtained from local measurements of experimental fires), the total CO 2 emissions from wildfires in Siberia in 2012 are estimated to be in the range from 262 to 477 Tg C, with the optimal (maximum likelihood) value of 354 Tg C. Sensitivity test cases featuring different assumptions regarding the injection height and diurnal variations of emissions indicate that the derived estimates of the total CO 2 emissions in Siberia are robust with respect to the modelling options (the different estimates vary within less than 10% of their magnitude). The obtained CO 2 emission estimates for several years are compared with the independent estimates provided by the GFED3.1 and GFASv1.0 global emission inventories. It is found that our "top-down" estimates for the total annual biomass burning CO 2 emissions in the period from 2007 to 2011 in Siberia are by factors of 2.3 and 1.7 larger than the respective bottom-up estimates; these discrepancies cannot be fully explained by uncertainties in our estimates. There are also considerable differences in the spatial distribution of the different emission estimates; some of those differences have a systematic character and require further analysis.

Beschreibung: Enhancement of aerosols in UTLS over the Tibetan Plateau induced by deep convection during the Asian summer monsoon Atmospheric Chemistry and Physics Discussions, 14, 3169-3191, 2014 Author(s): Q. S. He, C. C. Li, J. Z. Ma, H. Q. Wang, X. L. Yan, Z. R. Liang, and G. M. Qi Vertical profiles of aerosol extinction coefficients were measured by an Micro Pulse Lidar at Naqu (31.5° N, 92.1° E, 4508 m a.m.s.l.), a meteorological station located on the central part of the Tibetan Plateau during summer 2011. Observations show a persistent maximum in aerosol extinction coefficients in the upper troposphere–lower stratosphere (UTLS) within an anticyclone during the Asian summer monsoon. These aerosol layers were generally located at an altitude of 18–19 km m.s.l., 1–2 km higher than the tropopause, with broad layer depth ranging approximately 3–4 km. Variations in these aerosols are found to be closely related to the intensity of underlying deep convection. Efficient vertical transport resulting from the most intensive convection is considered to be most important for the enhancement of aerosols observed near the tropopause. Temporal variations in aerosol layer in UTLS over the Plateau show a significant peak at midnight. This further indicates that deep convection plays an important role in the accumulation of aerosols in UTLS over the Tibetan Plateau.

Beschreibung: Analysis of the global atmospheric methane budget using ECHAM-MOZ simulations for present-day, pre-industrial time and the Last Glacial Maximum Atmospheric Chemistry and Physics Discussions, 14, 3193-3230, 2014 Author(s): A. Basu, M. G. Schultz, S. Schröder, L. Francois, X. Zhang, G. Lohmann, and T. Laepple Atmospheric methane concentrations increased considerably from pre-industrial (PI) to present times largely due to anthropogenic emissions. However, firn and ice core records also document a notable rise of methane levels between the Last Glacial Maximum (LGM) and the pre-industrial era, the exact cause of which is not entirely clear. This study investigates these changes by analyzing the methane sources and sinks at each of these climatic periods. Wetlands are the largest natural source of methane and play a key role in determining methane budget changes in particular in the absence of anthropogenic sources. Here, a simple wetland parameterization suitable for coarse-scale climate simulations over long periods is introduced, which is derived from a high-resolution map of surface slopes together with various soil hydrology parameters from the CARAIB vegetation model. This parameterization was implemented in the chemistry general circulation model ECHAM5-MOZ and multi-year time slices were run for LGM, PI and present-day (PD) climate conditions. Global wetland emissions from our parameterization are 72 Tg yr −1 (LGM), 115 Tg yr −1 (PI), and 132 Tg yr −1 (PD). These estimates are lower than most previous studies, and we find a stronger increase of methane emissions between LGM and PI. Taking into account recent findings that suggest more stable OH concentrations than assumed in previous studies, the observed methane distributions are nevertheless well reproduced under the different climates. Hence, this is one of the first studies where a consistent model approach has been successfully applied for simulating methane concentrations over a wide range of climate conditions.

Beschreibung: Estimating greenhouse gas fluxes from an agriculture-dominated landscape using multiple planetary boundary layer methods Atmospheric Chemistry and Physics Discussions, 14, 3231-3267, 2014 Author(s): X. Zhang, X. Lee, T. J. Griffis, J. M. Baker, and W. Xiao Quantification of regional greenhouse gas (GHG) fluxes is essential for establishing mitigation strategies and evaluating their effectiveness. Here, we used multiple top-down approaches and multiple trace gas observations at a tall tower to estimate GHG regional fluxes and evaluate the GHG fluxes derived from bottom-up approaches. We first applied the eddy covariance, equilibrium, inverse modeling (CarbonTracker), and flux aggregation methods using three years of carbon dioxide (CO 2 ) measurements on a 244 m tall tower in the Upper Midwest, USA. We then applied the equilibrium method for estimating methane (CH 4 ) and nitrous oxide (N 2 O) fluxes with one-month high-frequency CH 4 and N 2 O gradient measurements on the tall tower and one-year concentration measurements on a nearby tall tower, and evaluated the uncertainties of this application. The results indicate that: (1) the flux aggregation, eddy covariance, the equilibrium method, and the CarbonTracker product all gave similar seasonal patterns of the regional CO 2 flux (10 5 –10 6 km 2 ), but that the equilibrium method underestimated the July CO 2 flux by 52–69%. (2) The annual budget varied among these methods from 74 to −131 g C-CO 2 m −2 yr −1 , indicating a large uncertainty in the annual CO 2 flux estimation. (3) The regional CH 4 and N 2 O emissions according to a top-down method were at least six and two times higher than the emissions from a bottom-up inventory (Emission Database for Global Atmospheric Research), respectively. (4) The global warming potentials of the CH 4 and N 2 O emissions were equal in magnitude to the cooling benefit of the regional CO 2 uptake. The regional GHG budget, including both biological and anthropogenic origins, is estimated at 7 ± 160 g CO 2 eq m −2 yr −1 .

Beschreibung: Determination of α-pinene-derived organic nitrate yields: particle phase partitioning and hydrolysis Atmospheric Chemistry and Physics Discussions, 14, 3301-3335, 2014 Author(s): J. D. Rindelaub, K. M. McAvey, and P. B. Shepson The hydroxyl radical oxidation of α-pinene under high NO x conditions was studied in a photochemical reaction chamber to investigate organic nitrate (RONO 2 ) production and partitioning between the gas and particle phases. We report an organic nitrate yield of 26 ± 7% from the oxidation of this monoterpene in the presence of nitric oxide (NO). However, the organic nitrate yield was found to be highly dependent on both chamber relative humidity (RH) and seed aerosol acidity, likely as a result of particle phase hydrolysis. The particle phase loss of organic nitrates perturbs the gas-particle equilibrium within the system, leading to decreased RONO 2 yields in both the gas and particle phases at elevated RH and an apparent non-equilibrium partitioning mechanism. This resulted in smaller apparent partition coefficients of the total organic nitrate species under high chamber RH. The hydrolysis of particle phase organic nitrates at low chamber relative humidity in this study implies that aerosol partitioning of organic nitrates may be an important sink for atmospheric NO x and may have a significant impact on regional air quality.

Beschreibung: Impacts of the East Asian summer monsoon on interannual variations of summertime surface-layer ozone concentrations over China Atmospheric Chemistry and Physics Discussions, 14, 3269-3300, 2014 Author(s): Y. Yang, H. Liao, and J. Li We apply a global three-dimensional Goddard Earth Observing System (GEOS) chemical transport model (GEOS-Chem) driven by NASA/GEOS-4 assimilated meteorological fields to quantify the impacts of the East Asian summer monsoon (EASM) on interannual variations of summertime surface-layer O 3 concentrations over China. With anthropogenic emissions fixed at year 2005 levels, model simulation for years 1986–2006 shows that the changes in meteorological parameters alone lead to interannual variations in surface-layer O 3 concentrations by 2–5% over central eastern China, 1–3% in northwestern China, and 5–10% over the Tibetan Plateau as well as the border and coastal areas of South China, as the interannual variations are relative to the average O 3 concentrations over the 21 yr. Over 1986–2006, O 3 concentration averaged over the whole China is found to correlate positively with the EASM index with a large correlation coefficient of +0.75, indicating that JJA O 3 concentrations are lower (or higher) in weaker (or stronger) EASM years. Relative to JJA surface-layer O 3 concentrations in the strongest EASM years (1990, 1994, 1997, 2002, and 2006), O 3 levels in the weakest EASM years (1988, 1989, 1996, 1998, and 2003) are lower over almost whole China with a nation mean lower O 3 concentration by 2.0 ppbv (or 4%). Regionally, the largest percentage differences in O 3 concentration between the weakest and strongest EASM years are found to exceed 6% in northeastern China, southwestern China, and over the Tibetan Plateau. Sensitivity studies show that the difference in transboundary transport of O 3 is the most dominant factor that leads to lower O 3 concentrations in the weakest EASM years than in the strongest EASM years, which, together with the enhanced vertical convections in the weakest EASM years, explain about 80% of the differences in surface-layer O 3 concentrations between the weakest and strongest EASM years. We also find that the changes in the EASM strength are as important as the changes in anthropogenic emissions over 1986–2006 in influencing JJA surface-layer O 3 concentrations in China.

Beschreibung: Ice nucleation and its effect on the atmospheric transport of fungal spores from the classes Agaricomycetes , Ustilaginomycetes , and Eurotiomycetes Atmospheric Chemistry and Physics Discussions, 14, 5013-5059, 2014 Author(s): D. I. Haga, S. M. Burrows, R. Iannone, M. J. Wheeler, R. Mason, J. Chen, E. A. Polishchuk, U. Pöschl, and A. K. Bertram Ice nucleation on fungal spores may affect the frequency and properties of ice and mixed-phase clouds. We studied the ice nucleation properties of 12 different species of fungal spores chosen from three classes: Agaricomycetes , Ustilaginomycetes , and Eurotiomycetes . Agaricomycetes include many types of mushroom species and are cosmopolitan. Ustilaginomycetes are agricultural pathogens and have caused widespread damage to crops. Eurotiomycetes are found on all types of decaying material and include important human allergens. We focused on these classes since they are thought to be abundant in the atmosphere and because there is very little information on the ice nucleation ability of these classes of spores in the literature. All of the fungal spores investigated were found to cause freezing of water droplets at temperatures warmer than homogeneous freezing. The cumulative number of ice nuclei per spore was 0.001 at temperatures between −19 °C and −29 °C, 0.01 between −25.5 °C and −31 °C, and 0.1 between −26 °C and −36 °C. On average, the order of ice nucleating ability for these spores is Ustilaginomycetes 〉 Agaricomycetes ≃ Eurotiomycetes . We show that at temperatures below −20 °C, all of the fungal spores studied here are less efficient ice nuclei compared to Asian mineral dust on a per surface area basis. We used our new freezing results together with data in the literature to compare the freezing temperatures of spores from the phyla Basidiomycota and Ascomycota , which together make up 98% of known fungal species found on Earth. The data show that within both phyla ( Ascomycota and Basidiomycota ) there is a wide range of freezing properties, and also that the variation within a phylum is greater than the variation between the average freezing properties of the phyla. Using a global chemistry–climate transport model, we investigated whether ice nucleation on the studied spores, followed by precipitation, can influence the atmospheric transport and global distributions of these spores in the atmosphere. Simulations show that inclusion of ice nucleation scavenging of these fungal spores in mixed-phase clouds can decrease the annual mean concentrations of fungal spores in near-surface air over the oceans and polar regions and decrease annual mean mixing ratios in the upper troposphere.

Beschreibung: Technical Note: Particulate reactive oxygen species concentrations and their association with environmental conditions in an urban, subtropical climate Atmospheric Chemistry and Physics Discussions, 14, 5061-5080, 2014 Author(s): S. S. Khurshid, J. A. Siegel, and K. A. Kinney Reactions between hydrocarbons and ozone or hydroxyl radicals lead to the formation of oxidized species, including reactive oxygen species (ROS), and secondary organic aerosol (SOA) in the troposphere. ROS can be carried deep into the lungs by small aerodynamic particles where they can cause oxidative stress and cell damage. While environmental studies have focused on ROS in the gas-phase and rainwater, it is also important to determine concentrations of ROS on respirable particles. Samples of PM 2.5 collected over 3 h at midday on 40 days during November 2011 and September 2012 show that the particulate ROS concentration in Austin, Texas ranged from a minimum value of 0.02 nmol H 2 O 2 (m 3 air) −1 in December to 3.81 nmol H 2 O 2 (m 3 air) −1 in September. Results from correlation tests and linear regression analysis on particulate ROS concentrations and environmental conditions (which included ozone and PM 2.5 concentrations, temperature, relative humidity, precipitation and solar radiation) indicate that ambient particulate ROS is significantly influenced by the ambient ozone concentration, temperature and incident solar radiation. Particulate ROS concentrations measured in this study were in the range reported by other studies in the US, Taiwan and Singapore. This study is one of the first to assess seasonal variations in particulate ROS concentrations and helps explain the influence of environmental conditions on particulate ROS concentrations.