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25 pags., 14 figs., 2 tabs., Tropospheric ozone ((Formula presented.)) is an important greenhouse gas and a surface pollutant. The future evolution of (Formula presented.) abundances and chemical processing are uncertain due to a changing climate, socioeconomic developments, and missing chemistry in global models. Here, we use an Earth System Model with natural halogen chemistry to investigate the changes in the (Formula presented.) budget over the 21st century following Representative Concentration Pathway (RCP)6.0 and RCP8.5 climate scenarios. Our results indicate that the global tropospheric (Formula presented.) net chemical change (NCC, chemical gross production minus destruction) will decrease (Formula presented.), notwithstanding increasing or decreasing trends in ozone production and loss. However, a wide range of surface NCC variations (from −60 (Formula presented.) to 150 (Formula presented.)) are projected over polluted regions with stringent abatements in (Formula presented.) precursor emissions. Water vapor and iodine are found to be key drivers of future tropospheric (Formula presented.) destruction, while the largest changes in (Formula presented.) production are determined by the future evolution of peroxy radicals. We show that natural halogens, currently not considered in climate models, significantly impact on the present-day and future global (Formula presented.) burden reducing (Formula presented.) 30–35 Tg (11–15 (Formula presented.)) of tropospheric ozone throughout the 21st century regardless of the RCP scenario considered. This highlights the importance of including natural halogen chemistry in climate model projections of future tropospheric ozone., EC, H2020, H2020 Priority Excellent Science, H2020 European Research Council (ERC). Grant Number: ERC-2016-COG726349; NSF, Office of Science of the US Department of Energy, PICT-2016-0714 (ANPCyT) i-COOP-B20331 (CSIC + CONICET)

Forest and vegetation fires, used as tools for agriculture and deforestation, are a major source of air pollutants and can cause serious air quality issues in many parts of Asia. Actions to reduce fire may offer considerable, yet largely unrecognized, options for rapid improvements in air quality. In this study, we used a combination of regional and global air quality models and observations to examine the impact of forest and vegetation fires on air quality degradation and public health in Southeast Asia (including Mainland Southeast Asia and south‐eastern China). We found that eliminating fire could substantially improve regional air quality across Southeast Asia by reducing the population exposure to fine particulate matter (PM2.5) concentrations by 7% and surface ozone concentrations by 5%. These reductions in PM2.5 exposures would yield a considerable public health benefit across the region; averting 59,000 (95% uncertainty interval (95UI): 55,200–62,900) premature deaths annually. Analysis of subnational infant mortality rate data and PM2.5 exposure suggested that PM2.5 from fires disproportionately impacts poorer populations across Southeast Asia. We identified two key regions in northern Laos and western Myanmar where particularly high levels of poverty coincide with exposure to relatively high levels of PM2.5 from fires. Our results show that reducing forest and vegetation fires should be a public health priority for the Southeast Asia region., Key Points Eliminating forest and vegetation fires could substantially improve regional air quality in Mainland Southeast AsiaReducing exposure to particulate and ozone pollution from fires would yield a considerable public health benefit across Southeast AsiaParticulate air pollution from fires disproportionately impacts poorer populations across Southeast Asia

The COVID‐19 outbreak in 2020 prompted strict lockdowns, reduced human activity, and reduced emissions of air pollutants. We measured volatile organic compounds (VOCs) using a proton‐transfer‐reaction mass spectrometry instrument in Changzhou, China from 8 January through 27 March, including periods of pre‐lockdown, strict measures (level 1), and more relaxed measures (level 2). We analyze the data using positive matrix factorization and resolve four factors: textile industrial emissions (62 ± 10% average reduction during level 1 relative to pre‐lockdown), pharmaceutical industrial emissions (40 ± 20%), traffic emissions (71 ± 10%), and secondary chemistry (20 ± 20%). The two industrial sources showed different responses to the lockdown, so emissions from the industrial sector should not be scaled uniformly. The quantified changes in VOCs due to the lockdowns constrain emission inventories and inform chemistry‐transport models, particularly for sectors where activity data are sparse, as the effects of lockdowns on air quality are explored., Key Points We measured and quantified volatile organic compounds (VOCs) in Changzhou, China during the COVID‐19 lockdownsTraffic‐related VOC emissions were reduced by 70%, which agrees with reductions in traffic countsTextile and pharmaceutical industry VOC emissions responded differently, so industrial emissions should not be scaled uniformly

International audience; Ammonia (NH3) and nitrogen oxides (NOx: nitrogen dioxide [NO2] + nitric oxide [NO]) play important roles in atmospheric chemistry. Throughout most of Africa, emissions of these gases are predominantly from soils and biomass burning. Here we use observations of tropospheric NO2 vertical column densities (VCDs) from the Ozone Monitoring Instrument from 2005 through 2017 and atmospheric NH3 VCDs from the Infrared Atmospheric Sounding Interferometer from 2008 through 2017 to evaluate seasonal variation of NO2 and NH3 VCDs across Africa and in seven African ecoregions. In regions where mean annual precipitation (MAP) is under 500 mm yr−1, we find that NO2 and NH3 VCDs are positively related to monthly precipitation, and where MAP is between 500 and 1,750 mm yr−1 or higher, NO2 VCDs are negatively related to monthly precipitation. In dry ecoregions, temperature and precipitation were important predictors of NH3 and NO2 VCDs, likely related to variation in soil emissions. In mesic ecoregions, monthly NO2 VCDs were strongly related to burned area, suggesting that biomass burning drives seasonality. NH3 VCDs in mesic ecoregions were positively related to both monthly temperature and monthly carbon monoxide (CO) VCDs, suggesting that a mixture of soil and biomass burning emissions influenced NH3 seasonality. In northern mesic ecoregions, monthly temperature explained most of the variance in monthly NH3 VCDs, suggesting that soil sources, including animal excreta, determined NH3 seasonality. In southern mesic ecoregions, monthly CO VCDs explained more variation in NH3 VCDs than temperature, suggesting that biomass burning may have greater influence over NH3 seasonality.

Elevated concentrations of atmospheric bromine are known to cause ozone depletion in the Arctic, which is most frequently observed during springtime. We implement a detailed description of bromine and chlorine chemistry within the WRF‐Chem 4.1.1 model, and two different descriptions of Arctic bromine activation: (1) heterogeneous chemistry on surface snow on sea ice, triggered by ozone deposition to snow (Toyota et al., 2011 https://doi.org/10.5194/acp-11-3949-2011), and (2) heterogeneous reactions on sea salt aerosols emitted through the sublimation of lofted blowing snow (Yang et al., 2008, https://doi.org/10.1029/2008gl034536). In both mechanisms, bromine activation is sustained by heterogeneous reactions on aerosols and surface snow. Simulations for spring 2012 covering the entire Arctic reproduce frequent and widespread ozone depletion events, and comparisons with observations of ozone show that these developments significantly improve model predictions during the Arctic spring. Simulations show that ozone depletion events can be initiated by both surface snow on sea ice, or by aerosols that originate from blowing snow. On a regional scale, in spring 2012, snow on sea ice dominates halogen activation and ozone depletion at the surface. During this period, blowing snow is a major source of Arctic sea salt aerosols but only triggers a few depletion events., Key Points Halogen activation and its role in Arctic surface ozone depletion events (ODEs) is modeled using WRF‐ChemTwo halogen activation mechanisms are implemented (1) surface snow and (2) blowing snowA spring 2012 case study indicates that both mechanisms can trigger near‐surface ODEs, but that surface snow dominates

Ship emissions contribute to air pollution, increasing the adverse health impacts on people living in coastal cities. We estimated the impacts caused by ship emissions, both on air quality and human health, in 2015 and future (2030) within the Pearl River Delta (PRD) region of China. In addition, we assessed the potential health benefits of implementing an Emission Control Area (ECA) in the region by predicting avoided premature mortality with and without an ECA. In 2015, ship emissions increased PM2.5 concentrations and O3 mixing ratios by 1.4 μg/m3 and 1.9 ppb, respectively, within the PRD region. This resulted in 466 and 346 excess premature acute deaths from PM2.5 and O3, respectively. Premature mortality from chronic exposures was even more significant, with 2,085 and 852 premature deaths from ship‐related PM2.5 and O3, respectively. In 2030, we projected the future ship emissions with and without an ECA, using two possible land scenarios. With an ECA, we predicted 76% reductions in SO2 and 13% reductions in NOx from the shipping sector. Assuming constant land emissions from 2015 in 2030 (2030 Constant scenario), we found that an ECA could avoid 811 PM2.5‐related and 108 O3‐related deaths from chronic exposures. Using 2030 Projected scenario for land emissions, we found that an ECA would avoid 1,194 PM2.5‐related and 160 O3‐related premature deaths in 2030. In both scenarios, implementing an ECA resulted in 30% fewer PM2.5‐related premature deaths and 10% fewer O3‐related premature deaths, illustrating the importance of reducing ship emissions., Key Points In 2015, ship emissions increased summer PM2.5 and O3 by 1.4 μg/m3 and 1.9 ppb, respectively, within the Pearl River Delta (PRD), ChinaShip emissions caused over 2,500 PM2.5‐related and 1,200 O3‐related premature deaths in the PRD region in 2015Implementing an Emission Control Area in the PRD region in 2030 could reduce mortality due to PM2.5 and O3 exposures by 30% and 10%, respectively

SO2 column densities from Ozone Monitoring Instrument provide important information on emission trends and missing sources, but there are discrepancies between different retrieval products. We employ three Ozone Monitoring Instrument SO2 retrieval products (National Aeronautics and Space Administration (NASA) standard (SP), NASA prototype, and BIRA) to study the magnitude and trend of SO2 emissions. SO2 column densities from these retrievals are most consistent when viewing angles and solar zenith angles are small, suggesting more robust emission estimates in summer and at low latitudes. We then apply a hybrid 4D‐Var/mass balance emission inversion to derive monthly SO2 emissions from the NASA SP and BIRA products. Compared to HTAPv2 emissions in 2010, both posterior emission estimates are lower in United States, India, and Southeast China, but show different changes of emissions in North China Plain. The discrepancies between monthly NASA and BIRA posterior emissions in 2010 are less than or equal to 17% in China and 34% in India. SO2 emissions increase from 2005 to 2016 by 35% (NASA)–48% (BIRA) in India, but decrease in China by 23% (NASA)–33% (BIRA) since 2008. Compared to in situ measurements, the posterior GEOS‐Chem surface SO2 concentrations have reduced NMB in China, the United States, and India but not in South Korea in 2010. BIRA posteriors have better consistency with the annual growth rate of surface SO2 measurement in China and spatial variability of SO2 concentration in China, South Korea, and India, whereas NASA SP posteriors have better seasonality. These evaluations demonstrate the capability to recover SO2 emissions using Ozone Monitoring Instrument observations., Key Points SO2 retrievals are most consistent when VZA and SZA are small; treatment of cloud and radiative transfer can cause differences up to 4 DUTop‐down SO2 emissions are more robust between NASA and BIRA retrievals in China and India where SO2 concentrations are highSO2 emissions continuously increase in India from 2005 to 2017 but start to decrease in China from 2008

Air quality models provide spatial fields of wet deposition (WD) and dry deposition that explicitly account for the transport and transformation of emissions from thousands of sources. However, many sources of uncertainty in the air quality model including errors in emissions and meteorological inputs (particularly precipitation) and incomplete descriptions of the chemical and physical processes governing deposition can lead to bias and error in the simulation of WD. We present an approach to bias correct Community Multiscale Air Quality model output over the contiguous United States using observation‐based gridded precipitation data generated by the Parameter‐elevation Regressions on Independent Slopes Model and WD observations at the National Atmospheric Deposition Program National Trends Network sites. A cross‐validation analysis shows that the adjusted annual accumulated WD for NO3 −, NH4 +, and SO4 2− from 2002 to 2012 has less bias and higher correlation with observed values than the base model output without adjustment. Temporal trends in observed WD are captured well by the adjusted model simulations across the entire contiguous United States. Consistent with previous trend analyses, WD NO3 − and SO4 2− are shown to decrease during this period in the eastern half of the United States, particularly in the Northeast, while remaining nearly constant in the West. Trends in WD of NH4 + are more spatially and temporally heterogeneous, with some positive trends in the Great Plains and Central Valley of CA and slightly negative trends in the south., Key Points Modeled estimates of the wet deposition across the United States are improved by fusion with gridded precipitation data and observed wet depositionThe fused wet deposition estimates evaluate well against observed values and maintain the spatial heterogeneity in the original model outputTemporal trends in observed wet deposition of nitrate, ammonium, and sulfate from 2002 to 2012 are well captured by the adjusted model output

Formic acid (HCOOH) is among the most abundant carboxylic acids in the atmosphere, but its budget is poorly understood. We present eddy flux, vertical gradient, and soil chamber measurements from a mixed forest and apply the data to better constrain HCOOH source/sink pathways. While the cumulative above‐canopy flux was downward, HCOOH exchange was bidirectional, with extended periods of net upward and downward flux. Net above‐canopy fluxes were mostly upward during warmer/drier periods. The implied gross canopy HCOOH source corresponds to 3% and 38% of observed isoprene and monoterpene carbon emissions and is 15× underestimated in a state‐of‐science atmospheric model (GEOS‐Chem). Gradient and soil chamber measurements identify the canopy layer as the controlling source of HCOOH or its precursors to the forest environment; below‐canopy sources were minor. A correlation analysis using an ensemble of marker volatile organic compounds suggests that secondary formation, not direct emission, is the major source driving ambient HCOOH., Key Points Bidirectional exchange of HCOOH was observed over a mixed forest, with a canopy‐level source; soil and below‐canopy sources were minorThe gross canopy HCOOH source is 3% and 38% of isoprene and monoterpene C emissions and is underestimated fifteenfold in a current modelCorrelation analysis points to secondary formation, not direct emission, as the major HCOOH source in this forest ecosystem

We study the drivers behind the global atmospheric methane (CH4) increase observed after 2006. Candidate emission and sink scenarios are constructed based on proposed hypotheses in the literature. These scenarios are simulated in the TM5 tracer transport model for 1984–2016 to produce three‐dimensional fields of CH4 and δ 13C‐CH4, which are compared with observations to test the competing hypotheses in the literature in one common model framework. We find that the fossil fuel (FF) CH4 emission trend from the Emissions Database for Global Atmospheric Research 4.3.2 inventory does not agree with observed δ 13C‐CH4. Increased FF CH4 emissions are unlikely to be the dominant driver for the post‐2006 global CH4 increase despite the possibility for a small FF emission increase. We also find that a significant decrease in the abundance of hydroxyl radicals (OH) cannot explain the post‐2006 global CH4 increase since it does not track the observed decrease in global mean δ 13C‐CH4. Different CH4 sinks have different fractionation factors for δ 13C‐CH4, thus we can investigate the uncertainty introduced by the reaction of CH4 with tropospheric chlorine (Cl), a CH4 sink whose abundance, spatial distribution, and temporal changes remain uncertain. Our results show that including or excluding tropospheric Cl as a 13 Tg/year CH4 sink in our model changes the magnitude of estimated fossil emissions by ∼20%. We also found that by using different wetland emissions based on a static versus a dynamic wetland area map, the partitioning between FF and microbial sources differs by 20 Tg/year, ∼12% of estimated fossil emissions., Key Points Increased fossil fuel emissions are unlikely the dominant driver for post‐2006 global CH4 increaseA significant decrease in the abundance of hydroxyl radicals (OH) cannot explain the post‐2006 global CH4 increaseCH4 emission attributions are sensitive to the uncertainties in OH fractionation, tropospheric Cl sink, and wetland areas

While multiple information sources exist concerning surface‐level air pollution, no individual source simultaneously provides large‐scale spatial coverage, fine spatial and temporal resolution, and high accuracy. It is, therefore, necessary to integrate multiple data sources, using the strengths of each source to compensate for the weaknesses of others. In this study, we propose a method incorporating outputs of NASA’s GEOS Composition Forecasting model system with satellite information from the TROPOMI instrument and ground measurement data on surface concentrations. Although we use ground monitoring data from the Environmental Protection Agency network in the continental United States, the model and satellite data sources used have the potential to allow for global application. This method is demonstrated using surface measurements of nitrogen dioxide as a test case in regions surrounding five major US cities. The proposed method is assessed through cross‐validation against withheld ground monitoring sites. In these assessments, the proposed method demonstrates major improvements over two baseline approaches which use ground‐based measurements only. Results also indicate the potential for near‐term updating of forecasts based on recent ground measurements., Key Points Multiple air quality data sources (GOES‐CF model, TROPOMI satellite, Environmental Protection Agency monitors) are combined to improve city‐scale NO2 forecastsForecasts using combined data outperform forecasts using ground‐based measurements onlyUpdating of forecasts based on residuals against the most recent ground measurements further improves short‐term forecasting

17 pags., 9 figs., 1 tab., Ocean-going ships supply products from one region to another and contribute to the world’s economy. Ship exhaust contains many air pollutants and results in significant changes in marine atmospheric composition. The role of reactive halogen species (RHS) in the troposphere has received increasing recognition and oceans are the largest contributors to their atmospheric burden. However, the impact of shipping emissions on RHS and that of RHS on ship-originated air pollutants have not been studied in detail. Here, an updated Weather Research Forecasting coupled with Chemistry model is utilized to explore the chemical interactions between ship emissions and oceanic RHS over the East Asia seas in summer. The emissions and resulting chemical transformations from shipping activities increase the level of NO and NO at the surface, increase O in the South China Sea, but decrease O in the East China Sea. Such changes in pollutants result in remarkable changes in the levels of RHS (>200% increase of chlorine; ∼30% and ∼5% decrease of bromine and iodine, respectively) as well as in their partitioning. The abundant RHS, in turn, reshape the loadings of air pollutants (∼20% decrease of NO and NO; ∼15% decrease of O) and those of the oxidants (>10% reduction of OH and HO; ∼40% decrease of NO) with marked patterns along the ship tracks. We, therefore, suggest that these important chemical interactions of ship-originated emissions with RHS should be considered in the environmental policy assessments of the role of shipping emissions in air quality and climate., This study received funding from the European Research Council Executive Agency under the European Union’s Horizon 2020 Research and Innovation Program (Project ERC-2016- COG 726349 CLIMAHAL) and was supported by the Consejo Superior de Investigaciones Científicas (CSIC) of Spain and The Program for Professor of Special Appointment (Eastern Scholar) at Shanghai Institutions of Higher Learning and Shanghai Thousand Talents Program. The development and maintenance of the WRF-Chem model are conducted by NOAA/ESRL/GSD in active collaboration with other institutes. Computing resources, support, and data storage were provided by the Climate Simulation Laboratory at NCAR’s Computational and Information Systems Laboratory (CISL), sponsored by the NSF. International cooperation between CSIC (Spain) and CONICET (Argentina) was supported by the i-COOP program (B20331). R.P.F. would like to thank ANPCyT (PICT 2015-0714), UNCuyo (SeCTyP M032/3853), and UTN (PID 4920-194/2018) for financial support. IITM is funded by the Ministry of Earth Sciences, Government of India.

Global solid biofuel stove emissions strongly impact air quality, climate change, and human health. However, investigations of the impacts of global solid biofuel stove emissions on human health associated with PM2.5 (particulate matter with aerodynamic diameter ≤2.5 μm) and ozone (O3) are limited. Here, we quantify the impacts of global solid biofuel stove emissions on ambient PM2.5 and O3 air quality and the associated human health effects for the year 2010, using the Community Atmosphere Model coupled with Chemistry version 5.3. Annual mean surface PM2.5 concentrations from global solid biofuel stove emissions averaged over 2006–2010 are up to 23.1 μg m−3, with large impacts found over China, India, sub‐Saharan Africa, and eastern and central Europe. For surface O3 impacts, we find that global solid biofuel stove emissions lead to increases in surface O3 concentrations by up to 5.7 ppbv for China, India, and sub‐Saharan Africa, and negligible impacts or reductions of up to 0.5 ppbv for the US, Europe, and parts of South America. Global solid biofuel stove emissions for the year 2010 contribute to 382,000 [95% confidence interval (95CI): 349,000–409,000] annual premature deaths associated with PM2.5 and O3 exposure, with the corresponding years of life lost as 8.10 million years (95CI: 7.38–8.70 million years). Our study highlights air quality and human health benefits of mitigating emissions from the global solid biofuel stove sector, especially over populous regions of low‐income and middle‐income countries, through promoting clean household energy programs for the residential energy supply., Key Points Solid biofuel stove emissions contribute significantly to surface ambient air quality over China, India, and sub‐Saharan AfricaGlobally 382,000 annual premature deaths are attributable to ambient PM2.5 and ozone exposure from solid biofuel stove emissionsPromoting clean household energy programs over low‐income and middle‐income countries to improve air quality and human health is recommended

Throughout spring and summer 2020, ozone stations in the northern extratropics recorded unusually low ozone in the free troposphere. From April to August, and from 1 to 8 kilometers altitude, ozone was on average 7% (≈4 nmol/mol) below the 2000–2020 climatological mean. Such low ozone, over several months, and at so many stations, has not been observed in any previous year since at least 2000. Atmospheric composition analyses from the Copernicus Atmosphere Monitoring Service and simulations from the NASA GMI model indicate that the large 2020 springtime ozone depletion in the Arctic stratosphere contributed less than one‐quarter of the observed tropospheric anomaly. The observed anomaly is consistent with recent chemistry‐climate model simulations, which assume emissions reductions similar to those caused by the COVID‐19 crisis. COVID‐19 related emissions reductions appear to be the major cause for the observed reduced free tropospheric ozone in 2020., Plain Language Summary: Worldwide actions to contain the COVID‐19 virus have closed factories, grounded airplanes, and have generally reduced travel and transportation. Less fuel was burnt, and less exhaust was emitted into the atmosphere. Due to these measures, the concentration of nitrogen oxides and volatile organic compounds (VOCs) decreased in the atmosphere. These substances are important for photochemical production and destruction of ozone in the atmosphere. In clean or mildly polluted air, reducing nitrogen oxides and/or VOCs will reduce the photochemical production of ozone and result in less ozone. In heavily polluted air, in contrast, reducing nitrogen oxides can increase ozone concentrations, because less nitrogen oxide is available to destroy ozone. In this study, we use data from three types of ozone instruments, but mostly from ozonesondes on weather balloons. The sondes fly from the ground up to 30 kilometers altitude. In the first 8 km, we find significantly reduced ozone concentrations in the northern extratropics during spring and summer of 2020, less than in any other year since at least 2000. We suggest that reduced emissions due to the COVID‐19 crisis have lowered photochemical ozone production and have caused the observed ozone reductions in the troposphere., Key Points: In spring and summer 2020, stations in the northern extratropics report on average 7% (4 nmol/mol) less tropospheric ozone than normal Such low tropospheric ozone, over several months, and at so many sites, has not been observed in any previous year since at least 2000 Most of the reduction in tropospheric ozone in 2020 is likely due to emissions reductions related to the COVID‐19 pandemic, NASA | Earth Sciences Division (NASA Earth Science Division) http://dx.doi.org/10.13039/100014573, Gouvernement du Canada | Natural Sciences and Engineering Research Council of Canada (NSERC) http://dx.doi.org/10.13039/501100000038, Australian Research Council, Fonds De La Recherche Scientifique ‐ FNRS (FNRS) http://dx.doi.org/10.13039/501100002661, Deutsche Forschungsgemeinschaft (DFG) http://dx.doi.org/10.13039/501100001659, Bundesministerium für Wirtschaft und Energie (BMWi) http://dx.doi.org/10.13039/501100006360

10 pags., 4 figs., Many Chemistry-Climate Models (CCMs) include a simplified treatment of brominated very short-lived (VSL) species by assuming CHBr as a surrogate for VSL. However, neglecting a comprehensive treatment of VSL in CCMs may yield an unrealistic representation of the associated impacts. Here, we use the Community Atmospheric Model with Chemistry (CAM-Chem) CCM to quantify the tropospheric and stratospheric changes between various VSL chemical approaches with increasing degrees of complexity (i.e., surrogate, explicit, and full). Our CAM-Chem results highlight the improved accuracy achieved by considering a detailed treatment of VSL photochemistry, including sea-salt aerosol dehalogenation and heterogeneous recycling on ice-crystals. Differences between the full and surrogate schemes maximize in the lowermost stratosphere and midlatitude free troposphere, resulting in a latitudinally dependent reduction of ∼1–7 DU in total ozone column and a ∼5%–15% decrease of the OH/HO ratio. We encourage all CCMs to include a complete chemical treatment of VSL in the troposphere and stratosphere., This study has been funded by the European Union's Horizon 2020 Re-search and Innovation program (Project ‘ERC-2016-COG 726349 CLIMAHAL’), and supported by the Consejo Superior de Investigaciones Científicas of Spain. Computing resources and support are provided by the Computational and Information System Laboratory (CISL,2017). R. P. Fernandez would like to thank financial support from CONICET, ANPCyT (PICT 2015-0714), UNCuyo (SeCTyP M032/3853) and UTN (PID 4920-194/2018). NCAR is sponsored by NSF under grant number 1852977. R. J. Salawitch appreciates support from the NASA (grant ACMP 80NSSC19K0983). The authors thank two anonymous reviewers for their constructive comments

Satellite HCHO data are widely used as a reliable proxy of non‐methane volatile organic compounds (NMVOCs) to constrain underlying emissions and chemistry. Here, we examine global significant changes in HCHO columns at the early stage of the COVID‐19 pandemic (January–April 2020) compared with the same period in 2019 with observations from the TROPOspheric Monitoring Instrument (TROPOMI). HCHO columns decline (11.0%) in the Northern China Plain (NCP) because of a combination of meteorological impacts, lower HCHO yields as NOx emission plunges (by 36.0%), and reduced NMVOC emissions (by 15.0%) resulting from the lockdown. HCHO columns change near Beijing (+8.4%) due mainly to elevated hydroxyl radical as NOx emission decreases in a NOx‐saturated regime. HCHO columns change in Australia (+17.5%), Northeastern Myanmar of Southeast Asia (+14.9%), Central Africa (+7.8%), and Central America (+18.9%), consistent with fire activities. Our work also points to other changes related to temperature and meteorological variations., Key Points We detect significant changes in HCHO columns worldwide at the early stage of the COVID‐19 pandemicWe see evidence of changing atmospheric oxidizing capacity and NMVOC emissions in the Northern China Plain due to the massive lockdownBesides anthropogenic emission, temperature and open fires are vital factors dominating variations in HCHO columns

Satellite nitrogen dioxide (NO2) measurements are used extensively to infer nitrogen oxide emissions and their trends, but interpretation can be complicated by background contributions to the NO2 column sensed from space. We use the step decrease of US anthropogenic emissions from the COVID‐19 shutdown to compare the responses of NO2 concentrations observed at surface network sites and from satellites (Ozone Monitoring Instrument [OMI], Tropospheric Ozone Monitoring Instrument [TROPOMI]). After correcting for differences in meteorology, surface NO2 measurements for 2020 show decreases of 20% in March–April and 10% in May–August compared to 2019. The satellites show much weaker responses in March–June and no decrease in July–August, consistent with a large background contribution to the NO2 column. Inspection of the long‐term OMI trend over remote US regions shows a rising summertime NO2 background from 2010 to 2019 potentially attributable to wildfires., Key Points COVID‐19 US shutdown led to 22%–26% reductions of surface NO2 in March–April 2020Satellite NO2 data show muted response to COVID‐19 shutdown because of NO2 background contribution to tropospheric column sensed from spaceSummertime NO2 background has been rising in the US over the past decade and complicates the interpretation of trends in satellite NO2 data

Air pollution in megacities represents one of the greatest environmental challenges. Our observed results show that the dramatic NOx decrease (77%) led to significant O3 increases (a factor of 2) during the COVID‐19 lockdown in megacity Hangzhou, China. Model simulations further demonstrate large increases of daytime OH and HO2 radicals and nighttime NO3 radical, which can promote the gas‐phase reaction and nocturnal multiphase chemistry. Therefore, enhanced NO3 − and SO4 2− formation was observed during the COVID‐19 lockdown because of the enhanced oxidizing capacity. The PM2.5 decrease was only partially offset by enhanced aerosol formation with its reduction reaching 50%. In particular, NO3 − decreased largely by 68%. PM2.5 chemical analysis reveals that vehicular emissions mainly contributed to PM2.5 under normal conditions in Hangzhou. Whereas, stationary sources dominated the residual PM2.5 during the COVID‐19 lockdown. This study provides evidence that large reductions in vehicular emissions can effectively mitigate air pollution in megacities., Key Points Air pollutants PM10, PM2.5, NOx, CO, and SO2 decreased whereas O3 increased during the COVID‐19 lockdown in megacity HangzhouEnhanced NO3 − and SO4 2− formation caused by the enhanced oxidizing capacity partially offset the decrease of PM2.5 in megacity HangzhouThe contribution of vehicular emissions to PM2.5 was over stationary sources under normal conditions in megacity Hangzhou

We apply an online‐coupled meteorology‐chemistry model (WRF‐Chem) embedded with an improved process analysis to examine aerosol‐radiation feedback (ARF) impacts on effectiveness of emission control due to Coronavirus Disease 2019 (COVID‐19) lockdown over North China Plain. Emission reduction alone induces PM2.5 decrease by 16.3 μg m−3 and O3 increase by 10.2 ppbv during COVID‐19 lockdown. The ARF enhances PM2.5 decrease by 2.7 μg m−3 (16.6%) and O3 increase by 0.8 ppbv (7.8%). The ARF‐induced enhancement of PM2.5 decline is mostly attributed to aerosol chemistry process, while enhancement of O3 rise is ascribed to physical advection and vertical mixing processes. A set of sensitivity experiments with emission reductions in different degrees indicate that the ARF‐induced enhancements of PM2.5 declines (O3 rises) follow a robust linear relationship with the emission‐reduction‐induced PM2.5 decreases. The fitted relationship has an important implication for assessing the effectiveness of emission abatement at any extent., Key Points Emission reduction due to COVID‐19 lockdown induces PM2.5 decrease by 16.3 μg m−3 (O3 increase by 10.2 ppbv) over North China PlainAerosol‐radiation feedback enhances emission‐reduction‐induced PM2.5 decrease by 16.6% (O3 increase by 7.8%) during COVID‐19 lockdownThe enhancement of PM2.5 decline (O3 rise) is ascribed to aerosol chemistry process (physical advection and vertical mixing processes)

On 19 March 2020, California put in place Stay‐At‐Home orders to reduce the spread of SARS‐CoV‐2. As a result, decreases up to 50% in traffic occurred across the South Coast Air Basin (SoCAB). We report that, compared to the 19 March to 30 June period of the last 5 years, the 2020 concentrations of PM2.5 and NOx showed an overall reduction across the basin. O3 concentrations decreased in the western part of the basin and generally increased in the downwind areas. The NOx decline in 2020 (approximately 27% basin‐wide) is in addition to ongoing declines over the last two decades (on average 4% less than the −6.8% per year afternoon NO2 concentration decrease) and provides insight into how air quality may respond over the next few years of continued vehicular reductions. The modest changes in O3 suggests additional mitigation will be necessary to comply with air quality standards., Key Points SoCAB maximum 1‐hr NOx and 24‐hr PM2.5 concentrations decreased 27% and 29%, respectively, between 19 March and 30 June of 2015–2019 and 2020The 8‐hr daily maximum O3 showed inconsistent changes across the basin during the COVID‐19 associated decrease of atmospheric NOx concentrationsDuring a shift to a NOx‐limited regime, a better understanding of VOC emission sources is needed to improve air quality in the SoCAB

This study estimates the influence of anthropogenic emission reductions on nitrogen dioxide ((Formula presented.)) and ozone ((Formula presented.)) concentration changes in Germany during the COVID-19 pandemic period using in-situ surface and Sentinel-5 Precursor TROPOspheric Monitoring Instrument (TROPOMI) satellite column measurements and GEOS-Chem model simulations. We show that reductions in anthropogenic emissions in eight German metropolitan areas reduced mean in-situ (& column) (Formula presented.) concentrations by 23 (Formula presented.) (& 16 (Formula presented.)) between March 21 and June 30, 2020 after accounting for meteorology, whereas the corresponding mean in-situ (Formula presented.) concentration increased by 4 (Formula presented.) between March 21 and May 31, 2020, and decreased by 3 (Formula presented.) in June 2020, compared to 2019. In the winter and spring, the degree of (Formula presented.) saturation of ozone production is stronger than in the summer. This implies that future reductions in (Formula presented.) emissions in these metropolitan areas are likely to increase ozone pollution during winter and spring if appropriate mitigation measures are not implemented. TROPOMI (Formula presented.) concentrations decreased nationwide during the stricter lockdown period after accounting for meteorology with the exception of North-West Germany which can be attributed to enhanced (Formula presented.) emissions from agricultural soils., Journal of Geophysical Research: Atmospheres, 126 (19), ISSN:0148-0227, ISSN:2169-897X

We use a global transport model and satellite retrievals of the carbon dioxide (CO2) column average to explore the impact of CO2 emissions reductions that occurred during the economic downturn at the start of the Covid‐19 pandemic. The changes in the column averages are substantial in a few places of the model global grid, but the induced gradients are most often less than the random errors of the retrievals. The current necessity to restrict the quality‐assured column retrievals to almost cloud‐free areas appears to be a major obstacle in identifying changes in CO2 emissions. Indeed, large changes have occurred in the presence of clouds and, in places that were cloud‐free in 2020, the comparison with previous years is hampered by different cloud conditions during these years. We therefore recommend to favor all‐weather CO2 monitoring systems, at least in situ, to support international efforts to reduce emissions., Key Points Covid‐19 impacted the CO2 column mostly in the vicinity of a few emission locations that changed over time.These places have not been well observed by the OCO‐2 satellite because of frequent or persistent cloud conditions.To support the Paris Agreement on climate, priority should be given on the development of all‐weather carbon‐monitoring systems.

We use the global Community Earth System Model to investigate the response of secondary pollutants (ozone O3, secondary organic aerosols SOA) in different parts of the world in response to modified emissions of primary pollutants during the COVID‐19 pandemic. We quantify the respective effects of the reductions in NOx and in volatile organic carbon (VOC) emissions, which, in most cases, affect oxidants in opposite ways. Using model simulations, we show that the level of NOx has been reduced by typically 40% in China during February 2020 and by similar amounts in many areas of Europe and North America in mid‐March to mid‐April 2020, in good agreement with space and surface observations. We show that, relative to a situation in which the emission reductions are ignored and despite the calculated increase in hydroxyl and peroxy radicals, the ozone concentration increased only in a few NOx‐saturated regions (northern China, northern Europe, and the US) during the winter months of the pandemic when the titration of this molecule by NOx was reduced. In other regions, where ozone is NOx‐controlled, the concentration of ozone decreased. SOA concentrations decrease in response to the concurrent reduction in the NOx and VOC emissions. The model also shows that atmospheric meteorological anomalies produced substantial variations in the concentrations of chemical species during the pandemic. In Europe, for example, a large fraction of the ozone increase in February 2020 was associated with meteorological anomalies, while in the North China Plain, enhanced ozone concentrations resulted primarily from reduced emissions of primary pollutants., Plain Language Summary: With the reduction in economic activities following the COVID‐19 pandemic outbreak in early 2020, most emissions of air pollutants (i.e., nitrogen oxides [NOx], carbon monoxide [CO], sulfur dioxide [SO2], volatile organic carbon [VOC], black carbon [BC], organic carbon [OC]) have decreased substantially during several months in different regions of the world. This unintended global experiment offered a glimpse into a potential future in which air quality would be improved. Here, a global atmospheric model is used to assess the changes in the chemical composition of the atmosphere during the pandemic period and in the related chemical processes that lead to the formation of ozone (O3) and secondary organic aerosols (SOA). The study illustrates the nonlinearity of the air quality response to reduced NOx and VOC emissions, which depends on the chemical environment including the background level of nitrogen oxides. Meteorological variability can lead to anomalies in the concentration of chemical species with magnitudes that are as large or even larger than the perturbations due to COVID‐induced changes in the emissions., Key Points: During the COVID‐19 lockdown, the atmospheric concentration of primary pollutants (NOx, VOCs, CO, SO2) was considerably reduced The concentration of secondary pollutants increased in NOx‐saturated areas and decreased in NOx‐limited areas The response of the chemical system depends on the relative changes in NOx and VOC emissions, and is affected by weather variability, AQ‐WATCH European project, HORIZON 2020 Research and Innovation Action, Hong Kong Research Grants Council

Atmospheric aerosol over the North Atlantic Ocean impacts regional clouds and climate. In this work, we use a set of sun photometer observations of aerosol optical depth (AOD) located on the Graciosa and Cape Verde islands, along with the GEOS‐Chem chemical transport model to investigate the sources of these aerosol and their transport over the North Atlantic Ocean. At both locations, the largest simulated contributor to aerosol extinction is the local source of sea‐salt aerosol. In addition to this large source, we find that signatures consistent with long‐range transport of anthropogenic, biomass burning, and dust emissions are apparent throughout the year at both locations. Model simulations suggest that this signal of long‐range transport in AOD is more apparent at higher elevation locations; the influence of anthropogenic and biomass burning aerosol extinction is particularly pronounced at the height of Pico Mountain, near the Graciosa Island site. Using a machine learning approach, we further show that simulated observations at these three sites (near Graciosa, Pico Mountain, and Cape Verde) can be used to predict the simulated background aerosol imported into cities on the European mainland, particularly during the local winter months, highlighting the utility of background AOD monitoring for understanding downwind air quality., Key Points Sun photometer AOD observations at island sites characterize long‐range transport of aerosol species across the North Atlantic OceanModel simulations suggest that higher elevation observations provide a pronounced signature of anthropogenic and biomass burning aerosolThese island sites contain information useful for predicting imported aerosol to the European continent

We use TROPOMI (TROPOspheric Monitoring Instrument) tropospheric nitrogen dioxide (NO2) measurements to identify cropland soil nitrogen oxide (NOx = NO + NO2) emissions at daily to seasonal scales in the U.S. Southern Mississippi River Valley. Evaluating 1.5 years of TROPOMI observations with a box model, we observe seasonality in local NOx enhancements and estimate maximum cropland soil NOx emissions (15–34 ng N m−2 s−1) early in growing season (May–June). We observe soil NOx pulsing in response to daily decreases in volumetric soil moisture (VSM) as measured by the Soil Moisture Active Passive (SMAP) satellite. Daily NO2 enhancements reach up to 0.8 × 1015 molecules cm−2 4–8 days after precipitation when VSM decreases to ~30%, reflecting emissions behavior distinct from previously defined soil NOx pulse events. This demonstrates that TROPOMI NO2 observations, combined with observations of underlying process controls (e.g., soil moisture), can constrain soil NOx processes from space., Key Points Daily TROPOMI data provide new opportunities to observe regional cropland NOx emissions from spaceSoil NOx pulsing is identified throughout the growing season with a NOx maximum observed when soils dry to ~30% volumetric soil moistureCropland NOx emissions peak at the onset of the growing season as determined by TROPOMI NO2 enhancements and a box model framework

We present an assessment of the impacts on atmospheric composition and radiative forcing of short‐lived pollutants following a worldwide decrease in anthropogenic activity and emissions comparable to what has occurred in response to the COVID‐19 pandemic, using the global composition‐climate model United Kingdom Chemistry and Aerosols Model (UKCA). Emission changes reduce tropospheric hydroxyl radical and ozone burdens, increasing methane lifetime. Reduced SO2 emissions and oxidizing capacity lead to a decrease in sulfate aerosol and increase in aerosol size, with accompanying reductions to cloud droplet concentration. However, large reductions in black carbon emissions increase aerosol albedo. Overall, the changes in ozone and aerosol direct effects (neglecting aerosol‐cloud interactions which were statistically insignificant but whose response warrants future investigation) yield a radiative forcing of −33 to −78 mWm−2. Upon cessation of emission reductions, the short‐lived climate forcers rapidly return to pre‐COVID levels; meaning, these changes are unlikely to have lasting impacts on climate assuming emissions return to pre‐intervention levels., Key Points Emission reductions are likely to have led to a global reduction in short‐lived climate forcers and tropospheric oxidizing capacityReductions in O3 and aerosol from both lower emissions and decreased sulfate oxidation resulted in a net negative radiative forcingThe radiative impacts are small and short‐lived. Longer term climate impacts must come through future sustained emission reductions

Efforts to stem the spread of COVID‐19 in China hinged on severe restrictions to human movement starting 23 January 2020 in Wuhan and subsequently to other provinces. Here, we quantify the ancillary impacts on air pollution and human health using inverse emissions estimates based on multiple satellite observations. We find that Chinese NOx emissions were reduced by 36% from early January to mid‐February, with more than 80% of reductions occurring after their respective lockdown in most provinces. The reduced precursor emissions increased surface ozone by up to 16 ppb over northern China but decreased PM2.5 by up to 23 μg m−3 nationwide. Changes in human exposure are associated with about 2,100 more ozone‐related and at least 60,000 fewer PM2.5‐related morbidity incidences, primarily from asthma cases, thereby augmenting efforts to reduce hospital admissions and alleviate negative impacts from potential delayed treatments., Key Points We quantified the ancillary impacts of the COVID‐19 lockdown on air pollution and human health using multiple satellite observationsRapid reductions in Chinese NOx and SO2 emissions increased surface ozone by 16 ppb over northern China but decreased PM2.5 nationwideThese changes increased by about 60 ozone‐related but decreased by about 5,000 PM2.5‐related hospital admissions

Understanding the past, present, and future evolution of methane remains a grand challenge. Here we have used a hierarchy of models, ranging from simple box models to a chemistry‐climate model (CCM), UM‐UKCA, to assess the contemporary and possible future atmospheric methane burden. We assess two emission data sets for the year 2000 deployed in UM‐UKCA against key observational constraints. We explore the impact of the treatment of model boundary conditions for methane and show that, depending on other factors, such as CO emissions, satisfactory agreement may be obtained with either of the CH4 emission data sets, highlighting the difficulty in unambiguous choice of model emissions in a coupled chemistry model with strong feedbacks. The feedbacks in the CH4‐CO‐OH system, and their uncertainties, play a critical role in the projection of possible futures. In a future driven by large increases in greenhouse gas forcing, increases in tropospheric temperature drive, an increase in water vapor, and, hence, [OH]. In the absence of methane emission changes this leads to a significant decrease in methane compared to the year 2000. However, adding a projected increase in methane emissions from the RCP8.5 scenario leads to a large increase in methane abundance. This is modified by changes to CO and NOx emissions. Clearly, future levels of methane are uncertain and depend critically on climate change and on the future emission pathways of methane and ozone precursors. We highlight that further work is needed to understand the coupled CH4‐CO‐OH system in order to understand better future methane evolution., Key Points We study methane in present and future climate in a model employing flux‐based treatment of methane emissionsWe examine the source and importance of feedbacks in the CH4‐CO‐OH system using a 0‐D box model and a 3‐D chemistry‐climate modelWe examine methane levels in future climate and the role played by climate and emissions changes

Sulfur compounds are an important constituent of particulate matter, with impacts on climate and public health. While most sulfur observed in particulate matter has been assumed to be sulfate, laboratory experiments reveal that hydroxymethanesulfonate (HMS), an adduct formed by aqueous phase chemical reaction of dissolved HCHO and SO2, may be easily misinterpreted in measurements as sulfate. Here we present observational and modeling evidence for a ubiquitous global presence of HMS. We find that filter samples collected in Shijiazhuang, China, and examined with ion chromatography within 9 days show as much as 7.6 μg m−3 of HMS, while samples from Singapore examined 9–18 months after collection reveal ~0.6 μg m−3 of HMS. The Shijiazhuang samples show only minor traces of HMS 4 months later, suggesting that HMS had decomposed over time during sample storage. In contrast, the Singapore samples do not clearly show a decline in HMS concentration over 2 months of monitoring. Measurements from over 150 sites, primarily derived from the IMPROVE network across the United States, suggest the ubiquitous presence of HMS in at least trace amounts as much as 60 days after collection. The degree of possible HMS decomposition in the IMPROVE observations is unknown. Using the GEOS‐Chem chemical transport model, we estimate that HMS may account for 10% of global particulate sulfur in continental surface air and over 25% in many polluted regions. Our results suggest that reducing emissions of HCHO and other volatile organic compounds may have a co‐benefit of decreasing particulate sulfur., Key Points New and reanalyzed observations suggest a ubiquitous global presence of hydroxymethanesulfonate (HMS) in particulate matterGEOS‐Chem simulations suggest HMS may comprise over 25% of particulate sulfur in many polluted regions, especially in continental winterReductions of formaldehyde and other volatile organic compounds may have the co‐benefit of reducing particulate sulfur

TROPOMI satellite data show substantial drops in nitrogen dioxide (NO2) during COVID‐19 physical distancing. To attribute NO2 changes to NOX emissions changes over short timescales, one must account for meteorology. We find that meteorological patterns were especially favorable for low NO2 in much of the U.S. in spring 2020, complicating comparisons with spring 2019. Meteorological variations between years can cause column NO2 differences of ~15% over monthly timescales. After accounting for sun angle and meteorological considerations, we calculate that NO2 drops ranged between 9.2 – 43.4% among twenty cities in North America, with a median of 21.6%. Of the studied cities, largest NO2 drops (>30%) were in San Jose, Los Angeles, and Toronto, and smallest drops (, Plain‐Language Summary Nitrogen dioxide (NO2) is an air pollutant whose prevalence in urban areas is linked to fossil fuel combustion. The NO2 concentrations in our atmosphere are primarily a function of the magnitude of nitrogen oxide (NOx) emissions and weather factors such as sun angle, wind speed, and temperature. In this work, we developed two novel methods to account for weather impacts on daily pollution levels during COVID‐19 precautions. Once we accounted for favorable weather conditions that in some cases kept air pollution low independent of tail‐pipe emissions, calculated air pollutant emission reductions varied dramatically (9 – 43%) among twenty North American cities. Results can be used to understand factors contributing to inconsistent NO2 changes during physical distancing, which can inform the effectiveness of COVID‐19 protocols and aid future policy development. These methodologies will allow us to respond more quickly in future unintended experiments when emissions change suddenly.

Spaceborne NO2 column observations from two high‐resolution instruments, TROPOMI onboard Sentinel‐5 Precursor and OMI on Aura, reveal unprecedented NO2 decreases over China, South Korea, Western Europe and the U.S. as a result of public health measures enforced to contain the coronavirus disease outbreak (Covid‐19) in January‐April 2020. The average NO2 column drop over all Chinese cities amounts to ‐40% relative to the same period in 2019, and reaches up to a factor of ~2 at heavily hit cities, e.g. Wuhan, Jinan, while the decreases in Western Europe and the U.S. are also significant (‐20 to ‐38%). In contrast with this, although Iran is also strongly affected by the disease, the observations do not show evidence of lower emissions, reflecting more limited health measures., Key Points Satellite NO2 data show substantial decreases by 40% on average over Chinese cities due to lockdown measures against the Covid‐19 outbreakWestern Europe and U.S. display robust NO2 decreases in 2020, 20‐38% relative to the same period in 2019Satellite NO2 observations above Iran, a region strongly affected by coronavirus, do not show clear evidence of lower emissions

During the COVID‐19 outbreak that took place in early 2020, the economic activities in China were drastically reduced and accompanied by a strong reduction in the emission of primary air pollutants. On the basis of measurements made at the monitoring stations operated by the China National Environmental Monitoring Center, we quantify the reduction in surface PM2.5, NO2, CO and SO2 concentrations in northern China during the lockdown, which started on 23 January 2020. We find that, on the average, the levels of surface PM2.5 and NO2 have decreased by approximately 35 and 60 percent, respectively, between the period 1‐22 January 2020 and the period 23 January‐29 February 2020. At the same time, the mean ozone concentration has increased by a factor 1.5–2. In urban area of Wuhan, where drastic measures were adopted to limit the spread of the coronavirus, similar changes in the concentrations of PM2.5, NO2 and ozone are found., Key Points The levels of PM2.5 and NO2 in northern China have decreased by 35 and 60 percent following the coronavirus outbreak of early 2020.Simultaneously, the ozone concentration, a secondary pollutant responsible for severe health problems, has increased by a factor 1.5‐2.The same type of behavior was observed specifically in the city of Wuhan, where the virus outbreak originated.

Improved observational understanding of urban CO2 emissions, a large and dynamic global source of fossil CO2, can provide essential insights for both carbon cycle science and mitigation decision making. Here we compare three distinct global CO2 emissions inventory representations of urban CO2 emissions for five Middle Eastern cities (Riyadh, Mecca, Tabuk, Jeddah, and Baghdad) and use independent satellite observations from the Orbiting Carbon Observatory‐2 (OCO‐2) satellite to evaluate the inventory representations of afternoon emissions. We use the column version of the Stochastic Time‐Inverted Lagrangian Transport (X‐STILT) model to account for atmospheric transport and link emissions to observations. We compare XCO2 simulations with observations to determine optimum inventory scaling factors. Applying these factors, we find that the average summed emissions for all five cities are 100 MtC year−1 (50–151, 90% CI), which is 2.0 (1.0, 3.0) times the average prior inventory magnitudes. The total adjustment of the emissions of these cities comes out to ~7% (0%, 14%) of total Middle Eastern emissions (~700 MtC year−1). We find our results to be insensitive to the prior spatial distributions in inventories of the cities' emissions, facilitating robust quantitative assessments of urban emission magnitudes without accurate high‐resolution gridded inventories., Key Points We use satellite CO2 observations and three global emissions inventories to assess emissions representations of five Middle Eastern citiesEmissions estimates optimized with satellite CO2 observations are insensitive to prior inventory spatial distributionsEmissions estimates suggest afternoon Middle East urban inventory representations are too low

It is a puzzle as to why more severe haze formed during the New Year Holiday in 2020 (NYH‐20), when China was in an unprecedented state of shutdown to contain the coronavirus (COVID‐19) outbreak, than in 2019 (NYH‐19). We performed a comprehensive measurement and modeling analysis of the aerosol chemistry and physics at multiple sites in China (mainly in Shanghai) before, during, and after NYH‐19 and NYH‐20. Much higher secondary aerosol fraction in PM2.5 were observed during NYH‐20 (73%) than during NYH‐19 (59%). During NYH‐20, PM2.5 levels correlated significantly with the oxidation ratio of nitrogen (r 2 = 0.77, p < 0.01), and aged particles from northern China were found to impede atmospheric new particle formation and growth in Shanghai. A markedly enhanced efficiency of nitrate aerosol formation was observed along the transport pathways during NYH‐20, despite the overall low atmospheric NO2 levels., Key Points Higher concentrations and distinct compositions of aerosol particles were observed during the COVID‐19 shutdownFast formation of secondary inorganic aerosol contributed to high aerosol mass loadingLonger‐range, regional transport facilitated and enhanced particulate nitrate formation

The global gasoline and diesel fuel vehicle fleets impose substantial impacts on air quality, human health, and climate change. Here we quantify the global radiative forcing and human health impacts of the global gasoline and diesel sectors using the NCAR CESM global chemistry‐climate model for year 2015 emissions from the IIASA GAINS inventory. Net global radiative effects of short‐lived climate forcers (including aerosols, ozone, and methane) from the gasoline and diesel sectors are +13.6 and +9.4 mW m−2, respectively. The annual mean net aerosol contributions to the net radiative effects of gasoline and diesel are −9.6 ± 2.0 and +8.8 ± 5.8 mW m−2. Aerosol indirect effects for the gasoline and diesel road vehicle sectors are −16.6 ± 2.1 and −40.6 ± 4.0 mW m−2. The fractional contributions of short‐lived climate forcers to the total global climate impact including carbon dioxide on the 20‐year time scale are similar, 14.9% and 14.4% for gasoline and diesel, respectively. Global annual total PM2.5‐ and ozone‐induced premature deaths for gasoline and diesel sectors approach 115,000 (95% CI: 69,000–153,600) and 122,100 (95% CI: 78,500–157,500), with corresponding years of life lost of 2.10 (95% CI: 1.23–2.66) and 2.21 (95% CI: 1.47–2.85) million years. Substantial regional variability of premature death rates is found for the diesel sector when the regional health effects are normalized by the annual total regional vehicle distance traveled. Regional premature death rates for the gasoline and diesel sectors, respectively, vary by a factor of eight and two orders of magnitude, with India showing the highest for both gasoline and diesel sectors., Key Points The net radiative effects for the gasoline and diesel sectors over a 20‐year time scale are +91.4 and +65.7 mW m−2, respectivelyGlobal annual total premature deaths of 115,000 and 122,100 are attributable to the gasoline and diesel sectorsThere exists substantial regional variability of premature death rates for the diesel sector

Supplement containing data related to the 2020 Global Carbon Budget from the Global Carbon Project. The original article is Friedlingstein et al: Global Carbon Budget 2020, Earth Syst. Sci. Data Discussions, 2020. https://doi.org/10.5194/essd-12-3269-2020. Further information is available on: http://www.globalcarbonproject.org/carbonbudget. File Global_Carbon_Budget_2020v1.0.xlsx includes the following: 1. Summary 2. Global Carbon Budget 3. Fossil fuel emissions by Fuel Type 4. Land-use change emissions 5. Ocean Sink 6. Terrestrial sink 7. Historical Budget. File National_Carbon_Emissions_2020v1.0.xlsx includes the following: 1. Summary 2. Territorial emissions 3. Consumption emissions 4. Emissions transfers 5. Country definitions 6. Disaggregation 7. Aggregation

The direct radiative forcing of black carbon aerosol (BC) on the Earth system remains unsettled, largely due to the uncertainty with physical properties of BC throughout their lifecycle. Here we show that ambient chamber measurements of BC properties provide a novel constraint on the crude BC aging representation in climate models. Observational evidence for significant absorption enhancement of BC can be reproduced when the aging processes in the four‐mode version of the Modal Aerosol Module (MAM4) aerosol scheme in the Community Atmosphere Model version 5 are calibrated by the recent in situ chamber measurements. An observation‐based scaling method is developed in the aging timescale calculation to alleviate the influence of biases in the simulated model chemical composition. Model sensitivity simulations suggest that the different monolayer settings in the BC aging parameterization of MAM4 can cause as large as 26% and 24% differences in BC burden and radiative forcing, respectively. We also find that an increase in coating materials (e.g., sulfate and secondary organic aerosols) reduces BC lifetime by increasing the hygroscopicity of the mixture but enhances its absorption, resulting in a net increase in BC direct radiative forcing. Our results suggest that accurate simulations of BC aging processes as well as other aerosol species are equally important in reducing the uncertainty of BC forcing estimation., Key Points Atmospheric black carbon aging processes in CAM5 are constrained by chamber experimentsThe calibrated model shows that coating results in net enhancement in BC forcing in spite of a reduction in BC lifetimeModeled BC direct radiative forcing varies up to 26% due to the uncertainties in BC aging parameterization and coating aerosol mass

The concentrations of atmospheric ammonia ([NH3]) have been observed to be increasing over the United States in the last decade, especially in Eastern United States. It is important to understand this temporal trend and variation due to the role of NH3 in particle formation and its ecological effects. Here the long‐term trend of [NH3] over the United States is investigated using GEOS‐Chem, a global 3‐D tropospheric chemistry model, and is corroborated with empirical evidence from the Ammonia Monitoring Network. Model simulations, consistent with observations, show increase in [NH3] over the United States from 2001 to 2016, with magnitude largest in the East (~5% to 12%/year) and smallest in the West (~0% to 5%/year). Reasons for this are examined, and evidence for the role of decreasing SO2 and NOx emissions in increasing [NH3] is provided. The contributions of meteorology and NH3 emission changes to the [NH3] increase appear to be small during the period. Our sensitivity study suggests that decreasing SO2 and NOx emissions over the United States owing to stringent regulations explain about 2/3 and 1/3 of the increase in [NH3], respectively. This effect is different for various NH3 and SO2 and NOx regimes. Given the continued reduction of SO2 and NOx emissions due to U.S. regulations mainly aimed at PM2.5 reduction, the present results are important towards better assessing the environmental impact of emission controlling policies., Key Points This work represents the first modeling study of long‐term trend in gas‐phase ammonia concentration ([NH3]) over the United StatesModel simulations, consistent with observations, show increase in [NH3] over the United States, with magnitude of relative change largest in the East and smallest in the WestDecreasing SO2 and NOx emissions respectively contribute to 2/3 and 1/3 of the increase in [NH3] over the United States

Southeast Asia experiences frequent fires in fuel‐rich tropical peatlands, leading to extreme episodes of regional haze with high concentrations of fine particulate matter (PM2.5) impacting human health. In a study published recently, the first field measurements of PM2.5 emission factors for tropical peat fires showed larger emissions than from other fuel types. Here we report even higher PM2.5 emission factors, measured at newly ignited peat fires in Malaysia, suggesting that current estimates of fine particulate emissions from peat fires may be underestimated by a factor of 3 or more. In addition, we use both field and laboratory measurements of burning peat to provide the first mechanistic explanation for the high variability in PM2.5 emission factors, demonstrating that buildup of a surface ash layer causes the emissions of PM2.5 to decrease as the peat fire progresses. This finding implies that peat fires are more hazardous (in terms of aerosol emissions) when first ignited than when still burning many days later. Varying emission factors for PM2.5 also have implications for our ability to correctly model the climate and air quality impacts downwind of the peat fires. For modelers able to implement a time‐varying emission factor, we recommend an emission factor for PM2.5 from newly ignited tropical peat fires of 58 g of PM2.5 per kilogram of dry fuel consumed (g/kg), reducing exponentially at a rate of 9%/day. If the age of the fire is unknown or only a single value may be used, we recommend an average value of 24 g/kg., Key Points In this study we show that emissions of PM2.5 from Malaysian peat fires are likely 3 times larger than previously assumedWe show that the emissions of fine particulate matter from peat fires in the field decrease rapidly with the age of the fireWe show that the likely cause is the accumulation of an ash layer as the peat burns below the surface

New treatments for organic aerosol (OA) formation have been added to a modified version of the CESM/CAM5 model (CESM‐NCSU). These treatments include a volatility basis set treatment for the simulation of primary and secondary organic aerosols (SOAs), a simplified treatment for organic aerosol (OA) formation from glyoxal, and a parameterization representing the impact of new particle formation (NPF) of organic gases and sulfuric acid. With the inclusion of these new treatments, the concentration of oxygenated organic aerosol increases by 0.33 µg m−3 and that of primary organic aerosol (POA) decreases by 0.22 µg m−3 on global average. The decrease in POA leads to a reduction in the OA direct effect, while the increased OOA increases the OA indirect effects. Simulations with the new OA treatments show considerable improvement in simulated SOA, oxygenated organic aerosol (OOA), organic carbon (OC), total carbon (TC), and total organic aerosol (TOA), but degradation in the performance of HOA. In simulations of the current climate period, despite some deviations from observations, CESM‐NCSU with the new OA treatments significantly improves the magnitude, spatial pattern, seasonal pattern of OC and TC, as well as, the speciation of TOA between POA and OOA. Sensitivity analysis reveals that the inclusion of the organic NPF treatment impacts the OA indirect effects by enhancing cloud properties. The simulated OA level and its impact on the climate system are most sensitive to choices in the enthalpy of vaporization and wet deposition of SVOCs, indicating that accurate representations of these parameters are critical for accurate OA‐climate simulations., Key Points VBS and other OA updates significantly improve OA model performanceOA‐climate interactions are the most sensitive to enthalpy of vaporization and semivolatile organic compounds wet depositionThe updated CESM‐NCSU OA treatments reasonably simulate OA in the current climate

Marine cloud brightening (MCB) is proposed to offset global warming by emitting sea salt aerosols to the tropical marine boundary layer, which increases aerosol and cloud albedo. Sea salt aerosol is the main source of tropospheric reactive chlorine (Cly) and bromine (Bry). The effects of additional sea salt on atmospheric chemistry have not been explored. We simulate sea salt aerosol injections for MCB under two scenarios (212–569 Tg/a) in the GEOS‐Chem global chemical transport model, only considering their impacts as a halogen source. Globally, tropospheric Cly and Bry increase (20–40%), leading to decreased ozone (−3 to −6%). Consequently, OH decreases (−3 to −5%), which increases the methane lifetime (3–6%). Our results suggest that the chemistry of the additional sea salt leads to minor total radiative forcing compared to that of the sea salt aerosol itself (~2%) but may have potential implications for surface ozone pollution in tropical coastal regions., Key Points Sea salt aerosol emissions for Marine Cloud Brightening geoengineering are implemented in a global chemical transport modelThis leads to changes in global tropospheric Bry and Cly (+20 to 40%), ozone (−3 to −6%), OH (−2 to −4%), and methane lifetime (+3 to 6%)Chemistry of the added sea salt leads to minor total radiative forcing (−20 to −50 mW/m2) but may have implications for ozone pollution