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This study analyses three extreme ozone (O3) episodes that occurred in Barcelona (NE Spain) during the summers of 2015, 2018, and 2019. These episodes exceeded the EU's hourly information threshold (180 µg m−3) for the first time since at least the year 2000, raising concerns due to Barcelona's large population. By employing experimental data and various modelling tools, our main objective is to elucidate the underlying phenomena of these recent O3 episodes and improve predictive capabilities. The findings indicate that the factors contributing to these occurrences are largely consistent across episodes. These factors, with estimated O3 contributions specified for particular instances, comprise (i) initial O3 accumulation in surrounding coastal areas; (ii) weekend occurrence, accompanied by the corresponding weekend effect (+15 µg m−3); and (iii) the prevalence of Tramontana meteorological conditions during above-normal temperatures, which (iv) force the convergence of multiregional polluted air masses to the city (+45–65 µg m−3). Major source areas include regions of southern France through the Gulf of Lion, the interior of the Mediterranean, and eastern Spanish coastal regions, including Barcelona's pollution plume. Some of these factors, which may manifest in the days preceding the episodes, are observable or can be anticipated. This study enhances understanding of the mechanisms driving extreme O3 episodes recently observed in Barcelona and provides valuable insights for prediction., This research has received financial support from the Spanish Ministry of Ecological Transition and Demographic Challenge, the Generalitat de Catalunya (grant no. AGAUR 2021 SGR 00447), the University of the Basque Country (UPV/EHU) (grant no. GIU21/050) and Eduardo Torre-Pascual's Margarita Salas grant (grant no. MARSA21/23). The article processing charges for this open-access publication were covered by the CSIC Open Access Publication Support Initiative through its Unit of Information Resources for Research (URICI)., Postprint (published version)

Gaseous and particulate chlorine species play an important role in modulating tropospheric oxidation capacity, aerosol water uptake, visibility degradation, and human health. The lack of recent global continental chlorine emissions has hindered modeling studies of the role of chlorine in the atmosphere. Here, we develop a comprehensive global emission inventory of gaseous HCl and particulate Cl-(pCl), including 35 sources categorized in six source sectors based on published up-to-date activity data and emission factors. These emissions are gridded at a spatial resolution of 0.1° × 0.1° for the years 1960 to 2014. The estimated emissions of HCl and pCl in 2014 are 2354 (1661-3201) and 2321 (930-3264) Gg Cl a-1, respectively. Emissions of HCl are mostly from open waste burning (38%), open biomass burning (19%), energy (19%), and residential (13%) sectors, and the major sources classified by fuel type are combustion of waste (43%), biomass (32%), and coal (25%). Emissions of pCl are mostly from biofuel (29%) and open biomass burning processes (44%). The sectoral and spatial distributions of HCl and pCl emissions are very heterogeneous along the study period, and the temporal trends are mainly driven by the changes in emission factors, energy intensity, economy, and population.

Megacities and other major population centres (MPCs) worldwide are major sources of air pollution, both locally as well as downwind. The overall assessment and prediction of the impact of MPC pollution on tropospheric chemistry are challenging. The present work provides an overview of the highlights of a major new contribution to the understanding of this issue based on the data and analysis of the EMeRGe (Effect of Megacities on the transport and transformation of pollutants on the Regional to Global scales) international project. EMeRGe focuses on atmospheric chemistry, dynamics, and transport of local and regional pollution originating in MPCs. Airborne measurements, taking advantage of the long range capabilities of the High Altitude and LOng Range Research Aircraft (HALO, https://www.halo-spp.de, last access: 22 March 2022), are a central part of the project. The synergistic use and consistent interpretation of observational data sets of different spatial and temporal resolution (e.g. from ground-based networks, airborne campaigns, and satellite measurements) supported by modelling within EMeRGe provide unique insight to test the current understanding of MPC pollution outflows. In order to obtain an adequate set of measurements at different spatial scales, two field experiments were positioned in time and space to contrast situations when the photochemical transformation of plumes emerging from MPCs is large. These experiments were conducted in summer 2017 over Europe and in the inter-monsoon period over Asia in spring 2018. The intensive observational periods (IOPs) involved HALO airborne measurements of ozone and its precursors, volatile organic compounds, aerosol particles, and related species as well as coordinated ground-based ancillary observations at different sites. Perfluorocarbon (PFC) tracer releases and model forecasts supported the flight planning, the identification of pollution plumes, and the analysis of chemical transformations during transport, The HALO deployment during EMeRGe was funded by a consortium comprising the German Research Foundation (DFG) Priority Program HALO-SPP 1294, the Institute of Atmospheric Physics of DLR, the Max Planck Society (MPG), and the Helmholtz Association. Flora Kluge, Benjamin Schreiner, and Klaus Pfeilsticker acknowledge the support given by the DFG through the project nos. PF 384-16, PF 384-17, and PG 385-19. Ralf Koppmann and Marc Krebsbach acknowledge DFG funding through project no. KR3861_1-1. Katja Bigge acknowledges additional funding from the Heidelberg Graduate School for Physics. Johannes Schneider, Katharina Kaiser, and Stephan Borrmann acknowledge funding through the DFG (project no. 316589531). Lisa Eirenschmalz and Hans Schlager acknowledge support by DFG through project MEPOLL (SCHL1857/4-1). Anna B. Kalisz Hedegaard would like to thank DAAD and DLR for a Research Fellowship. Hans Schlager acknowledge financial support by the DLR TraK (Transport and Climate) project. Michael Sicard acknowledges support from the EU (GA nos. 654109, 778349, 871115, and 101008004) and the Spanish Government (ref. nos. CGL2017-90884-REDT, PID2019-103886RB-I00, RTI2018-096548-B-I00, and MDM-2016-0600). Midhun George, Yangzhuoran Liu, M. Dolores Andrés Hernández, and John Phillip Burrows acknowledge financial support from the University of Bremen. FLEXPART simulations were performed on the HPC cluster Aether at the University of Bremen, financed by DFG within the scope of the Excellence Initiative. Anne-Marlene Blechschmidt was partly funded through the CAMS-84 project. Jennifer Wolf acknowledges support from the German Federal Ministry for Economic Affairs and Energy – BMWi (project Digitally optimized Engineering for Services – DoEfS; contract no. 20X1701B). Theresa Harlass thanks DLR VOR for funding the young investigator research group “Greenhouse Gases”. Mariano Mertens, Patrick Jöckel, and Markus Kilian acknowledge resources of the Deutsches Klimarechenzentrum (DKRZ) granted b, Peer Reviewed, Article signat per 53 autors/es: M. Dolores Andrés Hernández (1), Andreas Hilboll (2), Helmut Ziereis (3), Eric Förster (4), Ovid O. Krüger (5), Katharina Kaiser (6,7), Johannes Schneider (7), Francesca Barnaba (8), Mihalis Vrekoussis (2,18), Jörg Schmidt (9), Heidi Huntrieser (3), Anne-Marlene Blechschmidt (1), Midhun George (1), Vladyslav Nenakhov (1,a), Theresa Harlass (3), Bruna A. Holanda (5), Jennifer Wolf (3), Lisa Eirenschmalz (3), Marc Krebsbach (10), Mira L. Pöhlker (5,b), Anna B. Kalisz Hedegaard (3,2), Linlu Mei (1), Klaus Pfeilsticker (11), Yangzhuoran Liu (1), Ralf Koppmann (10), Hans Schlager (3), Birger Bohn (12), Ulrich Schumann (3), Andreas Richter (1), Benjamin Schreiner (11), Daniel Sauer (3), Robert Baumann (3), Mariano Mertens (3), Patrick Jöckel (3), Markus Kilian (3), Greta Stratmann (3,c,) Christopher Pöhlker (5), Monica Campanelli (8), Marco Pandolfi (13), Michael Sicard (14,15), José L. Gómez-Amo (16), Manuel Pujadas (17), Katja Bigge (11), Flora Kluge (11), Anja Schwarz (9), Nikos Daskalakis (2), David Walter (5), Andreas Zahn (4), Ulrich Pöschl (5), Harald Bönisch (4), Stephan Borrmann (6,7), Ulrich Platt (11), and John P. Burrows (1) // (1) Institute of Environmental Physics, University of Bremen, Bremen, Germany; (2) Laboratory for Modeling and Observation of the Earth System, Institute of Environmental Physics, Bremen, Germany; (3) Deutsches Zentrum für Luft- und Raumfahrt (DLR), Institut für Physik der Atmosphäre, Oberpfaffenhofen, Germany; (4) Atmospheric Trace Gases and Remote Sensing, Karlsruhe Institute of Technology, Institute of Meteorology and Climate Research, Karlsruhe, Germany; (5) Multiphase Chemistry Department, Max Planck Institute for Chemistry, Mainz, Germany; (6) Institute for Atmospheric Physics, Johannes Gutenberg University, Mainz, Germany, (7) Particle Chemistry Department, Max Planck Institute for Chemistry, Mainz, Germany; (8) National Research Council of Italy, Institute of Atmospheric Sciences and Climate (CNR, Postprint (published version)

Tropospheric O3 is a noxious ambient pollutant that severely damages human health. In this study, a considerable number of model simulations with comprehensive designs have been conducted to investigate the formation and depletion of tropospheric O3 over the Guangdong-Hong Kong-Macau Greater Bay Area (GBA) and the Asia-Pacific Region. Station-based observational results indicate that, over the last two decades (2000–2019), O3 concentration significantly increases over the GBA, with a rate of 0.5–1.0 ppbv yr-1. A matrix of emission reduction experiments is then designed and executed to figure out the formation of O3 pollution over the GBA. According to the simulation results, to reduce ~10 ppbv O3 formations, dramatic elimination (>25%) in NOx or anthropogenic Volatile Organic Compounds (AVOCs) emission is required. O3 formation is more sensitive to NOx reduction compared to AVOCs reduction in most cases, while during some episodes reducing AVOCs is a more effective way to impede O3 formation. Meanwhile, the lack of halogen chemical processes over the coastal region limits our ability to accurately simulate O3 concentration. Implementing the CMAQ model with novel bromine and iodine mechanisms, we show distinct halogen-mediated O3 depletion features over the land and ocean in the Asia-Pacific region. Across a coastline-centric 400-km-wide belt from onshore to offshore, averaged maximum gradient of O3 loss reaches 1.1 ppbv/100 km at surface level. To further analyze the relationship between the coastal O3 depletion and oceanic airflow, a novel Lagrangian-based concept of Ocean Air Occupation Ratio (OAOR), defined as the total period that an oceanic air parcel resides within a specific area, has been proposed and investigated. We show that the OAOR increases at the largest rate of 10–15%/100 km across a coastline-centr

Our understanding of the processes that control the burden and budget of tropospheric ozone has changed dramatically over the last 60 years. Models are the key tools used to understand these changes, and these underscore that there are many processes important in controlling the tropospheric ozone budget. In this critical review, we assess our evolving understanding of these processes, both physical and chemical. We review model simulations from the International Global Atmospheric Chemistry Atmospheric Chemistry and Climate Model Intercomparison Project and Chemistry Climate Modelling Initiative to assess the changes in the tropospheric ozone burden and its budget from 1850 to 2010. Analysis of these data indicates that there has been significant growth in the ozone burden from 1850 to 2000 (approximately 43 ± 9%) but smaller growth between 1960 and 2000 (approximately 16 ± 10%) and that the models simulate burdens of ozone well within recent satellite estimates. The Chemistry Climate Modelling Initiative model ozone budgets indicate that the net chemical production of ozone in the troposphere plateaued in the 1990s and has not changed since then inspite of increases in the burden. There has been a shift in net ozone production in the troposphere being greatest in the northern mid and high latitudes to the northern tropics, driven by the regional evolution of precursor emissions. An analysis of the evolution of tropospheric ozone through the 21st century, as simulated by Climate Model Intercomparison Project Phase 5 models, reveals a large source of uncertainty associated with models themselves (i.e., in the way that they simulate the chemical and physical processes that control tropospheric ozone). This structural uncertainty is greatest in the near term (two to three decades), but emissions scenarios dominate uncertainty in the longer term (2050¿2100) evolution of tropospheric ozone. This intrinsic model uncertainty prevents robust predictions of near-term change

Mercury (Hg), a global contaminant, is emitted mainly in its elemental form Hgto the atmosphere where it is oxidized to reactive Hgcompounds, which efficiently deposit to surface ecosystems. Therefore, the chemical cycling between the elemental and oxidized Hg forms in the atmosphere determines the scale and geographical pattern of global Hg deposition. Recent advances in the photochemistry of gas-phase oxidized Hgand Hgspecies postulate their photodissociation back to Hgas a crucial step in the atmospheric Hg redox cycle. However, the significance of these photodissociation mechanisms on atmospheric Hg chemistry, lifetime, and surface deposition remains uncertain. Here we implement a comprehensive and quantitative mechanism of the photochemical and thermal atmospheric reactions between Hg, Hg, and Hgspecies in a global model and evaluate the results against atmospheric Hg observations. We find that the photochemistry of Hgand Hgleads to insufficient Hg oxidation globally. The combined efficient photoreduction of Hgand Hgto Hgcompetes with thermal oxidation of Hg, resulting in a large model overestimation of 99% of measured Hgand underestimation of 51% of oxidized Hg and ∼66% of Hgwet deposition. This in turn leads to a significant increase in the calculated global atmospheric Hg lifetime of 20 mo, which is unrealistically longer than the 3-6-mo range based on observed atmospheric Hg variability. These results show that the Hgand Hgphotoreduction processes largely offset the efficiency of bromine-initiated Hgoxidation and reveal missing Hg oxidation processes in the troposphere.

Our understanding of the processes that control the burden and budget of tropospheric ozone has changed dramatically over the last 60 years. Models are the key tools used to understand these changes, and these underscore that there are many processes important in controlling the tropospheric ozone budget. In this critical review, we assess our evolving understanding of these processes, both physical and chemical. We review model simulations from the International Global Atmospheric Chemistry Atmospheric Chemistry and Climate Model Intercomparison Project and Chemistry Climate Modelling Initiative to assess the changes in the tropospheric ozone burden and its budget from 1850 to 2010. Analysis of these data indicates that there has been significant growth in the ozone burden from 1850 to 2000 (approximately 43 + 9%) but smaller growth between 1960 and 2000 (approximately 16 + 10%) and that the models simulate burdens of ozone well within recent satellite estimates. The Chemistry Climate Modelling Initiative model ozone budgets indicate that the net chemical production of ozone in the troposphere plateaued in the 1990s and has not changed since then inspite of increases in the burden. There has been a shift in net ozone production in the troposphere being greatest in the northern mid and high latitudes to the northern tropics, driven by the regional evolution of precursor emissions. An analysis of the evolution of tropospheric ozone through the 21st century, as simulated by Climate Model Intercomparison Project Phase 5 models, reveals a large source of uncertainty associated with models themselves (i.e., in the way that they simulate the chemical and physical processes that control tropospheric ozone). This structural uncertainty is greatest in the near term (two to three decades), but emissions scenarios dominate uncertainty in the longer term (2050–2100) evolution of tropospheric ozone. This intrinsic model uncertainty prevents robust predictions of near-term change

The authors wish to thank WOUDC, GAW, EMEP, WDCGG, CASTNET-EPA, NADP and EANET for the provision of measurement stations. Also, thanks go to the free use of the MOPITT CO data obtained from the NASA Langley Research Center Atmospheric Science Data Center. SCIAMACHY radiances have been provided by ESA. This work is funded by grants CGL2013-46736-R, Supercomputación and e-ciencia Project (CSD2007-0050) from the Consolider-Ingenio 2010 program of the Spanish Ministry of Economy and Competitiveness. Further support was provided by the SEV-2011-00067 grant of the Severo Ochoa Program, awarded by the Spanish Government. A.H. received funding from the Earth System Science Research School (ESSReS), an initiative of the Helmholtz Association of German research centres (HGF) at the AlfredWegener Institute for Polar and Marine Research. All the numerical simulations were performed with the MareNostrum Supercomputer hosted by the Barcelona Supercomputing Center. We also thank Beatriz Monge-Sanz for providing the COPCAT coefficients., Peer Reviewed, Postprint (author's final draft)

Hydroxyl radical (OH) is the primary oxidant in the troposphere, initiating the removal of numerous atmospheric species including greenhouse gases, pollutants that are detrimental to human health, and ozone-depleting substances. Because of the complexity of OH chemistry, models vary widely in their OH chemistry schemes and resulting methane (CH4) lifetimes. The current state of knowledge concerning global OH abundances is often contradictory. This body of work encompasses three projects that investigate tropospheric OH from a modeling perspective, with the goal of improving the tropospheric community’s knowledge of the atmospheric lifetime of CH4. First, measurements taken during the airborne CONvective TRansport of Active Species in the Tropics (CONTRAST) field campaign are used to evaluate OH in global models. A box model constrained to measured variables is utilized to infer concentrations of OH along the flight track. Results are used to evaluate global model performance, suggest against the existence of a proposed “OH Hole” in the tropical Western Pacific, and investigate implications of high O3/low H2O filaments on chemical transport to the stratosphere. While methyl chloroform-based estimates of global mean OH suggest that models are overestimating OH, we report evidence that these models are actually underestimating OH in the tropical Western Pacific. The second project examines OH within global models to diagnose differences in CH4 lifetime. I developed an approach to quantify the roles of OH precursor field differences (O3, H2O, CO, NOx, etc.) using a neural network method. This technique enables us to approximate the change in CH4 lifetime resulting from variations in individual precursor fields. The dominant factors driving CH4 lifetime differences between models are O3, CO, and J(O3-O1D). My third project evaluates the effect of climate change on global fields of OH using an empirical model. Observations of H2O and O3 from satellite instruments are comb

Biofuels are considered to be an environmental friendly alternative to fossil fuels as they have the potential to reduce the global emissions of greenhouse gases. Studies have showed that an increased use of ethanol could alter the atmospheric chemical composition and enhance urban ozone concentrations resulting in higher human mortality rates. In recent years furanic compounds have been considered as second generation biofuels as they can be produced from non-food biomass. This project aims to improve the understanding of the impact of furanic biofuels in the atmosphere. Laboratory studies have been performed in smog chambers at Copenhagen Centre for Atmospheric Research and the University of Oslo, considering the reactions of furan, 2,3-dihydrofuran and 2,5-dihydrofuran with ozone and chlorine radicals. The furanic ozonolysis mechanism was further studied using computational methods. The results of the relative rate studies show that furan reacts slower with both ozone and Cl than the two DHF do, while the reactions of 2,3-DHF with ozone and Cl is faster than the corresponding reactions of 2,5-DHF. When comparing the results with literature data it was found that furans will mainly decompose due to tropospheric reactions with OH. The furanic Cl reaction may become important, close to local chlorine sources, in highly polluted areas were elevated VOC concentrations result in increased competition for the tropospheric OH reaction. Furthermore it was found that 2,3-DHF react immediately with Cl2, while furan and 2,5-DHF did not. The results of the product study propose that furanic ozonolysis produces shorter oxygenated organic compounds, such as aldehydes and carboxylic acids. This study shows that furanic biofuels mainly decompose into atmospheric compounds associated with elevated tropospheric ozone levels and urban air pollution. Emission of these compounds to the atmosphere can therefore be expected to result in elevated levels of ground level ozone., Biobränslen anses vara ett miljövänligt alternativ till fossila bränslen eftersom de har potential att minska de globala utsläppen av växthusgaser. Studier har visat att en ökad användning av etanol kan komma att påverka atmosfärens kemiska sammansättning och på så sätt bidra till förhöjda koncentrationer av marknära ozon, en luftförorening som är känd för att orsaka ökad dödlighet. Furaner kan framställas från bland annat mat- och skogsavfall och har därför introducerats som en andra generationens biobränslen. Detta projekt syftar till att förbättra förståelsen för hur furan-baserade biobränslen kan komma att påverka atmosfären. Laborationer har utförts i gaskammare vid Köpenhamns Universitet och Universitet i Oslo för att studera hur furan, 2,3-dihydrofuran och 2,5-dihydrofuran reagerar med ozon och fria klorradikaler. Vidare har den kemiska mekanismen för ozonolys av furaner studerats med hjälp av kemiska beräkningsmetoder. De experimentella resultaten tyder på att furan reagerar långsammare med ozon och klor än vad de båda DHF gör, 2,3-DHF reagerar dessutom snabbare med ozon och klor än vad 2,5-DHF gör. Resultaten jämfördes med tidigare forskning och det kunde konstateras att furaner huvudsakligen reagerar med OH i troposfären. Däremot kan klorreaktioner komma att bli betydelsefulla nära lokala klorkällor, i kraftigt förorenade områden med ökad konkurrens om OH. Vidare visade studien att 2,3-DHF reagerar omedelbart med Cl2, men att varken furan och 2,5-DHF gör det. Produktstudien tyder på att ozonolys av furaner producerar korta syreinnehållande kolföreningar, så som aldehyder och karboxylsyror. Denna studie visar att furan-baserade biobränslen huvudsakligen bryts ner till kemiska föreningar som är associerade med luftföroreningar och förhöjda ozonhalter i troposfären. Utsläpp av olika furaner till atmosfären förväntas därför att resultera i förhöjda nivåer av marknära ozon.

Biomass burning is an important contributor to global total emissions of NOx (NO+NO2). Generally bottom-up fire emissions models calculate NOx emissions by multiplying fuel consumption estimates with static biome-specific emission factors, defined in units of grams of NO per kilogram of dry matter consumed. Emission factors are a significant source of uncertainty in bottom-up fire emissions modeling because relatively few observations are available to characterize the large spatial and temporal variability of burning conditions. In this paper we use NO2 tropospheric column observations from the Ozone Monitoring Instrument (OMI) from the year 2005 over South America to calculate monthly NOx emission factors for four fire types: deforestation, savanna/grassland, woodland, and agricultural waste burning. In general, the spatial patterns in NOx emission factors calculated in this work are consistent with emission factors derived from in situ measurements from the region but are more variable than published biome-specific global average emission factors widely used in bottom-up fire emissions inventories such as the Global Fire Emissions Database (GFED). Satellite-based NOx emission factors also indicate substantial temporal variability in burning conditions. Overall, we found that deforestation fires have the lowest NOx emission factors, on average 30% lower than the emission factors used in GFED v3. Agricultural fire NOx emission factors were the highest, on average a factor of 1.8 higher than GFED v3 values. For savanna, woodland, and deforestation fires, early dry season NOx emission factors were a factor of ~1.5–2 higher than late dry season emission factors. A minimum in the NOx emission factor seasonal cycle for deforestation fires occurred in August, the time period of severe drought in South America in 2005, supporting the hypothesis that prolonged dry spells may lead to an increase in the contribution of smoldering combustion from large-diameter fuels, offsetti

The global methane (CH4) growth rate showed large variations after the eruption of Mount Pinatubo in June 1991. Both sources and sinks of tropospheric CH4 were altered following the eruption, by feedback processes between climate and tropospheric photochemistry. Such processes include Ultra Violet (UV) radiative changes due to the presence of volcanic sulfur dioxide (SO2) and sulphate aerosols in the stratosphere, and due to stratospheric ozone depletion. Changes in temperature and water vapour in the following years caused changes in tropospheric chemistry, as well as in natural emissions. We present a sensitivity study that investigates the relative effects that these processes had on tropospheric CH4 concentrations, using a simple one-dimensional chemistry model representative for the global tropospheric column. To infer the changes in UV radiative fluxes, the chemistry model is coupled to a radiative transfer model. We find that the overall effect of natural processes after the eruption on the CH4 growth rate is dominated by the reduction in CH4 lifetime due to stratospheric ozone depletion. However, all the other processes are found to have non-negligible effects, and should therefore be taken into account in order to obtain a good estimate of CH4 concentrations after Pinatubo. We find that the overall effect was a small initial increase in the CH4 growth rate after the eruption, followed by a decrease of about 7 ppb yr(-1) by mid-1993. When changes in anthropogenic emissions are employed according to emission inventories, an additional decrease of about 5 ppb yr(-1) in the CH4 growth rate is obtained between the years 1991 and 1993. The results using the simplified single column model are in good qualitative agreement with observed changes in the CH4 growth rate. Further analysis, taking into account changes in the dynamics of the atmosphere, variations in emissions from biomass burning, and in biogenic emissions of non-methane volatile organic compounds (NMVO

An assessment of the global distribution of nitrogen oxides (NOx) is required for a satisfactory description of tropospheric chemistry and in the evaluation of the global impact of increasing anthropogenic emissions of NOx. In the mathematical models utilized for this purpose, it is necessary to have the natural as well as man-made sources of NOx in the atmosphere as inputs. Thunderstorms are a main natural source of NOx in the atmosphere and it may be the dominant source of NOx in the troposphere in equatorial and tropical South Pacific., QC 20170622

This study presents the results of a broad intercomparison of a total of 15 global aerosol models within the AeroCom project. Each model is compared to observations related to desert dust aerosols, their direct radiative effect, and their impact on the biogeochemical cycle, i.e., aerosol optical depth (AOD) and dust deposition. Additional com parisons to Angstrom exponent (AE), coarse mode AOD and dust surface concentrations are included to extend the assessment of model performance and to identify common biases present in models. These data comprise a benchmark dataset that is proposed for model inspection and future dust model development. There are large differences among the global models that simulate the dust cycle and its impact on climate. In general, models simulate the climatology of vertically integrated parameters (AOD and AE) within a factor of two whereas the total deposition and surface concentration are reproduced within a factor of 10. In addition, smaller mean normalized bias and root mean square errors are obtained for the climatology of AOD and AE than for total deposition and surface concentration. Characteristics of the datasets used and their uncertainties may influence these differences. Large uncertainties still exist with respect to the deposition fluxes in the southern oceans. Further measurements and model studies are necessary to assess the general model performance to reproduce dust deposition in ocean regions sensible to iron contributions. Models overestimate the wet deposition in regions dominated by dry deposition. They generally simulate more realistic surface concentration at stations downwind of the main sources than at remote ones. Most models simulate the gradient in AOD and AE between the different dusty regions. However the seasonality and magnitude of both variables is better simulated at African stations than Middle East ones. The models simulate the offshore transport of West Africa throughout the year but they overestimat

Atmospheric formaldehyde (H2CO) is an intermediate product common to the degradation of many volatile organic compounds and therefore it is a central component of the tropospheric chemistry. While the global formaldehyde background is due to methane oxidation, emissions of non-methane volatile organic compounds (NMVOCs) from biogenic, biomass burning and anthropogenic continental sources result in important and localised enhancements of the H2CO concentration. Recent spaceborne nadir sensors provide an opportunity to quantify the abundance of tropospheric formaldehyde at the global scale, and thereby to improve our knowledge of NMVOC emissions. This is essential for a better understanding of the processes that control the production and the evolution of tropospheric ozone, a key actor in air quality and climate change, but also of the hydroxyl radical OH, the main cleansing agent of our troposphere. For this reason, H2CO satellite observations are increasingly used in combination with tropospheric chemistry transport models to constrain NMVOC emission inventories in so-called top-down inversion approaches. Such inverse modelling applications require well characterised satellite data products consistently retrieved over long time periods.This work reports on global observations of formaldehyde columns retrieved from the successive solar backscatter nadir sensors GOME, SCIAMACHY and GOME-2, respectively launched in 1995, 2002 and 2006. The retrieval procedure is based on the differential optical absorption spectroscopy technique (DOAS). Formaldehyde concentrations integrated along the mean atmospheric optical path are derived from the recorded spectra in the UV region, and further converted to vertical columns by means of calculated air mass factors. These are obtained from radiative transfer simulations, accounting for cloud coverage, surface properties and best-guess H2CO profiles, the latter being derived from the IMAGES chemistry transport model. A key task of, Doctorat en Sciences de l'ingénieur, info:eu-repo/semantics/nonPublished

A novel approach for measuring vertical profiles of HCs and particle number concentrations was described and applied in the low troposphere over Milan (Italy) during typical spring and summer days. Particle profiles yielded nearly homogeneous concentrations below the mixing height, with level-to-ground concentration ratios of 92-97%, while HCs showed a more pronounced decrease (74-95%). Vertical mixing and photochemical loss of HCs were demonstrated to cause the e gradients. Much lower concentrations were observed for the profiles above the mixing height, where the HC mixtures showed also a different composition, which was partially explained by the horizontal advection of air with HC sources different to those prevailing at the site. The application of pseudo-firs: order kinetics for reactions between HCs and the hydroxyl radical allowed for the estimation of the vertical mixing time scale in the order of 100 +/- 20 min

We apply a four-dimensional variational (4D-VAR) data assimilation system to optimize carbon monoxide (CO) emissions for 2003 and 2004 and to reduce the uncertainty of emission estimates from individual sources using the chemistry transport model TM5. The system is designed to assimilate large (satellite) datasets, but in the current study only a limited amount of surface network observations from the National Oceanic and Atmospheric Administration Earth System Research Laboratory (NOAA/ESRL) Global Monitoring Division (GMD) is used to test the 4D-VAR system. By design, the system is capable to adjust the emissions in such a way that the posterior simulation reproduces background CO mixing ratios and large-scale pollution events at background stations. Uncertainty reduction up to 60 % in yearly emissions is observed over well-constrained regions and the inferred emissions compare well with recent studies for 2004. However, with the limited amount of data from the surface network, the system becomes data sparse resulting in a large solution space. Sensitivity studies have shown that model uncertainties (e.g., vertical distribution of biomass burning emissions and the OH field) and the prior inventories used, influence the inferred emission estimates. Also, since the observations only constrain total CO emissions, the 4D-VAR system has difficulties in separating anthropogenic and biogenic sources in particular. The inferred emissions are validated with NOAA aircraft data over North America and the agreement is significantly improved from the prior to posterior simulation. Validation with the Measurements Of Pollution In The Troposphere (MOPITT) instrument version 4 (V4) shows a slight improved agreement over the well-constrained Northern Hemisphere and in the tropics (except for the African continent). However, the model simulation with posterior emissions underestimates MOPITT CO total columns on the remote Southern Hemisphere (SH) by about 10 %. This is caused by a

We combine CO column measurements from the MOPITT, AIRS, SCIAMACHY, and TES satellite instruments in a full-year (May 2004-April 2005) global inversion of CO sources at 4 degrees x 5 degrees spatial resolution and monthly temporal resolution. The inversion uses the GEOS-Chem chemical transport model (CTM) and its adjoint applied to MOPITT, AIRS, and SCIAMACHY. Observations from TES, surface sites (NOAA/GMD), and aircraft (MOZAIC) are used for evaluation of the a posteriori solution. Using GEOS-Chem as a common intercomparison platform shows global consistency between the different satellite datasets and with the in situ data. Differences can be largely explained by different averaging kernels and a priori information. The global CO emission from combustion as constrained in the inversion is 1350 Tg a(-1). This is much higher than current bottom-up emission inventories. A large fraction of the correction results from a seasonal underestimate of CO sources at northern mid-latitudes in winter and suggests a larger-than-expected CO source from vehicle cold starts and residential heating. Implementing this seasonal variation of emissions solves the long-standing problem of models underestimating CO in the northern extratropics in winter-spring. A posteriori emissions also indicate a general underestimation of biomass burning in the GFED2 inventory. However, the tropical biomass burning constraints are not quantitatively consistent across the different datasets.

We combine CO column measurements from the MOPITT, AIRS, SCIAMACHY, and TES satellite instruments in a full-year (May 2004-April 2005) global inversion of CO sources at 4 degrees x 5 degrees spatial resolution and monthly temporal resolution. The inversion uses the GEOS-Chem chemical transport model (CTM) and its adjoint applied to MOPITT, AIRS, and SCIAMACHY. Observations from TES, surface sites (NOAA/GMD), and aircraft (MOZAIC) are used for evaluation of the a posteriori solution. Using GEOS-Chem as a common intercomparison platform shows global consistency between the different satellite datasets and with the in situ data. Differences can be largely explained by different averaging kernels and a priori information. The global CO emission from combustion as constrained in the inversion is 1350 Tg a(-1). This is much higher than current bottom-up emission inventories. A large fraction of the correction results from a seasonal underestimate of CO sources at northern mid-latitudes in winter and suggests a larger-than-expected CO source from vehicle cold starts and residential heating. Implementing this seasonal variation of emissions solves the long-standing problem of models underestimating CO in the northern extratropics in winter-spring. A posteriori emissions also indicate a general underestimation of biomass burning in the GFED2 inventory. However, the tropical biomass burning constraints are not quantitatively consistent across the different datasets.

In-situ airborne measurements of trace gases, aerosol size distributions, chemistry and optical properties were conducted over Mexico and the Eastern North Pacific during MILAGRO and INTEX-B. Heterogeneous reactions between secondary aerosol precursor gases and mineral dust lead to sequestration of sulfur, nitrogen and chlorine in the supermicrometer particulate size range. Simultaneous measurements of aerosol size distributions and weak-acid soluble calcium result in an estimate of 11 wt% of CaCO3 for Asian dust. During transport across the North Pacific, ~5–30% of the CaCO3 is converted to CaSO4 or Ca(NO3)2 with an additional ~4% consumed through reactions with HCl. The 1996 to 2008 record from the Mauna Loa Observatory confirm these findings, indicating that, on average, 19% of the CaCO3 has reacted to form CaSO4 and 7% has reacted to form Ca(NO3)2 and ~2% has reacted with HCl. In the nitrogen-oxide rich boundary layer near Mexico City up to 30% of the CaCO3 has reacted to form Ca(NO3)2while an additional 8% has reacted with HCl. These heterogeneous reactions can result in a ~3% increase in dust solubility which has an insignificant effect on their optical properties compared to their variability in-situ. However, competition between supermicrometer dust and submicrometer primary aerosol for condensing secondary aerosol species led to a 25% smaller number median diameter for the accumulation mode aerosol. A 10–25% reduction of accumulation mode number median diameter results in a 30–70% reduction in submicrometer light scattering at relative humidities in the 80–95% range. At 80% RH submicrometer light scattering is only reduced ~3% due to a higher mass fraction of hydrophobic refractory components in the dust-affected accumulation mode aerosol. Thus reducing the geometric mean diameter of the submicrometer aerosol has a much larger effect on aerosol optical properties than changes to the hygroscopic:hydrophobic mass fractions of the accumulation mode aerosol. In

We know that, at least in the short term, a rise in temperature exponentially increases the emission rates of most biogenic volatile organic compounds (BVOCs). It does so by enhancing the enzymatic activities of synthesis, by raising the BVOC vapour pressure, and by decreasing the resistance of the diffusion pathway (Tingey et al., 1991). BVOC emissions are thus expected to increase strongly with globally rising temperatures (IPCC, 2007). By applying the most frequently used algorithms of emission response to temperature (Guenther et al., 1995), it is easy to estimate that climate warming over the past 30 yr (IPCC, 2007) could have already increased BVOC global emissions by 10%. A further 2–3°C rise in the mean global temperature, which is predicted to occur early this century (IPCC, 2007), could increase BVOC global emissions by an additional 30–45%. Furthermore, global warming in boreal and temperate environments not only means warmer average and warmer winter temperatures, but also implies an extended plant activity season (Peñuelas & Filella, 2001), further increasing the total annual emissions. The biological and environmental effects of such increases in emissions can be substantial (Peñuelas & Llusià, 2003). However, many investigations are conducted in the laboratory, and we still do not know very much about the emissions of BVOCs in the field in response to warming. We know even less about medium- to long-term responses of BVOC emissions to warming, and about some regions of our planet, such as the Arctic, that are likely to experience the most pronounced effects of climatic warming. Tiiva et al. have started to fill this gap in our knowledge in this issue of New Phytologist (pp. 853–863). They have measured the emissions of isoprene, the most emitted reactive BVOC from vegetation, at a subarctic heath that they have experimentally subjected to a 3–4°C air temperature enhancement for 8 yr. Tiiva et al. have measured increases in emissions ranging between 56

Doctorat en Sciences, info:eu-repo/semantics/nonPublished

Measurements of several short-lived photochemical species (e.g., OH, HO2, and CH2O) were obtained from the DC-8 and P3-B aircraft during the NASA Transport and Chemical Evolution over the Pacific (TRACE-P) campaign. To assess fast photochemical theory over the east Asian coast and western Pacific, these measurements are compared to predictions using a photochemical time-dependent box model constrained by coincident measurements of long-lived tracers and physical parameters. Both OH and HO2 are generally overpredicted by the model throughout the troposphere, which is a different result from previous field campaigns. The calculated-to-observed ratio of OH shows an altitude trend, with OH overpredicted by 80% in the upper troposphere and by 40-60% in the middle troposphere. Boundary layer and lower tropospheric OH ratios decrease from middle tropospheric values to 1.07 for the DC-8 and to 0.70 for the P3-B. HO2 measured on the DC-8 is overpredicted by a median of 23% and shows no trend in the agreement with altitude. Three subsets of data which compose 12% of the HO2 measurements represent outliers with respect to calculated-to-observed ratios: stratospherically influenced air, upper tropospheric data with NO > 135 pptv, and data from within clouds. Pronounced underpredictions of both HO2 and OH were found for stratospherically influenced air, which is in contrast to previous studies showing good agreement of predicted and observed HOx, in the stratosphere. Observational evidence of heterogeneous uptake of HO2 within low and middle tropospheric clouds is presented, though there is no indication of significant HO2 uptake within higher-altitude clouds. Model predictions of CH2O are in good agreement with observations in the median for background concentrations, but a large scatter exists. Factors contributing to this scatter are examined, including the limited availability of some important constraining measurements, particularly CH3OOH. Some high concentrations of CH

We investigate the influence of mineral dust on tropospheric chemistry in the present climate at the global scale. The analysis examines the effects of dust on photolysis and heterogeneous uptake, operating independently and together. In numerical experiments the size-resolved, time-varying mineral dust distribution predicted by the global Dust Entrainment and Deposition (DEAD) model perturbs the gas phase species in a global chemical transport model (University of California at Irvine (UCI) CTM). We find that the photolysis perturbation dominates limited regions in the low to middle troposphere, while heterogeneous uptake dominates the rest of atmosphere. Coupling of the photochemical and heterogeneous effects of dust is weak in the global mean but moderate in dusty regions, where coupling is sometimes responsible for more than 20% of local O3 changes. Ozone and odd-nitrogen concentrations are perturbed in opposite directions by photolysis and heterogeneous chemistry, resulting in a weak net change. However, both processes decrease the concentrations of OH and HO2. The global mean change due to dust is −0.7% for tropospheric O3, −11.1% for OH, −5.2% for HO2, and −3.5% for HNO3. Large seasonal signals are present near dust source regions. Over the North African region and tropical Atlantic Ocean downwind, OH decreases by −66.8%, six times more than the global mean reduction. Interestingly, net photolysis-induced annual mean O3 changes are greater in the Southern Hemisphere than in the Northern Hemisphere, where significantly more dust and O3 precursors reside. In polar regions, O3 change is dominated by transported O3 and is not sensitive to local dust concentration. O3 change due to photolysis depends not only on dust vertical structure but also on the availability of O3 precursors. O3 change due to heterogeneous reactions on dust is sensitive to dust vertical structure, mainly through the influence of temperature on uptake rates.

Aerosols affect the global budgets of O3, OH, and CH4 in part through their alteration of photolysis rates and in part through their direct chemical interactions with gases (i.e., “heterogeneous chemistry”). The first effect is evaluated here with a global tropospheric chemistry transport model using recently developed global climatologies of tropospheric aerosols: a satellite-derived aerosol climatology over the oceans by advanced very high resolution radiometer and a model-generated climatology for land plus oceans by the Center for Climate System Research. Globally averaged, the impact of aerosols on photolysis alone is to increase tropospheric O3 by 0.63 Dobson units and increase tropospheric CH4 by 130 ppb (via tropospheric OH decreases of 8%). These greenhouse gas increases lead to an aerosol indirect effect (counting both natural and anthropogenic aerosols) of +0.08 W/m2. Although the CH4 increases are, of course, global, the changes in tropospheric OH and O3 are mainly regional, with the largest impacts in northwest Africa for January and in India and southern Africa for July. The influence of aerosols is greater in July than in January and greater in the Northern Hemisphere than in the Southern Hemisphere, as expected given the pollution sources in the Northern Hemisphere. The predominant impact is due to the aerosols over land; aerosols over the ocean contribute less than a third to globally integrated changes.

Trois instruments ont été développés dans le cadre de ce travail pour la mesure par spectroscopie d'absorption différentielle dans le domaine UV-visible. Le premier utilise un spectromètre par transformée de Fourier, les deux autres des spectromètres à réseau associés soit à une barrette de photodiodes soit à un détecteur de type CCD. Ces instruments ont été conçus dans le but de fournir des mesures de divers constituants (03, SO2, NO2, HNO2, H2CO, toluène, benzène) de manière automatique et en utilisant des trajets d'absorption courts ( < 1 km). Les performances de chacun de ces instruments ont été évaluées au cours de différentes campagnes de mesure. Le spectromètre par transformée de Fourier s'avère être un outil performant pour de telles mesures, son principal avantage étant de posséder une calibration en longueur d'onde interne, précise et reproductible. Les instruments utilisant un spectromètre à réseau associé à un détecteur multi-éléments présentent un certain nombre d'inconvénients rendant peu aisées les mesures troposphériques sur de courtes distances. Ces inconvénients sont liés soit a, Doctorat en sciences appliquées, info:eu-repo/semantics/nonPublished

De manière à mieux comprendre l'impact des émissions anthropiques sur la troposphère reculée, les concentrations d'un nombre important de composés atmosphériques ont été mesurés dans la troposphère libre au-dessus d'Hawaii durant la campagne Mauna Loa Observatory Photochemistry Experiment (MLOPEX) accomplie au cours des années 1991 et 1992. Le constituant chimique fondamental pour évaluer cet impact est le radical hydroxyle OH qui fut mesuré au printemps et en été de l'année 1992. La variation diurne de la direction du vent génère pendant la journée un mélange des masses d'air de la couche limite planétaire avec la troposphère libre. Actuellement les modèles régional et global de chimie troposphérique ne peuvent tenir compte à la fois du transport à grande échelle et du mélange local. Nous avons développé un modèle tridimensionnel qui nous permet d'analyser la chimie et la dynamique troposphérique à ces deux échelles. Pour ce faire, nous avons utilisé une grille non-structurée qui offre un moyen efficace de caractériser la région d'Hawaii à l'aide d'une haute résolution et le restant de l'hémisphère Nord avec une résolution qui décroît au fur et à mesure que l'on s'éloigne d'Hawaii. La distribution de 46 composés gazeux avec 138 réactions, incluant une chimie détaillée des hydrocarbures non-méthaniques (isoprène, éthane, éthène, propène et alpha-pinène) est calculée avec un pas de temps de 20 minutes. A l'aide de notre modèle nous avons simulé une période de huit jours pour chacune des saisons. Les résultats des simulations ont été comparés aux observations et interprétés à l'aide d'études de rétro-trajectoires, de traceurs passifs et de bilans chimiques local et régional de l'ozone et de ses précurseurs., Doctorat en sciences appliquées, info:eu-repo/semantics/nonPublished