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Tropospheric levels of ozone and its precursors have risen in Asia since 2000. Satellite observations and chemistry–transport simulations suggest that transport of these pollutants to North America partly offsets benefits from stricter regulation. Rapid population growth and industrialization have driven substantial increases in Asian ozone precursor emissions over the past decade 1 , with highly uncertain impacts on regional and global tropospheric ozone levels. According to ozonesonde measurements 2 , 3 , tropospheric ozone concentrations at two Asian sites have increased by 1 to 3% per year since 2000, an increase thought to contribute to positive trends in the ozone levels observed at North America’s West Coast 4 , 5 . However, model estimates of the Asian contribution to North American ozone levels are not well-constrained by observations 6 , 7 . Here we interpret Aura satellite measurements of tropospheric concentrations of ozone and its precursor NO 2 , along with its largest natural source, stratospheric ozone, using the TM5 global chemistry–transport model. We show that tropospheric ozone concentrations over China have increased by about 7% between 2005 and 2010 in response to two factors: a rise in Chinese emissions by about 21% and increased downward transport of stratospheric ozone. Furthermore, we find that transport from China of ozone and its precursors has offset about 43% of the 0.42 DU reduction in free-tropospheric ozone over the western United States that was expected between 2005 and 2010 as a result of emissions reductions associated with federal, state and local air quality policies. We conclude that global efforts may be required to address regional air quality and climate change.

We provide a comprehensive description of the high-resolution version of the TM5-MP global chemistry transport model, which is to be employed for deriving highly resolved vertical profiles of nitrogen dioxide (NO2), formaldehyde (CH2O), and sulfur dioxide (SO2) for use in satellite retrievals from platforms such as the Ozone Monitoring Instrument (OMI) and the Sentinel-5 Precursor, and the TROPOspheric Monitoring Instrument (tropOMI). Comparing simulations conducted at horizontal resolutions of 3° × 2° and 1° × 1° reveals differences of ±20% exist in the global seasonal distribution of 222Rn, being larger near specific coastal locations and tropical oceans. For tropospheric ozone (O3), analysis of the chemical budget terms shows that the impact on globally integrated photolysis rates is rather low, in spite of the higher spatial variability of meteorological data fields from ERA-Interim at 1° × 1°. Surface concentrations of O3 in high-NOx regions decrease between 5 and 10% at 1° × 1° due to a reduction in NOx recycling terms and an increase in the associated titration term of O3 by NO. At 1° × 1°, the net global stratosphere-troposphere exchange of O3 decreases by ∼ 7%, with an associated shift in the hemispheric gradient. By comparing NO, NO2, HNO3 and peroxy-acetyl-nitrate (PAN) profiles against measurement composites, we show that TM5-MP captures the vertical distribution of NOx and long-lived NOx reservoirs at background locations, again with modest changes at 1° × 1°. Comparing monthly mean distributions in lightning NOx and applying ERA-Interim convective mass fluxes, we show that the vertical re-distribution of lightning NOx changes with enhanced release of NOx in the upper troposphere. We show that surface mixing ratios in both NO and NO2 are generally underestimated in both low- and high-NOx scenarios. For Europe, a negative bias exists for [NO] at the surface across the whole domain, with lower biases at 1° × 1° at only ∼ 20% of sites. For NO2, biases are more variable, with lower (higher) biases at 1° × 1° occurring at ∼ 35% ( ∼ 20%) of sites, with the remainder showing little change. For CH2O, the impact of higher resolution on the chemical budget terms is rather modest, with changes of less than 5%. The simulated vertical distribution of CH2O agrees reasonably well with measurements in pristine locations, although column-integrated values are generally underestimated relative to satellite measurements in polluted regions. For SO2, the performance at 1° × 1° is principally governed by the quality of the emission inventory, with limited improvements in the site-specific biases, with most showing no significant improvement. For the vertical column, improvements near strong source regions occur which reduce the biases in the integrated column. For remote regions missing biogenic source terms are inferred.

The daily analyses and forecasts of atmospheric composition delivered by the Copernicus Atmosphere Monitoring Service (CAMS) are produced by the ECMWF Integrated Forecasting System configured for COMPOsition (IFS-COMPO). In 2023 this system was upgraded to Cy48 which solves explicitly for stratospheric chemistry through a module extracted from the Belgian Assimilation System for Chemical ObsErvations (BASCOE). In 2024 the system was further upgraded to Cy49 which improves the representation of stratospheric composition with an adjusted parameterization of Polar Stratospheric Clouds (PSC), updated chemical rates for heterogeneous chemistry, and the implementation of missing processes to simulate an accurate distribution of sulfate aerosols in the stratosphere. Here we report on these improvements and evaluate the resulting stratospheric composition in chemical forecast mode, where the model is constrained by assimilation of meteorological observations but not by assimilation of composition observations. These evaluations comprise 13 gas-phase species and sulfate aerosols in three case studies: a global-scale assessment during a quiescent period (July 2023 to May 2024) in the context of the operational upgrade of the CAMS system; the evolution of key tracers related to polar ozone depletion during the winter and spring seasons across several years; and the evolution of stratospheric aerosols over the three years following the June 1991 Mount Pinatubo eruption. The model captures the rapid increase of the sulfate burden after the Pinatubo eruption, with the peak of stratospheric sulfate burden timed correctly, gradual recovery, and expected vertical profiles for quiescent periods. A scorecard assessment of chemical forecasts in the stratosphere of IFS-COMPO Cy49 highlights good performance for O3, CH4, N2O, and H2O and adequate performance for HCl, ClO, BrO and BrONO2 in the polar lower stratosphere. The model performance is poorer for HNO3, N2O5, NO2 and ClONO2, highlighting the need to improve the representation of heterogeneous chemistry, particularly the interactivity between aerosols and gas-phase composition, and refine the parameterization of PSC to better capture their impact on gas-phase composition. Overestimations of CH4 and N2O in the upper stratosphere are potentially related to the Brewer–Dobson Circulation, and long-standing biases of NO2 and O3 in the upper stratosphere remain unresolved. Despite these points for further development, IFS-COMPO will be a useful tool for studies of the couplings between stratospheric aerosols and gas-phase chemistry. The current cycle paves the way for assimilating stratospheric composition observations beyond ozone.

Top-down estimates of surface NOX emissions were derived for 23 European cities based on the downwind plume decay of tropospheric nitrogen dioxide (NO2) columns from the LOTOS-EUROS (Long Term Ozone Simulation-European Ozone Simulation) chemistry transport model (CTM) and from Ozone Monitoring Instrument (OMI) satellite retrievals, averaged for the summertime period (April-September) during 2013. Here we show that the top-down NOX emissions derived from LOTOS-EUROS for European urban areas agree well with the bottom-up NOX emissions from the MACC-III inventory data (R(exp 2) = 0.88) driving the CTM demonstrating the potential of this method. OMI top-down NOX emissions over the 23 European cities are generally lower compared with the MACC-III emissions and their correlation is slightly lower (R(exp 2) = 0.79). The uncertainty on the derived NO2 lifetimes and NOX emissions are on average ~55% for OMI and ~63% for LOTOS-EUROS data. The downwind NO2 plume method applied on both LOTOS-EUROS and OMI tropospheric NO2 columns allows to estimate NOX emissions from urban areas, demonstrating that this is a useful method for real-time updates of urban NOX emissions with reasonable accuracy.

The Copernicus Atmosphere Monitoring Service (CAMS) provides routine analyses and forecasts of trace gases and aerosols on a global scale. The core is the European Centre for Medium Range Weather Forecasts (ECMWF) Integrated Forecast System (IFS), where modules for atmospheric chemistry and aerosols have been introduced and which allows for data assimilation of satellite retrievals of composition.We have updated both the homogeneous and heterogeneous NOx chemistry applied in the three independent tropospheric–stratospheric chemistry modules maintained within CAMS, referred to as IFS(CB05BASCOE), IFS(MOCAGE) and IFS(MOZART). Here we focus on the evaluation of main trace gas products from these modules that are of interest as markers of air quality, namely lower-tropospheric O3, NO2 and CO, with a regional focus over the contiguous United States.Evaluation against lower-tropospheric composition reveals overall good performance, with chemically induced biases within 10 ppb across species for regions within the US with respect to a range of observations. The versions show overall equal or better performance than the CAMS reanalysis, which includes data assimilation. Evaluation of surface air quality aspects shows that annual cycles are captured well, albeit with variable seasonal biases. During wintertime conditions there is a large model spread between chemistry schemes in lower-tropospheric O3 (∼ 10 %–35 %) and, in turn, oxidative capacity related to NOx lifetime differences. Analysis of differences in theHNO3 and PAN formation, which act as reservoirs for reactive nitrogen, revealed a general underestimate in PAN formation over polluted regions, likely due to too low organic precursors. Particularly during wintertime, the fraction of NO2 sequestered into PAN has a variability of 100 % across chemistry modules, indicating the need for further constraints. Notably, a considerable uncertainty in HNO3 formation associated with wintertime N2O5 conversion on wet particle surfaces remains.In summary, this study has indicated that the chemically induced differences in the quality of CAMS forecast products over the United States depends on season, trace gas, altitude and region. While analysis of the three chemistry modules in CAMS provide a strong handle on uncertainties associated with chemistry modeling, the further improvement of operational products additionally requires coordinated development involving emissions handling, chemistry and aerosol modeling, complemented with data-assimilation efforts.

This work documents and evaluates the tropospheric gas-phase chemical mechanism MOGUNTIA in the three-dimensional chemistry transport model TM5-MP. Compared to the modified CB05 (mCB05) chemical mechanism previously used in the model, MOGUNTIA includes a detailed representation of the light hydrocarbons (C1–C4) and isoprene, along with a simplified chemistry representation of terpenes and aromatics. Another feature implemented in TM5-MP for this work is the use of the Rosenbrock solver in the chemistry code, which can replace the classical Euler backward integration method of the model. Global budgets of ozone (O3), carbon monoxide (CO), hydroxyl radicals (OH), nitrogen oxides (NOx), and volatile organic compounds (VOCs) are analyzed, and their mixing ratios are compared with a series of surface, aircraft, and satellite observations for the year 2006. Both mechanisms appear to be able to satisfactorily represent observed mixing ratios of important trace gases, with the MOGUNTIA chemistry configuration yielding lower biases than mCB05 compared to measurements in most of the cases. However, the two chemical mechanisms fail to reproduce the observed mixing ratios of light VOCs, indicating insufficient primary emission source strengths, oxidation that is too fast, and/or a low bias in the secondary contribution to C2–C3 organics via VOC atmospheric oxidation. Relative computational memory and time requirements of the different model configurations are also compared and discussed. Overall, the MOGUNTIA scheme simulates a large suite of oxygenated VOCs that are observed in the atmosphere at significant levels. This significantly expands the possible applications of TM5-MP.

Secondary Inorganic Aerosol (SIA) makes up a considerable fraction of the total particulate matter exposure and, thus, is an important product from any forecasting system of atmospheric composition and air quality. The subsequent loss to the surface of SIA via dry and wet deposition determines the duration of the exposure time for humans and the extent of acidification imposed on sensitive ecosystems. Here we provide a description and evaluation of the most recent updates made towards aerosol production, aerosol scavenging and wet deposition components of the global Integrated Forecast System-COMPOsition (IFS-COMPO) chemical forecasting system, which is used as part of the Copernicus Atmosphere Monitoring Service. The implementation of the EQSAM4Clim simplified thermodynamic module in IFS-COMPO, for use in cycle 49R1, changes the efficacy of phase transfer of SIA precursor gases (sulphur dioxide, nitric acid and ammonia) which significantly impacts the respective SIA particulate concentrations by changing the fraction converted into SIA. Comparisons made against observational composites at the surface for Europe, the US, and Southeast Asia during 2018 show reductions in the global yearly mean bias statistics for both sulphates and nitrates. Updating the IFS-COMPO model towards cycle 49r1 increases both the burden and lifetime of sulphate and ammonium particles by one third. Coupling EQSAM4Clim into IFS-COMPO provides a better description of the partitioning between state phases involving ammonia and ammonium across regions, whereas changes for sulphate are minimal. For nitric acid and nitrates, the partitioning changes significantly, leading to lower particulate concentrations and a corresponding increase in gas-phase nitric acid with an associated improvement in surface nitrate. There is also a shift in the particle size distribution, with less nitrate production in the coarse mode and more in the fine mode. The impact on the total regional wet deposition values is generally positive, except for sulphates in the US and ammonium particles in Southeast Asia which are strongly influenced by the precursor emission estimates. This provides confidence that this update to IFS-COMPO has the ability to provide accurate deposition fluxes of S and N at global scale.

The rapid development of the economy and the implementation of environmental policies adapted in India have led to fast changes of regional SO2 emissions. We present a monthly SO2 emission inventory for India covering December 2018 to November 2023 based on the Tropospheric Monitoring Instrument (TROPOMI) Level-2 COBRA SO2 dataset, using an improved flux-divergence method and estimated local SO2 lifetime, which includes both its chemical loss and dry deposition. We update the methodology to use the daily CAMS model output estimates of the hydroxyl-radical distribution as well as the measured dry deposition velocity to account for the variability in the tropospheric SO2 lifetime. It is the first effort to derive the local SO2 lifetime for application in the divergence method. The results show the application of the local SO2 lifetime improves the accuracy of SO2 emissions estimation when compared to calculations using a constant lifetime. Our improved flux-divergence method reduced the spreading of the point-source emissions compared to the standard flux-divergence method. Our derived averaged SO2 emissions covering the recent 5 years are about 5.2 Tg yr−1 with a monthly mean uncertainty of 40 %, which is lower than the bottom-up emissions of 11.0 Tg yr−1 from CAMS-GLOB-ANT v5.3. The total emissions from the 92 largest point-source emissions are estimated to be 2.9 Tg yr−1, lower than the estimation of 5.2 Tg yr−1 from the global SO2 catalog MSAQSO2LV4. We claim that the variability in the SO2 lifetime is important to account for in estimating top-down SO2 emissions.

The atmospheric composition forecasting system used to produce the Copernicus Atmosphere Monitoring Service (CAMS) forecasts of global aerosol and trace gas distributions, the Integrated Forecasting System (IFS-COMPO), undergoes periodic upgrades. In this study we describe the development of the future operational cycle 49R1 and focus on the implementation of the thermodynamical model EQSAM4Clim version 12, which represents gas–aerosol partitioning processes for the nitric acid–nitrate and ammonia–ammonium couples and computes diagnostic aerosol, cloud, and precipitation pH values at the global scale. This information on aerosol acidity influences the simulated tropospheric chemistry processes associated with aqueous-phase chemistry and wet deposition. The other updates of cycle 49R1 concern wet deposition, sea-salt aerosol emissions, dust optics, and size distribution used for the calculation of sulfate aerosol optics. The implementation of EQSAM4Clim significantly improves the partitioning of reactive nitrogen compounds, decreasing surface concentrations of both nitrate and ammonium in the particulate phase, which reduces PM2.5 biases for Europe, the US, and China, especially during summertime. For aerosol optical depth there is generally a decrease in the simulated wintertime biases and for some regions an increase in the summertime bias. Improvements in the simulated Ångström exponent are noted for almost all regions, resulting in generally good agreement with observations. The diagnostic aerosol and precipitation pH calculated by EQSAM4Clim have been compared to ground observations and published simulation results. For precipitation pH, the annual mean values show relatively good agreement with the regional observational datasets, while for aerosol pH the simulated values over continents are quite close to those simulated by ISORROPIA II. The use of aerosol acidity has a relatively smaller impact on the aqueous-phase production of sulfate compared to the changes in gas-to-particle partitioning induced by the use of EQSAM4Clim.

The Equilibrium Simplified Aerosol Model for Climate version 12 (EQSAM4Clim-v12) has recently been revised to provide an accurate and efficient method for calculating the acidity of atmospheric particles. EQSAM4Clim is based on an analytical concept that is not only sufficiently fast for chemical weather prediction applications but also free of numerical noise, which also makes it attractive for air quality forecasting. EQSAM4Clim allows the calculation of aerosol composition based on the gas–liquid–solid and the reduced gas–liquid partitioning with the associated water uptake for both cases and can therefore provide important information about the acidity of the aerosols. Here we provide a comprehensive description of the recent changes made to the aerosol acidity parameterization (referred to as a version 12) which builds on the original EQSAM4Clim. We evaluate the pH improvements using a detailed box model and compare them against previous model calculations and both ground-based and aircraft observations from the USA and China, covering different seasons and scenarios. We show that, in most cases, the simulated pH is within reasonable agreement with the reference results of the Extended Aerosol Inorganics Model (E-AIM) and of satisfactory accuracy.