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Intergovernmental Panel on Climate Change (IPCC) assessments are the trusted source of scientific evidence for climate negotiations taking place under the United Nations Framework Convention on Climate Change (UNFCCC), including the first global stocktake under the Paris Agreement that will conclude at COP28 in December 2023. Evidence-based decision-making needs to be informed by up-to-date and timely information on key indicators of the state of the climate system and of the human influence on the global climate system. However, successive IPCC reports are published at intervals of 5–10 years, creating potential for an information gap between report cycles. We follow methods as close as possible to those used in the IPCC Sixth Assessment Report (AR6) Working Group One (WGI) report. We compile monitoring datasets to produce estimates for key climate indicators related to forcing of the climate system: emissions of greenhouse gases and short-lived climate forcers, greenhouse gas concentrations, radiative forcing, surface temperature changes, the Earth's energy imbalance, warming attributed to human activities, the remaining carbon budget, and estimates of global temperature extremes. The purpose of this effort, grounded in an open data, open science approach, is to make annually updated reliable global climate indicators available in the public domain (https://doi.org/10.5281/zenodo.8000192, Smith et al., 2023a). As they are traceable to IPCC report methods, they can be trusted by all parties involved in UNFCCC negotiations and help convey wider understanding of the latest knowledge of the climate system and its direction of travel. The indicators show that human-induced warming reached 1.14 [0.9 to 1.4] ∘C averaged over the 2013–2022 decade and 1.26 [1.0 to 1.6] ∘C in 2022. Over the 2013–2022 period, human-induced warming has been increasing at an unprecedented rate of over 0.2 ∘C per decade. This high rate of warming is caused by a combination of greenhouse gas emissions being at an all-time high of 54 ± 5.3 GtCO2e over the last decade, as well as reductions in the strength of aerosol cooling. Despite this, there is evidence that increases in greenhouse gas emissions have slowed, and depending on societal choices, a continued series of these annual updates over the critical 2020s decade could track a change of direction for human influence on climate.

International audience; Due to the lack of observations, the budgets of key chemical species in the upper troposphere/lower stratosphere region are still highly uncertain. Hence, the large scale gradient of CO/CO2/CH4 or the abundance and feature of oxygenated compounds are poorly captured by the global chemistry-transport models. The CARIBIC project relies on the use of a passenger aircraft for making frequent atmospheric chemistry measurements mainly in the tropopause region. It provides accurate and simultaneous observations of several compounds in the upper troposphere/lower stratosphere. These airborne observations, nowadays covering several years, are used in this work to evaluate the ability of two global models (TM5 and LMDz-INCA) to simulate the seasonal variations, inter-hemispheric differences and vertical distributions of CH4, CO and CO2 in this region. Furthermore, as CARIBIC provides original measurements of oxygenated compounds, the distribution and budget of such compounds which control the radical production in the UTLS, is also investigated. This work is performed in the framework of the GEOmon EU project.

The SPS-OKEANOS (Outsized Kitecraft for Exploration and AstroNautics in the Outer Solar system) is under study in Japan to rendezvous with and land on a Jupiter Trojan asteroid [1]. It is primarily an engineering mission to demonstrate advanced space technology but also to conduct key science for understanding the solar system origin and evolution by conducting indepth measurements there. The concept is complementary to LUCY [2], multi-flybys to six Jupiter Trojans, that was selected as a Discovery class mission, aiming at understanding the variation and diversity of Jupiter Trojans. The SPS-OKEANOS mission is jointly studied between engineers and scientists both from Japan and Europe [3]. The scientific objectives and the strawman payloads for this mission are described here

To quantify changes in air pollution over Europe at the 2050 horizon, we designed a comprehensive modelling system that captures the external factors considered to be most relevant, and that relies on up-to-date and consistent sets of air pollution and climate policy scenarios. Global and regional climate as well as global chemistry simulations are based on the recent representative concentration pathways (RCP) produced for the Fifth Assessment Report (AR5) of the IPCC (Intergovernmental Panel on Climate Change) whereas regional air quality modelling is based on the updated emissions scenarios produced in the framework of the Global Energy Assessment. We explored two diverse scenarios: a reference scenario where climate policies are absent and a mitigation scenario which limits global temperature rise to within 2 °C by the end of this century. This first assessment of projected air quality and climate at the regional scale based on CMIP5 (5th Coupled Model Intercomparison Project) climate simulations is in line with the existing literature using CMIP3. The discrepancy between air quality simulations obtained with a climate model or with meteorological reanalyses is pointed out. Sensitivity simulations show that the main factor driving future air quality projections is air pollutant emissions, rather than climate change or intercontinental transport of pollution. Whereas the well documented "climate penalty" that weights upon ozone (increase of ozone pollution with global warming) over Europe is confirmed, other features appear less robust compared to the literature, such as the impact of climate on PM2.5. The quantitative disentangling of external factors shows that, while several published studies focused on the climate penalty bearing upon ozone, the contribution of the global ozone burden is somewhat overlooked in the literature.

Dry deposition is a key component of surface–atmosphere exchange of compounds, acting as a sink for several chemical species. Meteorological factors, chemical properties of the trace gas considered and land surface properties are strong drivers of dry deposition efficiency and variability. Under both climatic and anthropogenic pressure, the vegetation distribution over the Earth has been changing a lot over the past centuries and could be significantly altered in the future. In this study, we perform a modeling investigation of the potential impact of land-cover changes between the present day (2006) and the future (2050) on dry deposition velocities at the surface, with special interest for ozone (O3) and nitric acid (HNO3), two compounds which are characterized by very different physicochemical properties. The 3-D chemistry-transport model LMDz-INCA is used, considering changes in vegetation distribution based on the three future projections, RCPs 2.6, 4.5 and 8.5, and present-day (2007) meteorology. The 2050 RCP 8.5 vegetation distribution leads to a rise of up to 7 % (+0.02 cm s−1) in the surface deposition velocity calculated for ozone (Vd,O3) and a decrease of −0.06 cm s−1 in the surface deposition velocity calculated for nitric acid (Vd,HNO3) relative to the present-day values in tropical Africa and up to +18 and −15 %, respectively, in Australia. When taking into account the RCP 4.5 scenario, which shows dramatic land-cover change in Eurasia, Vd,HNO3 increases by up to 20 % (annual-mean value) and reduces Vd,O3 by the same magnitude in this region. When analyzing the impact of surface dry deposition change on atmospheric chemical composition, our model calculates that the effect is lower than 1 ppb on annual-mean surface ozone concentration for both the RCP 8.5 and RCP 2.6 scenarios. The impact on HNO3 surface concentrations is more disparate between the two scenarios regarding the spatial repartition of effects. In the case of the RCP 4.5 scenario, a significant increase of the surface O3 concentration reaching locally by up to 5 ppb (+5 %) is calculated on average during the June–August period. This scenario also induces an increase of HNO3 deposited flux exceeding locally 10 % for monthly values. Comparing the impact of land-cover change to the impact of climate change, considering a 0.93 °C increase of global temperature, on dry deposition velocities, we estimate that the strongest increase over lands occurs in the Northern Hemisphere during winter, especially in Eurasia, by +50 % (+0.07 cm s−1) for Vd,O3 and +100 % (+0.9 cm s−1) for Vd,HNO3. However, different regions are affected by both changes, with climate change impact on deposition characterized by a latitudinal gradient, while the land-cover change impact is much more heterogeneous depending on vegetation distribution modification described in the future RCP scenarios. The impact of long-term land-cover changes on dry deposition is shown to be significant and to differ strongly from one scenario to another. It should therefore be considered in biosphere–atmospheric chemistry interaction studies in order to have a fully consistent picture.

Ozone (O-3) precursor emissions influence regional and global climate and air quality through changes in tropospheric O-3 and oxidants, which also influence methane (CH4) and sulfate aerosols (SO42-). We examine changes in the tropospheric composition of O-3, CH4, SO42- and global net radiative forcing (RF) for 20% reductions in global CH4 burden and in anthropogenic O-3 precursor emissions (NOx, NMVOC, and CO) from four regions (East Asia, Europe and Northern Africa, North America, and South Asia) using the Task Force on Hemispheric Transport of Air Pollution Source-Receptor global chemical transport model (CTM) simulations, assessing uncertainty (mean +/- 1 standard deviation) across multiple CTMs. We evaluate steady state O-3 responses, including long-term feedbacks via CH4. With a radiative transfer model that includes greenhouse gases and the aerosol direct effect, we find that regional NOx reductions produce global, annually averaged positive net RFs (0.2 +/- 0.6 to 1.7 +/- 2 mWm(-2)/TgN yr(-1)), with some variation among models. Negative net RFs result from reductions in global CH4 (-162.6 +/- 2 mWm(-2) for a change from 1760 to 1408 ppbv CH4) and regional NMVOC (-0.4 +/- 0.2 to -0.7 +/- 0.2 mWm(-2)/Tg C yr(-1)) and CO emissions (-0.13 +/- 0.02 to -0.15 +/- 0.02 mWm(-2)/Tg CO yr(-1)). Including the effect of O-3 on CO2 uptake by vegetation likely makes these net RFs more negative by -1.9 to -5.2 mWm(-2)/Tg N yr(-1), -0.2 to -0.7 mWm(-2)/Tg C yr(-1), and -0.02 to -0.05 mWm(-2)/Tg CO yr(-1). Net RF impacts reflect the distribution of concentration changes, where RF is affected locally by changes in SO42-, regionally to hemispherically by O-3, and globally by CH4. Global annual average SO42- responses to oxidant changes range from 0.4 +/- 2.6 to -1.9 +/- 1.3 Gg for NOx reductions, 0.1 +/- 1.2 to -0.9 +/- 0.8 Gg for NMVOC reductions, and -0.09 +/- 0.5 to -0.9 +/- 0.8 Gg for CO reductions, suggesting additional research is needed. The 100-year global warming potentials (GWP(100)) are calculated for the global CH4 reduction (20.9 +/- 3.7 without stratospheric O-3 or water vapor, 24.2 +/- 4.2 including those components), and for the regional NOx, NMVOC, and CO reductions (-18.7 +/- 25.9 to -1.9 +/- 8.7 for NOx, 4.8 +/- 1.7 to 8.3 +/- 1.9 for NMVOC, and 1.5 +/- 0.4 to 1.7 +/- 0.5 for CO). Variation in GWP(100) for NOx, NMVOC, and CO suggests that regionally specific GWPs may be necessary and could support the inclusion

International audience; Satellites data can be combined in sophisticated inversion systems that take the impact of the chemistry of the lower atmosphere into account. In order to improve our knowledge of the emissions of greenhouse gases and of their precursors at the global scale, we have developed a multi-species inversion system of the emission fluxes and 3D-production of the gases involved in the oxidation chain of methane (CH4 ), in particular carbon monoxide (CO) and formaldehyde (HCHO), reacting with hydroxyl radicals (OH). The inverse modelling approach offers an attractive solution to the problem of estimating the gaseous emissions, that allows to correct the prior inventories. For the first time, this study aims at estimating the CO emissions for the past ten years (from March 2000 to December 2009), using our multi-species inversion system and the MOPITT Version 4 retrievals. Our validation strategy consists in comparing our posterior-modelled concentrations with several sets of independent measurements: surface measurements, aircraft and satellite. The posterior emissions, with a global 10-year average of 1368 TgCO/yr, are 37% higher than the prior ones, built from the EDGAR 3.2 and the GFEDv2 inventories (1005 TgCO/yr on average). In addition, they present some significant and challenging seasonal variations in the northern hemisphere that are not present in our prior nor in others major inventories (IPCC AR5, RETRO, EPA, EMEP). Our results exhibit some large interannual variability due to biomass burning emissions, climate, and socioeconomic factors. Indeed, the highest annual budget of the last decade is seen for year 2007 with 1457 TgCO/yr, because of fires over South East Asia and South American Temperate. Then, the CO emissions show a dramatic decrease (1277 TgCO/yr in 2009), that could be connected to the economic recession.

International audience; Tropospheric ozone and aerosols are major climate forcing agents but the quantification of their forcings are still highly uncertain. In the framework of the ACC-MIP, a global decadal dataset describing the emissions of aerosols and ozone precursors and consistent with those provided for long-lived GHG, was made available. It allowed chemistry-climate models to simulate the evolution of short-lived GHG over the historical period (1850-2005) and for 4 future projections (Representative Concentration Pathways). This work describes the results obtained with the LMDz-INCA model using these state-of-the-art emissions and considering past and future climate change trajectories in a General Circulation Model. These concentrations are currently used as forcing by the two French Earth system models (IPSL-CM5 and CNRM-CM5) to simulate the climate evolution in the CMIP5 framework. The variability and trends of the concentrations along with their relevance will be discussed. In parallel to these projections considering the anthropogenic emissions changes, a short insight on the potential impact of natural isoprene emission changes on ozone will be shown.

International audience; In order to study the spatial and temporal variations of the emissions of greenhouse gases and of their precursors , we developed a data assimilation system and applied it to infer emissions of CH 4 , CO and H 2 for one year. It is based on an atmospheric chemical transport model and on a simplified scheme for the oxidation chain of hydrocarbons , including methane, formaldehyde, carbon monoxide and molecular hydrogen together with methyl chloroform. The methodology is exposed and a first attempt at evaluating the inverted fluxes is made. Inversions of the emission fluxes of CO, CH 4 and H 2 and concentrations of HCHO and OH were performed for the year 2004, using surface concentration measurements of CO, CH 4 , H 2 and CH 3 CCl 3 as constraints. Independent data from ship and aircraft measurements and satellite retrievals are used to evaluate the results. The total emitted mass of CO is 30% higher after the inversion , due to increased fluxes by up to 35% in the Northern Hemisphere. The spatial distribution of emissions of CH 4 is modified by a decrease of fluxes in boreal areas up to 60%. The comparison between mono-and multi-species inversions shows that the results are close at a global scale but may significantly differ at a regional scale because of the interactions between the various tracers during the inversion.

International audience; Within ACCENT, a European Network of Excellence, eighteen atmospheric models from the U.S., Europe, and Japan calculated present (2000) and future (2030) concentrations of ozone at the Earth's surface with hourly temporal resolution. Comparison of model results with surface ozone measurements in 14 world regions indicates that levels and seasonality of surface ozone in North America and Europe are characterized well by global models, with annual average biases typically within 5–10 nmol/mol. However, comparison with rather sparse observations over some regions suggest that most models overestimate annual ozone by 15–20 nmol/mol in some locations. Two scenarios from the International Institute for Applied Systems Analysis (IIASA) and one from the Intergovernmental Panel on Climate Change Special Report on Emissions Scenarios (IPCC SRES) have been implemented in the models. This study focuses on changes in near-surface ozone and their effects on human health and vegetation. Different indices and air quality standards are used to characterise air quality. We show that often the calculated changes in the different indices are closely inter-related. Indices using lower thresholds are more consistent between the models, and are recommended for global model analysis. Our analysis indicates that currently about two-thirds of the regions considered do not meet health air quality standards, whereas only 2–4 regions remain below the threshold. Calculated air quality exceedances show moderate deterioration by 2030 if current emissions legislation is followed and slight improvements if current emissions reduction technology is used optimally. For the "business as usual" scenario severe air quality problems are predicted. We show that model simulations of air quality indices are particularly sensitive to how well ozone is represented, and improved accuracy is needed for future projections. Additional measurements are needed to allow a more quantitative assessment of the risks to human health and vegetation from changing levels of surface ozone.

Fifteen chemistry-transport models are used to quantify, for the first time, the export of oxidised nitrogen (NOy) to and from four regions (Europe, North America, South Asia, and East Asia), and to estimate the uncertainty in the results. Between 12 and 24% of the NOx emitted is exported from each region annually. The strongest impact of each source region on a foreign region is: Europe on East Asia, North America on Europe, South Asia on East Asia, and East Asia on North America. Europe exports the most NOy, and East Asia the least. East Asia receives the most NOy from the other regions. Between 8 and 15% of NOx emitted in each region is transported over distances larger than 1000 km, with 3-10% ultimately deposited over the foreign regions.

REanalysis of the TROpospheric chemical composition over thepast 40 years (RETRO)Objectives:• exploit (often under-utilised) existing data sets from ground based stations,aircraft, and satellite instruments, integrating these into common datasets,• develop tools for the analysis, interpretation and exploitation of the data,• formulate recommendations for future measurement strategies,• assess changes in trace compound emissions and their effect on troposphericchemical composition and aerosols, and the associated radiative forcing, overthe past 40 years,• provide an assessment of uncertainties caused by climate variability,• evaluate emission control strategies in Europe,• predict changes over the next 20 years in tropospheric composition, andradiative forcing through model studies using the emission scenarios definedfor the IPCC 2001 climate assessment,• analyze the magnitude of intercontinental pollutant transport.Scientific achievements:• first detailed, comprehensive and consistent data sets on global emissionsfrom fossil and biofuel combustion and from open vegetation burningcovering the time period 1960-2000; available as gridded data sets with0.5°×0.5° and monthly mean resolution,• first global long-term atmospheric chemistry integrations with several stateof-the-art models using the ERA-40 meteorological data, the RETROemissions and other constrains in a consistent and well-documented manner,• analysis of key parameters controlling the interannual and seasonalvariability and the longer-term trends in the tropospheric composition relatedto ozone and its precursors,• development of new software tools for the analysis of observational data andmodel results; standardisation of model output and data formats anddefinition of model evaluation metrics and skill scores,• development of a comprehensive data base for tropospheric compositionobservations with complete metadata definition and a user-friendly interfacefor data access,• multi-model analysis of specific scenarios related to power generation andthe traffic sector in OECD countries,• contributions to the IPCC 4th assessment report through participation in amulti-model assessment study coordinated by the ACCENT network ofexcellenceMain deliverables:• gridded data sets of global emissions from 1960 to 2000 (D1-4)• gridded data sets of simulated concentrations of ozone and ozone precursorsfrom 1960 to 2000 (D4-3)• data base of atmospheric observationsreport on the RETRO emission inventories (D1-6)o report on trend analysis based on observational data (D2-3)o reports on sensitivity studies (D3-2, D3-3 and D3-4)o report on the long-term reanalysis simulations (D4-4)o reports on changes in UV and radiative forcing (D5-1 and D5-2)o analysis of past policy measures including scenario studies (D5-5)All RETRO deliverables are available on the RETRO ftp serverftp://ftp.retro.enes.org/pub or via the RETRO web pages http://retro.enes.org.Socio-economic relevance and policy implications:Understanding past trends in large-scale air pollution is a key requirement toformulate future strategies to enhance air quality in Europe and elsewhere. Due tothe scarcity of observational data (particularly for years before 1990), numericalmodels are the only way to obtain comprehensive and consistent information onpast trends in tropospheric ozone and related substances. The model simulationsin RETRO also allow for evaluating the impact of important parameters on theinterannual variability of air pollution in Europe. From specific sensitivity studiesthe roles of meteorological variability, variability in anthropogenic emissions andvariability in wildland fire emissions could be determined. Scenario calculationson the impact of certain policy measures to reduce emissions from powergeneration or from the traffic sector provide a direct answer to the potentialconsequences of the introduction of more stringent air pollution abatementmeasures. Due to the effects of intercontinental transport of air pollution, localmeasures may not always achieve the expected results. These issues can only beaddressed with comprehensive, well-constrained global model simulations as theywere pioneered in the RETRO project. Several of the tools and methodologiesdeveloped in RETRO are now applied in ongoing projects with a more immediatepolicy link (especially the FP6 integrated project GEMS and the multi-modelassessment activity under the auspices of the task force on hemispheric transportof air pollution).Conclusions:• RETRO contributed significant developments to advance the state-of-the-artin global atmospheric chemistry modelling in terms of new long-termemissions data and the first comprehensive long-term multi-modelintegrations of atmospheric chemistry and transport,• The RETRO models generally show very good consistency in terms ofinterannual variability and trend patterns, but they occasionally differsignificantly in the absolute values of the simulated concentrations of ozoneand its precursors,• Comparison with observations shows that the RETRO models often capturethe atmospheric variability patterns rather well and that the mean modelgenerally provides a reasonable description of the chemical state of theatmosphere. Some exceptions are noted, in particular the absence of anincreasing ozone trend over central Europe in the 1990s which is clearly seenfrom measurements at several mountain stations,• According to the RETRO simulations, summertime boundary layerconcentrations of ozone over Europe increased between 5 and 15 ppb (10-20%) between 1960 and the year 2000. Further abatement measures in thetraffic sector (introduction of the EURO 5 standard in all OECD countries)could lead to a reduction of summertime ozone by 8-10%.• A renewed effort following on the RETRO activities would be highlydesirable in order to further expand the available emission data sets(inclusion of SO2, aerosol precursors and greenhouse gases and review ofnatural emissions), to further investigate the underlying causes fortropospheric composition change and to include the analysis of feedbacksbetween tropospheric ozone, climate change and aerosols.Dissemination of results:• more than 30 articles appeared in peer reviewed scientific journals with director indirect contributions from the RETRO project,• RETRO project results were presented in more than 100 conferencecontributions and presentations at international meetings,• a specific session on “tropospheric trends and variability” was inaugurated atthe EGU General Symposium 2004 and was repeated in 2006 and 2007,• RETRO emission data sets are being used in several ongoing researchprojects (e.g. GEMS and TFHTAP, GREENCYCLES, etc.),• All RETRO data sets and reports are available on the RETRO ftp serverftp://ftp.retro.enes.org/pub or via the RETRO web pages athttp://retro.enes.org.

We present a systematic comparison of tropospheric NO2 from 17 global atmospheric chemistry models with three state-of-the-art retrievals from the Global Ozone Monitoring Experiment (GOME) for the year 2000. The models used constant anthropogenic emissions from IIASA/EDGAR3.2 and monthly emissions from biomass burning based on the 1997–2002 average carbon emissions from the Global Fire Emissions Database (GFED). Model output is analyzed at 10:30 local time, close to the overpass time of the ERS-2 satellite, and collocated with the measurements to account for sampling biases due to incomplete spatiotemporal coverage of the instrument. We assessed the importance of different contributions to the sampling bias: correlations on seasonal time scale give rise to a positive bias of 30–50% in the retrieved annual means over regions dominated by emissions from biomass burning. Over the industrial regions of the eastern United States, Europe and eastern China the retrieved annual means have a negative bias with significant contributions (between –25% and +10% of the NO2 column) resulting from correlations on time scales from a day to a month. We present global maps of modeled and retrieved annual mean NO2 column densities, together with the corresponding ensemble means and standard deviations for models and retrievals. The spatial correlation between the individual models and retrievals are high, typically in the range 0.81–0.93 after smoothing the data to a common resolution. On average the models underestimate the retrievals in industrial regions, especially over eastern China and over the Highveld region of South Africa, and overestimate the retrievals in regions dominated by biomass burning during the dry season. The discrepancy over South America south of the Amazon disappears when we use the GFED emissions specific to the year 2000. The seasonal cycle is analyzed in detail for eight different continental regions. Over regions dominated by biomass burning, the timing of the seasonal cycle is generally well reproduced by the models. However, over Central Africa south of the Equator the models peak one to two months earlier than the retrievals. We further evaluate a recent proposal to reduce the NOx emission factors for savanna fires by 40% and find that this leads to an improvement of the amplitude of the seasonal cycle over the biomass burning regions of Northern and Central Africa. In these regions the models tend to underestimate the retrievals during the wet season, suggesting that the soil emissions are higher than assumed in the models. In general, the discrepancies between models and retrievals cannot be explained by a priori profile assumptions made in the retrievals, neither by diurnal variations in anthropogenic emissions, which lead to a marginal reduction of the NO2 abundance at 10:30 local time (by 2.5–4.1% over Europe). Overall, there are significant differences among the various models and, in particular, among the three retrievals. The discrepancies among the retrievals (10–50% in the annual mean over polluted regions) indicate that the previously estimated retrieval uncertainties have a large systematic component. Our findings imply that top-down estimations of NOx emissions from satellite retrievals of tropospheric NO2 are strongly dependent on the choice of model and retrieval.

We present a systematic comparison of tropospheric NO2 from 17 global atmospheric chemistry models with three state-of-the-art retrievals from the Global Ozone Monitoring Experiment (GOME) for the year 2000. The models used constant anthropogenic emissions from IIASA/EDGAR3.2 and monthly emissions from biomass burning based on the 1997 - 2002 average carbon emissions from the Global Fire Emissions Database (GFED). Model output is analyzed at 10: 30 local time, close to the overpass time of the ERS-2 satellite, and collocated with the measurements to account for sampling biases due to incomplete spatiotemporal coverage of the instrument. We assessed the importance of different contributions to the sampling bias: correlations on seasonal time scale give rise to a positive bias of 30 - 50% in the retrieved annual means over regions dominated by emissions from biomass burning. Over the industrial regions of the eastern United States, Europe and eastern China the retrieved annual means have a negative bias with significant contributions ( between - 25% and + 10% of the NO2 column) resulting from correlations on time scales from a day to a month. We present global maps of modeled and retrieved annual mean NO2 column densities, together with the corresponding ensemble means and standard deviations for models and retrievals. The spatial correlation between the individual models and retrievals are high, typically in the range 0.81 - 0.93 after smoothing the data to a common resolution. On average the models underestimate the retrievals in industrial regions, especially over eastern China and over the Highveld region of South Africa, and overestimate the retrievals in regions dominated by biomass burning during the dry season. The discrepancy over South America south of the Amazon disappears when we use the GFED emissions specific to the year 2000. The seasonal cycle is analyzed in detail for eight different continental regions. Over regions dominated by biomass burning, the timing of the seasonal cycle is generally well reproduced by the models. However, over Central Africa south of the Equator the models peak one to two months earlier than the retrievals. We further evaluate a recent proposal to reduce the NOx emission factors for savanna fires by 40% and find that this leads to an improvement of the amplitude of the seasonal cycle over the biomass burning regions of Northern and Central Africa. In these regions the models tend to underestimate the retrievals during the wet season, suggesting that the soil emissions are higher than assumed in the models. In general, the discrepancies between models and retrievals cannot be explained by a priori profile assumptions made in the retrievals, neither by diurnal variations in anthropogenic emissions, which lead to a marginal reduction of the NO2 abundance at 10: 30 local time ( by 2.5 - 4.1% over Europe). Overall, there are significant differences among the various models and, in particular, among the three retrievals. The discrepancies among the retrievals ( 10 - 50% in the annual mean over polluted regions) indicate that the previously estimated retrieval uncertainties have a large systematic component. Our findings imply that top-down estimations of NOx emissions from satellite retrievals of tropospheric NO2 are strongly dependent on the choice of model and retrieval.

We use 23 atmospheric chemistry transport models to calculate current and future (2030) deposition of reactive nitrogen (NOy, NHx) and sulfate (SOx) to land and ocean surfaces. The models are driven by three emission scenarios: (1) current air quality legislation (CLE); (2) an optimistic case of the maximum emissions reductions currently technologically feasible (MFR); and (3) the contrasting pessimistic IPCC SRES A2 scenario. An extensive evaluation of the present-day deposition using nearly all information on wet deposition available worldwide shows a good agreement with observations in Europe and North America, where 60–70% of the model-calculated wet deposition rates agree to within ±50% with quality-controlled measurements. Models systematically overestimate NHx deposition in South Asia, and underestimate NOy deposition in East Asia. We show that there are substantial differences among models for the removal mechanisms of NOy, NHx, and SOx, leading to ±1 s variance in total deposition fluxes of about 30% in the anthropogenic emissions regions, and up to a factor of 2 outside. In all cases the mean model constructed from the ensemble calculations is among the best when comparing to measurements. Currently, 36–51% of all NOy, NHx, and SOx is deposited over the ocean, and 50–80% of the fraction of deposition on land falls on natural (nonagricultural) vegetation. Currently, 11% of the world's natural vegetation receives nitrogen deposition in excess of the “critical load” threshold of 1000 mg(N) m-2 yr-1. The regions most affected are the United States (20% of vegetation), western Europe (30%), eastern Europe (80%), South Asia (60%), East Asia (40%), southeast Asia (30%), and Japan (50%). Future deposition fluxes are mainly driven by changes in emissions, and less importantly by changes in atmospheric chemistry and climate. The global fraction of vegetation exposed to nitrogen loads in excess of 1000 mg(N) m-2 yr-1 increases globally to 17% for CLE and 25% for A2. In MFR, the reductions in NOy are offset by further increases for NHx deposition. The regions most affected by exceedingly high nitrogen loads for CLE and A2 are Europe and Asia, but also parts of Africa.

International audience; The objective of this work was to develop and assess an automatic procedure to generate reduced chemical schemes for the atmospheric photooxidation of volatile organic carbon (VOC) compounds. The procedure is based on (i) the development of a tool for writing the fully explicit schemes for VOC oxidation (see companion paper Aumont et al., 2005), (ii) the application of several commonly used reduction methods to the fully explicit scheme, and (iii) the assessment of resulting errors based on direct comparison between the reduced and full schemes. The reference scheme included seventy emitted VOCs chosen to be representative of both anthropogenic and biogenic emissions, and their atmospheric degradation chemistry required more than two million reactions among 350000 species. Three methods were applied to reduce the size of the reference chemical scheme: (i) use of operators, based on the redundancy of the reaction sequences involved in the VOC oxidation, (ii) grouping of primary species having similar reactivities into surrogate species and (iii) grouping of some secondary products into surrogate species. The number of species in the final reduced scheme is 147, this being small enough for practical inclusion in current three-dimensional models. Comparisons between the fully explicit and reduced schemes, carried out with a box model for several typical tropospheric conditions, showed that the reduced chemical scheme accurately predicts ozone concentrations and some other aspects of oxidant chemistry for both polluted and clean tropospheric conditions.