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

We quantify the contributions of emissions from the transport sector to tropospheric ozone and the hydroxyl radical (OH) by means of model simulations with a global chemistry-climate model equipped with a source attribution method. For the first time we applied a method which also allows for quantifying contributions to OH which is invariant upon disaggregation or recombination and additive. Based on these quantified contributions, we analyse the ozone radiative forcing (RF) and methane lifetime reductions attributable to emissions from the transport sectors. The contributions were analysed for each transport sector separately and for 2015 as well as for 2050 under the Shared Socioeconomic Pathways (SSPs) SSP1-1.9, SSP2-4.5, and SSP3-7.0. In line with previous publications using the source attribution approach, we quantify ozone RF attributable to emissions from land transport, shipping, and aviation for the year 2015 of 121, 60, and 31 mWm-2 , respectively. At the same time, we diagnose a relative reduction in methane lifetime due to transport emissions of 14.3 % (land transport), 8.5 % (shipping), and 3.8 % (aviation). These reductions are significantly larger than reported by previous studies due to the application of the source attribution method. Compared to 2015, only SSP1-1.9 shows a strong decrease in ozone RF and methane lifetime reduction attributable to the entire transport sector in 2050. For the projections of SSP2-4.5, we find similar effects of the total transport sector as for 2015, while the effects in SSP3-7.0 increase compared to 2015. This small change in the effects for the two projections compared to 2015 is caused by two main factors. Firstly, aviation emissions are projected to increase in SSP2-4.5 (increase of 107 %) and SSP3-7.0 (+86 %) compared to 2015, resulting in projected ozone RF of 55 mWm-2 (+78 %) and 50 mWm-2 (+61 %) for the year 2050 from aviation emissions. Secondly, the non-linear effects of atmospheric chemistry in polluted regions such as Europe and North America lead to rather small reductions in ozone and OH in response to emission reductions, especially from land transport emissions. In addition, the increase in emissions from land transport in other parts of the world, particularly in South Asia, leads to an increased contribution of ozone and OH. In particular, ozone formed by land transport emissions from South Asia causes strong RF that partially offsets the reductions in Europe and North America. Moreover, our results show that besides the non-linear response, lack of international cooperation, as in the SSP3-7.0 projection, hinders mitigation of ground-level ozone. [ABSTRACT FROM AUTHOR]

Sustained near‐surface ozone pollution despite stringent emission controls in China has drawn wide attention. However, how such rapid regional emission reductions affect global tropospheric ozone remains unexplored. Here, using an ensemble of measurements and global model simulations, we find Chinese emission reductions during 2013–2020 have increased domestic surface ozone; particularly in North China, they have decreased the global tropospheric ozone burden by 2.7 Tg (54% of China's anthropogenic contribution and 7% of global anthropogenic contribution in 2019) and ozone radiative forcing by 8.8 mW m−2 (64% of China's anthropogenic contribution and 9% of global anthropogenic contribution in 2019). Nitrogen oxides (NOx) associated nonlinear chemistry and height‐dependent atmospheric transport patterns primarily cause the contrasting impacts. Future Chinese NOx emission controls would continue to decrease global tropospheric ozone, while regional surface ozone may still increase unless NOx reductions exceed ∼53% of the 2019 level. Our results reveal the substantial benefit of regional emission reductions in China on global tropospheric ozone mitigation. Plain Language Summary: Tropospheric ozone is an important air pollutant and greenhouse gas and mainly produced by atmospheric photochemical reactions. Its levels are largely impacted by anthropogenic emission sources. Over the past decade, anthropogenic emissions in China have greatly declined, which may alter tropospheric ozone on a global scale. Using numerical simulations from a global chemical transport model, we show that emission reductions in China have substantially decreased the global tropospheric ozone burden over 2013–2020. The emission reductions have reduced the global anthropogenic tropospheric ozone radiative forcing and the Chinese contributions by 9% and 64%, respectively, although the emission reductions increased domestic surface ozone over North China. The differences in the nitrogen oxides (NOx) associated ozone formation regimes and atmospheric transport mechanisms over the surface of North China and the free troposphere primarily cause the contrasting impacts. We estimate that further emission reductions of NOx in China would cause nonlinear domestic and global tropospheric ozone changes. Our assessment demonstrates that emission reductions in China help global tropospheric ozone mitigation. Deep emission reductions are needed to further reduce domestic ozone pollution in China. Key Points: Emission reductions in China have substantially decreased the global tropospheric ozone burden and its radiative forcingDifferences in ozone chemistry and atmospheric transport mechanisms cause contrasting domestic and global impactsFuture NOx reductions in China would further mitigate global tropospheric ozone despite short‐term domestic surface ozone increases [ABSTRACT FROM AUTHOR]

Atmospheric hydrogen concentrations have been increasing in recent decades. Hydrogen is radiatively inert, but it is chemically reactive and exerts an indirect radiative forcing through chemistry that perturbs the concentrations of key species within the troposphere, including ozone. Using the atmospheric version of the United Kingdom Earth System Model, we analyse the impact of 10% increased surface concentrations of hydrogen on ozone production and loss. We also analyse the impact of this hydrogen in atmospheres with lower anthropogenic emissions of nitrogen oxides (80% and 30% of present-day anthropogenic surface emissions), as this is a likely outcome of the transition from fossil fuels towards cleaner technologies. In each case, we also assess the changes in hydroxyl radical concentration and hence methane lifetime and calculate the net impact on the hydrogen tropospheric global warming potential (GWP). We find that the hydrogen tropospheric GWP100 will change relatively little with decreases in surface anthropogenic NOx emissions (9.4 and 9.1 for our present day and 30% anthropogenic emissions, respectively). The current estimate for hydrogen GWP100 can therefore be applied to future scenarios of differing NOx, although this conclusion may be impacted by future changes in emissions of other reactive species. [ABSTRACT FROM AUTHOR]

Biomass burning (BB) is a major source of trace gases and particles in the atmosphere, influencing air quality, radiative balance, and climate. Previous studies have mainly focused on the BB emissions of carbon and nitrogen species with less attention on chlorine. Reactive chlorine chemistry has significant effects on atmospheric chemistry and air quality. However, quantitative information on chlorine emissions from BB, particularly the long-term trend and associated atmospheric impacts, is limited both on regional and global scales. Here, we report a long-term (2001–2018) high-resolution BB emission inventory for the major chlorine-containing compounds (HCl, chloride, and CH3Cl) in Asia based on satellite observations. We estimate an average of 730 Gg yr−1 chlorine emitted from BB activity in Asia, with China contributing the largest share at 24.2% (177 Gg yr−1), followed by Myanmar at 18.7% and India at 18.3%. Distinct seasonal patterns and significant spatial and interannual variability are observed, mainly driven by human-mediated changes in agricultural activity. By incorporating the newly developed chlorine emission inventory into a global chemistry-climate model (CAM-Chem), we find that the BB-chlorine emissions lead to elevated levels of HCl and CH3Cl (monthly average up to 2062 and 1421 parts per trillion by volume (pptv), respectively), subsequently resulting in noticeable changes in oxidants (up to 3.1% in O3 and 17% in OH radicals). The results demonstrate that BB is not only a significant source of air pollutants but also of oxidants, suggesting a larger role of BB emissions in the atmospheric chemistry and oxidation process than previously appreciated. In light of the projected increase in BB activity toward the end of the century and the extensive control of anthropogenic emissions worldwide, the contribution of BB emissions may become fundamental to air quality composition in the future. [ABSTRACT FROM AUTHOR]

Satellite-based observations of free-tropospheric water vapour isotopologue ratios (HDO / H 2 O, expressed in form of the δ value δ D) with good global and temporal coverage have become available recently. We investigate the potential of these observations for constraining the uncertainties of the atmospheric analyses fields of specific humidity (q), temperature (T), and δ D and of variables that capture important properties of the atmospheric water cycle, namely the vertical velocity (ω), the latent heating rate (Q2), and the precipitation rate (Prcp). Our focus is on the impact of the δ D observations relative to the impact achieved by the observation of q and T , which are much more easily observed by satellites and are routinely in use for atmospheric analyses. For our investigations we use an Observing System Simulation Experiment; i.e. we simulate the satellite observations of q , T , and δ D with known uncertainties and coverage (e.g. observations are not available for cloudy conditions, i.e. at locations where the atmosphere is vertically unstable). Then we use the simulated observations within a Kalman-filter-based assimilation framework in order to evaluate their potential for improving the quality of atmospheric analyses. The study is made for low latitudes (30° S to 30° N) and for 40 d between mid-July and the end of August 2016. We find that q observations generally have the largest impacts on the analyses' quality and that T observations have stronger impacts overall than δ D observations. We show that there is no significant impact of δ D observations for stable atmospheric conditions; however, for very unstable conditions, the impact of δ D observations is significant and even slightly stronger than the respective impact of T observations. Very unstable conditions are rare but are related to extreme events (e.g. storms, flooding); i.e. the δ D observations significantly impact the analyses' quality of the events that have the largest societal consequences. The fact that no satellite observations are available at the location and time of the unstable conditions indicates a remote impact of δ D observations that are available elsewhere. Concerning real-world applications, we conclude that the situation of δ D satellite observations is very promising but that further improving the model's linkage between convective processes and the larger-scale δ D fields might be needed for optimizing the assimilation impact of real-world δ D observations. [ABSTRACT FROM AUTHOR]

​​​​​​​Volatile organic compounds (VOCs) play a key role in tropospheric chemistry, giving rise to secondary products such as highly oxygenated organic molecules (HOMs) and secondary organic aerosols (SOAs). HOMs, a group of low-volatility gas-phase products, are formed through the autoxidation process of peroxy radicals (RO 2) originating from the oxidation of VOCs. The measurement of HOMs is made by a NO 3- ToFCIMS instrument, which also detects other species like small highly oxygenated VOCs (e.g., dicarboxylic acids) and sulfuric acid (H 2 SO 4). The instrument response to HOMs is typically estimated using H 2 SO 4 , as HOMs are neither commercially available nor easily synthesized in the laboratory. The resulting calibration factor is then applied to quantify all species detected using this technique. In this study, we explore the sensitivity of the instrument to commercially available small organic compounds, primarily dicarboxylic acids, given the limitations associated with producing known amounts of HOMs for calibration. We compare these single-compound calibration factors to the one obtained for H 2 SO 4 under identical operational conditions. The study found that the sensitivity of the NO 3- ToFCIMS varies depending on the specific type of organic compound, illustrating how a single calibration factor derived from sulfuric acid is clearly inadequate for quantifying all detected species using this technique. The results highlighted substantial variability in the calibration factors for the tested organic compounds, with 4-nitrocatechol exhibiting the highest sensitivity and pyruvic acid the lowest. The obtained sulfuric acid calibration factor agreed well with the previous values from the literature. In summary, this research emphasized the need to develop reliable and precise calibration methods for progressively oxygenated reaction products measured with a NO 3- chemical-ionization mass spectrometer (CIMS), for example, HOMs. [ABSTRACT FROM AUTHOR]

The Geostationary Environment Monitoring Spectrometer (GEMS) over Asia is the first geostationary Earth orbit instrument in the virtual constellation of sensors for atmospheric chemistry and composition air quality research and applications. For the first time, the hourly observations enable studies of diurnal variation in several important trace gas and aerosol pollutants including nitrogen dioxide (NO2), which is the focus of this work. NO2 is a regulated pollutant and an indicator of anthropogenic emissions in addition to being involved in tropospheric ozone chemistry and particulate matter formation. We present new quantitative measures of NO2 tropospheric column diurnal variation which can be greater than 50 % of the column amount, especially in polluted environments. The NO2 distribution is seen to change hourly and can be quite different from what would be seen by a once-a-day low-Earth-orbit satellite observation. We use GEMS data in combination with TROPOspheric Monitoring Instrument (TROPOMI) satellite and Pandora ground-based remote sensing measurements and Multi-Scale Infrastructure for Chemistry and Aerosols (Version 0, MUSICAv0) 3D chemical transport model analysis to examine the NO2 diurnal variation in January and June 2023 over Northeast Asia and Seoul, South Korea, study regions to distinguish the different emissions, chemistry, and meteorological processes that drive the variation. Understanding the relative importance of these processes will be key to including pollutant diurnal variation in models aimed at determining true pollutant exposure levels for air quality studies. The work presented here also provides a path for investigating similar NO2 diurnal cycles in the new Earth Venture Instrument-1 Tropospheric Emissions: Monitoring Pollution (TEMPO) data over North America, and later over Europe with Sentinel-4. [ABSTRACT FROM AUTHOR]

Tropospheric ozone is a major air pollutant and greenhouse gas. It is also the primary precursor of OH, the main tropospheric oxidant. Global atmospheric chemistry models show large differences in their simulations of tropospheric ozone budgets. Here we implement the widely used GEOS-Chem atmospheric chemistry module as an alternative to CAM-chem within the Community Earth System Model version 2 (CESM2). We compare the resulting GEOS-Chem and CAM-chem simulations of tropospheric ozone and related species within CESM2 to observations from ozonesondes, surface sites, the ATom-1 aircraft campaign over the Pacific and Atlantic, and the KORUS-AQ aircraft campaign over the Seoul Metropolitan Area. We find that GEOS-Chem and CAM-chem within CESM2 have similar tropospheric ozone budgets and concentrations usually within 5 ppb but important differences in the underlying processes including (1) photolysis scheme (no aerosol effects in CAM-chem), (2) aerosol nitrate photolysis, (3) N2O5 cloud uptake, (4) tropospheric halogen chemistry, and (5) ozone deposition to the oceans. Global tropospheric OH concentrations are the same in both models, but there are large regional differences reflecting the above processes. Carbon monoxide is lower in CAM-chem (and lower than observations), at least in part because of higher OH concentrations in the Northern Hemisphere and insufficient production from isoprene oxidation in the Southern Hemisphere. CESM2 does not scavenge water-soluble gases in convective updrafts, leading to some upper-tropospheric biases. Comparison to KORUS-AQ observations shows an overestimate of ozone above 4 km altitude in both models, which at least in GEOS-Chem is due to inadequate scavenging of particulate nitrate in convective updrafts in CESM2, leading to excessive NO production from nitrate photolysis. The KORUS-AQ comparison also suggests insufficient boundary layer mixing in CESM2. This implementation and evaluation of GEOS-Chem in CESM2 contribute to the MUSICA vision of modularizing tropospheric chemistry in Earth system models. [ABSTRACT FROM AUTHOR]

Tropospheric ozone (O3) is an important greenhouse gas that is also hazardous to human health. The formation of O3 is sensitive to the levels of its precursors NOx (≡NO + NO2) and peroxy radicals, for example, generated by the oxidation of volatile organic compounds (VOCs). A better understanding of this sensitivity will show how changes in the levels of these trace gases could affect O3 levels today and in the future, and thus air quality and climate. In this study, we investigate O3 sensitivity in the tropical troposphere based on in situ observations of NO, HO2 and O3 from four research aircraft campaigns between 2015 and 2023. These are OMO (Oxidation Mechanism Observations), ATom (Atmospheric Tomography Mission), CAFE Africa (Chemistry of the Atmosphere Field Experiment in Africa) and CAFE Brazil, in combination with simulations using the EMAC atmospheric chemistry—climate model. We use the metric α(CH3O2) together with NO to investigate the O3 formation sensitivity. We show that O3 formation is generally NOx‐sensitive in the lower and middle tropical troposphere and is in a transition regime in the upper troposphere. By distinguishing observations impacted by lightning or not we show that NO from lightning is the most important driver of O3 sensitivity in the tropics. NOx‐sensitive chemistry predominates in regions without lightning impact, with α(CH3O2) ranging between 0.56 and 0.82 and observed average O3 levels between 35 and 55 ppbv. Areas affected by lightning exhibit strongly VOC‐sensitive O3 chemistry with α(CH3O2) of about 1 and average O3 levels between 55 and 80 ppbv. Plain Language Summary: Ozone (O3) in the troposphere is both an air pollutant and a greenhouse gas. It is formed from nitrogen oxides (NOx) and volatile organic compounds (VOCs). The formation can be sensitive to either of these precursors depending on their abundance. Considering the high relevance of O3 in regard to human health and global warming, it is important to understand this sensitivity of O3 formation, which allows to predict future changes in O3. Here, we investigate O3 formation sensitivity toward NOx and VOCs in the tropical troposphere based on aircraft measurements during four research campaigns between 2015 and 2023, and a global model. We include observations of NO, HO2 (hydroperoxyl radicals) and O3 over South America, the Middle East and the Pacific, Atlantic and Indian Ocean. We find that O3 formation is sensitive to NOx in the lower tropical troposphere. In the upper tropical troposphere, lightning events control O3 chemistry and promote strong VOC‐sensitive O3 formation. Key Points: α(CH3O2) correlated with NO is a powerful metric for indicating O3 sensitivity and is valid throughout the troposphereO3 chemistry in the remote tropical lower troposphere is found to be NOx‐sensitiveNO emissions from lightning drive O3 sensitivity in the tropical upper troposphere and induce highly VOC‐sensitive chemistry [ABSTRACT FROM AUTHOR]

The decay of methyl chloroform, a banned ozone-depleting substance, has provided a clear observational metric of mean tropospheric hydroxyl radical (OH) abundance. Almost all current global chemistry models calculate about 15% too much OH and thus too rapid methane loss. Methane is a short-lived climate forcer, critical to achieving global warming targets, and this error affects our model projections of climate change. New observations of water vapor absorption in the ultraviolet region (290 to 350 nanometers) imply reductions in sunlight with key photolysis rates decreasing by 8 to 12% in the near-surface tropical atmosphere. Incorporation of this new mechanism in a chemistry-transport model reduces OH and methane loss by only 4%, but combined with other proposed mechanisms, such as tropospheric halogen chemistry (7%), we may be able to resolve this conundrum. [ABSTRACT FROM AUTHOR]

We investigate the performance of two widely used chemistry-transport models (CTMs) with different chemical mechanisms in reproducing the ambient maximum daily 8-h average ozone (MDA8 O 3 ) burden over Central Europe. We explore a base case setup with boundary conditions (BC) for meteorology from the ERA-Interim reanalysis and chemical BC from CAM-Chem as well as effects of alterations in these BC based on global model fields. Our results show that changes in meteorological BC strongly affect the correlation with observations but only marginally affect the model biases, while changes in chemical BC increase model biases while correlation patterns remain largely unchanged. Furthermore, our study highlights that CTM choice (and choice of chemical mechanism) has a similar or even larger impact on MDA8 O 3 levels as the impact of altered BC. In summary, our study calls for a multi-model strategy combining different CTM and BC combinations to explore the bandwidth of MDA8 O 3 distributions and thus uncertainty in hindcasts and future projections, in analogy to climate studies considering ensemble simulations under the same anthropogenic emissions but with slightly different initial conditions. [ABSTRACT FROM AUTHOR]

The hydroxyl radical (OH) fuels atmospheric chemical cycling as the main sink for methane and a driver of the formation and loss of many air pollutants, but direct OH observations are sparse. We develop and evaluate an observation-based proxy for short-term, spatial variations in OH (ProxyOH) in the remote marine troposphere using comprehensive measurements from the NASA Atmospheric Tomography (ATom) airborne campaign. ProxyOH is a reduced form of the OH steady-state equation representing the dominant OH production and loss pathways in the remote marine troposphere, according to box model simulations of OH constrained with ATom observations. ProxyOH comprises only eight variables that are generally observed by routine ground- or satellite-based instruments. ProxyOH scales linearly with in situ [OH] spatial variations along the ATom flight tracks (median r2 = 0.90, interquartile range = 0.80 to 0.94 across 2-km altitude by 20° latitudinal regions). We deconstruct spatial variations in ProxyOH as a first-order approximation of the sensitivity of OH variations to individual terms. Two terms modulate within-region ProxyOH variations-water vapor (H2O) and, to a lesser extent, nitric oxide (NO). This implies that a limited set of observations could offer an avenue for observation-based mapping of OH spatial variations over much of the remote marine troposphere. Both H2O and NO are expected to change with climate, while NO also varies strongly with human activities. We also illustrate the utility of ProxyOH as a process-based approach for evaluating intermodel differences in remote marine tropospheric OH.

This article examines the relationship between hydrogen and carbon monoxide in the atmosphere, particularly in the context of the increasing use of hydrogen energy. The study finds that the interactions between these two gases are complex and understanding them is crucial for assessing the environmental impacts of a shift to hydrogen energy. The research reveals that hydrogen can affect the levels of carbon monoxide, formaldehyde, hydroxyl, methane, ozone, and stratospheric water vapor in the atmosphere. The findings suggest that the warming effect of hydrogen leakage should be considered when expanding hydrogen energy. The article also discusses the interlinkage between hydrogen and carbon monoxide through incomplete combustion reactions and how carbon monoxide emissions can be used to estimate global hydrogen emissions. The study provides valuable insights into the relationship between hydrogen and carbon monoxide, contributing to our understanding of the potential environmental impacts of hydrogen energy. The research was funded by the HYDROGEN project and the Research Council of Norway. [Extracted from the article]

The oxygen isotope anomaly (Δ17O = δ17O - 0.52 × δ18O > 0) has proven to be a robust tool for probing photochemical cycling and atmospheric formation pathways of oxidized reactive nitrogen (NOy). Several studies have developed modeling techniques to implicitly model Δ17O of NOy molecules based on numerous assumptions that may not always be valid. Thus, these models may be oversimplified and limit our ability to compare model Δ17O values of NOy with observations. In this work, we introduce a novel method for explicitly tracking Δ17O transfer and propagation into NOy and odd oxygen (Ox), integrated into the Regional Atmospheric Chemistry Mechanism, version 2 (RACM2). Termed ICOIN-RACM2 (InCorporating Oxygen Isotopes of NOy in RACM2), this new model includes the addition of 55 new species and 729 replicate reactions to represent the propagation of Δ17O derived from O3 into NOy and Ox. Employing this mechanism within a box model, we simulate Δ17O for various NOy and Ox molecules for chamber experiments with varying initial nitrogen oxides (NOx = NO + NO2) and α -pinene conditions, revealing response shifts in Δ17O linked to distinct oxidant conditions. Furthermore, diel cycles are simulated under two summertime scenarios, representative of an urban and rural site, revealing pronounced Δ17O diurnal patterns for several NOy components and substantial Δ17O differences associated with pollution levels (urban vs. rural). Overall, the proposed mechanism offers the potential to assess NOy oxidation chemistry in chamber studies and air quality campaigns through Δ17O model comparisons against observations. The integration of this mechanism into a 3-D atmospheric chemistry transport model is expected to notably enhance our capacity to model and anticipate Δ17O across landscapes, consequently refining model representations of atmospheric chemistry and tropospheric oxidation capacity. [ABSTRACT FROM AUTHOR]

In this study, we utilized a 20-bin WRF-Chem (Weather Research and Forecasting coupled with Chemistry) regional model to investigate the contributions of chemical drivers to the growth of new particles, as well as to simulate the three-dimensional dynamics of new particle formation (NPF) events over the North China Plain during a summer campaign in 2019. The model replicated the occurrence of NPF and the growth pattern of newly formed particles, as well as the performance to meet the benchmarks, i.e., absolute mean fractional bias ≤ 50 % and mean fractional error ≤ 75 %, in replicating number concentrations of particles in the 10–40 nm range in five events between 29 June and 6 July 2019. Therefore, we further analyzed three NPF events with distinct particle growth characteristics. In these instances, the model overpredicted daytime condensation of H2SO4 vapor and nighttime formation of NH4NO3. These resulted in overestimation of the hygroscopicity parameter of nanometer particles. Nevertheless, the model performance met the benchmarks for reproducing cloud condensation nuclei (CCN) at a supersaturation (SS) of 0.4 % on NPF days. This was because the overestimation of inorganics was offset by the model underestimation of CCN originating from submicron particles. Additionally, three-dimensional simulations of NPF events demonstrated some key findings. First, NPF consistently begins in the upper parts of the planetary boundary layer (PBL) before expanding. Second, during daytime organics dominate growth of new particles in the PBL, whereas in the free troposphere the primary chemical drivers are inorganic species. However, to confirm these findings, vertical observations are required. [ABSTRACT FROM AUTHOR]

Zenith tropospheric delay (ZTD) is an important atmospheric parameter in radio‐space‐geodetic techniques such as Global Navigation Satellite System (GNSS), which is pivotal for GNSS positioning, navigation and meteorology. The Vienna Mapping Function (VMF) data server is a widely utilized source for implementing ZTD, offering two types of models, that is, the empirical one and the discrete one with Grid‐wise and Site‐wise models. Therefore, to evaluate the accuracy of these models becomes the focus of this article. Specifically, this study investigates their performances in terms of calculation of ZTD, using the hourly values derived from the International GNSS Service data as references. The results show that the root mean square err (RMSE) of the Site‐wise, Grid‐wise and global pressure and temperature 3 model are 11.71/13.03/38.56 mm, respectively, indicating the discrete model performs generally better than the empirical model, and the Site‐wise model is the better of the two discrete models. From the perspective of spatial resolution, the performance of these three models in ZTD calculation shows obvious influences of latitude changes and elevation differences. From the temporal analysis, the accuracy of the discrete model shows differences over different UTC epochs, while the empirical model can only express the seasonal ZTD characteristics with the average RMSE at different epochs being similar, the specifically values are 39.67, 39.26, 39.38 and 39.18 mm at UTC 0:00, 6:00, 12:00 and 18:00, respectively. The histogram and boxplot well indicate the accuracy differences of the three models in different seasons. Additionally, the time series of three models at different latitudes were also explored in this research. These explorations are conducive to the selection of appropriate models for calculating ZTD based on specific requirements. Key Points: The discrete and empirical troposphere delay models of the VMF data server is comprehensibly evaluated using the global International GNSS Service (IGS) dataThe Site‐wise of discrete model exhibits the best accuracy and is recommended first, followed by the Grid‐wide and the empirical modelEach model shows different influences by the latitude, elevation, seasons and epochs [ABSTRACT FROM AUTHOR]

Remote sensing based on satellites can provide long-term, consistent, and global coverage of NO2 (an important atmospheric air pollutant) as well as other trace gases. However, satellite data often miss data due to factors including but not limited to clouds, surface features, and aerosols. Moreover, one of the longest continuous observational platforms of NO2 observations from space, OMI, has suffered from missing data over certain rows since 2007, significantly reducing spatial coverage. This work uses the OMI based OMNO2 product, as well as an NO2 product from GOME-2 in combination with machine learning (XGBoost) and spatial interpolation (DINEOF) method to produce a 16-year global daily high spatial-temporal coverage merged tropospheric NO2 dataset (HSTCM-NO2, https://doi.org/10.5281/zenodo.10968462 , Qin et al., 2024), which increases the global spatial coverage of NO2 by ~60 % compared to the original OMINO2 data. The HSTCM-NO2 dataset is validated using upward looking observations of NO2 (MAX-DOAS), other satellites (TROPOMI), and reanalysis products. The comparisons show that HSTCM-NO2 maintains a good correlation with the magnitude of other observational datasets, except for under heavily polluted conditions (>6×1015 molec.cm-2). This work also introduces a new validation technique to validate coherent spatial and temporal signals (EOF) and validates that the HSTCM-NO2 are not only consistent with the original OMNO2 data, but in some parts of the world effectively fill in missing gaps and yield a superior result when analyzing long-range atmospheric transport of NO2. The few differences are also reported to be related to areas in which the original OMNO2 signal was very low, which has been shown elsewhere, but not from this perspective, further validating that applying a minimum cutoff to retrieved NO2 data is essential. The reconstructed data product can effectively extend the utilization value of the original OMNO2 data, and the data quality of HSTCM-NO2 can meet the needs of scientific research. [ABSTRACT FROM AUTHOR]

The hydroxyl radical (OH) largely controls the tropospheric self-cleansing capacity by reacting with gases harmful to the environment and human health. OH concentrations are determined locally by competing production and loss processes. Lacking strong observational constraints, models differ in how they balance these processes, such that the sign of past and future OH changes is uncertain. In a warmer climate, OH production will increase due to its water vapor dependence, partially offset by faster OH-methane loss. Weather-sensitive emissions will also likely increase, although their net impact on global mean OH depends on the balance between source (nitrogen oxides) and sink (reactive carbon) gases. Lightning activity increases OH, but its response to climate warming is of uncertain sign. To enable confident projections of OH, we recommend efforts to reduce uncertainties in kinetic reactions, in measured and modeled OH, in proxies for past OH concentrations, and in source and sink gas emissions. OH is strongly modulated by internal climate variability despite its lifetime of a few seconds at most, with implications for interpreting trends in methane. Improved kinetic constraints on key reactions would strengthen confidence in regional and global OH budgets, and in the response of OH to climate change. Future OH changes will depend on uncertain and compensating processes involving weather-sensitive chemistry and emissions, plus human choices. Technological solutions to climate change will likely impact tropospheric oxidizing capacity and merit further study prior to implementation. [ABSTRACT FROM AUTHOR]

South America is a large continent situated mostly in the Southern Hemisphere (SH) with complex topography and diverse emissions sources. However, the atmospheric chemistry of this region has been historically understudied. Here, we employ the Multi‐Scale Infrastructure for Chemistry and Aerosols, a novel global circulation model with regional refinement capabilities and full chemistry, to explore the sources and distribution of the carbon monoxide (CO) tropospheric column in South America during 2019, and also to assess the effect that South American primary emissions have over the rest of the world. Most of the CO over South America can be explained either by non‐methane volatile organic compounds (NMVOC) secondary chemical production or by biomass burning emissions, with biomass burning as the main explanation for the variability in CO. Biomass burning in Central Africa is a relevant contributor to CO in all of the continent, including the southern tip. Biogenic emissions play a dual role in CO concentrations: they provide volatile organic compounds that contribute to the secondary CO production, but they also destroy OH, which limits the chemical production and destruction of CO. As a net effect, the lifetime of CO is extended to ∼120 days on average over the Amazon, while still being in the range of 30–60 days in the rest of South America. Plain Language Summary: We use the Multi‐Scale Infrastructure for Chemistry and Aerosols, a global model with regional refinement, to study the origins of carbon monoxide (CO) in South America during 2019. The main sources of CO are the secondary production from non‐volatile organic compounds and the biomass burning primary emissions. The main source of temporal variability in the whole are the biomass burning emissions. We show that biomass burning in central Africa is a relevant source of South American CO during all year in all of the continent, including the furthermost south. Key Points: We employ the Multi‐Scale infrastructure for Chemistry and Aerosols to quantify the budget of CO in South America during 2019Most of the variability in the CO burden is explained by the variability in biomass burning emissionsBiomass burning in Central Africa is a relevant contributor to CO in all of the continent, including the southern region [ABSTRACT FROM AUTHOR]

Global chemistry-climate models (CCMs) play an important role in assessing the climate and air pollution implications of aerosols and chemically reactive gases. Evaluating these models under past conditions and constraining historical sources and sinks necessitates reliable records of atmospheric mixing ratios spanning preindustrial times. Such precious records were recently obtained for carbon monoxide (CO) documenting for the first time the evolution of this reactive compound over the industrial era. In this study, we compare the simulated atmospheric surface CO mixing ratios ([CO]) from two different sets of CCMs and emissions in the frame of CMIP5 and of CMIP6 (Coupled Model Intercomparison Project Phases 5 and 6) with recent bipolar ice archive reconstructions for the period spanning 1850 to present. We analyze how historical (1850–2014) [CO] outputs from 16 (Atmospheric Chemistry and Climate Model Intercomparison Project) models and 6 AerChemMIP (Aerosol Chemistry Model Intercomparison Project) models over Greenland and Antarctica are able to capture both absolute values and trends recorded in multi-site ice archives. While most models underestimate [CO] at high northern latitudes, a reduction in this bias is observed from ACCMIP to AerChemMIP exercises. Over the 1980–2010 CE period, trends in ice archive and firn air observations and AerChemMIP outputs align remarkably well at high northern and southern latitudes, indicating improved quantification of CO anthropogenic emissions and the main CO sink (OH oxidation) compared to ACCMIP. From 1850 to 1980 CE, AerChemMIP models and observations consistently show increasing [CO] in both the Northern Hemisphere (NH) and Southern Hemisphere (SH), suggesting a robust understanding of the CO budget evolution. However, a divergence in the [CO] growth rate emerges in NH between models and observations over the 1920–1975 CE period, attributed to uncertainties in CO emission factors (EF), particularly EF for RCO (Residential, Commercial and Others) and transportation sectors, although we cannot totally rule out that the CO record based on Greenland ice archives may be biased high by CO chemical production processes occurring in the ice prior the measurements (i.e., in situ CO production). In the Southern Hemisphere, AerChemMIP models simulate an increase in atmospheric [CO] from 1850 to 1980 CE closely reproducing the observations (22±10 ppb and 13±7 ppb, respectively). Such agreement supports CMIP6 biomass burning CO emission inventories which do not reveal a peak in CO emissions in the late 19th century. Furthermore, both SH models and observations reveal an accelerated growth rate in [CO] during 1945–1980 CE relative to 1980–1945 CE, likely linked to increased anthropogenic transportation emissions. [ABSTRACT FROM AUTHOR]

The Measurements Of Pollution In The Troposphere (MOPITT) is an ideal instrument to understand the impact of (1) assimilating multispectral and joint retrievals versus single spectral products, (2) assimilating satellite profile products versus column products, and (3) assimilating multispectral and joint retrievals versus assimilating individual products separately. We use the Community Atmosphere Model with chemistry with the Data Assimilation Research Testbed (CAM-chem + DART) to assimilate different MOPITT carbon monoxide (CO) products to address these three questions. Both anthropogenic and fire CO emissions are optimized in the data assimilation experiments. The results are compared with independent CO observations from TROPOspheric Monitoring Instrument (TROPOMI), the Total Carbon Column Observing Network (TCCON), NOAA Carbon Cycle Greenhouse Gases (CCGG) sites, In-service Aircraft for a Global Observing System (IAGOS), and Western wildfire Experiment for Cloud chemistry, Aerosol absorption and Nitrogen (WE-CAN). We find that (1) assimilating the MOPITT joint (multispectral; near-IR and thermal IR) column product leads to better model–observation agreement at and near the surface than assimilating the MOPITT thermal-IR-only column retrieval. (2) Assimilating column products has a larger impact and improvement for background and large-scale CO compared to assimilating profile products due to vertical localization in profile assimilation. However, profile assimilation can outperform column assimilations in fire-impacted regions and near the surface. (3) Assimilating multispectral and joint products results in similar or slightly better agreement with observations compared to assimilating the single spectral products separately. [ABSTRACT FROM AUTHOR]

Aerosols erupted from volcanoes may significantly alter climate because they can impact cloud dynamics, radiation budget, and tropospheric chemistry. The optical and physical characteristics of the Spatio-temporal variation of aerosols and gases' impact on the atmosphere during the recent La Palma eruption have been examined in the current paper from 19 September to 31 October 2021. The result shows that before the La Palma eruption, the AOD value was 0.1 showing a clear sky. However, after the eruption daily mean AOD reached the highest values of 0.7 and 0.56, showing that La Palma had high aerosol loading when the eruption occurred. The 0.4 value Aerosol index was observed. The coarse mode aerosols contributed more to the overall aerosol load than fine mode aerosols, confirming their existence above La Palma. Analysis revealed that mixed aerosols were present over La Palma. Due to the powerful ash plume's ability to scatter sunlight, the high value of Single-Scattering Albedo (~ 0.99) was detected. The maximum concentration of SO2, HCHO, NO2, O3, and CO was found to be 60 mmol/m2, 120 µmol/m2, 70 µmol/m2, 129 mmol/m2, and 50 mmol/m2, respectively, over the La Palma region. The ash plume's trajectory by the forward trajectory and the Hybrid Single Particle Lagrangian Integrated Trajectory dispersion model showed the plume moving in the southwest direction. [ABSTRACT FROM AUTHOR]

Nitrogen oxides (NO x= NO + NO 2) play a critical role in regulating tropospheric chemistry, yet NO x emission estimates are subject to large uncertainties, casting doubt on our ability to accurately model secondary pollutants such as ozone. Bottom-up emissions inventories are subject to a number of uncertainties related to estimates of emission activities, scaling factors, and fuel sources. Here, we provide an additional constraint on NO x emissions and trends using nitrate wet deposition (NWD) fluxes from the United States National Atmospheric Deposition Program (NADP) and the European Monitoring and Evaluation Programme (EMEP). We use these NWD measurements to evaluate anthropogenic and total NO x trends and magnitudes in the global Community Emissions Data System (CEDS) emissions inventory and the GEOS-Chem chemical transport model from 1980–2020. Over both the United States and Europe, observed NWD trends track well with anthropogenic NO x emissions from the CEDS inventory until 2010, after which NWD trends level out in contrast to continued decreases in CEDS. After 2010, NWD trends are able to reproduce total NO x emissions trends when the influences of both anthropogenic and background sources are considered. Observed NWD fluxes are also able to capture NO x emissions decreases over the 2020 COVID-19 lockdown period and are consistent with satellite and surface measurements of NO 2. These results suggest that NWD fluxes constrain total NO x emissions well, whether trends are driven by anthropogenic or background sources. We further compare modeled and observed NWD to provide an additional line of evidence for potential overestimates of anthropogenic NO x in emissions inventories. Over the United States, we find that NWD is overestimated in summer from 1980–2017 by 15 %–20 % on average (interquartile range: 11 %–31 %), with overestimates most prominent in the eastern US after 2000 (20 % on average), implying an overestimate of NO x emissions in the CEDS inventory (0.5×0.5 ° resolution). Over Europe, we find that modeled NWD is overestimated in all seasons from 1980–2017, with the strongest average overestimates occurring in summer and fall (175 % and 170 %, respectively). These overestimates may be reduced by cutting anthropogenic NO x emissions by 50 % in CEDS over Europe (i.e., cutting the 1980–2017 average annual emissions from 2.6 to 1.3 Tg N), but summertime and fall NO x may still need to be reduced further for observations and models to align. Overestimates may extend to other inventories such as the EMEP inventory, which estimates comparable but lower emissions than CEDS, with a 1990–2017 average of 2.1 Tg N relative to the CEDS 1990–2017 average of 2.4 Tg N. We find that NO x emission reductions over Europe improve model ozone at the surface, reducing the model summertime ozone overestimate from 14 % to 2 %. [ABSTRACT FROM AUTHOR]

Instruments for air quality observations on geostationary satellites provide multiple observations per day and allow for the analysis of the diurnal variation of important air pollutants such as nitrogen dioxide (NO2) over large areas. The South Korean instrument GEMS on the GK2B satellite was launched in February 2020 and is the first instrument in geostationary orbit that delivers hourly daytime observations of NO2. The measurements with a spatial resolution of 3.5 km x 8 km cover a large part of Asia. This study compares one year of tropospheric NO2 vertical column density (VCD) observations of the operational GEMS L2 product, the scientific GEMS IUP-UB product, the operational TROPOMI product, and ground-based DOAS measurements in South Korea. The GEMS L2 tropospheric NO2 VCDs overestimate the VCDs retrieved from the ground-based observations with a median relative difference of +64 % and a correlation coefficient of 0.75. The median relative difference is -1 % for the GEMS IUP-UB product and -14 % for the TROPOMI product. The evaluation of the GEMS IUP-UB product and the operational TROPOMI product with ground-based measurements is in good agreement with correlation coefficients of 0.82 and 0.88. The scatter in the GEMS products can be reduced when observations are limited to the TROPOMI overpass time. The observed diurnal variations of the tropospheric NO2 VCDs show a maximum of NO2 during the late morning for urban sites, whereas rural sites show weak or almost no diurnal changes. Investigations of the seasonal diurnal variability show with a minimum in the observed tropospheric NO2 VCDs around noon the importance of chemical loss of NO2 in summer. Most variability is seen in spring and autumn, which dominate the average annual diurnal cycle. Observations under low wind conditions show strong enhancements of NO2 over the day, especially at polluted sites during winter. This indicates that under calm conditions, dilution and the less effective chemical loss in winter do not balance the accumulating emissions. The impact of transport processes is illustrated by the diurnal variability at a rural site following mean wind patterns for specific seasons and observation times. Analyzing the weekday-weekend effect, good agreement was found between the different products. However, the GEMS L2 product while agreeing with the other data sets during weekdays shows significantly less reduction on weekends. Our investigations show that the observed diurnal evolution of NO2 varies significantly at the different measurement sites, with good agreement between the GEMS IUP-UB and ground-based observations. The diurnal variability of tropospheric NO2 VCDs depends on chemistry, emissions, and transport into and out of the measurement region. To interpret the sources and sinks of NO2 requires that all of these factors are considered. [ABSTRACT FROM AUTHOR]

Bromine activation (the production of Br in an elevated oxidation state) promotes ozone destruction and mercury removal in the global troposphere and commonly occurs in both springtime polar boundary layers, often accompanied by nearly complete ozone destruction. The chemistry and budget of active bromine compounds (e.g., Br2 , BrCl , BrO , HOBr) reflect the cycling of Br and affect its environmental impact. Cyanogen bromide (BrCN) has recently been measured by iodide ion high-resolution time-of-flight mass spectrometry (I- CIMS), and trifluoro methoxide ion time-of-flight mass spectrometry (CF3O- CIMS) during the NASA Atmospheric Tomography Mission second, third, and fourth deployments (NASA ATom), and could be a previously unquantified participant in active Br chemistry. BrCN mixing ratios ranged from below the detection limit (1.5 pptv) up to as high as 36 pptv (10 s average) and enhancements were almost exclusively confined to the polar boundary layers in the Arctic winter and in both polar regions during spring and fall. The coincidence of BrCN with active Br chemistry (often observable BrO , BrCl and O3 loss) and high CHBr3/CH2Br2 ratios imply that much of the observed BrCN is from atmospheric Br chemistry rather than a biogenic source. Likely BrCN formation pathways involve the heterogeneous reactions of active Br (Br2 , HOBr) with reduced nitrogen compounds, for example hydrogen cyanide (HCN / CN-), on snow, ice, or particle surfaces. Competitive reaction calculations of HOBr reactions with Cl- / Br- and HCN / CN- in solution, as well as box model calculations with bromine chemistry, confirm the viability of this formation channel and show a distinct pH dependence, with BrCN formation favored at higher pH values. Gas-phase loss processes of BrCN due to reaction with radical species are likely quite slow and photolysis is known to be relatively slow (BrCN lifetime of ∼ 4 months in midlatitude summer). These features, and the lack of BrCN enhancements above the polar boundary layer, imply that surface reactions must be the major loss processes. The fate of BrCN determines whether BrCN production fuels or terminates bromine activation. BrCN reactions with other halogens (Br- , HOCl , HOBr) may perpetuate the active Br cycle; however, preliminary laboratory experiments showed that BrCN did not react with aqueous bromide ion (< 0.1 %) to reform Br2. Liquid-phase reactions of BrCN are more likely to convert Br to bromide (Br-) or form a C – Br bonded organic species, as these are the known condensed-phase reactions of BrCN and would therefore constitute a loss of atmospheric active Br. Thus, further study of the chemistry of BrCN will be important for diagnosing polar Br cycling. [ABSTRACT FROM AUTHOR]

We analyze tropical ozone (O3) and carbon monoxide (CO) distributions in the upper troposphere (UT) and their temporal changes for 2005–2020 using Aura Microwave Limb Sounder (MLS) observations and chemistry climate models. The models are the Whole Atmosphere Community Climate Model (WACCM6) and two variants of the Community Atmosphere Model with Chemistry (CAM-chem), each variant using different anthropogenic emissions. Upper tropospheric trends and variability diagnostics are obtained from multiple linear regression analyses. We compare the model and MLS annual climatologies, focusing on 147 and 215 hPa pressure levels; climatological values generally fall within 10–20 % of each other, with both positive and negative differences for O3, and with models generally underestimating observed CO. In the northern hemisphere tropics, we find significantly poorer model fits to the observed phasing of CO seasonal changes at 215 hPa than at 147 hPa. This discrepancy is much smaller for the comparison of modeled and Measurements of Pollution in the Troposphere (MOPITT) V9J CO columns. We also find that the sensitivity of UT CO to El Niño / Southern Oscillation (ENSO) is positive at all tropical longitudes, in contrast to the dipolar longitudinal structure that exists for UT O3 ENSO sensitivity. MLS O3 has a zonal mean trend at 20° S–20° N of +0.39 ± 0.28 % yr-1; CAM-chem and WACCM have similar trends, though the WACCM trend is somewhat smaller. Our analyses for specific latitude/longitude bins yield positive trends up to 1.4 % yr-1 over Indonesia and East of that region, as well as over tropical Africa and the tropical Atlantic. We find broad similarities between the mapped MLS-derived UT O3 trends and corresponding mapped trends of tropospheric column ozone. Positive tropical UT mapped O3 trends are generally captured by the models, although in a more muted way. There is room for improvements in modeled tropical UT CO trends. Indeed, the MLS zonal mean CO trend is -0.25 ± 0.30 % yr-1, whereas the corresponding modeled CO trends are near zero (0.0 ± 0.14 % yr-1) when the anthropogenic emissions used in CAM-chem and WACCM are taken from Community Emissions Data System (CEDS) version 2. The non-CEDS version of CAM-chem yields CO UT trends of 0.22 ± 0.19 % yr-1, in contrast to the negative MLS CO trends throughout the tropics. The negative MLS tropical UT CO trends for 2005–2020 agree with (but tend to be smaller in magnitude than) previously published total column CO trends. Decreasing CO emissions from anthropogenic and biomass burning sources have previously been suggested as the main causes of tropospheric CO decreases, although significant regional emission trend variations exist. [ABSTRACT FROM AUTHOR]

The Geostationary Environment Monitoring Spectrometer (GEMS) over Asia is the first geostationary Earth orbit instrument in the virtual constellation of sensors for atmospheric chemistry and composition air quality research and applications. For the first time, the hourly observations enable studies of diurnal variation of several important trace gas and aerosol pollutants including nitrogen dioxide (NO2) which is the focus of this work. NO2 is a regulated pollutant and an indicator of anthropogenic emissions in addition to being involved in tropospheric ozone chemistry and particulate matter formation. We present new quantitative measures of NO2 tropospheric column diurnal variation which can be greater than 50 % of the column amount especially in polluted environments. The NO2 distribution is seen to change hourly and can be quite different from what would be seen by a once-a-day low Earth orbit satellite observation. We use GEMS data in combination with TROPOMI satellite and Pandora ground-based remote sensing measurements and MUSICAv0 3D chemical transport model analysis to examine the NO2 diurnal variation in January and June 2023 over Northeast Asia and Seoul, South Korea, study regions to distinguish the different emissions, chemistry, and meteorological processes that drive the variation. Understanding the relative importance of these processes will be important for including pollutant diurnal variation in models aimed at determining true pollutant exposure levels for air quality studies. The work presented here also provides a path for investigating similar NO2 diurnal cycles in the new TEMPO data over North America, and later over Europe with S-4. [ABSTRACT FROM AUTHOR]

• We developed an offline method to measure HO 2 uptake kinetics onto ambient aerosols. • γ HO 2 is strongly RH and pH dependent and inversely correlated with [HO2] 0. • Different ambient aerosol components have collective effects to γ HO 2. • HO 2 self-reaction and transition metal-catalyzed reactions both contributed to γ HO 2. The heterogeneous loss of HO 2 radicals onto ambient aerosols plays an important role in tropospheric chemistry. However, sparse investigation of the dominating parameters controlling the HO 2 uptake coefficients onto ambient aerosols (γ HO 2) has largely hindered the application of the measured γ HO 2 to the global spatial prediction. Here we induced an offline method using LFP-LIF technique to measure the kinetics of HO 2 uptake onto ambient aerosols collected in summertime and wintertime in Yokohama city, a regional urban site near Tokyo, Japan. By controlling the dominating parameters which influence γ HO 2 , we were able to investigate the detailed HO 2 uptake mechanism. We characterized the chemical composition of the collected ambient aerosols, including organics, inorganics, transition metals ions, etc. and modeled γ HO 2 using different mechanisms. Results show that γ HO 2 increased with the increase in RH, and the aerosol states ("dry" or wet/aqueous) have large effects on γ HO 2. With fixed RH and aerosol chemical composition, γ HO 2 was highly dependent on pH and inversely correlated with [HO 2 ] 0. By combing the measured γ HO 2 values with the modeled ones, we found that both the HO 2 self-reaction and transition metal-catalyzed reactions should be accounted for to yield a single parameterization to predict γ HO 2 , and different chemical compositions may have collective effects on γ HO 2. Results may serve for extending the γ HO 2 values measured at one observation site to different environmental conditions, which will help us to achieve more accurate modeling results concerning secondary pollutant formation (i.e., ozone). [ABSTRACT FROM AUTHOR]

This study conducts a spatio-temporal analysis of tropospheric nitrogen dioxide (NO2) and total carbon monoxide (CO) concentrations in the Punjab and Haryana regions of India and Pakistan, using datasets from the Sentinel 5-Precursor (S5P) satellite. These regions, marked by diverse economic growth factors including population expansion, power generation, transportation, and agricultural practices, face similar challenges in atmospheric pollution, particularly evident in major urban centers like Delhi and Lahore, identified as pollution hotspots. The study also spotlights pollution associated with power plants. In urban areas, tropospheric NO2 levels are predominantly elevated due to vehicular emissions, whereas residential activities mainly contribute to CO pollution. However, precisely attributing urban CO sources is complex due to its longer atmospheric residence time and intricate circulation patterns. Notably, the burning of rice crop residue in November significantly exacerbates winter pollution episodes and smog, showing a more pronounced correlation with total CO than with tropospheric NO2 levels. The temporal analysis indicates that the months from October to December witness peak pollution, contrasted with the relatively cleaner period during the monsoon months of July to September. The severe pollution in the OND quarter is attributed to factors such as variations in boundary layer height and depletion of OH radicals. Furthermore, the study highlights the positive impact of the COVID-19 lockdown on air quality, with a significant decrease in NO2 concentrations during April, 2020 (Delhi: 59%, Lahore: 58%). However, the reduction in total CO columns was less significant. The study also correlates lockdown stringency with tropospheric NO2 columns (R2: 0.37 for Delhi, 0.25 for Lahore, 0.22 for Rawalpindi/Islamabad), acknowledging the influence of various meteorological and atmospheric variables. The research highlights the significant impact of crop residue burning on winter pollution levels, particularly on total CO concentrations. The study also shows the notable effect of the COVID-19 lockdown on air quality, significantly reducing NO2 levels. Additionally, it explores the correlation between lockdown stringency and tropospheric NO2 columns, considering various meteorological factors. [ABSTRACT FROM AUTHOR]

Heterogeneous chemical cycles of pyrogenic nitrogen and halides influence tropospheric ozone and affect the stratosphere during extreme Pyrocumulonimbus (PyroCB) events. We report field‐derived N2O5 uptake coefficients, γ(N2O5), and ClNO2 yields, φ(ClNO2), from two aircraft campaigns observing fresh smoke in the lower and mid troposphere and processed/aged smoke in the upper troposphere and lower stratosphere (UTLS). Derived φ(ClNO2) varied across the full 0–1 range but was typically <0.5 and smallest in a PyroCB (<0.05). Derived γ(N2O5) was low in agricultural smoke (0.2–3.6 × 10−3), extremely low in mid‐tropospheric wildfire smoke (0.1 × 10−3), but larger in PyroCB processed smoke (0.7–5.0 × 10−3). Aged biomass burning aerosol in the UTLS had a higher γ(N2O5) of 17 × 10−3 that increased with sulfate and liquid water, but that was 1–2 orders of magnitude lower than values for aqueous sulfuric aerosol used in stratospheric models. Plain Language Summary: The injection of reactive material into Earth's atmosphere from fires affects atmospheric composition at regional and hemispheric scales. Reported stratospheric ozone depletion during extreme events, such as the 2020 Australian wildfires, illustrates one example of fire impacts and the role of heterogeneous (gas‐particle) processes. We report field quantification of rates and product yields from airborne observations of smoke. Extremely slow heterogeneous reaction rates on young smoke increase with transport and aging, but upper atmospheric values are still a factor of 10 slower than parameterizations used in stratospheric models. Heterogeneous production of ClNO2, a major lower atmospheric chlorine activation pathway, may be active on biomass burning aerosol in the upper atmosphere. Key Points: ClNO2 formation is active on biomass burning (BB) particles but decreases with transport to the upper troposphere and lower stratosphere (UTLS)N2O5 uptake coefficients are low on young BB smoke and increase with transport through a PyroCB and UTLS agingN2O5 uptake coefficients on aged BB particles in the UTLS are significantly lower than those used in model parameterizations [ABSTRACT FROM AUTHOR]

Chemistry Climate Models (CCMs) are essential tools for characterizing and predicting the role of atmospheric composition and chemistry in Earth's climate system. This study demonstrates the use of airborne in situ observations to diagnose the representation of chemical composition and transport by CCMs. Process‐based diagnostics using dynamical and chemical coordinates are presented which minimize the spatial and temporal sampling differences between airborne in situ measurements and CCM grid points. The chosen process is the chemical impact of the Asian summer monsoon (ASM), where deep convection serves as a rapid transport pathway for surface emissions to reach the upper troposphere and lower stratosphere (UTLS). We examine two CCM configurations for their representation of the ASM UTLS using a set of airborne observations from south Asia. The diagnostics reveal good model performance at representing tropospheric tracer distribution throughout the troposphere and lower stratosphere, and excellent representation of chemical aging in the lower stratosphere when chemical loss is dominated by photolysis. Identified model limitations include the use of zonally averaged mole fraction boundary conditions for species with sufficiently short tropospheric lifetimes, which may obscure enhanced regional emissions sources. Overall, the diagnostics underscore the skill of current‐generation models at representing pollution transport from the boundary layer to the stratosphere via the ASM mechanism, and demonstrate the strength of airborne in situ observations toward characterizing this representation. Plain Language Summary: The chemical composition of Earth's atmosphere is important to understand for future climate prediction. This study establishes an approach for evaluating the representation of chemical composition in global climate models, and demonstrates the capabilities of the approach using a set of observations collected by research aircraft. We specifically target an evaluation of the Asian summer monsoon, a process with a well‐documented transport pathway for chemical species near the surface to reach the upper atmosphere. In doing so, we identify specific areas where focused model improvement is needed. Key Points: Process‐based diagnostics for model evaluation using airborne in situ observations are presentedThe Asian summer monsoon is explored for its role in impacting global composition and climateThe diagnostics use dynamical and chemical coordinates to identify model strengths and limitations [ABSTRACT FROM AUTHOR]

Volatile organic compounds (VOCs) play a key role in tropospheric chemistry, giving rise to secondary products such as highly oxygenated organic molecules (HOMs) and secondary organic aerosols (SOA). HOMs, a group of low-volatility gas-phase products, are formed through the autoxidation process of peroxy radicals (RO2) originating from the oxidation of VOCs. The measurement of HOMs is made by a NO3¯ ToFCIMS instrument, which also detects other species like small highly oxygenated VOCs (e.g. dicarboxylic acids) and sulfuric acid (H2SO4). The instrument response to HOMs is typically estimated using H2SO4, as HOMs are neither commercially available nor easily synthesized in the laboratory. The resulting calibration factor is then applied to quantify all species detected using this technique. In this study, we explore the sensitivity of the instrument to commercially available small organic compounds, primarily dicarboxylic acids, given the limitations associated with producing known amounts of HOMs for calibration. We compare these single compound calibration factors to the one obtained for H2SO4 under identical operational conditions. The study found that the sensitivity of the NO3¯ ToFCIMS varies depending on the specific type of organic compound, illustrating how a single calibration factor derived from sulfuric acid is clearly inadequate for quantifying all detected species using this technique. The results highlighted substantial variability in the calibration factors for the tested organic compounds, with 4-nitrocatechol exhibiting the highest sensitivity, and pyruvic acid the lowest. The obtained sulfuric acid calibration factor agreed well with the previous values from the literature. In summary, this research emphasized the need to develop reliable and precise calibration methods for progressively oxygenated reaction products measure with NO3¯ CIMS, for example, HOMs. [ABSTRACT FROM AUTHOR]

Oxygenated volatile organic compounds (OVOCs) are reactive species and the primary precursors of free radicals; thus, OVOCs play important roles in tropospheric chemistry. @Tibet field campaigns 2021 discovered surprisingly high levels and activity contributions of OVOCs at Lulang, a site with high vegetation cover and strong solar ultraviolet radiation on the southeast of the Tibetan Plateau (TP). The 13 OVOCs detected accounted for 49 % of the total VOCs (TVOCs; average level of 11.7 ± 4.4 ppb), and the levels of these OVOCs exhibited typical diurnal variation, with high values in the daytime and a peak at approximately 12:00. OVOCs contributed 65 % and 63 % to VOC- k OH and the ozone formation potential, respectively, and thus had a strong influence on atmospheric chemical processes. Two independent methods were used to determine the contributions of various sources and revealed consistent conclusions regarding the importance of biological sources there. The source apportionment results obtained through positive matrix factorization indicated that sunlight-impacted and direct plant emission sources both related to plant sources contributed 47 % of the TVOCs and 65 % of the OVOCs. OVOC source fitting through the photochemical age parameterization method also indicated that biogenic sources made the largest contribution (67 %) to OVOCs and revealed a clear peak at noon. In addition, biomass burning sources were found to be closely related to the VOC background because biomass burning is highly prevalent across the whole TP; these sources made the second greatest contribution (33 %) to the TVOCs and contributed more than 23 % of OVOCs. [ABSTRACT FROM AUTHOR]

The Geostationary Carbon Cycle Observatory (GeoCarb) was selected as NASA's second Earth Venture Mission (EVM-2). The scientific objectives of GeoCarb were to advance our knowledge of the carbon cycle, in particular, land–atmosphere fluxes of the greenhouse gases carbon dioxide (CO 2) and methane (CH 4) and the effects of these fluxes on the Earth's radiation budget. GeoCarb would retrieve column-integrated dry-air mole fractions of CO 2 (XCO2), CH 4 (XCH4) and CO (XCO), important for understanding tropospheric chemistry), in addition to solar-induced fluorescence (SIF), from hyperspectral resolution measurements in the O 2 A-band at 0.76 µ m, the weak CO 2 band at 1.6 µ m, the strong CO 2 band at 2.06 µ m, and a CH 4 /CO band at 2.32 µ m. Unlike its predecessors (OCO-2/3, GOSAT-1/2, TROPOMI), GeoCarb would be in a geostationary orbit with a sub-satellite point centered over the Americas. This orbital configuration combined with its high-spatial-resolution imaging capabilities would provide an unprecedented view of these quantities on spatial and temporal scales accurate enough to resolve sources and sinks to improve land–atmosphere CO 2 and CH 4 flux calculations and reduce the uncertainty of these fluxes. This paper will present a description of the GeoCarb instrument and the L2 retrieval algorithms which will be followed by simulation experiments to determine an error budget for each target gas. Several sources of uncertainty will be explored, including that from the instrument calibration parameters for radiometric gain, the instrument line shape (ILS), the polarization, and the geolocation pointing, in addition to forward model parameters including meteorology and spectroscopy, although there are some other instrument-related sources of uncertainty that are left out for this study, including that from "smile", the keystone effect, stray light, detector persistence, and scene inhomogeneity. The results indicate that the errors (1 σ) are less than the instrument's multi-sounding precision requirements of 1.2 ppm, 10 ppb, and 12 ppb (10 %), for XCO2 , XCH4 , and XCO , respectively. In particular, when considering the sources of uncertainty separately and in combination (all sources included), we find overall RMSEs of 1.06 ppm for XCO2 , 8.2 ppb for XCH4 , and 2.5 ppb for XCO , respectively. Additionally, we find that, as expected, errors in XCO2 and XCH4 are dominated by forward model and other systematic errors, while errors in XCO are dominated by measurement noise. It is important to note that the GeoCarb mission was canceled by NASA; however, the instrument is still in development and will be delivered to NASA, in full, with the hope that it will eventually be adopted in a future mission proposal. [ABSTRACT FROM AUTHOR]

Carbon monoxide (CO) is a regulated air pollutant that impacts tropospheric chemistry and is an important indicator of the incomplete combustion of carbon-based fuels. In this study, we used four years (2019–2022) of winter and spring (January–May) atmospheric CO observations to quantify and characterize city-scale CO enhancements (ΔCO) from the New York City metropolitan area (NYCMA). We observed large variability in ΔCO, roughly 60 % of which was explained by atmospheric transport from the surrounding surface areas to the measurement sites, with the remaining 40 % due to changes in emissions on sub-monthly timescales. We evaluated the CO emissions from the Emissions Database for Global Atmospheric Research (EDGAR), which has been used to scale greenhouse gas emissions, and found the emissions are much too low in magnitude. During the COVID-19 shutdown in spring 2020, we observed a flattening of the diurnal pattern of CO emissions, consistent with reductions in daytime transportation. Our results highlight the role of meteorology in driving the variability of air pollutants and show that the transportation sector is unlikely to account for the non-shutdown observed CO emissions magnitude and variability, an important distinction to determine the sources of combustion emissions in urban regions like the NYCMA. [ABSTRACT FROM AUTHOR]

The atmospheric composition forecasting system used to produce the CAMS forecasts of global aerosol and trace gases distributions, IFS-COMPO, undergoes periodic upgrades. In this paper we describe the development of the future operational cycle 49R1, and focus on the implementation of the thermodynamical model EQSAM4Clim version 12 for describing gas-aerosol partitioning processes for nitrate and ammonium and for providing diagnostic aerosol, cloud and precipitation pH values at global scale. This information on aerosol acidity influences tropospheric chemistry processes associated with aqueous phase chemistry and wet deposition. The other updates to cycle 49R1 include modifications to the description of Desert Dust, Sea-salt aerosols, Carbonaceous aerosols and the size description for the calculation of aerosol optics. The implementation of EQSAM4Clim significantly improves the partitioning of reactive nitrogen compounds decreasing surface concentrations of both nitrate and ammonium, which reduces PM2.5 biases for Europe, U.S. and China, especially during summertime. For aerosol optical depth there is generally a decrease in the simulated biases for wintertime, and for some regions an increase in the bias for summertime. Improvements in the simulated Ångström exponent is noted for almost all regions, resulting in generally a good agreement with observations. The diagnostic aerosol and precipitation pH calculated by EQSAM4Clim have been compared against results from previous simulations (for aerosol pH) and against ground observations (for precipitation pH), with the temporal distribution in the annual mean values showing good agreement against the regional observational datasets. The use of aerosol acidity only has a relatively smaller impact on the aqueous-phase production of sulphate when compared to the changes in gas-to-particle partitioning brought by the use of EQSAM4Clim. [ABSTRACT FROM AUTHOR]

This study investigates the adjustment of large‐scale localized buoyancy anomalies in mid‐latitude regions and the nonlinear evolution of associated condensation patterns in both adiabatic and moist‐convective environments. This investigation is carried out utilizing the two‐layer idealized moist‐convective thermal rotating shallow water (mcTRSW) model. Our investigation reveals that the presence of a circular positive potential temperature anomaly in the lower layer initiates an anticyclonic high‐pressure rotation, accompanied by a negative buoyancy anomaly in the upper layer, resulting in an anisotropic northeast–southwest tilted circulation of heat flux. The evolution of eddy heat fluxes, such as poleward heat flux, energy, and meridional elongation of the buoyancy field, heavily depends on the perturbation's strength, size, and vertical structure. The heatwave initiates atmospheric instability, leading to precipitation systems such as rain bands and asymmetric latent heat release due to moist convection in a diabatic environment. This creates a comma cloud pattern in the upper troposphere and a comma‐shaped buoyancy anomaly in the lower layer, accompanied by the emission of inertia gravity waves. The southern and eastern sectors of the buoyancy anomaly show an upward flux, generating a stronger cross‐equatorial flow and inertia‐gravity waves in a southward and eastward direction. Furthermore, the simulations reveal a similar asymmetric pattern of total condensed liquid water content distribution, accompanied by the intensification of moist convection as rain bands. This intensification is more pronounced in barotropic structures than in baroclinic configurations with stagnant upper layers. This study highlights the importance of considering moist convection and its effects on atmospheric and oceanic flows in mid‐latitude regions, as well as the role of buoyancy anomalies in generating heatwaves and precipitation patterns. [ABSTRACT FROM AUTHOR]

Seasonal and interannual variations of the aerosol optical depth (AOD) and the total carbon monoxide content in the atmospheric column have been determined by measurements of the MODIS and Measurements of Pollution in the Troposphere (MOPITT) instruments of the Terra satellite for several cities of Irkutsk oblast and Buryatia with a high level of atmospheric pollution for 2010–2021. For Irkutsk, a decrease in AOD interannual value from 0.278 in 2012 to 0.14 in 2020 with a general negative trend is shown. However, in 2021, the interannual AOD value for Irkutsk increased to 0.22. For Irkutsk, a negative trend of interannual variations in the total CO content in the atmospheric column is shown. When comparing the long-term seasonal variations of AOD according to ground measurements by a CIMEL photometer in Tory (Buryatia) and the OMI/Aura satellite data, the proximity of AOD values for the summer and autumn seasons and a significant excess of AOD according to satellite data over ground measurements for the spring months are shown. The largest Spearman correlation coefficient between ground and satellite measurements of AOD was for 2021 (ρs = 0.64). [ABSTRACT FROM AUTHOR]

Bromine monoxide (BrO) plays an important role in tropospheric chemistry. The state‐of‐the‐science TROPOspheric Monitoring Instrument (TROPOMI) offers the potential to monitor atmospheric composition with a fine spatial resolution of up to 5.5 × 3.5 km2. We present here the retrieval of tropospheric BrO columns from TROPOMI. We implement a stratospheric correction scheme using a climatological approach based on the latest GEOS‐Chem High Performance chemical transport model, and improve the tropospheric air mass factor calculation with TROPOMI surface albedo data accounting for the geometrical dependency. Our product presents a good level of consistency in comparison with measurements from ground‐based zenith‐sky differential optical absorption spectroscopy (r = 0.67), aircrafts (r = 0.46), and satellites (similar spatial distributions of BrO columns). Furthermore, our retrieval captures BrO enhancements in the polar springtime with values up to 7.8 × 1013 molecules cm−2 and identifies small‐scale emission sources such as volcanoes and salt marshes. Based on TROPOMI data, we probe a blowing snow aerosol bromine mechanism in which the snow salinity is reduced to better match simulation and observation. Our TROPOMI tropospheric BrO product contributes high‐resolution global information to studies investigating atmospheric bromine chemistry. Plain Language Summary: Bromine monoxide (BrO) is an important species that affects the global chemistry of the troposphere. However, global observations of tropospheric BrO remain challenging and limited due to the short lifetime and low abundance. In this study, we present a global high‐spatial‐resolution tropospheric BrO column product from the TROPOspheric Monitoring Instrument. We describe the retrieval algorithm and present a comprehensive verification and evaluation. In addition, we use the data set to investigate sources and sinks on a daily scale for measurement scenarios of BrO enhancements, such as polar sea ice, volcanic plumes, and salt marshes. We additionally optimize salinity, the key parameter in modeling blow snow aerosol bromine emissions, by comparing simulation and observation. Our work provides unique information to studies exploring atmospheric bromine chemistry. Key Points: We present the retrieval and evaluation of tropospheric BrO columns from TROPOspheric Monitoring Instrument (TROPOMI)Our high‐resolution BrO product identifies small‐scale emission sources on a daily scaleA blowing snow aerosol bromine scheme with reduced snow salinity improves agreement between the model and TROPOMI [ABSTRACT FROM AUTHOR]

The reactions of limonene with various free radicals (•OCH3, •OBr, •SH, •OOH, and •OOCH3) were investigated along the 273.15–312.15 K temperature range. To that purpose the density functional theory was used, at the M06‐2X/6–311+g(d,p) level. Two reaction mechanisms, hydrogen atom transfer (HAT) and radical adduct formation (RAF) were considered. It was found that the relative reactivity of the studied radicals toward limonene is: •SH > •OBr > •OCH3 > •OOH > •OOCH3. HAT was identified as the dominant mechanism for •OOH and •OOCH3, while RAF contributes the most to the reactions involving •OCH3, •OBr, and •SH. The obtained Arrhenius expressions are: k(•OCH3) = 1.58 × 10−13 e−1.59/RT, k(•OBr) = 3.55 × 10−12 e+1.82/RT, k(•SH) = 3.30 × 10−11 e+0.79/RT, k(•OOH) = 1.33 × 10−15 e−5.99/RT, and k(•OOCH3) = 5.88 × 10−17 e−6.26/RT. According to them, the reactions of •OBr and •SH become slower as temperature rises from 273.15 to 312.15 K, while for the other radicals the reactions rate increases with temperature. The subsequent tropospheric fate of the most abundant •OBr adduct was also investigated in the same temperature range, considering O2 addition to this radical (step 2) and the reaction of the peroxyl radical yielded in this step 2 with NO. The latter is predicted to take place in two steps: the NO addition (3a) and the NO2 elimination (3b). The corresponding Arrhenius expression are k2 = 3.56 × 10−15 e+1.43/RT and k3b = 1.35 × 1014 e−31.65/RT. Step 3a was found to be barrierless. To our best knowledge, all the data provided here is reported for the first time. Thus, it would hopefully contribute to enhance the knowledge necessary for the full understanding (and accurate modeling) of the troposphere. [ABSTRACT FROM AUTHOR]

Associations between tropospheric ozone (O3) and climate variations have been extensively investigated worldwide. However, given the lack of historical O3 monitoring data, the knowledge gaps regarding the influences of climate variations on long-term O3 trends in China remain. The present study used a tropospheric O3 dataset from the summers of 1999 to 2017 simulated by an atmospheric chemistry model to explore the linkage between summer O3 and a dominant atmospheric circulation system – the Western Pacific Subtropical High (WPSH) pressure – on an interannual basis in China. During this period, both WPSH strength and O3 concentrations in eastern and central China illustrated a growing trend. An EOF analysis was conducted to examine significant summer O3 characteristics and patterns and their potential connections with the WPSH. We find that the correlation between the first principal component of summer ozone concentration in the EOF analysis and the WPSH reached 0.56 (P≤0.01) in China from 1999 to 2017. We show that the WPSH determines interannual fluctuations of summer O3 , whereas O3 precursor emissions contribute primarily to the O3 long-term trend. Our results reveal that the WPSH plays a vital role in O3 perturbation in the eastern seaboard regions and inland China. Precursor emissions made more significant contributions of up to 60 % to increasing O3 trends in the inland urban agglomerations than coastal regions in eastern and southern China. The strongest contribution of meteorological conditions associated with the WPSH to summer O3 occurred in the Yangtze River Delta (YRD), accounting for over 9 % to ozone perturbations from 1999 to 2017. We find that the effect of the WPSH on regional O3 depends on the spatial proximity to the WPSH. We attributed the effects of the WPSH on O3 interannual variations to the changes in air temperature, precipitation, and winds associated with the WPSH's intensity and positions. [ABSTRACT FROM AUTHOR]

Under future net-zero emission policies, reductions in emissions of ozone (O 3) precursors are expected to alter the temporal and spatial distributions of tropospheric O 3. In this study, we quantify changes in the tropospheric O 3 budget and in the spatiotemporal distribution of surface O 3 in east Asia and the contributions of regional emissions, intercontinental transport and climate change between the present day and 2060 under a net-zero scenario using the NCAR Community Earth System Model (CESM) with online tagging of O 3 and its precursors. The results reveal that the global tropospheric O 3 burden is likely to decrease by more than 20 %, from 316 Tg in the present day to 247 Tg in 2060, under a net-zero scenario. The burden of stratospheric O 3 in the troposphere is expected to increase from 69 to 77 Tg. The mean lifetime of tropospheric O 3 is expected to increase by 2 d (∼10 %). Changes in climate under a net-zero pathway are relatively small and only lead to small increases in tropospheric O 3. Over eastern China, surface O 3 increases in winter due to the weakened titration of O 3 by NO associated with reduced anthropogenic NO emissions and due to enhanced stratospheric input. In summer, surface O 3 decreases by more than 30 ppbv, and peak concentrations shift from July to May. Local contributions from anthropogenic emissions to surface O 3 over east Asia are highest in summer but drop substantially, from 30 % to 14 %, under a net-zero scenario. The contribution of biogenic NO sources is enhanced and forms the dominant contributor to future surface O 3 , especially in summer (∼40 %). This enhanced contribution is mainly due to the increased O 3 production efficiency under lower anthropogenic precursor emissions. Over eastern China, local anthropogenic contributions decrease from 50 % to 30 %. The decreases in surface O 3 are strongly beneficial and are more than sufficient to counteract the increases in surface O 3 observed in China over recent years. This study thus highlights the important co-benefits of net-zero policies that target climate change in addressing surface O 3 pollution over east Asia. [ABSTRACT FROM AUTHOR]

We present a chemistry-climate model with a tagged stratospheric nitrous oxide (N2O) tracer that predicts distinct seasonal cycles in tropospheric N2O caused by descent of N2O-depleted stratospheric air in polar regions. We identify similar phenomena in recently available aircraft profiles from global campaigns and routine monitoring. Long-term atmospheric measurements from the National Oceanic Atmospheric Administration (NOAA) global surface monitoring network provide additional support for a significant impact on surface N2O originating from the stratosphere. In the northern hemisphere, the NOAA surface N2O atmospheric growth rate anomaly is negatively correlated with the previous winter's polar lower stratospheric temperature. This negative correlation is consistent with increased (decreased) transport in years with a strong (weak) Brewer Dobson circulation of warm, N2O-depleted air from the middle and upper stratosphere into the lower stratosphere, with subsequent cross-tropopause transport of the N2O-depleted air into the troposphere. In the southern hemisphere, polar lower stratospheric temperature is correlated to monthly summertime anomalies in tropospheric N2O as it descends into its seasonal minimum, a result that is supported by aircraft data as well as the chemistry-climate model. However, the N2O atmospheric growth rate anomaly in the southern hemisphere is better correlated to the stratospheric quasi-biennial oscillation (QBO) index, as well as the El Niño Southern Oscillation index, than to polar lower stratospheric temperature. These hemispheric differences in the factors influencing the N2O atmospheric growth rate are consistent with known atmospheric dynamics and the complex interaction of the QBO with the Brewer Dobson circulation. [ABSTRACT FROM AUTHOR]

A brief overview of the work of Russian scientists in the field of atmospheric chemistry in 20190–2022 is presented, including work on the chemistry of the troposphere, the chemistry of the ozone layer, the study of heterophase processes, and the chemical aspects of climate and its change. The review was prepared in the Commission on Atmospheric Chemistry of the Section of Meteorology and Atmospheric Sciences of the National Geophysical Committee. [ABSTRACT FROM AUTHOR]

Quantifying the level of CO2, the main greenhouse gas (GHG), is essential for research on regional and global climate change, especially in the densely populated North China Plain with its severe CO2 emissions. In this study, 12 airborne flights were managed and conducted during the autumn–winter period of 2018–2019 in downtown Shijiazhuang and its surrounding areas, which are representative of the typical urban conditions in the North China Plain, to explore the spatial and temporal distributions of CO2. The results showed that the measured columnar averages of CO2 ranged between 399.9 ± 1.5 and 443.8 ± 31.8 ppm; the average of the 12 flights was 412.1 ppm, slightly higher than the globally averaged 410.5 ± 0.20 ppm and the 2 background concentrations of 411.6 ± 2.1 ppm and 411.4 ± 0.2 ppm in low-latitude Mauna Loa and middle-latitude Waliguan in 2019, indicating the potential influences of anthropogenic activities. The typical stratification of the planetary boundary layer (PBLH), residual layer (RL), and elevated inversion layer (IL) was crucial in constraining the high CO2 concentrations. This illustrated that the warming effect of CO2 within the PBLH may also have some influences on regulating the thermal structure of the low troposphere. Based on a backward trajectory analysis, it was evidenced that there were three different categories of air masses for autumn and one category for winter. Both trajectories in the PBL, i.e., below 1000 m, from the local and southern areas with tremendous anthropogenic emissions (autumn) and from the western regions (winter) led to comparatively high levels of CO2, but the mid-tropospheric CO2 concentrations above 1000 m were commonly homogeneously distributed, with higher levels appearing in winter because the concentration in the free troposphere followed the global seasonal pattern, with a summer minimum and winter maximum as a result of the seasonality of the net CO2 exchange and the balance between photosynthesis and respiration. These results provide an in-depth understanding of the vertical concentrations of tropospheric CO2 in the North China Plain, which will offer scientific references for the evaluation of carbon accounting and carbon emissions. [ABSTRACT FROM AUTHOR]

At present, evidence of the associations between carbon monoxide (CO) and respiratory diseases (RD) in Northwest China is limited and controversial. The aim of this study is to evaluate the impact of ambient CO on outpatient visits for RD in Lanzhou, China. The daily amount of outpatient visits for total and cause-specific RD, air pollutant, and weather variables were collected in Lanzhou, China from 1st January 2013 to 31st December 2019. A generalized additive model and distributed lag nonlinear model were used to assess associations between CO and outpatient visits for RD. During the study period, a total of 1,623,361 RD outpatient visits were recorded. For each interquartile range (IQR) (0.77 mg/m3) increase in CO, the relative risk (RR) was 1.163 (95% CI: 1.138, 1.188) for total RD at lag07, 1.153 (95% CI: 1.128,1.179) for upper respiratory tract infection (URTI) at lag07, 1.379 (95% CI: 1.338,1.422) for pneumonia at lag07, 1.029 (95% CI: 0.997,1.062) for chronic obstructive pulmonary disease (COPD) lag04, 1.068 (95% CI: 1.028,1.110) for asthma lag03, and 1.212 (95% CI: 1.178,1.247) for bronchitis lag07, respectively. In the subgroup analyses, the impacts of CO were more pronounced on total RD, pneumonia, COPD, and bronchitis in males than females, while the opposite was true in URTI and asthma. The impact of CO on RD was the strongest for children under 15 years-of-age. We also found significantly stronger effects during cold seasons compared to warm seasons. In addition, we observed a roughly linear exposure–response curve between CO and RD with no threshold effect. This study in Lanzhou revealed a remarkable association between CO level and an elevated risk of total and cause-specific RD outpatient visits, especially for pneumonia. [ABSTRACT FROM AUTHOR]

In recent years, anthropogenic activities and climate change have significantly increased exposure of plants to environmental stresses (single or multiple) and pollutants, which negatively affect plant growth, survival, and productivity. Plants may activate an armament of defenses against such environmental stresses. Isoprene, the most abundant biogenic volatile organic compound emitted by plants, is supposed to induce stress tolerance directly, by quenching reactive oxygen species, or indirectly by strengthening photosynthetic membranes and reprogramming expression of genes that are involved in antioxidant defense mechanisms. On the other hand, isoprene is also involved in tropospheric chemistry that contributes to the production of air pollutants when mixing with anthropogenic gases. In this review, we summarized current knowledge about the impact of air and soil pollutants on isoprene emission from plants, focusing on possible feedback and feedforward mechanisms that may affect whole ecosystem functioning and evolution of plant species. Despite limited available information, especially about long-term effects of soil pollutants, it may be speculated that isoprene generally improves fitness of plants challenged by air and soil pollutants, and their interaction with other organisms. [ABSTRACT FROM AUTHOR]

Tropospheric chlorine chemistry can strongly impact the atmospheric oxidation capacity and composition, especially in urban environments. To account for these reactions, the gas- and aqueous-phase Cl chemistry of the community atmospheric chemistry box model Chemistry As A Boxmodel Application/Module Efficiently Calculating the Chemistry of the Atmosphere (CAABA/MECCA) has been extended. In particular, an explicit mechanism for ClNO2 formation following N2O5 uptake to aerosols has been developed. The updated model has been applied to two urban environments with different concentrations of NOx (NO + NO2): New Delhi (India) and Leicester (United Kingdom). The model shows a sharp build-up of Cl at sunrise through Cl2 photolysis in both the urban environments. Besides Cl2 photolysis, ClO + NO reaction and photolysis of ClNO2 and ClONO are also prominent sources of Cl in Leicester. High- NOx conditions in Delhi tend to suppress the nighttime build-up of N2O5 due to titration of O3 and thus lead to lower ClNO2 , in contrast to Leicester. Major loss of ClNO2 is through its uptake on chloride, producing Cl2 , which consequently leads to the formation of Cl through photolysis. The reactivities of Cl and OH are much higher in Delhi; however, the Cl / OH reactivity ratio is up to ≈ 9 times greater in Leicester. The contribution of Cl to the atmospheric oxidation capacity is significant and even exceeds (by ≈ 2.9 times) that of OH during the morning hours in Leicester. Sensitivity simulations suggest that the additional consumption of volatile organic compounds (VOCs) due to active gas- and aqueous-phase chlorine chemistry enhances OH , HO2 , and RO2 near sunrise. The simulation results of the updated model have important implications for future studies on atmospheric chemistry and urban air quality. [ABSTRACT FROM AUTHOR]