ALBERT

Description: On the representation of immersion and condensation freezing in cloud models using different nucleation schemes Atmospheric Chemistry and Physics Discussions, 12, 7167-7209, 2012 Author(s): B. Ervens and G. Feingold Ice nucleation in clouds is often observed at temperatures 〉235 K, pointing to heterogeneous freezing as a predominant mechanism. Many models deterministically predict the number concentration of ice particles as a function of temperature and/or supersaturation. Laboratory experiments at constant temperature and/or supersaturation often report heterogeneous freezing as a stochastic, time-dependent process that follows classical nucleation theory which might appear to contradict singular freezing behavior. We explore the extent to which the choice of nucleation scheme (deterministic/stochastic, single/multiple contact angles θ) affects the prediction of the frozen ice nuclei (IN) fraction and cloud evolution. A box model with constant temperature and supersaturation is used to mimic published laboratory experiments of immersion freezing of kaolinite (~243 K), and the fitness of different nucleation schemes. Sensitivity studies show that agreement of all five schemes is restricted to the narrow parameter range (time, temperature, IN diameter) in the original laboratory studies. The schemes are implemented in an adiabatic parcel model that includes feedbacks of the formation and growth of drops and ice particles on supersaturation during the ascent of an air parcel. Model results show that feedbacks of droplets and ice on supersaturation limit ice nucleation events, often leading to smaller differences in number concentration of ice particles and ice water content (IWC) between stochastic and deterministic approaches than expected from the box model studies. However, the different parameterizations of θ distributions and time-dependencies are highly sensitive to IN size and can lead to great differences in predicted ice number concentrations and IWC between the different schemes. Finally, since the choice of nucleation scheme determines the temperature range over which nucleation occurs, at habit-prone temperatures (~253 K) different onset temperatures of freezing create variability in the initial inherent growth ratio of ice particles, which can lead to amplification or reduction in differences in predicted IWC.

Description: A multi-model assessment of the efficacy of sea spray geoengineering Atmospheric Chemistry and Physics Discussions, 12, 7125-7166, 2012 Author(s): K. J. Pringle, K. S. Carslaw, T. Fan, G.W. Mann, A. Hill, P. Stier, K. Zhang, and H. Tost Artificially increasing the albedo of marine clouds by the mechanical emission of sea spray aerosol has been proposed as a geoengineering technique to slow the warming caused by anthropogenic greenhouse gases. A previous global model study found that only modest increases and sometimes even decreases in cloud drop number (CDN) concentrations would result from plausible emission scenarios. Here we extend that work to examine the conditions under which decreases in CDN can occur, and use three independent global models to quantify maximum achievable CDN changes. We find that decreases in CDN can occur when at least three of the following conditions are met: the injected particle number is 250–300 nm, the background aerosol loading is large (≥150 cm −3 ) and the in-cloud updraught velocity is low (

Description: Observations of glyoxal and formaldehyde as metrics for the anthropogenic impact on rural photochemistry Atmospheric Chemistry and Physics Discussions, 12, 6049-6084, 2012 Author(s): J. P. DiGangi, S. B. Henry, A. Kammrath, E. S. Boyle, L. Kaser, R. Schnitzhofer, M. Graus, A. Turnipseed, J.-H. Park, R. J. Weber, R. S. Hornbrook, C. A. Cantrell, R. L. Maudlin III, S. Kim, Y. Nakashima, G. M. Wolfe, Y. Kajii, E. C. Apel, A. H. Goldstein, A. Guenther, T. Karl, A. Hansel, and F. N. Keutsch We present simultaneous fast, in-situ measurements of formaldehyde and glyoxal from two rural campaigns, BEARPEX 2009 and BEACHON-ROCS, both located in Pinus Ponderosa forests with emissions dominated by biogenic volatile organic compounds (VOCs). Despite considerable variability in the formaldehyde and glyoxal concentrations, the ratio of glyoxal to formaldehyde, R GF , displayed a very regular diurnal cycle over nearly 2 weeks of measurements. The only deviations in R GF were toward higher values and were the result of a biomass burning event during BEARPEX 2009 and very fresh anthropogenic influence during BEACHON-ROCS. Other rapid changes in glyoxal and formaldehyde concentrations have hardly any affect on R GF and could reflect transitions between low and high NO regimes. The trend of increased R GF from both anthropogenic reactive VOC mixtures and biomass burning compared to biogenic reactive VOC mixtures is robust due to the short timescales over which the observed changes in R GF occurred. Satellite retrievals, which suggest higher R GF for biogenic areas, are in contrast to our observed trends. It remains important to address this discrepancy, especially in view of the importance of satellite retrievals and in-situ measurements for model comparison. In addition, we propose that R GF , together with the absolute concentrations of glyoxal and formaldehyde, represents a useful metric for biogenic or anthropogenic reactive VOC mixtures. In particular, R GF yields information about not simply the VOCs in an airmass, but the VOC processing that directly couples ozone and secondary organic aerosol production.

Description: Characterization of aerosol and cloud water at a mountain site during WACS 2010: secondary organic aerosol formation through oxidative cloud processing Atmospheric Chemistry and Physics Discussions, 12, 6019-6047, 2012 Author(s): A. K. Y. Lee, K. L. Hayden, P. Herckes, W. R. Leaitch, J. Liggio, A. M. Macdonald, and J. P. D. Abbatt The water-soluble fractions of aerosol samples and cloud water collected during Whistler Aerosol and Cloud Study (WACS 2010) were analyzed using an Aerodyne aerosol mass spectrometer (AMS). This is the first study to report AMS organic spectra of re-aerosolized cloud water, and to make direct comparison between the AMS spectra of cloud water and aerosol samples collected at the same location. In general, the aerosol and cloud organic spectra were very similar, indicating that the cloud water organics likely originated from secondary organic aerosol (SOA) formed nearby. By using a photochemical reactor to oxidize both aerosol filter extracts and cloud water, we find evidence that fragmentation of aerosol water-soluble organics increases their volatility during oxidation. By contrast, enhancement of AMS-measurable organic mass by up to 30% was observed during aqueous-phase photochemical oxidation of cloud water organics. We propose that additional SOA material was produced by functionalizing dissolved organics via OH oxidation, where these dissolved organics are sufficiently volatile that they are not usually part of the aerosol. This work points out that water-soluble organic compounds of intermediate volatility (IVOC), such as cis -pinonic acid, produced via gas-phase oxidation of monoterpenes, can be important aqueous-phase SOA precursors in a biogenic-rich environment.

Description: Evaluation of chemical transport model predictions of primary organic aerosol for air masses classified by particle-component-based factor analysis Atmospheric Chemistry and Physics Discussions, 12, 5939-6018, 2012 Author(s): C. A. Stroud, M. D. Moran, P. A. Makar, S. Gong, W. Gong, J. Zhang, J. G. Slowik, J. P. D. Abbatt, G. Lu, J. R. Brook, C. Mihele, Q. Li, D. Sills, K. B. Strawbridge, M. L. McGuire, and G. J. Evans Observations from the 2007 Border Air Quality and Meteorology Study (BAQS-Met 2007) in southern Ontario (ON), Canada, were used to evaluate Environment Canada's regional chemical transport model predictions of primary organic aerosol (POA). Environment Canada's operational numerical weather prediction model and the 2006 Canadian and 2005 US national emissions inventories were used as input to the chemical transport model (named AURAMS). Particle-component-based factor analysis was applied to aerosol mass spectrometer measurements made at one urban site (Windsor, ON) and two rural sites (Harrow and Bear Creek, ON) to derive hydrocarbon-like organic aerosol (HOA) factors. Co-located carbon monoxide (CO), PM 2.5 black carbon (BC), and PM 1 SO 4 measurements were also used for evaluation and interpretation, permitting a detailed diagnostic model evaluation. At the urban site, good agreement was observed for the comparison of daytime campaign PM 1 POA and HOA mean values: 1.1 μg m −3 vs. 1.2 μg m −3 , respectively. However, a POA overprediction was evident on calm nights due to an overly-stable model surface layer. Biases in model POA predictions trended from positive to negative with increasing HOA values. This trend has several possible explanations, including (1) underweighting of urban locations in particulate matter (PM) spatial surrogate fields, (2) overly-coarse model grid spacing for resolving urban-scale sources, and (3) lack of a model particle POA evaporation process during dilution of vehicular POA tail-pipe emissions to urban scales. Furthermore, a trend in POA bias was observed at the urban site as a function of the BC/HOA ratio, suggesting a possible association of POA underprediction for diesel combustion sources. For several time periods, POA overprediction was also observed for sulphate-rich plumes, suggesting that our model POA fractions for the PM 2.5 chemical speciation profiles may be too high for these point sources. At the rural Harrow site, significant underpredictions in PM 1 POA concentration were found compared to observed HOA concentration and were associated, based on back-trajectory analysis, with (1) transport from the Detroit/Windsor urban complex, (2) longer-range transport from the US Midwest, and (3) biomass burning. Daytime CO concentrations were significantly overpredicted at Windsor but were unbiased at Harrow. Collectively, these biases provide support for a hypothesis that combines a current underweighting of PM spatial surrogate fields for urban locations with insufficient model vertical mixing for sources close to the urban measurement sites. The magnitude of the area POA emissions sources in the US and Canadian inventories (e.g., food cooking, road and soil dust, waste disposal burning) suggests that more effort should be placed at reducing uncertainties in these sectors, especially spatial and temporal surrogates.

Description: The sudden stratospheric warming of the Arctic winter 2009/2010: comparison to other recent warm winters Atmospheric Chemistry and Physics Discussions, 12, 7243-7271, 2012 Author(s): J. Kuttippurath and G. Nikulin The Arctic winter 2009/10 was moderately cold in December. A minor warming occurred around mid-December due to a wave 2 amplification split the lower stratospheric vortex into two lobes. The vortices merged again and formed a relatively large vortex in a few days. The temperatures began to rise by mid-January and triggered a major sudden stratospheric warming (SSW) by the reversal of westerlies in late (24–26) January, driven by a planetary wave 1 with a peak amplitude of about 100 m 2 s −2 at 60° N/10 hPa. The momentum flux associated with this warming showed the largest value in the recent winters, about 450 m 2 s −2 at 60° N/10 hPa. The associated vortex split confined to altitudes below 10 hPa and hence, the major warming (MW) was a vortex displacement event. Large amounts of Eliassen-Palm (EP) and wave 2 EP fluxes (3.9 ×10 5 kg s −2 ) are found shortly before the MW event at 100 hPa over 45–75° N, suggesting a tropospheric preconditioning of the MW event. We observe an increase in SSWs in the Arctic in recent years, as there were 6 MWs in 6 out of the 7 winters of 2003/04–2009/10, which confirms the conclusions of previous studies on the SSWs in winters prior to 2003/04. Each MW event was unique as far as its evolution and related polar processes were concerned. As compared to the MWs in the recent Arctic winters, the strongest MW was observed in 2008/09 and was initiated by a wave 2 event. A detailed diagnosis of ozone loss during the past fifteen years shows that the loss is inversely proportional to the intensity and timing of SSWs in each winter, where early MWs lead to minimal loss. The ozone loss shows a good correlation with the zonal mean amplitude of zonal winds in January over 60–90° N, suggesting a proxy for MWs in the Arctic winters.

Description: Observing the continental-scale carbon balance: assessment of sampling complementarity and redundancy in a terrestrial assimilation system by means of quantitative network design Atmospheric Chemistry and Physics Discussions, 12, 7211-7242, 2012 Author(s): T. Kaminski, P. J. Rayner, M. Voßbeck, M. Scholze, and E. Koffi This paper investigates the relationship between the heterogeneity of the terrestrial carbon cycle and the optimal design of observing networks to constrain it. We combine the methods of quantitative network design and carbon-cycle data assimilation to a hierarchy of increasingly heterogeneous descriptions of the European terrestrial biosphere as indicated by increasing diversity of plant functional types. We employ three types of observations, flask measurements of CO 2 concentrations, continuous measurements of CO 2 and pointwise measurements of CO 2 flux. We show that flux measurements are extremely efficient for relatively homogeneous situations but not robust against increasing or unknown complexity. Here a hybrid approach is necessary and we recommend its use in the development of integrated carbon observing systems.

Description: Isoprene emissions in Africa inferred from OMI observations of formaldehyde columns Atmospheric Chemistry and Physics Discussions, 12, 7475-7520, 2012 Author(s): E. A. Marais, D. J. Jacob, T. P. Kurosu, K. Chance, J. G. Murphy, C. Reeves, G. Mills, S. Casadio, D. B. Millet, M. P. Barkley, F. Paulot, and J. Mao We use 2005–2009 satellite observations of formaldehyde (HCHO) columns from OMI to infer biogenic isoprene emissions at monthly 1 × 1° resolution over the African continent. Our work includes new approaches to remove biomass burning influences using OMI absorbing aerosol optical depth data (to account for transport of fire plumes) and anthropogenic influences using AATSR satellite data for persistent small-flame fires (gas flaring). The resulting biogenic HCHO columns (Ω HCHO ) follow closely the distribution of vegetation patterns in Africa. We infer isoprene emission ( E ISOP ) from the local sensitivity S =ΔΩ HCHO /Δ E ISOP derived with the GEOS-Chem chemical transport model using two alternate isoprene oxidation mechanisms, and verify the validity of this approach using AMMA aircraft observations over West Africa and a longitudinal transect across central Africa. Displacement error (smearing) is diagnosed by anomalously high values of S and the corresponding data are removed. We find significant sensitivity of S to NO x under low-NO x conditions that we fit to a linear function of tropospheric column NO 2 from OMI. We estimate a 40% error in our inferred isoprene emissions under high-NO x conditions and 40–90% under low-NO x conditions. Comparison to the state-of-science MEGAN inventory indicates a large overestimate of central African rainforest emissions in that inventory.

Description: Global distribution and climate forcing of marine organic aerosol – Part 2: Effects on cloud properties and radiative forcing Atmospheric Chemistry and Physics Discussions, 12, 7453-7474, 2012 Author(s): B. Gantt, J. Xu, N. Meskhidze, Y. Zhang, A. Nenes, S. J. Ghan, X. Liu, R. Easter, and R. Zaveri In the first part of this paper series (Meskhidze et al., 2011), a treatment of marine organic aerosols (including primary organic aerosol, secondary organic aerosols, and methane sulfonate) was implemented into the Community Atmosphere Model version 5 (CAM5) with a 7-mode Modal Aerosol Module. A series of simulations was conducted to quantify the changes in aerosol and cloud condensation nuclei concentrations in the marine boundary layer. In this study, changes in the cloud microphysical properties and radiative forcing resulting from marine organic aerosols are assessed. Model simulations show that the anthropogenic aerosol indirect forcing (AIF) predicted by CAM5 is decreased in absolute magnitude by up to ~0.10 W m −2 (8%) when marine organic aerosols are included. Changes in the AIF from marine organic aerosols are associated with small global increases in low-level in-cloud droplet number concentration and liquid water path of ~1.3 cm −3 (~1.6%) and 0.2 g m −2 (0.5%), respectively. Areas especially sensitive to changes in cloud properties due to marine organic aerosol include the Southern Ocean, North Pacific Ocean, and North Atlantic Ocean, all of which are characterized by high marine organic emission rates. As climate models are particularly sensitive to the background aerosol concentration, this small but non-negligible change in the AIF due to marine organic aerosols provides a notable link for ocean-ecosystem marine low-level cloud interactions and may be a candidate for consideration in future earth system models.

Description: Ice nuclei in marine air: bioparticles or dust? Atmospheric Chemistry and Physics Discussions, 12, 4373-4416, 2012 Author(s): S. M. Burrows, C. Hoose, U. Pöschl, and M. G. Lawrence Ice nuclei can influence the properties of clouds and precipitation, but their sources and distribution in the atmosphere are still not well known. Particularly little attention has been paid to IN sources in marine environments, although anecdotal evidence suggests that IN populations in remote marine regions may be dominated by biological particles associated with sea spray. In this exploratory model study, we aim to bring attention to this long-neglected topic and identify promising target regions for future field campaigns. We assess the likely global distribution of marine biological ice nuclei using a combination of historical observations, satellite data and model output. By comparing simulated marine biological IN distributions and dust IN distributions, we predict strong regional differences in the importance of marine biological IN relative to dust IN. Our analysis suggests that marine biological IN are most likely to play a dominant role in determining IN concentrations over the Southern Ocean, so future field campaigns aimed at investigating marine biological IN should target that region. Climate-related changes in the abundance and emission of biological marine IN could affect marine cloud properties, thereby introducing previously unconsidered feedbacks that influence the hydrological cycle and the Earth's energy balance. Furthermore, marine biological IN may be an important aspect to consider in proposals for marine cloud brightening by artificial sea spray production.

Description: An observation-based approach to identify local natural dust events from routine aerosol ground monitoring Atmospheric Chemistry and Physics Discussions, 12, 4279-4310, 2012 Author(s): D. Q. Tong, M. Dan, T. Wang, and P. Lee Dust is a major component of atmospheric aerosols in many parts of the world. Although there exist many routine aerosol monitoring networks, it is often difficult to obtain dust records from these networks, because these monitors are either deployed far away from dust active regions (most likely collocated with dense population) or contaminated by anthropogenic sources and other natural sources, such as wildfires and vegetation detritus. Here we propose a new approach to identify local dust events relying solely on aerosol mass and composition from general-purpose aerosol measurements. Through analyzing the chemical and physical characteristics of aerosol observations during satellite-detected dust episodes, we select five indicators to be used to identify local dust records: (1) high PM 10 concentrations; (2) low PM 2.5 /PM 10 ratio; (3) higher concentrations and percentage of crustal elements; (4) lower percentage of anthropogenic pollutants; and (5) low enrichment factors of anthropogenic elements. After establishing these identification criteria, we conduct hierarchical cluster analysis for all validated aerosol measurement data over 68 IMPROVE sites in the Western United States. A total of 182 local dust events were identified over 30 of the 68 locations from 2000 to 2007. These locations are either close to the four US Deserts, namely the Great Basin Desert, the Mojave Desert, the Sonoran Desert, and the Chihuahuan Desert, or in the high wind power region (Colorado). During the eight-year study period, the total number of dust events displays an interesting four-year activity cycle (one in 2000–2003 and the other in 2004–2007). The years of 2003, 2002 and 2007 are the three most active dust periods, with 46, 31 and 24 recorded dust events, respectively, while the years of 2000, 2004 and 2005 are the calmest periods, all with single digit dust records. Among these deserts, the Chihuahua Desert (59 cases) and the Sonoran Desert (62 cases) are by far the most active source regions. In general, the Chihuahua Desert dominates dust activities in the first half of the eight-year period while the Sonoran Desert in the second half. The monthly frequency of dust events shows a peak from March to July and a second peak in autumn from September to November. The large quantity of dust events occurring in summertime also suggests the prevailing impact of windblown dust across the year. This seasonal variation is consistent with previous model simulations over the United States.

Description: Quantifying retrieval uncertainties in the CM-SAF cloud physical property algorithm with simulated SEVIRI observations Atmospheric Chemistry and Physics Discussions, 12, 4311-4340, 2012 Author(s): B. J. Jonkheid, R. A. Roebeling, and E. van Meijgaard The uncertainties in the cloud physical properties derived from satellite observations make it difficult to interpret model evaluation studies. In this paper, the uncertainties in the cloud water path (CWP) retrievals derived with the cloud physical properties retrieval algorithm (CPP) of the climate monitoring satellite application facility (CM-SAF) are investigated. To this end, a numerical simulator of MSG-SEVIRI observations was developed that calculates the reflectances at 0.64 and 1.63 μm for a wide range of cloud parameters, satellite viewing geometries and surface albedos. These reflectances are used as input to CPP, and the retrieved values of CWP are compared to the original input of the simulator. It is shown that the CWP retrievals are very sensitive to the assumptions made in the CPP code. The CWP retrieval errors are generally small for unbroken single-phase clouds with COT 〉10, with retrieval errors of ~3% for liquid water clouds to ~10% for ice clouds. When both liquid water and ice clouds are present in a pixel, the CWP retrieval errors increase dramatically; depending on the cloud, this can lead to uncertainties of 40–80%. CWP retrievals also become more uncertain when the cloud does not cover the entire pixel, leading to errors of ~50% for cloud fractions of 0.75 and even larger errors for smaller cloud fractions. Thus, the satellite retrieval of cloud physical properties of broken clouds and multi-phase clouds is complicated by inherent difficulties, and the proper interpretation of such retrievals requires extra care.

Description: Enhanced cold-season warming in semi-arid regions Atmospheric Chemistry and Physics Discussions, 12, 4627-4653, 2012 Author(s): J. Huang, X. Guan, and F. Ji This study examined surface air temperature trends over global land from 1901–2009. It is found that the warming trend was particularly enhanced, in the boreal cold season (November to March) over semi-arid regions (with precipitation of 200–600 mm yr −1 ), showing a temperature increase of 1.53 °C as compared to the global annual mean temperature increase of 1.13 °C over land. In mid-latitude semi-arid areas of Europe, Asia, and North America, temperatures in the cold season increased by 1.41, 2.42, and 1.5 °C, respectively. The semi-arid regions contribute 44.46% to global annual-mean land-surface temperature trend. The mid-latitude semi-arid regions in the Northern Hemisphere accounting contribute by 27.0% of the total, with the mid-latitude semi-arid areas in Europe, Asia, and North America accounting for 6.29%, 13.81%, and 6.85%, respectively. Such enhanced semi-arid warming (ESAW) may cause these regions to become drier and warmer.

Description: Humidity-dependent phase state of SOA particles from biogenic and anthropogenic precursors Atmospheric Chemistry and Physics Discussions, 12, 4447-4476, 2012 Author(s): E. Saukko, A. T. Lambe, P. Massoli, Koop, T., J. P. Wright, D. R. Croasdale, D. A. Pedernera, T. B. Onasch, A. Laaksonen, P. Davidovits, D. R. Worsnop, and A. Virtanen The physical phase state (solid, semi-solid, or liquid) of secondary organic aerosol (SOA) particles has important implications for a number of atmospheric processes. We report the phase state of SOA particles spanning a wide range of oxygen to carbon ratios (O/C), used here as a surrogate for SOA oxidation level, produced in a flow tube reactor by photo-oxidation of various atmospherically relevant surrogate anthropogenic and biogenic volatile organic compounds (VOCs). The phase state of laboratory-generated SOA was determined by the particle bounce behavior after inertial impaction on a polished steel substrate. The measured bounce fraction was evaluated as a function of relative humidity and SOA oxidation level (O/C) measured by an Aerodyne high resolution time of flight aerosol mass spectrometer (HR-ToF AMS). The main findings of the study are: (1) Biogenic and anthropogenic SOA particles are found to be solid or semi-solid until a relative humidity of at least 50 % RH at impaction is reached. (2) Long-chain alkanes produce liquid SOA particles when generated at low oxidation level of O/C

Description: Carbonaceous aerosol AAE inferred from in-situ aerosol measurements at the Gosan ABC super site, and the implications for brown carbon aerosol Atmospheric Chemistry and Physics Discussions, 12, 4507-4539, 2012 Author(s): C. E. Chung, S.-W. Kim, M. Lee, S.-C. Yoon, and S. Lee Carbon mass of aerosols and its division between EC and OC sources were continuously measured at hourly intervals from October 2009 to June 2010. During this 9-month period, we also measured the aerosol absorption coefficient at 7 wavelengths and obtained PM mass density data at 1-h resolution. The measurement was made at the Gosan ABC super site, which is an ideal location for monitoring long-range transported aerosols from China. We remove the absorption data with significant dust influence using the mass ratio of PM 10 to PM 2.5 . The remaining data shows an Absorption Ångström Exponent (AAE) of about 1.27, which we suggest represent the average carbonaceous aerosol (CA) AAE at Gosan. CA AAE is highest in winter, in which the monthly value is near 1.4. We find a positive correlation between the mass ratio of OC to EC and CA AAE, and successfully increase the correlation by filtering out data associated with weak absorption signal. After the filtering, absorption coefficient is regressed on OC and EC mass densities. Black carbon (BC) and organic aerosol (OA) absorption cross sections per carbon mass are found to be 5.1 (4.2–6.0) and 1.4 (1.1–1.8) m 2 g −1 at 520 nm respectively. From the estimated BC & OA MAC, we find that OA contributes about 45% to CA absorption at 520 nm. OA AAE is found to be 1.7 (1.4–2.1). Compared with a previous estimate of OA MAC and AAE, our estimates at Gosan strongly suggest that the strongly-absorbing so-called brown carbon spheres are either unrelated to biomass burning or absent near the emission source.

Description: Organic molecular markers and signature from wood combustion particles in winter ambient aerosols: aerosol mass spectrometer (AMS) and high time-resolved GC-MS measurements in Augsburg, Germany Atmospheric Chemistry and Physics Discussions, 12, 4831-4866, 2012 Author(s): M. Elsasser, M. Crippa, J. Orasche, P. F. DeCarlo, M. Oster, M. Pitz, T. L. Gustafson, J. B. C. Pettersson, J. Schnelle-Kreis, A. S. H. Prévôt, and R. Zimmermann The impact of wood combustion on ambient aerosols was investigated in Augsburg, Germany during a winter measurement campaign of a six-week period. Special attention was paid to the high time resolution observations of wood combustion with different mass spectrometric methods. Here we present and compare the results from an Aerodyne aerosol mass spectrometer (AMS) and gas chromatographic – mass spectrometric (GC-MS) analysed PM 1 filters on an hourly basis. This includes source apportionment of the AMS derived organic mass by using positive matrix factorisation (PMF) and analysis of levoglucosan as wood combustion marker, respectively. In the measurement period nitrate and organics are the main contributors to the defined submicron particle mass with 28% and 35%, respectively. To the latter wood combustion organic aerosol (WCOA) contributes 23% on average and 27% in the evening and night-time. Conclusively, wood combustion has a strong influence on the organics and overall aerosol composition. Levoglucosan accounts for 14% of WCOA mass with a higher percentage in comparison to other studies. The ratio between the mass of levoglucosan and organic carbon amounts to 0.06. This study is unique in the one-hour time resolution comparison between the wood combustion results of the AMS and the GC-MS analysed filter method at a PM 1 particle size range. This comparison of the concentration courses of the PMF WCOA factor, levoglucosan estimated by the AMS data and the levoglucosan measured by GC-MS is highly correlated, and a detailed discussion on the contributors to the wood combustion marker ion at mass-to-charge ratio 60 will be given. This offers a suitable application possibility for the description of the wood combustion course by the WCOA factor and the levoglucosan concentration estimated by AMS data. However, quantitative description of the levoglucosan concentration estimated by the AMS data is difficult due to the offset of latter compared to measured levoglucosan by the GC-MS.

Description: An isotope view on ionising radiation as a source of sulphuric acid Atmospheric Chemistry and Physics Discussions, 12, 5039-5064, 2012 Author(s): M. B. Enghoff, N. Bork, S. Hattori, C. Meusinger, M. Nakagawa, J. O. P. Pedersen, S. Danielache, Y. Ueno, M. S. Johnson, N. Yoshida, and H. Svensmark Sulphuric acid is an important factor in aerosol nucleation and growth. It has been shown that ions enhance the formation of sulphuric acid aerosols, but the exact mechanism has remained undetermined. Furthermore some studies have found a deficiency in the sulphuric acid budget, suggesting a missing source. In this study the production of sulphuric acid from SO 2 through a number of different pathways is investigated. The production methods are standard gas phase oxidation by OH radicals produced by ozone photolysis with UV light, liquid phase oxidation by ozone, and gas phase oxidation initiated by gamma rays. The distributions of stable sulphur isotopes in the products and substrate were measured using isotope ratio mass spectrometry. All methods produced sulphate enriched in 34 S and we find a δ 34 S value of 8.7 ± 0.4‰ (1 standard deviation) for the UV-initiated OH reaction. Only UV light (Hg emission at 253.65 nm) produced a clear non-mass-dependent excess of 33 S. The pattern of isotopic enrichment produced by gamma rays is similar, but not equal, to that produced by aqueous oxidation of SO 2 by ozone. This, combined with the relative yields of the experiments, suggests a mechanism in which ionising radiation may lead to hydrated ion clusters that serve as nanoreactors for S(IV) to S(VI) conversion.

Description: Quantification of diesel exhaust gas phase organics by a thermal desorption proton transfer reaction mass spectrometer Atmospheric Chemistry and Physics Discussions, 12, 5389-5423, 2012 Author(s): M. H. Erickson, H. W. Wallace, and B. T. Jobson A new approach was developed to measure the total abundance of long chain alkanes (C 12 and above) in urban air using thermal desorption with a proton transfer reaction mass spectrometer (PTR-MS). These species are emitted in diesel exhaust and may be important precursors to secondary organic aerosol production in urban areas. Long chain alkanes undergo dissociative proton transfer reactions forming a series of fragment ions with formula C n H 2n+1 . The yield of the fragment ions is a function of drift conditions. At a drift field strength of 80 Townsends, the most abundant ion fragments from C 10 to C 16 n -alkanes were m/z 57, 71 and 85. The PTR-MS is insensitive to n -alkanes less than C 8 but displays an increasing sensitivity for larger alkanes. Higher drift field strengths yield greater normalized sensitivity implying that the proton affinity of the long chain n -alkanes is less than H 2 O. Analysis of diesel fuel shows the mass spectrum was dominated by alkanes (C n H 2 n +1 ), monocyclic aromatics, and an ion group with formula C n H 2 n −1 ( m/z 97, 111, 125, 139). The PTR-MS was deployed in Sacramento, CA during the Carbonaceous Aerosols and Radiative Effects Study field experiment in June 2010. The ratio of the m/z 97 to 85 ion intensities in ambient air matched that found in diesel fuel. Total diesel exhaust alkane concentrations calculated from the measured abundance of m/z 85 ranged from the method detection limit of ~1 μg m −3 to 100 μg m −3 in several air pollution episodes. The total diesel exhaust alkane concentration determined by this method was on average a factor of 10 greater than the sum of alkylbenzenes associated with spark ignition vehicle exhaust.

Description: Arctic climate response to forcing from light-absorbing particles in snow and sea ice in CESM Atmospheric Chemistry and Physics Discussions, 12, 5341-5388, 2012 Author(s): N. Goldenson, S. J. Doherty, C. M. Bitz, M. M. Holland, B. Light, and A. J. Conley The presence of light-absorbing aerosol particles deposited on arctic snow and sea ice influences the surface albedo, causing greater shortwave absorption, warming, and loss of snow and sea ice, lowering the albedo further. The Community Earth System Model version 1 (CESM1) now includes the radiative effects of light-absorbing particles in snow on land and sea ice and in sea ice itself. We investigate the model response to the deposition of black carbon and dust to both snow and sea ice. For these purposes we employ a slab ocean version of CESM1, using the Community Atmosphere Model version 4 (CAM4), run to equilibrium for year 2000 levels of CO 2 and fixed aerosol deposition. We construct experiments with and without aerosol deposition, with dust or black carbon deposition alone, and with varying quantities of black carbon and dust to approximate year 1850 and 2000 deposition fluxes. The year 2000 deposition fluxes of both dust and black carbon cause 1–2 °C of surface warming over large areas of the Arctic Ocean and sub-Arctic seas in autumn and winter and in patches of Northern land in every season. Atmospheric circulation changes are a key component of the surface-warming pattern. Arctic sea ice thins by on average about 30 cm. Simulations with year 1850 aerosol deposition are not substantially different from those with year 2000 deposition, given constant levels of CO 2 . The climatic impact of particulate impurities deposited over land exceeds that of particles deposited over sea ice. Even the surface warming over the sea ice and sea ice thinning depends more upon light-absorbing particles deposited over land. For CO 2 doubled relative to year 2000 levels, the climate impact of particulate impurities in snow and sea ice is substantially lower than for the year 2000 equilibrium simulation.

Description: Modelling of organic aerosols over Europe (2002–2007) using a volatility basis set (VBS) framework with application of different assumptions regarding the formation of secondary organic aerosol Atmospheric Chemistry and Physics Discussions, 12, 5425-5485, 2012 Author(s): R. Bergström, H. A. C. Denier van der Gon, A. S. H. Prévôt, K. E. Yttri, and D. Simpson A new organic aerosol (OA) module has been implemented into the EMEP chemical transport model. Four different volatility basis set (VBS) schemes have been tested in long-term simulations for Europe, covering the six years 2002–2007. Different assumptions regarding partitioning of primary OA (POA) and aging of POA and secondary OA (SOA), have been explored. Model results are compared to filter measurements, AMS-data and source-apportionment studies, as well as to other model studies. The present study indicates that many different sources contribute significantly to OA in Europe. Fossil POA and oxidised POA, biogenic and anthropogenic SOA (BSOA and ASOA), residential burning of biomass fuels and wildfire emissions may all contribute more than 10% each over substantial parts of Europe. Simple VBS based OA models can give reasonably good results for summer OA but more observational studies are needed to constrain the VBS parameterisations and to help improve emission inventories. The volatility distribution of primary emissions is an important issue for further work. This study shows smaller contributions from BSOA to OA in Europe than earlier work, but relatively greater ASOA. BVOC emissions are highly uncertain and need further validation. We can not reproduce winter levels of OA in Europe, and there are many indications that the present emission inventories substantially underestimate emissions from residential wood burning in large parts of Europe.

Description: Dissolved organic carbon (DOC) and select aldehydes in cloud and fog water: the role of the aqueous phase in impacting trace gas budgets Atmospheric Chemistry and Physics Discussions, 12, 33083-33125, 2012 Author(s): B. Ervens, Y. Wang, J. Eagar, W. R. Leaitch, A. M. Macdonald, K. T. Valsaraj, and P. Herckes Cloud and fog droplets efficiently scavenge and process water-soluble compounds and thus modify the chemical composition of the gas and particle phases. The concentrations of dissolved organic carbon (DOC) in the aqueous phase reach concentrations on the order of ~10 mg C L −1 which is typically on the same order of magnitude as the sum of inorganic anions. Aldehydes and carboxylic acids typically comprise a large fraction of DOC because of their high solubility. The dissolution of species in the aqueous phase can lead to (i) the removal of species from the gas phase preventing their processing by gas phase reactions (e.g. photolysis of aldehydes) and (ii) the formation of unique products that do not have any efficient gas phase sources (e.g. dicarboxylic acids). We present measurements of DOC and select aldehydes in fog water at high elevation and intercepted clouds in a biogenically-impacted location (Whistler, Canada) and in fog water in a more polluted area (Davis, CA). Concentrations of formaldehyde, glyoxal and methylglyoxal were in the micromolar range and comprised ≤2% each individually of the DOC. Comparison of the DOC and aldehyde concentrations to those at other locations shows good agreement and reveals highest levels for both in anthropogenically impacted regions. Based on this overview, we conclude that the fraction of organic carbon (dissolved and insoluble inclusions) in the aqueous phase comprises 1–~40% of total organic carbon. Higher values are observed to be associated with aged air masses where organics are expected to be more highly oxidized and thus more soluble. Accordingly, the aqueous/gas partitioning ratio expressed here as an effective Henry's law constant for DOC ( K H *DOC ) increases by an order of magnitude from 7×10 3 M atm −1 to 7×10 4 M atm −1 during the ageing of air masses. The measurements are accompanied by photochemical box model simulations. They suggest that the scavenging of aldehydes by the aqueous phase can reduce HO 2 gas phase levels by two orders of magnitude due to a weaker net source of HO 2 production from aldehyde photolysis in the gas phase. Despite the high solubility of dialdehydes (glyoxal, methylglyoxal), their impact on the HO 2 budget by scavenging is

Description: Limitations of wind extraction from 4-D-Var assimilation of trace gases Atmospheric Chemistry and Physics Discussions, 12, 32985-33023, 2012 Author(s): D. R. Allen, K. W. Hoppel, G. E. Nedoluha, D. D. Kuhl, N. L. Baker, L. Xu, and T. E. Rosmond Time-dependent variational data assimilation allows the possibility of extracting wind information from observations of long-lived trace gases. Since trace gas observations are not available at sufficient resolution for deriving feature-track winds, they must be combined with model background information to produce an analysis. If done with time-dependent variational assimilation, wind information may be extracted via the adjoint of the linearized tracer continuity equation. This paper presents idealized experiments that illustrate the mechanics of tracer-wind extraction and demonstrate some of the limitations of this procedure. We first examine tracer-wind extraction using a simple one-dimensional advection equation. The analytic solution for a single trace gas observation is discussed along with numerical solutions for multiple observations. The limitations of tracer-wind extraction are then explored using highly idealized ozone experiments performed with a development version of the Navy Global Environmental Model (NAVGEM) in which stratospheric globally-distributed hourly stratospheric ozone profiles are assimilated in a single 6-h update cycle in January 2009. Starting with perfect background ozone conditions, but imperfect dynamical conditions, ozone errors develop over the 6-h background window. Wind increments are introduced in the analysis in order to reduce the differences between background ozone and ozone observations. For "perfect" observations (unbiased and no random error), this results in root mean square (RMS) vector wind error reductions of up to ∼ 3 m s −1 in the winter hemisphere and tropics. Wind extraction is more difficult in the summer hemisphere due to weak ozone gradients and smaller background wind errors. The limitations of wind extraction are also explored for observations with imposed random errors and for limited sampling patterns. As expected, the amount of wind information extracted degrades as observation errors or data voids increase. In the case of poorly specified observation error covariances, assimilation of ozone data with imposed errors may result in erroneous wind increments, since the assimilation is constrained too tightly to the noisy observations.

Description: Mass deposition fluxes of Saharan mineral dust to the tropical northeast Atlantic Ocean: an intercomparison of methods Atmospheric Chemistry and Physics Discussions, 12, 33025-33081, 2012 Author(s): N. Niedermeier, A. Held, T. Müller, B. Heinold, K. Schepanski, I. Tegen, K. Kandler, M. Ebert, S. Weinbruch, K. Read, J. Lee, K. W. Fomba, K. Müller, H. Herrmann, and A. Wiedensohler The aim of this study is to determine the mass deposition flux of mineral dust to the tropical northeast Atlantic Ocean at the Cape Verde Atmospheric Observatory (CVAO) on the island Sao Vicente for January 2009. Five different methods were applied to estimate the deposition flux, using different meteorological and microphysical measurements, remote sensing, and regional dust transport simulations. The set of observations comprises micrometeorological measurements with an ultra-sonic anemometer and profile measurements using 2-D anemometers at two different heights, and microphysical measurements of the size-resolved mass concentrations of mineral dust. In addition, the total mass concentration of mineral dust was derived from absorption photometer observations and passive sampling. The regional dust model COSMO-MUSCAT was used for simulations of dust emission and transport, including dry and wet deposition processes. The four observation-based methods yield a monthly average deposition flux of mineral dust of 12–29 ng m −2 s −1 . The simulation results come close to the upper range of the measurements with an average value of 47 ng m −2 s −1 . It is shown, that the mass deposition flux of mineral dust obtained by the combination of micrometeorological (ultra-sonic anemometer) and microphysical measurements (particle mass size distribution of mineral dust) is within 5% to modeled mass deposition fluxes when the mineral dust is relatively homogenously distributed over the investigated area.

Description: Intercomparison of shortwave radiative transfer schemes in global aerosol modeling: results from the AeroCom Radiative Transfer Experiment Atmospheric Chemistry and Physics Discussions, 12, 32631-32706, 2012 Author(s): C. A. Randles, S. Kinne, G. Myhre, M. Schulz, P. Stier, J. Fischer, L. Doppler, E. Highwood, C. Ryder, B. Harris, J. Huttunen, Y. Ma, R. T. Pinker, B. Mayer, D. Neubauer, R. Hitzenberger, L. Oreopoulos, D. Lee, G. Pitari, G. Di Genova, J. Quaas, Fred G. Rose, S. Kato, S. T. Rumbold, I. Vardavas, N. Hatzianastassiou, C. Matsoukas, H. Yu, F. Zhang, H. Zhang, and P. Lu In this study we examine the performance of 31 global model radiative transfer schemes in cloud-free conditions with prescribed gaseous absorbers and no aerosols (Rayleigh atmosphere), with prescribed scattering-only aerosols, and with more absorbing aerosols. Results are compared to benchmark results from high-resolution, multi-angular line-by-line radiation models. For purely scattering aerosols, model bias relative to the line-by-line models in the top-of-the atmosphere aerosol radiative forcing ranges from roughly −10 to 20%, with over- and underestimates of radiative cooling at higher and lower sun elevation, respectively. Inter-model diversity (relative standard deviation) increases from ~10 to 15% as sun elevation increases. Inter-model diversity in atmospheric and surface forcing decreases with increased aerosol absorption, indicating that the treatment of multiple-scattering is more variable than aerosol absorption in the models considered. Aerosol radiative forcing results from multi-stream models are generally in better agreement with the line-by-line results than the simpler two-stream schemes. Considering radiative fluxes, model performance is generally the same or slightly better than results from previous radiation scheme intercomparisons. However, the inter-model diversity in aerosol radiative forcing remains large, primarily as a result of the treatment of multiple-scattering. Results indicate that global models that estimate aerosol radiative forcing with two-stream radiation schemes may be subject to persistent biases introduced by these schemes, particularly for regional aerosol forcing.

Description: Immersion freezing of birch pollen washing water Atmospheric Chemistry and Physics Discussions, 12, 32911-32943, 2012 Author(s): S. Augustin, S. Hartmann, B. Pummer, H. Grothe, D. Niedermeier, T. Clauss, J. Voigtländer, L. Tomsche, H. Wex, and F. Stratmann In the present study, the immersion freezing behavior of birch pollen, i.e. its ice nucleating active (INA) macromolecules, was investigated at the Leipzig Aerosol Cloud Interaction Simulator (LACIS). For that, washing water of two different birch pollen samples with different regional origin (Northern birch and Southern birch) were used. The immersion freezing of droplets generated from the pollen washing water was already observed at temperatures higher than −20 °C, for both samples. Main differences between the Northern birch pollen and the Southern birch pollen were obvious in a temperature range, between −18 °C and −24 °C, where the ice fraction increased with decreasing temperature. There, the Northern birch pollen washing water featured two different slopes, with one being steeper and one being similar to the slope of the Southern birch pollen washing water. As we assume single INA macromolecules being the reason for the ice nucleation, we concluded that the Northern birch pollen are able to produce at least two different types of INA macromolecules. We were able to determine the heterogeneous nucleation rates for both INA macromolecule types and so could explain the ice nucleation behavior of both, the Southern and the Northern birch pollen washing water.

Description: Evidence and quantitation of aromatic organosulfates in ambient aerosols in Lahore, Pakistan Atmospheric Chemistry and Physics Discussions, 12, 32795-32823, 2012 Author(s): S. Kundu, T. A. Quraishi, G. Yu, C. Suarez, F. N. Keutsch, and E. A. Stone Organosulfates are important components of atmospheric organic aerosols, yet their structures, abundances, sources and formation processes are not adequately understood. This study presents the identification and quantitation of benzyl sulfate in atmospheric aerosols, which is the first reported atmospheric organosulfate with aromatic carbon backbone. Benzyl sulfate was identified and quantified in fine particulate matter (PM 2.5 ) collected in Lahore, Pakistan during 2007–2008. An authentic standard of benzyl sulfate was synthesized, standardized, and identified in atmospheric aerosols using ultra-performance liquid chromatography (UPLC) coupled with quadrupole time-of-flight (Q-ToF) mass spectrometry (MS). Benzyl sulfate was quantified in aerosol samples using UPLC coupled to negative electrospray ionization triple quadrupole (TQ) MS. The highest benzyl sulfate concentrations were recorded in November and January 2007 (0.50 ± 0.11 ng m −3 ) whereas the lowest concentration was observed in July (0.05 ± 0.02 ng m −3 ). To evaluate matrix effects, benzyl sulfate concentrations were determined using external calibration and the method of standard addition; comparable concentrations were detected by the two methods, which ruled out significant matrix effects in benzyl sulfate quantitation. Three additional organosulfates with m / z 187, 201 and 215 were qualitatively identified as aromatic organosulfates with additional methyl substituents by high-resolution mass measurements and tandem MS. The observed aromatic organosulfates form a homologous series analogous to toluene, xylene, and trimethylbenzene, which are abundant anthropogenic volatile organic compounds (VOC), suggesting that aromatic organosulfates may be formed by secondary reactions. Further studies are needed to elucidate the sources and formation pathways of aromatic organosulfates in the atmosphere.

Description: OH and HO 2 radical chemistry during PROPHET 2008 and CABINEX 2009 – Part 1: Measurements and model comparison Atmospheric Chemistry and Physics Discussions, 12, 33165-33218, 2012 Author(s): S. M. Griffith, R. F. Hansen, S. Dusanter, P. S. Stevens, M. Alaghmand, S. B. Bertman, M. A. Carroll, M. Erickson, M. Galloway, N. Grossberg, J. Hottle, J. Hou, B. T. Jobson, A. Kammrath, F. N. Keutsch, B. L. Lefer, L. H. Mielke, A. O'Brien, P. B. Shepson, M. Thurlow, W. Wallace, N. Zhang, and X. L. Zhou Hydroxyl (OH) and hydroperoxyl (HO 2 ) radicals are key species driving the oxidation of volatile organic compounds that can lead to the production of ozone and secondary organic aerosols. Previous measurements of these radicals in forest environments with high isoprene, low NO x conditions have shown serious discrepancies with modeled concentrations, bringing into question the current understanding of isoprene oxidation chemistry in these environments. During the summers of 2008 and 2009, OH and peroxy radical concentrations were measured using a laser-induced fluorescence instrument as part of the PROPHET (Program for Research on Oxidants: PHotochemistry, Emissions, and Transport) and CABINEX (Community Atmosphere-Biosphere INteractions EXperiment) campaigns at a forested site in northern Michigan. Supporting measurements of photolysis rates, volatile organic compounds, NO x (NO + NO 2 ) and other inorganic species were used to constrain a zero-dimensional box model based on the Regional Atmospheric Chemistry Mechanism, modified to include the Mainz Isoprene Mechanism (RACM-MIM). The CABINEX model OH predictions were in good agreement with the measured OH concentrations, with an observed-to-modeled ratio near one (0.70 ± 0.31) for isoprene mixing ratios between 1–2 ppb on average. The measured peroxy radical concentrations, reflecting the sum of HO 2 and isoprene-based peroxy radicals, were generally lower than predicted by the box model in both years.

Description: Estimation of volatile organic compound emissions for Europe using data assimilation Atmospheric Chemistry and Physics Discussions, 12, 33219-33263, 2012 Author(s): M. R. Koohkan, M. Bocquet, Y. Roustan, Y. Kim, and C. Seigneur The emission of volatile organic compounds (VOCs) over western Europe for the year 2005 are estimated via inverse modelling, by assimilation of in situ observations of concentration and compared to a standard emission inventory. The study focuses on fifteen VOC species: five aromatics, six alkanes, two alkenes, one alkyne and one biogenic diene. The inversion relies on a validated fast adjoint of the chemistry transport model used to simulate the fate and transport of these VOCs. The assimilated ground-based measurements over Europe are provided by the European Monitoring and Evaluation Programme (EMEP) network. The background emissions errors and the prior observational errors are estimated by maximum likelihood approaches. The positivity assumptions on the VOC emission fluxes is pivotal for a successful inversion and this maximum likelihood approach consistently accounts for the positivity of the fluxes. For most species, the retrieval leads to a significant reduction of the bias, which underlines the misfit between the standard inventories and the observed concentrations. The results are validated through a forecast test and a cross-validation test. It is shown that the statistically consistent non-Gaussian approach based on a reliable estimation of the errors offers the best performance. The efficiency in correcting the inventory depends on the lifetime of the VOCs. In particular, it is shown that the use of in-situ observations using a sparse monitoring network to estimate emissions of isoprene is inadequate because its short chemical lifetime significantly limits the spatial radius of influence of the monitoring data. For species with longer lifetime (a few days), successful, albeit partial, emission corrections can reach regions hundreds of kilometres away from the stations. Domainwide corrections of the emissions inventories of some VOCs are significant, with underestimations on order of a factor of two of propane, ethane, ethylene and acetylene.

Description: A new source of oxygenated organic aerosol and oligomers Atmospheric Chemistry and Physics Discussions, 12, 29069-29098, 2012 Author(s): J. Liggio and S.-M. Li A large oxygenated organic uptake to aerosols was observed when exposing ambient urban air to inorganic acidic and non-acidic sulfate seed aerosol. For non-acidic seed aerosol the uptake was attributed to the direct condensation of primary vehicle exhaust gases, and was correlated to the initial seed sulfate mass. The uptake of primary oxygenated organic gases to aerosols in this study represents a significant amount of organic aerosol (OA) when compared to that reported for primary organic aerosol (POA), but is considerably more oxygenated (O : C ~ 0.3) than traditional POA. Consequently, a fraction of measured ambient oxygenated OA, which correlate with secondary sulfate, may in fact be of a primary, rather than secondary source. These results represent a new source of oxygenated OA on neutral aerosol and imply that the uptake of primary organic gases will occur in the ambient atmosphere, under dilute conditions, and in the presence of pre-existing SO 4 aerosols. Under acidic seed aerosol conditions, oligomer formation was observed with the uptake of organics being enhanced by a factor of three or more compared to neutral aerosols, and in less than 2 min. This resulted in a trajectory in Van Krevelen space towards higher O : C (slope ~ −1.5), despite a lack of continual gas-phase oxidation in this closed system. The results demonstrate that high molecular weight species will form on acidic aerosols at the ambient level and mixture of organic gases, but are otherwise unaffected by subsequent aerosol neutralization, and that aerosol acidity will affect the organic O : C via aerosol-phase reactions. These new processes under both neutral and acidic conditions can contribute to ambient OA mass and the evolution of ambient aerosol O : C ratios and may be important for properly representing organic aerosol O : C ratios in air quality and climate models.

Description: Characterization of photochemical pollution at different elevations in mountainous areas in Hong Kong Atmospheric Chemistry and Physics Discussions, 12, 29025-29067, 2012 Author(s): H. Guo, Z. H. Ling, K. Cheung, F. Jiang, D. W. Wang, I. J. Simpson, T. J. Wang, X. M. Wang, S. M. Saunders, and D. R. Blake To advance our understanding on the factors that affect photochemical pollution at different elevations in mountainous areas, concurrent systematic field measurements (September to November 2010) were conducted at a mountain site and at an urban site at the foot of the mountain in Hong Kong. The mixing ratios of air pollutants were greater at the foot of the mountain (i.e. Tsuen Wan urban site, TW) than near the summit (i.e. Tai Mao Shan mountain site, TMS), except for ozone. In total, only 1 O 3 episode day was observed at TW, whereas 21 O 3 episode days were observed at TMS. The discrepancy of O 3 at the two sites was attributed to the mixed effects of NO titration, vertical meteorological conditions, regional transport and mesoscale circulations. The lower NO levels at TMS and the smaller differences of "oxidant" O x (O 3 + NO 2 ) than O 3 between the two sites suggested that variations of O 3 at the two sites were partly attributed to different degree of NO titration. In addition, analysis of vertical structure of meteorological variables revealed that the inversion layer at the range of altitudes of 500–1000 m might be another factor that caused the high O 3 levels at TMS. Furthermore, analyses of the wind fields and the levels of air pollutants in different air flows indicated that high O 3 concentrations at TMS were somewhat influenced by regional air masses from the highly polluted Pearl River Delta (PRD) region. In particular, the analysis of diurnal profiles and correlations of gaseous pollutants suggested influence of mesoscale circulations which was further confirmed using the Master Chemical Mechanism moving box model (Mbox) and the Weather Research and Forecasting (WRF) model. By investigating the correlations of observed O 3 and NO x *, as well as the ratios of VOC/NO x , it was concluded that photochemical O 3 formation was VOC-sensitive or both NO x and VOC-sensitive at TMS, while it was VOC-sensitive at TW.

Description: Cloud-resolving modeling of aerosol indirect effects in idealized radiative-convective equilibrium with interactive and fixed sea surface temperature Atmospheric Chemistry and Physics Discussions, 12, 29099-29127, 2012 Author(s): M. F. Khairoutdinov and C.-E. Yang The study attempts to evaluate the aerosol indirect effects over tropical oceans in regions of deep convection applying a three-dimensional cloud-resolving model run over a doubly-periodic domain. The Tropics are modeled using a radiative-convective equilibrium idealization when the radiation, turbulence, cloud microphysics, and surface fluxes are explicitly represented while the effects of large-scale circulation are ignored. The aerosol effects are modeled by varying the number concentration of cloud condensation nuclei (CCN) at 1% supersaturation, which serves as a proxy for the aerosol amount in the environment, over a wide range, starting from pristine maritime (50 cm −3 ) to polluted (1000 cm −3 ) conditions. No direct effects of aerosol on radiation are included. Two sets of simulations have been run to equilibrium: fixed (non-interactive) sea surface temperature (SST) and interactive SST as predicted by a simple slab-ocean model responding to the surface radiative fluxes and surface enthalpy flux. Both sets of experiments agree on the tendency to make the shortwave cloud forcing more negative and reduce the longwave cloud forcing in response to increasing CCN concentration. These, in turn, tend to cool the SST in interactive-SST case. It is interesting that the absolute change of the SST and most other bulk quantities depends only on relative change of CCN concentration; that is, same SST change can be the result of doubling CCN concentration regardless of clean or polluted conditions. It is found that the 10-fold increase of CCN concentration can cool the SST by as much as 1.5 K. This is quite comparable to 2 K warming obtained in a simulation for clean maritime conditions, but doubled CO 2 concentration. Qualitative differences between the interactive and fixed SST cases have been found in sensitivity of the hydrological cycle to the increase in CCN concentration; namely, the precipitation rate shows some tendency to increase in fixed SST case, but robust tendency to decrease in interactive SST case.

Description: Formation of organic aerosol in the Paris region during the MEGAPOLI summer campaign: evaluation of the Volatility-Basis-Set approach within the CHIMERE model Atmospheric Chemistry and Physics Discussions, 12, 29475-29533, 2012 Author(s): Q. J. Zhang, M. Beekmann, F. Drewnick, F. Freutel, J. Schneider, M. Crippa, A. S. H. Prévôt, U. Baltensperger, L. Poulain, A. Wiedensohler, J. Sciare, V. Gros, A. Borbon, A. Colomb, V. Michoud, J.-F. Doussin, H. A. C. Denier van der Gon, M. Haeffelin, J.-C. Dupont, G. Siour, H. Petetin, B. Bessagnet, S. N. Pandis, A. Hodzic, O. Sanchez, C. Honoré, and O. Perrussel Results of the chemistry transport model CHIMERE are compared with the measurements performed during the MEGAPOLI summer campaign in the Greater Paris Region in July, 2009. The Volatility-Basis-Set approach (VBS) is implemented into this model, taking into account the volatility of primary organic aerosol (POA) and the chemical aging of semi-volatile organic species. Organic aerosol is the main focus and is simulated with three different configurations related to the volatility of POA and the scheme of secondary organic aerosol (SOA) formation. In addition, two types of emission inventories are used as model input in order to test the uncertainty related to the emissions. Predictions of basic meteorological parameters and primary and secondary pollutant concentrations are evaluated and four pollution regimes according to the air mass origin are defined. Primary pollutants are generally overestimated, while ozone is consistent with observations. Sulfate is generally overestimated, while ammonium and nitrate levels are well simulated with the refined emission data set. As expected, the simulation with non-volatile POA and a single-step SOA formation mechanism largely overestimates POA and underestimates SOA. Simulation of organic aerosol with the VBS approach taking into account the aging of semi-volatile organic compounds (SVOC) shows the best correlation with measurements. All observed high concentration events are reproduced by the model mostly after long range transport, indicating that long range transport of SOA to Paris is well reproduced. Depending on the emission inventory used, simulated POA levels are either reasonable or underestimated, while SOA levels tend to be overestimated. Several uncertainties related to the VBS scheme (POA volatility, SOA yields, the aging parameterization), to emission input data, and to simulated OH levels can be responsible for this behavior. Despite these uncertainties, the implementation of the VBS scheme into the CHIMERE model allowed for much more realistic organic aerosol simulations for Paris during summer time. The advection of SOA from outside Paris is mostly responsible for the highest OA concentration levels. During advection of polluted air masses from north-east (Benelux and Central Europe), simulations indicate high levels of both anthropogenic and biogenic SOA fractions, while biogenic SOA dominates during days with advection from Southern France and Spain.

Description: Ice water content of arctic, midlatitude, and tropical cirrus – Part 2: Extension of the database and new statistical analysis Atmospheric Chemistry and Physics Discussions, 12, 29443-29474, 2012 Author(s): A. E. Luebke, L. M. Avallone, C. Schiller, C. Rolf, and M. Krämer Ice clouds are known to be major contributors to radiative forcing in the Earth's atmosphere, yet describing their microphysical properties in climate models remains challenging. Among these properties, the ice water content (IWC) of cirrus clouds is of particular interest both because it is measurable and because it can be directly related to a number of other radiatively important variables such as extinction and effective radius. This study expands upon the work of Schiller et al. (2008), extending a climatology of IWC by combining datasets from several European and US airborne campaigns and ground-based lidar measurements over Jülich, Germany. The relationship between IWC and temperature is further investigated using the new merged dataset and probability distribution functions (PDFs). A PDF-based formulation allows for representation of not only the mean values of IWC, but also the variability of IWC within a temperature band. The IWC-PDFs are found to be bimodal over the whole cirrus temperature range, which might be attributed to different cirrus formation mechanisms such as heterogeneous and homogeneous freezing. The PDFs of IWC are further compared to distributions of cirrus ice crystal number and mass mean radius, which show that the general relationship between IWC and temperature appears to be influenced much more by particle number than by particle size.

Description: Systematic investigation of bromine monoxide in volcanic plumes from space by using the GOME-2 instrument Atmospheric Chemistry and Physics Discussions, 12, 29325-29389, 2012 Author(s): C. Hörmann, H. Sihler, N. Bobrowski, S. Beirle, M. Penning de Vries, U. Platt, and T. Wagner During recent years, volcanic emissions turned out to be a natural source of bromine compounds in the atmosphere. While the inital formation process of bromine monoxide (BrO) has been successfully studied in local ground-based measurements at quiescent degassing volcanoes worldwide, literature on the chemical evolution of BrO on large spatial and temporal scales is sparse. The first space-based observation of a volcanic BrO plume following the Kasatochi eruption in 2008 demonstrated the capability of satellite instruments to monitor such events on a global scale. In this study, we systematically examined GOME-2 observations from January 2007 until June 2011 for significantly enhanced BrO slant column densities (SCDs) in the vicinity of volcanic plumes. In total, 772 plumes from at least 37 volcanoes have been found by using sulphur dioxide (SO 2 ) as a tracer for a volcanic plume. All captured SO 2 plumes were subsequently analysed for a simultaneous enhancement of BrO and the data were checked for a possible spatial correlation between the two species. Additionally, the mean BrO/SO 2 ratios for all volcanic plumes have been calculated by the application of a bivariate linear fit. A total number of 64 volcanic plumes from at least 11 different volcanoes showed clear evidence for BrO of volcanic origin, revealing large differences in the BrO/SO 2 ratios (ranging from some 10 −5 to several 10 −4 ) and the spatial distribution of both species. A close correlation between SO 2 and BrO occurred only for some of the observed eruptions or just in certain parts of the examined plumes. For other cases, only a rough spatial relationship was found. We discuss possible explanations for the occurrence of the different spatial SO 2 and BrO distributions in aged volcanic plumes.

Description: A one-year comprehensive chemical characterisation of fine aerosol (PM 2.5 ) at urban, suburban and rural background sites in the region of Paris (France) Atmospheric Chemistry and Physics Discussions, 12, 29391-29442, 2012 Author(s): M. Bressi, J. Sciare, V. Ghersi, N. Bonnaire, J. B. Nicolas, J.-E. Petit, S. Moukhtar, A. Rosso, N. Mihalopoulos, and A. Féron Studies describing the chemical composition of fine aerosol (PM 2.5 ) in urban areas are often conducted during few weeks only, and at one sole site, giving thus a narrow view of their temporal and spatial characteristics. This paper presents a one-year (11 September 2009–10 September 2010) survey of the daily chemical composition of PM 2.5 in the region of Paris, which is the second most populated "Larger Urban Zone" in Europe. Five sampling sites representative of suburban (SUB), urban (URB), northeast (NER), northwest (NWR) and south (SOR) rural backgrounds were implemented. The major chemical components of PM 2.5 were determined including elemental carbon (EC), organic carbon (OC), and the major ions. OC was converted to organic matter (OM) using the chemical mass closure methodology, which leads to conversion factors of 1.95 for the SUB and URB sites, and 2.05 for the three rural ones. On average, gravimetrically determined PM 2.5 annual mass concentrations are 15.2, 14.8, 12.6, 11.7 and 10.8 μg m −3 for SUB, URB, NER, NWR and SOR sites, respectively. The chemical composition of fine aerosol is very homogeneous at the five sites and is composed of OM (38–47%), nitrate (17–22%), non-sea-salt sulfate (13–16%), ammonium (10–12%), EC (4–10%), mineral dust (2–5%) and sea salt (3–4%). This chemical composition is in agreement with those reported in the literature for most European environments. On the annual scale, Paris (URB and SUB sites) exhibits its highest PM 2.5 concentrations during late autumn, winter and early spring (higher than 15 μg m −3 on average, from December to April), intermediates during late spring and early autumn (between 10 and 15 μg m −3 during May, June, September, October, and November) and the lowest during summer (below 10 μg m −3 during July and August). PM levels are mostly homogeneous at the regional scale, on the whole duration of the project (e.g. for URB plotted against NER sites: slope = 1.06, r 2 = 0.84, n = 330), suggesting the importance of mid- or long-range transport, and regional instead of local scale phenomena. During this one-year project, two third of the days exceeding the PM 2.5 2015 EU annual limit value of 25 μg m −3 were due to continental import from countries located northeast, east of France. This result questions the efficiency of local, regional and even national abatement strategies during pollution episodes, pointing the need for a wider collaborative work with the neighbourhood countries on these topics. Nevertheless, emissions of local anthropogenic sources lead to higher levels at the URB and SUB sites compared to the others (e.g. 26% higher on average at the URB than at the NWR site for PM 2.5 , during the whole campaign), which can even be emphasised by specific meteorological conditions such as low boundary layer heights. OM and secondary inorganic species (nitrate, non-sea-salt sulfate and ammonium, noted SIA) are mainly imported by mid- or long-range transport (e.g. for NWR plotted against URB sites: slope = 0.79, r 2 = 0.72, n = 335 for OM, and slope = 0.91, r 2 = 0.89, n = 335 for SIA) whereas EC is primarily locally emitted (e.g. for SOR plotted against URB sites: slope = 0.27; r 2 = 0.03; n = 335). This database will serve deepest investigations of carbonaceous aerosols, metals as well as the main sources and geographical origins of PM in the region of Paris.

Description: Elucidating multipollutant exposure across a complex metropolitan area by systematic deployment of a mobile laboratory Atmospheric Chemistry and Physics Discussions, 12, 31585-31627, 2012 Author(s): I. Levy, C. Mihele, G. Lu, J. Narayan, N. Hilker, and J. R. Brook In urban areas, air quality is the outcome of multiple emission sources, each emitting a different combination of air pollutants. The result is a complex mixture of pollutants with a different spatiotemporal variability for each constituent. Studies exploring average spatial patterns across urban areas typically rely on air quality monitoring networks of a few sites, short multi-site saturation monitoring campaigns measuring a limited number of pollutants and/or air quality models. Each of these options has limitations. This study elucidates the main complexities of urban air quality with respect to small scale spatial differences for multiple pollutants so as to gain a better understanding of the variability in exposure estimates in urban areas. Mobile measurements of 23 air pollutants were taken at high resolution in Montreal, Quebec, Canada, and examined with respect to space, time and their interrelationships. The same route was systematically followed on 34 measurement days spread over different seasons and measurements were compared to adjacent air quality monitoring network stations. This approach allowed linkage of the mobile measurements to the network observations and to generate average maps that provide reliable information on the typical, annual average spatial pattern. Sharp differences in the spatial distribution were found to exist between different pollutants on the sub-urban scale, i.e. the neighbourhood to street scales, even for pollutants usually associated with the same specific sources. Nearby microenvironments may have a wide range in average pollution levels varying by up to 300%, which may cause large misclassification errors in estimating chronic exposures in epidemiological studies. For example, NO 2 measurements next to a main road microenvironment are shown to be 210–265% higher than levels measured at a nearby urban background monitoring site, while black carbon is higher by 180–200% and ultrafine particles are 300% higher. For some pollutants (e.g. SO 2 and benzene), there is good correspondence on a large scale due to similar emission sources, but differences on a small scale in proximity to these sources. Moreover. hotspots of different pollutants were identified and quantified. These results demonstrate the ability of an independent heavily instrumented mobile laboratory survey to quantify the representativeness of the monitoring sites to unmonitored locations, reveal the complex relationships between pollutants and understand chronic multi-pollutant exposure patterns associated with outdoor concentrations in an urban environment.

Description: Exceptional emissions of NH 3 and HCOOH in the 2010 Russian wildfires Atmospheric Chemistry and Physics Discussions, 12, 31561-31584, 2012 Author(s): Y. R'Honi, L. Clarisse, C. Clerbaux, D. Hurtmans, V. Duflot, S. Turquety, Y. Ngadi, and P.-F. Coheur In July 2010, several hundred forest and peat fires broke out across Central Russia during its hottest summer on record. Here, we analyze these wildfires using observations of the Infrared Atmospheric Sounding Interferometer (IASI). Carbon monoxide (CO), ammonia (NH 3 ) and formic acid (HCOOH) total columns are presented for the year 2010. Maximum total columns have been observed reaching over 40 (for CO and HCOOH) and 200 (for NH 3 ) times higher than typical background values. The temporal evolution of NH 3 and HCOOH enhancement ratios relative to CO are presented. Strong evidence of secondary formation of HCOOH is found, with enhancement ratios exceeding 10 times reported emission ratios in fresh plumes. We estimate the total emitted masses for the period July–August 2010 over the center of Western Russia; they are 19–33 Tg (CO), 0.7–2.6 Tg (NH 3 ) and 0.9–3.9 Tg (HCOOH). For NH 3 andHCOOH, these quantities are comparable to what is emitted in the course of a whole year by all extratropical forest fires.

Description: Climatology of middle atmospheric water vapour above the ALOMAR observatory in northern Norway Atmospheric Chemistry and Physics Discussions, 12, 31531-31560, 2012 Author(s): K. Hallgren, P. Hartogh, and C. Jarchow We have been observing the water vapour line at 22.235 GHz above ALOMAR in northern Norway (69° N, 16° E) since early 1996 with ground-based microwave spectrometers (WASPAM and cWASPAM) and will here describe a climatology based on these observations. Maintenance, different spectrometers and upgrades of the hardware have slightly changed the instruments. Therefore great care has been taken to make sure the different datasets are compatible with each other. In order to maximise the sensitivity at high altitude for the older instrument a long integration time (168 h) was chosen. The complete dataset was thereafter recompiled into a climatology which describes the yearly variation of water vapour at polar latitudes on a weekly basis. The atmosphere is divided into 16 layers between 40–80 km, each 2.5 km thick. The dataset, spanning 15 yr from 1996 to 2010, enabled us to investigate the long-term behaviour of water vapour at these latitudes. By comparing the measurements from every year to the climatological mean we were also able to look for indications of trends in the dataset at different altitudes during the time period of our observations. In general there is a weak negative trend which differs slightly at different altitudes. There are however no drifts in the annual variation of water vapour from the point of view of onset of summer and winter. We compare our climatology to the reference water vapour profiles from AFGL, a free and easy accessible reference atmosphere. There are strong deviations between our observations and the reference profile, therefore we publish our climatological dataset in a table in the paper.

Description: Observationally-constrained estimates of global small-mode AOD Atmospheric Chemistry and Physics Discussions, 12, 31663-31698, 2012 Author(s): K. Lee and C. E. Chung Small aerosols are mostly anthropogenic, and an area average of the small-mode aerosol optical depth (sAOD) is a powerful and independent measure of anthropogenic aerosol emission. We estimate AOD and sAOD globally on a monthly time scale from 2001 to 2010 by integrating satellite-based (MODIS and MISR) and ground-based (AERONET) observations. For sAOD, three integration methods were developed to maximize the influence of AERONET data and ensure consistency between MODIS, MISR and AERONET sAOD data. We evaluated each method by applying the technique with fewer AERONET data and comparing its output with the unused AERONET data. The best performing method gives an overall error of 13 ± 2%, compared with an overall error of 62% in simply using MISR sAOD, and this method takes advantage of an empirical relationship between the Ångström exponent (AE) and fine mode fraction (FMF). This relationship is obtained by analyzing AERONET data. Using our integrated data, we find that the global 2001–2010 average of 500 nm AOD and sAOD is 0.17 and 0.094, respectively. sAOD over eastern China is several times as large as the global average. The linear trend from 2001 to 2010 is found to be slightly negative in global AOD or global sAOD. In India and eastern China combined, however, sAOD increased by more than 4% against a backdrop of decreasing AOD and large-mode AOD. On the contrary to India and China, the west (Western Europe and US/Canada combined) is found to have a sAOD reduction of −20%. These results quantify the overall anthropogenic aerosol emission reduction in the west, and rapidly deteriorating conditions in Asia. Moreover, our results in the west are consistent with the so-called surface brightening phenomenon in the recent decades.

Description: ACE-FTS observations of pyrogenic trace species in boreal biomass burning plumes during BORTAS Atmospheric Chemistry and Physics Discussions, 12, 31629-31661, 2012 Author(s): K. A. Tereszchuk, G. González Abad, C. Clerbaux, J. Hadji-Lazaro, D. Hurtmans, P.-F. Coheur, and P. F. Bernath To further our understanding of the effects of biomass burning emissions on atmospheric composition, the Quantifying the impact of BOReal forest fires on Tropospheric oxidants over the Atlantic using Aircraft and Satellites (BORTAS) campaign was conducted on 12 July to 3 August 2011 during the Boreal forest fire season in Canada. The simultaneous aerial, ground and satellite measurement campaign sought to record instances of Boreal biomass burning to measure the tropospheric volume mixing ratios (VMRs) of short- and long-lived trace molecular species from biomass burning emissions. The goal was to investigate the connection between the composition and the distribution of these pyrogenic outflows and their resulting perturbation to atmospheric chemistry, with particular focus on oxidant species to determine the overall impact on the oxidizing capacity of the free troposphere. Measurements of pyrogenic trace species in Boreal biomass burning plumes were made by the Atmospheric Chemistry Experiment Fourier Transform Spectrometer (ACE-FTS) onboard the Canadian Space Agency (CSA) SCISAT-1 satellite during the BORTAS campaign. Even though most biomass burning smoke is typically confined to the boundary layer, emissions are often injected directly into the upper troposphere via fire-related convective processes, thus allowing space-borne instruments to measure these pyrogenic outflows. An extensive set of 15 molecules, CH 3 OH, CH 4 , C 2 H 2 , C 2 H 6 , C 3 H 6 O, CO, HCN, HCOOH, HNO 3 , H 2 CO, NO, NO 2 , OCS, O 3 and PAN have been analyzed. Included in this analysis is the calculation of age-dependent sets of enhancement ratios for each of the species.

Description: Validation of an hourly resolved global aerosol model in answer to solar electricity generation information needs Atmospheric Chemistry and Physics Discussions, 12, 31917-31953, 2012 Author(s): M. Schroedter-Homscheidt and A. Oumbe Solar energy applications need global aerosol optical depth (AOD) information to derive historic surface solar irradiance databases from geostationary meteorological satellites reaching back to the 1980's. This paper validates the MATCH/DLR model originating in the climate community against AERONET ground measurements. Hourly or daily mean AOD model output is evaluated individually for all stations in Europe, Africa and the Middle East – an area highly interesting for solar energy applications being partly dominated by high aerosol loads. Overall, a bias of 0.02 and a root mean square error of 0.23 are found for daily mean AOD values, while the RMSE increases to 0.28 for hourly mean AOD values. Large differences between various regions and stations are found providing a feedback loop for the aerosol modelling community. The difference in using daily means versus hourly resolved modelling with respect to hourly resolved observations is evaluated. Nowadays state of the art in solar resource assessment relies on monthly turbidity or AOD climatologies while at least hourly resolved irradiance time series are needed by the solar sector. Therefore, the contribution of higher temporally modelled AOD is evaluated.

Description: Classifying organic materials by oxygen-to-carbon elemental ratio to predict the activation regime of cloud condensation nuclei (CCN) Atmospheric Chemistry and Physics Discussions, 12, 31829-31870, 2012 Author(s): M. Kuwata, W. Shao, R. Lebouteiller, and S. T. Martin The governing highly soluble, slightly soluble, or insoluble activation regime of organic compounds as cloud condensation nuclei (CCN) was examined as a function of oxygen-to-carbon elemental ratio (O : C). New data were collected for adipic, pimelic, suberic, azelaic and pinonic acids. Secondary organic materials (SOMs) produced by α-pinene ozonolysis and isoprene photo-oxidation were also included in the analysis. The saturation concentrations C of the organic compounds in aqueous solutions served as the key parameter for delineating regimes of CCN activation, and the values of C were tightly correlated to the O : C ratios. The highly soluble, slightly soluble, and insoluble regimes of CCN activation were found to correspond to ranges of [O : C] 〉 0.6, 0.2 〈 [O : C] 〈 0.6, and [O : C] 〈 0.2, respectively. These classifications were evaluated against CCN activation data of isoprene-derived SOM (O : C = 0.69–0.72) and α-pinene-derived SOM (O : C = 0.38–0.48). Isoprene-derived SOM had highly soluble activation behavior, consistent with its high O : C ratio. For α-pinene-derived SOM, although CCN activation can be modeled as a highly soluble mechanism, this behavior was not predicted by the O : C ratio, for which a slightly soluble mechanism was anticipated. Complexity in chemical composition, resulting in continuous water uptake and the absence of a deliquescence transition that can thermodynamically limit CCN activation, might explain the differences of α-pinene-derived SOM compared to the behavior of pure organic compounds. The present results suggest that atmospheric particles dominated by hydrocarbon-like organic components do not activate (i.e. insoluble regime) whereas those dominated by oxygenated organic components activate (i.e. highly soluble regime).

Description: Secondary organic aerosol formation from gasoline passenger vehicle emissions investigated in a smog chamber Atmospheric Chemistry and Physics Discussions, 12, 31725-31765, 2012 Author(s): E. Z. Nordin, A. C. Eriksson, P. Roldin, P. T. Nilsson, J. E. Carlsson, M. K. Kajos, H. Hellén, C. Wittbom, J. Rissler, J. Löndahl, E. Swietlicki, B. Svenningsson, M. Bohgard, M. Kulmala, M. Hallquist, and J. Pagels Gasoline vehicles have elevated emissions of volatile organic compounds during cold starts and idling and have recently been pointed out as potentially the main source of anthropogenic secondary organic aerosol (SOA) in megacities. However, there is a lack of laboratory studies to systematically investigate SOA formation in real-world exhaust. In this study, SOA formation from pure aromatic precursors, idling and cold start gasoline exhaust from one Euro II, one Euro III and one Euro IV passenger vehicles were investigated using photo-oxidation experiments in a 6 m 3 smog chamber. The experiments were carried out at atmospherically relevant organic aerosol mass concentrations. The characterization methods included a high resolution aerosol mass spectrometer and a proton transfer mass spectrometer. It was found that gasoline exhaust readily forms SOA with a signature aerosol mass spectrum similar to the oxidized organic aerosol that commonly dominates the organic aerosol mass spectra downwind urban areas. After 4 h aging the formed SOA was 1–2 orders of magnitude higher than the Primary OA emissions. The SOA mass spectrum from a relevant mixture of traditional light aromatic precursors gave f 43 (mass fraction at m/z = 4 3) approximately two times higher than to the gasoline SOA. However O : C and H : C ratios were similar for the two cases. Classical C 6 –C 9 light aromatic precursors were responsible for up to 60% of the formed SOA, which is significantly higher than for diesel exhaust. Important candidates for additional precursors are higher order aromatic compounds such as C 10 , C 11 light aromatics, naphthalene and methyl-naphthalenes.

Description: Long-term changes of tropospheric NO 2 over megacities derived from multiple satellite instruments Atmospheric Chemistry and Physics Discussions, 12, 31767-31828, 2012 Author(s): A. Hilboll, A. Richter, and J. P. Burrows Tropospheric NO 2 , a key pollutant in particular in cities, has been measured from space since the mid-1990s by the GOME, SCIAMACHY, OMI, and GOME-2 instruments. These data provide a unique global long-term data set of tropospheric pollution. However, the measurements differ in spatial resolution, local time of measurement, and measurement geometry. All these factors can severely impact the retrieved NO 2 columns, which is why they need to be taken into account when analysing time series spanning more than one instrument. In this study, we present several ways to explicitly account for the instrumental differences in trend analyses of the NO 2 columns derived from satellite measurements, while preserving their high spatial resolution. Both a physical method, based on spatial averaging of the measured earthshine spectra and extraction of a resolution pattern, and statistical methods, including instrument-dependent offsets in the fitted trend function, are developed. These methods are applied to data from GOME and SCIAMACHY separately, to the combined time series and to an extended data set comprising also GOME-2 and OMI measurements. All approaches show consistent trends of tropospheric NO 2 for a selection of areas on both regional and city scales, for the first time allowing consistent trend analysis of the full time series at high spatial resolution and significantly reducing the uncertainties of the retrieved trend estimates compared to previous studies. We show that measured tropospheric NO 2 columns have been strongly increasing over China, the Middle East, and India, with values over East Central China triplicating from 1996 to 2011. All parts of the developed world, including Western Europe, the United States, and Japan, show significantly decreasing NO 2 amounts in the same time period. On a megacity level, individual trends can be as large as +27 ± 3.7% yr −1 and +20 ± 1.9% yr −1 in Dhaka and Baghdad, respectively, while Los Angeles shows a very strong decrease of −6.0 ± 0.37% yr −1 . Most megacities in China, India, and the Middle East show increasing NO 2 columns of +5–10% yr −1 , leading to a doubling to triplication within the observed period. While linear trends derived with the different methods are consistent, comparison of the GOME and SCIAMACHY time series as well as inspection of time series over individual areas shows clear indication of non-linear changes in NO 2 columns in response to rapid changes in technology used and the economical situation.

Description: Measurements of natural deposition ice nuclei in Córdoba, Argentina Atmospheric Chemistry and Physics Discussions, 12, 31699-31723, 2012 Author(s): M. L. López and E. E. Ávila Ice nucleation in the atmosphere is of practical and fundamental importance since ice crystals influence the release of snow, rain and hail. Suspended aerosols in the atmosphere can initiate freezing at temperatures below −15 °C. In this work we describe an experimental device designed to measure the concentration of natural ice nuclei under controlled temperature and supersaturation conditions. The measurements were performed at Córdoba City, for temperatures between −15 °C and −30 °C and the sampled air was supersaturated with respect to ice and subsaturated with respect to liquid water; under these conditions the deposition ice nuclei were quantified. There are few studies reported in the literature regarding measurements of deposition ice nuclei concentration and, to our knowledge, there are no previous laboratory data of this kind of ice nuclei for T 〈 −20 °C. The results show that the number of deposition ice nuclei increases at colder temperatures and higher supersaturations. These results are in general in good agreement with results previously reported by other authors. A fitting function which depends on temperature and supersaturation is proposed to parameterize the results obtained in the present work.

Description: Wintertime peroxyacetyl nitrate (PAN) in the megacity Beijing: the role of photochemical and meteorological processes Atmospheric Chemistry and Physics Discussions, 12, 31871-31916, 2012 Author(s): H. Zhang, X. Xu, W. Lin, and Y. Wang Peroxyacetyl nitrate (PAN) is one of the key photochemical pollutants and acts as an important reservoir for the peroxyacetyl (PA) radical and nitrogen oxides (NO x ) over cold and less polluted regions. Previous measurements of PAN in Asian megacities were scarce and mainly conducted for relatively short periods in summer. In this study, we present and analyze the measurements of PAN, O 3 , NO x , CO, and some meteorological variables, made at an urban site (CMA) in Beijing from 25 January to 22 March 2010. During the observations, the hourly concentration of PAN varied from 0.23 to 3.51 ppb, with an average of 0.70 ppb. Both PAN and O 3 showed small but significant diurnal cycle, with PAN peaking around 17:00 LT, three hours later than O 3 . The observed concentration of PAN is well correlated with that of NO x but not O 3 . These phenomena indicate that the variations of the winter concentrations of PAN and O 3 in urban Beijing are decoupled with each other. Wind conditions and transport of air masses exert very significant impacts on O 3 , PAN, and other species. The strong WNW-N winds caused elevated concentrations of surface O 3 and lower concentrations of PAN, NO x , and CO. Weak winds from the other directions led to enhanced levels of PAN, NO x , and CO and decreased level of O 3 . Air masses arriving at our site originated either from the boundary layer over the highly polluted N-S-W sector or from the free troposphere over the W-N sector. The descending free-tropospheric air was rich in O 3 , with an average PAN/O 3 ratio smaller than 0.031, while the boundary layer air over the polluted sector contained higher levels of PAN and primary pollutants, with an average PAN/O 3 ratio of 0.11. These facts related with meteorological conditions, specifically the air transport conditions, can well explain the observed PAN-O 3 decoupling. The impact of meso-scale transport is demonstrated using a case during 21–22 February 2010. In addition to transport, photochemical production is important to PAN in the winter boundary layer over Beijing. The PA concentration is estimated from the measurements of PAN and related variables. The estimated PA concentration for three days with stable atmospheric condition, 7 February, 23 February, and 11 March, are in the range of 0–0.012, 0–0.036, and 0–0.040 ppt, respectively. We found that both the formation reaction and thermal decomposition contributed significantly to PAN's variation. The results here suggest that even in the colder period, both photochemical production and thermal decomposition of PAN in the polluted boundary layer over Beijing are not negligible, with the production exceeding the decomposition.

Description: A case study of the highly time-resolved evolution of aerosol chemical and optical properties in urban Shanghai, China Atmospheric Chemistry and Physics Discussions, 12, 31955-31990, 2012 Author(s): Y. Huang, L. Li, J. Li, X. Wang, H. Chen, J. Chen, X. Yang, D. S. Gross, H. Wang, L. Qiao, and C. Chen Characteristics of the chemical and optical properties of aerosols in urban Shanghai and their relationship were studied over a three-day period in October 2011. A suite of real-time instruments, including an Aerosol Time-Of-Flight Mass Spectrometer (ATOFMS), a Monitor for AeRosols and GAses (MARGA), a Cavity Ring Down Spectrometer (CRDS), a nephelometer and a Scanning Mobility Particle Sizer (SMPS), was employed to follow the quick changes of the aerosol properties within the 72-h sampling period. The origin of the air mass arriving in Shanghai during this period shifted from the East China Sea to the northwest area of China, offering a unique opportunity to observe the evolution of aerosols influenced by regional transport from the most polluted areas in China. According to the meteorological conditions and temporal characterizations of the chemical and optical properties, the sampling period was divided into three periods. During Period 1 (00:00–23:00, 13 October), the aerosols in urban Shanghai were mainly fresh and the single scattering albedo varied negatively with the emission of elemental carbon, indicating that local sources dominated. Period 2 (23:00 on 13 October to 10:00 on 15 October) was impacted by regionally transported pollutants and had the highest particulate matter (PM) mass loading and the lowest particle acidity, characterized by large fractions of aged particles and high secondary ion (nitrate, sulfate and ammonium) mass concentrations. Two sub-periods were identified in Period 2 based on the scattering efficiency of PM 1 mass. The comparison of these sub-periods highlights the influence of particle mixing state on aerosol optical properties. Period 3 (from 10:00 on 15 October to 00:00 on 16 October) had a low PM 1 /PM 10 ratio and a new particle formation event. We directly observed the influence of regionally transported pollutants on local aerosol properties and demonstrate that the PM mass extinction efficiency is largely determined by the chemical components and mixing states of the aerosol.

Description: Remote sensing of ice crystal asymmetry parameter using multi-directional polarization measurements – Part 2: Application to the Research Scanning Polarimeter Atmospheric Chemistry and Physics Discussions, 12, 32063-32107, 2012 Author(s): B. van Diedenhoven, B. Cairns, A. M. Fridlind, A. S. Ackerman, and T. J. Garrett A new method to retrieve ice cloud asymmetry parameters from multi-directional polarized reflectance measurements is applied to measurements of the airborne Research Scanning Polarimeter (RSP) obtained during the CRYSTAL-FACE campaign in 2002. The method assumes individual hexagonal ice columns and plates serve as proxies for more complex shapes and aggregates. The closest fit is searched in a look-up table of simulated polarized reflectances computed for cloud layers that contain individual, randomly oriented hexagonal columns and plates with a virtually continuous selection of aspect ratios and distortion. The asymmetry parameter, aspect ratio and distortion of the hexagonal particle that leads to the best fit with the measurements are considered the retrieved values. Two cases of thick convective clouds and two cases of thinner anvil cloud layers are analyzed. Median asymmetry parameters retrieved by the RSP range from 0.76 to 0.78, and are generally smaller that those currently assumed in most climate models and satellite retrievals. In all cases the measurements indicate roughened ice crystals, which is consistent with previous findings. Retrieved aspect ratios in three of the cases range from 0.9 to 1.6, indicating compact particles dominate the cloud-top shortwave radiation. Retrievals for the remaining case indicate plate-like ice crystals with aspect ratios around 0.3. The RSP retrievals are qualitatively consistent with the CPI images obtained in the same cloud layers. Retrieved asymmetry parameters are compared to those determined in situ by the Cloud Integrating Nephelometer (CIN). For two cases, the median values of asymmetry parameter retrieved by CIN and RSP agree within 0.01, while for the two other cases RSP asymmetry parameters are about 0.03–0.05 greater than those obtained by the CIN. Part of this bias might be explained by vertical variation of the asymmetry parameter.

Description: Biogenic volatile organic compound emissions during BEARPEX 2009 measured by eddy covariance and flux-gradient similarity methods Atmospheric Chemistry and Physics Discussions, 12, 25081-25120, 2012 Author(s): J.-H. Park, S. Fares, R. Weber, and A. H. Goldstein The Biosphere Effects on AeRosols and Photochemistry EXperiment (BEARPEX) took place in Blodgett Forest, a Ponderosa pine forest in the Sierra Nevada Mountains of California, during summer 2009. We deployed a Proton Transfer Reaction – Mass Spectrometer (PTR-MS) to measure fluxes and concentrations of biogenic volatile organic compounds (BVOCs). Eighteen ion species including the major BVOC expected at the site were measured sequentially at 5 heights to observe their vertical gradient from the forest floor to above the canopy. Fluxes of the 3 dominant BVOCs methanol, 2-Methyl-3-butene-2-ol (MBO), and monoterpenes, were measured above the canopy by the eddy covariance method. Canopy scale fluxes were also determined by the flux-gradient similarity method ( K -theory). A universal K ( K univ ) was determined as the mean of individual K 's calculated from the measured fluxes divided by vertical gradients for methanol, MBO, and monoterpenes. This K univ was then multiplied by the gradients of each observed ion species to compute their fluxes. The flux-gradient similarity method showed very good agreement with the Eddy Covariance method. Fluxes are presented for all measured species and compared to historical measurements from the same site, and used to test emission algorithms used to model fluxes at the regional scale. MBO was the dominant emission observed followed by methanol, monoterpenes, acetone, and acetaldehyde. The flux-gradient similarity method is shown to be a useful, and we recommend its use especially in experimental conditions when fast measurement of BVOC species is not available.

Description: Black carbon physical properties and mixing state in the European megacity Paris Atmospheric Chemistry and Physics Discussions, 12, 25121-25180, 2012 Author(s): M. Laborde, M. Crippa, T. Tritscher, Z. Jurányi, P. F. DeCarlo, B. Temime-Roussel, N. Marchand, S. Eckhardt, A. Stohl, U. Baltensperger, A. S. H. Prévôt, E. Weingartner, and M. Gysel Aerosol hygroscopicity and black carbon (BC) properties were characterised during wintertime in Paris, one of the biggest European megacities. Hygroscopic growth factor (GF) distributions, characterised by distinct modes of more-hygroscopic background aerosol and non- or slightly hygroscopic aerosol of local (or regional) origin, revealed an increase of the relative contribution of the local sources compared to the background aerosol with decreasing particle size. BC particles in Paris were mainly originating from fresh traffic emissions, whereas biomass burning was only a minor contribution. The mass size distribution of the BC cores peaked on average at a BC core mass equivalent diameter of D MEV ≈150 nm. The BC particles were moderately coated (Δ coat ≈30 nm on average for BC cores with D MEV =160–260 nm) and an average mass absorption coefficient (MAC) of ~8.6 m 2 g −1 at the wavelength λ = 880 nm was observed. Different time periods were selected to investigate the properties of BC particles as a function of source and air mass type. The traffic emissions were found to be non-hygroscopic (GF ≈ 1.0), and essentially all particles with a dry mobility diameter larger than D 0 = 110 nm contained a BC core. BC from traffic emissions was further characterised by literally no coating (Δ coat ≈2 nm), the smallest maximum of the BC core mass size distribution ( D MEV ≈100 nm) and the smallest MAC (~7.3 m 2 g −1 at λ = 880 nm). The biomass burning aerosol was slightly more-hygroscopic than the traffic emissions (with a distinct slightly hygroscopic mode peaking at GF≈1.1–1.2). Furthermore, only a minor fraction (⩽10%) of the slightly hygroscopic particles with GF⩾1.1 (and D 0 = 265 nm) contained a detectable BC core. The BC particles from biomass burning were found to have a medium coating thickness as well as slightly larger mean BC core sizes and MAC values compared to traffic emissions. The aerosol observed under the influence of aged air masses and air masses from Eastern Continental Europe was dominated by a more-hygroscopic mode peaking at GF≈1.6. Most particles (95%) with a D 0 = 265 nm, in this mode, did not contain a detectable BC core. A significant fraction of the BC particles had a substantial coating with non-refractory aerosol components. MAC values of ~8.8 m 2 g −1 and ~8.3 m 2 g −1 at λ = 880 nm and mass mean BC core diameters of 150 nm and 200 nm were observed for the aged and continental air mass types, respectively. The reason for the larger BC core sizes compared to the fresh emissions – transport effects or a different BC source – remains unclear. The dominant fraction of the BC-containing particles was found to have no or very little coating with non-refractory matter. The lack of coatings is consistent with the observation that the BC particles are non- or slightly hygroscopic, which makes them poor cloud condensation nuclei.

Description: Linkages between ozone depleting substances, tropospheric oxidation and aerosols Atmospheric Chemistry and Physics Discussions, 12, 25551-25572, 2012 Author(s): A. Voulgarakis, D. T. Shindell, and G. Faluvegi Coupling between the stratosphere and the troposphere allows changes in stratospheric ozone abundances to affect tropospheric chemistry. Large-scale effects from such changes on chemically produced tropospheric aerosols have not been systematically examined in past studies. We use a composition-climate model to investigate potential past and future impacts of changes in stratospheric Ozone Depleting Substances (ODS) on tropospheric oxidants and sulfate aerosols. In most experiments, we find significant responses in tropospheric photolysis and oxidants, with small but significant effects on methane radiative forcing. The response of sulfate aerosols is sizeable when examining the effect of increasing future nitrous oxide (N 2 O) emissions. We also find that without the regulation of chlorofluorocarbons (CFCs) through the Montreal Protocol, sulfate aerosols could have increased by 2050 by a comparable amount to the decreases predicted due to relatively stringent sulfur emissions controls. The historical radiative forcing of CFCs through their indirect effects on methane (−22.6 mW m −2 ) and sulfate aerosols (−3.0 mW m −2 ) discussed here is non-negligible when compared to known historical CFC forcing. Our results stress the importance of accounting for stratosphere-troposphere, gas-aerosol and composition-climate interactions when investigating the effects of changing emissions on atmospheric composition and climate.

Description: CLARA-SAL: a global 28-yr timeseries of Earth's black-sky surface albedo Atmospheric Chemistry and Physics Discussions, 12, 25573-25615, 2012 Author(s): A. Riihelä, T. Manninen, V. Laine, K. Andersson, and F. Kaspar We present a novel 28-yr dataset of Earth's black-sky surface albedo, derived from AVHRR instruments. The dataset is created using algorithms to separately derive the surface albedo for different land use areas globally. Snow, sea ice, open water and vegetation are all treated independently. The product features corrections for the atmospheric effect in satellite-observed surface radiances, a BRDF correction for the anisotropic reflectance properties of natural surfaces, and a novel topography correction of geolocation and radiometric accuracy of surface reflectance observations over mountainous areas. The dataset is based on a homogenized AVHRR radiance timeseries. The product is validated against quality-controlled in situ observations of clear-sky surface albedo at various BSRN sites around the world. Snow and ice albedo retrieval validation is given particular attention using BSRN sites over Antarctica, Greenland Climate Network stations on the Greenland Ice Sheet (GrIS), as well as sea ice albedo data from the SHEBA and Tara expeditions. The product quality is found to be comparable to other previous long-term surface albedo datasets from AVHRR.

Description: A unified approach to aerosol remote sensing and type specification in the infrared Atmospheric Chemistry and Physics Discussions, 12, 26871-26928, 2012 Author(s): L. Clarisse, P.-F. Coheur, F. Prata, J. Hadji-Lazaro, D. Hurtmans, and C. Clerbaux Atmospheric aerosols impact air quality and global climate. Space based measurements are the best way to observe their spatial and temporal distributions, and can also be used to gain better understanding of their chemical, physical and optical properties. Aerosol composition is the key parameter affecting the refractive index, which determines how much radiation is scattered and absorbed. Composition of aerosols is unfortunately not measured by state of the art satellite remote sounders. Here we use high resolution infrared measurements for aerosol type differentiation, exploiting, in that part of spectrum, the dependency of their refractive index on wavelength. We review existing detection methods and present a unified detection method based on linear discrimination analysis. We demonstrate this method on measurements of the Infrared Atmospheric Sounding Interferometer (IASI) and six different aerosol types, namely volcanic ash, windblown sand, ice crystals, sulfuric acid droplets, ammonium sulfate and smoke particles. The detection of the last three types is unprecedented in the infrared in nadir mode, but is very promising, especially for sulfuric acid droplets which are detected in the lower troposphere and up to 6 months after injection in the upper troposphere/lower stratosphere.

Description: Comparison of three vertically resolved ozone data bases: climatology, trends and radiative forcings Atmospheric Chemistry and Physics Discussions, 12, 26561-26605, 2012 Author(s): B. Hassler, P. J. Young, R. W. Portmann, G. E. Bodeker, J. S. Daniel, K. H. Rosenlof, and S. Solomon Climate models that do not simulate changes in stratospheric ozone concentrations require ozone input fields to accurately calculate UV fluxes and stratospheric heating rates. In this study, three different global ozone time series that are available for this purpose are compared: the data set of Randel and Wu (2007) (RW07), Cionni et al. (2011) (SPARC), and Bodeker et al. (2012) (BDBP). The latter is a very recent data set, based on the comprehensive ozone measurement database described by Hassler et al. (2008). All three data sets represent multiple-linear regression fits to vertically resolved ozone observations, resulting in a patially and temporally continuous stratospheric ozone field covering at least the period from 1979 to 2005. The main difference between the data sets result from using different observations and including different basis functions for the regression model fits. These three regression-based data sets are compared against observations from ozonesondes and satellites to compare how the data sets represent concentrations, trends, and interannual variability. In the Southern Hemisphere polar region, RW07 and SPARC underestimate the ozone depletion in spring as seen in ozonesonde measurements. A piecewise linear trend regression is performed to estimate the 1979–1996 ozone decrease globally, covering a period of extreme depletion in most regions. BDBP seems to overestimate Arctic and tropical ozone loss over this period somewhat relative to the available measurements, whereas these appear to be underestimated in RW07 and SPARC. In most regions, the three data sets yield ozone values that are within the range of the different observations that serve as input to the regressions. However, the differences among the three suggest that there are large uncertainties in ozone trends. These result in differences of almost a factor of four in radiative forcing, which is important for the resulting climate changes.

Description: On the export of reactive nitrogen from Asia: NO x partitioning and effects on ozone Atmospheric Chemistry and Physics Discussions, 12, 24955-24984, 2012 Author(s): T. H. Bertram, A. E. Perring, P. J. Wooldridge, J. Dibb, M. A. Avery, and R. C. Cohen The partitioning of reactive nitrogen (NO y ) was measured over the remote North Pacific during spring 2006. We use these observations to assess the impact of increasing emissions of nitrogen oxides in East Asia on ozone (O 3 ) production rates over the remote Pacific and the intercontinental transport of O 3 and its precursors to North America. Aircraft observations of speciated NO y , made between 25° and 55° N, confirm a controlling role for peroxyacyl nitrates in NO x production in aged Asian outflow, accounting for more than 60% of NO y above 5 km, while thermal dissociation limits their contribution to less than 10% in the lower troposphere. The observations reveal the extreme sensitivity of the remote Pacific to future changes in NO x loadings, with an experimentally determined crossover point between net O x destruction and net O x production of 60 pptv NO x . Using simultaneous observations of speciated NO y and wind speed, we calculate the flux of reactive nitrogen through the meridional plane of 150° W (between 25° and 55° N) to be 0.007 ± 0.002 Tg N day −1 , which provides an upper limit of 15% on the export efficiency of NO y from East Asia. Analysis of the subsiding plumes in the sampling domains suggests that episodic dry subsidence events play an important role in the intercontinental transport of ozone and its precursors from East Asia to North America.

Description: The Atmospheric Chemistry and Canopy Exchange Simulation System (ACCESS): model description and application to a temperate deciduous forest canopy Atmospheric Chemistry and Physics Discussions, 12, 24765-24820, 2012 Author(s): R. D. Saylor Forest canopies are primary emission sources of biogenic volatile organic compounds (BVOCs) and have the potential to significantly influence the formation and distribution of secondary organic aerosol (SOA) mass. Biogenically-derived SOA formed as a result of emissions from the widespread forests across the globe may affect air quality in populated areas, degrade atmospheric visibility, and affect climate through direct and indirect forcings. In an effort to better understand the formation of SOA mass from forest emissions, a 1-D column model of the physical and chemical processes occurring within and just above a vegetative canopy has been created. This model, the Atmospheric Chemistry and Canopy Exchange Simulation System (ACCESS), includes processes accounting for the emission of BVOCs from the canopy, turbulent vertical transport within and above the canopy and throughout the height of the planetary boundary layer (PBL), near-explicit representation of chemical transformations, mixing with the background atmosphere and bi-directional exchange between the atmosphere and canopy and the atmosphere and forest floor. The model formulation of ACCESS is described in detail and results are presented for an initial application of the modeling system to Walker Branch Watershed, an isoprene-emission-dominated forest canopy in the Southeastern United States which has been the focal point for previous chemical and micrometeorological studies. Model results of isoprene profiles and fluxes are found to be consistent with previous measurements made at the simulated site and with other measurements made in and above mixed deciduous forests in the Southeastern United States. Sensitivity experiments exploring how canopy concentrations and fluxes of gas-phase precursors of SOA are affected by background anthropogenic nitrogen oxides suggest potentially significant non-linearities in the chemical and physical system of the canopy which may have an impact on the relative magnitude of SOA formed through aqueous- versus gas-phase pathways as a function of anthropogenic influence.

Description: Anthropogenic, biomass burning, and volcanic emissions of black carbon, organic carbon, and SO 2 from 1980 to 2010 for hindcast model experiments Atmospheric Chemistry and Physics Discussions, 12, 24895-24954, 2012 Author(s): T. Diehl, A. Heil, M. Chin, X. Pan, D. Streets, M. Schultz, and S. Kinne Two historical emission inventories of black carbon (BC), primary organic carbon (OC), and SO 2 emissions from land-based anthropogenic sources, ocean-going vessels, air traffic, biomass burning, and volcanoes are presented and discussed for the period 1980–2010. These gridded inventories are provided to the internationally coordinated AeroCom Phase II multi-model hindcast experiments. The horizontal resolution is 0.5°×0.5° and 1.0°×1.0°, while the temporal resolution varies from daily for volcanoes to monthly for biomass burning and aircraft emissions, and annual averages for land-based and ship emissions. One inventory is based on inter-annually varying activity rates of land-based anthropogenic emissions and shows strong variability within a decade, while the other one is derived from interpolation between decadal endpoints and thus exhibits linear trends within a decade. Both datasets capture the major trends of decreasing anthropogenic emissions over the USA and Western Europe since 1980, a sharp decrease around 1990 over Eastern Europe and the former USSR, and a steep increase after 2000 over East and South Asia. The inventory differences for the combined anthropogenic and biomass burning emissions in the year 2005 are 34% for BC, 46% for OC, and 13% for SO 2 . They vary strongly depending on species, year and region, from about 10% to 40% in most cases, but in some cases the inventories differ by 100% or more. Differences in emissions from wild-land fires are caused only by different choices of the emission factors for years after 1996 which vary by a factor of about 1 to 2 for OC depending on region, and by a combination of emission factors and the amount of dry mass burned for years up to 1996. Volcanic SO 2 emissions, which are only provided in one inventory, include emissions from explosive, effusive, and quiescent degassing events for 1167 volcanoes.

Description: The global 3-D distribution of tropospheric aerosols as characterized by CALIOP Atmospheric Chemistry and Physics Discussions, 12, 24847-24893, 2012 Author(s): D. M. Winker, J. L. Tackett, B. J. Getzewich, Z. Liu, M. A. Vaughan, and R. R. Rogers The CALIOP lidar, carried on the CALIPSO satellite, has been acquiring global atmospheric profiles since June 2006. This dataset now offers the opportunity to characterize the global 3-D distribution of aerosol as well as seasonal and interannual variations, and confront aerosol models with observations in a way that has not been possible before. With that goal in mind, a monthly global gridded dataset of daytime and nighttime aerosol extinction profiles has been constructed. Averaged aerosol profiles for cloud-free and all-sky conditions are reported separately. This 6-yr dataset characterizes the global 3-dimensional distribution of tropospheric aerosol. Vertical distributions are seen to vary with season, as both source strengths and transport mechanisms vary. In most regions, clear-sky and all-sky mean aerosol profiles are found to be quite similar, implying a lack of correlation between high semi-transparent cloud and aerosol in the lower troposphere. An initial evaluation of the accuracy of the aerosol extinction profiles is presented. Detection limitations and the representivity of aerosol profiles in the upper troposphere are of particular concern. While results are preliminary, we present evidence that the monthly-mean gridded CALIOP aerosol profiles are representative for aerosol extinction greater than about 0.001 km −1 and up to an altitude of 4–6 km in most cases. The work described here forms an initial global 3-D aerosol climatology which hopefully will be extended and improved over time.

Description: Characteristics of atmospheric total gaseous mercury (TGM) observed in urban Nanjing, China Atmospheric Chemistry and Physics Discussions, 12, 25037-25080, 2012 Author(s): J. Zhu, T. Wang, R. Talbot, H. Mao, C. B. Hall, X. Yang, C. Fu, B. Zhuang, S. Li, Y. Han, and X. Huang Long-term continuous measurements of total gaseous mercury (TGM = gaseous elemental mercury (GEM) + reactive gaseous mercury, RGM) were conducted simultaneously along with meteorological variables and a suite of trace gases at an urban site in Nanjing, China from 18 January to 31 December 2011. Measurements were conducted using a high resolution mercury vapor analyzer (Tekran 2537B) with 5-min time resolution. The average concentration of TGM was 7.9 ± 7.0 ng m −3 with a range of 0.8–180 ng m −3 over the study period. TGM concentrations followed a typical lognormal pattern dominated by a range of 3–7 ng m −3 , which was significantly higher than the continental background values (~1.5 ng m −3 ) in Northern Hemisphere. The mean seasonal TGM concentrations decreased in the following order: summer, spring, fall, and winter. This seasonal pattern was quite different from measurements at most other sites around the world. We attributed high monthly average concentrations to the re-volatilization of deposited mercury during the warm season due to high temperatures and greater solar radiation. Previous Modeling studies suggested that Nanjing and the surrounding region have the largest Chinese natural emissions during the summer. Positive correlations between temperature, solar radiation, and TGM concentration combined with no correlation between CO and TGM in summer provide a strong indication that natural sources are important in Nanjing. While most sharp peaks were caused by anthropogenic sources. TGM concentrations in Nanjing exhibited a noticeable diurnal pattern with a sharp increase after sunrise and peak of greater than 8 m −3 during 7–10 a.m. LT. Further, seasonally averaged diurnal cycles of TGM exhibited considerably different patterns with the largest variation in spring and insignificant fluctuations in winter. Using HYSPLIT backwards trajectories from six clusters, it was indicated that the highest TGM concentrations, 11.9 m −3 , was derived from lacal air masses. The cleanest air masses, with an average TGM concentration of 4.7 and 5.9 m −3 , were advected from the north via fast transport facilitated by sweeping synoptic flows.

Description: The effects of recent control policies on trends in emissions of anthropogenic atmospheric pollutants and CO 2 in China Atmospheric Chemistry and Physics Discussions, 12, 24985-25036, 2012 Author(s): Y. Zhao, J. Zhang, and C. P. Nielsen To examine the effects of China's national policies of energy conservation and emission control during 2005–2010, inter-annual emission trends of gaseous pollutants, primary aerosols, and CO 2 are estimated with a bottom-up framework. The control measures led to improved energy efficiency and/or increased penetration of emission control devices at power plants and other important industrial sources, yielding reduced emission factors for all evaluated species except NO x . The national emissions of anthropogenic SO 2 , CO, and total primary PM (particulate matter) in 2010 are estimated to have been 89%, 108%, and 86% of those in 2005, respectively, suggesting successful emission control of those species despite fast growth of the economy and energy consumption during the period. The emissions of NO x and CO 2 , however, are estimated to have increased by 48% and 43%, respectively, indicating that they remain largely determined by the growth of energy use, industrial production, and vehicle populations. Based on application of a Monte-Carlo framework, estimated uncertainties of SO 2 and PM emissions increased from 2005 to 2010, resulting mainly from weakly understood average SO 2 removal efficiency in flue gas desulfurization (FGD) systems in the power sector, and unclear changes in the penetration levels of dust collectors at industrial sources, respectively. While emission trends determined by bottom-up methods can be generally verified by observations from both ground stations and satellites, clear discrepancies exist for given regions and seasons, indicating a need for more accurate spatial and time distributions of emissions. Limitations of current emission control polices are analyzed based on the estimated emission trends. Compared with control of total PM, there are fewer gains in control of fine particles and carbonaceous aerosols, the PM forms most responsible for damages to public health and effects on radiative forcing. A decrease of alkaline base cations as primary PM that is much faster than that of SO 2 may have raised the acidification risks to ecosystems, indicating further control of acid precursors is required. Moreover, with relatively strict controls in developed urban areas, air pollution challenges have been expanding to less-developed neighboring regions. There is a great need in the future for multi-pollutant control strategies that combine recognition of diverse environmental impacts both in urban and rural areas with emission abatement of multiple species in concert.

Description: Measurements of reactive trace gases and variable O 3 formation rates in some South Carolina biomass burning plumes Atmospheric Chemistry and Physics Discussions, 12, 25255-25328, 2012 Author(s): S. K. Akagi, R. J. Yokelson, I. R. Burling, S. Meinardi, I. Simpson, D. R. Blake, G. R. McMeeking, A. Sullivan, T. Lee, S. Kreidenweis, S. Urbanski, J. Reardon, D. W. T. Griffith, T. J. Johnson, and D. R. Weise In October–November 2011 we measured trace gas emission factors from seven prescribed fires in South Carolina (SC), US, using two Fourier transform infrared spectrometer (FTIR) systems and whole air sampling (WAS) into canisters followed by gas-chromatographic analysis. A total of 97 trace gas species were quantified from both airborne and ground-based sampling platforms, making this one of the most detailed field studies of fire emissions to date. The measurements include the first emission factors for a suite of monoterpenes produced by heating vegetative fuels during field fires. The first quantitative FTIR observations of limonene in smoke are reported along with an expanded suite of monoterpenes measured by WAS including α-pinene, β-pinene, limonene, camphene, 4-carene, and myrcene. The known chemistry of the monoterpenes and their measured abundance of 0.4–27.9% of non-methane organic compounds (NMOCs) and ~21% of organic aerosol (mass basis) suggests that they impacted secondary formation of ozone (O 3 ), aerosols, and small organic trace gases such as methanol and formaldehyde in the sampled plumes in first few hours after emission. The variability in the initial terpene emissions in the SC fire plumes was high and, in general, the speciation of the initially emitted gas-phase NMOCs was 13–195% different from that observed in a similar study in nominally similar pine forests in North Carolina ~20 months earlier. It is likely that differences in stand structure and environmental conditions contributed to the high variability observed within and between these studies. Similar factors may explain much of the variability in initial emissions in the literature. The ΔHCN/ΔCO emission ratio, however, was found to be fairly consistent with previous airborne fire measurements in other coniferous-dominated ecosystems, with the mean for these studies being 0.90 ± 0.06%, further confirming the value of HCN as a biomass burning tracer. The SC results also support an earlier finding that C 3 -C 4 alkynes may be of use as biomass burning indicators on the time-scale of hours to a day. It was possible to measure the downwind chemical evolution of the plume on four of the fires and significant O 3 formation (ΔO 3 /ΔCO from 10–90%) occurred in all of these plumes within two hours. The slowest O 3 production was observed on a cloudy day with low co-emission of NO x . The fastest O 3 production was observed on a sunny day when the downwind plume almost certainly incorporated significant additional NO x by passing over the Columbia, SC metropolitan area. Due to rapid plume dilution, it was only possible to acquire high-quality downwind data for two other trace gas species (formaldehyde and methanol) during two of the fires. In all four of these cases, significant increases in formaldehyde and methanol were observed in

Description: Selected topics on interactions between cirrus clouds and embedded contrails Atmospheric Chemistry and Physics Discussions, 12, 25237-25254, 2012 Author(s): K. Gierens Persistent contrails and natural cirrus clouds often coexist in the upper troposphere and contrails can be embedded within cirrus clouds. The present paper deals with some questions regarding the interaction of cirrus clouds and embedded contrails. I have selected only questions that can be answered by analytical means. I find that (1) the emission index for water vapour is only slightly changed when an aircraft crosses a cirrus cloud, (2) that contrail formation is not affected by an ambient cirrus, (3) that cirrus ice crystals entrained into the trailing wing tip vortex do not efficiently retard the sublimation of contrail ice crystals, and (4) that cirrus can start to dissolve an embedded contrail after a couple of hours by aggregation.

Description: Biological aerosol particle concentrations and size distributions measured in pristine tropical rainforest air during AMAZE-08 Atmospheric Chemistry and Physics Discussions, 12, 25181-25236, 2012 Author(s): J. A. Huffman, B. Sinha, R. M. Garland, A. Snee-Pollmann, S. S. Gunthe, P. Artaxo, S. T. Martin, M. O. Andreae, and U. Pöschl As a part of the AMAZE-08 campaign during the wet season in the rainforest of Central Amazonia, an ultraviolet aerodynamic particle sizer (UV-APS) was operated for continuous measurements of fluorescent biological aerosol particles (FBAP). In the coarse particle size range (〉 1 μm) the campaign median and quartiles of FBAP number and mass concentration were 7.3 × 10 4 m −3 (4.0–13.2 × 10 4 m −3 ) and 0.72 μg m −3 (0.42–1.19 μg mm −3 ), respectively, accounting for 24% (11–41%) of total particle number and 47% (25–65%) of total particle mass. During the five-week campaign in February–March 2008 the concentration of coarse-mode Saharan dust particles was highly variable. In contrast, FBAP concentrations remained fairly constant over the course of weeks and had a consistent daily pattern, peaking several hours before sunrise, suggesting observed FBAP was dominated by nocturnal spore emission. This conclusion was supported by the consistent FBAP number size distribution peaking at 2.3 μm, also attributed to fungal spores and mixed biological particles by scanning electron microscopy (SEM), light microscopy and biochemical staining. A second primary biological aerosol particle (PBAP) mode between 0.5 and 1.0 μm was also observed by SEM, but exhibited little fluorescence and no fungal staining. This mode consisted of single bacterial cells, brochosomes and various fragments of biological material. Particles liquid-coated with mixed organic-inorganic material constituted a large fraction of observations, and these coatings contained salts likely from primary biological origin. We provide key support for the suggestion that real-time laser-induce fluorescence (LIF) techniques provide size-resolved concentrations of FBAP as a lower limit for the atmospheric abundance of biological particles. We also show that primary biological particles, fungal spores in particular, are key fractions of supermicron aerosol in the Amazon and that, especially when coated by mixed inorganic material, could contribute significantly to hydrological cycling in such regions of the globe.

Description: Size distributions of dicarboxylic acids, ketocarboxylic acids, α-dicarbonyls and fatty acids in atmospheric aerosols from Tanzania, East Africa during wet and dry seasons Atmospheric Chemistry and Physics Discussions, 12, 25657-25701, 2012 Author(s): S. L. Mkoma and K. Kawamura Atmospheric aerosol samples of PM 2.5 and PM 10 were collected during the wet and dry seasons in 2011 from a rural site in Tanzania and analysed for water-soluble dicarboxylic acids, ketocarboxylic acids, α-dicarbonyls and fatty acids using a gas chromatography (GC) and GC/mass spectrometry. Here we report the size distribution and sources of diacids and related compounds for wet and dry seasons. Oxalic acid (C 2 ) was found as the most abundant diacid species followed by succinic and/or malonic acids whereas glyoxylic acid and glyoxal were the dominant ketoacids and α-dicarbonyls, respectively in both seasons in PM 2.5 and PM 10 . Mean concentration of C 2 in PM 2.5 (121.5± 46.6 ng m −3 ) was lower in wet season than dry season (258.1± 69.5 ng m −3 ). Similarly, PM 10 samples showed lower concentration of C 2 (168.6 ± 42.4 ng m −3 ) in wet season than dry season (292.4± 164.8 ng m −3 ). Relative abundances of C 2 in total diacids were 65.4% and 67.1% in PM 2.5 and 64.6% and 63.9% in PM 10 in the wet and dry seasons, respectively. Total concentrations of diacids (289–362 m −3 ), ketoacids (37.8–53.7ng m −3 ), and α-dicarbonyls (5.7–7.8 ng m −3 ) in Tanzania are higher to those reported at a rural background site in Nylsvley (South Africa) but comparable or lower to those reported from sites in Asia and Europe. Diacids and ketoacids were found to be present mainly in the fine fraction in both seasons (total α-dicarbonyls in the dry season), suggesting a production of organic aerosols from pyrogenic sources and photochemical oxidations. The averaged contributions of total diacid carbon to aerosol total carbon were 1.4% in PM 2.5 and 2.1% in PM 10 in wet season and 3.3% in PM 2.5 and 3.9% in PM 10 in dry season whereas those to water-soluble organic carbon were 2.2% and 4.7% inPM 2.5 and 3.1% and 5.8% in PM 10 during the wet and dry seasons, respectively. These ratios suggest an enhanced photochemical oxidation of organic precursors and heterogeneous reactions on aerosols under strong solar radiation and high humidity. Correlations among organic components and relations between source tracers with diacids and related compounds in both seasons showed influence of mixed sources from natural biogenic emissions, biomass burning, biofuel combustion, and enhanced photochemical production.

Description: Aerosol classification by airborne high spectral resolution lidar observations Atmospheric Chemistry and Physics Discussions, 12, 25983-26028, 2012 Author(s): S. Groß, M. Esselborn, B. Weinzierl, M. Wirth, A. Fix, and A. Petzold During four aircraft field experiments with the DLR research aircraft Falcon in 1998 (LACE), 2006 (SAMUM-1) and 2008 (SAMUM-2 and EUCAARI), airborne High Spectral Resolution Lidar (HSRL) and in situ measurements of aerosol microphysical and optical properties were performed. Altogether, the properties of six different aerosol types and aerosol mixtures – Saharan mineral dust, Saharan dust mixtures, Canadian biomass burning aerosol, African biomass burning aerosol, anthropogenic pollution aerosol, and marine aerosol have been studied. On the basis of this extensive HSRL data set, we present an aerosol classification scheme which is also capable to identify mixtures of different aerosol types. We calculated mixing lines that allowed us to determine the contributing aerosol types. The aerosol classification scheme was validated with in-situ measurements and backward trajectory analyses. Our results demonstrate that the developed aerosol mask is capable to identify complex stratifications with different aerosol types throughout the atmosphere.

Description: The diurnal evolution of the urban heat island of Paris: a model-based case study during Summer 2006 Atmospheric Chemistry and Physics Discussions, 12, 25941-25981, 2012 Author(s): H. Wouters, K. De Ridder, N. P. M. van Lipzig, M. Demuzere, and D. Lauwaet The urban heat island (UHI) over Paris during summer 2006 was simulated using the Advanced Regional Prediction System (ARPS) updated with a simple urban parametrization at a horizontal resolution of 1 km. Two integrations were performed, one with the urban land cover of Paris and another in which Paris was replaced by cropland. The focus is on a five-day clear-sky period, for which the UHI intensity reaches its maximum. The diurnal evolution of the UHI intensity was found to be adequately simulated for this five day period. The maximum difference at night in 2-m temperature between urban and rural areas stemming from the urban heating is reproduced with a relative error of less than 10%. The UHI has an ellipsoidal shape and stretches along the prevailing wind direction. The maximum UHI intensity of 6.1 K occurs at 23:00 UTC located 6 km downstream of the city centre and this largely remains during the whole night. An idealized one-column model study demonstrates that the nocturnal differential sensible heat flux, even though much smaller than its daytime value, is mainly responsible for the maximum UHI intensity. The reason for this nighttime maximum is that additional heat is only affecting a shallow layer of 150 m. At the same time, an idealized study shows that the orography around the city of Paris induces an uplift. This leads to a considerable nocturnal adiabatic cooling over cropland. In contrast, this uplift has little effect on the mixed-layer temperature over the city. About twenty percent of the total maximum UHI intensity is estimated to be caused by this uplift.

Description: Effects of internal mixing and aggregate morphology on optical properties of black carbon using a discrete dipole approximation model Atmospheric Chemistry and Physics Discussions, 12, 26401-26434, 2012 Author(s): B. Scarnato, S. Vahidinia, D. T. Richard, and T. W. Kirchstetter According to recent studies, internal mixing of black carbon (BC) with other aerosol materials in the atmosphere alters its aggregate shape, absorption of solar radiation, and radiative forcing. These mixing state effects are not yet fully understood. In this study, we characterize the morphology and mixing state of bare BC and BC internally mixed with sodium chloride (NaCl) using electron microscopy and examine the sensitivity of optical properties to BC mixing state and aggregate morphology using a discrete dipole approximation model (DDSCAT). DDSCAT predicts a higher mass absorption coefficient, lower single scattering albedo (SSA), and higher absorption Angstrom exponent (AAE) for bare BC aggregates that are lacy rather than compact. Predicted values of SSA at 550 nm range between 0.18 and 0.27 for lacy and compact aggregates, respectively, in agreement with reported experimental values of 0.25 ± 0.05. The variation in absorption with wavelength does not adhere precisely to a power law relationship over the 200 to 1000 nm range. Consequently, AAE values depend on the wavelength region over which they are computed. In the 300 to 550 nm range, AAE values ranged in this study from 0.70 for compact to 0.95 for lacy aggregates. The SSA of BC internally mixed with NaCl (100–300 nm in radius) is higher than for bare BC and increases with the embedding in the NaCl. Internally mixed BC SSA values decrease in the 200–400 nm wavelength range, a feature also common to the optical properties of dust and organics. Linear polarization features are also predicted in DDSCAT and are dependent on particle morphology. The bare BC (with a radius of 80 nm) presents in the linear polarization a bell shape feature, which is a characteristic of the Rayleigh regime (for particles smaller than the wavelength of incident radiation). When BC is internally mixed with NaCl (100–300 nm in radius), strong depolarization features for near-VIS incident radiation are evident, such as a decrease in the intensity and multiple modes at different angles corresponding to different mixing states. DDSCAT, being flexible on the geometry and refractive index of the particle, can be used to study the effect of mixing state and complex morphology on optical properties of realistic BC aggregates. This study shows that DDSCAT predicts morphology and mixing state dependent optical properties that have been reported previously and are relevant to radiative transfer and climate modeling and interpretation of remote sensing measurements.

Description: Modeling of 2008 Kasatochi volcanic sulfate direct radiative forcing: assimilation of OMI SO 2 plume height data and comparison with MODIS and CALIOP observations Atmospheric Chemistry and Physics Discussions, 12, 26435-26475, 2012 Author(s): J. Wang, S. Park, J. Zeng, K. Yang, S. Carn, N. Krotkov, and A. H. Omar Volcanic SO 2 column amount and injection height retrieved from the Ozone Monitoring Instrument (OMI) with the Extended Iterative Spectral Fitting (EISF) technique are used to initialize a global chemistry transport model (GEOS-Chem) to simulate the atmospheric transport and lifecycle of volcanic SO 2 and sulfate aerosol from the 2008 Kasatochi eruption, and to subsequently estimate the direct shortwave, top-of-the-atmosphere radiative forcing of the volcanic sulfate aerosol. Analysis shows that the integrated use of OMI SO 2 plume height in GEOS-Chem yields: (a) good agreement of the temporal evolution of 3-D volcanic sulfate distributions between model simulations and satellite observations from the Moderate Resolution Imaging Spectroradiometer (MODIS) and Cloud-Aerosol Lidar with Orthogonal Polarisation (CALIOP), and (b) a e-folding time for volcanic SO 2 that is consistent with OMI measurements, reflecting SO 2 oxidation in the upper troposphere and stratosphere is reliably represented in the model However, a consistent (~25%) low bias is found in the GEOS-Chem simulated SO 2 burden, and is likely due to a high (~20%) bias of cloud liquid water amount (as compared to the MODIS cloud product) and the resultant stronger SO 2 oxidation in the GEOS meteorological data during the first week after eruption when part of SO 2 underwent aqueous-phase oxidation in clouds. Radiative transfer calculations show that the forcing by Kasatochi volcanic sulfate aerosol becomes negligible 6 months after the eruption, but its global average over the first month is −1.3 W m −2 with the majority of the forcing-influenced region located north of 20° N, and with daily peak values up to −2 W m −2 on days 16–17. Sensitivity experiments show that every 2 km decrease of SO 2 injection height in the GEOS-Chem simulations will result in a ~25% decrease in volcanic sulfate forcing; similar sensitivity but opposite sign also holds for a 0.03 μm increase of geometric radius of the volcanic aerosol particles. Both sensitivities highlight the need to characterize the SO 2 plume height and aerosol particle size from space. While more research efforts are warranted, this study is among the first to assimilate both satellite-based SO 2 plume height and amount into a chemical transport model for an improved simulation of volcanic SO 2 and sulfate transport.

Description: An advanced scheme for wet scavenging and liquid-phase chemistry in a regional online-coupled chemistry transport model Atmospheric Chemistry and Physics Discussions, 12, 26099-26142, 2012 Author(s): C. Knote and D. Brunner Clouds are reaction chambers for atmospheric trace gases and aerosols, and the associated precipitation is a major sink for atmospheric constituents. The regional chemistry-climate model COSMO-ART has been lacking a description of wet scavenging of gases and aqueous-phase chemistry. In this work we present a coupling of COSMO-ART with a wet scavenging and aqueous-phase chemistry scheme. The coupling is made consistent with the cloud microphysics scheme of the underlying meteorological model COSMO. While the choice of the aqueous-chemistry mechanism is flexible, the effects of a simple sulfur oxidation scheme are shown in the application of the coupled system in this work. We give details explaining the coupling and extensions made, then present results from idealized flow-over-hill experiments in a 2-D model setup and finally results from a full 3-D simulation. Comparison against measurement data shows that the scheme efficiently reduces SO 2 trace gas concentrations by 0.3 ppbv (−30%) on average, while leaving O 3 and NO x unchanged. PM 10 aerosol mass, which has been overestimated previously, is now in much better agreement with measured values due to a stronger scavenging of coarse particles. While total PM 2.5 changes only little, chemical composition is improved notably. Overestimations of nitrate aerosols are reduced by typically 0.5–1 μg m −3 (up to −2 μg m −3 in the Po Valley) while sulfate mass is increased by 1–1.5 μg m −3 on average (up to 2.5 μg m −3 in Eastern Europe). The effect of cloud processing of aerosols on its size distribution, i. e. a shift towards larger diameters, is observed. Compared against wet deposition measurements the system underestimates the total wet deposited mass for the simulated case study. We find that while evaporation of cloud droplets dominates in higher altitudes, evaporation of precipitation can contribute up to 50% of total evaporated mass near the surface.

Description: Estimation of aerosol particle distributions with Kalman Filtering – Part 1: Theory, general aspects and statistical validity Atmospheric Chemistry and Physics Discussions, 12, 18853-18887, 2012 Author(s): T. Viskari, E. Asmi, P. Kolmonen, H. Vuollekoski, T. Petäjä, and H. Järvinen Aerosol characteristics can be measured with different instruments providing observations that are not trivially inter-comparable. Extended Kalman Filter (EKF) is introduced here as a method to estimate aerosol particle number size distributions from multiple simultaneous observations. The focus here in Part 1 of the work was on general aspects of EKF in the context of Differential Mobility Particle Sizer (DMPS) measurements. Additional instruments and their implementations are discussed in Part 2 of the work. University of Helsinki Multi-component Aerosol model (UHMA) is used to propagate the size distribution in time. At each observation time (10 min apart), the time evolved state is updated with the raw particle mobility distributions, measured with two DMPS systems. EKF approach was validated by calculating the bias and the standard deviation for the estimated size distributions with respect to the raw measurements. These were compared to corresponding bias and standard deviation values for distributions calculated with a mathematical inversion method. Despite the assumptions made in the EKF implementation, EKF was found to be more accurate than the mathematical inversion in terms of bias, and compatible in terms of standard deviation. Potential further improvements of the EKF implementation are discussed.

Description: Cost effective determination of vehicle emission factors using on-road measurements Atmospheric Chemistry and Physics Discussions, 12, 18715-18740, 2012 Author(s): N. Hudda, S. Fruin, R. J. Delfino, and C. Sioutas To evaluate the success of vehicle emissions regulations, trends in both fleet-wide average emissions as well as high-emitter emissions are needed, but it is challenging to capture the full spread of vehicle emission factors (EFs) with chassis dynamometer, tunnel or remote sensing studies. We developed an efficient and cost-effective method using real-time on-road pollutant measurements from a mobile platform, which when linked with real-time traffic data, allows calculating both the average and spread of EFs for light-duty gasoline-powered vehicles (LDV) and heavy-duty diesel-powered vehicles (HDV). This is the first study in California to report EFs under a wide range of real-driving conditions on multiple freeways and it captured much or most of the variability in EFs due to inter-vehicle differences. Fleet average LDV EFs were generally in agreement with most recent studies and an order of magnitude lower than HDV EFs, but over an order of magnitude or more spread was observed for both LDV and HDV EFs. HDV EFs reflected relatively rapid decreases occurring in diesel emissions in Los Angeles/California, and HDV EFs on I-710, a primary route used for goods movement and a focus of additional truck fleet turnover incentives, were lower than on other freeways. When freeway emission rates (ER) were quantified as the product of EF and vehicle activity rates per mile of freeway, ERs were found to be generally similar in magnitude. Despite a two- to three-fold difference in HDV fractions between freeways, higher LDV volumes largely offset this difference.

Description: Classification and investigation of Asian aerosol properties Atmospheric Chemistry and Physics Discussions, 12, 18927-18958, 2012 Author(s): T. Logan, B. Xi, X. Dong, Z. Li, and M. Cribb Ongoing urbanization and industrialization in East Asia have generated a wide variety of aerosols in the atmosphere and have consequently added more uncertainty when evaluating global climate change. To classify different types of aerosols and investigate their physical and chemical properties, four AErosol RObotic NETwork (AERONET) sites have been selected to represent aerosol properties dominated by mixed complex particle types (Xianghe and Taihu), desert-urban (SACOL), and biomass (Mukdahan) over East Asia during the 2001–2010 period. The volume size distribution, aerosol optical depth [τ (λ) and τ abs (λ)], Ångström exponent (α and α abs ), and the single scattering co-albedo [ω oabs (λ)] and α(ω oabs ) parameters over the four selected sites have been analyzed. These parameters are used to (a) investigate the aerosol properties and their seasonal variations over the four selected sites, (b) discern the different absorptive characteristics of BC, OC, and mineral dust particles using α abs440-870 and α (ω oabs440-870 ), and (c) develop an aerosol clustering method involving α 440-870 and ω oabs440 . A strong mineral dust influence is seen at the Xianghe, Taihu, and SACOL sites during the spring months (MAM) as given by coarse mode size distribution dominance, declining α 440-870 , and elevated α abs440-870 and α (ω oabs440-870 ) values. A weakly absorbing pollution (OC and biomass) aerosol dominance is seen in the summer (JJA) and autumn (SON) months as given by a strong fine mode influence, increasing α 440-870 , and declining α abs440-870 and α (ω oabs440-870 ) values. A winter season (DJF) shift toward strongly absorbing BC particles is observed at Xianghe and Taihu (elevated α 440-870 , increase in α abs440-870 and α(ω oabs440-870 )). At Mukdahan, a fine mode biomass particle influence is observed year round as given by the volume size distribution, elevated α 440-870 (higher than the other sites), low α abs440-870 and negative α (ω oabs440-870 ) values indicating weakly absorbing OC particles. The α(ω oabs ) parameter is also shown to have less overlap in values than α abs in discerning influences from OC, BC, biomass and mineral dust particles. The clustering method using α 440-870 and ω oabs440 illustrates four groups of aerosols: Cluster I – fine mode, weakly absorbing pollution particles, Cluster II – fine mode, strongly absorbing pollution particles, Cluster III – coarse mode, strongly absorbing mineral dust particles, and Cluster IV – biomass particles with similar characteristics as Cluster II but less absorbing. This method has shown that aerosol mixtures are both seasonal and regional combinations of particles that were either locally generated or transported from other source regions and should be implemented over other AERONET sites in the future.

Description: A 60-yr record of atmospheric carbon monoxide reconstructed from Greenland firn air Atmospheric Chemistry and Physics Discussions, 12, 18993-19037, 2012 Author(s): V. V. Petrenko, P. Martinerie, P. Novelli, D. M. Etheridge, I. Levin, Z. Wang, T. Blunier, J. Chappellaz, J. Kaiser, P. Lang, L. P. Steele, S. Hammer, J. Mak, R. L. Langenfelds, J. Schwander, J. P. Severinghaus, E. Witrant, G. Petron, M. O. Battle, G. Forster, W. T. Sturges, J.-F. Lamarque, K. Steffen, and J. W. C. White We present a reconstruction of the Northern Hemisphere (NH) high latitude atmospheric carbon monoxide (CO) mole fraction from Greenland firn air. Firn air samples were collected at three deep ice core sites in Greenland (NGRIP in 2001, Summit in 2006 and NEEM in 2008). CO records from the three sites agree well with each other as well as with recent atmospheric measurements, indicating that CO is well preserved in the firn at these sites. CO atmospheric history was reconstructed back to the year 1950 from the measurements using a combination of two forward models of gas transport in firn and an inverse model. The reconstructed history suggests that Arctic CO was already higher in 1950 than it is today. CO mole fractions rose gradually until the 1970s and peaked in the 1970s or early 1980s, followed by a decline to today's levels. We compare the CO history with the atmospheric histories of methane, light hydrocarbons, molecular hydrogen, CO stable isotopes and hydroxyl radical (OH), as well as with published CO emission inventories and results of a historical run from a chemistry-transport model. We find that the reconstructed Greenland CO history cannot be reconciled with available emission inventories unless large changes in OH are assumed. We argue that the available CO emission inventories chronically underestimate NH emissions, and fail to capture the emission decline starting in the late 1970s, which was most likely due to reduced emissions from road transportation in North America and Europe.

Description: Characteristics of tropospheric ozone depletion events in the Arctic spring: analysis of the ARCTAS, ARCPAC, and ARCIONS measurements and satellite BrO observations Atmospheric Chemistry and Physics Discussions, 12, 16219-16257, 2012 Author(s): J.-H. Koo, Y. Wang, T. P. Kurosu, K. Chance, A. Rozanov, A. Richter, S. J. Oltmans, A. M. Thompson, J. W. Hair, M. A. Fenn, A. J. Weinheimer, T. B. Ryerson, S. Solberg, L. G. Huey, J. Liao, J. E. Dibb, J. A. Neuman, J. B. Nowak, R. B. Pierce, M. Natarajan, and J. Al-Saadi Arctic ozone depletion events (ODEs) are due to catalytic ozone loss driven by halogen chemistry. The presence of ODEs is affected not only by in situ chemistry but also by transport including advection of ozone-poor air mass and vertical mixing. To better characterize the ODEs, we analyze the combined set of surface, ozonesonde, and aircraft in situ measurements of ozone and bromine compounds during the Arctic Research of the Composition of the Troposphere from Aircraft and Satellites (ARCTAS) and the Aerosol, Radiation, and Cloud Processes affecting Arctic Climate (ARCPAC) experiments (April 2008). Tropospheric BrO columns retrieved from satellite measurements and back trajectories calculations are used to investigate the characteristics of observed ODEs. The implications of the analysis results for the validation of the retrieval of tropospheric column BrO are also discussed. Time-lagged correlation analysis between in situ (surface and ozonesonde) measurements of ozone and satellite derived tropospheric BrO indicates that the ODEs are due to either local halogen-driven ozone loss or short-range (~1 day) transport from nearby regions with ozone depletion. The effect of in situ halogen-driven loss is also evident in the diurnal variation of surface ozone concentrations at Alert, Canada. High-BrO regions revealed by satellite measurements tend to be collocated with first-year sea ice, particularly over the Chukchi Sea. Aircraft observations indicate low-ozone air mass transported from these high-BrO regions. Correlation analyses of ozone with potential temperature and time-lagged tropospheric BrO column show that the vertical extent of local ozone loss is surprisingly deep (1–2 km) at Resolute and Churchill, Canada. The unstable boundary layer during ODEs at Churchill could potentially provide a source of free tropospheric BrO through convective transport and explain the significant negative correlation between free tropospheric ozone and tropospheric BrO column at this site.

Description: Simultaneous assimilation of satellite NO 2 , O 3 , CO, and HNO 3 data for the analysis of tropospheric chemical composition and emissions Atmospheric Chemistry and Physics Discussions, 12, 16131-16218, 2012 Author(s): K. Miyazaki, H. J. Eskes, K. Sudo, M. Takigawa, M. van Weele, and K. F. Boersma We have developed an advanced chemical data assimilation system to combine observations of chemical compounds from multiple satellites. NO 2 , O 3 , CO, and HNO 3 measurements from the OMI, TES, MOPITT, and MLS satellite instruments are assimilated into the global chemical transport model CHASER for the years 2006–2007. The CHASER data assimilation system (CHASER-DAS), based on the local ensemble transform Kalman filter technique, simultaneously optimizes the chemical species, as well as the emissions of O 3 precursors, while taking their chemical feedbacks into account. With the available datasets, an improved description of the chemical feedbacks can be obtained, especially related to the NO x -CO-OH-O 3 set of chemical reactions. Comparisons against independent satellite, aircraft, and ozonesonde data show that the data assimilation results in substantial improvements for various chemical compounds. These improvements include a reduced negative tropospheric NO 2 column bias (by 40–85%), a reduced negative CO bias in the Northern Hemisphere (by 40–90%), and a reduced positive O 3 bias in the middle and upper troposphere (from 30–40% to within 10%). These changes are related to increased tropospheric OH concentrations by 5–15% in the tropics and the Southern Hemisphere in July. Observing System Experiments (OSEs) have been conducted to quantify the relative importance of each data set on constraining the emissions and concentrations. The OSEs confirm that the assimilation of individual data sets results in a strong influence on both assimilated and non-assimilated species through the inter-species error correlation and the chemical coupling described by the model.

Description: Discrimination of biomass burning smoke and clouds in MAIAC algorithm Atmospheric Chemistry and Physics Discussions, 12, 18651-18670, 2012 Author(s): A. Lyapustin, S. Korkin, Y. Wang, B. Quayle, and I. Laszlo The multi-angle implementation of atmospheric correction (MAIAC) algorithm makes aerosol retrievals from MODIS data at 1 km resolution providing information about the fine scale aerosol variability. This information is required in different applications such as urban air quality analysis, aerosol source identification etc. The quality of high resolution aerosol data is directly linked to the quality of cloud mask, in particular detection of small (sub-pixel) and low clouds. This work continues research in this direction, describing a technique to detect small clouds and introducing the "smoke test" to discriminate the biomass burning smoke from the clouds. The smoke test relies on a relative increase of aerosol absorption at MODIS wavelength 0.412 μm as compared to 0.47–0.67 μm due to multiple scattering and enhanced absorption by organic carbon released during combustion. This general principle has been successfully used in the OMI detection of absorbing aerosols based on UV measurements. This paper provides the algorithm detail and illustrates its performance on two examples of wildfires in US Pacific North-West and in Georgia/Florida of 2007.

Description: Impact of Gobi desert dust on aerosol chemistry of Xi'an, inland China during spring 2009: differences in composition and size distribution between the urban ground surface and the mountain atmosphere Atmospheric Chemistry and Physics Discussions, 12, 21355-21397, 2012 Author(s): G. H. Wang, B. H. Zhou, C. L. Cheng, J. J. Cao, J. J. Meng, J. J. Li, J. Tao, R. J. Zhang, and P. Q. Fu Composition and size distribution of atmospheric aerosols from Xi'an city (~400 m, altitude) in inland China during the spring of 2009 including a massive dust event on 24 April were measured and compared with a parallel measurement at the summit (2060 m, altitude) of Mt. Hua, an alpine site nearby Xi'an. EC, OC and major ions in the city were 2–22 times higher than those on the mountaintop during the whole sampling period. Sulfate was the highest species in the nonevent time in Xi'an and Mt. Hua, followed by nitrate, OC and NH 4 + . In contrast, OC was the most abundant in the event at both sites, followed by sulfate, nitrate and Ca 2+ . Compared to those on the urban ground surface aerosols in the elevated troposphere over Mt. Hua contain more sulfate and less nitrate, because HNO 3 is formed faster than H 2 SO 4 and thus long-range transport of HNO 3 is less significant than that of H 2 SO 4 . An increased water-soluble organic nitrogen (WSON) was observed for the dust samples from Xi'an, indicating a significant deposition of anthropogenic WSON onto dust and/or an input of biogenic WSON from Gobi desert. As far as we know, it is for the first time to perform a simultaneous observation of aerosol chemistry between the ground surface and the free troposphere in inland East Asia. Our results showed that fine particles are more acidic on the mountaintop than on the urban ground surface in the nonevent, mainly due to continuous oxidation of SO 2 to produce H 2 SO 4 during the transport from lowland areas to the alpine atmosphere. However, we found the urban fine particles became more acidic in the event than in the nonevent, in contrast to the mountain atmosphere, where fine particles were less acidic when dust was present. The opposite changes in acidity of fine particles at both sites during the event are mostly caused by enhanced heterogeneous formation of nitrate onto dust in the urban air and decreased formation of nitrate in the mountain troposphere. In comparison to those during the nonevent Cl − and NO 3 − in the urban air during the event significantly shifted toward coarse particles. Such redistributions were further pronounced on the mountaintop when dust was present, resulting in both ions almost entirely staying in coarse particles. On the contrary, no significant spatial difference in size distribution of SO 4 2− was found between the urban ground surface and the mountain atmosphere, dominating in the fine mode (

Description: High resolution mapping of combustion processes and implications for CO 2 emissions Atmospheric Chemistry and Physics Discussions, 12, 21211-21239, 2012 Author(s): R. Wang, S. Tao, P. Ciais, H. Z. Shen, Y. Huang, H. Chen, G. F. Shen, B. Wang, W. Li, Y. Y. Zhang, Y. Lu, D. Zhu, Y. C. Chen, X. P. Liu, W. T. Wang, X. L. Wang, W. X. Liu, B. G. Li, and S. L. Piao High-resolution mapping of fuel combustion and CO 2 emission provides valuable information for inferring terrestrial carbon balance, modeling pollutant transport, and developing mitigation strategies. Previous inventories included only a limited number of fuel types and anthropogenic emissions were mapped using national population proxies which may distort the geographical distribution within countries. In this study, a sub-national disaggregation method (SDM) was applied to establish a global 0.1°×0.1° geo-referenced inventory of fuel combustion (PKU-FUEL) and a corresponding CO 2 emission inventory (PKU-CO 2 ) based upon 64 fuel sub-types for the year 2007. Uncertainties of the new inventories were evaluated using a Monte Carlo method. The total combustion CO 2 emission in 2007 was 11.2 (9.11 and 13.3 as 5th and 95th percentiles) Pg C yr −1 . By replacing national disaggregation with sub-national disaggregation in this study, the average 95th minus 5th percentile ranges of CO 2 emission for all grids can be reduced from 417 to 68.2 Mg km −2 yr −1 , indicating a significant reduction in uncertainty, because the uneven distribution of per-capita fuel consumptions within countries has been taken into account by using the sub-national fuel consumption data directly. Significant difference in per-capita CO 2 emissions between urban and rural areas was found in developing nations (2.09 vs. 0.600 Mg C cap −1 yr −1 ), but not in developed ones (3.57 vs. 3.42 Mg C cap −1 yr −1 ), suggesting strong influence of the rapid urbanization of these countries on the carbon emission. By using the CO 2 emission product, a new spatial pattern of terrestrial carbon sink was derived and the impact of sub-national disaggregation is discussed.

Description: Composition and evolution of volcanic aerosol from eruptions of Kasatochi, Sarychev and Eyjafjallajökull in 2008–2010 based on CARIBIC observations Atmospheric Chemistry and Physics Discussions, 12, 21481-21516, 2012 Author(s): S. M. Andersson, B. G. Martinsson, J. Friberg, C. A. M. Brenninkmeijer, A. Rauthe-Schöch, M. Hermann, P. F. J. van Velthoven, and A. Zahn Large volcanic eruptions impact significantly on climate and lead to ozone depletion due to injection of particles and gases into the stratosphere where their residence times are long. In this the composition of volcanic aerosol is an important but inadequately studied factor. Samples of volcanically influenced aerosol have been collected following the Kasatochi (Alaska), Sarychev (Russia) and also during the Eyjafjallajökull (Iceland) eruptions in the period 2008–2010. Sampling was conducted by the CARIBIC platform during regular flights at an altitude of 10–12 km as well as during dedicated flights through the volcanic clouds from the eruption of Eyjafjallajökull in spring 2010. Elemental concentrations of the collected aerosol were obtained by accelerator-based analysis. Aerosol from the Eyjafjallajökull volcanic clouds was identified by high concentrations of sulfur and elements pointing to crustal origin, and confirmed by trajectory analysis. Signatures of volcanic influence were also used to detect volcanic aerosol in stratospheric samples collected following the Sarychev and Kasatochi eruptions. In total it was possible to identify 17 relevant samples collected between 1 and more than 100 days following the eruptions studied. The volcanic aerosol mainly consisted of ash, sulfate and included a carbonaceous component. Samples collected in the volcanic cloud from Eyjafjallajökull were dominated by the ash and sulfate component (~45% each) while samples collected in the tropopause region and LMS mainly consisted of sulfate (50–77%) and carbon (21–43%). These fractions were increasing/decreasing with the age of the aerosol. Because of the long observation period, it was possible to analyze the evolution of the relationship between the ash and sulfate components of the volcanic aerosol. From this analysis the residence time (1/ e ) of sulfur dioxide in the studied volcanic cloud was estimated to be 45 days.

Description: Coupling field and laboratory measurements to estimate the emission factors of identified and unidentified trace gases for prescribed fires Atmospheric Chemistry and Physics Discussions, 12, 21517-21578, 2012 Author(s): R. J. Yokelson, I. R. Burling, J. B. Gilman, C. Warneke, C. E. Stockwell, J. de Gouw, S. K. Akagi, S. P. Urbanski, P. Veres, J. M. Roberts, W. C. Kuster, J. Reardon, D. W. T. Griffith, T. J. Johnson, S. Hosseini, J. W. Miller, D. R. Cocker III, H. Jung, and D. R. Weise An extensive program of experiments focused on biomass burning emissions began with a laboratory phase in which vegetative fuels commonly consumed in prescribed fires were collected in the southeastern and southwestern US and burned in a series of 71 fires at the US Forest Service Fire Sciences Laboratory in Missoula, Montana. The particulate matter (PM 2.5 ) emissions were measured by gravimetric filter sampling with subsequent analysis for elemental carbon (EC), organic carbon (OC), and 38 elements. The trace gas emissions were measured by an open-path Fourier transform infrared (OP-FTIR) spectrometer, proton-transfer-reaction mass spectrometry (PTR-MS), proton-transfer ion-trap mass spectrometry (PIT-MS), negative-ion proton-transfer chemical-ionization mass spectrometry (NI-PT-CIMS), and gas chromatography with MS detection (GC-MS). 204 trace gas species (mostly non-methane organic compounds – NMOC) were identified and quantified with the above instruments. Many of the 182 species quantified by the GC-MS have rarely, if ever, been measured in smoke before. An additional 153 significant peaks in the unit mass resolution mass spectra were quantified, but either could not be identified or most of the signal at that molecular mass was unaccounted for by identifiable species. In a second, "field" phase of this program, airborne and ground-based measurements were made of the emissions from prescribed fires that were mostly located in the same land management units where the fuels for the lab fires were collected. A broad variety, but smaller number of species (21 trace gas species and PM 2.5 ) was measured on 14 fires in chaparral and oak savanna in the southwestern US, as well as pine forest understory in the southeastern US and Sierra Nevada mountains of California. The field measurements of emission factors (EF) are useful both for modeling and to examine the representativeness of our lab fire EF. The lab EF/field EF ratio for the pine understory fuels was not statistically different from one, on average. However, our lab EF for "smoldering compounds" emitted from the semiarid shrubland fuels should likely be increased by a factor of ~2.7 to better represent field fires. Based on the lab/field comparison, we present emission factors for 357 pyrogenic species (including unidentified species) for 4 broad fuel types: pine understory, semiarid shrublands, coniferous canopy, and organic soil. To our knowledge this is the most comprehensive measurement of biomass burning emissions to date and it should enable improved representation of smoke composition in atmospheric models. The results support a recent estimate of global NMOC emissions from biomass burning that is much higher than widely used estimates and they provide important insights into the nature of smoke. 31–72% of the mass of gas-phase NMOC species was attributed to species that we could not identify. These unidentified species are not represented in most models, but some provision should be made for the fact that they will react in the atmosphere. In addition, the total mass of gas-phase NMOC divided by the mass of co-emitted PM 2.5 averaged about three (range ~2.0–8.7). About 35–64% of the NMOC were likely semivolatile or of intermediate volatility. Thus, the gas-phase NMOC represent a large reservoir of potential precursors for secondary formation of ozone and organic aerosol. For the single lab fire in organic soil about 28% of the emitted carbon was present as gas-phase NMOC and ~72% of the mass of these NMOC was unidentified, highlighting the need to learn more about the emissions from smoldering organic soils. The mass ratio of total NMOC to "NO x as NO" ranged from 11 to 267, indicating that NO x -limited O 3 production would be common in evolving biomass burning plumes. The fuel consumption per unit area was 7.0 ± 2.3 Mg ha −1 and 7.7 ± 3.7 Mg ha −1 for pine-understory and semiarid shrubland prescribed fires, respectively.

Description: A data assimilative perspective of oceanic mesoscale eddy evolution during VOCALS-REx Atmospheric Chemistry and Physics Discussions, 12, 20901-20930, 2012 Author(s): A. C. Subramanian, A. J. Miller, B. D. Cornuelle, E. Di Lorenzo, R. A. Weller, and F. Straneo Oceanic observations collected during the VOCALS-REx cruise time period, 1–30 November 2008, are assimilated into a regional ocean model (ROMS) using 4DVAR and then analyzed for their dynamics. Nonlinearities in the system prevent a complete 30-day fit, so two 15-day fits for 1–15 November and 16–30 November are executed using the available observations of hydrographic temperature and salinity, along with satellite fields of SST and sea-level height anomaly. The fits converge and reduce the cost function significantly, and the results indicated that ROMS is able to successfully reproduce both large-scale and smaller-scale features of the flows observed during the 76° W, 19° S. The ROMS fits capture this eddy as an isolated rotating 3-D vortex with a strong subsurface signature in velocity, temperature and anomalously low salinity. The eddy has an average temperature anomaly of approximately −0.5 °C over a depth range from 50–600 m and features a cold anomaly of approximately −1 °C near 150 m depth. The eddy moves northwestward and elongates during the second 15-day fit. It exhibits a strong signature in the Okubo-Weiss parameter, which indicates significant nonlinearity in its evolution. The heat balance for the period of the cruise from the ocean state estimate reveals that the horizontal advection and the vertical mixing processes are the dominant terms that balance the temperature tendency of the upper layer of the ocean locally in time and space. Areal averages, however, around the eddies and around the cruise tracks, suggest that vertical mixing processes generally balance the surface heating, indicating only a small role for lateral advective processes in this region.

Description: Future methane, hydroxyl, and their uncertainties: key climate and emission parameters for future predictions Atmospheric Chemistry and Physics Discussions, 12, 20931-20974, 2012 Author(s): C. D. Holmes, M. J. Prather, O. A. Søvde, and G. Myhre Accurate prediction of future methane abundances following a climate scenario requires understanding the lifetime changes driven by anthropogenic emissions, meteorological factors, and chemistry-climate feedbacks. Uncertainty in any of these influences or the underlying processes implies uncertainty in future abundance and radiative forcing. We simulate methane lifetime in multiple models over the period 1997–2009, adding sensitivity tests to determine key variables that drive the year-to-year variability. Across three atmospheric chemistry and transport models – UCI CTM, GEOS-Chem, and Oslo CTM3 – we find that temperature, water vapor, ozone column, biomass burning and lightning NO x are the dominant sources of interannual changes in methane lifetime. We also evaluate the model responses to forcings that have impacts on decadal time scales, such as methane feedback, and anthropogenic NO x emissions. In general, these different CTMs show similar sensitivities to the driving variables. We construct a parametric model that reproduces most of the interannual variability of each CTM and use it to predict methane lifetime from 1980 through 2100 following a specified emissions and climate scenario (RCP 8.5). The parametric model propagates uncertainties through all steps and provides a foundation for predicting methane abundances in any climate scenario. Our sensitivity tests also enable a new estimate of the methane global warming potential (GWP), accounting for stratospheric ozone effects, including those mediated by water vapor. We estimate the 100-yr GWP to be 32.

Description: Future impact of traffic emissions on atmospheric ozone and OH based on two scenarios Atmospheric Chemistry and Physics Discussions, 12, 20975-21012, 2012 Author(s): Ø. Hodnebrog, T. K. Berntsen, O. Dessens, M. Gauss, V. Grewe, I. S. A. Isaksen, B. Koffi, G. Myhre, D. Olivié, M. J. Prather, F. Stordal, S. Szopa, Q. Tang, P. van Velthoven, and J. E. Williams The future impact of traffic emissions on atmospheric ozone and OH has been investigated separately for the three sectors AIRcraft, maritime SHIPping and ROAD traffic. To reduce uncertainties we present results from an ensemble of six different atmospheric chemistry models, each simulating the atmospheric chemical composition in a possible high emission scenario (A1B), and with emissions from each transport sector reduced by 5% to estimate sensitivities. Our results are compared with optimistic future emission scenarios (B1 and B1 ACARE), presented in a companion paper, and with the recent past (year 2000). Present-day activity indicates that anthropogenic emissions so far evolve closer to A1B than the B1 scenario. As a response to expected changes in emissions, AIR and SHIP will have increased impacts on atmospheric O 3 and OH in the future while the impact of ROAD traffic will decrease substantially as a result of technological improvements. In 2050, maximum aircraft-induced O 3 occurs near 80° N in the UTLS region and could reach 9 ppbv in the zonal mean during summer. Emissions from ship traffic have their largest O 3 impact in the maritime boundary layer with a maximum of 6 ppbv over the North Atlantic Ocean during summer in 2050. The O 3 impact of road traffic emissions in the lower troposphere peaks at 3 ppbv over the Arabian Peninsula, much lower than the impact in 2000. Radiative Forcing (RF) calculations show that the net effect of AIR, SHIP and ROAD combined will change from a~marginal cooling of −0.38 ± 13 mW m −2 in 2000 to a relatively strong cooling of −32 ± 8.9 (B1) or −31 ± 20 mW m −2 (A1B) in 2050, when taking into account RF due to changes in O 3 , CH 4 and CH 4 -induced O 3 . This is caused both by the enhanced negative net RF from SHIP, which will change from −20 ± 5.4 mW m −2 in 2000 to −31 ± 4.8 (B1) or −40 ± 11 mW m −2 (A1B) in 2050, and from reduced O 3 warming from ROAD, which is likely to turn from a positive net RF of 13 ± 7.9 mW m −2 in 2000 to a slightly negative net RF of −2.9 ± 1.7 (B1) or −3.3 ± 3.8 (A1B) mW m −2 in the middle of this century. The negative net RF from ROAD is temporary and induced by the strong decline in ROAD emissions prior to 2050, which only affects the methane cooling term due to the longer lifetime of CH 4 compared to O 3 . The O 3 RF from AIR in 2050 is strongly dependent on scenario and ranges from 19 ± 6.8 (B1 ACARE) to 62 ± 13.6 mW m −2 (A1B). There is also a considerable span in the net RF from AIR in 2050, ranging from −0.54 ± 4.6 (B1 ACARE) to 12 ± 11 (A1B) mW m −2 compared to 6.5 ± 2.1 mW m −2 in 2000.

Description: Aerosol decadal trends – Part 2: In-situ aerosol particle number concentrations at GAW and ACTRIS stations Atmospheric Chemistry and Physics Discussions, 12, 20849-20899, 2012 Author(s): A. Asmi, M. Collaud Coen, J. A. Ogren, E. Andrews, P. Sheridan, A. Jefferson, E. Weingartner, U. Baltensperger, N. Bukowiecki, H. Lihavainen, N. Kivekäs, E. Asmi, P. P. Aalto, M. Kulmala, A. Wiedensohler, W. Birmili, A. Hamed, C. O'Dowd, S. G. Jennings, R. Weller, H. Flentje, A. Mari Fjaeraa, M. Fiebig, C. Lund Myhre, A. G. Hallar, and P. Laj We have analysed the trends of total aerosol particle number concentrations ( N ) measured at long-term measurement stations involved either in the Global Atmosphere Watch (GAW) and/or EU infrastructure project ACTRIS. The sites are located in Europe, North America, Antarctica, and on Pacific Ocean islands. The majority of the sites showed clear decreasing trends both in the full-length time-series, and in the intra-site comparison period of 2001–2010, especially during the winter months. Several potential driving processes for the observed trends were studied, and even though there are some similarities between N trends and air temperature changes, the most likely cause of many Northern Hemisphere trends was found to be decreases in the anthropogenic emissions of primary particles, SO 2 or some co-emitted species. We could not find a consistent agreement between the trends of N and particle optical properties in the few stations with long timeseries of all of these properties. The trends of N and the proxies for cloud condensation nuclei (CCN) were generally consistent in the few European stations where the measurements were available. This work provides a useful comparison analysis for modelling studies of trends in aerosol number concentrations.

Description: A multi-instrument comparison of integrated water vapour measurements at a high latitude site Atmospheric Chemistry and Physics Discussions, 12, 21013-21063, 2012 Author(s): S. A. Buehler, S. Östman, C. Melsheimer, G. Holl, S. Eliasson, V. O. John, T. Blumenstock, F. Hase, G. Elgered, U. Raffalski, T. Nasuno, M. Satoh, M. Milz, and J. Mendrok We compare measurements of integrated water vapour (IWV) over a subarctic site (Kiruna, Northern Sweden) from five different sensors and retrieval methods: Radiosondes, Global Positioning System (GPS), ground-based Fourier-transform infrared (FTIR) spectrometer, ground-based microwave radiometer, and satellite-based microwave radiometer (AMSU-B). Additionally, we compare also to ERA-Interim model reanalysis data. GPS-based IWV data have the highest temporal coverage and resolution and are chosen as reference data set. All datasets agree reasonably well, but the ground-based microwave instrument only if the data are cloud-filtered. We also address two issues that are general for such intercomparison studies, the impact of different lower altitude limits for the IWV integration, and the impact of representativeness error. We develop methods for correcting for the former, and estimating the random error contribution of the latter. A literature survey reveals that reported systematic differences between different techniques are study-dependent and show no overall consistent pattern. Further improving the absolute accuracy of IWV measurements and providing climate-quality time series therefore remain challenging problems.

Description: Diurnal variation of stratospheric HOCl, ClO and HO 2 at the equator: comparison of 1-D model calculations with measurements of satellite instruments Atmospheric Chemistry and Physics Discussions, 12, 21065-21104, 2012 Author(s): M. Khosravi, P. Baron, J. Urban, L. Froidevaux, A. I. Jonsson, Y. Kasai, K. Kuribayashi, C. Mitsuda, D. P. Murtagh, H. Sagawa, M. L. Santee, T. O. Sato, M. Shiotani, M. Suzuki, T. von Clarmann, K. A. Walker, and S. Wang The diurnal variation of HOCl and the related species ClO, HO 2 and HCl measured by satellites has been compared with the results of a one-dimensional photochemical model. The study compares the data from various limb-viewing instruments with model simulations from the middle stratosphere to the lower mesosphere. Data from three sub-millimeter instruments and two infrared spectrometers are used, namely from the Sub-Millimeter Radiometer (SMR) on board Odin, the Microwave Limb Sounder (MLS) on board Aura, the Superconducting Submillimeter-wave Limb-Emission Sounder (SMILES) on the International Space Station, the Michelson Interferometer for Passive Atmospheric Sounding (MIPAS) on board ENVISAT, and the Atmospheric Chemistry Experiment Fourier Transform Spectrometer (ACE-FTS) on board SCISAT. Inter-comparison of the measurements from instruments on sun-synchronous satellites (SMR, MLS, MIPAS) and measurements from solar occultation instruments (ACE-FTS) is challenging since the measurements correspond to different solar zenith angles (or local times). However, using a model which covers all solar zenith angles and the new SMILES instrument which measures at all local times over a period of several months provides the possibility to indirectly compare the diurnally variable species. The satellite data were averaged for latitudes of 20° S to 20° N for the SMILES observation period from November 2009 to April 2010 and were compared at three altitudes: 35, 45 and 55 km. This study presents the first evaluation of HO 2 Odin/SMR data and also the first comparison of the new SMILES data and the latest version of MLS (version 3.3) with other satellite observations. The MISU-1D model has been run for conditions and locations of the observations. The diurnal cycle features for the species investigated here are generally well reproduced by the model. The satellite observations and the model generally agree well in terms of absolute mixing ratios as well as differences between the day and night values. This confirms that gas phase chemistry of these species based on latest recommendations of reaction rate constants is fairly well understood.

Description: Radiative forcing in the ACCMIP historical and future climate simulations Atmospheric Chemistry and Physics Discussions, 12, 21105-21210, 2012 Author(s): D. T. Shindell, J.-F. Lamarque, M. Schulz, M. Flanner, C. Jiao, M. Chin, P. Young, Y. H. Lee, L. Rotstayn, G. Milly, G. Faluvegi, Y. Balkanski, W. J. Collins, A. J. Conley, S. Dalsoren, R. Easter, S. Ghan, L. Horowitz, X. Liu, G. Myhre, T. Nagashima, V. Naik, S. Rumbold, R. Skeie, K. Sudo, S. Szopa, T. Takemura, A. Voulgarakis, and J.-H. Yoon A primary goal of the Atmospheric Chemistry and Climate Model Intercomparison Project (ACCMIP) was to characterize the short-lived drivers of preindustrial to 2100 climate change in the current generation of climate models. Here we evaluate historical and future radiative forcing in the 10 ACCMIP models that included aerosols, 8 of which also participated in the Coupled Model Intercomparison Project phase 5 (CMIP5). The models generally reproduce present-day climatological total aerosol optical depth (AOD) relatively well. They have quite different contributions from various aerosol components to this total, however, and most appear to underestimate AOD over East Asia. The models generally capture 1980–2000 AOD trends fairly well, though they underpredict AOD increases over the Yellow/Eastern Sea. They appear to strongly underestimate absorbing AOD, especially in East Asia, South and Southeast Asia, South America and Southern Hemisphere Africa. We examined both the conventional direct radiative forcing at the tropopause (RF) and the forcing including rapid adjustments (adjusted forcing; AF, including direct and indirect effects). The models' calculated all aerosol all-sky 1850 to 2000 global mean annual average RF ranges from −0.06 to −0.49 W m −2 , with a mean of −0.26 W m −2 and a median of −0.27 W m −2 . Adjusting for missing aerosol components in some models brings the range to −0.12 to −0.62 W m −2 , with a mean of −0.39 W m −2 . Screening the models based on their ability to capture spatial patterns and magnitudes of AOD and AOD trends yields a quality-controlled mean of −0.42 W m −2 and range of −0.33 to −0.50 W m −2 (accounting for missing components). The CMIP5 subset of ACCMIP models spans −0.06 to −0.49 W m −2 , suggesting some CMIP5 simulations likely have too little aerosol RF. A substantial, but not well quantified, contribution to historical aerosol RF may come from climate feedbacks (35 to −58 %). The mean aerosol AF during this period is −1.12 W m −2 (median value −1.16 W m −2 , range −0.72 to −1.44 W m −2 ), indicating that adjustments to aerosols, which include cloud, water vapor and temperature, lead to stronger forcing than the aerosol direct RF. Both negative aerosol RF and AF are greatest over and near Europe, South and East Asia and North America during 1850 to 2000. AF, however, is positive over both polar regions, the Sahara, and the Karakoram. Annual average AF is stronger than 0.5 W m −2 over parts of the Arctic and more than 1.5 W m −2 during boreal summer. Examination of the regional pattern of RF and AF shows that the multi-model spread relative to the mean of AF is typically the same or smaller than that for RF over areas with substantial forcing. Historical aerosol RF peaks in nearly all models around 1980, declining thereafter. Aerosol RF declines greatly in most models over the 21st century and is only weakly sensitive to the particular Representative Concentration Pathway (RCP). One model, however, shows approximate stabilization at current RF levels under RCP 8.5, while two others show increasingly negative RF due to the influence of nitrate aerosols (which are not included in most models). Aerosol AF, in contrast, continues to become more negative during 1980 to 2000 despite the turnaround in RF. Total anthropogenic composition forcing (RF due to well-mixed greenhouse gases (WMGHGs) and ozone plus aerosol AF) shows substantial masking of greenhouse forcing by aerosols towards the end of the 20{th} century and in the early 21st century at the global scale. Regionally, net forcing is negative over most industrialized and biomass burning regions through 1980, but remains strongly negative only over East and Southeast Asia by 2000 and only over a very small part of Southeast Asia by 2030 (under RCP8.5). Net forcing is strongly positive by 1980 over the Sahara, Arabian peninsula, the Arctic, Southern Hemisphere South America, Australia and most of the oceans. Both the magnitude of and area covered by positive forcing expand steadily thereafter. There is no clear relationship between aerosol AF and climate sensitivity in the CMIP5 subset of ACCMIP models. There is a clear link between the strength of aerosol+ozone forcing and the global mean historical climate response to anthropogenic non-WMGHG forcing (ANWF). The models show ~20–35% greater climate sensitivity to ANWF than to WMGHG forcing, at least in part due to geographic differences in climate sensitivity. These lead to ~50% more warming in the Northern Hemisphere in response to increasing WMGHGs. This interhemispheric asymmetry is enhanced for ANWF by an additional 10–30%. At smaller spatial scales, response to ANWF and WMGHGs show distinct differences.

Description: Aerosol decadal trends – Part 1: In-situ optical measurements at GAW and IMPROVE stations Atmospheric Chemistry and Physics Discussions, 12, 20785-20848, 2012 Author(s): M. Collaud Coen, E. Andrews, A. Asmi, U. Baltensperger, N. Bukowiecki, D. Day, M. Fiebig, A. M. Fjaeraa, H. Flentje, A. Hyvärinen, A. Jefferson, S. G. Jennings, G. Kouvarakis, H. Lihavainen, C. Lund Myhre, W. C. Malm, N. Mihapopoulos, J. V. Molenar, C. O'Dowd, J. A. Ogren, B. A. Schichtel, P. Sheridan, A. Virkkula, E. Weingartner, R. Weller, and P. Laj Currently many ground-based atmospheric stations include in-situ measurements of aerosol physical and optical properties, resulting in more than 20 long-term (〉10 yr) aerosol measurement sites in the Northern Hemisphere and Antarctica. Most of these sites are located at remote locations and monitor the aerosol particle number concentration, wavelength-dependent light scattering, backscattering, and absorption coefficients. The existence of these multi-year datasets enables the analysis of long-term trends of these aerosol parameters of the derived light scattering Ångström exponent and backscatter fraction. Since the aerosol variables are not normally distributed, three different methods (the seasonal Mann-Kendall test associated with the Sen's slope, the generalized least squares fit associated with an autoregressive bootstrap algorithm for confidence intervals, and the least-mean square fit applied to logarithms of the data) were applied to detect the long-term trends and their magnitudes for each month. To allow a comparison among measurement sites with varying length of data records, trends on the most recent 10 and 15 yr periods were calculated. No significant trends were found for the three continental European sites. Statistically significant trends were found for the two European marine sites but the signs of the trends varied with aerosol property and location. Statistically significant decreasing trends for both scattering and absorption coefficient were found for most North American stations, although positive trends were found for a few desert and high-altitude sites. No significant trends in scattering coefficient were found for the Arctic or Antarctic stations, whereas the Arctic station had a negative trend in absorption coefficient.

Description: Optical properties of Saharan dust aerosol and contribution from the coarse mode as measured during the Fennec 2011 aircraft campaign Atmospheric Chemistry and Physics Discussions, 12, 26783-26842, 2012 Author(s): C. L. Ryder, E. J. Highwood, P. D. Rosenberg, J. Trembath, J. K. Brooke, M. Bart, A. Dean, J. Crosier, J. Dorsey, H. Brindley, J. Banks, J. H. Marsham, J. B. McQuaid, H. Sodemann, and R. Washington New in-situ aircraft measurements of Saharan dust originating from Mali, Mauritania and Algeria taken during the Fennec 2011 aircraft campaign over a remote part of the Sahara Desert are presented. Size distributions extending to 300 μm are shown, representing measurements extending further into the coarse mode than previously published for airborne Saharan dust. A significant coarse mode was present in the size distribution measurements with effective diameter ( d eff ) from 2.3 to 19.4 μm and coarse mode volume median diameter ( d vc ) from 5.8 to 45.3 μm. The mean size distribution had a larger relative proportion of coarse mode particles than previous aircraft measurements. The largest particles (with d eff 〉 12 μm, or d vc 〉 25 μm) were only encountered within 1 km of the ground. Number concentration, mass loading and extinction coefficient showed inverse relationships to dust age since uplift. Dust particle size showed a weak exponential relationship to dust age. Two cases of freshly uplifted dust showed quite different characteristics of size distribution and number concentration. SSA values at 550 nm calculated from the measured size distributions revealed high absorption ranging from 0.70 to 0.97 depending on the refractive index. SSA was found to be strongly related to d eff . New instrumentation revealed that direct measurements, behind Rosemount inlets, overestimate SSA by up to 0.11 when d eff is greater than 2 μm. This is caused by aircraft inlet inefficiencies and sampling losses. Previous measurements of SSA from aircraft measurements may also have been overestimates for this reason. Radiative transfer calculations indicate that the range of SSAs during Fennec 2011 can lead to underestimates in shortwave atmospheric heating rates by 2.0 to 2.9 times if the coarse mode is neglected. This will have an impact on Saharan atmospheric dynamics and circulation, which should be taken into account by numerical weather prediction and climate models.

Description: Application of the Statistical Oxidation Model (SOM) to secondary organic aerosol formation from photooxidation of C 12 Alkanes Atmospheric Chemistry and Physics Discussions, 12, 27077-27109, 2012 Author(s): C. D. Cappa, X. Zhang, C. L. Loza, J. S. Craven, L. D. Yee, and J. H. Seinfeld Laboratory chamber experiments are the main source of data on the mechanism of oxidation and the secondary organic aerosol (SOA) forming potential of volatile organic compounds. Traditional methods of representing the SOA formation potential of an organic do not fully capture the dynamic, multi-generational nature of the SOA formation process. We apply the Statistical Oxidation Model (SOM) of Cappa and Wilson (2012) to model the formation of SOA from the formation of the four C 12 alkanes, dodecane, 2-methyl undecane, cyclododecane and hexylcyclohexane, under both high- and low-NO x conditions, based upon data from the Caltech chambers. In the SOM, the evolution of reaction products is defined by the number of carbon ( N C ) and oxygen ( N O ) atoms, and the model parameters are (1) the number of oxygen atoms added per reaction, (2) the decrease in volatility upon addition of an oxygen atom and (3) the probability that a given reaction leads to fragmentation of the molecules. Optimal fitting of the model to chamber data is carried out using the measured SOA mass concentration and the aerosol O : C atomic ratio. The use of the kinetic, multi-generational SOM is shown to provide insights into the SOA formation process and to offer promise for application to the extensive library of existing SOA chamber experiments that is available.

Description: Radical loss in the atmosphere from Cu-Fe redox coupling in aerosols Atmospheric Chemistry and Physics Discussions, 12, 27053-27076, 2012 Author(s): J. Mao, S. Fan, D. J. Jacob, and K. R. Travis The hydroperoxyl radical (HO 2 ) is a major precursor of OH and tropospheric ozone. OH is the main atmospheric oxidant, while tropospheric ozone is an important surface pollutant and greenhouse gas. Standard gas-phase models for atmospheric chemistry tend to overestimate observed HO 2 concentrations, and this has been tentatively attributed to heterogeneous uptake by aerosol particles. It is generally assumed that HO 2 uptake by aerosol involve conversion to H 2 O 2 , but this is of limited efficacy as an HO 2 sink because H 2 O 2 can photolyze to regenerate OH and from there HO 2 . Joint atmospheric observations of HO 2 and H 2 O 2 suggest that HO 2 uptake by aerosols may in fact not produce H 2 O 2 . Here we propose a catalytic mechanism involving coupling of the transition metal ions (TMI) Cu(I)/Cu(II) and Fe(II)/Fe(III) to rapidly convert HO 2 to H 2 O in aerosols. The implied HO 2 uptake significantly affects global model predictions of tropospheric OH, ozone, and other species, improving comparisons to observations, and may have a major and previously unrecognized impact on atmospheric oxidant chemistry.

Description: Technical Note: New methodology for measuring viscosities in small volumes characteristic of environmental chamber particle samples Atmospheric Chemistry and Physics Discussions, 12, 27021-27051, 2012 Author(s): L. Renbaum-Wolff, J. W. Grayson, and A. K. Bertram Herein, a method for the determination of viscosities of small sample volumes is introduced, with important implications for the viscosity determination of particle samples from environmental chambers (used to simulate atmospheric conditions). The amount of sample needed is 〈 1 μl, and the technique is capable of determining viscosities (η) ranging between 10 −3 and 10 3 Pascal seconds (Pa s) in samples that cover a range of chemical properties and with real-time relative humidity and temperature control; hence, the technique should be well-suited for determining the viscosities, under atmospherically relevant conditions, of particles collected from environmental chambers. In this technique, supermicron particles are first deposited on an inert hydrophobic substrate. Then, insoluble beads (~1 μm in diameter) are embedded in the particles. Next, a flow of gas is introduced over the particles, which generates a shear stress on the particle surfaces. The sample responds to this shear stress by generating internal circulations, which are quantified with an optical microscope by monitoring the movement of the beads. The rate of internal circulation is shown to be a function of particle viscosity but independent of the particle material for a wide range of organic and organic-water samples. A calibration curve is constructed from the experimental data that relates the rate of internal circulation to particle viscosity, and this calibration curve is successfully used to predict viscosities in multicomponent organic mixtures.

Description: Dynamics of the chemical composition of rainwater throughout Hurricane Irene Atmospheric Chemistry and Physics Discussions, 12, 26995-27020, 2012 Author(s): K. M. Mullaugh, J. D. Willey, R. J. Kieber, R. N. Mead, and G. B. Avery Jr. Sequential sampling of rainwater from Hurricane Irene was carried out in Wilmington, NC, USA on 26 and 27 August 2011. Eleven samples were analyzed for pH, major ions (Cl − , NO 3 − , SO 4 2− , Na + , K + , Mg 2+ , Ca 2+ , NH 4 + ), dissolved organic carbon (DOC) and hydrogen peroxide (H 2 O 2 ). Hurricane Irene contributed 16% of the total rainwater and 18% of the total chloride wet deposition received in Wilmington NC during all of 2011. This work highlights the main physical factors influencing the chemical composition of tropical storm rainwater: wind speed, wind direction, air mass back trajectory and vertical mixing, time of day and total rain volume. Samples collected early in the storm, when winds blew out of the east, contained dissolved components indicative of marine sources (salts from sea spray and low DOC). The seasalt components in the samples had two maxima in concentration during the storm the first of which occurred before the volume of rain had sufficiently washed out seasalt from the atmosphere and the second when the air mass dipped to low elevations over the Atlantic Ocean followed by rapid vertical mixing. As the storm progressed and winds shifted to a westerly direction, the chemical composition of the rainwater became characteristic of terrestrial storms (high DOC and NH 4 + and low seasalt). This work demonstrates that tropical storms are not only responsible for significant wet deposition of marine components to land, but terrestrial components can also become entrained in rainwater, which can then be delivered to coastal waters via wet deposition. This study also underscores why analysis of one composite sample can lead to an incomplete interpretation of the factors that influence the chemically divergent analytes in rainwater during extreme weather events.

Description: Quantification of structural uncertainty in climate data records from GPS radio occultation Atmospheric Chemistry and Physics Discussions, 12, 26963-26994, 2012 Author(s): A. K. Steiner, D. Hunt, S.-P. Ho, G. Kirchengast, A. J. Mannucci, B. Scherllin-Pirscher, H. Gleisner, A. von Engeln, T. Schmidt, C. Ao, S. S. Leroy, E. R. Kursinski, U. Foelsche, M. Gorbunov, S. Heise, Y.-H. Kuo, K. B. Lauritsen, C. Marquardt, C. Rocken, W. Schreiner, S. Sokolovskiy, S. Syndergaard, and J. Wickert Global Positioning System (GPS) radio occultation (RO) provides continuous observations of the Earth's atmosphere since 2001 with global coverage, all-weather capability, and high accuracy and vertical resolution in the upper troposphere and lower stratosphere (UTLS). Precise time measurements enable long-term stability but careful processing is needed. Here we provide climate-oriented atmospheric scientists with multicenter-based results on the long-term stability of RO climatological fields for trend studies. We quantify the structural uncertainty of atmospheric trends estimated from the RO record, which arises from current processing schemes of six international RO processing centers, DMI Copenhagen, EUM Darmstadt, GFZ Potsdam, JPL Pasadena, UCAR Boulder, and WEGC Graz. Monthly-mean zonal-mean fields of bending angle, refractivity, dry pressure, dry geopotential height, and dry temperature from the CHAMP mission are compared for September 2001 to September 2008. We find that structural uncertainty is lowest in the tropics and mid-latitudes (50° S to 50° N) from 8 km to 25 km for all inspected RO variables. In this region, the structural uncertainty in trends over 7 yr is

Description: State transformations and ice nucleation in glassy or (semi-)solid amorphous organic aerosol Atmospheric Chemistry and Physics Discussions, 12, 27333-27366, 2012 Author(s): K. J. Baustian, M. E. Wise, E. J. Jensen, G. P. Schill, M. A. Freedman, and M. A. Tolbert Glassy or amorphous (semi-)solid organic aerosol particles have the potential to serve as surfaces for heterogeneous ice nucleation in cirrus clouds. Raman spectroscopy and optical microscopy have been used in conjunction with a cold stage to examine water uptake and ice nucleation on individual aqueous organic glass particles at atmospherically relevant temperatures (200–273 K). Three organic compounds considered proxies for atmospheric secondary organic aerosol (SOA) were used in this investigation: sucrose, citric acid and glucose. Internally mixed particles consisting of each organic species and ammonium sulfate were also investigated. Results from water uptake experiments were used to construct glass transition curves and state diagrams for each organic and corresponding mixture. A unique glass transition point on each state diagram, T g ', was used to quantify and compare results from this study to previous works. Values of T g ' determined for aqueous sucrose, glucose and citric acid glasses were 236 K, 230 K and 220 K, respectively. Values of T g ' for internally mixed organic/sulfate particles were always significantly lower; 210 K, 207 K and 215 K for sucrose/sulfate, glucose/sulfate and citric acid/sulfate, respectively. All investigated organic species were observed to serve as heterogeneous ice nuclei at tropospheric temperatures. Heterogeneous ice nucleation on pure organic particles occurred at S ice =1.1–1.4 for temperatures between 235 K and 200 K. Particles consisting of 1:1 organic-sulfate mixtures remained liquid over a greater range of conditions but were in some cases also observed to depositionally nucleate ice at temperatures below 202 K ( S ice =1.25–1.38). Glass transition curves constructed from experimental data were incorporated into the Community Aerosol Radiation Model for Atmospheres (CARMA) along with the predicted range of glass transition temperatures for atmospheric SOA from Koop et al. (2011). Model results suggest that organic and organic/sulfate aerosol will be glassy more than 60% of the time in the midlatitude upper troposphere and more than 40% of the time in the tropical tropopause region (TTL). At conditions favorable for ice formation ( S ice 〉1), particles in the TTL are expected to be glassy more than 50% of the time for temperatures below 200 K. Combined with the low saturation ratios measured for ice nucleation, this work suggests heterogeneous ice formation on glassy substances may play an important role in cirrus cloud formation.

Description: Receptor modelling of secondary particulate matter at UK sites Atmospheric Chemistry and Physics Discussions, 12, 27255-27295, 2012 Author(s): A. Charron, C. Degrendele, B. Laongsri, and R. M. Harrison Complementary approaches have been taken to better understand the sources and their spatial distribution for secondary inorganic (nitrate and sulphate) and secondary organic aerosol sampled at a rural site (Harwell) in the southern United Kingdom. A concentration field map method was applied to 1581 daily samples of chloride, nitrate and sulphate from 2006 to 2010, and 982 samples for organic carbon and elemental carbon from 2007 to 2010. This revealed a rather similar pattern of sources for nitrate, sulphate and secondary organic carbon within western/central Europe, which in the case of nitrate and sulphate, correlated significantly with EMEP emissions maps of NO x and SO 2 . A slightly more southerly source emphasis for secondary organic carbon may reflect the contribution of biogenic sources. Trajectory clusters confirm this pattern of behaviour with a major contribution from mainland European sources. Similar behaviours of, on the one hand, sulphate and organic carbon and, on the other hand, EC and nitrate showed that the former are more subject to regional influence than the latter in agreement with the slower atmospheric formation of sulphate and secondary organic aerosol than for nitrate, and the local/mesoscale influences upon primary EC. In a separate study, measurements of sulphate, nitrate, elemental and organic carbon were made in 100 simultaneously collected samples at Harwell and at a suburban site in Birmingham (UK). This showed a significant correlation in concentrations between the two sites for all of the secondary constituents, further indicating secondary organic aerosol to be a regional pollutant behaving similarly to sulphate and nitrate.

Description: Sensitivity of cloud condensation nuclei to regional changes in dimethyl-sulphide emissions Atmospheric Chemistry and Physics Discussions, 12, 27395-27423, 2012 Author(s): M. T. Woodhouse, G. W. Mann, K. S. Carslaw, and O. Boucher The atmospheric oxidation of dimethyl-sulphide (DMS) derived from marine phytoplankton is a significant source of marine sulphate aerosol. DMS has been proposed to regulate climate via changes in cloud properties, though recent studies have shown that present-day global cloud condensation nuclei (CCN) concentrations have only a weak dependence on the total emission flux of DMS. Here, we use a global aerosol microphysics model to examine how efficiently CCN are produced when DMS emissions are changed in different regions. We find that global CCN production per unit mass of sulphur emitted varies by more than a factor of 20 depending on which oceanic region the change in DMS emission flux is applied. The variation in CCN production efficiency depends upon where CCN production processes (DMS oxidation, SO 2 oxidation, nucleation and growth) are most efficient and removal processes (deposition) least efficient. The analysis shows that the production of aerosol sulphate through aqueous-phase oxidation of SO 2 limits the amount of H 2 SO 4 available for nucleation and condensational growth and therefore suppresses CCN formation, leading to the weak response of CCN to changes in DMS emission. Our results show that past and future changes in the spatial distribution of DMS emissions (through changes in phytoplankton or wind speed patterns) could exert a stronger control on climate than net increases in biological productivity.

Description: The spatial scale of ozone depletion events derived from an autonomous surface ozone network in coastal Antarctica Atmospheric Chemistry and Physics Discussions, 12, 27555-27588, 2012 Author(s): A. E. Jones, E. W. Wolff, N. Brough, S. J.-B. Bauguitte, R. Weller, M. Yela, M. Navarro-Comas, H. A. Ochoa, and N. Theys To probe the spatial extent of tropospheric ozone depletion events during Antarctic spring, a network of 10 autonomous ozone monitors was established around the Dronning Maud Land sector of Antarctica for a full calendar year. Together with manned stations in the area, the network covered a ~1200 km stretch of coast, as well as a transect ~300 km inland and to ~2000 m above sea level (a.s.l.). Here we present results from the spring period (August to October 2008). While some ozone depletion events were evident at only a single site, implying localised ozone destruction, others were evident across the network. The fact that, on occasions, ozone depletion events were observed at all coastal sites simultaneously, suggests the depleted air mass had a scale of at least 1200 km. As the ozone-poor air was advected from the Weddell Sea sea ice zone, the data imply that large areas over the Weddell Sea sea ice zone are significantly depleted in ozone on occasions during Antarctic spring.

Description: Iodine monoxide in the Western Pacific marine boundary layer Atmospheric Chemistry and Physics Discussions, 12, 27475-27519, 2012 Author(s): K. Großmann, U. Frieß, E. Peters, F. Wittrock, J. Lampel, S. Yilmaz, J. Tschritter, R. Sommariva, R. von Glasow, B. Quack, K. Krüger, K. Pfeilsticker, and U. Platt A latitudinal cross-section and vertical profiles of iodine monoxide (IO) are reported from the marine boundary layer of the Western Pacific. The measurements were taken using Multi-Axis Differential Optical Absorption Spectroscopy (MAX-DOAS) during the TransBrom cruise of the German research vessel Sonne , which led from Tomakomai, Japan (42° N, 141° E) through the Western Pacific to Townsville, Australia (19° S, 146° E) in October 2009. In the marine boundary layer within the tropics (between 20° N and 5° S), IO mixing ratios ranged between 1 and 2.2 ppt, whereas in the subtropics and at mid-latitudes typical IO mixing ratios were around 1 ppt in the daytime. The profile retrieval reveals that the bulk of the IO was located in the lower part of the marine boundary layer. Photochemical simulations indicate that the organic iodine precursors observed during the cruise (CH 3 I, CH 2 I 2 , CH 2 ClI, CH 2 BrI) are not sufficient to explain the measured IO mixing ratios. Reasonable agreement between measured and modelled IO can only be achieved, if an additional sea-air flux of inorganic iodine (e.g. I 2 ) is assumed in the model. Our observations add further evidence to previous studies that reactive iodine is an important oxidant in the marine boundary layer.

Description: Particle and gaseous emissions from individual diesel and CNG buses Atmospheric Chemistry and Physics Discussions, 12, 27737-27773, 2012 Author(s): Å. M. Hallquist, M. Jerksjö, H. Fallgren, J. Westerlund, and Å. Sjödin In this study size-resolved particle and gaseous emissions from 28 individual diesel-fuelled and 7 compressed natural gas (CNG)-fuelled buses, selected from an in-use bus fleet, were characterised for real-world dilution scenarios. The method used was based on using CO 2 as a tracer of exhaust gas dilution. The particles were sampled by using an extractive sampling method and analysed with high time resolution instrumentation EEPS (10 Hz) and CO 2 with non-dispersive infrared gas analyser (LI-840, LI-COR Inc. 1 Hz). The gaseous constituents (CO, HC and NO) were measured by using a remote sensing device (AccuScan RSD 3000, Environmental System Products Inc.). Nitrogen oxides, NO x , were estimated from NO by using default NO 2 /NO x ratios from the road vehicle emission model HBEFA 3.1. The buses studied were diesel-fuelled Euro II–V and CNG-fuelled Enhanced Environmental Friendly Vehicles (EEVs) with different after-treatment, including selective catalytic reduction (SCR), exhaust gas recirculation (EGR) and with and without diesel particulate filter (DPF). The primary driving mode applied in this study was accelerating mode. However, regarding the particle emissions also a constant speed mode was analysed. The investigated CNG buses emitted on average higher number of particles but less mass compared to the diesel-fuelled buses. Emission factors for number of particles (EF PN ) were EF PN, DPF = 8.0 ± 3.1 × 10 14 , EF PN, no DPF =2.8 ± 1.6 × 10 15 and EF PN, CNG = 7.8 ± 5.7 × 10 15 (kg fuel −1 ). In the accelerating mode size-resolved EFs showed unimodal number size distributions with peak diameters of 70–90 nm and 10 nm for diesel and CNG buses, respectively. For the constant speed mode bimodal average number size distributions were obtained for the diesel buses with peak modes of ~10 nm and ~60 nm. Emission factors for NO x expressed as NO 2 equivalents for the diesel buses were on average 27 ± 7 g (kg fuel) −1 and for the CNG buses 41 ± 26 g (kg fuel) −1 . An anti-relationship between EF NO x and EF PM was observed especially for buses with no DPF and there was a positive relationship between EF PM and EF CO .