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We retrieved column‐averaged dry air mole fractions of atmospheric carbon dioxide () from backscattered short‐wave infrared (SWIR) sunlight measured by the Japanese Greenhouse gases Observing SATellite (GOSAT). Over two years of retrieved from GOSAT is compared with inferred from collocated SWIR measurements by seven ground‐based Total Carbon Column Observing Network (TCCON) stations. The average difference between GOSAT and TCCON for individual TCCON sites ranges from −0.87 ppm to 0.77 ppm with a mean value of 0.1 ppm and standard deviation of 0.56 ppm. We find an average bias between all GOSAT and TCCON retrievals of −0.20 ppm with a standard deviation of 2.26 ppm and a correlation coefficient of 0.75. One year of was retrieved from GOSAT globally, which was compared to global 3‐D GEOS‐Chem chemistry transport model calculations. We find that the latitudinal gradient, seasonal cycles, and spatial variability of GOSAT and GEOS‐Chem agree well in general with a correlation coefficient of 0.61. Regional differences between GEOS‐Chem model calculations and GOSAT observations are typically less than 1 ppm except for the Sahara and central Asia where a mean difference between 2 to 3 ppm is observed, indicating regional biases in the GOSAT retrievals unobserved by the current TCCON network. Using a bias correction scheme based on linear regression these regional biases are significantly reduced, approaching the required accuracy for surface flux inversions. Key Points Bias and precision should be sufficient to allow improved surface flux estimates Globally, regional differences are found to be small, except over desert regions Retrievals should be useful for the inversion of CO2 surface fluxes

We report new short‐wave infrared (SWIR) column retrievals of atmospheric methane (XCH4) from the Japanese Greenhouse Gases Observing SATellite (GOSAT) and compare observed spatial and temporal variations with correlative ground‐based measurements from the Total Carbon Column Observing Network (TCCON) and with the global 3‐D GEOS‐Chem chemistry transport model. GOSAT XCH4 retrievals are compared with daily TCCON observations at six sites between April 2009 and July 2010 (Bialystok, Park Falls, Lamont, Orleans, Darwin and Wollongong). GOSAT reproduces the site‐dependent seasonal cycles as observed by TCCON with correlations typically between 0.5 and 0.7 with an estimated single‐sounding precision between 0.4–0.8%. We find a latitudinal‐dependent difference between the XCH4 retrievals from GOSAT and TCCON which ranges from 17.9 ppb at the most northerly site (Bialystok) to −14.6 ppb at the site with the lowest latitude (Darwin). We estimate that the mean smoothing error difference included in the GOSAT to TCCON comparisons can account for 15.7 to 17.4 ppb for the northerly sites and for 1.1 ppb at the lowest latitude site. The GOSAT XCH4 retrievals agree well with the GEOS‐Chem model on annual (August 2009 – July 2010) and monthly timescales, capturing over 80% of the zonal variability. Differences between model and observed XCH4 are found over key source regions such as Southeast Asia and central Africa which will be further investigated using a formal inverse model analysis. Key Points CH4 is now successfully being retrieved from GOSAT satellite Validation against ground‐based data shows good agreement Excellent agreement to model simulations as a first step towards inversions

Ozone (O3) pollution is a severe air quality issue in China, posing a threat to human health and ecosystems. The climate change will affect O3 levels by directly changing physical and chemical processes of O3 and indirectly changing natural emissions of O3 precursors. In this study, near‐surface O3 concentrations in China in 2030 and 2060 are predicted using the process‐based interpretable Extreme Gradient Boosting (XGBoost) model integrated with multi‐source data. The results show that the climate‐driven O3 levels over eastern China are projected to decrease by more than 0.4 ppb in 2060 under the carbon neutral scenario (SSP1‐1.9) compared with the high emission scenario (SSP5‐8.5). Among this reduction, 80% is attributed to the changes in physical and chemical processes of O3 related to a cooler climate, while the remaining 20% is attributed to the reduced biogenic isoprene emissions. Plain Language Summary O3 pollution is a severe air quality issue in China that threatens human health and ecosystem. Under the background of climate change, O3 pollution will continue to evolve in the future. Here, we predict near‐surface O3 concentrations in China in 2030 and 2060 based on an interpretable machine learning method, integrated with physical and chemical processes of O3, natural emissions of O3 precursors, and other multi‐source data. The direct (via changing physical and chemical processes of O3) and indirect (via changing natural emissions of O3 precursors) impacts of future climate change on O3 concentrations are quantitatively analyzed. It demonstrates that the climate‐driven O3 levels are projected to decrease by more than 0.4 ppb in 2060 over eastern China under a carbon neutral scenario relative to a high emission scenario. The changes in physical and chemical processes under climate change play a more important role in regulating O3 concentrations in the future than the changes in natural emissions. Key Points Climate change influences O3 pollution in China through changing physical and chemical processes and natural precursor emissions of O3 Physical and chemical processes play a dominant role in regulating future near‐surface O3 concentrations over eastern China Carbon neutral scenario is an ideal pathway for China to mitigate both climate change and O3 pollution in 2060

Volcanism is the largest natural source of mercury (Hg) to the biosphere. However, past Hg emission estimates have varied by three orders of magnitude. Here, we present an updated central estimate and interquartile range (232 Mg a−1; IQR: 170–336 Mg a−1) for modern volcanic Hg emissions based on advances in satellite remote sensing of sulfur dioxide (SO2) and an improved method for considering uncertainty in Hg:SO2 emissions ratios. Atmospheric modeling shows the influence of volcanic Hg on surface atmospheric concentrations in the extratropical Northern Hemisphere is 1.8 times higher than in the Southern Hemisphere. Spatiotemporal variability in volcanic Hg emissions may obscure atmospheric trends forced by anthropogenic emissions at some locations. This should be considered when selecting monitoring sites to inform global regulatory actions. Volcanic emission estimates from this work suggest the pre‐anthropogenic global atmospheric Hg reservoir was 580 Mg, 7‐fold lower than in 2015 (4,000 Mg). Plain Language Summary Volcanism is widely recognized as the most important natural source of mercury (Hg) globally, but existing emissions estimates contain substantial uncertainty. This study combines satellite observations of sulfur dioxide (SO2) in volcanic plumes and measured Hg:SO2 ratios to quantify the magnitude and spatiotemporal variability of global volcanic Hg emissions. Using a global model, we show that the spatial pattern of volcanic releases and atmospheric dynamics result in greater concentrations of volcanic Hg in the mid‐latitude Northern Hemisphere compared to the mid‐latitude Southern Hemisphere. Modeling results suggest that variability in volcanic Hg emissions at some locations may obscure trends in atmospheric Hg concentrations driven by human emissions. The influence of volcanic Hg emissions should therefore be considered during selection of global monitoring sites used to track the progress of regulatory actions designed to mitigate Hg pollution. Volcanic release estimates from this work suggest the natural atmospheric Hg reservoir was ∼7 times smaller than in 2015, reinforcing that humans have profoundly disrupted the global biogeochemical Hg cycle. Key Points Volcanic mercury emissions of 232 Mg a−1 (IQR: 170–336 Mg a−1) are estimated by indexing to sulfur dioxide from satellite remote sensing Over 90% of volcanic mercury emissions occur in the tropics and mid‐latitude Northern Hemisphere Volcanic emissions support a pre‐anthropogenic atmospheric mercury reservoir of approximately 580 Mg (7‐fold lower than in 2015)

The Indian Ocean Dipole (IOD) mode exerts distinct impacts on the climate in China and can further affect tropospheric ozone. Using long‐term GEOS‐Chem simulations, we found distinct changes in planetary boundary layer ozone throughout China during positive and negative phases of IOD. In summer, ozone shows synchronized increases except in southern China during positive IOD; the ozone increases are dominated by chemical production and transport in northern and western China, respectively. The increased precursor from biogenic emissions contributed to ozone chemical formation in the northern region, and the increased precipitation and decreased solar radiation hindered ozone production in southern China. Ozone changes show good symmetry over most regions during negative IOD. In autumn, the ozone reduction in southern China shares the same reason as summer, while the chemical increase over northern China is affected more by changes in solar radiation and relative humidity than in the precursor emission. Plain Language Summary Indian Ocean Dipole (IOD) is an important climate variability in the Indian Ocean; here, we found that the variation of IOD from its neutral state to positive or negative phases can strongly modulate ozone within planetary boundary layer (PBL) in China. In summer, positive IOD generally increases ozone over the entire China; northern and western China experience ozone increases mainly due to chemical reactions and regional transport, while southern China sees hindered ozone production from factors like increased rainfall. During autumn for positive events, the ozone reduction in southern China shares the same reason as summer, while northern China is affected more by local weather conditions such as sunlight and humidity rather than increases in reactants during summer. Ozone exhibits opposite changes in negative IOD events due to reverse effects. This research indicates that the IOD's phases influence PBL ozone in China, with varying effects depending on the regions and seasons. Key Points In summer, positive Indian Ocean Dipole (IOD) induces a synchronous increase in planetary boundary layer (PBL) ozone up to 5% throughout China, and vice versa for negative IOD PBL ozone increases over northern and western China in summer for positive IOD is dominated by precursor increase and regional transport PBL ozone increases in autumn are primarily driven by heightened chemical production influenced by meteorological conditions

Perchlorate has been observed in many environments on Earth and Mars but its sources remain poorly quantified. In this study, we use a global three‐dimensional chemical transport model to simulate perchlorate's gas‐phase photochemical production, atmospheric transport, and deposition on Earth's surface. Model predictions are compared to newly compiled observations of atmospheric concentrations, deposition flux, and oxygen isotopic composition of perchlorate. We find that the modeled gas‐phase production of perchlorate is consistent with reported stratospheric observations. Nevertheless, we show that this mechanism alone cannot explain the high levels of perchlorate observed at many near‐surface sites (aerosol concentrations >0.1 ng m−3 and deposition fluxes >10 g km−2 yr−1) or the low 17O‐excess observed in perchlorate sampled from pristine environments (<+18.4‰). We discuss four hypotheses to explain the model‐observation discrepancies and recommend laboratory and field observations to address key uncertainties in atmospheric sources of perchlorate. Plain Language Summary Perchlorate (ClO4−) pollution is an environmental issue because excessive exposure can affect the thyroid and disrupt hormonal balance, especially for infants. Perchlorate on Earth has both human and natural sources. Industrial perchlorate is used for explosives and rocket fuels. Perchlorate also occurs naturally and accumulates in many deserts on Earth and in the soil of Mars. Atmospheric chemistry has long been considered a source of natural perchlorate, but its contribution remains uncertain. In this study, we use a 3‐D atmospheric model to estimate how much of the perchlorate occurrence on Earth can be explained by known and plausible reactions between gases containing chlorine and oxygen. We find that these reactions can explain the abundance of stratospheric perchlorate. However, they cannot explain many tropospheric observations of perchlorate, especially those in Antarctica and urban areas. Our analysis of oxygen isotopic anomalies also suggests that stratospheric chemistry alone cannot account for all the natural perchlorate found in deserts. We discuss four possible explanations for the differences between observations and model predictions. We recommend some future research that can reduce the uncertainties in the sources of atmospheric perchlorate and improve our understanding of the occurrence of natural perchlorate in planetary atmospheres. Key Points We conduct the first global simulation of atmospheric perchlorate using a three‐dimensional chemical transport model Gas‐phase production of perchlorate in the stratosphere and its subsequent transport cannot explain observations at many surface sites Analysis of modeled and observed 17O excess suggests that non‐stratospheric sources are important for the occurrence of natural perchlorate

We use a suite of satellite observations (Moderate Resolution Imaging Spectroradiometer (MODIS), Multiangle Imaging Spectroradiometer (MISR), Cloud‐Aerosol Lidar With Orthogonal Polarization (CALIOP)) to investigate the processes of long‐range transport of dust represented in the global GEOS‐Chem model in 2006–2008. A multiyear mean of African dust transport is developed and used to test the representation of the variability in the model. We find that both MODIS and MISR correlate well with the majority of Aerosol Robotic Network observations in the region (r> 0.8). However, MODIS aerosol optical depth (AOD) appears to be biased low (>0.05) relative to MISR in Saharan regions during summer. We find that GEOS‐Chem captures much of the variability in AOD when compared with MISR and MODIS (r> 0.6) and represents the vertical structure in aerosol extinction over outflow regions well when compared to CALIOP. Including a realistic representation of the submicron‐size distribution of dust reduces simulated AOD by ∼25% over North Africa and improves agreement with observations. The lifetime of the simulated dust is typically a few days (25%–50%) shorter than inferred from MODIS observations, suggesting overvigorous wet removal, confirmed by comparison with rain rate observations from the Tropical Rainfall Measuring Mission satellite. The simulation captures the seasonality of deposition in Florida and the observed magnitude and variability of dust concentrations at Barbados from 2006 to 2008 (r = 0.74), indicating a good simulation of the impacts of North African dust on air quality in North America. We estimate that 218 ± 48 Tg of dust is annually deposited into the Atlantic and calculate a lower estimate for the dust deposited in the Caribbean and Amazon to be 26 ± 5 Tg yr−1 and 17 ± 5 Tg yr−1, respectively. This suggests that the dust deposition in the Amazon derived from satellites may be an upper limit. Key Points A better size representation of dust aerosol for dust optics Climatology of African mineral dust emissions from daily to annual time scales Estimates of dust deposition to the Americas and related uncertainties

We quantify the changes in the intensity of Brewer‐Dobson Circulation (BDC) during sudden stratospheric warming (SSW) and its impact on the tropical stratospheric thermal structure and ozone distribution by composite analysis using observations and a chemical‐transport model. An increase in the planetary wave activity and enhancement in BDC intensity before the central date of SSW is noticed. A positive ozone anomaly is observed in the tropical upper stratosphere. The tropical lower stratosphere shows a cooling (1–2 K) and negative ozone anomaly (∼0.1 ppmv) after ∼10 days from the central date. The polar stratosphere experiences a positive ozone anomaly, whereas the upper stratosphere shows ozone depletion due to the downwelling of NOx‐rich mesospheric air. The cold‐point tropopause temperature shows a cooling of ∼0.5 K for major warming which in turn dries the lower stratosphere. Plain Language Summary The Brewer‐Dobson circulation transports tropical air toward the polar stratosphere. During sudden stratospheric warming, the associated wave activity and changing zonal wind direction in the stratosphere alter the intensity of the circulation. The strength of the circulation increases before the warming, resulting in enhanced upwelling over the tropics. The upwelling leads to cooling across the stratosphere and decreased ozone concentrations in the lower stratosphere over the tropics. The lower temperatures over the upper stratosphere reduce the ozone depletion rate. Over the polar upper stratosphere, ozone depletion is observed due to the downwelling of NOx‐rich air and an increase in the lower stratosphere due to the transport of ozone‐rich air from higher levels. Key Points Change in the intensity of Brewer‐Dobson Circulation (BDC) due to sudden stratospheric warming Ozone transport due to change in the intensity of BDC using observations and modeling Cooling across the tropical stratosphere and decreased ozone concentrations due to upwelling

Detailed examination of the impact of modern space launches on the Earth's atmosphere is crucial, given booming investment in the space industry and an anticipated space tourism era. We develop air pollutant emissions inventories for rocket launches and re‐entry of reusable components and debris in 2019 and for a speculative space tourism scenario based on the recent billionaire space race. This we include in the global GEOS‐Chem model coupled to a radiative transfer model to determine the influence on stratospheric ozone (O3) and climate. Due to recent surge in re‐entering debris and reusable components, nitrogen oxides from re‐entry heating and chlorine from solid fuels contribute equally to all stratospheric O3 depletion by contemporary rockets. Decline in global stratospheric O3 is small (0.01%), but reaches 0.15% in the upper stratosphere (∼5 hPa, 40 km) in spring at 60–90°N after a decade of sustained 5.6% a−1 growth in 2019 launches and re‐entries. This increases to 0.24% with a decade of emissions from space tourism rockets, undermining O3 recovery achieved with the Montreal Protocol. Rocket emissions of black carbon (BC) produce substantial global mean radiative forcing of 8 mW m−2 after just 3 years of routine space tourism launches. This is a much greater contribution to global radiative forcing (6%) than emissions (0.02%) of all other BC sources, as radiative forcing per unit mass emitted is ∼500 times more than surface and aviation sources. The O3 damage and climate effect we estimate should motivate regulation of an industry poised for rapid growth. Plain Language Summary It is imperative that we understand the current and future risks to Earth's atmosphere posed by pollution from rocket launches and re‐entry heating of reusable and discarded rocket parts and historical debris. Rockets, unlike other anthropogenic pollution sources, emit gaseous and solid chemicals directly into the upper atmosphere. We compile inventories of these chemicals from rocket launches in 2019 and projections of future growth and speculative space tourism activity. We incorporate these in a 3D atmospheric chemistry model to simulate the impact on climate and the protective stratospheric ozone layer. We find that loss of ozone due to current rockets is small, but that routine space tourism launches may undermine progress made by the Montreal Protocol in reversing ozone depletion in the Arctic springtime upper stratosphere. The BC (or soot) particles from rockets are also of great concern, as these are almost five hundred times more efficient at warming the atmosphere than all other sources of soot combined. These findings demonstrate an urgent need to develop environmental regulation to mitigate damage from this rapidly growing industry. Key Points Air pollutant emission inventory for current space sector and future tourism input to a coupled chemistry and radiative transfer model Upper stratospheric Arctic ozone loss from launch chlorine and re‐entry nitrogen oxide emissions undermines Montreal Protocol success Warming efficiency of space tourism (soot) emissions about 500‐times greater than surface and aircraft sources of soot

Tropospheric reactive bromine (Bry) influences the oxidation capacity of the atmosphere by acting as a sink for ozone and nitrogen oxides. Aerosol acidity plays a crucial role in Bry abundances through acid‐catalyzed debromination from sea‐salt‐aerosol, the largest global source. Bromine concentrations in a Russian Arctic ice‐core, Akademii Nauk, show a 3.5‐fold increase from pre‐industrial (PI) to the 1970s (peak acidity, PA), and decreased by half to 1999 (present day, PD). Ice‐core acidity mirrors this trend, showing robust correlation with bromine, especially after 1940 (r = 0.9). Model simulations considering anthropogenic emission changes alone show that atmospheric acidity is the main driver of Bry changes, consistent with the observed relationship between acidity and bromine. The influence of atmospheric acidity on Bry should be considered in interpretation of ice‐core bromine trends. Plain Language Summary Reactive bromine in the air impacts major oxidants in our atmosphere, which remove pollutants and greenhouse gases and has changed over time in the Russian Arctic. Ice‐core bromine and acidity show a significant increase from pre‐industrial to the 1970s followed by a decrease. Our study suggests that human activities caused changes in bromine through the emissions of acidic gases from fossil fuel combustion. Considering relationships between atmospheric acidity and bromine is crucial to interpreting bromine variations in ice cores. Key Points Bromine (Br) concentrations increased 3.5‐fold from pre‐industrial to 1975 and declined 50% by 1999 in a Russian Arctic ice‐core A robust correlation between ice‐core Br and acidity highlights acidity’s key role in influencing the atmospheric Br budget Model shows acid‐catalyzed sea‐salt debromination is the largest source of reactive Br and drives ice‐core Br trends