ALBERT

Description: Sensing Hadley cell with space lidar Atmospheric Chemistry and Physics Discussions, 11, 16599-16610, 2011 Author(s): W. Sun and B. Lin This letter shows that the extent of the Hadley cell could reliably be estimated by measuring the height of the uppermost super-thin clouds in the troposphere with space-borne lidar. Through consecutive multi-year measurements of the height of the uppermost super-thin clouds, a good estimation of the expansion of the Hadley cell could be obtained.

Description: CARIBIC aircraft measurements of Eyjafjallajökull volcanic plumes in April/May 2010 Atmospheric Chemistry and Physics Discussions, 11, 16693-16744, 2011 Author(s): A. Rauthe-Schöch, A. Weigelt, M. Hermann, B. G. Martinsson, A. K. Baker, K.-P. Heue, C. A. M. Brenninkmeijer, A. Zahn, D. Scharffe, S. Eckhardt, A. Stohl, and P. F. J. van Velthoven The Civil Aircraft for the Regular Investigation of the Atmosphere Based on an Instrument Container (CARIBIC) project investigates physical and chemical processes in the Earth's atmosphere using a Lufthansa Airbus long-distance passenger aircraft. After the beginning of the explosive eruption of the Eyjafjallajökull volcano on Iceland on 14 April 2010, the first CARIBIC volcano-specific measurement flight was carried out over the Baltic Sea and Southern Sweden on 20 April. Two more flights followed: one over Ireland and the Irish Sea on 16 May and the other over the Norwegian Sea on 19 May 2010. During these three special mission flights the CARIBIC container proved its merits as a versatile and comprehensive flying laboratory. The elemental composition of particles collected over the Baltic Sea during the first flight (20 April) indicated the presence of volcanic ash. Over Northern Ireland and the Irish Sea (16 May), the DOAS system detected SO 2 and BrO co-located with volcanic ash particles that increased the aerosol optical depth. Over the Norwegian Sea (19 May), the optical particle counter detected a strong increase of particles larger than 400 nm diameter in a region where ash clouds were predicted by aerosol dispersion models. Aerosol particle samples collected over the Irish Sea and the Norwegian Sea showed large relative enhancements of the elements silicon, iron, titanium and calcium. Non-methane hydrocarbon concentrations in whole air samples collected on 16 May and 19 May 2010 showed a pattern of removal of several hydrocarbons that is typical for chlorine chemistry in the plumes. Comparisons of measured ash concentrations and simulations with the FLEXPART dispersion model demonstrate the difficulty of detailed volcanic ash dispersion modelling due to the large variability of the volcanic plume sources, extent and patchiness as well as the thin ash layers formed in the volcanic plumes.

Description: Explicit modeling of organic chemistry and secondary organic aerosol partitioning for Mexico City and its outflow plume Atmospheric Chemistry and Physics Discussions, 11, 17013-17070, 2011 Author(s): J. Lee-Taylor, S. Madronich, B. Aumont, M. Camredon, A. Hodzic, G. S. Tyndall, E. Apel, and R. A. Zaveri The evolution of organic aerosols (OA) in Mexico City and its outflow is investigated with the nearly explicit gas phase photochemistry model GECKO-A (Generator of Explicit Chemistry and Kinetics of Organics in the Atmosphere), wherein precursor hydrocarbons are oxidized to numerous intermediate species for which vapor pressures are computed and used to determine gas/particle partitioning in a chemical box model. Precursor emissions included observed C3–10 alkanes, alkenes, and light aromatics, as well as larger n -alkanes (up to C25) not directly observed but estimated by scaling to particulate emissions according to their volatility. Conditions were selected for comparison with observations made in March 2006 (MILAGRO). The model successfully reproduces the magnitude and diurnal shape for both primary (POA) and secondary (SOA) organic aerosols, with POA peaking in the early morning at 15–20 μg m −3 , and SOA peaking at 10–15 μg m −3 during mid-day. The majority (≥75 %) of the model SOA stems from the large n -alkanes, with the remainder mostly from the light aromatics. Simulated OA elemental composition reproduces observed H/C and O/C ratios reasonably well, although modeled ratios develop more slowly than observations suggest. SOA chemical composition is initially dominated by δ-hydroxy ketones and nitrates from the large alkanes, with contributions from peroxy acyl nitrates and, at later times when NO x is lower, organic hydroperoxides. The simulated plume-integrated OA mass continues to increase for several days downwind despite dilution-induced particle evaporation, since oxidation chemistry leading to SOA formation remains strong. In this model, the plume SOA burden several days downwind exceeds that leaving the city by a factor of 〉3. These results suggest significant regional radiative impacts of SOA.

Description: Nudging technique for scale bridging in air quality/climate atmospheric composition modelling Atmospheric Chemistry and Physics Discussions, 11, 17177-17199, 2011 Author(s): A. Maurizi, F. Russo, M. D'Isidoro, and F. Tampieri The interaction between air quality and climate involves dynamical scales that cover an immensely wide range. Bridging these scales in numerical simulations is fundamental in studies devoted to megacity/hot-spot impacts on climate. The nudging technique is proposed as a bridging method that can couple different models at different scales. Here, nudging is used to force low resolution chemical composition models using a high resolution run on critical areas. A one-year numerical experiment focused on the Po Valley hot spot is performed using the BOLCHEM model to asses the method. The results show that the model response is stable to perturbation induced by the nudging and that, if a high resolution run is taken as a reference, there is an increase in model skills of low resolution run when the technique is applied. This improvement depends on the species and the season. The effect spreads outside the forcing area and remains noticeable over an extension about 9 times larger.

Description: Measurements of ozone and its precursors in Beijing during summertime: impact of urban plumes on ozone pollution in downwind rural areas Atmospheric Chemistry and Physics Discussions, 11, 17337-17373, 2011 Author(s): J. Xu, J. Z. Ma, X. L. Zhang, X. B. Xu, X. F. Xu, W. L. Lin, Y. Wang, W. Meng, and Z. Q. Ma Sea-land and mount-valley circulations are the dominant mesoscale synoptic systems affecting the Beijing area during summertime. Under the influence of these two circulations, the prevailing wind is southwesterly from afternoon to midnight, and then changes to northeasterly till forenoon. In this study, surface ozone (O 3 ), carbon monoxide (CO), nitric oxide (NO), nitrogen dioxide (NO 2 ), nitrogen oxide (NO x ) and non-methane hydrocarbons (NMHCs) were measured at four sites located along the route of prevailing wind, including two upwind urban sites (Fengtai (FT) and Baolian (BL)), an upwind suburban site (Shunyi (SY)) and a downwind rural site (Shangdianzi (SDZ)) during 20 June–16 September 2007. The purpose is to improve our understanding of ozone photochemistry in urban and rural areas of Beijing and the influence of urban plumes on ozone pollution in downwind rural areas. It is found that ozone pollution was synchronism in the urban and rural areas of Beijing, coinciding with the regional-scale synoptic processes. Due to the high traffic density and local emissions, the average levels of reactive gases NO x and NMHCs at the non-rural sites were much higher than those at SDZ. The level of long-lived gas CO at SDZ was comparable to and slightly lower than it was at other sites. The daily-averaged ozone concentration at SDZ was much higher than at other sites due to weak titration. Ranking by OH loss rate coefficient ( L OH ), alkenes played a dominant role in total NMHCs reactivity at both urban and rural sites during the experiment, accounting for 48.6 % and 52.1 % of total L OH , respectively. The NMHCs data were also used to estimate the ozone potential formation (OFP) in Beijing. The leading contributors to ozone formation were aromatics at both urban and rural sites during the experiment, which accounts for 55.5 % and 49.4 % of total OFP, respectively. The ozone peak values are found to lag behind one site after another along the route of prevailing wind from SW to NE. Intersection analyses of trace gases reveal that polluted air masses arriving at SDZ were more aged with both higher O 3 and O x concentrations than those at BL. The results indicate that urban plume can transport not only O 3 but its precursors, the latter leading more photochemical O 3 production when being mixed with background atmosphere in the downwind rural area.

Description: Ice nucleation properties of volcanic ash from Eyjafjallajökull Atmospheric Chemistry and Physics Discussions, 11, 17201-17243, 2011 Author(s): C. R. Hoyle, V. Pinti, A. Welti, B. Zobrist, C. Marcolli, B. Luo, Á. Höskuldsson, H. B. Mattsson, T. Thorsteinsson, G. Larsen, and T. Peter The ice nucleation ability of volcanic ash particles collected close to the Icelandic volcano Eyjafjallajökull during its eruptions in April and May 2010 is investigated experimentally, in the immersion and deposition modes, and applied to atmospheric conditions by comparison with airborne measurements and microphysical model calculations. The number of ash particles which are active as ice nuclei (IN) is strongly temperature dependent, with a very small minority being active in the immersion mode at temperatures of 250–263 K. Average ash particles show only a moderate effect on ice nucleation, by inducing freezing at temperatures between 236 K and 240 K (i.e. approximately 3–4 K higher than temperatures required for homogeneous ice nucleation, measured with the same instrument). By scaling the results to aircraft and lidar measurements of the conditions in the ash plume days down wind of the eruption and by applying a simple microphysical model, it was found that the IN active in the immersion mode in the range 250–263 K generally occurred in atmospheric number densities at the lower end of those required to have an impact on ice cloud formation. However, 3–4 K above the homogeneous freezing point, immersion mode IN number densities a few days down wind of the eruption were sufficiently high to have a moderate influence on ice cloud formation. The efficiency of IN in the deposition mode was found to be poor except at very cold conditions ( 〈 238 K), when they reach an efficiency similar to that of mineral dust with the onset of freezing at 10 % supersaturation with respect to ice, and with the frozen fraction nearing its maximum value at a supersaturation 20 %. In summary, these investigations suggest volcanic ash particles to have only moderate effects on atmospheric ice formation.

Description: Novel application of satellite and in-situ measurements to map surface-level NO 2 in the Great Lakes region Atmospheric Chemistry and Physics Discussions, 11, 17245-17287, 2011 Author(s): C. J. Lee, J. R. Brook, G. J. Evans, R. V. Martin, and C. Mihele Ozone Monitoring Instrument (OMI) tropospheric NO 2 vertical column density data were used in conjunction with in-situ NO 2 concentrations collected by permanently installed monitoring stations to infer 24 h surface-level NO 2 concentrations at 0.1° (~ 11 km) resolution. The region examined included rural and suburban areas, and the highly industrialised area of Windsor, Ontario, which is situated directly across the US-Canada border from Detroit, MI. Photolytic NO 2 monitors were collocated with standard NO 2 monitors to provide qualitative data regarding NO z interference during the campaign. To test the accuracy of the OMI-inferred concentrations, two-week integrative NO 2 measurements were collected using passive monitors at 18 locations, approximating a 15 km grid across the region, for 7 consecutive two-week periods. When compared with these passive results, satellite-inferred concentrations showed an 18 % positive bias. The correlation of the passive monitor and OMI-inferred concentrations ( R = 0.69, n = 115) was stronger than that for the passive monitor concentrations and OMI column densities ( R = 0.52), indicating that using a sparse network of monitoring sites to estimate concentrations improves the direct utility of the OMI observations. OMI-inferred concentrations were then calculated for four years to show an overall declining trend in surface NO 2 concentrations in the region. Additionally, by separating OMI-inferred surface concentrations by wind direction, clear patterns in emissions and affected down-wind regions, in particular around the US-Canada border, were revealed.

Description: Variable lifetimes and loss mechanisms for NO 3 and N 2 O 5 during the DOMINO campaign: contrasts between marine, urban and continental air Atmospheric Chemistry and Physics Discussions, 11, 17825-17877, 2011 Author(s): J. N. Crowley, J. Thieser, M. Tang, G. Schuster, H. Bozem, Z. Hosaynali Beygi, H. Fischer, J. Diesch, F. Drewnick, S. Borrmann, W. Song, N. Yassaa, J. Williams, D. Pöhler, U. Platt, and J. Lelieveld Nighttime mixing ratios of boundary layer N 2 O 5 were determined using cavity-ring-down spectroscopy during the DOMINO campaign. Observation of N 2 O 5 was intermittent, with mixing ratios ranging from below the detection limit (~5 ppt) to ~500 ppt. A steady-state analysis constrained by measured mixing ratios of NO 2 and O 3 was used to derive NO 3 lifetimes and compare them to calculated rates of loss via gas-phase and heterogeneous reactions of both NO 3 and N 2 O 5 . Three distinct types of air masses were encountered, which were largely marine (Atlantic), continental or urban-industrial in origin. NO 3 lifetimes were longest in the Atlantic sector (up to ~30 min) but were very short (a few seconds) in polluted, air masses from the local city and petroleum-related industrial complex of Huelva. Air from the continental sector was an intermediate case. The high reactivity to NO 3 of the urban air mass was not accounted for by gas-phase and heterogeneous reactions, rates of which were constrained by measurements of NO, volatile organic species and aerosol surface area. In general, high NO 2 mixing ratios resulted in low NO 3 lifetimes, though heterogeneous processes (e.g. reaction of N 2 O 5 on aerosol) were generally less important than direct gas-phase losses of NO 3 . The presence of SO 2 at levels above ~2 ppb in the urban air sector was always associated with very low N 2 O 5 mixing ratios indicating either very short NO 3 lifetimes in the presence of combustion-related emissions or an important role for reduced sulphur species in urban, nighttime chemistry. High production rates coupled with low lifetimes of NO 3 imply an important contribution of nighttime chemistry to removal of both NO x and VOC.

Description: General overview: European Integrated project on Aerosol Cloud Climate and Air Quality interactions (EUCAARI) – integrating aerosol research from nano to global scales Atmospheric Chemistry and Physics Discussions, 11, 17941-18160, 2011 Author(s): M. Kulmala, A. Asmi, H. K. Lappalainen, U. Baltensperger, J.-L. Brenguier, M. C. Facchini, H.-C. Hansson, Ø. Hov, C. D. O'Dowd, U. Pöschl, A. Wiedensohler, R. Boers, O. Boucher, G. de Leeuw, H. Denier van den Gon, J. Feichter, R. Krejci, P. Laj, H. Lihavainen, U. Lohmann, G. McFiggans, T. Mentel, C. Pilinis, I. Riipinen, M. Schulz, A. Stohl, E. Swietlicki, E. Vignati, M. Amann, M. Amann, C. Alves, S. Arabas, P. Artaxo, D. C. S. Beddows, R. Bergström, J. P. Beukes, M. Bilde, J. F. Burkhart, F. Canonaco, S. Clegg, H. Coe, S. Crumeyrolle, B. D'Anna, S. Decesari, S. Gilardoni, M. Fischer, A. M. Fjæraa, C. Fountoukis, C. George, L. Gomes, P. Halloran, T. Hamburger, R. M. Harrison, H. Herrmann, T. Hoffmann, C. Hoose, M. Hu, U. Hõrrak, Y. Iinuma, T. Iversen, M. Josipovic, M. Kanakidou, A. Kiendler-Scharr, A. Kirkevåg, G. Kiss, Z. Klimont, P. Kolmonen, M. Komppula, J.-E. Kristjánsson, L. Laakso, A. Laaksonen, L. Labonnote, V. A. Lanz, K. E. J. Lehtinen, R. Makkonen, G. McMeeking, J. Merikanto, A. Minikin, S. Mirme, W. T. Morgan, E. Nemitz, D. O'Donnell, T. S. Panwar, H. Pawlowska, A. Petzold, J. J. Pienaar, C. Pio, C. Plass-Duelmer, A. S. H. Prévôt, S. Pryor, C. L. Reddington, G. Roberts, D. Rosenfeld, J. Schwarz, Ø. Seland, K. Sellegri, X. J. Shen, M. Shiraiwa, H. Siebert, B. Sierau, D. Simpson, J. Y. Sun, D. Topping, P. Tunved, P. Vaattovaara, V. Vakkari, J. P. Veefkind, A. Visschedijk, H. Vuollekoski, R. Vuolo, B. Wehner, J. Wildt, S. Woodward, D. R. Worsnop, G.-J. van Zadelhoff, A. A. Zardini, K. Zhang, P. G. van Zyl, V.-M. Kerminen, K. S. Carslaw, and S. N. Pandis In this paper we describe and summarize the main achievements of the European Aerosol Cloud Climate and Air Quality Interactions project (EUCAARI). EUCAARI started on 1 January 2007 and ended on 31 December 2010 leaving a rich legacy including: (a) a comprehensive database with a year of observations of the physical, chemical and optical properties of aerosol particles over Europe, (b) the first comprehensive aerosol measurements in four developing countries, (c) a database of airborne measurements of aerosols and clouds over Europe during May 2008, (d) comprehensive modeling tools to study aerosol processes fron nano to global scale and their effects on climate and air quality. In addition a new Pan-European aerosol emissions inventory was developed and evaluated, a new cluster spectrometer was built and tested in the field and several new aerosol parameterizations and computations modules for chemical transport and global climate models were developed and evaluated. This work enabled EUCAARI to improve our understanding of aerosol radiative forcing and air quality-climate interactions. The EUCAARI results can be utilized in European and global environmental policy to assess the aerosol impacts and the corresponding abatement strategies.

Description: A global climatology of tropospheric and stratospheric ozone derived from Aura OMI and MLS measurements Atmospheric Chemistry and Physics Discussions, 11, 17879-17911, 2011 Author(s): J. R. Ziemke, S. Chandra, G. Labow, P. K. Bhartia, L. Froidevaux, and J. C. Witte A global climatology of tropospheric and stratospheric column ozone is derived by combining six years of Aura Ozone Monitoring Instrument (OMI) and Microwave Limb Sounder (MLS) ozone measurements for the period October 2004 through December 2010. The OMI/MLS tropospheric ozone climatology exhibits large temporal and spatial variability which includes ozone accumulation zones in the tropical south Atlantic year-round and in the subtropical Mediterranean/Asia region in summer months. High levels of tropospheric ozone in the Northern Hemisphere also persist in mid-latitudes over the Eastern North American and Asian continents extending eastward over the Pacific Ocean. For stratospheric ozone climatology from MLS, largest ozone abundance lies in the Northern Hemisphere in the latitude range 70° N–80° N in February–April and in the Southern Hemisphere around 40° S–50° S during months August–October. The largest stratospheric ozone abundances in the Northern Hemisphere lie over North America and Eastern Asia extending eastward across the Pacific Ocean and in the Southern Hemisphere south of Australia extending eastward across the dateline. With the advent of many newly developing 3-D chemistry and transport models it is advantageous to have such a dataset for evaluating the performance of the models in relation to dynamical and photochemical processes controlling the ozone distributions in the troposphere and stratosphere. The OMI/MLS ozone gridded climatology data, both calculated mean values and RMS uncertainties are made available to the science community via the NASA total ozone mapping spectrometer (TOMS) website http://toms.gsfc.nasa.gov .

Description: Scattering and absorption by aerosols during EUCAARI-LONGREX: can airborne measurements and models agree? Atmospheric Chemistry and Physics Discussions, 11, 18487-18525, 2011 Author(s): E. J. Highwood, M. J. Northway, G. R. McMeeking, W. T. Morgan, D. Liu, S. Osborne, K. Bower, H. Coe, C. Ryder, and P. Williams Scattering and absorption by aerosol in anthropogenically perturbed air masses over Europe has been measured using instrumentation flown on the UK's BAe-146-301 large Atmospheric Research Aircraft (ARA) operated by the Facility for Airborne Atmospheric Measurements (FAAM) on 14 flights during the EUCAARI-LONGREX campaign in May 2008. The geographical and temporal variations of the derived shortwave optical properties of aerosol are presented. Values of single scattering albedo of dry aerosol at 550 nm varied considerably over the data set from 0.86 to near unity. Dry aerosol optical depths ranged from 0.03 to 0.24. An optical properties closure study comparing calculations from composition data and Mie scattering code with the measured properties is presented. Very good agreement (to within 30 %) can be achieved for scattering, but the modelling of absorption is shown to be sensitive to the refractive indices chosen for organic aerosols, and to a lesser extent black carbon. Agreement with the measured absorption can only be achieved if organic carbon is assumed to be only weakly absorbing. Hygroscopic growth curves derived from the wet nephelometer indicate moderate water uptake by the aerosol with a campaign mean f (RH) value (change in scattering) of 1.3 at 80 % relative humidity. This value is consistent with the major chemical components of the aerosol measured by the aerosol mass spectrometer (AMS), which are primarily mixed organics and nitrate and some sulphate. As expected the effect of humidity is to raise the single scattering albedo, and to increase the aerosol optical depth. This study represents an important new body of data regarding European aerosol amounts, composition and optical properties and additionally demonstrates the importance of airborne measurements of black carbon mass and aerosol hygroscopicity.

Description: Biological residues define the ice nucleation properties of soil dust Atmospheric Chemistry and Physics Discussions, 11, 16585-16598, 2011 Author(s): F. Conen, C. E. Morris, J. Leifeld, M. V. Yakutin, and C. Alewell Soil dust is a major driver of ice nucleation in clouds leading to precipitation. It consists largely of mineral particles with a small fraction of organic matter constituted mainly of remains of micro-organisms that participated in degrading plant debris before their own decay. Some micro-organisms have been shown to be much better ice nuclei than the most efficient soil mineral. Yet, current aerosol schemes in global climate models do not consider a difference between soil dust and mineral dust in terms of ice nucleation activity. Here, we show that particles from the clay and silt size fraction of four different soils naturally associated with 0.7 to 11.8 % organic carbon (w/w) can have up to four orders of magnitude more ice nuclei per unit mass active in the immersion freezing mode at −12 °C than montmorillonite, the most efficient pure clay mineral. Most of this activity was lost after heat treatment. Removal of biological residues reduced ice nucleation activity to, or below that of montmorillonite. Desert soils, inherently low in organic content, are a large natural source of dust in the atmosphere. In contrast, agricultural land use is concentrated on fertile soils with much larger organic matter contents than found in deserts. It is currently estimated that the contribution of agricultural soils to the global dust burden is less than 20 %. Yet, these disturbed soils can contribute ice nuclei to the atmosphere of a very different and much more potent kind than mineral dusts.

Description: ACE-FTS measurements of trace species in the characterization of biomass burning plumes Atmospheric Chemistry and Physics Discussions, 11, 16611-16637, 2011 Author(s): K. A. Tereszchuk, G. González Abad, C. Clerbaux, D. Hurtmans, P.-F. Coheur, and P. F. Bernath To further our understanding of the effects of biomass burning emission on atmospheric composition, we report measurements of trace species from biomass burning plumes made by the Atmospheric Chemistry Experiment Fourier Transform Spectrometer (ACE-FTS) instrument on the SCISAT-1 satellite. An extensive set of 15 molecules, C 2 H 2 , C 2 H 6 , CH 3 OH, CH 4 , CO, H 2 CO, HCN, HCOOH, HNO 3 , NO, NO 2 , N 2 O 5 , O 3 , OCS and SF 6 are used in our analysis. Even though most biomass burning smoke is typically confined to the boundary layer, much of these emissions are injected directly into the free troposphere via fire-related convective processes and transported away from the emission region. Further knowledge of the aging of biomass burning emission in the free troposphere is needed. Tracer-tracer correlations are made between known pyrogenic species in these plumes in an effort to classify them and follow their chemical evolution. Criteria such as age and type of biomass material burned are considered. Emission factors are derived and compared to airborne measurements of biomass burning from numerous ecosystems to validate the ACE-FTS data.

Description: Emission controls versus meteorological conditions in determining aerosol concentrations in Beijing during the 2008 Olympic Games Atmospheric Chemistry and Physics Discussions, 11, 16655-16691, 2011 Author(s): Y. Gao, X. Liu, C. Zhao, M. Zhang, and Y. Wang A series of emission control measures were undertaken in Beijing and the adjacent provinces in China during the 2008 Beijing Olympic Games on 8–24 August 2008. This provides a unique opportunity for investigating the effectiveness of emission controls on air pollution in Beijing. We conducted a series of numerical experiments over East Asia for the period of July to September 2008 using a coupled meteorology-chemistry model (WRF-Chem). Model can generally reproduce the observed variation of aerosol concentrations. Consistent with observations, modeled concentrations of aerosol species (sulfate, nitrate, ammonium, black carbon, organic carbon, total particulate matter) in Beijing were decreased by 30–50 % during the Olympic period compared to the other periods in July and August in 2008 and the same period in 2007. Model results indicate that emission controls were effective in reducing the aerosol concentrations by comparing simulations with and without emission controls. However, our analysis suggests that meteorological conditions (e.g., wind direction and precipitation) are at least as important as emission controls in producing the low aerosol concentrations appearing during the Olympic period. Transport from the regions surrounding Beijing determines the temporal variation of aerosol concentrations in Beijing. Based on the budget analysis, we suggest that to improve the air quality over Beijing, emission control strategy should focus on the regional scale instead of the local scale.

Description: Validity of satellite measurements used for the monitoring of UV radiation risk on health Atmospheric Chemistry and Physics Discussions, 11, 17375-17421, 2011 Author(s): F. Jégou, S. Godin-Beekman, M. P. Corrêa, C. Brogniez, F. Auriol, V. H. Peuch, M. Haeffelin, A. Pazmino, P. Saiag, F. Goutail, and E. Mahé In order to test the validity of ultraviolet index (UVI) satellite products and UVI model simulations for general public information, intercomparison involving three satellite instruments (SCIAMACHY, OMI and GOME-2), the Chemistry and Transport Model, Modélisation de la Chimie Atmosphérique Grande Echelle (MOCAGE), and ground-based instruments was performed in 2008 and 2009. The intercomparison highlighted a systematic high bias of ~1 UVI in the OMI clear-sky products compared to the SCIAMACHY and TUV model clear-sky products. The OMI and GOME-2 all-sky products are close to the ground-based observations with a low 6 % positive bias, comparable to the results found during the satellite validation campaigns. This result shows that OMI and GOME-2 all-sky products are well appropriate to evaluate the UV-risk on health. The study has pointed out the difficulty to take into account either in the retrieval algorithms or in the models, the large spatial and temporal cloud modification effect on UV radiation. This factor is crucial to provide good quality UV information. OMI and GOME-2 show a realistic UV variability as a function of the cloud cover. Nevertheless these satellite products do not sufficiently take into account the radiation reflected by clouds. MOCAGE numerical forecasts show good results during periods with low cloud covers, but are actually not adequate for overcast conditions; this is why Météo-France currently uses human-expertised cloudiness (rather than direct outputs from Numerical Prediction Models) together with MOCAGE clear-sky UV indices for its operational forecasts. From now on, the UV monitoring could be done using free satellite products (OMI, GOME-2) and operational forecast for general public by using modelling, as long as cloud forecasts and the parametrisation of the impact of cloudiness on UV radiation are adequate.

Description: Aerosol concentration and size distribution measured below, in, and above cloud from the DOE G-1 during VOCALS-REx Atmospheric Chemistry and Physics Discussions, 11, 17289-17336, 2011 Author(s): L. I. Kleinman, P. H. Daum, Y.-N. Lee, E. R. Lewis, A. J. Sedlacek III, G. I. Senum, S. R. Springston, J. Wang, J. Hubbe, J. Jayne, Q. Min, S. S. Yum, and G. Allen During the VOCALS Regional Experiment, the DOE G-1 aircraft was used to sample a varying aerosol environment pertinent to properties of stratocumulus clouds over a longitude band extending 800 km west from the Chilean coast at Arica. Trace gas and aerosol measurements are presented as a function of longitude, altitude, and dew point in this study. Spatial distributions are consistent with an upper atmospheric source for O 3 and South American coastal sources for marine boundary layer (MBL) CO and aerosol, most of which is acidic sulfate in agreement with the dominant pollution source being SO 2 from Cu smelters and power plants. Pollutant layers in the free troposphere (FT) can be a result of emissions to the north in Peru or long range transport from the west. At a given altitude in the FT (up to 3 km), dew point varies by 40 °C with dry air descending from the upper atmospheric and moist air having a BL contribution. Ascent of BL air to a cold high altitude results in the condensation and precipitation removal of all but a few percent of BL water along with aerosol that served as CCN. Thus, aerosol volume decreases with dew point in the FT. Aerosol size spectra have a bimodal structure in the MBL and an intermediate diameter unimodal distribution in the FT. Comparing cloud droplet number concentration (CDNC) and pre-cloud aerosol ( D p 〉 100 nm) gives a linear relation up to a number concentration of ~150 cm −3 , followed by a less than proportional increase in CDNC at higher aerosol number concentration. A number balance between below cloud aerosol and cloud droplets indicates that ~25 % of aerosol in the PCASP size range are interstitial (not activated). One hundred and two constant altitude cloud transects were identified and used to determine properties of interstitial aerosol. One transect is examined in detail as a case study. Approximately 25 to 50 % of aerosol with D p 〉 110 nm were not activated, the difference between the two approaches possibly representing shattered cloud droplets or unknown artifact. CDNC and interstitial aerosol were anti-correlated in all cloud transects, consistent with the occurrence of dry in-cloud areas due to entrainment or circulation mixing.

Description: Interannual variability of ozone and carbon monoxide at the Whistler high elevation site: 2002–2006 Atmospheric Chemistry and Physics Discussions, 11, 17621-17664, 2011 Author(s): A. M. Macdonald, K. G. Anlauf, W. R. Leaitch, and E. Chan In spring 2002, an atmospheric measurement site was established at the peak of Whistler Mountain in British Columbia, Canada to measure trace gases, particle chemistry and physics, and meteorology. This paper uses continuous measurements from March 2002 to December 2006 to investigate the influence of trans-Pacific transport and North American forest fires on both O 3 and CO at Whistler. Annual mean mixing ratios of O 3 and CO were 41 ppbv (monthly means of 35–48 ppbv) and 145 ppbv (monthly means of 113–177 ppbv) respectively with both species exhibiting an annual cycle of late-winter to early-spring maxima and summer minima. The absence of a broad summer O 3 peak differs from previously-reported high altitude sites in the western US. The highest monthly-averaged O 3 and CO mixing ratios relative to the 5-year monthly means were seen in fall 2002 and spring 2003 with increased O 3 and CO of 10 % and 25 % respectively. These increases correspond to anomalously-high values reported at other Northern Hemisphere sites and are attributed to fires in the Russian Federation. Air mass back trajectory analysis is used to associate the mean enhancements of O 3 and CO with trans-Pacific transported or North American air masses relative to the Pacific background. Mean values of the enhancements for March to June were 6 ppbv and 16 ppbv for O 3 and CO respectively. In summers 2002–2006, higher CO and O 3 mixing ratios were always observed in North American air masses and this relative enhancement co-varied for each year with the western US and Canada total wildfire area. The greatest enhancements in O 3 and CO were seen in 2004, a record year for forest fires in Alaska and the Yukon Territory. In August 2004, average O 3 and CO mixing ratios were 13 and 44 ppbv above background values.

Description: Atmospheric dust modeling from meso to global scales with the online NMMB/BSC-Dust model – Part 1: Model description, annual simulations and evaluation Atmospheric Chemistry and Physics Discussions, 11, 17551-17620, 2011 Author(s): C. Pérez, K. Haustein, Z. Janjic, O. Jorba, N. Huneeus, J. M. Baldasano, T. Black, S. Basart, S. Nickovic, R. L. Miller, J. P. Perlwitz, M. Schulz, and M. Thomson We describe and evaluate the NMMB/BSC-Dust, a new dust aerosol cycle model embedded online within the NCEP Non-hydrostatic Multiscale Model (NMMB). NMMB is a further evolution of the operational Non-hydrostatic Mesoscale Model (WRF-NMM), which together with other upgrades has been extended from meso to global scales. Its unified non-hydrostatic dynamical core is prepared for regional and global simulation domains. The new NMMB/BSC-Dust is intended to provide short to medium-range weather and dust forecasts from regional to global scales and represents a first step towards the development of a unified chemical-weather model. This paper describes the parameterizations used in the model to simulate the dust cycle including sources, transport, deposition and interaction with radiation. We evaluate monthly and annual means of the global configuration of the model against the AEROCOM dust benchmark dataset for year 2000 including surface concentration, deposition and aerosol optical depth (AOD), and we evaluate the daily AOD variability in a regional domain at high resolution covering Northern Africa, Middle East and Europe against AERONET AOD for year 2006. The NMMB/BSC-Dust provides a good description of the horizontal distribution and temporal variability of the dust. Daily AOD correlations at the regional scale are around 0.6–0.7 on average without dust data assimilation. At the global scale the model lies within the top range of AEROCOM dust models in terms of performance statistics for surface concentration, deposition and AOD. This paper discusses the current strengths and limitations of the modeling system and points towards future improvements.

Description: Sources and seasonality of atmospheric methanol based on tall tower measurements in the US Upper Midwest Atmospheric Chemistry and Physics Discussions, 11, 17473-17505, 2011 Author(s): L. Hu, M. J. Mohr, K. C. Wells, T. J. Griffis, D. Helmig, and D. B. Millet We present over one year of continuous atmospheric methanol measurements from the University of Minnesota tall tower Trace Gas Observatory (KCMP tall tower; 244 m a.g.l.), and interpret the dataset in terms of constraints on regional methanol sources and seasonality. The seasonal cycle of methanol concentrations observed at the KCMP tall tower is generally similar to that simulated by a global 3-D chemical transport model (GEOS-Chem, driven with MEGANv2.0 biogenic emissions) except the seasonal peak occurs ~1 month earlier in the observations, apparently reflecting a model underestimate of emission rates for younger versus older leaves. Based on a source tracer approach, which we evaluate using GEOS-Chem and with multiple tracers, we estimate that anthropogenic emissions account for approximately 40 % of ambient methanol abundance during winter and 10 % during summer. During daytime in summer, methanol concentrations increase exponentially with temperature, reflecting the temperature sensitivity of the biogenic source, and the observed temperature dependence is statistically consistent with that in the model. Nevertheless, summertime concentrations are underestimated by on average 35 % in the model for this region. The seasonal importance of methanol as a source of formaldehyde (HCHO) and carbon monoxide (CO) is highest in spring through early summer, when biogenic methanol emissions are high but isoprene emissions are still relatively low. During that time observed methanol concentrations account for on average 20 % of the total CO and HCHO production rates as simulated by GEOS-Chem, compared to 12 % later in the summer and 12 % on an annual average basis. The biased seasonality in the model means that the photochemical role for methanol early in the growing season is presently underestimated.

Description: Impact of lightning-NO on Eastern United States photochemistry during the summer of 2006 as determined using the CMAQ model Atmospheric Chemistry and Physics Discussions, 11, 17699-17757, 2011 Author(s): D. J. Allen, K. E. Pickering, R. W. Pinder, B. H. Henderson, K. W. Appel, and A. Prados A lightning-nitrogen oxide (NO) algorithm is developed for the regional Community Multiscale Air Quality Model (CMAQ) and used to evaluate the impact of lightning-NO emissions (LNO x ) on tropospheric photochemistry over the Eastern United States during the summer of 2006. The scheme assumes flash rates are proportional to the model convective precipitation rate but then adjusts the flash rates locally to match monthly average observations. Over the Eastern United States, LNO x is responsible for 20–25 % of the tropospheric nitrogen dioxide (NO 2 ) column. This additional NO 2 reduces the low-bias of simulated NO 2 columns with respect to satellite-retrieved Dutch Ozone Monitoring Instrument NO 2 (DOMINO) columns from 41 to 14 %. It also adds 10–20 ppbv to upper tropospheric ozone and 1.5–4.5 ppbv to 8-h maximum surface layer ozone, although, on average, the contribution of LNO x to surface ozone is 1–2 ppbv less on poor air quality days. Biases between modeled and satellite-retrieved tropospheric NO 2 columns vary greatly between urban and rural locations. In general, CMAQ overestimates columns at urban locations and underestimates columns at rural locations. These biases are consistent with in situ measurements that also indicate that CMAQ has too much NO 2 in urban regions and not enough in rural regions. However, closer analysis suggests that most of the differences between modeled and satellite-retrieved urban to rural ratios are likely a consequence of the horizontal and vertical smoothing inherent in columns retrieved by the Ozone Monitoring Instrument (OMI). Within CMAQ, LNO x increases wet deposition of nitrate by 50 % and total deposition of nitrogen by 11 %. This additional deposition reduces the magnitude of the CMAQ low-bias in nitrate wet deposition with respect to National Atmospheric Deposition monitors to near zero. In order to obtain an upper bound on the contribution of uncertainties in chemistry to upper tropospheric NO x low biases, sensitivity calculations with updated chemistry were run for the time period of the Intercontinental Chemical Transport Experiment (INTEX-A) field campaign (summer 2004). After adjusting for possible interferences in NO 2 measurements and averaging over the entire campaign, these updates reduced 7–9 km biases from 32 to 17 % and 9–12 km biases from 57 to 46 %. While these changes lead to better agreement, a considerable NO 2 low-bias remains in the uppermost troposphere.

Description: Interpreting elevated space-borne HCHO columns over the Mediterranean Sea using the OMI and SCIAMACHY sensors Atmospheric Chemistry and Physics Discussions, 11, 17913-17940, 2011 Author(s): A. Sabolis, N. Meskhidze, G. Curci, P. I. Palmer, and B. Gantt Formaldehyde (HCHO) is an oxidation product of a wide range of volatile organic compounds (VOCs) and important atmospheric constituent found in both the polluted urban atmosphere and remote background sites. In this study, remotely sensed data of HCHO vertical column densities are analyzed over the Mediterranean Sea using the Ozone Monitoring Instrument (OMI). Data analysis indicates a marked seasonal cycle with a summer maximum and winter minimum confined to the marine environment during a three year period (2005–2007) examined. A possible retrieval artifact associated with Saharan dust transport over the region is explored by changing intensity of Saharan dust sources in GEOS-Chem following the recommendation of Generoso et al. (2008). Recalculated air mass factors (AMF), based on the new values of aerosol loadings, lead to a reduction of the summertime "hot spot" in OMI retrieval of HCHO columns over the Mediterranean Sea; however, even after the correction, enhanced values are still present in this region. To explain these values, marine biogenic sources of VOCs are examined. Calculations indicate that emissions of phytoplankton-produced isoprene and monoterpenes are not likely to explain the enhanced HCHO columns over the Mediterranean Sea. To further understand spatial and seasonal variation of HCHO over the Mediterranean Sea, OMI HCHO columns are compared to those of the SCanning Imaging Absorption spectroMeter for Atmospheric CartograpHY (SCIAMACHY) sensor. Unlike OMI retrievals, over the Mediterranean Sea SCIAMACHY HCHO columns did not reveal clear seasonality during the three years and the two sensors did not agree within their retrieval uncertainty. Overall, comparison of OMI and SCIAMACHY HCHO columns were inconclusive. Moreover, retrievals of HCHO columns over other water bodies showed that the two sensors agree reasonably well over the Equatorial Pacific region, Gulf of Mexico, and the North Sea, but do not show similar magnitudes or seasonal variations over oligotrophic water bodies such as Mediterranean Sea, Northwestern and Southern Pacific Oceans. Model simulations in conjunction with measurements studies may be required to fully explore the complex mechanism of HCHO formation over the Mediterranean and its implications for the air quality in the region.

Description: Rate coefficients for the reaction of methylglyoxal (CH 3 COCHO) with OH and NO 2 and glyoxal (HCO) 2 with NO 3 Atmospheric Chemistry and Physics Discussions, 11, 18211-18248, 2011 Author(s): R. K. Talukdar, L. Zhu, K. J. Feierabend, and J. B. Burkholder Rate coefficients, k , for the gas-phase reaction of CH 3 COCHO (methylglyoxal) with the OH and NO 3 radicals and (CHO) 2 (glyoxal) with the NO 3 radical are reported. Rate coefficients for the OH + CH 3 COCHO ( k 1 ) reaction were measured under pseudo-first-order conditions in OH as a function of temperature (211–373 K) and pressure (100–220 Torr, He and N 2 bath gases) using pulsed laser photolysis to produce OH radicals and laser induced fluorescence to measure its temporal profile. k 1 was found to be independent of the bath gas pressure with k 1 (295 K) = (1.29 ± 0.13) × 10 −11 cm 3 molecule −1 s −1 and a temperature dependence that is well represented by the Arrhenius expression k 1 (T) = (1.74 ± 0.20) × 10 −12 exp[(590 ± 40)/T] cm 3 molecule −1 s −1 where the uncertainties are 2σ and include estimated systematic errors. Rate coefficients for the NO 3 + (CHO) 2 ( k 3 ) and NO 3 + CH 3 COCHO ( k 4 ) reactions were measured using a relative rate technique to be k 3 (296 K) = (3.7 ± 1.0) × 10 −16 cm 3 molecule −1 s −1 and k 4 (296 K) = (4.1 ± 1.2) × 10 −16 cm 3 molecule −1 s −1 . k 3 (T) was also measured using an absolute rate coefficient method under pseudo-first-order conditions at 296 and 353 K to be (4.2 ± 0.8) × 10 −16 and (7.9 ± 3.6) × 10 −16 cm 3 molecule −1 s −1 , respectively, in agreement with the relative rate result obtained at room temperature. The atmospheric implications of the OH and NO 3 reaction rate coefficients measured in this work are discussed.

Description: Photochemical production of ozone in Beijing during the 2008 Olympic Games Atmospheric Chemistry and Physics Discussions, 11, 16553-16584, 2011 Author(s): C. C.-K. Chou, C.-Y. Tsai, C.-C. Chang, P.-H. Lin, S. C. Liu, and T. Zhu As a part of the CAREBeijing-2008 campaign, observations of O 3 , oxides of nitrogen (NO x and NO y ), CO, and hydrocarbons (NMHCs) were carried out at the air quality observatory of the Peking University in Beijing, China during August 2008, including the period of the 29th Summer Olympic Games. The measurements were compared to those of the CAREBeijing-2006 campaign to evaluate the effectiveness of the air pollution control measures, which were conducted for improving the air quality in Beijing during the Olympics. The results indicate that significant reduction in the emissions of primary air pollutants had been achieved; the monthly averages of NO x , NO y , CO, and NMHCs reduced by 42.2, 56.5, 27.8, and 49.7 %, respectively. In contrast to the primary pollutants, the averaged mixing ratio of O 3 increased by 42.2 %. Nevertheless, it was revealed that the ambient levels of total oxidants (O x =O 3 +NO 2 +1.5NO z ) and NO z reduced by 21.3 and 77.4 %, respectively. The contradictions between O 3 and O x were further examined in two case studies. Ozone production rates of 30–70 ppbv hr −1 and OPEx of ~8 mole mole −1 were observed on a clear-sky day in spite of the reduced levels of precursors. In that case, it was found that the concentrations of O 3 increased with the increasing NO 2 /NO ratio, whereas the NO z concentrations leveled off when NO 2 /NO〉8. Consequently, the ratio of O 3 to NO z increased to above 10, indicating the shift from VOC-sensitive regime to NO x -sensitive regime. However, in the other case, it was found that the O 3 production was inhibited significantly due to substantial reduction in the ambient levels of NMHCs. According to the observations, it was suggested that the O 3 /O x production rates in Beijing should have been reduced for the reduction in the emissions of precursors during the Olympic period; however, the nighttime O 3 levels were increased for decline in the NO-O 3 titration, and the midday O 3 peak levels were elevated for the shift in the photochemical regime and the inhibition of NO z formation.

Description: ARCTAS-A ground-based observational campaign and meteorological context, interior Alaska, April 2008 Atmospheric Chemistry and Physics Discussions, 11, 16499-16552, 2011 Author(s): D. E. Atkinson, K. Sassen, M. Hayashi, C. F. Cahill, G. Shaw, D. Harrigan, and H. Fuelberg Arctic aerosol loading in interior Alaska displays a strong seasonality, with pristine conditions generally prevailing during winter months and increasing frequency of midlatitude air intrusions occurring in spring. By summer, local aerosol sources, like boreal forest fire smoke, may come into prominence. Long term aerosol research from the University of Alaska Fairbanks indicates that the period around April typically marks the beginning of the retreat of the Polar Front, opening the free exchange of midlatitude air. In April 2008 the NASA ARCTAS field campaign was conducted, supported in Fairbanks by comprehensive polarization (0.693 μm) lidar, surface and balloon-borne aerosol measurements, and synoptic weather analyzes. The data provided information on the vertical distribution and type of aerosol, the size distributions and chemical nature of the surface aerosol, as well as the large scale view of aerosol transport conditions to Alaska. We found evidence to suggest four major aerosol loading events in the 25 March–30 April 2008 timeframe: a typical Arctic haze event, several days of extremely clear conditions, rapid onset of a period dominated by Asian dust with some smoke, and a period dominated by Asian smoke. A focused case study analysis conducted on 19 April 2008 using a balloon-borne optical particle counter suggested that, on this day, the majority of the suspended particulate matter consisted of Asian dust although a contribution from Asian smoke cannot be ruled out on the basis of backtrack analysis. In the last week of April concentrations gradually decreased as synoptic conditions shifted away from favoring transport to Alaska.

Description: Theory of isotope fractionation on facetted ice crystals Atmospheric Chemistry and Physics Discussions, 11, 17423-17445, 2011 Author(s): J. Nelson Present models of the differential incorporation of isotopic water molecules into vapor-grown ice omit surface processes that may be important in temperature reconstructions. This article introduces a model that includes such surface processes and shows that differences in deposition coefficients for water isotopes can produce isotope fractionation coefficients that significantly differ from those of existing theory. For example, if the deposition coefficient of H 2 18 O differs by just 5 % from that of ordinary water (H 2 16 O), the resulting fractionation coefficient at 20 % supersaturation may deviate from the kinetic fractionation (KF) prediction by up to about ±17 ‰. Like the KF model, this "surface-kinetic" fractionation model generally predicts greater deviation from the equilibrium prediction at higher supersaturations; indeed, the sensitivity to supersaturation far exceeds that to temperature. Moreover, the model introduces possible new temperature dependencies from the deposition coefficients. These parameters need to be constrained by new laboratory measurements; nevertheless, the theory suggests that observed δ 18 O changes in ice samples are unlikely to be due solely to temperature changes.

Description: Flight-based chemical characterization of biomass burning aerosols within two prescribed burn smoke plumes Atmospheric Chemistry and Physics Discussions, 11, 17507-17550, 2011 Author(s): K. A. Pratt, S. M. Murphy, R. Subramanian, P. J. DeMott, G. L. Kok, T. Campos, D. C. Rogers, A. J. Prenni, A. J. Heymsfield, J. H. Seinfeld, and K. A. Prather Biomass burning represents a major global source of aerosols impacting direct radiative forcing and cloud properties. Thus, the goal of a number of current studies involves developing a better understanding of how the chemical composition and mixing state of biomass burning aerosols evolve during atmospheric aging processes. During the Ice in Cloud Experiment – Layer Clouds (ICE-L) in fall of 2007, smoke plumes from two small Wyoming Bureau of Land Management prescribed burns were measured by on-line aerosol instrumentation aboard a C-130 aircraft, providing a detailed chemical characterization of the particles. After ~2–4 min of aging, submicron smoke particles, produced primarily from sagebrush combustion, consisted predominantly of organics by mass, but were comprised primarily of internal mixtures of organic carbon, elemental carbon, potassium chloride, and potassium sulfate. Significantly, 100 % of the fresh biomass burning particles contained minor mass fractions of nitrate and sulfate, suggesting that hygroscopic material is incorporated very near or at the point of emission. The mass fractions of ammonium, sulfate, and nitrate increased with aging up to ~81–88 min and resulted in acidic particles, with both nitric acid and sulfuric acid present. Decreasing black carbon mass concentrations occurred due to dilution of the plume. Increases in the fraction of oxygenated organic carbon and the presence of dicarboxylic acids, in particular, were observed with aging. Cloud condensation nuclei measurements suggested all particles 〉100 nm were active at 0.5 % water supersaturation in the smoke plumes, confirming the relatively high hygroscopicity of the freshly emitted particles. For immersion/condensation freezing, ice nuclei measurements at −32 °C suggested activation of ~0.03–0.07 % of the particles with diameters greater than 500 nm.

Description: Wind tunnel experiments on the retention of trace gases during riming: nitric acid, hydrochloric acid, and hydrogen peroxide Atmospheric Chemistry and Physics Discussions, 11, 17447-17472, 2011 Author(s): N. von Blohn, K. Diehl, S. K. Mitra, and S. Borrmann Laboratory experiments were carried out in a vertical wind tunnel to study the retention of different atmospheric trace gases during riming. In the experiments, the rimed ice particles floated in a laminar air stream carrying a cloud of supercooled droplets with radii between 10 and 20 μm. Ice particles, dendritic ice crystals, and snow flakes with diameters between 6 mm and 1.5 cm were allowed to rime at temperatures between −5 and −12 °C where riming mainly proceeds in the atmosphere and with cloud liquid water contents between 1 and 1.5 g m −3 which are values typically found in atmospheric mixed phase clouds. Three trace species were investigated, nitric and hydrochloric acid, and hydrogen peroxide. They were present in the supercooled liquid droplets in concentrations from 1 to 120 ppmv, i.e. similar to the ones measured in cloud drops. The chemical analyses of the rimed ice particles allow to determine the trace species concentration in the ice phase. Together with the known liquid phase concentration the retention coefficients were calculated in terms of the amount of the species which remained in the ice phase after freezing. It was found that the highly soluble trace gases nitric and hydrochloric acid were retained nearly completely (98.6 ± 8 % and 99.7 ± 9 %, respectively) while for hydrogen peroxide a retention coefficient of 64.3 ± 11 % was determined. No influence of the riming temperature on the retention was found which can be explained by the fact that in the observed range of temperature and liquid water content riming proceeded in the dry growth regime.

Description: Predicting the relative humidities of liquid-liquid phase separation, efflorescence, and deliquescence of mixed particles of ammonium sulfate, organic material, and water using the organic-to-sulfate mass ratio of the particle and the oxygen-to-carbon elemental ratio of the organic component Atmospheric Chemistry and Physics Discussions, 11, 17759-17788, 2011 Author(s): A. K. Bertram, S. T. Martin, S. J. Hanna, M. L. Smith, A. Bodsworth, Q. Chen, M. Kuwata, A. Liu, Y. You, and S. R. Zorn Individual particles that on a mass basis consist dominantly of the components ammonium sulfate, organic material, and water are a common class of submicron particles found in today's atmosphere. Here we use (1) the organic-to-sulfate (org:sulf) mass ratio of the overall particle and (2) the oxygen-to-carbon (O:C) elemental ratio of the organic component as input variables in parameterisations that predict the critical relative humidity of several different types of particle phase transitions. These transitions include liquid-liquid phase separation (SRH), efflorescence (ERH), and deliquescence (DRH). Experiments were conducted by optical microscopy for 11 different oxygenated organic-ammonium sulfate systems covering the range 0.1 〈 org:sulf 〈12.8 and 0.29 〈 O:C 〈 1.33. These new data, in conjunction with other data already available in the literature, were used to develop the parameterisations SRH(org:sulf, O:C), ERH(org:sulf, O:C), and DRH(org:sulf, O:C). The parameterisations correctly predicted SRH within 15 % RH for 86 % of the measurements, ERH within 5 % for 86 % of the measurements, and DRH within 5 % for 95 % of the measurements. The applicability of the derived parameterisations beyond the training data set was tested against observations for organic-sulfate particles produced in an environmental chamber. The organic component consisted of secondary organic material produced by the oxidation of isoprene, α-pinene, and β-caryophyllene. The predictions of the parameterisations were also tested against data from the Southern Great Plains, Oklahoma, USA. The observed ERH and DRH values for both the chamber and field data agreed within 5 % RH with the value predicted by the parameterisations using the measured org:sulf and O:C ratios as the input variables.

Description: Accounting for non-linear chemistry of ship plumes in the GEOS-Chem global chemistry transport model Atmospheric Chemistry and Physics Discussions, 11, 17789-17823, 2011 Author(s): G. C. M. Vinken, K. F. Boersma, D. J. Jacob, and E. W. Meijer We present a computationally efficient approach to account for the non-linear chemistry occurring during the dispersion of ship exhaust plumes in a global 3-D model of atmospheric chemistry (GEOS-Chem). We use a plume-in-grid formulation where ship emissions age chemically for 5 h before being released in the global model grid. Besides reducing the original ship NO x emissions in GEOS-Chem, our approach also releases the secondary compounds ozone and HNO 3 , produced in the 5 h after the original emissions, into the model. We applied our improved method and also the widely used "instant dilution" approach to a 1-yr GEOS-Chem simulation of global tropospheric ozone-NO x -VOC-aerosol chemistry. We also ran simulations with the standard model, and a model without any ship emissions at all. Our improved GEOS-Chem model simulates up to 0.1 ppbv (or 90 %) more NO x over the North Atlantic in July than GEOS-Chem versions without any ship NO x emissions at all. "Instant dilution" overestimates NO x concentrations by 50 % (0.1 ppbv) and ozone by 10–25 % (3–5 ppbv) over this region. These conclusions are supported by comparing simulated and observed NO x and ozone concentrations in the lower troposphere over the Pacific Ocean. The comparisons show that the improved GEOS-Chem model simulates NO x concentrations in between the instant diluting model and the model with no ship emissions, and results in lower O 3 concentrations than the instant diluting model. The relative differences in simulated NO x and ozone between our improved approach and instant dilution are smallest over strongly polluted seas (e.g. North Sea), suggesting that accounting for in-plume chemistry is most relevant for pristine marine areas.

Description: A Lagrangian view of convective sources for transport of air across the Tropical Tropopause Layer: distribution, times and the radiative influence of clouds Atmospheric Chemistry and Physics Discussions, 11, 18161-18209, 2011 Author(s): A. Tzella and B. Legras The Tropical Tropopause Layer (TTL) is a key region controlling transport between the troposphere and the stratosphere. The efficiency of transport across the TTL depends on the continuous interaction between the large-scale advection and the small-scale intermittent convection that reaches the Level of Zero radiative Heating (LZH). The wide range of scales involved presents a significant challenge to determine the sources of convection and quantify transport across the TTL. Here, we use a simple Lagrangian model, termed TTL detrainment model , that combines a large ensemble of 200-day back trajectory calculations with high-resolution fields of brightness temperatures (provided by the CLAUS dataset) in order to determine the ensemble of trajectories that are detrained from convective sources. The trajectories are calculated using the ECMWF ERA-Interim winds and radiative heating rates, derived both under all-sky and clear-sky conditions, so that the radiative influence of clouds is established. We show that most trajectories are detrained near the mean LZH with the horizontal distributions of convective sources being highly-localized, even within the space defined by deep convection. As well as modifying the degree of source localization, the radiative heating from clouds facilitates the rapid upwelling of air across the TTL. However, large-scale motion near the fluctuating LZH can lead a significant proportion of trajectories to alternating clear-sky and cloudy regions, thus generating a large dispersion in the vertical transport times. The distributions of vertical transport times are wide and skewed and are largely insensitive to a bias of about ±1 km (∓5 K) in the altitude of cloud top heights (the main sensitivity appearing in the times to escape the immediate neighbourhood of the LZH) while seasonal and regional transport characteristics are only apparent at small time-scales. The strong horizontal mixing that characterizes the TTL ensures that most air of convective origin is well-mixed within the tropical and eventually within the extra-tropical lower-stratosphere.

Description: Probability density functions of long-lived tracer observations from satellite in the subtropical barrier region: data intercomparison Atmospheric Chemistry and Physics Discussions, 11, 18385-18432, 2011 Author(s): E. Palazzi, F. Fierli, G. P. Stiller, and J. Urban Past studies have shown that a clear relationship exists between the field of a passive tracer and the Probability Distribution Function (PDF) of tracer concentrations, which can be exploited to identify the position and variability of stratospheric barriers to isentropic mixing. In the present study, we focus on the dynamical barrier located in the subtropics. We calculate PDFs of the long-lived tracers nitrous oxide (N 2 O) and methane (CH 4 ) from different satellite instruments: the Microwave Limb Sounder (MLS) on board Aura, the Michelson Interferometer for Passive Atmospheric Sounding (MIPAS) on board Envisat, the Sub-Millimetre Radiometre (SMR) on board Odin and the Halogen Occultation Experiment (HALOE) on board UARS, overall covering the time period of 1992–2009. An analysis of the consistency among the different sets of data and their capability of identifying mixing regions and barrier to transport regions in the stratosphere and the subtropical barrier location is a prime aim of the present study. This is done looking at the morphological structure of the one- and two-dimensional PDFs of tracer concentrations measured by the different instruments. The latter differ in their spatial and temporal sampling and resolution, and there are some systematic differences in the determination of the subtropical barrier position that have been highlighted. However, the four satellite instruments offer an overall consistent picture of the subtropical barrier annual cycle. There is a strong seasonality consistently represented, characterized by the wintertime shift of the subtropical edge toward the summer hemisphere. However, the influence of the Quasi Biennial Oscillation (QBO) on isentropic transport and mixing, and by consequence, on the position of the subtropical barrier, is not equally represented in all satellite data using the methodology proposed.

Description: Column-integrated aerosol microphysical properties from AERONET Sun photometer over Southwestern Spain Atmospheric Chemistry and Physics Discussions, 11, 18349-18384, 2011 Author(s): N. Prats, V. E. Cachorro, A. Berjón, C. Toledano, and A. M. De Frutos The aim of the present work is to carry out a detailed analysis of columnar microphysical properties obtained from Cimel sun-photometer measurements in the Southwest of Spain within the frame of the AERONET-RIMA network. AERONET inversion products are analysed, in particular the particle size distribution together with their associated microphysical parameters for both fine and coarse modes: concentration, effective radius and the fine mode volume fraction. This work complements previous works based on aerosol optical depth (AOD) and the Ångström exponent (AE) for a global characterization of atmospheric aerosol in this representative area of Spain and Europe. The analysed dataset spans between February 2000 and October 2008. Time series and statistical analysis has been carried out for these parameters in order to assess their typical values and seasonality together with their relationships with the AOD and AE. Mean values of volume particle concentration are 0.06 ± 0.07 μm 3 μm −2 for total, 0.019 ± 0.015 μm 3 μm −2 for fine and 0.04 ± 0.06 μm 3 μm −2 for coarse mode; and of effective radius are 0.040 ± 0.19 μm for total, 0.14 ± 0.02 μm for fine and 1.96 ± 0.41 μm for coarse mode. The most relevant features are the clear bimodality of the volume particle size distribution, with a slight dominance of the coarse mode for the total climatology and under the prevailing atmospheric conditions of the site (coastal marine). There is a clear prevalence of the coarse mode in summer months, September and March in coincidence with the occurrence of desert dust intrusions and highest AOD values. During aerosol desert dust arrivals, the particle size distribution is practically mono-modal with strong prevalence of the coarse mode which also shows a shift of the modal radius to lower values. The size particle predominance defines the characteristic of the site and it has been analysed under two different approaches: with respect to particle number, using the Ångström exponent and with respect to particle volume, where the fine mode volume fraction V f / V t is taken.

Description: Uncertainty of the stratospheric/tropospheric temperature trends in 1979–2008: Multiple Satellite MSU, radiosonde, and reanalysis datasets Atmospheric Chemistry and Physics Discussions, 11, 16639-16654, 2011 Author(s): J. Xu and A. M. Powell Jr. The trends and spreads of tropospheric and stratospheric temperature are discussed in terms of three groups of datasets in 1979–2008. These datasets include (a) three satellite observations of Microwave Sounding Units (MSU) measurements, (b) five radiosonde observations and (c) five reanalysis products. The equivalent tropospheric and stratospheric temperature from radiosonde and reanalyses are calculated based on the vertical weighting function of the MSU channel 2 (CH 2 ) and channel 4 (CH 4 ) measurements, respectively. The results show that both cooling in the stratosphere and warming in troposphere significantly depends on the datasets and latitudes.

Description: Contrasting organic aerosol particles from boreal and tropical forests during HUMPPA-COPEC-2010 and AMAZE-08 using coherent vibrational spectroscopy Atmospheric Chemistry and Physics Discussions, 11, 16933-16966, 2011 Author(s): C. J. Ebben, I. S. Martinez, M. Shrestha, A. M. Buchbinder, A. L. Corrigan, A. Guenther, T. Karl, T. Petäjä, W. W. Song, S. R. Zorn, P. Artaxo, M. Kulmala, S. T. Martin, L. M. Russell, J. Williams, and F. M. Geiger We present the vibrational sum frequency generation spectra of organic particles collected in a boreal forest in Finland and a tropical forest in Brazil. These spectra are compared to those of secondary organic material produced in the Harvard Environmental Chamber. By comparing coherent vibrational spectra of a variety of terpene and olefin reference compounds, along with the secondary organic material synthesized in the environmental chamber, we show that submicron aerosol particles sampled in Southern Finland during HUMPPA-COPEC-2010 are composed to a large degree of material similar in chemical composition to synthetic α-pinene-derived material. For material collected in Brazil as part of AMAZE-08, the organic component is found to be chemically complex in the coarse mode but highly uniform in the fine mode. When combined with histogram analyses of the isoprene and monoterpene abundance recorded during the HUMPPA-COPEC-2010 and AMAZE-08 campaigns, the findings presented here indicate that if air is rich in monoterpenes, submicron-sized secondary aerosol particles that form under normal OH and O 3 concentration levels can be described in terms of their hydrocarbon content as being similar to α-pinene-derived model secondary organic aerosol particles. If the isoprene concentration dominates the chemical composition of organic compounds in forest air, then the hydrocarbon component of secondary organic material in the submicron size range is not simply well-represented by that of isoprene-derived model secondary organic aerosol particles but is more complex. Throughout the climate-relevant size range of the fine mode, however, we find that the chemical composition of the secondary organic particle material from such air is invariant with size, suggesting that the particle growth does not change the chemical composition of the hydrocarbon component of the particles in a significant way.

Description: Organic carbon and non-refractory aerosol over the remote Southeast Pacific: oceanic and combustion sources Atmospheric Chemistry and Physics Discussions, 11, 16895-16932, 2011 Author(s): L. M. Shank, S. Howell, A. D. Clarke, S. Freitag, V. Brekhovskikh, V. Kapustin, C. McNaughton, T. Campos, and R. Wood Submicron aerosol physical and chemical properties in remote marine air were measured from aircraft over the Southeast Pacific during VOCALS-REx in 2008 and the North Pacific during IMPEX in 2006, and aboard a ship in the Equatorial Pacific in 2009. A High Resolution – Particle Time of Flight Aerosol Mass Spectrometer (HR-ToF-AMS) measured non-refractory submicron aerosol composition during all campaigns. Sulfate (SO 4 ) and organics (Org), during VOCALS and the cruise show lower absolute values than those reported for previous "clean air" studies. In the marine boundary layer, average concentrations for SO 4 were 0.52 μg m −3 for the VOCALS region and 0.85 μg m −3 for the equatorial region while average Org concentrations were 0.10 and 0.07 μg m −3 , respectively. Campaign average Org/SO 4 ratios were 0.19 (VOCALS) and 0.08 (Equatorial Pacific), while previous studies report "clean marine" Org/SO 4 ratios between 0.25 and 0.40, and in some cases as high as 3.5. CO and black carbon (BC) measurements over the Southeast Pacific provided sensitive indicators of pollution, and were used to identify the least polluted air, which had average concentrations of SO 4 and Org of 0.14 and 0.01 μg m −3 , respectively, with an average Org/SO 4 of 0.10. Furthermore, under cleanest MBL conditions, identified by CO below 60 ppbv, we found a robust linear relationship between Org and combustion derived BC concentrations between 2 and 15 ng m −3 , suggesting little to no marine source of submicrometer Org to the atmosphere over the Eastern South Pacific. This suggests that identification of Org in clean marine air may require a BC threshold below 4 ng m −3 , an order of magnitude lower than has been used in prior studies. Data from IMPEX was constrained to similar clean air criterion, and resulted in an average Org/SO 4 ratio of 0.19.

Description: Future impact of non-land based traffic emissions on atmospheric ozone and OH – an optimistic scenario and a possible mitigation strategy Atmospheric Chemistry and Physics Discussions, 11, 16801-16859, 2011 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, J. A. Pyle, F. Stordal, S. Szopa, Q. Tang, P. van Velthoven, J. E. Williams, and K. Ødemark The impact of future emissions from aviation and shipping on the atmospheric chemical composition has been estimated using an ensemble of six different atmospheric chemistry models. This study considers an optimistic emission scenario (B1) taking into account e.g. rapid introduction of clean and resource-efficient technologies, and a mitigation option for the aircraft sector (B1 ACARE), assuming further technological improvements. Results from sensitivity simulations, where emissions from each of the transport sectors were reduced by 5 %, show that emissions from both aircraft and shipping will have a larger impact on atmospheric ozone and OH in near future (2025; B1) and for longer time horizons (2050; B1) compared to recent time (2000). However, the ozone and OH impact from aircraft can be reduced substantially in 2050 if the technological improvements considered in the B1 ACARE will be achieved. Shipping emissions have the largest impact in the marine boundary layer and their ozone contribution may exceed 4 ppb (scaled to 100 %) over the North Atlantic Ocean in the future (2050; B1) during northern summer (July). In the zonal mean, ship-induced ozone relative to the background levels may exceed 12 % near the surface. Corresponding numbers for OH are 6.0 × 10 5 molecules cm −3 and 30 %, respectively. This large impact on OH from shipping leads to a relative methane lifetime reduction of 3.92(±0.48) % on the global average in 2050 B1 (ensemble mean CH 4 lifetime is 8.0(±1.0) yr), compared to 3.68(±0.47) % in 2000. Aircraft emissions have about 4 times higher ozone enhancement efficiency (ozone molecules enhanced relative to NO x molecules emitted) than shipping emissions, and the maximum impact is found in the UTLS region. Zonal mean aircraft-induced ozone could reach up to 5 ppb at northern mid- and high latitudes during future summer (July 2050; B1), while the relative impact peaks during northern winter (January) with a contribution of 4.2 %. Although the aviation-induced impact on OH is lower than for shipping, it still causes a reduction in the relative methane lifetime of 1.68(±0.38) % in 2050 B1. However, for B1 ACARE the perturbation is reduced to 1.17(±0.28) %, which is lower than the year 2000 estimate of 1.30(±0.30) %. Based on the fully scaled perturbations we calculate net radiative forcings from the six models taking into account ozone, methane (including stratospheric water vapour), and methane-induced ozone changes. For the B1 scenario, shipping leads to a net cooling with radiative forcings of −28.0(±5.1) and −30.8(±4.8) mW m −2 in 2025 and 2050, respectively, due to the large impact on OH and thereby methane lifetime reductions. Corresponding values for the aviation sector shows a net warming effect with 3.8(±6.1) and 1.9(±6.3) mW m −2 , respectively, but with a small net cooling of −0.6(±4.6) mW m −2 for B1 ACARE in 2050.

Description: Formation of hydroxyl radical from San Joaquin Valley particles extracted in a cell-free solution Atmospheric Chemistry and Physics Discussions, 11, 16861-16894, 2011 Author(s): H. Shen and C. Anastasio Previous studies have suggested that the adverse health effects from ambient particulate matter (PM) are linked to the formation of reactive oxygen species (ROS) by PM. While hydroxyl radical ( • OH) is the most reactive of the ROS species, there are few quantitative studies of • OH generation from PM. Here we report on • OH formation from PM collected at an urban (Fresno) and rural (Westside) site in the San Joaquin Valley (SJV) of California. We quantified • OH in PM extracts using a cell-free, phosphate-buffered saline (PBS) solution with or without 50 μM ascorbate (Asc). The results show that generally the urban Fresno PM generates much more • OH than the rural Westside PM. The presence of Asc at a physiologically relevant concentration in the extraction solution greatly enhances • OH formation from all the samples. Fine PM (PM 2.5 ) generally makes more • OH than the corresponding coarse PM (PM cf , i.e., 2.5 to 10 μm) normalized by air volume collected, while the coarse PM typically generates more • OH normalized by PM mass. • OH production by SJV PM is reduced on average by (97 ± 6) % when the transition metal chelator desferoxamine (DSF) is added to the extraction solution, indicating a dominant role of transition metals. By measuring calibration curves of • OH generation from copper and iron, and quantifying copper and iron concentrations in our particle extracts, we find that PBS-soluble copper is primarily responsible for • OH production by the SJV PM, while iron often makes a significant contribution. Extrapolating our results to expected burdens of PM-derived • OH in human lung lining fluid suggests that typical daily PM exposures in the San Joaquin Valley are unlikely to result in a high amount of pulmonary • OH, although high PM events could produce much higher levels of • OH, which might lead to cytotoxicity.

Description: Singular vector decomposition for sensitivity analyses of tropospheric chemical scenarios Atmospheric Chemistry and Physics Discussions, 11, 16745-16799, 2011 Author(s): N. Goris and H. Elbern Observations of the chemical state of the atmosphere typically provide only sparse snapshots of the state of the system due to their insufficient temporal and spatial density. Therefore the measurement configurations need to be optimised to get a best possible state estimate. One possibility to optimise the state estimate is provided by observation targeting of sensitive system states, to identify measurement configurations of best value for forecast improvements. In recent years, numerical weather prediction adapted singular vector analysis with respect to initial values as a novel method to identify sensitive states. In the present work, this technique is transferred from meteorological to chemical forecast. Besides initial values, emissions are investigated as controlling variables. More precisely uncertainties in the amplitude of the diurnal profile of emissions are analysed, yielding emission factors as target variables. Singular vector analysis is extended to allow for projected target variables not only at final time but also at initial time. Further, special operators are introduced, which consider the combined influence of groups of chemical species. As a preparation for targeted observation calculations, the concept of adaptive observations is studied with a chemistry box model. For a set of six different scenarios, the VOC versus NO x limitation of the ozone formation is investigated. Results reveal, that the singular vectors are strongly dependent on start time and length of the simulation. As expected, singular vectors with initial values as target variables tend to be more sensitive to initial values, while emission factors as target variables are more sensitive to simulation length. Further, the particular importance of chemical compounds differs strongly between absolute and relative error growth.

Description: Chemical characteristics of inorganic ammonium salts in PM 2.5 $ in the atmosphere of Beijing (China) Atmospheric Chemistry and Physics Discussions, 11, 17127-17176, 2011 Author(s): A. Ianniello, F. Spataro, G. Esposito, I. Allegrini, M. Hu, and T. Zhu The atmospheric concentrations of gaseous HNO 3 , HCl and NH 3 and their relative salts have been measured during two field campaigns in the winter and in the summer of 2007 at Beijing (China), as part of CAREBEIJING (Campaigns of Air Quality Research in Beijing and Surrounding Region). In this study, annular denuder technique was used with integration times of 2 and 24 h to collect inorganic and soluble PM 2.5 without interferences from gas-particle and particle-particle interactions. The results were discussed from the standpoint of temporal and diurnal variations and meteorological effects. Fine particulate Cl − , NH 4 + and SO 4 2− exhibited distinct temporal variations, while fine particulate NO 3 − did not show much variation with respect to season. Daily mean concentrations of fine particulate NH 4 + and SO 4 2− were higher during summer (12.30 μg m −3 and 18.24 μg m −3 , respectively) than during winter (6.51 μg m −3 and 7.50 μg m −3 , respectively). Instead, daily mean concentrations of fine particulate Cl − were higher during winter (2.94 μg m −3 ) than during summer (0.79 μg m −3 ), while fine particulate NO 3 − showed similar variations both in winter (8.38 μg m −3 ) and in summer (9.62 μg m −3 ) periods. However, the presence of large amounts of fine particulate NO 3 − even in summer are due to higher local and regional concentrations of NH 3 in the atmosphere available to neutralize H 2 SO 4 and HNO 3 , which is consistent with the observation that the measured particulate species were neutralized. Indeed, the composition of fine particulate matter indicated the domination of (NH 4 ) 2 SO 4 during winter and summer periods. In addition, the high relative humidity conditions in summer period seemed to dissolve a significant fraction of HNO 3 and NH 3 enhancing fine particulate NO 3 − and NH 4 + in the atmosphere. All measured particulate species showed diurnal similar patterns during the winter and summer periods with higher peaks in the early morning, especially in summer, when humid and stable atmospheric conditions occurred. These diurnal variations were affected nearly by wind direction suggesting regional and local source influences. Indeed, the fine particulate species were also correlated with NO x and PM 2.5 , supporting the hypothesis that the traffic may be also an important source of secondary particles.

Description: The effects of atmospheric waves on the amounts of polar stratospheric clouds Atmospheric Chemistry and Physics Discussions, 11, 16967-17012, 2011 Author(s): M. Kohma and K. Sato A quantitative analysis on the relationship between atmospheric waves and polar stratospheric clouds (PSCs) in the 2008 austral winter and the 2007/2008 boreal winter is made using CALIPSO, COSMIC and Aura MLS observation data and reanalysis data. A longitude-time section of the frequency of PSC occurrence in the Southern Hemisphere indicates that PSC frequency is not regionally uniform and that high PSC frequency regions propagate eastward at different speeds from the background zonal wind. These features suggest a significant influence of atmospheric waves on PSC behavior. Next, three temperature thresholds for PSC existence are calculated using HNO 3 and H 2 O mixing ratios. Among the three, the T STS (a threshold for super cooled ternary solution)-based estimates of PSC frequency accord best with the observations in terms of the amount, spatial and temporal variation, in particular for the latitude range of 55° S–70° S in the Southern Hemisphere and for 55° N–85° N in the Northern Hemisphere. Moreover, the effects of planetary waves, synoptic-scale waves and gravity waves on PSC areal extent are separately examined using the T STS -based PSC estimates. The latitude range of 55° S–70° S is analyzed because the T STS -based estimates are not consistent with observations at higher latitudes ( 〈 75° S) above 18 km, and PSCs in lower latitudes are more important to the ozone depletion because of the earlier arrival of solar radiation in spring. It is shown that nearly 100 % of PSCs between 55° S and 70° S at altitudes of 16–24 km are formed by temperature modulation, which is influenced by planetary waves during winter. Although the effects of synoptic-scale waves on PSCs are limited, around an altitude of 12 km more than 60 % of the total PSC areal extent is formed by synoptic-scale waves. The effects of gravity waves on PSC areal extent are not large in the latitude range of 55° S–70° S. However, at higher latitudes, gravity waves act to increase PSC areal extent at an altitude of 15 km by about 30 % in September. Similar analyses are performed for the Northern Hemisphere. It is shown that almost all PSCs observed in the Northern Hemisphere are attributable to low temperature anomalies associated with planetary waves.

Description: Origins and composition of fine atmospheric carbonaceous aerosol in the Sierra Nevada Mountains, California Atmospheric Chemistry and Physics Discussions, 11, 17071-17125, 2011 Author(s): D. R. Worton, A. H. Goldstein, D. K. Farmer, K. S. Docherty, J. L. Jimenez, J. B. Gilman, W. C. Kuster, J. de Gouw, B. J. Williams, N. M. Kreisberg, S. V. Hering, G. Bench, M. McKay, K. Kristensen, M. Glasius, J. D. Surratt, and J. H. Seinfeld In this paper we report chemically resolved measurements of organic aerosol (OA) and related tracers during the Biosphere Effects on Aerosols and Photochemistry Experiment (BEARPEX) at the Blodgett Forest Research Station, California. OA contributed the majority of the mass to the fine atmospheric particles and was predominately oxygenated (OOA). The highest concentrations of OA were during sporadic wildfire influence when aged plumes were impacting the site. In situ measurements of particle phase molecular markers were dominated by secondary compounds and could be categorized into three factors or sources: (1) aged biomass burning emissions and oxidized urban emissions, (2) oxidation products of temperature-driven local biogenic emissions and (3) local light-driven emissions and oxidation products. There were multiple biogenic components that contributed to OA at this site whose contributions varied diurnally, seasonally and in response to changing meteorological conditions, e.g., temperature and precipitation events. Concentrations of isoprene oxidation products were larger when temperatures were higher due to more substantial emissions of isoprene and enhanced photochemistry. Methyl chavicol oxidation contributed similarly to OA during both identified meteorological periods. In contrast, the abundances of monoterpene oxidation products in the particle phase were greater during cooler conditions, even though emissions of the precursors were lower. Following the first precipitation event of the fall the abundances of the monoterpene oxidation products increased dramatically, although the mechanism is not known. OA was correlated with the anthropogenic tracers 2-propyl nitrate and carbon monoxide (CO), consistent with previous observations, while being comprised of mostly non-fossil carbon (〉75 %). The correlation between OA and an anthropogenic tracer does not necessarily identify the source of the carbon as being anthropogenic but instead suggests a coupling between the anthropogenic and biogenic components in the air mass that might be related to the source of the oxidant and/or the aerosol sulfate. Observations of organosulfates of isoprene and α-pinene provided evidence for the likely importance of aerosol sulfate in spite of neutralized aerosol. This is in contrast to laboratory studies where strongly acidic seed aerosols were needed in order to form these compounds. These compounds together represented only a minor fraction ( 〈 1 %) of the total OA mass and suggest that other mechanisms, e.g., NO x enhancement of oxidant levels, are more likely to be responsible for the majority of the anthropogenic enhancement of biogenic secondary organic aerosol observed at this site.

Description: Ice nucleation properties of fine ash particles from the Eyjafjallajökull eruption in April 2010 Atmospheric Chemistry and Physics Discussions, 11, 17665-17698, 2011 Author(s): I. Steinke, O. Möhler, A. Kiselev, M. Niemand, H. Saathoff, M. Schnaiter, J. Skrotzki, C. Hoose, and T. Leisner During the eruption of the Eyjafjallajökull volcano in the south of Iceland in April/May 2010, about 40 Tg of ash mass were emitted into the atmosphere. However, it was unclear whether volcanic ash particles with d 〈 10 μm facilitate the glaciation of clouds. Thus, ice nucleation properties of volcanic ash particles were investigated in AIDA (Aerosol Interaction and Dynamics in the Atmosphere) cloud chamber experiments simulating atmospherically relevant conditions. The ash sample that was used for our experiments had been collected at a distance of 58 km from the Eyjafjallajökull during the eruption period in April 2010. The temperature range covered by our ice nucleation experiments extended from 219 to 264 K, and both ice nucleation via immersion freezing and deposition nucleation could be observed. Immersion freezing was first observed at 252 K, whereas the deposition nucleation onset lay at 242 K and RH ice = 126 %. About 0.1 % of the volcanic ash particles were active as immersion freezing nuclei at a temperature of 249 K. For deposition nucleation, an ice fraction of 0.1 % was observed at around 233 K and RH ice = 116 %. Taking ice-active surface site densities as a measure for the ice nucleation efficiency, volcanic ash particles are similarly efficient ice nuclei in immersion freezing mode ( n s, imm ~ 10 9 m −2 at 247 K) compared to certain mineral dusts. For deposition nucleation, the observed ice-active surface site densities n s, dep were found to be 10 11 m −2 at 224 K and RH ice = 116 %. Thus, volcanic ash particles initiate deposition nucleation more efficiently than Asian and Saharan dust but appear to be poorer ice nuclei than ATD particles. Based on the experimental data, we have derived ice-active surface site densities as a function of temperature for immersion freezing and of relative humidity over ice and temperature for deposition nucleation.

Description: Primary versus secondary contributions to particle number concentrations in the European boundary layer Atmospheric Chemistry and Physics Discussions, 11, 18249-18318, 2011 Author(s): C. L. Reddington, K. S. Carslaw, D. V. Spracklen, M. G. Frontoso, L. Collins, J. Merikanto, A. Minikin, T. Hamburger, H. Coe, M. Kulmala, P. Aalto, H. Flentje, C. Plass-Dülmer, W. Birmili, A. Wiedensohler, B. Wehner, T. Tuch, A. Sonntag, C. D. O'Dowd, S. G. Jennings, R. Dupuy, U. Baltensperger, E. Weingartner, H.-C. Hansson, P. Tunved, P. Laj, K. Sellegri, J. Boulon, J.-P. Putaud, C. Gruening, E. Swietlicki, P. Roldin, J. S. Henzing, M. Moerman, N. Mihalopoulos, G. Kouvarakis, V. Ždímal, N. Zíková, A. Marinoni, P. Bonasoni, and R. Duchi It is important to understand the relative contribution of primary and secondary particles to regional and global aerosol so that models can attribute aerosol radiative forcing to different sources. In large-scale models, there is considerable uncertainty associated with treatments of particle formation (nucleation) in the boundary layer (BL) and in the size distribution of emitted primary particles, leading to uncertainties in predicted cloud condensation nuclei (CCN) concentrations. Here we quantify how primary particle emissions and secondary particle formation influence size-resolved particle number concentrations in the BL using a global aerosol microphysics model and observations made during the May 2008 campaign of the European Integrated Project on Aerosol Cloud Climate Air Quality Interactions (EUCAARI). Observations are available from the DLR Falcon 20 aircraft and from 15 ground sites of the European Supersites for Atmospheric Aerosol Research (EUSAAR) and the German Ultrafine Aerosol Network (GUAN). Measurements include total and non-volatile particle number concentrations and the particle size distribution between ~3 nm and ~1 μm. We tested four different parameterisations for BL nucleation and two assumptions for the emission size distribution of anthropogenic and wildfire carbonaceous particles. When we emit small carbonaceous particles (recommended by the Aerosol Intercomparison project, AEROCOM), the spatial distributions of campaign-mean number concentrations 〉50 nm ( N 50 ) and 〉100 nm ( N 100 ) dry diameter were well captured by the model ( R 2 ~0.9) and the normalised mean bias (NMB) was also small (−5 % for N 50 and 12 % for N 100 ). Emission of larger particles, which we consider to be more realistic for global models, results in equally good correlation but larger bias ( R 2 ~0.8, NMB = −51 % and −21 %), which could be partly but not entirely compensated by BL nucleation. The model also predicts the particle concentration frequency distribution fairly well, with an overlap of modelled and observed N 50 hourly histograms of ~60 % across all sites. However, the model-observation temporal correlation on an hourly time scale is poor ( R 2 ≤0.1) for this period. These comparisons show that caution is required when drawing conclusions about model realism from time or site-averaged data or frequency histograms when deterministic behaviour is not captured at individual sites. From this 1-month intensive European dataset it is not possible to determine a reliable estimate of the fraction of CCN-sized particles from primary and secondary sources, although the size of primary emitted particles is shown to be a major source of uncertainty.

Description: Mechanisms leading to oligomers and SOA through aqueous photooxidation: insights from OH radical oxidation of acetic acid Atmospheric Chemistry and Physics Discussions, 11, 18319-18347, 2011 Author(s): Y. Tan, Y. B. Lim, K. E. Altieri, S. P. Seitzinger, and B. J. Turpin Previous experiments have demonstrated that the aqueous OH radical oxidation of methylglyoxal produces low volatility products including oxalate and oligomers. These products are found predominantly in the particle phase in the atmosphere, suggesting that methylglyoxal is a precursor of secondary organic aerosol (SOA). Acetic acid is an important intermediate in aqueous methylglyoxal oxidation and a ubiquitous product of gas phase photochemistry, making it a potential "aqueous" SOA precursor in its own right. Altieri et al. (2008) proposed that acetic acid was the precursor of oligoesters observed in methylglyoxal oxidation. However, the fate of acetic acid upon aqueous-phase oxidation is not well understood. In this research, acetic acid at concentrations relevant to atmospheric waters (20 μM–10 mM) was oxidized by OH radical. Products were analyzed by ion chromatography (IC), electrospray ionization mass spectrometry (ESI-MS), and IC-ESI-MS. The formation of glyoxylic, glycolic, and oxalic acids were observed. In contrast to methylglyoxal oxidation, succinic acid and oligomers were not detected. Using results from these and methylglyoxal + OH radical experiments, radical mechanisms responsible for oligomer formation from methylglyoxal oxidation in clouds and wet aerosols are proposed. The importance of acetic acid/acetate as an SOA precursor is also discussed. We hypothesize that this and similar chemistry is central to the daytime formation of oligomers in wet aerosols.

Description: Amine reactivity with charged sulfuric acid clusters Atmospheric Chemistry and Physics Discussions, 11, 14637-14658, 2011 Author(s): B. R. Bzdek, D. P. Ridge, and M. V. Johnston The distribution of ionic species produced by electrospray of an ammonium sulfate solution in both positive and negative polarities is examined using Fourier transform ion cyclotron resonance mass spectrometry (FTICR-MS). Positively-charged ammonium bisulfate cluster composition differs significantly from negatively-charged cluster composition. For positively-charged clusters all sulfuric acid is neutralized to bisulfate, whereas for negatively-charged clusters the degree of sulfuric acid neutralization is cluster size-dependent. With increasing cluster size (and, therefore, a decreasing role of charge), both positively- and negatively-charged cluster compositions converge toward ammonium bisulfate. The reactivity of negatively-charged sulfuric acid-ammonia clusters with dimethylamine and ammonia are also investigated by FTICR-MS. Two series of negatively-charged clusters are investigated: [(HSO 4 )(H 2 SO 4 ) x ] − and [(NH 4 ) x (HSO 4 ) x +1 (H 2 SO 4 ) 3 ] − . Dimethylamine substitution for ammonia in [(NH 4 ) x (HSO 4 ) x +1 (H 2 SO 4 ) 3 ] − clusters is nearly collision-limited, and subsequent addition of dimethylamine to neutralize H 2 SO 4 is within one order of magnitude of the substitution rate. Dimethylamine addition to [(HSO 4 )(H 2 SO 4 ) x ] − clusters is either not observed or very slow. The results of this study indicate that amine chemistry will be evident and important only in large ambient negative ions (〉 m/z 400), whereas amine chemistry may be evident in small ambient positive ions. Addition of ammonia to unneutralized clusters occurs at a rate that is ~2–3 orders of magnitude slower than incorporation of dimethylamine either by substitution or addition. Therefore, in locations where amine levels are within a few orders of magnitude of ammonia levels, amine chemistry may compete favorably with ammonia chemistry.

Description: Seasonal variation in vertical volatile compounds air concentrations within a remote hemiboreal mixed forest Atmospheric Chemistry and Physics Discussions, 11, 14607-14635, 2011 Author(s): S. M. Noe, K. Hüve, Ü. Niinemets, and L. Copolovici The vertical distribution of ambient biogenic volatile organic compounds (BVOC) concentrations within a hemiboreal forest canopy was investigated over a period of one year. Variability in temporal and spatial isoprene concentrations can be mainly explained by biogenic emissions from deciduous trees, ranging from 0.1 to 7.5 μg m −3 . Monoterpene concentrations exceeded isoprene largely and ranged from 0.01 to 140 μg m −3 and during winter time anthropogenic contributions are likely. Variation in monoterpene concentrations found to be largest right above the ground and the vertical profile suggest a weak mixing leading to terpene accumulation in the lower canopy. Exceptionally high values were recorded during a heat wave in July 2010 with very high midday temperatures above 30 °C for several weeks. During summer months, monoterpene exceeded isoprene concentrations 6-fold and during winter 12-fold. The relative contribution of diverse monoterpene species to the ambient concentrations revealed a dominance of α-pinene in the lower and of limonene in the upper part of the canopy, both accounting for up to 70 % of the total monoterpene concentration during summer months. The main contributing monoterpene during wintertime was Δ 3 -carene accounting for 60 % of total monoterpene concentration in January. Possible biogenic monoterpene sources beside the foliage are the leaf litter, the soil and also resins exuding from stems. In comparison, the hemiboreal mixed forest canopy showed similar isoprene but higher monoterpene concentrations than the boreal forest and lower isoprene but substantially higher monoterpene concentrations than the temperate mixed forest canopies. These results have major implications for simulating air chemistry and secondary organic aerosol formation within and above hemiboreal forest canopies.

Description: The HNO 3 forming branch of the HO 2 + NO reaction: pre-industrial-to-present trends in atmospheric species and radiative forcings Atmospheric Chemistry and Physics Discussions, 11, 14801-14835, 2011 Author(s): O. A. Søvde, C. R. Hoyle, G. Myhre, and I. S. A. Isaksen Recent laboratory measurements have shown the existence of a HNO 3 forming branch of the HO 2 + NO reaction. This reaction is the main source of atmospheric O 3 , through the subsequent photolysis of NO 2 , as well as being a major source of OH. The branching of the reaction to HNO 3 reduces the formation of these species significantly, affecting O 3 abundances, climate and the oxidation capacity of the troposphere. The Oslo CTM2, a three-dimensional chemistry transport model, is used to calculate atmospheric composition and trends with and without the new reaction branch. Results for the present day atmosphere, when both temperature and pressure effects on the branching ratio are accounted for, show an increase of the global, annual mean methane lifetime by 10.9 %, resulting from a 14.1 % reduction in the global, annual mean OH concentration. Comparisons with measurements show that including the new branch improves the modelled O 3 , but that it is not possible to conclude whether the NO y distribution improves. We model an approximately 11 % reduction in the tropical tropospheric O 3 increase since pre-industrial times, as well as an 8 % decrease in the trend of OH concentration, when the new branch is accounted for. The radiative forcing due to changes in O 3 over the industrial era was calculated as 0.33 W m −2 , reducing to 0.26 W m −2 with the new reaction branch. These results are significant, and it is important that this reaction branching is confirmed by other laboratory groups.

Description: Aerosol composition and sources in the Central Arctic Ocean during ASCOS Atmospheric Chemistry and Physics Discussions, 11, 14837-14881, 2011 Author(s): R. Y.-W. Chang, C. Leck, M. Graus, M. Müller, J. Paatero, J. F. Burkhart, A. Stohl, L. H. Orr, K. Hayden, S.-M. Li, A. Hansel, M. Tjernström, W. R. Leaitch, and J. P. D. Abbatt Measurements of submicron aerosol chemical composition were made in the Central Arctic Ocean from 5 August to 8 September 2008 as a part of the Arctic Summer Cloud Ocean Study (ASCOS) using an aerosol mass spectrometer (AMS). The median levels of sulphate and organics for the entire study were 0.042 and 0.046 μg m −3 , respectively. Positive matrix factorisation was performed on the entire mass spectral time series and this enabled marine biogenic and continental sources of particles to be separated. These factors accounted for 33 % and 36 % of the sampled ambient aerosol mass, respectively, and they were both predominantly composed of sulphate, with 47 % of the sulphate apportioned to marine biogenic sources and 48 % to continental sources, by mass. Within the marine biogenic factor, the ratio of methane sulphonate to sulphate was 0.25 ± 0.02, consistent with values reported in the literature. The organic component of the continental factor was more oxidised than that of the marine biogenic factor, suggesting that it was more processed and had been present longer in the atmosphere than the organics in the marine biogenic factor. The remaining ambient aerosol mass was apportioned to an organic-rich factor that could have arisen from a combination of marine and continental sources.

Description: Black carbon aerosol mixing state, organic aerosols and aerosol optical properties over the UK Atmospheric Chemistry and Physics Discussions, 11, 14991-15027, 2011 Author(s): G. R. McMeeking, W. T. Morgan, M. Flynn, E. J. Highwood, K. Turnbull, J. Haywood, and H. Coe Black carbon (BC) aerosols absorb sunlight thereby leading to a positive radiative forcing and a warming of climate and can also impact human health through their impact on the respiratory system. The state of mixing of BC with other aerosol species, particularly the degree of internal/external mixing, has been highlighted as a major uncertainty in assessing its radiative forcing and hence its climate impact, but few in situ observations of mixing state exist. We present airborne single particle soot photometer (SP2) measurements of refractory BC (rBC) mass concentrations and mixing state coupled with aerosol composition and optical properties measured in urban plumes and regional pollution over the UK. All data were obtained using instrumentation flown on the UK's BAe-146-301 large Atmospheric Research Aircraft (ARA) operated by the Facility for Airborne Atmospheric Measurements (FAAM). We measured sub-micron aerosol composition using an aerosol mass spectrometer (AMS) and used positive matrix factorization to separate hydrocarbon-like (HOA) and oxygenated organic aerosols (OOA). We found a higher number fraction of thickly coated rBC particles in air masses with large OOA relative to HOA, higher ozone-to-nitrogen oxides (NO x ) ratios and large concentrations of total sub-micron aerosol mass relative to rBC mass concentrations. The more ozone- and OOA-rich air masses were associated with transport from continental Europe, while plumes from UK cities had higher HOA and NO x and fewer thickly coated rBC particles. We did not observe any significant change in the rBC mass absorption efficiency calculated from rBC mass and light absorption coefficients measured by a particle soot absorption photometer despite observing significant changes in aerosol composition and rBC mixing state. The contributions of light scattering and absorption to total extinction (quantified by the single scattering albedo; SSA) did change for different air masses, with lower SSA observed in urban plumes compared to regional aerosol (0.85 versus 0.9–0.95). We attribute these differences to the presence of relatively rapidly formed secondary aerosol, primarily OOA and ammonium nitrate, which must be taken into account in radiative forcing calculations.

Description: Technical Note: In-situ derivatization thermal desorption GC-TOFMS for direct analysis of particle-bound non-polar and polar organic species Atmospheric Chemistry and Physics Discussions, 11, 15255-15295, 2011 Author(s): J. Orasche, J. Schnelle-Kreis, G. Abbaszade, and R. Zimmermann An in-situ derivatization thermal desorption method followed by gas chromatography and time-of-flight mass spectrometry (IDTD-GC-TOFMS) was developed for determination of polar organic compounds. Hydroxyl and carboxyl groups of compounds such as anhydrous sugars, alcohols and phenols, fatty acids and resin acids are targets of the derivatization procedure. Derivatization is based on silylation with N-Methyl-N-trimethylsilyl-trifluoroacetamide (MSTFA) during the step of thermal desorption. The high temperature of 300 °C during desorption is utilized for the in-situ derivatization on the collection substrate (quartz fibre filters) accelerating the reaction rate. Thereby, the analysis time is as short as without derivatization. At first the filter surface is dampened with derivatization reagent before insertion of the sample into the thermal desorption unit. To ensure ongoing derivatization during thermal desorption the carrier gas is saturated with MSTFA until the desorption procedure is finished. The method introduced here was compared with direct thermal desorption gas chromatography time-of-flight-mass spectrometry (DTD-GC-TOFMS) and with solvent extraction (SE) procedures followed by gas chromatography and mass spectrometry. Comparisons were carried out with field samples originating from ambient aerosol collected on quartz fibre filters. Moreover, the methods have been applied on NIST Standard Reference Material Urban Dust (SRM 1649a).

Description: Impacts of changes in land use and land cover on atmospheric chemistry and air quality over the 21st century Atmospheric Chemistry and Physics Discussions, 11, 15469-15495, 2011 Author(s): S. Wu, L. J. Mickley, J. O. Kaplan, and D. J. Jacob The effects of future land use and land cover change on the chemical composition of the atmosphere and air quality are largely unknown. To investigate the potential effects associated with future changes in vegetation driven by atmospheric CO 2 concentrations, climate, and anthropogenic land use over the 21st century, we performed a series of model experiments combining a general circulation model with a dynamic global vegetation model and an atmospheric chemical-transport model. Our results indicate that climate- and CO 2 -induced changes in vegetation composition and density could lead to decreases in summer afternoon surface ozone of up to 10 ppb over large areas of the northern mid-latitudes. This is largely driven by the substantial increases in ozone dry deposition associated with changes in the composition of temperate and boreal forests where conifer forests are replaced by those dominated by broadleaf tree types, as well as a CO 2 -driven increase in vegetation density. Climate-driven vegetation changes over the period 2000–2100 lead to general increases in isoprene emissions, globally by 15 % in 2050 and 36 % in 2100. These increases in isoprene emissions result in decreases in surface ozone concentrations where the NO x levels are low, such as in remote tropical rainforests. However, over polluted regions, such as the northeastern United States, ozone concentrations are calculated to increase with higher isoprene emissions in the future. Increases in biogenic emissions also lead to higher concentrations of secondary organic aerosols, which increase globally by 10 % in 2050 and 20 % in 2100. Surface concentrations of secondary organic aerosols are calculated to increase by up to 1 μg m −3 for large areas in Eurasia. When we use a scenario of future anthropogenic land use change, we find less increase in global isoprene emissions due to replacement of higher-emitting forests by lower-emitting cropland. The global atmospheric burden of secondary organic aerosols changes little by 2100 when we account for future land use change, but both secondary organic aerosols and ozone show large regional changes at the surface.

Description: A comprehensive numerical study of aerosol-cloud-precipitation interactions in marine stratocumulus Atmospheric Chemistry and Physics Discussions, 11, 15497-15550, 2011 Author(s): Y.-C. Chen, L. Xue, Z. J. Lebo, H. Wang, R. M. Rasmussen, and J. H. Seinfeld Three-dimensional large-eddy simulations (LES) with detailed bin-resolved microphysics are performed to explore the diurnal variation of marine stratocumulus (MSc) clouds under clean and polluted conditions. The sensitivity of the aerosol-cloud-precipitation interactions to variation of sea surface temperature, free tropospheric humidity, large-scale divergence rate, and wind speed is assessed. The comprehensive set of simulations corroborates previous studies that (1) with moderate/heavy drizzle, an increase in aerosol leads to an increase in cloud thickness; and (2) with non/light drizzle, an increase in aerosol results in a thinner cloud, due to the pronounced effect on entrainment. It is shown that for higher SST, stronger large-scale divergence, drier free troposphere, or lower wind speed, the cloud thins and precipitation decreases. The sign and magnitude of the Twomey effect, droplet dispersion effect, cloud thickness effect, and overall cloud optical depth susceptibility to aerosol perturbations are evaluated by LES experiments and compared with analytical formulations. The Twomey effect emerges as dominant in total cloud susceptibility to aerosol perturbations. The dispersion effect, that of aerosol perturbations on the cloud droplet size spectrum, is positive (i.e., increase in aerosol leads to spectral narrowing) and accounts for 3 % to 10 % of the total cloud susceptibility at nighttime, with the largest influence in heavier drizzling clouds. The cloud thickness effect is negative (i.e., increase in aerosol leads to thinner cloud) for non/light drizzling cloud and positive for moderate/heavy drizzling clouds; the cloud thickness effect contributes 5 % to 22 % of the nighttime cloud susceptibility. The range of magnitude for each effect is more variable in the daytime owing to cloud thinning and decoupling. Overall, the cloud susceptibility is ~0.28 to 0.53 at night; an increase in aerosol concentration enhances cloud optical depth, especially with heavier precipitation and in a more pristine environment. The good agreement between LES experiments and analytical formulations suggests that the latter may be useful in evaluations of cloud susceptibility. The ratio of the magnitude of the cloud thickness effect to that of the Twomey effect depends on cloud base height and cloud thickness in unperturbed (clean) clouds.

Description: Analysis of ΔO 2 /ΔCO 2 ratios for the pollution events observed at Hateruma Island, Japan Atmospheric Chemistry and Physics Discussions, 11, 15631-15657, 2011 Author(s): C. Minejima, M. Kubo, Y. Tohjima, H. Yamagishi, Y. Koyama, S. Maksyutov, K. Kita, and H. Mukai In-situ observations of atmospheric CO 2 and O 2 concentrations at Hateruma Island (HAT, 24° N, 124° E) often show synoptic scale pollution events when air masses are transported from East Asian source regions. We calculate the regression slopes (-ΔO 2 /ΔCO 2 molar ratios) of the correlation plots between O 2 and CO 2 for selected pollution events observed between October 2006 and December 2008. The observed -ΔO 2 /ΔCO 2 ratios vary from 1.0 to 1.7. Categorizing the air mass origins for the pollution events by using back trajectory analysis, we find that there is a significant difference in the average -ΔO 2 /ΔCO 2 ratios between events from China (1.14±0.12, n = 25) and Japan/Korea (1.37±0.15, n = 16). These values are comparable to the -O 2 :CO 2 molar exchange ratios, which are estimated from the national fossil fuel inventories from CDIAC. Simulations using a particle dispersion model reveal that the pollution events at HAT are predominantly CO 2 emissions from the combustion of fossil fuels in East Asian countries, which is consistent with the above observational results. Although the average value of the model-predicted -ΔO 2 /ΔCO 2 ratios for Japan/Korea origin is underestimated in comparison with the observation, that for China origin agree well with the observation. The sensitivity experiment suggests that the -ΔO 2 /ΔCO 2 ratio at HAT reflects about 90% of the change in the -O 2 :CO 2 exchange ratio for the fossil carbon emissions from China.

Description: Cloud-system resolving model simulations of aerosol indirect effects on tropical deep convection and its thermodynamic environment Atmospheric Chemistry and Physics Discussions, 11, 15573-15629, 2011 Author(s): H. Morrison and W. W. Grabowski This paper presents results from 240-member ensemble simulations of aerosol indirect effects on tropical deep convection and its thermodynamic environment. Simulations using a two-dimensional cloud system-resolving model are run with pristine, polluted, or highly polluted aerosol conditions and large-scale forcing from a 6-day period of active monsoon conditions during the 2006 Tropical Warm Pool – International Cloud Experiment (TWP-ICE). Domain-mean surface precipitation is insensitive to aerosols primarily because the large-scale forcing is prescribed and dominates the water and static energy budgets. The spread of the top-of-atmosphere (TOA) shortwave and longwave radiative fluxes among different ensemble members for the same aerosol loading is surprisingly large, exceeding 25 W m −2 even when averaged over the 6-day period. This variability is caused by random fluctuations in the strength and timing of individual deep convective events. The ensemble approach demonstrates a small weakening of convection averaged over the 6-day period in the polluted simulations compared to pristine. Despite this weakening, the cloud top heights and anvil ice mixing ratios are higher in polluted conditions. This occurs because of the larger concentrations of cloud droplets that freeze, leading directly to higher ice particle concentrations, smaller ice particle sizes, and smaller fall velocities compared to simulations with pristine aerosols. Weaker convection in polluted conditions is a direct result of the changes in anvil ice characteristics and subsequent upper-tropospheric radiative heating and weaker tropospheric destabilization. Such a conclusion offers a different interpretation of recent satellite observations of tropical deep convection in pristine and polluted environments compared to the hypothesis of aerosol-induced convective invigoration. Sensitivity tests using the ensemble approach with modified microphysical parameters or domain configuration (horizontal gridlength, domain size) produce results that are similar to baseline, although there are quantitative differences in estimates of aerosol impacts on TOA radiative fluxes.

Description: Coastal precipitation formation and discharge based on TRMM observations Atmospheric Chemistry and Physics Discussions, 11, 15659-15696, 2011 Author(s): R. H. Heiblum, I. Koren, and O. Altaratz The interaction between breezes and synoptic gradient winds creates persistent convergence zones nearby coastlines. The low level convergence of moist air promotes the dynamical and microphysical processes responsible for the formation of clouds and precipitation. Our work focuses on the winter seasons of 1998–2011 in the Eastern Mediterrenean. During the winter the Mediterrenean sea is usually warmer than the adjacent land, resulting in frequent occurence of land breeze that opposes the commom synoptic winds. Using rain-rate vertical profiles from the Tropical Rainfall Measurement Mission (TRMM) satellite, we examined the spatial and temporal distribution of average hydrometeor mass in clouds as a funtion of the distance from coastlines. Results show that coastalines in the Eastern Mediterrenean are indeed favored areas for precipitation formation and discharge. The intra-seasonal and diurnal changes in the distribution of hydrometeor mass indicate that the land breeze is most likely the main responsible mechanism behind our results.

Description: TES ammonia retrieval strategy and global observations of the spatial and seasonal variability of ammonia Atmospheric Chemistry and Physics Discussions, 11, 16023-16074, 2011 Author(s): M. W. Shephard, K. E. Cady-Pereira, M. Luo, D. K. Henze, R. W. Pinder, J. T. Walker, C. P. Rinsland, J. O. Bash, L. Zhu, V. H. Payne, and L. Clarisse Presently only limited sets of tropospheric ammonia (NH 3 ) measurements in the Earth's atmosphere have been reported from satellite and surface station measurements, despite the well-documented negative impact of NH 3 on the environment and human health. Presented here is a detailed description of the satellite retrieval strategy and analysis for the Tropospheric Emission Spectrometer (TES) using simulations and measurements. These results show that: (i) the level of detectability for a representative boundary layer TES NH 3 mixing ratio value is ~0.3 ppbv, which typically corresponds to a profile that contains a maximum level value of ~1 ppbv; (ii) TES NH 3 retrievals provide at most one degree of freedom for signal (DOFS), with peak sensitivity between 700 and 900 mbar; (iii) TES NH 3 retrievals show significant spatial and seasonal variability of NH 3 globally; (iv) Initial comparisons of TES observations with GEOS-CHEM estimates show TES values being higher overall. Important differences and similarities between modeled and observed seasonal and spatial trends are noted, with discrepancies indicating areas where the timing and magnitude of modeled NH 3 emissions from agricultural sources, and to lesser extent biomass burning sources, need further study.

Description: Gas transport in firn: multiple-tracer characterisation and model intercomparison for NEEM, Northern Greenland Atmospheric Chemistry and Physics Discussions, 11, 15975-16021, 2011 Author(s): C. Buizert, P. Martinerie, V. V. Petrenko, J. P. Severinghaus, C. M. Trudinger, E. Witrant, J. L. Rosen, A. J. Orsi, M. Rubino, D. M. Etheridge, L. P. Steele, C. Hogan, J. C. Laube, W. T. Sturges, V. A. Levchenko, A. M. Smith, I. Levin, T. J. Conway, E. J. Dlugokencky, P. M. Lang, K. Kawamura, T. M. Jenk, J. W. C. White, T. Sowers, J. Schwander, and T. Blunier Compacted snow (firn) preserves a continuous record of atmospheric composition up to a century back in time. Firn air transport modeling is essential for interpretation of firn gas records. Each site needs to be characterised individually through a tuning procedure, in which the effective diffusivity at each depth is adjusted to optimise the agreement between modeled and measured mixing ratios of a selected reference gas (usually CO 2 ). We present the characterisation of the NEEM site, Northern Greenland (77.45° N 51.06° W), where an ensemble of ten reference tracers is used to constrain the diffusivity reconstruction. By analysing uncertainties in both data and the reference gas atmospheric histories, we can objectively assign weights to each of the gases used for the model tuning, and define a root mean square criterion that is minimised in the tuning. Each tracer constrains the firn profile differently through its unique atmospheric history and free air diffusivity, making our multiple-tracer characterisation method a clear improvement over the commonly used single-tracer tuning. Six firn air transport models are tuned to the NEEM site; all models successfully reproduce the data within a 1σ Gaussian distribution. The modern day Δage, i.e. the difference between gas age and ice age, is calculated to be 182 ± 8 yr. We find evidence that diffusivity does not vanish completely in the firn lock-in zone, as is commonly assumed. We further present the first intercomparison study of firn air models, where we introduce diagnostic scenarios designed to probe specific aspects of the model physics. Our results show that there are major differences in the way the models handle advective transport. Furthermore diffusive fractionation of isotopes in the firn is poorly constrained by the models, which has consequences for attempts to reconstruct the isotopic composition of trace gases back in time using firn air and ice core records.

Description: Isotope effects in N 2 O photolysis from first principles Atmospheric Chemistry and Physics Discussions, 11, 16075-16105, 2011 Author(s): J. A. Schmidt, M. S. Johnson, and R. Schinke For the first time, accurate potential energy surfaces allow N 2 O cross sections and isotopic fractionation spectra to be derived that are in agreement with available experimental data (without ad hoc shifting), extending knowledge to a much broader range of conditions. Absorption spectra of rare N- and O-isotopologues ( 15 N 14 N 16 O, 14 N 15 N 16 O, 15 N 2 16 O, 14 N 2 17 O and 14 N 2 18 O) calculated using wavepacket propagation are compared to the most abundant isotopologue ( 14 N 2 16 O). The fractionation constants as a function of wavelength and temperature are in excellent agreement with experimental data. The study shows that excitations from the 3rd excited bending state, (0,3,0), and the first combination band, (1,1,0), are important for explaining the isotope effect at wavelengths longer than 210 nm. Only a small amount of the mass independent oxygen isotope anomaly observed in atmospheric N 2 O samples can be explained as arising from photolysis.

Description: Measurements of aerosol charging states in Helsinki, Finland Atmospheric Chemistry and Physics Discussions, 11, 15875-15920, 2011 Author(s): S. Gagné, J. Leppä, T. Petäjä, M. J. McGrath, M. Vana, V.-M. Kerminen, L. Laakso, and M. Kulmala The charging state of aerosol populations was measured with an Ion-DMPS in Helsinki, Finland between December 2008 and February 2010. Based on the charging states, we calculated the ion-induced nucleation fraction to be around 0.8 % ± 0.9 %. We review the role of ion-induced nucleation and propose different explanations for a low ion-induced nucleation participation in urban areas. We present a new method to retrieve the average charging state for an event, and a given size. We also use a new theoretical framework that allows for different concentrations of small cluster ions for different polarities (polarity asymmetry). We extrapolate the ion-induced fraction using polarity symmetry and asymmetry. Finally, a method to calculate the growth rates from the variation of the charging state as a function of the particle diameter using polarity symmetry and asymmetry is presented and used on a selection of new particle formation events.

Description: The summertime Boreal forest field measurement intensive (HUMPPA-COPEC-2010): an overview of meteorological and chemical influences Atmospheric Chemistry and Physics Discussions, 11, 15921-15973, 2011 Author(s): J. Williams, J. Crowley, H. Fischer, H. Harder, M. Martinez, T. Petäjä, J. Rinne, J. Bäck, M. Boy, M. Dal Maso, J. Hakala, M. Kajos, P. Keronen, P. Rantala, J. Aalto, H. Aaltonen, J. Paatero, T. Vesala, H. Hakola, J. Levula, T. Pohja, F. Herrmann, J. Auld, E. Mesarchaki, W. Song, N. Yassaa, A. Nölscher, A. M. Johnson, T. Custer, V. Sinha, J. Thieser, N. Pouvesle, D. Taraborrelli, M. J. Tang, H. Bozem, Z. Hosaynali-Beygi, R. Axinte, R. Oswald, A. Novelli, D. Kubistin, K. Hens, U. Javed, K. Trawny, C. Breitenberger, P. J. Hidalgo, C. J. Ebben, F. M. Geiger, A. L. Corrigan, L. M. Russell, H. Ouwersloot, J. Vilà-Guerau de Arellano, L. Ganzeveld, A. Vogel, M. Beck, A. Bayerle, C. J. Kampf, M. Bertelmann, F. Köllner, T. Hoffmann, J. Valverde, D. González, M.-L. Riekkola, M. Kulmala, and J. Lelieveld This paper describes the background, instrumentation, goals, and the regional influences on the HUMPPA-COPEC intensive field measurement campaign, conducted at the Boreal forest research station SMEAR II (Station for Measuring Ecosystem-Atmosphere Relation) in Hyytiälä, Finland from 12 July–12 August 2010. The prevailing meteorological conditions during the campaign are examined and contrasted with those of the past six years. Back trajectory analyses show that meteorological conditions at the site were characterized by a higher proportion of southerly flow. As a result the summer of 2010 was anomalously warm and high in ozone making the campaign relevant for the analysis of possible future climates. A comprehensive land use analysis, provided on both 5 and 50 km scales, shows that the main vegetation types surrounding the site on both the regional and local scales are: coniferous forest (Scots pine and/or Norway spruce); mixed forest (Birch and conifers); and woodland scrub (e.g. Willows, Aspen); indicating that the campaign results can be taken as representative of the Boreal forest ecosystem. In addition to the influence of biogenic emissions, the measurement site was occasionally impacted by sources other than vegetation. Specific tracers have been used here to identify the time periods when such sources have impacted the site namely: biomass burning (acetonitrile and CO), urban anthropogenic pollution (pentane and SO 2 ) and the nearby Korkeakoski sawmill (enantiomeric ratio of chiral monoterpenes). None of these sources dominated the study period, allowing the Boreal forest summertime emissions to be assessed and contrasted with various other source signatures.

Description: Chemical ozone loss in Arctic and Antarctic polar winter/spring season derived from SCIAMACHY limb measurements 2002–2009 Atmospheric Chemistry and Physics Discussions, 11, 6555-6599, 2011 Author(s): T. Sonkaew, C. von Savigny, K.-U. Eichmann, M. Weber, A. Rozanov, H. Bovensmann, and J. P. Burrows Stratospheric ozone profiles are retrieved for the period 2002–2009 from SCIAMACHY measurements of limb-scattered solar radiation in the Hartley and Chappuis absorption bands of ozone. This data set is used to determine the chemical ozone loss in both the Arctic and Antarctic polar vortices using the vortex average method. The chemical ozone loss at isentropic levels between 450 K and 600 K is derived from the difference between observed ozone abundances and the ozone modelled considering diabatic cooling, but no chemical ozone loss. The results show chemical ozone losses of up to 20–40% between the beginning of January and the end of March inside the Arctic polar vortex. Strong inter-annual variability of the Arctic ozone loss is observed, with the cold winters 2004/2005 and 2006/2007 showing the largest chemical ozone losses. The ozone mass loss inside the polar vortex is also estimated. In the coldest Arctic winter 2004/2005 the total ozone mass loss is about 30 million tons inside the polar vortex between the 450 K and 600 K isentropic levels from the beginning of January until the end of March. The Antarctic vortex averaged ozone loss as well as the size of the polar vortex do not vary much from year to year. At the 475 K isentropic level ozone losses of 70–80% between mid-August and mid-November are observed every year inside the vortex, also in the anomalous year 2002. The total ozone mass loss inside the Antarctic polar vortex between the 450 K and 600 K isentropic levels is about 55–75 million tons for the period between mid-August and mid-November. Comparisons of the vertical variation of ozone loss derived from SCIAMACHY observations with several independent techniques for the Arctic winter 2004/2005 show very good agreement.

Description: Hygroscopic properties of the ambient aerosol in southern Sweden – a two year study Atmospheric Chemistry and Physics Discussions, 11, 6601-6650, 2011 Author(s): E. O. Fors, E. Swietlicki, B. Svenningsson, A. Kristensson, G. P. Frank, and M. Sporre The hygroscopic growth of the atmospheric aerosol is a critical parameter for quantifying the anthropogenic radiative forcing. Until now, there has been a lack of long term measurements due to limitations in instrumental techniques. In this work, for the first time the seasonal variation of the hygroscopic properties of a continental background aerosol has been described, based on more than two years of continuous measurements. In addition to this, the diurnal variation of the hygroscopic growth has been investigated, as well as the seasonal variation in CCN concentration. These physical properties of the aerosol have been measured with a Hygroscopic Tandem Differential Mobility Analyzer (H-TDMA), a Differential Mobility Particle Sizer (DMPS), and a Cloud Condensation Nuclei Counter (CCNC). The results show that smaller particles are generally less hygroscopic than larger ones, and that there is a clear difference in the hygroscopic properties between the Aitken and the accumulation mode. A seasonal cycle was found for all particle sizes. In general, the average hygroscopic growth is lower during wintertime, due to an increase in the relative abundance of less hygroscopic or hydrophobic particles. Monthly averages showed that the hygroscopic growth factors of the two dominating hygroscopic modes (one barely hygroscopic and one more hygroscopic) were relatively stable. The hygroscopic growth additionally showed a diurnal cycle, with higher growth factors during day time. CCN predictions based on H-TDMA data underpredicted the activated CCN concentration with 7% for 1% water supersaturation ( s ). The underprediction increases with decreasing $s$, most likely due to a combination of measurement and modeling uncertainties. It was found that although the aerosol is often externally mixed, recalculating to an internal mixture with respect to hygroscopicity did not change CCN parameterizations significantly.

Description: Microwave Limb Sounder observations of biomass-burning products from the Australian bush fires of February 2009 Atmospheric Chemistry and Physics Discussions, 11, 6531-6554, 2011 Author(s): H. C. Pumphrey, M. L. Santee, N. J. Livesey, M. J. Schwartz, and W. G. Read The large bush fires which occurred in southeast Australia in February 2009 were unusually destructive. However, they were also unusual in the amounts of various combustion products which were injected directly into the stratosphere. We report the observations by the Microwave Limb Sounder (MLS) instrument on the Aura satellite of some of these combustion products. The highest quality observations are of CO; these clearly show a large region of enhanced mixing ratios to the north of New Zealand which remains in that region for about ten days before drifting westwards and finally dissipating over the Atlantic about a month after the fire. Back trajectories run from the points where MLS observes enhanced CO pass close to the site of the fire. The MLS observations of CH 3 CN and HCN resemble those of CO except for their poorer vertical resolution and more limited vertical range. An apparent enhancement in ClO is also observed by MLS, but detailed analysis of the measured radiances reveals this feature to be a signature of CH 3 OH, which is not currently retrieved by the MLS data processing system. The fires of February 2009 are the only event of this type and magnitude in the 6-yr MLS record.

Description: Seasonal impact of natural and anthropogenic emissions on the highest glacier of the Eastern European Alps Atmospheric Chemistry and Physics Discussions, 11, 6493-6530, 2011 Author(s): J. Gabrieli, L. Carturan, P. Gabrielli, C. Turetta, N. Kehrwald, G. Cozzi, H. Staffler, R. Dinale, R. Seppi, G. dalla Fontana, L. Thompson, and C. Barbante In June 2009, we conducted the first extensive glaciological survey of Alto dell'Ortles, the uppermost glacier of Mt. Ortles which at 3905 meters above sea level (m a.s.l.) is the highest summit of the Eastern European Alps. We analyzed snow samples collected from a 4.5 m snow-pit at 3830 m a.s.l. Here, we present a comprehensive data set including a large suite of trace elements and ionic compounds that comprise the atmospheric depositions over the past few years. Trace element concentrations measured in snow samples are extremely low with mean concentrations at pg g −1 level. Only Al and Fe present median values of 1.8 and 3.3 ng g −1 , with maximum concentrations of 21 and 25 ng g −1 . The median EFc values for Be, Rb, Sr, Ba, U, Li, Al, Ca, Cr, Mn, Fe, Co, Ga and V are lower than 10 suggesting that these elements originated mainly from soil and mineral aerosol. EFc higher than 100 are reported for Zn (118), Ag (135), Bi (185), Sb (401) and Cd (514), demonstrating the predominance of non-crustal depositions and suggesting an anthropogenic origin. Our data show that the physical stratigraphy and the chemical signals of several species were well preserved in the uppermost snow of the Alto dell'Ortles glacier. A clear seasonality emerges from the data as the summer snow is more affected by anthropogenic and marine contributions while the winter aerosol flux is dominated by crustal sources. For trace elements, the largest mean EFc seasonal variations are displayed by V (with a factor of 3.8), Sb (3.3), Cu (3.3), Pb (2.9), Bi (2.8), Cd (2.1), Zn (1.9), Ni (1.8), Ag (1.8), As (1.7) and Co (1.6). The chemical data are also discussed in light of the atmospheric stability and back-trajectories analyses in order to explain the observed seasonal variability and how human activities impact the high altitude environment in the Eastern Alps. The origin and behavior of air masses as inferred from the evaluation of 48-h back-trajectories show significant seasonal differences. However, the large changes in trace elements concentrations seem to be more related to the vertical structure of the troposphere at a regional scale rather than the synoptic weather patterns.

Description: Microphysical, macrophysical and radiative signatures of volcanic aerosols in trade wind cumulus observed by the A-Train Atmospheric Chemistry and Physics Discussions, 11, 6415-6455, 2011 Author(s): T. Yuan, L. A. Remer, and H. Yu Increased aerosol concentrations can raise planetary albedo not only by reflecting sunlight and increasing cloud albedo, but also by changing cloud amount. However, detecting aerosol effect on cloud amount has been elusive to both observations and modeling due to potential buffering mechanisms and convolution of meteorology. Here through a natural experiment provided by long-term degassing of a low-lying volcano and use of A-Train satellite observations, we show aerosols are associated strongly with modification of trade cumulus cloud fields including decreased droplet size, decreased precipitation efficiency and increased cloud amount. We also demonstrate that the observed microphysical and macrophysical changes cannot be explained by synoptic meteorology or the orographic effect of the Hawaiian Islands. The "total shortwave aerosol forcing", resulting from direct and indirect forcings including both cloud albedo and cloud amount, is almost an order of magnitude higher than aerosol direct forcing alone. The precipitation reduction associated with enhanced aerosol leads to large changes in the energetics of air-sea exchange. Our results represent the first observational evidence of large-scale increase of cloud amount due to aerosols in a trade cumulus regime, which can be used to constrain the representation of aerosol-cloud interactions in climate models. The findings also have implications for volcano-climate interactions and climate mitigation research.

Description: Simultaneous retrieval of aerosol and cloud properties during the MILAGRO field campaign Atmospheric Chemistry and Physics Discussions, 11, 6363-6413, 2011 Author(s): K. Knobelspiesse, B. Cairns, J. Redemann, R. W. Bergstrom, and A. Stohl Estimation of Direct Climate Forcing (DCF) due to aerosols in cloudy areas has historically been a difficult task, mainly because of a lack of appropriate measurements. The Aerosol Polarimetry Sensor (APS), on the upcoming NASA Glory mission, has the potential to retrieve both cloud and aerosol properties because of its polarimetric, multiple view angle, and multi spectral observations. The APS airborne prototype is the Research Scanning Polarimeter (RSP), which has similar characteristics and can be used to demonstrate APS capabilities. In the spring of 2006, the RSP was deployed on an aircraft based in Veracruz, Mexico, as part of the Megacity Initiative: Local and Global Research Observations (MILAGRO) field campaign. On March 13th, the RSP over flew an aerosol layer lofted above a low altitude marine stratocumulus cloud close to shore in the Gulf of Mexico. We investigate the feasibility of retrieving aerosol properties over clouds using these data. Our approach is to first determine cloud droplet size distribution using the angular location of the cloud bow and other features in the polarized reflectance. The selected cloud was then used in a multiple scattering radiative transfer model optimization to determine the aerosol optical properties and fine tune the cloud size distribution. In this scene, we were able to retrieve aerosol optical depth, the fine mode aerosol size distribution and the cloud droplet size distribution to a degree of accuracy required for climate modeling. This required assumptions about the aerosol vertical distribution and the optical properties of the coarse aerosol size mode. A sensitivity study was also performed to place this case study in the context of the potential for future systematic APS observations of this kind, which found that the aerosol complex refractive index can also be observed accurately if the aerosol optical depth is larger than roughly 0.8 at a wavelength of 0.555 μm.

Description: Yields of hydrogen peroxide from the reaction of hydroxyl radical with organic compounds in solution and ice Atmospheric Chemistry and Physics Discussions, 11, 6457-6492, 2011 Author(s): T. Hullar and C. Anastasio Hydrogen peroxide (HOOH) is a significant oxidant in atmospheric condensed phases (e.g., cloud and fog drops, aqueous particles, and snow) that photolyzes to form hydroxyl radical ( · OH). · OH can react with organics in aqueous phases to form organic peroxyl radicals and ultimately reform HOOH, but the efficiency of this process in atmospheric aqueous phases, as well as snow and ice, is not well understood. We investigate HOOH formation from · OH radical attack of 10 environmentally relevant organic compounds: formaldehyde, formate, glycine, phenylalanine, benzoic acid, octanol, octanal, octanoic acid, octanedioic acid, and 2-butoxyethanol. Liquid and ice samples with and without nitrate (as an · OH source) were illuminated using simulated solar light, and HOOH formation rates were measured as a function of pH and temperature. For most compounds, the formation rate of HOOH without nitrate were the same as the background formation rate in blank water (i.e., illumination of the organic species does not produce HOOH directly), while formation rates with nitrate were greater than the water control (i.e., reactions of OH with the organic species forms HOOH). Yields of HOOH, defined as the rate of HOOH production divided by the rate of · OH production, ranged from essentially zero (glycine) to 0.24 (octanal), with an average of 0.12 ± 0.05 (95% CI). HOOH production rates and yields were higher at lower pH values. There was no temperature dependence of the HOOH yield for formaldehyde or octanedioic acid between −5 to 20 °C and ice samples had approximately the same HOOH yield as the aqueous solutions. In contrast, HOOH yields in formate solutions were higher at 5 and 10 °C compared to −5 and 20 °C. Yields of HOOH in ice for solutions containing nitrate and either phenylalanine, benzoate, octanal, or octanoic acid were indistinguishable from zero. Our HOOH yields were approximately half that found in previous studies conducted using γ-radiolysis, but this difference might be due to the much lower (and more environmentally relevant) · OH formation rates in our experiments.

Description: The 2005 Study of Organic Aerosols at Riverside (SOAR-1): instrumental intercomparisons and fine particle composition Atmospheric Chemistry and Physics Discussions, 11, 6301-6362, 2011 Author(s): K. S. Docherty, A. C. Aiken, J. A. Huffman, I. M. Ulbrich, P. F. DeCarlo, D. Sueper, D. R. Worsnop, D. C. Snyder, B. D. Grover, D. J. Eatough, A. H. Goldstein, P. J. Ziemann, and J. L. Jimenez Multiple state-of-the-art instruments sampled ambient aerosol in Riverside, California during the 2005 Study of Organic Aerosols at Riverside (SOAR) to investigate sources and chemical composition of fine particles (PM f ) in the inland region of Southern California. This paper briefly summarizes the spatial, meteorological and gas-phase conditions during SOAR-1 (15 July–15 August) and provides detailed intercomparisons of complementary measurements and average PM f composition during this period. Daily meteorology and gas-phase species concentrations were highly repetitive with meteorological and gas-phase species concentrations displaying clear diurnal cycles and weekday/weekend contrast, with organic aerosol (OA) being the single largest component contributing approximately one-third of PM f mass. In contrast with historical characterizations of OA in the region, several independent source apportionment efforts attributed the vast majority (~80%) of OA mass during SOAR-1 to secondary organic aerosol (SOA). Given the collocation of complementary aerosol measurements combined with a dominance of SOA during SOAR-1, this paper presents new results on intercomparisons among several complementary measurements and on PM f composition during this period. Total non-refractory submicron (NR-PM 1 ) measurements from a high-resolution aerosol mass spectrometer (HR-AMS) are compared with measurements by tapered element oscillating microbalances (TEOM) including a filter dynamics measurement system (TEOM FDMS ). NR-PM 1 is highly correlated with PM 2.5 TEOM FDMS measurements and accounts for the bulk of PM 2.5 mass with the remainder contributed primarily by refractory material. In contrast, measurements from a heated TEOM show substantial losses of semi-volatile material, including ammonium nitrate and semi-volatile organic material. Speciated HR-AMS measurements are also consistent and highly correlated with several complementary measurements, including those of a collocated compact AMS (C-AMS). Finally, elemental analysis (EA) of HR-AMS OA spectra allows direct comparison of HR-AMS organic carbon (OC) with measurements from two collocated Sunset thermal-optical semi-continuous monitors, and investigation of the elemental composition of OA in Riverside. While HR-AMS and base OC measurements from both Sunset instruments are similar within the combined uncertainties, a correction intended to account for the loss of semivolatile OC from the Sunset yields OC measurements ~30% higher than either HR-AMS or base Sunset measurements. Oxygen is the main heteroatom of ambient OA during SOAR-1 with a minimum atomic O/C of 0.28 during the morning rush hour and maximum of 0.42 during the afternoon. H/C is broadly anti-correlated with O/C, while N/C and S/C (excluding organonitrate (ON) and organosulfate (OS) functionalities) are far lower than O/C at about 0.015 and ~0.001, respectively. O/C, N/C, and S/C increase by 21%, a factor of 2, and a factor of 30, respectively, while H/C changes little when ON and OS estimates are included. This implies that ON account for ~1/2 of the organic nitrogen while OS dominate organic sulfur at this location. Accounting for the estimated ON and OS also improves the agreement between anions and cations measured by HR-AMS by ~8%, while amines have a very small impact (1%) on this balance.

Description: Size-resolved aerosol emission factors and new particle formation/growth activity occurring in Mexico City during the MILAGRO 2006 Campaign Atmospheric Chemistry and Physics Discussions, 11, 6651-6705, 2011 Author(s): A. J. Kalafut-Pettibone, J. Wang, W. E. Eichinger, A. Clarke, S. A. Vay, D. R. Blake, and C. O. Stanier Measurements of the aerosol size distribution from 11 nm to 2.5 microns were made in Mexico City in March, 2006, during the MILAGRO field campaign. Observations at the T0 research site could often be characterized by morning conditions with high particle mass concentrations, low mixing heights, and highly correlated particle number and CO 2 concentrations, indicative that particle number is controlled by primary emissions. Average size-resolved and total number- and volume-based emission factors for combustion sources impacting T0 have been determined using a comparison of peak sizes in number and CO 2 concentration peaks. The number emission and volume emission factors for particles from 11 nm to 494 nm are 1.23 × 10 15 particles, and 7.54 × 10 11 cubic microns per kg of carbon, respectively. Uncertainty on the number emission factor is approximately a factor of 1.5. The mode of the number emission factor was between 25 and 32 nm, while the mode of the volume factor was between 0.25 and 0.32 microns. These emission factors are reported as log normal model parameters and are compared with multiple emission factors from the literature. In Mexico City in the afternoon, the CO 2 concentration drops during ventilation of the polluted layer, and the coupling between CO 2 and particle number breaks down, especially during new particle formation events when particle number is no longer controlled by primary emissions. Using measurements of particle number and CO 2 taken aboard the NASA DC-8, this emission factor was applied to the MCMA plume; the primary emission factor predicts less than 50% of the total particle number and the surplus particle count is not correlated with photochemical age. Primary particle volume and number in the size range 0.1–2 μm are similarly too low to predict the observed volume distribution. Contrary to the case for number, the apparent secondary volume increases with photochemical age. The size distribution of the apparent increase, with a mode at ~250 nm, is reported.

Description: Emission sources contributing to tropospheric ozone over equatorial Africa during the summer monsoon Atmospheric Chemistry and Physics Discussions, 11, 13769-13827, 2011 Author(s): I. Bouarar, K. S. Law, M. Pham, C. Liousse, H. Schlager, T. Hamburger, C. E. Reeves, J.-P. Cammas, P. Nédéléc, S. Szopa, F. Ravegnani, S. Viciani, F. D'Amato, A. Ulanovsky, and A. Richter A global chemistry-climate model LMDz_INCA is used to investigate the contribution of African and Asian emissions to tropospheric ozone over central and West Africa during the summer monsoon. The model results show that ozone in this region is most sensitive to lightning NO x and to central African biomass burning emissions. However, other emission categories also contribute significantly to regional ozone. The maximum ozone changes due to lightning NO x occur in the upper troposphere between 400 hPa and 200 hPa over West Africa and downwind over the Atlantic Ocean. Biomass burning emissions mainly influence ozone in the lower and middle troposphere over central Africa, and downwind due to westward transport. Biogenic emissions of volatile organic compounds, which can be uplifted from the lower troposphere into higher altitudes by the deep convection that occurs over West Africa during the monsoon season, dominate the ozone changes in the upper troposphere and lower stratosphere region. Convective uplift of soil NO x emissions over the Sahel region also makes a significant contribution to ozone in the upper troposphere. Concerning African anthropogenic emissions, they make a lower contribution to ozone compared to the other emission categories. The model results indicate that most ozone changes due to African emissions occur downwind, especially over the Atlantic Ocean, far from the emission regions. The influence of Asian emissions should also be taken into account in studies of the ozone budget over Africa since they make a considerable contribution to ozone concentrations above 150 hPa. Using IPCC AR5 (Intergovernmental Panel on Climate Change; Fifth Assessment Report) estimates of anthropogenic emissions for 2030 over Africa and Asia, the model calculations suggest largest changes in ozone due to the growth of emissions over Asia than over Africa over the next 20 years.

Description: Fragmentation vs. functionalization: chemical aging and organic aerosol formation Atmospheric Chemistry and Physics Discussions, 11, 13693-13721, 2011 Author(s): H. J. Chacon-Madrid and N. M. Donahue The transformation process that a carbon backbone undergoes in the atmosphere is complex and dynamic. Understanding all these changes for all the species in detail is impossible; however, choosing different molecules that resemble progressively higher stages of oxidation or aging and studying them can give us an insight into general characteristics and mechanisms. Here we determine secondary organic aerosol (SOA) mass yields of two sequences of molecules reacting with the OH radical at high NO x . Each sequence consists of species with similar vapor pressures but a succession of oxidation states. The first sequence consists of n -pentadecane, n -tridecanal, 2-, 7-tridecanone, and pinonaldehyde. The second sequence consists of n -nonadecane, n -heptadecanal and cis -pinonic acid. Oxidized molecules tend to have lower relative SOA mass yields; however, oxidation state alone was not enough to predict how efficiently a molecule forms SOA. Certain functionalities are able to fragment more easily than others, and even the position of these functionalities on a molecule can have an effect. n -Alkanes tend to have the highest yields, and n -aldehydes the lowest. n -Ketones have slightly higher yields when the ketone moiety is located on the side of the molecule and not in the center. In general, oxidation products remain efficient SOA sources, though fragmentation makes them less effective than comparable alkanes.

Description: Photochemical modeling of glyoxal at a rural site: observations and analysis from BEARPEX 2007 Atmospheric Chemistry and Physics Discussions, 11, 13655-13691, 2011 Author(s): A. J. Huisman, J. R. Hottle, M. M. Galloway, J. P. DiGangi, K. L. Coens, W. S. Choi, I. C. Faloona, J. B. Gilman, W. C. Kuster, J. de Gouw, N. C. Bouvier-Brown, A. H. Goldstein, B. W. LaFranchi, R. C. Cohen, G. M. Wolfe, J. A. Thornton, K. S. Docherty, D. K. Farmer, M. J. Cubison, J. L. Jimenez, J. Mao, W. H. Brune, and F. N. Keutsch We present ~one month of high time-resolution, direct, in situ measurements of gas-phase glyoxal acquired during the BEARPEX 2007 field campaign. The research site, located on a ponderosa pine plantation in the Sierra Nevada mountains, is strongly influenced by biogenic volatile organic compounds (BVOCs); thus this data adds to the few existing measurements of glyoxal in BVOC-dominated areas. The short lifetime of glyoxal of ~1 h, the fact that glyoxal mixing ratios are much higher during high temperature periods, and the results of a photochemical model demonstrate that glyoxal is strongly influenced by BVOC precursors during high temperature periods. A zero-dimensional box model using near-explicit chemistry from the Leeds Master Chemical Mechanism v3.1 is used to investigate the processes controlling glyoxal chemistry during BEARPEX 2007. The model shows that MBO is the most important glyoxal precursor (~67%), followed by isoprene (~26%) and methylchavicol (~6%), a precursor previously not commonly considered for glyoxal production. The model calculates a noon lifetime for glyoxal of ~0.9 h, making glyoxal well suited as a local tracer of VOC oxidation in a forested rural environment; however, the modeled glyoxal mixing ratios over-predict measured glyoxal by a factor 2 to 5. Although several parameters, such as an approximation for advection and increased glyoxal loss to aerosol can improve the model measurement discrepancy, reduction in OH is by far the most effective. Reducing OH to half the measured values, which is suggested by preliminary OH measurements using a different technique, decreases the glyoxal over-prediction from a factor of 2.4 to 1.1, as well as the overprediction of HO 2 from a factor of 1.64 to 1.14.

Description: Carbon monoxide (CO) and ethane (C 2 H 6 ) trends from ground-based solar FTIR measurements at six European stations, comparison and sensitivity analysis with the EMEP model Atmospheric Chemistry and Physics Discussions, 11, 13723-13767, 2011 Author(s): J. Angelbratt, J. Mellqvist, D. Simpson, J. E. Jonson, T. Blumenstock, T. Borsdorff, P. Duchatelet, F. Forster, F. Hase, E. Mahieu, J. Notholt, A. K. Petersen, U. Raffalski, C. Servais, R. Sussman, and T. Warneke Trends in the CO and C 2 H 6 partial columns (~0–15 km) have been estimated from four European ground-based solar FTIR stations for the 1996–2006 time period. The CO trends from the four stations Jungfraujoch, Zugspitze, Harestua and Kiruna have been estimated to −0.45±0.16 % yr −1 , −1.00±0.24 % yr −1 , −0.62±0.19 % yr −1 and −0.61±0.16 % yr −1 , respectively. The corresponding trends for C 2 H 6 are −1.51±0.23 % yr −1 , −2.11±0.30 % yr −1 , −1.09±0.25 % yr −1 and −1.14±0.18 % yr −1 . To find possible reasons for the CO trends, the global-scale EMEP MSC-W chemical transport model has been used in a series of sensitivity scenarios. It is shown that the trends are consistent with the combination of a 20 % decrease in the anthropogenic CO emissions seen in Europe and North America during the 1996-2006 period and a 20 % increase in the anthropogenic CO emissions in East Asia, during the same time period. The possible impacts of CH 4 and biogenic volatile organic compounds (BVOCs) are also considered. The European and global-scale EMEP model have been evaluated against the measured CO and C 2 H 6 partial columns from Jungfraujoch, Zugspitze, Bremen, Harestua, Kiruna and Ny-Ålesund. The European model reproduces, on average the measurements at the different sites fairly well and within 10–22 % deviation for CO and 14–31 % deviation for C 2 H 6 . Their seasonal amplitude is captured within 6–35 % and 9–124 % for CO and C 2 H 6 , respectively. However, 61–98 % of the CO and C 2 6 partial columns in the European model are shown to arise from the boundary conditions, making the global-scale model a more suitable alternative when modeling theses two species. In the evaluation of the global model the average partial columns for year 2006 have shown to be within 1–9 % and 37–50 % for CO and C 2 6 , respectively. The global model sensitivity for assumptions done in this paper is also analyzed.

Description: Possible effect of extreme solar energetic particle event of 20 January 2005 on polar stratospheric aerosols: direct observational evidence Atmospheric Chemistry and Physics Discussions, 11, 14003-14029, 2011 Author(s): I. A. Mironova, I. G. Usoskin, G. A. Kovaltsov, and S. V. Petelina Energetic cosmic rays are the main source of ionization of the low-middle atmosphere, leading to associated changes in atmospheric properties. Via the hypothetical influence of ionization on aerosol growth and facilitated formation of cloud condensation nuclei, this may be an important indirect link relating solar variability to climate. This effect is highly debated, however, since the proposed theoretical mechanisms still remain illusive and qualitative, and observational evidence is inconclusive and controversial. Therefore, important questions regarding the existence and magnitude of the effect, and particularly the fraction of aerosol particles that can be formed and grow large enough to influence cloud condensation nuclei (CCN), are still open. Here we present empirical evidence of the possible effect caused by cosmic rays upon polar stratospheric aerosols, based on a case study of an extreme solar energetic particle (SEP) event of 20 January 2005. Using aerosol data obtained over polar regions from different satellites with optical instruments that were operating during January 2005, such as the Stratospheric Aerosol and Gas Experiment III (SAGE III), and Optical Spectrograph and Infrared Imaging System (OSIRIS), we found a significant simultaneous change in aerosol properties in both the southern and northern polar regions in temporal association with the SEP event. We speculate that ionization of the atmosphere, which was abnormally high during this extreme SEP event, might have led to formation of new particles and/or growth of preexisting ultrafine particles up to the size of CCN. However, a detailed interpretation of the effect is left for subsequent studies. This is the first time high vertical resolution measurements have been used to provide evidence for the probable production of stratospheric CCN from cosmic ray induced ionization.

Description: Modeling ozone plumes observed downwind of New York City over the North Atlantic Ocean during the ICARTT field campaign Atmospheric Chemistry and Physics Discussions, 11, 14031-14089, 2011 Author(s): S.-H. Lee, S.-W. Kim, M. Trainer, G. J. Frost, S. A. McKeen, O. R. Cooper, F. Flocke, J. S. Holloway, J. A. Neuman, T. Ryerson, C. J. Senff, A. L. Swanson, and A. M. Thompson Transport and chemical transformation of well-defined New York City (NYC) urban plumes over the North Atlantic Ocean were studied using aircraft measurements collected on 20–21 July 2004 during the ICARTT (International Consortium for Atmospheric Research on Transport and Transformation) field campaign and WRF-Chem (Weather Research and Forecasting-Chemistry) model simulations. The strong NYC urban plumes were characterized by carbon monoxide (CO) mixing ratios of 350–400 parts per billion by volume (ppbv) and ozone (O 3 ) levels of about 100 ppbv near New York City on 20 July in the WP-3D in-situ and DC-3 lidar aircraft measurements. On 21 July, the two aircraft captured strong urban plumes with about 350 ppbv CO and over 150 ppbv O 3 (~160 ppbv maximum) about 600 km downwind of NYC over the North Atlantic Ocean. The measured urban plumes extended vertically up to about 2 km near New York City, but shrank to 1–1.5 km over the stable marine boundary layer (MBL) over the North Atlantic Ocean. The WRF-Chem model reproduced ozone formation processes, chemical characteristics, and meteorology of the measured urban plumes near New York City (20 July) and in the far downwind region over the North Atlantic Ocean (21 July). The quasi-Lagrangian analysis of transport and chemical transformation of the simulated NYC urban plumes using WRF-Chem results showed that the pollutants can be efficiently transported in (isentropic) layers in the lower atmosphere ( ~3 km) than models that only accounted for North American anthropogenic/biogenic and wildfire contributions to background ozone. The comparison between models and observations show that chemical IC/BCs must be properly specified to achieve accurate model results.

Description: Solar response in tropical stratospheric ozone: a 3-D chemical transport model study using ERA reanalyses Atmospheric Chemistry and Physics Discussions, 11, 13975-14001, 2011 Author(s): S. Dhomse, M. P. Chipperfield, W. Feng, and J. D. Haigh We have used an off-line 3-D chemical transport model (CTM), to investigate the 11-year solar cycle response in tropical stratospheric ozone. The model is forced with European Centre for Medium-Range Weather Forecasts (ECMWF) (re)analysis (ERA-40/Operational and ERA-Interim) data for 1978–2005 time period. We have compared the modelled solar response in ozone to observational data from three satellite instruments, Solar Backscatter UltraViolet instrument (SBUV), Stratospheric Aerosol and Gas Experiment (SAGE) and Halogen Occultation Experiment (HALOE). A significant difference is seen between simulated and observed ozone during the 1980s, which is probably due to inhomogeneities in the ERA-40 reanalyses. In general, the model with ERA-Interim dynamics shows better agreement with the observations from 1990 onwards than ERA-40. Overall both standard model simulations are partially able to simulate a "double peak"-structured ozone solar response profile with a minimum around 30 km, and these are in better agreement with HALOE than SBUV or SAGE. The largest model-observation differences occur in the upper stratosphere where SBUV and SAGE show a significant (up to 4 %) solar response whereas the standard model and HALOE do not. This is partly due to a positive solar response in the ECMWF upper stratosphere analysed temperatures which reduces the modelled ozone signal. The large positive upper stratosphere response seen in SAGE/SBUV can be reproduced in a model run with fixed dynamical fields (i.e. no inter-annual meteorological changes). As this run effectively assumes no long-term temperature changes (solar-induced or otherwise) it should provide an upper limit of the ozone solar response. Overall, full quantification of the upper stratosphere ozone solar response is limited by differences in the observed dataset and by uncertainties in the solar response in the stratospheric temperatures. In the lower stratosphere we find that transport by analysed winds, which contain information about the Quasi-Biennial Oscillation (QBO), can lead to a large ozone solar response. However, the run with fixed dynamical fields also produces a positive solar response (up to 2 %) in line with the SAGE and SBUV observations.

Description: Ab initio studies of O 2 − (H 2 O) n and O 3 − (H 2 O) n anionic molecular clusters, n ≤12 Atmospheric Chemistry and Physics Discussions, 11, 13947-13973, 2011 Author(s): N. Bork, T. Kurtén, M. B. Enghoff, J. O. P. Pedersen, K. V. Mikkelsen, and H. Svensmark An ab initio study of gaseous clusters of O 2 − and O 2 − with water is presented. Based on thorough scans of configurational space, we determine the thermodynamics of cluster growth. The results are in good agreement with benchmark computational methods and existing experimental data. We find that anionic O 2 − (H 2 O) n and O 3 − (H 2 O) n clusters are thermally stabilized at typical atmospheric conditions for at least n = 5. The first 4 water molecules are strongly bound to the anion due to delocalization of the excess charge while stabilization of more than 4 H 2 O is due to normal hydrogen bonding. Although clustering up to 12 H 2 O, we find that the O 2 and O 3 anions retain at least ca. 80 % of the charge and are located at the surface of the cluster. The O 2 − and O 3 − speicies are thus accessible for further reactions. Finally, the thermodynamics of a few relevant cluster reactions are considered.

Description: Examination of aerosol distributions and radiative effects over the Bay of Bengal and the Arabian Sea region during ICARB using satellite data and a general circulation model Atmospheric Chemistry and Physics Discussions, 11, 13911-13946, 2011 Author(s): R. Cherian, C. Venkataraman, S. Ramachandran, J. Quaas, and S. Kedia In this paper we analyse aerosol loading and its direct radiative effects over the Bay of Bengal (BoB) and Arabian Sea (AS) regions for the Integrated Campaign on Aerosols, gases and Radiation Budget (ICARB) undertaken during 2006, using satellite data from the MODerate Resolution Imaging Spectroradiometer (MODIS) on board the Terra and Aqua satellites, the Aerosol Index from the Ozone Monitoring Instrument (OMI) on board the Aura satellite, and the European-Community Hamburg (ECHAM5.5) general circulation model extended by Hamburg Aerosol Module (HAM). By statistical comparison with large-scale satellite data sets, we firstly show that the ship-based ICARB observations are representative for the entire northern Indian Ocean during the pre-monsoon season. In a second step, the modelled aerosol distributions were evaluated by a comparison with the measurements from the ship-based sunphotometer, and the satellites. It was found that the model reproduces the observed spatial and temporal variability in aerosol optical depth (AOD) and simulated AODs to a large extent. However, AOD was systematically underestimated during high-pollution episodes, especially in the BoB leg. We show that this underprediction of AOD is mostly due to deficiencies in the coarse mode, where the model showed that dust was the dominant component. The analysis of simulated dust AOD along with the OMI Aerosol Index showed that the too low dust emissions from the Thar Desert in the model are the main cause for this deficiency. Thirdly, we analysed the spatio-temporal variability of AOD comparing the ship-based observations to the large-scale satellite observations and simulations. It was found that most of the variability along the track was due to geographical patterns, with minor influence by single events. Aerosol fields were homogeneous enough to yield a good statistical agreement between satellite data at a 1° spatial, but only twice-daily temporal resolution, and the ship-based sunphotometer data at a much finer spatial, but daily-average temporal resolution. Finally, we estimated the shortwave aerosol radiative forcing. We found that the cruise represents well the regional-seasonal mean forcings. Constraining simulated forcings using the observed AOD distributions yields a regional-seasonal mean aerosol forcing of −8.6, −21.4 and +12.9 W m −2 at the top of the atmosphere (TOA), at the surface (SUR) and in the atmosphere (ATM), respectively, for the BoB region, and over the AS, of, −6.8, −12.8, and +6 W m −2 at TOA, SUR, and ATM, respectively.

Description: Springtime carbon episodes at Gosan background site revealed by total carbon, stable carbon isotopic composition, and thermal characteristics of carbonaceous particles Atmospheric Chemistry and Physics Discussions, 11, 13867-13910, 2011 Author(s): J. Jung and K. Kawamura In order to investigate the carbon episodes at Gosan background super-site (33.17° N, 126.10° E) in East Asia during spring of 2007 and 2008, total suspended particles (TSP) were collected and analyzed for particulate organic carbon, elemental carbon, total carbon (TC), total nitrogen (TN), and stable carbon isotopic composition (δ 13 C) of TC. The carbon episodes at the Gosan site were categorized as long-range transported anthropogenic pollutant (LTP) from Asian continent, Asian dust (AD) accompanying with LTP, and local pollen episodes. The stable carbon isotopic composition of TC (δ 13 C TC ) was found to be lowest during the pollen episodes (range: −26.2 ‰ to −23.5 ‰, avg.: −25.2 ± 0.9 ‰), followed by the LTP episodes (range: −23.5 ‰ to −23.0 ‰, avg.: −23.3 ± 0.3 ‰) and the AD episodes (range: −23.3 to −20.4 %, avg.: −21.8 ± 2.0 ‰). The δ 13 C TC of the airborne pollens (−28.0 ‰) collected at the Gosan site showed value similar to that of tangerine fruit (−28.1 ‰) produced from Jeju Island. Based on the carbon isotope mass balance equation and the TN and TC regression approach, we found that ∼40–45 % of TC in the TSP samples during the pollen episodes was attributed to airborne pollens from Japanese cedar trees planted around tangerine farms in Jeju Island. The δ 13 C of citric acid in the airborne pollens (−26.3 ‰) collected at the Gosan site was similar to that in tangerine fruit (−27.4 ‰). The negative correlation between the citric acid-carbon/TC ratios and δ 13 C TC were obtained during the pollen episodes. These results suggest that citric acid emitted from tangerine fruit may be adsorbed on the airborne pollens and then transported to the Gosan site. Based on the thermal evolution pattern of organic aerosols during the carbon episodes, we found that organic aerosols originated from East China are more volatile on heating and are more likely to form pyrolized organic carbon than the pollen-enriched organic aerosols and organic aerosols originated from Northeast China. Since thermal evolution patterns of organic aerosols are highly influenced by their molecular weight, they can be used as additional information on the formation of secondary organic aerosols during the long-range atmospheric transport and the source regions of organics.

Description: The Brewer-Dobson circulation and total ozone from seasonal to decadal time scales Atmospheric Chemistry and Physics Discussions, 11, 13829-13865, 2011 Author(s): M. Weber, S. Dikty, J. P. Burrows, H. Garny, M. Dameris, A. Kubin, J. Abalichin, and U. Langematz The effect of the winter Brewer-Dobson circulation (BDC) on the seasonal and decadal evolution of total ozone in both hemispheres is investigated using satellite total ozone data and outputs from two chemistry-climate models (CCM). Combining data from both hemispheres a linear relationship between the winter cumulative extratropical 100 hPa eddy heat flux and the ozone ratio with respect to fall ozone levels exists and is statistically significant for tropical as well as polar ozone. The high correlation at high latitudes persists well into the summer months until the onset of the next winter season. The anti-correlation of the cumulative eddy heat flux with tropical ozone ratios, however, breaks down in spring as the polar vortex erodes and changes to a weak positive correlation similar to that observed at high latitudes. The inter-annual variability and decadal evolution of ozone in each hemisphere in winter, spring, and summer are therefore driven by the cumulative effect of the previous winter's meridional circulation. This compact linear relationship is also found in two different CCMs (EMAC-FUB, DLR-E39C-A) indicating that current models realistically describe the variability in stratospheric circulation and its climate effect on total ozone. Both models show a positive trend in the winter mean eddy heat flux (and winter BDC strength) in both hemispheres until year 2050, however the inter-annual variability (peak-to-peak) is two to three times larger than the mean change between 1960 and 2050. It is, therefore, possible to detect a shift in this compact linear relationship related to past and future changes in the stratospheric halogen load. A similar shift is difficult to derive from observational data since the satellite era now spanning more than thirty years is still fairly short.

Description: Minor effect of physical size sorting on iron solubility of transported mineral dust Atmospheric Chemistry and Physics Discussions, 11, 14309-14338, 2011 Author(s): Z. B. Shi, M. T. Woodhouse, K. S. Carslaw, M. D. Krom, G. W. Mann, A. R. Baker, I. Savov, G. Fones, B. Brooks, T. D. Jickells, and L. G. Benning Observations show that the fractional solubility of Fe (FS-Fe, ratio of dissolved to total Fe) in dust aerosol increases dramatically from ~0.1% in regions of high dust mass concentration to 80% in remote regions where concentrations are low. Here, we combined laboratory geochemical measurements with global aerosol model simulations to test the hypothesis that the increase in FS-Fe is due to physical size sorting during transport. We determined the FS-Fe and fractional solubility of Al (FS-Al) in size-fractionated dust generated from two representative soil samples collected from known Saharan dust source regions using a customized dust re-suspension and collection system. The results show that the FS-Fe is size-dependent and ranges from 0.1–0.3 % in the coarse size fractions (〉1 μm) to ~0.2–0.8 % in the fine size fractions ( 100 μg m −3 ) to ~0.2 % at low concentrations (

Description: Evaluation of a three-dimensional chemical transport model (PMCAMx) in the European domain during the EUCAARI May 2008 campaign Atmospheric Chemistry and Physics Discussions, 11, 14183-14220, 2011 Author(s): C. Fountoukis, P. N. Racherla, H. A. C. Denier van der Gon, P. Polymeneas, P. E. Haralabidis, A. Wiedensohler, C. Pilinis, and S. N. Pandis PMCAMx-2008, a detailed three dimensional chemical transport model (CTM), was applied to Europe to simulate the mass concentration and chemical composition of particulate matter (PM) during May 2008. The model includes a state-of-the-art organic aerosol module which is based on the volatility basis set framework treating both primary and secondary organic components to be semivolatile and photochemically reactive. The model performance is evaluated against high time resolution aerosol mass spectrometer (AMS) ground and airborne measurements. Overall, organic aerosol is predicted to account for 32% of total PM 1 at ground level during May 2008, followed by sulfate (30%), crustal material and sea-salt (14%), ammonium (13%), nitrate (7%), and elemental carbon (4%). The model predicts that fresh primary OA (POA) is a small contributor to organic PM concentrations in Europe during late spring, and that oxygenated species (oxidized primary and biogenic secondary) dominate the ambient OA. The Mediterranean region is the only area in Europe where sulfate concentrations are predicted to be much higher than the OA, while organic matter is predicted to be the dominant PM 1 species in Central and Northern Europe. The comparison of the model predictions with the ground measurements in four measurement stations is encouraging. The model reproduces more than 94% of the daily averaged data and more than 87% of the hourly data within a factor of 2 for PM 1 OA. The model tends to predict relatively flat diurnal profiles for PM 1 OA in many areas, both rural and urban, in agreement with the available measurements. The model performance against the high time resolution airborne measurements at multiple altitudes and locations is as good as its performance against the ground level hourly measurements. There is no evidence of missing sources of OA aloft over Europe during this period.

Description: Observations of in-situ generated gravity waves during a stratospheric temperature enhancement (STE) event Atmospheric Chemistry and Physics Discussions, 11, 14221-14232, 2011 Author(s): A. J. Gerrard, Y. Bhattacharya, and J. P. Thayer Observations of in-situ generated atmospheric gravity waves associated with a stratospheric temperature enhancement (STE) are presented. Two sets of gravity waves are observed by molecular-aerosol lidar in conjunction with the early December 2000 STE event above Sondrestrom, Greenland. The first set of gravity waves shows downward phase progression with a vertical wavelength of ~8 km while the second set shows upward phase progression with a vertical wavelength of ~9 km. With estimates of the background wind fields from synoptic analyses, the various intrinsic gravity wave parameters of these two wave structures are found. The observed waves compare well to numerical modeling predictions, though the potential observation of a downward propagating wave would be unexpected.

Description: Multiple-sulfur isotope effects during photolysis of carbonyl sulfide Atmospheric Chemistry and Physics Discussions, 11, 14233-14258, 2011 Author(s): Y. Lin, M. S. Sim, and S. Ono Laboratory experiments were carried out to determine sulfur isotope effects during ultraviolet photolysis of carbonyl sulfide (OCS) to carbon monoxide (CO) and elemental sulfur (S 0 ). The OCS gas at 3.7 to 501 mbar was irradiated with or without a N 2 bath gas using a 150 W Xe arc lamp. Sulfur isotope ratios for the product S 0 and residual OCS were analyzed by an isotope ratio mass-spectrometer with SF 6 as the analyte gas. The isotope effect after correction for the reservoir effects is −6.8 ‰ for the ratio 34 S/ 32 S, where product S 0 is depleted in heavy isotopes. The magnitude of the overall isotope effect is not sensitive to the addition of N 2 but increases to −9.5 ‰ when radiation of λ 〉285 nm is used. The measured isotope effect reflects that of photolysis as well as the subsequent sulfur abstraction (from OCS) reaction. The magnitude of isotope effects for the abstraction reaction is estimated by transition state theory to be between −18.9 and −3.1 ‰ for 34 S which gives the photolysis isotope effect as −10.5 to +5.3 ‰. The measured isotope effects are found to be δ 33 S/δ 34 S = 0.534±0.005 and δ 36 S/δ 34 S = 1.980±0.021. These values are largely mass-dependent but statistically differ from canonical values for mass-dependent fractionation of 0.515 and 1.90, respectively. The result demonstrates that the OCS photolysis may not produce large isotope effect of more than about 10 \permil, and can be the major source of background stratospheric sulfate aerosol (SSA) during volcanic quiescence.

Description: Contributions of regional and intercontinental transport to surface ozone in Tokyo Atmospheric Chemistry and Physics Discussions, 11, 10403-10448, 2011 Author(s): M. Yoshitomi, O. Wild, and H. Akimoto Japan lies downwind of the Asian continent and for much of the year air quality is directly influenced by emissions of ozone precursors over these heavily-populated and rapidly-industrializing regions. This study examines the extent to which oxidant transport from regional and distant anthropogenic sources influences air quality in Japan in springtime, when these contributions are largest. We find that European and North American contributions to surface ozone over Japan in spring are persistent, averaging 3.5±1.1 ppb and 2.8±0.5 ppb respectively, and are greatest in cold continental outflow conditions following the passage of cold fronts. Contributions from China are larger, 4.0±2.8 ppb, and more variable, as expected for a closer source region, and are generally highest near cold fronts preceding the influence of more distant sources. The stratosphere provides a varying but ever-present background of ozone of about 11.2±2.5 ppb during spring. Local sources over Japan and Korea have a relatively small impact on mean ozone, 2.4±7.6 ppb, but this masks a strong diurnal signal, and local sources clearly dominate during episodes of high daytime ozone. By examining the meteorological mechanisms that favour transport from different source regions, we demonstrate that while maximum foreign influence generally does not occur at the same time as the greatest buildup of oxidants from local sources, it retains a significant influence under these conditions. It is thus clear that while meteorological boundaries provide some protection from foreign influence during oxidant outbreaks in Tokyo, these distant sources still make a substantial contribution to exceedance of the Japanese ozone air quality standard in springtime.

Description: Isoprene oxidation mechanisms: measurements and modelling of OH and HO 2 over a South-East Asian tropical rainforest during the OP3 field campaign Atmospheric Chemistry and Physics Discussions, 11, 10343-10401, 2011 Author(s): D. Stone, M. J. Evans, P. M. Edwards, R. Commane, T. Ingham, A. R. Rickard, D. M. Brookes, J. Hopkins, R. J. Leigh, A. C. Lewis, P. S. Monks, D. Oram, C. E. Reeves, D. Stewart, and D. E. Heard Forests are the dominant source of volatile organic compounds into the atmosphere, with isoprene being the most significant species. The oxidation chemistry of these compounds is a significant driver of local, regional and global atmospheric composition. Observations made over Borneo during the OP3 project in 2008, together with an observationally constrained box model are used to assess our understanding of this oxidation chemistry. In line with previous work in tropical forests, we find that the standard model based on MCM chemistry significantly underestimates the observed OH concentrations. Geometric mean observed to modelled ratios of OH and HO 2 in airmasses impacted with isoprene are 5.32 −4.43 +3.68 and 1.18 −0.30 +0.30 respectively, with 68% of the observations being within the specified variation. We implement a variety of mechanistic changes into the model, including epoxide formation and unimolecular decomposition of isoprene peroxy radicals, and assess their impact on the model success. We conclude that none of the current suggestions can simultaneously remove the bias from both OH and HO 2 simulations and believe that detailed laboratory studies are now needed to resolve this issue.

Description: Comparing the effectiveness of recent algorithms to fill and smooth incomplete and noisy time series Atmospheric Chemistry and Physics Discussions, 11, 14259-14308, 2011 Author(s): J. P. Musial, M. M. Verstraete, and N. Gobron Geophysical time series often feature missing data or data acquired at irregular times. Procedures are needed to either resample these series at systematic time intervals or to generate reasonable estimates at specified times in order to meet specific user requirements or to facilitate subsequent analyses. Interpolation methods have long been used to address this problem, taking into account the fact that available measurements also include errors of measurement or uncertainties. This paper inspects some of the currently used approaches to fill gaps and smooth time series (smoothing splines, Singular Spectrum Analysis and Lomb-Scargle) by comparing their performance in either reconstructing the original record or in minimizing the Mean Absolute Error (MAE) between the underlying model and the available data, using both artificially-generated series or well-known publicly available records. Some methods make no assumption on the type of variability in the data while others hypothesize the presence of at least some dominant frequencies. It will be seen that each method exhibits advantages and drawbacks, and that the choice of an approach largely depends on the properties of the underlying time series and the objective of the research.

Description: Thermodynamics and kinetics of the hydrolysis of atmospherically relevant organonitrates and organosulfates Atmospheric Chemistry and Physics Discussions, 11, 14383-14416, 2011 Author(s): K. S. Hu, A. I. Darer, and M. J. Elrod The presence of alcohol, organonitrate, and organosulfate species related to the gaseous precursor isoprene in ambient secondary organic aerosol (SOA) has stimulated investigations of the nature of SOA-phase chemical processing. Recent work has suggested that certain isoprene-derived organonitrates are able to efficiently convert to organosulfates and alcohols on ambient SOA. In order to better understand the structure activity relationships previously observed for the isoprene-derived organonitrates and organosulfates, the hydrolysis reactions of a number of monofunctional and difunctional organonitrates and organosulfates with varying carbon substitution properties were investigated. Nuclear magnetic resonance techniques were used to study the bulk phase aqueous reactions of these organonitrates and organosulfates in order to determine hydrolysis reaction rate and, in some cases, thermodynamics information. Electronic structure calculations were also carried out to determine the enthalpy of hydrolysis for these species, and for the previously studied isoprene-derived species. The results suggest that while organonitrates and organosulfates are thermodynamically unstable with respect to the corresponding alcohols at standard state, only the tertiary organonitrates (and perhaps some tertiary organosulfates) are able to efficiently hydrolyze on SOA timescales and acidities.

Description: Impact of dust aerosols on Hurricane Helene's early development through the deliquescent heterogeneous freezing mode Atmospheric Chemistry and Physics Discussions, 11, 14339-14381, 2011 Author(s): H. Zhang, I. N. Sokolik, and J. A. Curry An ice nucleation parameterization accounting for the deliquescent heterogeneous freezing (DHF) mode was implemented into the Weather Research Forecast (WRF) model. The DHF mode refers to the freezing process for internally mixed aerosols with soluble and insoluble species that can serve as both cloud condensation nuclei (CCN) and ice nuclei (IN), such as dust. A modified version of WRF was used to examine the effect of Saharan dust on the early development of Hurricane Helene (2006) via acting as CCN and IN. The WRF simulations showed the tendency of DHF mode to promote ice formation at lower altitudes in strong updraft cores, increase the local latent heat release, and produce more low clouds and less high clouds. The inclusion of dust acting as CCN and IN through the DHF mode modified the storm intensity, track, hydrometeor distribution, cloud top temperature (hence the storm radiative energy budget), and precipitation and latent heat distribution. However, changes in storm intensity, latent heating rate, and total precipitation exhibit nonlinear dependence on the dust concentration. Improvement in the representation of atmospheric aerosols and cloud microphysics has the potential to contribute to better prediction of tropical cyclone development.

Description: Projections of UV radiation changes in the 21st century: impact of ozone recovery and cloud effects Atmospheric Chemistry and Physics Discussions, 11, 10769-10797, 2011 Author(s): A. F. Bais, K. Tourpali, A. Kazantzidis, H. Akiyoshi, S. Bekki, P. Braesicke, M. P. Chipperfield, M. Dameris, V. Eyring, H. Garny, D. Iachetti, P. Jöckel, A. Kubin, U. Langematz, E. Mancini, M. Michou, O. Morgenstern, T. Nakamura, P. A. Newman, G. Pitari, D. A. Plummer, E. Rozanov, T. G. Shepherd, K. Shibata, W. Tian, and Y. Yamashita Surface erythemal solar irradiance (UV-Ery) from 1960 to 2100 has been derived using radiative transfer calculations and projections of ozone, temperature and cloud change from 14 chemistry climate models (CCM), as part of the CCMVal-2 activity of SPARC. Our calculations show the influence of ozone depletion and recovery on erythemal irradiance. In addition, we investigate UV-Ery changes caused by climate changes due to increasing greenhouse gas concentrations. The latter include effects on both stratospheric ozone and cloud changes. The derived estimates provide a global picture of the likely changes in erythemal irradiance during the 21st century. Uncertainties arise from the assumed scenarios, different parameterizations – particularly of cloud effects on UV-Ery – and from the diversity in the CCM projections. The calculations suggest that relative to 1980 annually mean UV-Ery in the 2090s will be on average ~12% lower at high latitudes in both hemispheres, ~3% lower at mid latitudes, and marginally higher (~1%) in the tropics. The largest reduction (~16%) is projected for Antarctica in October. Cloud effects result in additional 2–3% reduction in UV-Ery at high latitudes, but they slightly moderate it at mid-latitudes (~1%). The year of return of erythemal irradiance to values of certain milestones (1965 and 1980) depends largely on the return of column ozone to the corresponding levels and is associated with large uncertainties mainly due to the spread of the model projections. The inclusion of cloud effects in the calculations has only a small effect of the return years. At mid and high latitudes, changes in clouds and stratospheric ozone dynamics due to greenhouse gases will sustain the erythemal irradiance at levels below those in 1965, despite the removal of ozone depleting substances. At high northern latitudes, the projected decreases in cloud transmittance towards the end of the 21st century will likely reduce the yearly average surface erythemal irradiance by up to 10% with respect to the 1960s.

Description: Yields of oxidized volatile organic compounds during the OH radical initiated oxidation of isoprene, methyl vinyl ketone, and methacrolein under high–NO x conditions Atmospheric Chemistry and Physics Discussions, 11, 10693-10720, 2011 Author(s): M. M. Galloway, A. J. Huisman, L. D. Yee, A. W. H. Chan, C. L. Loza, J. H. Seinfeld, and F. N. Keutsch We present first-generation and total production yields of glyoxal, methylglyoxal, glycolaldehyde, and hydroxyacetone from the oxidation of isoprene, methyl vinyl ketone (MVK), and methacrolein (MACR) with OH under high NO x conditions. Several of these first-generation yields are not included in commonly used chemical mechanisms, such as the Leeds Master Chemical Mechanism (MCM) v. 3.1. Inclusion of first-generation production of glyoxal, glycolaldehyde and hydroxyacetone from isoprene and methylglyoxal from MACR greatly improves performance of an MCM based model during the initial part of the experiments. In order to further improve performance of the MCM based model, higher generation glyoxal production was reduced by lowering the first-generation yield of glyoxal from C5 carbonyls. The results suggest that glyoxal production from reaction of OH with isoprene under high NO x conditions can be approximated by inclusion of a first-generation production term together with secondary production only via glycolaldehyde. Analogously, methylglyoxal production can be approximated by a first-generation production term from isoprene, and secondary production via MVK, MACR and hydroxyacetone. The first-generation yields reported here correspond to less than 5% of the total oxidized yield from isoprene and thus only have a small effect on the fate of isoprene. However, due to the abundance of isoprene, the combination of first-generation yields and reduced higher generation production of glyoxal from C5 carbonyls is important for models which include the production of the small organic molecules from isoprene.

Description: Variability of aerosol optical properties in the Western Mediterranean Basin Atmospheric Chemistry and Physics Discussions, 11, 14091-14125, 2011 Author(s): M. Pandolfi, M. Cusack, A. Alastuey, and X. Querol Aerosol light scattering, black carbon (BC) and particulate matter (PM) concentrations were measured at Montseny, a regional background site in the Western Mediterranean Basin (WMB) which is part of the European Supersite for Atmospheric Aerosol Research (EUSAAR). Off line analyses of 24 h PM filters collected with Hi-Vol instruments were performed for the determination of the main chemical components of PM. Measurements of BC were used to calculate the light absorption properties of atmospheric particles. Single Scattering Albedo (SSA) at 635 nm was estimated starting from aerosol scattering and absorption measurements, while Ångström exponents were calculated by means of the three wavelengths (450 nm, 525 nm, 635 nm) aerosol light scattering measurements from Nephelometer. Mean scattering and hemispheric backscattering coefficients (@ 635 nm) were 26.8 ± 23.3 Mm −1 and 4.3 ± 2.7 Mm −1 , respectively and the mean aerosol absorption coefficient was 2.8 ± 2.2 Mm −1 . Mean values of Single Scattering Albedo (SSA) and Ångström exponent (calculated from 450 nm to 635 nm) at MSY were 0.90 ± 0.05 and 1.2 ± 0.6, respectively. A clear relationship was observed between the PM 1 /PM 10 and PM 2.5 /PM 10 ratios as a function of the calculated Ångström exponents. Mass scattering cross sections for fine mass and sulfate at 635 nm were calculated in 2.8 ± 0.5 m 2 g −1 and 11.8 ± 2.2 m 2 g −1 respectively, while the mean aerosol absorption cross section was estimated around 10.4 ± 2.0 m 2 g −1 . The variability in aerosol optical properties in the WMB were largely explained by the origin and ageing of air masses over the measurement site. The sea breeze played an important role in transporting pollutants from the developed WMB coastlines towards inland rural areas, changing the optical properties of aerosols. Aerosol scattering and backscattering coefficients increased by around 40 % in the afternoon when the sea breeze was fully developed while the absorption coefficient increased by more than 100 % as a consequence of the increase in BC concentration at MSY observed under sea breeze circulation. The analysis of the Ångström (Å) exponent as a function of the origin the air masses revealed that polluted winter anticyclonic conditions and summer recirculation scenarios typical of the WMB led to an increase of fine particles in the atmosphere (Å = 1.4 ± 0.1) while the aerosol optical properties under Atlantic Advection episodes and Saharan dust outbreaks were clearly dominated by coarser particles (Å = 0.7 ± 0.3).

Description: Observations of volatile organic compounds during ARCTAS – Part 1: Biomass burning emissions and plume enhancements Atmospheric Chemistry and Physics Discussions, 11, 14127-14182, 2011 Author(s): R. S. Hornbrook, D. R. Blake, G. S. Diskin, H. E. Fuelberg, S. Meinardi, T. Mikoviny, G. W. Sachse, S. A. Vay, A. J. Weinheimer, C. Wiedinmyer, A. Wisthaler, A. Hills, D. D. Riemer, and E. C. Apel Mixing ratios of a large number of volatile organic compounds (VOCs) were observed by the Trace Organic Gas Analyzer (TOGA) on board the NASA DC-8 as part of the Arctic Research of the Composition of the Troposphere from Aircraft and Satellites (ARCTAS) field campaign. Many of these VOCs were observed concurrently by one or both of two other VOC measurement techniques on board the DC-8: proton-transfer-reaction mass spectrometry (PTR-MS) and whole air canister sampling (WAS). A comparison of these measurements to the data from TOGA indicates good agreement for the majority of co-measured VOCs. The ARCTAS study, which included both spring and summer deployments, provided opportunities to sample a large number of biomass burning (BB) plumes with origins in Asia, California and Central Canada, ranging from very recent emissions to plumes aged one week or more. For this analysis, identified BB plumes were grouped by flight, source region and, in some cases, time of day, generating 40 individual plume groups, each consisting of one or more BB plume interceptions. Normalized excess mixing ratios (EMRs) to CO were determined for each of the 40 plume groups for up to 19 different VOCs or VOC groups, many of which show significant variability, even within relatively fresh plumes. This variability demonstrates the importance of assessing BB plumes both regionally and temporally, as emissions can vary from region to region, and even within a fire over time. Comparisons with literature confirm that variability of EMRs to CO over an order of magnitude for many VOCs is consistent with previous observations. However, this variability is often diluted in the literature when individual observations are averaged to generate an overall regional EMR from a particular study. Previous studies give the impression that emission ratios are generally consistent within a given region, and this is not necessarily the case, as our results show. For some VOCs, earlier assumptions may lead to significant under-prediction of emissions in fire emissions inventories. Notably, though variable between plumes, observed EMRs of individual light alkanes are highly correlated within BB emissions. Using the NCAR master mechanism chemical box model initialized with concentrations based on two observed scenarios, i.e., fresh Canadian BB and fresh Californian BB, both plumes are expected to experience primarily decreases in oxygenated VOCs during the first 2.5 days, such that any production in the plumes of these compounds is less than the chemical loss. Comparisons of the modeled EMRs to the observed EMRs from BB plumes estimated to be three days in age or less indicate overall good agreement and, for most compounds, no significant difference between BB plumes in these two regions.

Description: Hygroscopicity and composition of Alaskan Arctic CCN during April 2008 Atmospheric Chemistry and Physics Discussions, 11, 21789-21834, 2011 Author(s): R. H. Moore, R. Bahreini, C. A. Brock, K. D. Froyd, J. Cozic, J. S. Holloway, A. M. Middlebrook, D. M. Murphy, and A. Nenes We present a comprehensive characterization of cloud condensation nuclei (CCN) sampled in the Alaskan Arctic during the 2008 Aerosol, Radiation, and Cloud Processes affecting Arctic Climate (ARCPAC) project, a component of the POLARCAT and International Polar Year (IPY) initiatives. Four distinct air mass types were sampled including relatively pristine Arctic background conditions as well as biomass burning and anthropogenic pollution plumes. Despite differences in chemical composition, inferred aerosol hygroscopicities were fairly invariant and ranged from κ = 0.1–0.3 over the atmospherically-relevant range of water vapor supersaturations studied. Analysis of the individual mass spectral m/z 43 and 44 peaks from an aerosol mass spectrometer show the organic aerosols sampled to be well-oxygenated, consistent with with long-range transport and aerosol aging processes. However, inferred hygroscopicities are less than would be predicted based on previous parameterizations of biogenic oxygenated organic aerosol, suggesting an upper limit on organic aerosol hygroscopicity above which κ is less sensitive to the O:C ratio. Most Arctic aerosol act as CCN above 0.1 % supersaturation, although the data suggest the presence of an externally-mixed, non-CCN-active mode comprising approximately 0–20 % of the aerosol number. CCN closure was assessed using measured size distributions, bulk chemical composition measurements, and assumed aerosol mixing states; CCN predictions tended toward overprediction, with the best agreement (± 0–20 %) obtained by assuming the aerosol to be externally-mixed with soluble organics. Closure also varied with CCN concentration, and the best agreement was found for CCN concentrations above 100 cm −3 with a 1.5- to 3-fold overprediction at lower concentrations.

Description: Measurement of ambient aerosol hydration state at Great Smoky Mountains National Park in the Southeastern United States Atmospheric Chemistry and Physics Discussions, 11, 21877-21933, 2011 Author(s): N. F. Taylor, D. R. Collins, C. W. Spencer, D. H. Lowenthal, B. Zielinska, V. Samburova, and N. Kumar We present results from two field deployments of a unique tandem differential mobility analyzer (TDMA) configuration with two primary capabilities: identifying alternative stable or meta-stable ambient aerosol hydration states associated with hysteresis in aerosol hydration behavior and determining the actual Ambient hydration State (AS-TDMA). This data set is the first to fully classify the ambient hydration state of aerosols despite recognition that hydration state significantly impacts the roles of aerosols in climate, visibility and heterogeneous chemistry. The AS-TDMA was installed at a site in eastern Tennessee on the border of Great Smoky Mountains National Park for projects during the summer of 2006 and winter of 2007–2008. During the summer, 12 % of the aerosols sampled in continuous AS-TDMA measurements were found to posses two possible hydration states under ambient conditions. In every case, the more hydrated of the possible states was occupied. The remaining 88 % did not posses multiple possible states. In continuous measurements during the winter, 49 % of the aerosols sampled possessed two possible ambient hydration states; the remainder possessed only one. Of those aerosols with multiple possible ambient hydration states, 65 % occupied the more hydrated state; 35 % occupied the less hydrated state. This seasonal contrast is supported by differences in the fine particulate (PM 2.5 ) composition and ambient RH as measured during the two study periods. In addition to seasonal summaries, this work includes case studies depicting the variation of hydration state with changing atmospheric conditions.

Description: Stratospheric impact on tropospheric ozone variability and trends: 1990–2009 Atmospheric Chemistry and Physics Discussions, 11, 22719-22770, 2011 Author(s): P. G. Hess and R. Zbinden We evaluate the influence of stratospheric ozone on the interannual variability and trends in tropospheric ozone from 30–90° N between 1990 and 2009 using ozone measurements and a global chemical transport model (the Community Atmospheric Model with chemistry) with input meteorology from the National Center for Environmental Prediction. The model simulation uses constant interannual emissions. Both the model and measurements indicate that on large spatial scales stratospheric interannual ozone variability drives significant tropospheric variability and contributes to long-term tropospheric ozone trends. To diagnose the measured variability we utilized measurements from ozonesondes and the Measurements of OZone and water vapour by in-service Airbus airCraft programme (MOZAIC) north of 30° N. We identify a regionally robust 150 hPa ozone signal from measurements over Canadian, Northern European and Central European regions and at 500 hPa over Canadian, Northern European and Eastern US regions. Averaged over these regions, the 150 hPa interannual ozone variability explains 69 % of the interannual variability at 500 hPa. The simulated stratospheric signal explains 81 % of the simulated variability over these same regions. Simulated and measured ozone are significantly correlated over these regions and the simulation suggests that the ozone record over these regions is representative of the overall hemispheric 500 hPa ozone record from 30–90° N. The measured 500 hPa trends averaged over these three regions between 1990 and 2000 and 1990 and 2009 are 0.73 (±0.51) ppbv yr −1 and 0.27 (±0.19) ppbv yr −1 , respectively. The simulated trends in 1990–2000 and 1990–2009 are 0.29±0.10 ppbv yr −1 and 0.13±0.05 ppbv yr −1 , respectively; however, these trends are substantially larger when the model is sampled for missing data exactly as the measurements are. Simulated stratospheric ozone accounts for 79 % of the simulated 500 hPa trend between 1990 and 2000 and 100 % of the simulated trend between 1990 and 2009. Due to the importance of local meteorology and emissions at the surface it is difficult to isolate the stratospheric component of measured surface ozone variability. Overall when averaged between 30–90° N simulated surface interannual ozone trends are 0.18 ppbv yr −1 and 0.07 ppbv yr −1 between 1990 and 1999, and between 1990 and 2009, respectively. We have identified a number of surface sites where the measured interannual ozone variability is correlated with the 150 hPa ozone signal. Most notably these sites include the high mountain sites over Europe and Macehead, Ireland. Over Macehead the measured 150 hPa ozone signal explains 40 % of the interannual variability of the unfiltered measured ozone record. The simulated and measured ozone are highly correlated over Macehead. The Macehead measured and simulated unfiltered ozone trends between 1990 and 2000 are 0.28 (±0.33) and 0.17 (±0.13) ppbv yr −1 respectively; between 1990 and 2009 the measured and simulated trends are 0.18 (±0.11) and 0.08 (±0.06) ppbv yr −1 , respectively. Increases in the simulated stratospheric ozone component accounts for 53 % and 75 % of the overall modeled trend for the two periods at Macehead.

Description: Advances and limitations of atmospheric boundary layer observations with GPS occultation over Southeast Pacific Ocean Atmospheric Chemistry and Physics Discussions, 11, 22857-22891, 2011 Author(s): F. Xie, D. L. Wu, C. O. Ao, A. J. Mannucci, and E. R. Kursinski The typical atmospheric boundary layer (ABL) over the southeast (SE) Pacific Ocean is featured with a strong temperature inversion and a sharp moisture gradient across the ABL top. The strong moisture and temperature gradients result in a sharp refractivity gradient that can be precisely detected by the Global Positioning System (GPS) radio occultation (RO) measurements. In this paper, the Constellation Observing System for Meteorology, Ionosphere and Climate (COSMIC) GPS RO soundings, radiosondes and the high-resolution ECMWF analysis over the SE Pacific are analyzed. COSMIC RO is able to detect a wide range of ABL height variations (1–2 km) as observed from the radiosondes. Whereas, the ECMWF analyses systematically underestimate ABL heights. The sharp refractivity gradient at the ABL top frequently exceeds the critical refraction (e.g., −157 N-unit km −1 ) and becomes the so-called ducting condition, which results in systematic RO refractivity bias (or called N-bias) inside the ABL. Simulation study using refractivity profiles based on radiosondes reveals that the N-biases are significant and the magnitudes of biases are vertical resolution dependent. The N-bias is also the primary cause of the systematically smaller refractivity gradient (rarely exceeding −110 N-unit km −1 ) at the ABL top from RO measurement. However, the N-bias seems not affect the ABL height detection. Instead, the very sharp refractivity gradient and the large RO bending angle due to ducting allow reliable detection of ABL height from GPS RO. The seasonal mean climatology of ABL heights derived from a nine-month composite of COSMIC RO soundings over the SE Pacific reveals significant differences from the ECMWF analysis. Both show the deepening of ABL height from the shallow stratocumulus near the coast to a much higher trade wind inversion further off the coast. However, COSMIC RO shows systematically higher ABL heights overall and reveals different locations of the minimum and maximum ABL heights as compared to the ECMWF analysis. The significantly decreasing number of COSMIC RO soundings at lower latitudes along with the lower percentage of RO soundings penetrating into the lowest 500 m above mean-sea-level (a.m.s.l.), result in generally small sampling errors in the mean ABL climatology and will not affect the morphology of RO ABL height climatology. The difference of ABL height climatology between COSMIC RO and ECMWF analysis over SE Pacific is significant and requires further studies.

Description: Immersion mode heterogeneous ice nucleation by an illite rich powder representative of atmospheric mineral dust Atmospheric Chemistry and Physics Discussions, 11, 22801-22856, 2011 Author(s): S. L. Broadley, B. J. Murray, R. J. Herbert, J. D. Atkinson, S. Dobbie, E. Condliffe, and L. Neve Atmospheric dust rich in illite is transported globally from arid regions and may impact cloud properties through the nucleation of ice. We present measurements of ice nucleation in water droplets containing known quantities of an illite rich powder under atmospherically relevant conditions. The illite rich powder used here, NX illite, has a similar mineralogical composition to atmospheric mineral dust sampled in remote locations, i.e. dust which has been subject to long range transport, cloud processing and sedimentation. Arizona Test Dust has a significantly different mineralogical composition and we suggest that NX illite is a better surrogate of natural atmospheric dust. Heterogeneous nucleation by NX illite was observed, using optical microscopy, to occur dominantly between 246 K and the homogeneous freezing limit and higher freezing temperatures were observed with larger surface areas of NX illite present within the droplets. It is shown that there is strong particle to particle variability in terms of ice nucleating ability with a few particles dominating ice nucleation at high surface areas. In fact, this work suggests that the bulk of atmospheric mineral dust particles are less efficient at nucleating ice than assumed in parameterisation currently used in models. For droplets containing ≤2 × 10 −6 cm 2 of NX illite, freezing temperatures did not noticeably change when the cooling rate was varied by an order of magnitude. The data obtained during cooling experiments (with surface areas ≤2 × 10 −6 cm 2 ) is shown to be inconsistent with the single component stochastic model, but is well described by the singular model ( n s (236.2 K ≤ T ≤ 247.5 K) = exp(6.53043 × 10 4 − 8.2153088 × 10 2 T + 3.446885376 T 2 − 4.822268 × 10 −3 T 3 ). However, droplets continued to freeze when the temperature was held constant, which is inconsistent with the time independent singular model. We show that this apparent discrepancy can be resolved using a multiple component stochastic model in which it is assumed there are many types of nucleation sites, each with a unique temperature dependent nucleation coefficient. Cooling rate independence can be achieved with this time dependent model if the nucleation rate coefficients increase very rapidly with decreasing temperature, thus reconciling our measurement of nucleation at constant temperature with the cooling rate independence.

Description: Optimal estimation of tropospheric H 2 O and δD with IASI/METOP Atmospheric Chemistry and Physics Discussions, 11, 16107-16146, 2011 Author(s): M. Schneider and F. Hase We present an optimal estimation retrieval for tropospheric H 2 O and δD applying thermal nadir spectra measured by the instrument IASI (Infrared Atmospheric Sounding Interferometer) flown on EUMETSAT's polar orbiter METOP. We document that the IASI spectra allow for retrieving H 2 O profiles between the surface and the upper troposphere as well as middle tropospheric δD values. A theoretical error estimation suggests a precision for H 2 O of better than 35 % in the lower troposphere and of better than 15 % in the middle and upper troposphere, respectively, whereby surface emissivity and atmospheric temperature uncertainties are the leading error sources. For the middle tropospheric δD values we estimate a precision of 15–20‰, with the measurement noise being the dominating error source. We compare our IASI products to a large number of quasi coincident radiosonde in-situ and ground-based FTS (Fourier Transform Spectrometer) remote sensing measurements and find no significant bias between the H 2 O and δD data obtained by the different techniques. Furthermore, the scatter between the different data sets confirms our theoretical precision estimates.

Description: In-situ observation of Asian pollution transported into the Arctic lowermost stratosphere Atmospheric Chemistry and Physics Discussions, 11, 16265-16310, 2011 Author(s): A. Roiger, H. Schlager, A. Schäfler, H. Huntrieser, M. Scheibe, H. Aufmhoff, O. R. Cooper, H. Sodemann, A. Stohl, J. Burkhart, M. Lazzara, C. Schiller, K. S. Law, and F. Arnold On a research flight on 10 July 2008, the German research aircraft Falcon sampled an air mass with unusually high carbon monoxide (CO), peroxyacetyl nitrate (PAN) and water vapour (H 2 O) mixing ratios in the Arctic lowermost stratosphere. The air mass was encountered twice at an altitude of 11.3 km, ~800 m above the dynamical tropopause. In-situ measurements of ozone, NO, and NO y indicate that this layer was a mixed air mass containing both air from the troposphere and stratosphere. Backward trajectory and Lagrangian particle dispersion model analysis suggest that the Falcon sampled the top of a polluted air mass originating from the coastal regions of East Asia. The anthropogenic pollution plume experienced strong up-lift in a warm conveyor belt (WCB) located over the Russian east-coast. Subsequently the Asian air mass was transported across the North Pole into the sampling area, elevating the local tropopause by up to ~3 km. Mixing with surrounding Arctic stratospheric air most likely took place during the horizontal transport when the tropospheric streamer was stretched into long and narrow filaments. The mechanism illustrated in this study possibly presents an important pathway to transport pollution into the polar tropopause region.