Search

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]

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]

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]

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]

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]

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]

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]

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]

Year‐round precipitation in coastal East Antarctica and Antarctic Peninsula was used to investigate the seasonal patterns in sources of atmospheric perchlorate (ClO4− ${{\text{ClO}}_{4}}^{-}$). Although featuring distinct climates, the two locations exhibit similar annual mean and seasonal cycles of ClO4− ${{\text{ClO}}_{4}}^{-}$ concentration, with higher values in autumn and lower concentrations in winter and spring. Tropospheric formation dominates atmospheric ClO4− ${{\text{ClO}}_{4}}^{-}$ in spring and summer, which is influenced by both oxidants levels and environmental conditions (e.g., air humidity). Tropospheric ClO4− ${{\text{ClO}}_{4}}^{-}$ production may also be promoted by elevated levels of oxidants brought by air mass from the interior Antarctic ice sheet in spring and summer. The autumn concentration maximum may originate from ClO4− ${{\text{ClO}}_{4}}^{-}$ produced in the stratosphere through reactions between reactive chlorine and ozone during spring and summer. In winter, the stratospheric input may contribute to ClO4− ${{\text{ClO}}_{4}}^{-}$ via polar stratospheric clouds sedimentation. Plain Language Summary: Perchlorate (ClO4− ${{\text{ClO}}_{4}}^{-}$) is an inorganic anion with a persistent presence in the environment, where its exposure can pose a significant health risk to humans. Environmental ClO4− ${{\text{ClO}}_{4}}^{-}$ is derived from both man‐made and natural sources. Natural ClO4− ${{\text{ClO}}_{4}}^{-}$, widespread in the environment, is thought to be formed in the atmosphere. However, knowledge on the sources and, in particular, the formation mechanisms of natural ClO4− ${{\text{ClO}}_{4}}^{-}$ is quite limited. Antarctica, with negligible man‐made sources, is one of the best regions for investigations on natural ClO4− ${{\text{ClO}}_{4}}^{-}$. Year‐round precipitation samples were collected at China Zhongshan Station located in coastal East Antarctica and China Great Wall Station on King George Island, Antarctic Peninsula. Results show that atmospheric ClO4− ${{\text{ClO}}_{4}}^{-}$ concentration presents obvious seasonal cycles, which are related to variations in sources. A significant amount of ClO4− ${{\text{ClO}}_{4}}^{-}$ is associated with tropospheric chemistry in spring and summer. Precursor levels, environmental conditions, and air mass from the interior Antarctica may influence tropospheric ClO4− ${{\text{ClO}}_{4}}^{-}$ formation. The maximum concentration in autumn may originate from ClO4− ${{\text{ClO}}_{4}}^{-}$ formed in the stratosphere during spring and summer, and the stratospheric input may also contribute to atmospheric ClO4− ${{\text{ClO}}_{4}}^{-}$ in winter. Key Points: Antarctic atmospheric perchlorate (ClO4− ${{\text{ClO}}_{4}}^{-}$) concentration at different locations exhibits a clear seasonal cycleThe highest concentration in autumn is likely related to ClO4− ${{\text{ClO}}_{4}}^{-}$ produced in the stratosphere in spring and summerMajor source of ClO4− ${{\text{ClO}}_{4}}^{-}$ in spring and summer is tropospheric formation influenced by both oxidants and environmental conditions [ABSTRACT FROM AUTHOR]

With the development of advanced data assimilation and computing techniques, many modern global reanalysis datasets aim to resolve the atmospheric mesoscale spectrum. However, large uncertainties remain with respect to the representation of mesoscale motions in these reanalysis datasets, for which a clear understanding is lacking. The aforementioned challenges have served as a strong motivation to reveal and quantify their mesoscale differences. This study presents the first comprehensive global intercomparison of the tropospheric and stratospheric mesoscale kinetic energy and its spectra over two selected periods of summer and winter events among six leading high-resolution atmospheric reanalysis products: European Centre for Medium-Range Weather Forecasts (ECMWF) Reanalysis v5 (ERA5), China Meteorological Administration Reanalysis, Modern-Era Retrospective Analysis for Research and Applications version 2, National Centers for Environmental Prediction's Climate Forecast System version 2 (CFSv2), Japanese 55-year Reanalysis, and ECMWF Reanalysis-Interim. A state-of-the-art global operational model is adopted as a supplementary reference. Although all reanalysis datasets can reproduce broad distribution characteristics that are grossly consistent with the 9 km model, there are substantial discrepancies among them in magnitudes. The ability to capture mesoscale signals is closely linked to their resolutions, but it is also impacted by other factors, including, but not limited to, the selected types of energy, seasons, altitudes, latitudes, model diffusions, parametrization schemes, moist condition, assimilation methods, and observation inputs. Moreover, all datasets illustrate conclusive behaviors for the prevalence of the rotational component in the troposphere, whereas only very few products fail to exhibit the dominance of the divergent component in the stratosphere. Overall, stratospheric ERA5 and CFSv2 outperform the other reanalysis datasets, and only these two can reproduce the feature of the canonical kinetic energy spectrum with a distinct shift from a steeper slope (approximately -3) at lower wave numbers to a shallower slope (approximately -5/3) at higher wave numbers. In addition, the relative disparities among datasets increase dramatically with height, and they are more pronounced in the divergent component. It is also found that the correlations among these datasets are much weaker in the Tropics. [ABSTRACT FROM AUTHOR]

Seasonal change of atmospheric potential oxygen (APO ∼ O2 + CO2) is a tracer for air‐sea O2 flux with little sensitivity to the terrestrial exchange of O2 and CO2. In this study, we present the tropospheric distribution and inventory of APO in each hemisphere with seasonal resolution, using O2 and CO2 measurements from discrete airborne campaigns between 2009 and 2018. The airborne data are represented on a mass‐weighted isentropic coordinate (Mθe) as an alternative to latitude, which reduces the noise from synoptic variability in the APO cycles. We find a larger seasonal amplitude of APO inventory in the Southern Hemisphere relative to the Northern Hemisphere, and a larger amplitude in high latitudes (low Mθe) relative to low latitudes (high Mθe) within each hemisphere. With a box model, we invert the seasonal changes in APO inventory to yield estimates of air‐sea flux cycles at the hemispheric scale. We found a larger seasonal net outgassing of APO in the Southern Hemisphere (518 ± 52.6 Tmol) than in the Northern Hemisphere (342 ± 52.1 Tmol). Differences in APO phasing and amplitude between the hemispheres suggest distinct physical and biogeochemical mechanisms driving the air‐sea O2 fluxes, such as fall outgassing of photosynthetic O2 in the Northern Hemisphere, possibly associated with the formation of the seasonal subsurface shallow oxygen maximum. We compare our estimates with four model‐ and observation‐based products, identifying key limitations in these products or in the tools used to create them. Plain Language Summary: A better understanding of the air‐sea O2 fluxes facilitates the study of marine productivity, global carbon cycle and ocean heat transport. Seasonal air‐sea exchange of O2 has been estimated by combining precise measurements of atmospheric O2 and CO2 into atmospheric potential oxygen (APO ∼ O2 + CO2). Using APO observations from nine global airborne campaigns between 2009 and 2018, we resolved the seasonal cycle of atmospheric APO concentration in multiple pressure and latitude bands, yielding estimates of the tropospheric APO inventory, and area‐integrated air‐sea APO flux of each hemisphere. To a first approximation, the ocean is a source of APO in the spring and summer but a sink in the fall and winter, tracking the seasonal warming and cooling of the ocean as well as different ocean biogeochemistry and ventilation regimes. In addition, these cycles show clear asymmetry between hemispheres and display a progressive shift in the seasonal phase and amplitude across latitudes. It is therefore important to understand the physical and biogeochemical processes that lead to these differences. Key Points: From airborne atmospheric potential oxygen (APO) data, we resolve seasonal cycles of APO and hemisphere‐integrated air‐sea APO fluxesWe resolved clear hemispheric differences in the seasonal amplitude and shape of the APO flux cyclesThis study identifies limitations in the Garcia and Keeling O2 flux climatology, pointing to the need for an improved climatology [ABSTRACT FROM AUTHOR]

We present a comparison of atmospheric transport models that simulate carbonyl sulfide (COS). This is part II of the ongoing Atmospheric Transport Model Inter‐comparison Project (TransCom–COS). Differently from part I, we focus on seven model intercomparison by transporting two recent COS inversions of NOAA surface data within TM5‐4DVAR and LMDz models. The main goals of TransCom‐COS part II are (a) to compare the COS simulations using the two sets of optimized fluxes with simulations that use a control scenario (part I) and (b) to evaluate the simulated tropospheric COS abundance with aircraft‐based observations from various sources. The output of the seven transport models are grouped in terms of their vertical mixing strength: strong and weak mixing. The results indicate that all transport models capture the meridional distribution of COS at the surface well. Model simulations generally match the aircraft campaigns HIAPER Pole‐To‐Pole Observations (HIPPO) and Atmospheric Tomography Mission (ATom). Comparisons to HIPPO and ATom demonstrate a gap between observed and modeled COS over the Pacific Ocean at 0–40°N, indicating a potential missing source in the free troposphere. The effects of seasonal continental COS uptake by the biosphere, observed on HIPPO and ATom over oceans, is well reproduced by the simulations. We found that the strength of the vertical mixing within the column as represented in the various atmospheric transport models explains much of the model to model differences. We also found that weak‐mixing models transporting the optimized flux derived from the strong‐mixing TM5 model show a too strong seasonal cycle at high latitudes. Plain Language Summary: Carbonyl sulfide (COS) is a significant sulfur‐containing trace gas in the atmosphere, which makes it important for studying climate change. One of the reasons it is worth investigating is because plants take up COS in a similar way as CO2 during photosynthesis. However, the atmospheric sources and sinks of COS are not well understood. To address this knowledge gap, we evaluated the state‐of‐the‐art optimized surface COS fluxes from the inverse models TM5‐4DVAR and LMDz, and then seven atmospheric transport models were used to simulate COS mole fractions by transporting the optimized fluxes under the TransCom‐COS protocol. The results showed good agreement between the simulated COS and COS observations on independent platforms. The study also revealed that COS drawdown due to plant uptake can be observed over Pacific and Atlantic Oceans. However, discrepancies between the model simulations and observations were mainly found in free troposphere, emphasizing the need for further investigation into COS chemistry and model transport differences. These findings provide important reference for further investigation of COS global distribution and budget analysis. Key Points: Simulations in seven models propagating optimized carbonyl sulfide (COS) fluxes derived from two inversions agree with independent observationsSimulated and observed COS drawdowns are captured in boundary layer over the Pacific and Atlantic Oceans due to plant uptake over landsWeak vertical mixing models using fluxes optimized from the fast‐mixing TM5 model overestimate the COS seasonal amplitude at high latitudes [ABSTRACT FROM AUTHOR]

Simulating whole atmosphere dynamics, chemistry, and physics is computationally expensive. It can require high vertical resolution throughout the middle and upper atmosphere, as well as a comprehensive chemistry and aerosol scheme coupled to radiation physics. An unintentional outcome of the development of one of the most sophisticated and hence computationally expensive model configurations is that it often excludes a broad community of users with limited computational resources. Here, we analyze two configurations of the Community Earth System Model Version 2, Whole Atmosphere Community Climate Model Version 6 (CESM2(WACCM6)) with simplified "middle atmosphere" chemistry at nominal 1 and 2° horizontal resolutions. Using observations, a reanalysis, and direct model comparisons, we find that these configurations generally reproduce the climate, variability, and climate sensitivity of the 1° nominal horizontal resolution configuration with comprehensive chemistry. While the background stratospheric aerosol optical depth is elevated in the middle atmosphere configurations as compared to the comprehensive chemistry configuration, it is comparable among all configurations during volcanic eruptions. For any purposes other than those needing an accurate representation of tropospheric organic chemistry and secondary organic aerosols, these simplified chemistry configurations deliver reliable simulations of the whole atmosphere that require 35% and 86% fewer computational resources at nominal 1 and 2° horizontal resolution, respectively. Plain Language Summary: Modeling the entire atmosphere, or at least from the surface to an altitude of 140 km (87 miles), takes a lot of computer resources. Simulating 1 year can require the equivalent of thousands of personal computers running for 1 day, for example, which is only realistic for researchers with access to a supercomputer. There are many people who would like to simulate the whole atmosphere to study climate change, space weather, and extreme events, but even with access to a supercomputer, it is still computationally expensive to run. We examined a whole atmosphere model with a simpler chemistry scheme, and at a lower horizontal resolution, to see if it still reproduces major features of climate and climate change. The two configurations perform similarly to the high resolution simulation with complex chemistry, with some minor and understandable differences. Anyone looking to simulate the whole atmosphere, using fewer computational resources, can do so confidently using the described model configurations, as long as they are aware of some of the deficiencies. Key Points: There are differences in stratospheric aerosol optical depth between comprehensive and simplified chemistry configurationsSimplifying the chemistry scheme generally has smaller global impacts than coarsening the horizontal resolutionAll configurations have similar climate sensitivities and responses to forcings [ABSTRACT FROM AUTHOR]

Polar mesospheric clouds (PMC), or noctilucent clouds, can be observed over high latitudes with the naked eye from the ground or from space near the summer solstice. PMC are considered a direct and sensitive indicator of climate change and have been reported to appear more frequently in recent decades. How PMC will change in the future under the influence of natural variability and anthropogenic forcing is uncertain. In this study, we utilize model output from the Whole Atmosphere Community Climate Model under several shared socioeconomic pathway (SSP) scenarios and input the water vapor, temperature, and pressure information into a 0‐d PMC model to project the trend and variation of PMC over the 21st century, and their relationship to future changes of temperature, water vapor, and the solar cycle. The 0‐d model calculations indicate that PMC ice water content (IWC) will increase and PMC will extend to lower latitudes under high SSP scenarios. Under these scenarios, more mesospheric water vapor leads to an increased IWC of PMC over the polar region, and colder mesopause temperature leads to more PMC over the mid‐latitudes. There is a significant anti‐correlation between the solar cycle and PMC IWC over the 21st century, but the anti‐correlation is not always significant on the decadal scale. Finally, methane oxidation in the stratosphere and water vapor entering from the troposphere are both responsible for future changes in mesospheric water vapor and thus PMC. Plain Language Summary: Polar mesospheric clouds (PMC), also known as noctilucent clouds, are a direct and sensitive indicator of climate change and have been reported to appear more frequently in recent decades. Our numerical model projection of future changes in PMC indicates a possible increase in their ice water content (IWC) with higher future greenhouse gas emissions and a decrease with lower greenhouse gas emissions. The IWC of the PMC can be influenced by the solar cycle, mesospheric water vapor, and temperature. With more greenhouse gas emissions and global surface warming, more water vapor will enter the middle atmosphere or form via the oxidation of methane, thus increasing PMC IWC over the polar region. The associated middle atmospheric cooling increases the IWC of PMC over the mid‐latitudes, and PMC can extend to lower latitudes where PMC have not been commonly seen before. Key Points: Future projections of Polar mesospheric clouds (PMC) for high greenhouse gas emission scenarios indicate an increasing ice water content (IWC) over the 21st centuryThe variation of PMC is mainly determined by mesospheric water vapor over polar region and by mesospheric temperature over mid‐latitudesOver the 21st century, PMC IWC anti‐correlates with the solar cycle. The correlation is not always significant on a decadal time scale [ABSTRACT FROM AUTHOR]

During polar spring, periods of elevated tropospheric bromine drive near complete removal of surface ozone. These events impact the tropospheric oxidative capacity and are an area of active research with multiple approaches for representing the underlying processes in global models. We present a method for parameterizing emissions of molecular bromine (Br2) over the Arctic using satellite retrievals of bromine monoxide (BrO) from the Ozone Monitoring Instrument (OMI). OMI retrieves column BrO with daily near global coverage, and we use the GEOS‐Chem chemical mechanism, run online within the Goddard Earth Observing System Earth System Model to identify hotspots of BrO likely associated with polar processes. To account for uncertainties in modeling background BrO, hotspots are only identified where the difference between OMI and modeled columns exceeds a statistical threshold. The resulting hotspot columns are a lower‐limit for the portion of OMI BrO attributable to bromine explosion events. While these hotspots are correlated with BrO measured in the lower troposphere over the Arctic Ocean, a case study of missing detections of near‐surface BrO is identified. Daily flux of Br2 is estimated from hotspot columns of BrO using internal model parameters. When the emissions are applied, BrO hotspots are modeled with a 5% low bias. The sensitivity of the resulting ozone simulations to the treatment of background uncertainties in the BrO column is demonstrated. While periods of isolated, large (>50%) decreases in surface ozone are modeled, this technique does not simulate the low ozone observed at coastal stations and consistently underestimates ozone loss during March. Plain Language Summary: During polar spring, high levels of bromine‐containing molecules drive near complete removal of surface ozone (O3), impacting the chemistry of the troposphere and the biological uptake of mercury. Global models currently have multiple mechanisms for representing the underlying processes that produce brominated molecules in polar regions. We estimate molecular bromine (Br2) emissions from measurements of bromine monoxide (BrO) collected over the Arctic by a satellite instrument. An atmospheric model, run without polar emissions of Br2, is used to estimate how much of the satellite BrO signal is due to background processes in the stratosphere and troposphere and isolate the portion of the signal likely associated with Arctic emissions. We account for uncertainties in the model representation of background BrO using a statistical threshold. Because of the catalytic nature of bromine‐mediated ozone depletion, we focus our initial efforts on developing a lower‐limit estimate of Arctic emissions. The amount of BrO attributed to polar processes and the resulting impact on O3 are sensitive to the magnitude of the statistical threshold, with a better representation of surface O3 achieved with a lower threshold. While the satellite‐based emissions result in periodic decreases in surface O3 in late spring, modeled O3 is consistently high with respect to observations, particularly during early spring. Key Points: BrO hotspots are isolated from satellite signals using modeled columns of BrO and a bias threshold to account for model uncertaintiesWe estimate Arctic Br2 emissions from BrO signals and demonstrate the sensitivity of modeled O3 to the BrO hotspot detection thresholdSimulations with satellite‐based Br2 emissions overestimate springtime Arctic surface O3 with few ozone depleting events modeled in March [ABSTRACT FROM AUTHOR]

We have coupled the GEOS‐Chem tropospheric‐stratospheric chemistry mechanism and the Community Aerosol and Radiation Model for Atmospheres (CARMA), a sectional aerosol microphysics module, within the NASA Goddard Earth Observing System Chemistry‐Climate Model (GEOS CCM) in order to simulate the interactions between stratospheric chemistry and aerosol microphysics. We use observations of the 1991 Mount Pinatubo volcanic cloud to evaluate this new version of the GEOS CCM. The GEOS‐Chem chemistry module is used to simulate the oxidation of sulfur dioxide (SO2) more realistically than assuming hydroxyl radical (OH) fields are constant, as OH concentrations in the plume decrease dramatically in the weeks following the eruption. CARMA simulates sulfate aerosols with dynamic microphysical and optical properties. The CARMA‐calculated aerosol surface area is coupled to the chemistry module from GEOS‐Chem for the calculation of heterogeneous chemistry. We use a set of observational and theoretical constraints for Pinatubo to evaluate the performance of this new version of the GEOS CCM. These simulations are specifically compared with satellite and in‐situ observations and provide insights into the connections between the gas‐phase chemistry and the aerosol microphysics of the early plume and how they impact the climatic and chemical changes following a large volcanic eruption. A second, smaller eruption is also included in these simulations, the 15 August 1991, eruption of Cerro Hudson in Chile, which we find essential in explaining the aerosol optical depth in the Southern Hemisphere in 1991. Plain Language Summary: We have simulated two volcanic eruptions, the 1991 eruptions of Pinatubo and Cerro Hudson, using a new version of a computer program called the NASA GEOS Chemistry‐Climate Model. This model helps us understand how certain particles from volcanic eruptions, called sulfate aerosols, change sizes and impact the atmosphere. We have compared the results of this model with real‐world observations of the volcanic particles from these two eruptions. We show that the model is able to recreate similar conditions to those seen in 1991 by satellites and observations made from balloons. Additionally, the model shows that a chemical called hydroxyl radical, a key component in the development of volcanic sulfate aerosols, significantly decreases in the volcanic plume in the weeks after the eruption. Key Points: We developed and applied a setup of the NASA Goddard Earth Observing System (GEOS) Chemistry‐Climate Model (CCM) that simulates the 1991 Pinatubo aerosol loading and transportThe updated microphysics‐chemistry GEOS‐CCM realistically simulates aerosol optical depth (AOD) observations made following the 1991 eruptionsThe simulations also support the hypothesis that depletion of hydroxyl radical in volcanic plumes can slow the growth of sulfate aerosol [ABSTRACT FROM AUTHOR]

Reactive halogens (X + XO, X = I, Br or Cl) catalytically destroy a fraction of tropospheric ozone under present‐day (PD) conditions, however, their distribution and potential impact on tropospheric ozone under pre‐industrial (PI) conditions remain largely unexplored. This study uses the Community Atmosphere Model with Chemistry (CAM‐Chem) to investigate the effect of anthropogenically amplified natural emissions of halogenated species and their subsequent chemistry on tropospheric ozone under PI and PD atmospheric conditions. Model results show that the global tropospheric ozone depletion due to natural halogens is slightly more sensitive in PI than PD, with percentage changes in tropospheric ozone burden (TOB) of −14.1 ± 0.6% for PI and −12.9 ± 0.6% for PD. Individually, the role of iodine and chlorine in ozone depletion is equivalent in both periods (ΔTOBI: ∼−7% and ΔTOBCl: ∼−2.5%), while bromine plays a larger role in PI (ΔTOBBr: −5.5 ± 0.6%) versus PD (ΔTOBBr: −4.3 ± 0.7%). The increase in anthropogenic ozone precursor emissions from PI to PD has amplified the natural emission of inorganic halogens and led to a shift in the partitioning of inorganic halogens from reactive to reservoir species. Consequently, halogen‐driven ozone depletion from the surface to the free troposphere is larger in PI than PD. In contrast, in the upper troposphere, the ozone depletion is larger in PD influenced mainly by stratospheric intrusion of reactive halogens from long‐lived species. This study highlights the importance of including a complete chemical coupling of natural halogens and atmospheric pollutants in chemistry‐climate models to adequately assess their effects on tropospheric ozone in a changing climate. Plain Language Summary: Halogens (I, Br and Cl) emitted from natural sources catalytically destroy a fraction of tropospheric ozone, a trace gas that plays a key role in atmospheric chemistry, both in determining the oxidative capacity and as a component of photochemical smog, affecting air quality and public health. Previous studies have explored the effect of halogens on ozone in present and future time, while the role of halogens in the pre‐industrial atmosphere is quite uncertain. Based on a state‐of‐the‐art coupled chemistry‐climate model, we explore the effect of both natural sources and chemistry of reactive halogens on tropospheric ozone under ambient conditions representative of the pre‐industrial and present‐day. Model results show that the increase in tropospheric pollutant emissions since pre‐industrial times amplifies halogen natural emissions and affects the tropospheric role of halogens, resulting in a larger impact on near‐surface ozone in less polluted conditions such as pre‐industrial compared to present‐day. Key Points: Natural halogens induce a larger near‐surface tropospheric ozone depletion under Pre‐Industrial than Present‐Day atmospheric conditionsThe role of natural halogens in ozone depletion is governed by iodine followed by bromine and to a lesser extent by chlorineAnthropogenic ozone precursor emissions induce changes in both natural emissions and inorganic partitioning of tropospheric halogens [ABSTRACT FROM AUTHOR]

The usually short lifetime of convective storms and their rapid development during unstable weather conditions makes forecasting these storms challenging. It is necessary, therefore, to improve the procedures for estimating the storms' expected life cycles, including the storms' lifetime, size, and intensity development. We present an analysis of the life cycles of convective cells in Germany, focusing on the relevance of the prevailing atmospheric conditions. Using data from the radar‐based cell detection and tracking algorithm KONRAD of the German Weather Service, the life cycles of isolated convective storms are analysed for the summer half‐years from 2011 to 2016. In addition, numerous convection‐relevant atmospheric ambient variables (e.g., deep‐layer shear, convective available potential energy, lifted index), which were calculated using high‐resolution COSMO‐EU assimilation analyses (0.0625°), are combined with the life cycles. The statistical analyses of the life cycles reveal that rapid initial area growth supports wider horizontal expansion of a cell in the subsequent development and, indirectly, a longer lifetime. Specifically, the information about the initial horizontal cell area is the most important predictor for the lifetime and expected maximum cell area during the life cycle. However, its predictive skill turns out to be moderate at most, but still considerably higher than the skill of any ambient variable is. Of the latter, measures of midtropospheric mean wind and vertical wind shear are most suitable for distinguishing between convective cells with short lifetime and those with long lifetime. Higher thermal instability is associated with faster initial growth, thus favouring larger and longer living cells. A detailed objective correlation analysis between ambient variables, coupled with analyses discriminating groups of different lifetime and maximum cell area, makes it possible to gain new insights into their statistical connections. The results of this study provide guidance for predictor selection and advancements of nowcasting applications. [ABSTRACT FROM AUTHOR]

Peroxy radicals (RO2* = HO2 + RO2) play key roles in forming secondary air pollutants such as ozone, yet model underprediction of RO2* is a challenging radical closure problem. In this study, RO2* were measured by a dual‐channel peroxy radical chemical amplification system during an ozone episode in October 2018 at an urban site in the Pearl River Delta region, China. The box model based on the Master Chemical Mechanism severely underpredicted RO2* levels, particularly at night and under high nitric oxide (NO) conditions. The observed‐to‐modeled ratio of RO2* increased from ∼3 under 1 ppbv NO to ∼46 under >10 ppbv NO with a missing RO2* source up to 5.8 ppbv hr−1. Observation data were used to constrain model predictions, and the results reveal that constraining nitrous acid (HONO) or glyoxal/methylglyoxal could not improve predictions, while constraining nitrate radicals (NO3) or other oxygenated volatile organic compounds (OVOCs), particularly phenolic compounds and improvements in their gas‐phase mechanisms, could more effectively increase model‐simulated RO2* concentrations. When OVOCs, NO3, and HONO were constrained, the simulated RO2* concentrations increased to the greatest extent with an observed‐to‐modeled RO2* ratio of 1.9 during the day and 1.3 at night, mainly due to the interaction between OVOCs and NO3 radicals. As the underestimated NO3 levels and the unmeasured reactive organic gases, as well as their unknown oxidation mechanisms, are among the major reasons for the underestimation of RO2*, upgraded atmospheric chemistry involving more OVOC species and more accurate NO3 would improve model‐simulated RO2* concentrations, especially during nighttime. Plain Language Summary: Peroxy radicals (RO2* = HO2 + RO2), including hydroperoxy radicals (HO2) and organic peroxy radicals (RO2), are mainly formed by atmospheric oxidative degradation of trace gases, such as formaldehyde, carbon monoxide (CO) and volatile organic compounds (VOCs). They are important intermediates for the tropospheric forming secondary air pollutants, yet model underprediction of RO2* is a challenging radical closure problem. Here, we conducted a field campaign during an ozone episode in October 2018 at an urban site in the Pearl River Delta region and revealed that RO2* concentrations simulated by the AtChem2‐MCM were much lower than those observed, particularly at nighttime and under high nitric oxide conditions. We found that using the observations of oxygenated volatile organic compounds (OVOCs) and nitrate radicals to constrain the model could more effectively reduce the gap between the simulated and observed RO2* radicals. Our results highlight that, more attention should be given to OVOCs that have not been measured or whose oxidation mechanisms are not well understood, but can be photolyzed or oxidized to form RO2*, such as phenolic compounds, for the in‐depth understanding of tropospheric RO2* chemistry. Key Points: Peroxy radical levels were severely underpredicted by the MCM model, especially at night and under high‐NO conditionsIncorporating an extra HONO source could slightly improve model's prediction of peroxy radical levelsConstraining both oxygenated volatile organic compounds and nitrate radicals can minimize the gap between modeled and measured peroxy radical levels [ABSTRACT FROM AUTHOR]

We conduct a series of eight 45‐day experiments with a global storm‐resolving model (GSRM) to test the sensitivity of relative humidity R $\mathcal{R}$ in the tropics to changes in model resolution and parameterizations. These changes include changes in horizontal and vertical grid spacing as well as in the parameterizations of microphysics and turbulence, and are chosen to capture currently existing differences among GSRMs. To link the R $\mathcal{R}$ distribution in the tropical free troposphere with processes in the deep convective regions, we adopt a trajectory‐based assessment of the last‐saturation paradigm. The perturbations we apply to the model result in tropical mean R $\mathcal{R}$ changes ranging from 0.5% to 8% (absolute) in the mid troposphere. The generated R $\mathcal{R}$ spread is similar to that in a multi‐model ensemble of GSRMs and smaller than the spread across conventional general circulation models, supporting that an explicit representation of deep convection reduces the uncertainty in tropical R $\mathcal{R}$. The largest R $\mathcal{R}$ changes result from changes in parameterizations, suggesting that model physics represent a major source of humidity spread across GSRMs. The R $\mathcal{R}$ in the moist tropical regions is particularly sensitive to vertical mixing processes within the tropics, which impact R $\mathcal{R}$ through their effect on the last‐saturation temperature rather than their effect on the evolution of the humidity since last‐saturation. In our analysis the R $\mathcal{R}$ of the dry tropical regions strongly depends on the exchange with the extratropics. The interaction between tropics and extratropics could change with warming and presage changes in the radiatively sensitive dry regions. Plain Language Summary: Water vapor is the most important greenhouse gas in the atmosphere. Therefore, for the prediction of future warming it is important that climate models capture the distribution of atmospheric humidity and its change under warming. However, climate models currently strongly disagree in their representation of humidity, causing uncertainty in climate predictions. A recent study has shown that, while there is better agreement among the newest generation of climate models, so called global storm‐resolving models, the remaining inter‐model differences are still relevant and therefore need to be better understood. To narrow down the causes of these differences, in this study we examine how much the humidity in a storm‐resolving model changes in response to changes in different model components, which are chosen to reflect the differences that currently exist between models. We find the largest humidity changes in response to changes in the model's representation of sub‐grid scale processes. In storm‐resolving models these are turbulent motions and cloud microphysics. Our results suggest that differences in the representation of these processes cause a major part of the humidity differences between storm‐resolving models. Key Points: Sensitivity experiments suggest that parameterizations are the major source of relative humidity spread across global storm‐resolving modelsVertical mixing processes strongly impact the humidity of the moist tropics by affecting last‐saturation statistics within the tropicsCompared to the rest of the tropics, the humidity of the dry tropics is more sensitive to the pathways of exchange with the extratropics [ABSTRACT FROM AUTHOR]

Observed surface organic aerosols (OA) concentrations slightly increased in the western US (WUS) but significantly decreased in the eastern US (EUS) in summer, and continuously decreased in winter over the US region. To understand the driving factors for the long‐term surface OA trend, we apply a revised version of the Community Atmosphere Model version 6 with comprehensive tropospheric and stratospheric chemistry representation, considering the heterogeneous formation of isoprene‐epoxydiol‐derived secondary organic aerosols (SOAIE) and fast photolysis rate of monoterpene‐derived secondary organic aerosols (MTSOA) to diagnose the OA evolution in 1988–2019. Compared to older versions, the revised model better reproduces the climatology, seasonal cycle, and long‐term trend of surface OA as evaluated against the Interagency Monitoring of Protected Visual Environments measurements. We find the decrease in EUS summertime OA is likely attributed to the interplay between SOAIE and MTSOA. With anthropogenic emissions reduction, primary organic aerosols (POA) declined, SOAIE decreased along with sulfate, while MTSOA increased along with biogenic emissions driven by a warming climate. POA from wildfires with a significant trend of 2.9% yr−1 and considerable interannual variation of 62.8% drive the statistically insignificant but increasing WUS summertime OA, while anthropogenic POA dominates the decreasing wintertime OA in the US. Through sensitivity experiments, we find MTSOA show linear responses to the increasing monoterpenes emissions and negligible responses to NOx emissions reduction due to the mutual offsets between MTSOA components from different oxidation pathways. This study reveals the increasingly important role of MTSOA in summertime OA under a warming climate. Plain Language Summary: As the major components of fine particles, organic aerosols (OA) increased in the western United States and decreased in the eastern United States in the summer, and kept decreasing in the winter in the past decades. The driving factors for the long‐term trend of OA and their components remain unclear and are investigated by conducting a series of long‐term simulations. We find the isoprene‐epoxydiol‐derived secondary organic aerosols decrease with sulfate emission controls, which is partly offset by the increasing monoterpene‐derived secondary organic aerosols (MTSOA) under global warming and the statistically insignificant increase of primary organic aerosols driven by wildfires in summer. In winter, anthropogenic emissions dominate the declining surface OA. We also find MTSOA are more sensitive to increasing biogenic emissions than anthropogenic emissions reduction. Our results reveal the important role of MTSOA in total summertime OA under a warming climate. Key Points: Monoterpene‐derived and isoprene‐epoxydiol‐derived secondary organic aerosols (SOA) contribute to the descending surface summertime organic aerosols (OA) in the eastern USPrimary organic aerosols (POA) from wildfires drive the statistically insignificant but increasing trend of surface summertime OA in the western USMonoterpene‐derived SOA linearly respond to increasing monoterpenes emissions but negligibly respond to NOx emissions reduction [ABSTRACT FROM AUTHOR]

Isoprene, arguably the most studied biogenically emitted gas, is thought to have a large impact on tropospheric composition. Other naturally emitted species have been considered to play a less important role. Here the GEOS‐Chem model is used to compare the impacts of isoprene and iodine emissions on present‐day tropospheric composition. Removing isoprene emissions leads to a 3.4% decrease in tropospheric O3 burden, a smaller absolute change than the 5.9% increase from removing iodine emissions. Iodine has a negligible impact on global mean OH concentrations and methane lifetime (+0.6% and +0.05%) whereas isoprene has a substantial impact on both (−4.3% and −4.2%). Isoprene emissions and chemistry are seen as essential for tropospheric chemistry models, but iodine is often not. We suggest that iodine should receive greater attention in model development and experimental research to allow improved predictions of past, present, and future tropospheric O3. Plain Language Summary: Natural emissions from the Earth's surface play a large role in determining the chemistry of the atmosphere, influencing air quality, and climate change. Considerable attention is given to land‐based emissions, notably of isoprene, which are emitted in vast quantities by trees and other vegetation and can impact the concentration of O3, aerosols and the hydroxyl radical. Historically, less emphasis has been on the influence of emissions of other compounds. We show that for one aspect of atmospheric composition (the globally averaged O3 concentrations) emissions of iodine from the ocean are likely at least as important as isoprene emissions and may be more so. As such, there should be an increased focus on better understanding the emissions and chemistry of iodine species (and other halogens) and embedding this information into our simulations of the Earth system. Key Points: Present‐day inorganic iodine emissions decrease tropospheric O3 substantially more than biogenic isoprene emissions increase itAt the surface, the impact of iodine on O3 exceeds that for isoprene, and is larger in defining the background for Europe and North AmericaIodine has a negligible impact on OH concentrations and CH4 lifetimes compared to isoprene [ABSTRACT FROM AUTHOR]

Ozone in the troposphere is a pollutant and greenhouse gas, and it is crucial to better understand its transport from the ozone‐rich stratosphere. Tropopause folding, wherein stratospheric air intrudes downward into the troposphere, enables stratosphere‐to‐troposphere ozone transport (STT). However, systematic analysis of the relationship between folding and tropospheric ozone, using data that can both capture folding's spatial scales and accurately represent tropospheric chemistry, is limited. Here, we compare folding in high‐resolution reanalysis ERA5 (0.25° horizontal, <21 hPa vertical) and low‐resolution chemical reanalysis CAMSRA (0.75°, <40 hPa), against CAMSRA ozone, over 1 year. Folding becomes dramatically more frequent at high resolution, with vertical resolution overwhelmingly responsible. Deeper, more filamentary folding is almost entirely unrepresented at low resolution. Higher‐resolution folding is better‐correlated with tropospheric ozone (especially along midlatitude storm tracks, where deep folding is most common); STT is therefore likely more attributable to tropopause folding than coarsely‐resolved folding can capture. Plain Language Summary: "Tropopause folding" refers to high‐altitude atmospheric events wherein the "tropopause" (the boundary separating the troposphere, the lowest atmospheric layer, from the stratosphere above it) is perturbed, "folding" downward and allowing stratospheric air to intrude into the troposphere. These intrusions enable stratosphere‐to‐troposphere transport (STT) of ozone, a pollutant and greenhouse gas in the troposphere—however, a thorough understanding of the relationship between folding and ozone STT has been limited due to the difficulty of combining global yet detailed meteorological data with high‐fidelity chemical data. Here, we identify folding occurrences in both high‐ and low‐resolution representations of atmospheric motion throughout 1 year, and assess how strongly each folding data set is related to estimated tropospheric ozone. A high‐resolution view reveals that folding events are much more frequent and widespread—and penetrate further into the troposphere, becoming thinner—than possible to represent at lower resolution. Moreover, folding at higher resolution is more closely correlated with nearby tropospheric ozone amounts. Folding may therefore exert influence over a larger proportion of ozone STT than is suggested by coarse representations of folding. Furthermore, our results identify the importance of vertical (rather than horizontal) resolution in representing such features and atmospheric transport. Key Points: Tropopause folding is much more frequent at higher reanalysis resolution, due almost entirely to vertical resolutionNearly 90% of folding in ERA5 (nearly 100% of Deep folding) is unrepresented in CAMSRA (at the resolution of ERA‐Interim)Higher‐resolution folding is more strongly correlated with tropospheric ozone, driven by deeper and more filamentary folding [ABSTRACT FROM AUTHOR]

We present a new version (v5.0) of the NOAA Center for Satellite Applications and Research (STAR) mid‐tropospheric temperature (TMT) time series. This data set uses a backward‐merging approach to intercalibrate 16 satellite‐based microwave sounding records. The instrument observations included those from the Microwave Sounding Unit (MSU) during 1979–2004, Advanced Microwave Sounding Unit‐A (AMSU‐A) during 1998–2017, and Advanced Technology Microwave Sounder (ATMS) from 2011 to present. A TMT time series during 2002–present based on satellite microwave observations in stable sun‐synchronous orbits was used as a reference in the backward merging process in which earlier satellites were adjusted and merged to the reference. Observations from earlier satellites were recalibrated to remove their calibration drifting errors relative to the reference using sequential overlapping observations. This included removal of spurious warming drifts in the MSU observations onboard NOAA‐11, NOAA‐12, and NOAA‐14 and a spurious cooling drift in the NOAA‐15 AMSU‐A observations. Temperature changes resulting from diurnal sampling drifts were corrected using an observation‐based semi‐physical model developed in this study. Other adjustments included channel frequency differences between MSU and AMSU‐A companion channels and instrument blackbody warm target effect on observed radiances. These adjustments resulted in inter‐consistent TMT records spanning MSU, AMSU‐A, and ATMS. The merged time series produced a global mean TMT trend of 0.092 ± 0.043 K/decade during 1979–2021 and a total tropospheric trend of 0.142 ± 0.045 K/decade after removal of a stratospheric cooling effect in TMT. Remarkably, the total tropospheric trends during the latest half period were nearly doubled the earlier half period over the global ocean. Plain Language Summary: Long‐term observations of global atmospheric temperatures from satellite microwave sounders play a vital role in climate change research. These observations involved multiple satellites spanning several decades. Careful intersatellite calibration and bias correction are needed to derive inter‐consistent records from multi‐satellite observations for reliable climate change detection. Here we develop a new version of the NOAA Center for Satellite Applications and Research (STAR) mid‐tropospheric temperature (TMT) time series for climate trend investigation. The time series includes instrument observations from three generations of satellite microwave sounders from 1979 to present. Intersatellite biases from several error sources were removed or minimized, including instrument degradation, changes in satellite local observation time, and spectral band differences between different generations of microwave sounders, etc. A unique feature in the time series was satellite merging started from the latest backward to the earlier ones. A TMT time series during 2002‐present was used as a reference in the backward merging, which was based on satellite microwave sounder observations with fixed local observation time. The reference TMT has a high accuracy in trend detection, allowing intercalibration and trend detection with better accuracy in time series of the entire period from 1979 to present. Key Points: A new version of the NOAA Center for Satellite Applications and Research (STAR) mid‐tropospheric layer temperature time series was developedInstrument recalibration has removed spurious warming drifts in observations from NOAA‐11 to NOAA‐14 and spurious cooling drifts in NOAA‐15The new record yields a trend of 0.14 K/decade during 1979–2021 with an even greater rate of warming after the year 2002 (0.22 K/decade) [ABSTRACT FROM AUTHOR]

We present a comparison of atmospheric transport model (ATM) simulations for carbonyl sulfide (COS), within the framework of the atmospheric tracer transport model intercomparison project "TransCom‐COS." Seven ATMs participated in the experiment and provided simulations of COS mixing ratios over the years 2010–2018, using state‐of‐the‐art surface fluxes for various components of the COS budget: biospheric sink, oceanic source, sources from fire and industry. The main goal of TransCom‐COS is to investigate the impact of the transport uncertainty and emission distribution in simulating the spatio‐temporal variability of tropospheric COS mixing ratios. A control case with seasonal surface fluxes of COS was constructed. The results indicate that the COS mixing ratios are underestimated by at least 50 parts per trillion (ppt) in the tropics, pointing to a missing tropical source. In summer, the mixing ratios are overestimated by at least 50 ppt above 40°N, pointing to a likely missing sink in the high northern latitudes. Regarding the latitudinal profile, the model spread is greater than 60 ppt above 40°N in boreal summer. Regarding the seasonal amplitude, the model spread reaches 50 ppt at 6 out of 15 sites, compared to an observed seasonal amplitude of 100 ppt. All models simulated a too late minimum by at least 2–3 months at two high northern‐latitude sites, likely owing to errors in the seasonal cycle in the ocean emissions. This study highlighted the shortcomings in the COS global budget that need to be resolved before using COS as a photosynthesis tracer. Plain Language Summary: In this study, we evaluate the state‐of‐the‐art fluxes for various components of the carbonyl sulfide (COS) budget: biospheric sink, oceanic source, sources from fire and industry. A control case with seasonal surface fluxes of COS was constructed. Seven atmospheric transport models provided simulations of COS mixing ratios. Then, the simulated mixing ratios were evaluated against atmospheric measurements at several surface sites. Results show that all models fail to capture the observed latitudinal distribution and that the model spread is small compared to the model‐observation mismatch. In summer, the overestimated mixing ratios above 40°N point to a likely missing sink in the high northern latitudes. The underestimated mixing ratios in the tropics point to a missing tropical source. This study highlighted the shortcomings in the COS global budget that need to be resolved before using COS as a photosynthesis tracer. Key Points: The model‐observation mismatch suggests there is a missing source in the tropics and a missing sink in the high northern latitude in summerAt northern latitude sites, the model spread in seasonal amplitude reaches 50 ppt compared to a mean seasonal amplitude of about 100 pptThe diurnal rectifier effect is small, decreasing the seasonal amplitude by up to 20% at continental sites [ABSTRACT FROM AUTHOR]

A lightning nitrogen oxides (LNOx) emissions model using satellite‐observed lightning optical energy is introduced for utilization in Air Quality modeling systems. The effort supports assessments of air‐quality/climate coupling as related to the influence of LNOx on atmospheric chemistry. The Geostationary Lightning Mapper (GLM), International Space Station Lightning Imaging Sensor (ISS‐LIS), and the Tropical Rainfall Measuring Mission (TRMM) LIS data are used to examine the efficacy of the method, extend the previously derived LNOx record, and demonstrate a path for using ISS‐LIS observations to cross‐calibrate regional LNOx estimates from the future global constellation of geostationary lightning observations. A detailed evaluation of the GLM data set is provided to establish the robustness of observations for LNOx estimates and to make preliminary assessments of the LNOx emissions model. Seasonal and geographical variation, land/ocean contrast, and annual fluctuation in the GLM observed lightning activity and flash optical energy are provided. GLM detection substantially degrades with the increase in the field of view, resulting in 44% more flashes and 40% less optical energy observation by GLM‐16 (compared to GLM‐17) to the east of the middle‐longitude between the two mappers (106.2°W). Regular horizontal striations are found in the optical energy product. On average, GLM flashes matched to the cloud‐to‐ground flashes have ∼30% longer duration, 50%–70% more extension, and ≥100% higher optical energy compared to the unmatched flashes (assumed to be intra‐cloud). The results from summer‐long chemical transport simulations using LNOx generated from the emission model agrees with previous studies and shows consistency across the GLM/LIS data sets. Plain Language Summary: A three‐dimensional lightning nitrogen oxides (LNOx) emissions model is introduced for use in Air Quality modeling. This model uses satellite‐observed lightning optical energy with the empirical vertical distribution. By using the observed optical energy, the model generates the actual LNOx features for intra‐cloud/cloud‐to‐ground flash fluctuations as well as geographic, land/ocean, and seasonal variations. The model performed consistently when tested using data from the Geostationary Lightning Mapper (GLM), the International Space Station Lightning Imaging Sensor (ISS‐LIS), and the Tropical Rainfall Measuring Mission (TRMM) LIS. This experiment paves the way for cross‐calibrating regional LNOx estimates using ISS‐LIS data for future global geostationary lightning observations. In our detailed analysis of the GLM data set, we found significant detection degradation near the edge of the instrument's field of view. East of the midpoint between the two mappers over the contiguous United States (106.2°W), GLM‐16 (east) detected 44% more lightning strikes and 40% less optical energy than that detected by GLM‐17 (west). Additionally, we found consistent horizontal striations in the optical energy product. Applications utilizing the data may have to compensate for these deficiencies. Key Points: Detailed Geostationary Lightning Mapper (GLM) data evaluation that shows the GLM detection efficiency, field of view edge detection degradation, striation artifacts in detected lightning optical energyGeographical and temporal variation in lightning activity and flash optical energyLNOx emission model that mirrors geographical, land/ocean, seasonal, and intra‐cloud/cloud‐to‐ground variabilities exhibited in the detected flash optical energy [ABSTRACT FROM AUTHOR]

Kinetic integration of large and stiff chemical mechanisms is a computational bottleneck in models of atmospheric chemistry. It requires implicit solution of the coupled system of kinetic differential equations with time‐consuming construction and inversion of the Jacobian matrix. We present here a new version of the Kinetic Pre‐Processor (KPP 3.0.0) for fast integration of chemical kinetics featuring a range of improvements over previous versions in performance, diagnostics, versatility, and community openness. KPP 3.0.0 includes a new adaptive auto‐reduction solver to decrease the size of any mechanism locally and on the fly under conditions where full complexity is not needed, by partitioning species as "fast" or "slow" based on their local production and loss rates. Previous implementations of this adaptive solver suffered from excessive overhead in the repeated construction of the local Jacobian matrix or were hard‐wired to specific mechanisms. Here we retain the general applicability of the method to any mechanism and avoid overhead by using pre‐computed Jacobian matrix terms for the full mechanism and cropping the matrix locally to remove the slow species with no change in memory allocation. We apply this adaptive solver within KPP 3.0.0 to the GEOS‐Chem global 3‐D model of atmospheric chemistry and demonstrate a 32% reduction in solver time while maintaining a mean error lower than 1% for key species in the troposphere. Plain Language Summary: Calculating chemical evolution in global atmospheric chemistry models is computationally expensive because the chemical mechanisms typically include hundreds of species to account for all conditions from urban to remote. However, the full chemical complexity is not needed under most conditions. Here we have developed an adaptive auto‐reduction chemical solver that reduces any mechanism on the fly depending on local conditions and without significant computational overhead. We apply this adaptive solver as an option in a new version 3.0.0 of the Kinetic Pre‐Processor (KPP) chemical solver software package that also includes a number of updates relative to previous versions. The adaptive solver achieves a 32% reduction in solver time in a global model simulation while incurring less than 1% average errors for key species. Key Points: An updated version 3.0.0 of the Kinetic Pre‐Processor (KPP) integrator of chemical kinetics for atmospheric models has been developedKPP 3.0.0 features an adaptive solver option for reducing chemical mechanisms locally and on the fly where full complexity is not neededThe adaptive solver implemented in the global GEOS‐Chem model shows a 32% speedup with errors less than 1% for key tropospheric species [ABSTRACT FROM AUTHOR]

A long and complete series of monthly rainfall amounts corresponding to Barcelona city (Catalonia, NE Spain), exceeding two centuries (years 1786–2019), is analysed in detail. The obtained results of periodicity (annual scale), time trends (monthly, seasonal and annual scales), statistical distribution (seasonal and annual scale) and fractal/multifractal structures and self‐similarity at monthly scale depict the complex structure of this pluviometric regime, which is characterized by moderate increasing and decreasing trends on rain amounts, varying from +0.08 mm·year−1 (February) to −0.07 mm·year−1 (September) and quite evident changes on the pluviometric trends at annual and seasonal scales when the rainfall data are analysed for segments of 50 years from 1800 up to 2019. A good example could be the relevant change on the annual scale time trend, from +0.77 mm·year−1 (years 1800–1850) to −0.17 mm·year−1 (years 1950–2019). Clear evidences of decreasing pluviometry for spring, summer and autumn for the last segment (1950–2019) in comparison with the other three segments, including years 1800–1950, are also detected. Additionally, increasing rainfall patterns complexity, expected difficulties on monthly rainfall forecasting and the increasing irregularity of monthly amounts is also detected by interpreting fractal and multifractal results. Irregularity increases on the monthly rainfall series and on the rainfall regime complexity derived from multifractal parameters, could be associated with the very notable increase of CO2 emissions into the atmosphere, globally varying from 51.1 × 106 metric tonnes (year 1820) to 36.6 × 109 metric tonnes (year 2019) and the tropospheric concentration increasing from 280.8 ppm (year 1850) to 397.5 ppm (year 2014), being the probable relationship between tropospheric concentrations and changes on rainfall patterns the objective of future researches. [ABSTRACT FROM AUTHOR]

Atmospheric Infrared Sounder (AIRS) aboard the National Aeronautics and Space Administration (NASA) Aqua satellite has been operating since September 2002. Its information content, superb instrument performance, and dense sampling pattern make the AIRS radiances an invaluable data set for climate studies. The trends of global‐mean, nadir‐view, clear‐sky AIRS radiances from 2003 to 2020 are studied here, together with the counterparts of synthetic radiances based on two reanalyzes, European Centre for Medium‐Range Weather Forecasts Reanalysis V5 (ECMWF ERA5) and NASA Goddard Earth Observing System V5.4.1 (GEOS‐5.4.1; a reanalysis product without assimilation of hyperspectral radiances such as AIRS). The AIRS observation shows statistically significant negative trends in most of its CO2 channels, positive but non‐significant trends in the channels over the window regions, and statistically significant positive trends in some of its H2O channels. The best agreements between observed and simulated radiance trends are seen over the CO2 tropospheric channels, while the observed and simulated trends over the CO2 stratospheric channels are opposite. ERA5 results largely agree with the AIRS observation over the H2O channels. The comparison in the H2O channels helps reveal a data continuity issue in the GEOS‐5.4.1. Contributions from individual variables to the radiance trends are also assessed by performing separate simulations. This study provides the first synopsis of the global‐mean trend of AIRS radiances over all its thermal‐IR channels. Plain Language Summary: Using 18 years of satellite hyperspectral observations, we depict, for the first time, a synopsis of the trend of global‐mean, clear‐sky infrared radiances from ∼6 to ∼16 μm. Statistically significant trends are seen in the carbon dioxide and water vapor absorption bands but not over the infrared window region. The physical explanations of such trends are discussed and supported with simulation sensitivity studies. Such observed trends, channel‐by‐channel, are also compared with those derived from synthetic radiances based on reanalysis data sets. Discrepancies between observed and reanalysis‐based trends are also discussed. Key Points: The 18‐year trends of global‐mean, clear‐sky radiances at all Atmospheric Infrared Sounder (AIRS) thermal‐infrared channels are analyzedAIRS observation shows statistically significant negative trends in most of its CO2 channels and positive trends in some of its H2O channelsThe best agreements between observations and reanalysis are seen over the CO2 tropospheric channels [ABSTRACT FROM AUTHOR]

Lower atmospheric gravity waves (GWs) can significantly impact waves in the middle and upper atmospheres and are vital for turbulence generation. This paper puts the spotlight on the spatial–temporal variability of tropospheric GW total energy (ET) and its potential sources above four regions of interest (ROIs) gathered from high‐resolution radiosonde observations from the China Radiosonde Network during the years 2016–2019. The seasonality of ET above four ROIs shows different characteristics and is dependent on latitudes and underlying terrains, reaching its maximum identified in the winter at middle latitudes. Interestingly, the annual cycles of the maximal ET shift from 35°N in October to 25°N in March of the next year, triggered by the shift in the winter subtropical jet. Based on the random forests regressor, the jet stream between 200 and 125 hPa likely serves as the primary source for the observed GWs above the ROIs with low and middle latitudes, with relative contributions of around 60%. However, the Kelvin–Helmholtz instability between 800 and 125 hPa could be the most recognized source of GWs and contributes around 68.4% to the observed energy. During the rainy season, the ET under scenarios of convective precipitation is around 20% larger than the other. As well, as the near‐surface or low‐level wind interacts with a mountain barrier over the Tibetan Plateau region, 12.4% of the observed ET is attributed to the strength of the low‐level wind. Plain Language Summary: The gravity wave (GW) is one of the most important waves in the atmosphere and acts as a triggering source to turbulence. However, the tropospheric GWs in the context of China has seldomly been investigated by using high‐resolution radiosonde data set. This analysis shows that the GW total energy exhibit obvious seasonal various at low and middle latitudes, with maximal identified in the winter and minimal in the summer. The jet stream in the upper troposphere is the most important source for GW at low and middle latitudes and gives rise to a southward propagation of the maximal GW energy in cold season. In the summer of southern China, the convective precipitation could contribute to the enhancement of GW energy. In addition, 12.4% of the observed GW energy is attributed to the strength of the low‐level wind over the Tibetan Plateau. Key Points: Jet stream is the dominant source for gravity waves (GWs) at low and middle latitudes and triggers southward movement of GW energy core during cold seasonsOver the Tibetan Plateau, Kelvin–Helmholtz instabilities and terrain‐induced flows contribute to the intensive GW activitiesDuring the summertime of southern China, convective precipitation could contribute to the enhancement of energy of about 20% [ABSTRACT FROM AUTHOR]

The morphological features of sprites are closely related to the polarity of their causative lightning strokes. Using the machine learning method, we develop a model with an accuracy of 93.8% to identify the polarity of sprite‐producing cloud‐to‐ground (CG) lightning strokes for events recorded during the Imager of Sprites and Upper Atmospheric Lightning (ISUAL) mission. Approximately 17% of the sprites are identified to be produced by negative CG lightning strokes. The global distribution of the polarity of sprite‐producing CG lightning strokes suggests that the ratio of sprites produced by negative CG lightning strokes relative to sprites produced by positive ones varies with latitude and sea‐land distribution. Sprites produced by negative CG lightning strokes appear to be generated in the tropical regions below 20° latitude and the oceanic area. Moreover, the proportion of sprites produced by negative CG lightning strokes over Africa and North America are much smaller than that over the rest of the continents and the sea. Plain Language Summary: Sprites are lightning‐induced optical transient emissions in the mesosphere. The morphological features of sprites are observed to be quite complex but closely related to the polarity of their causative cloud‐to‐ground (CG) lightning strokes. In our previous studies, the polarity of sprite‐producing CG lightning strokes has been determined for hundreds of events with the concurrent ground‐based measurement of associated lightning sferics. Based on the machine learning method, we develop a model to identify the polarity of sprite‐producing CG lightning strokes for events observed by the Imager of Sprites and Upper Atmospheric Lightning (ISUAL) mission. With such a model, the polarity of sprite‐producing CG lightning strokes without ground‐based observations can be recognized. In total, 258 sprites produced by negative CG lightning strokes, roughly 17% of the whole data set (1522) are identified. The proportion of sprites produced by negative CG lightning strokes greatly varies with latitude and sea‐land distribution, and it is greater in the tropical regions below 20° latitude and also larger over the sea. Moreover, the proportion of sprites produced by negative CG lightning strokes over Africa and North America is much smaller than that over the rest of the continents and the sea. Key Points: A model based on machine learning method is developed to determine the polarity of Imager of Sprites and Upper Atmospheric Lightning sprites, with an accuracy of 93.8%The ratio of negative sprites to positive sprites varies with latitude and sea‐land distributionThe proportion of negative sprites over Africa and North America are much smaller than that over the rest of the continents and the sea [ABSTRACT FROM AUTHOR]

Nitrogen (N) fertilizer application to agricultural soils results in substantial emissions of nitric oxide (NO), a key substance in tropospheric chemistry involved in climate forcing and air pollution. However, the estimates of global cropland NO emissions remain uncertain due to a lack of information on direct NO emission factors (EFds) of applied N for various cropping systems at seasonal or annual scales. Here we quantified the crop-specific seasonal and annual-scale NO EFds through synthesizing 1094 measurements from 125 field-based studies worldwide. The global mean crop-specific seasonal EFd was 0.53%, with the highest for vegetables (0.75%). Among cereal crops, the EFd of maize (0.45%) or wheat (0.47%) was about three times higher than for rice (0.12%). At annual scale, the mean EFd across all cropping systems was 0.58%, with tea plantations having the highest (1.54%). For other cropping systems, the annual-scale EFds ranged from 0.02% to 1.07%. Besides crop type, also soil organic carbon, total N, and pH as well as N fertilizer type were the main factors explaining the variations of NO EFds. Based on obtained specific EFds for each crop type, we estimated that NO emissions due to the use of synthetic fertilizers from global croplands are about 0.42-0.62 Tg N year-1. Our budgets are relatively lower if compared to estimates derived by the use of IPCC defaults for NO emissions (0.72-1.66 Tg N year-1) or reported elsewhere (0.67-1.04 Tg N year-1). In our estimates, cash crops (vegetable, tea and orchard), which cover only 9% of the world cropland area, contributed about 31% to total NO emissions from global fertilized croplands. Overall, our meta-analysis provides improved crop-specific NO EFds reflecting current stage of knowledge. The work also highlights the relative importance of cash crop production as sources for atmospheric NO, that is, agricultural systems on which mitigation efforts may focus. [ABSTRACT FROM AUTHOR]

Nitrous oxide (N2O) is the third most important anthropogenic greenhouse gas and a major ozone‐depleting substance. Its main sources include anthropogenic activities (mostly agriculture) and natural emissions from ocean and soils. However, emission estimates for individual sources are highly variable due to uncertainties in N2O lifetime estimates and partitioning among sources. We derive annual global N2O emissions for 1990–2019 using NOAA Global Monitoring Laboratory (GML) surface N2O observations and the N2O lifetime calculated in the NASA GEOS‐5 chemistry climate model. The inferred global mean N2O emissions has gradually increased from ∼15.8 TgN/yr in the early 1990s to ∼17.8 TgN/yr in the 2010s. This implies that anthropogenic N2O emissions have grown rapidly from ∼6.7 TgN/yr in the 1990s to about ∼8.7 TgN/yr in the 2010s, a ∼30% increase. With specially designed N2O isotopic tracers in 3‐D GEOSCCM, we estimate that, on global average, stratospheric enrichment contributes about +7.7‰/yr, +7.6‰/yr, +8.0‰/yr to tropospheric δ15Nα, δ15Nβ, and δ18O budget, respectively. To balance the global mean isotopic signature for pre‐industrial terrestrial sources of δ15Nα ∼ 6.7‰, δ15Nβ ∼ −12.6‰, δ18O ∼ 35.4‰, our 3‐dimensional isotopic budget simulation using the GEOSCCM suggests global mean anthropogenic isotopic signatures in the recent decades are δ15Nα ∼ −18‰, δ15Nβ ∼ −20‰, δ18O ∼ 19‰. These anthropogenic isotopic estimates are significantly lighter than results from one‐box atmospheric model‐based estimates with the largest difference seen for δ15Nβ. More surface isotopic measurements are needed to better quantify the N2O isotopic signatures. Plain Language Summary: Nitrous oxide (N2O) is a trace gas that plays an important role in Earth's atmosphere, impacting the chemical composition and radiation budget. In pre‐industrial time, N2O in the atmosphere primarily comes from ocean and soil emissions. Atmospheric N2O level has increased by about 20% since mid 1800s as a result of human activities, mostly agriculture. Its growth rate in recent decades has been notably greater than before. Using a 3‐Dimensional Chemistry Climate Model, we were able to determine that the notable increase in N2O growth rate is a result of rapidly growing anthropogenic emissions started in the mid‐1990s. In addition to the atmospheric mass balance budget of N2O, we were also able to complete a full 3‐Dimensional isotopic budget of 15N and 18O isotopologues for N2O for 2000‐2019 and estimated the contribution of isotopically‐heavy stratospheric enrichment and isotopically‐light anthropogenic sources. The increase in recent anthropogenic emissions has introduced detectable changes in the observed isotopic trends. Key Points: Global Nitrous oxide emissions increased from ∼15.8 TgN/yr from the early 1990s to ∼17.8 TgN/yr in the 2010s due to increasing anthropogenic sourcesStratospheric enrichment contributes about +7.7‰/yr, +7.6‰/yr, +8.0‰/yr to the tropospheric δ15Nα, δ15Nβ, and δ18O budgetThe 3‐dimensional model estimated global mean isotopic signatures for anthropogenic sources are δ15N−α ∼ −18‰, δ15N−β ∼ −20‰, and δ18O ∼ 19‰ [ABSTRACT FROM AUTHOR]

During the 12 years of the Imager of Sprites and Upper Atmospheric Lightning (ISUAL) experiment onboard the Taiwanese FORMOSAT‐2 satellite (2004/07/01–2016/6/20), 297,349 transient events were recorded, but only 48,414 events (16.3%) were manually classified as transient luminous events (TLEs). Since the main target of ISUAL is to observe TLEs, only the top ∼0.82% of intense lightning can meet the trigger threshold, which means the ISUAL experiment can also serve as a space‐borne intense lightning survey. An autonomous lightning identification algorithm was developed in this work to screen and locate lightning flashes in images, and 292,248 intense lightning discharges were identified successfully. Combining the statistical analysis of lightning events and unique spectral observations of ISUAL, the global distribution of intense lightning with spectral characteristics was constructed. The intense lightning occurrence rate of ISUAL was 1.29 fl s−1, and the land‐to‐ocean ratio 1.27. Intense lightning is most active near the equator, and the bimodal pattern peaks in April and September, different from the typical lightning behavior that occurs in the northern hemisphere during the summertime from July to August. In addition, the spectral statistics show that more than 95% of intense lightning is accompanied by nitrogen band emissions (1PN2, 2PN2, 1NN2+). Far‐ultraviolet photons mainly come from the radiation processes in the mesosphere to the ionosphere, and its radiation flux may correlate with lightning energies. Further, a significant 777.4 nm background enhancement is recognized in South America, which may affect the trigger of space‐borne 777.4 nm lightning surveys. Plain Language Summary: The Imager of Sprites and Upper Atmospheric Lightning (ISUAL) is an experiment onboard the Taiwanese FORMOSAT‐2 satellite designed to explore transient luminous events (TLEs) as its main mission objectives. Since most TLEs are driven by intense lightning, this mission can also act as an intense lightning survey. ISUAL recorded nearly 300,000 transient events dominated by lightning in 12 years. An autonomous lightning identification algorithm, developed in this work, successfully registered most of the intense lightning from captured images. The intense lightning occurrence rate of ISUAL derived was 1.29 fl s−1, and the land/ocean ratio 1.27, which is lower than typical lightning. The seasonal variation of ISUAL lightning also indicated that intense lightning is active near the equator and peaks in April and September. The spectrophotometer of ISUAL provides spectral features of energetic lightning and shows that most of the intense lightning is accompanied by nitrogen emissions. Key Points: The Imager of Sprite and Upper Atmospheric Lightning is the first global space‐borne intense lightning survey with spectral informationIntense lightning is active over the oceans near the equator and has a bimodal pattern in April and SeptemberThe occurrences of intense lightning are usually accompanied by nitrogen band emissions [ABSTRACT FROM AUTHOR]

A major computational barrier in global modeling of atmospheric chemistry is the numerical integration of the coupled kinetic equations describing the chemical mechanism. Machine-learned (ML) solvers can offer order of magnitude speedup relative to conventional implicit solvers but past implementations have suffered from fast error growth and only run for short simulation times (<1 month). A successful ML solver for global models must avoid error growth over yearlong simulations and allow for reinitialization of the chemical trajectory by transport at every time step. Here, we explore the capability of a neural network solver equipped with an autoencoder to achieve stable full-year simulations of tropospheric oxidant chemistry in the global 3-D Goddard Earth Observing System (GEOS)-Chem model, replacing its standard mechanism (228 species) by the Super-Fast mechanism (12 species) to avoid the curse of dimensionality. We find that online training of the ML solver within GEOS-Chem is important for accuracy, whereas offline training from archived GEOS-Chem inputs/outputs produces large errors. After online training, we achieve stable 1-year simulations with five-fold speedup compared to the standard implicit Rosenbrock solver with global tropospheric normalized mean biases of -0.3% for ozone, 1% for hydrogen oxide radicals, and -5% for nitrogen oxides. The ML solver captures the diurnal and synoptic variability of surface ozone at polluted and clean sites. There are however large regional biases for ozone and NOx under remote conditions where chemical aging leads to error accumulation. These regional biases remain a major limitation for practical application, and ML emulation would be more difficult in a more complex mechanism. Plain Language Summary Global models of atmospheric chemistry are computationally expensive. A bottleneck is the chemical solver that integrates the large-dimensional coupled systems of kinetic equations describing the chemical mechanism. Machine learning (ML) could be transformative for reducing the cost of an atmospheric chemistry simulation by replacing the chemical solver with a faster emulator. However, past work found that ML chemical solvers experience rapid error growth and become unstable over time. Here, we present results achieving for the first time a stable full-year global simulation of atmospheric chemistry with 3 months seasonal ML solvers and with five-fold speedup in computational performance over the reference simulation. We show that online training of the ML solver synchronously with the atmospheric chemistry model simulation produces considerably more stable results than offline training from a static data set of simulation results. Although our work represents an important step for using ML solvers in global atmospheric chemistry models, more work is needed to extend it to large chemical mechanisms and to reduce errors during long-term chemical aging. [ABSTRACT FROM AUTHOR]

Lightning processes generate a diverse collection of optical pulses from current traversing the lightning channels. These signals are then broadened spatially and temporally via scattering in the clouds. The resulting waveforms measured from space with instruments like the photodiode detector (PDD) on the Fast On‐orbit Recording of Transient Events (FORTE) satellite have a variety of shapes. In this study, we use coincident optical and Radio‐Frequency measurements to document the properties of optical PDD waveforms associated with different types of lightning, estimate delays from scattering in the clouds, and comment on how pulse shape impacts optical lightning detection. We find that the attributes of optical pulses recorded by the PDD are consistent with prior studies, but vary globally and with event amplitude. The most powerful lightning tends to be single‐peaked with faster rise times (median: ∼100 µs) and shorter effective widths (median: ∼400 µs) than normal lightning. Particularly dim events, meanwhile, include cases of broad optical waveforms with sustained optical emission throughout the PDD record, which the pixelated FORTE Lightning Location System (LLS) instrument has difficulty detecting. We propose that this is due to the optical signal being divided between individual LLS pixels that are each, individually, not bright enough to trigger. We use PDD waveforms and Monte Carlo radiative transfer modeling to demonstrate that increasing the temporal and spatial resolution of a pixelated lightning imager will make it more difficult to detect these broad/dim pulses as their energy becomes divided between additional pixels/integration frames. Plain Language Summary: Lightning generates pulses of light following the extreme heating produced by its electrical currents. These emissions must then transmit through the cloud to reach the lightning sensors on satellites. Variations in current and the severity of scattering lead to many different types of optical lightning waveforms being recorded from orbit. We use Fast On‐orbit Recording of Transient Events measurements to document the different types of optical waveforms that can result from lightning. Our global results generally agree with prior studies with more limited samples. However, we also find that these pulse characteristics vary across the globe and with the intensity and origin of the light source. These variations stem from physical differences in the flash, the amount of scattering that occurs within the clouds, and the ability of the sensor to detect a pulse of a given shape—with pixelated sensors less able to resolve faint and broad lightning emissions than wide‐Field of View instruments. Key Points: Variations in optical emission and scattering in the surrounding clouds create a diverse collection of optical lightning waveformsLarge‐footprint sensors such as the Fast On‐orbit Recording of Transient Events photodiode detector detect broad/dim emissions because all pulse energy contributes to detectionIncreasing the spatial or temporal resolution of pixelated instruments offers greater pulse detail at the cost of missing broad/dim events [ABSTRACT FROM AUTHOR]

Stratosphere‐troposphere exchange (STE) of ozone represents a significant source term in the tropospheric ozone budget and can impact surface ozone concentrations, tropospheric oxidation capacity, and methane lifetime. Using the Whole Atmosphere Community Climate Model 6, changes in the air mass and ozone STEs in the Last Glacial Maximum (LGM) as compared with preindustrial (PI) climate are investigated. We use dynamic isentropic surfaces that are determined by fitting to the tropical tropopauses as the upper boundary of the lowermost stratosphere in a mass budget approach, a method particularly suitable for estimating air mass and ozone STEs across different climates. Relative to the PI, the magnitude of ozone STE in the LGM is decreased by 14%–19%, 18%–24%, 18%–23%, 16%–21%, and 15%–21% over the Northern hemisphere extratropics, Southern hemisphere extratropics, the tropics, the extratropics, and the globe, respectively. The extratropical and global decreases are mainly caused by decreased ozone in the extratropical lower stratosphere associated with a weakening of Brewer‐Dobson circulation, while changes in air mass fluxes play a minor role because the effects of weakening Brewer‐Dobson circulation and increased isentropic density partly cancel each other. Analysis of the modeled tropospheric ozone budget indicates that the ozone STE in the LGM is 28% of the tropospheric ozone production rate, as compared to about 9% in the modern climate (year 2000) and 19% in the PI. Key Points: Global ozone stratosphere‐troposphere exchange decreases by 15%–21% in simulations of the Last Glacial Maximum (LGM) versus preindustrialThis decrease is mainly caused by decreased ozone in extratropical lower stratosphere due to a weakening of Brewer‐Dobson circulationStratosphere‐troposphere ozone exchange in the LGM is 28% of the tropospheric ozone production rate, as compared to 19% in preindustrial [ABSTRACT FROM AUTHOR]

When a lightning flash is propagating in the atmosphere it is known that especially the negative leaders emit a large number of very high frequency (VHF) radio pulses. It is thought that this is due to streamer activity at the tip of the growing negative leader. In this work, we have investigated the dependence of the strength of this VHF emission on the altitude of such emission for two lightning flashes as observed by the Low Frequency ARray (LOFAR) radio telescope. We find for these two flashes that the extracted amplitude distributions are consistent with a power‐law, and that the amplitude of the radio emissions decreases very strongly with source altitude, by more than a factor of 2 from 1 km altitude up to 5 km altitude. In addition, we do not find any dependence on the extracted power‐law with altitude, and that the extracted power‐law slope has an average around 3, for both flashes. Key Points: We measure the amplitude distribution of lightning very high frequency (VHF) pulsesThe VHF pulse amplitude spectrum follows a power‐lawThe top 10‐percentile amplitude decreases with altitude [ABSTRACT FROM AUTHOR]

Accurate tropospheric delays from Numerical Weather Models (NWM) are an important input to space geodetic techniques, especially for precise real‐time Global Navigation Satellite Systems, which are indispensable to earthquake and tsunami early warning systems as well as weather forecasting. The NWM‐based tropospheric delays are currently provided either site‐specific with a limited spatial coverage, or on two‐dimensional grids close to the Earth surface, which cannot be used for high altitudes. We introduce a new method of representing NWM‐derived tropospheric zenith hydrostatic and wet delays. A large volume of NWM‐derived data is parameterized with surface values and additional two or three coefficients for their vertical scaling to heights up to 14 km. A precision of 1–2 mm is achieved for reconstructing delays to the NWM‐determined delays at any altitudes. The method can efficiently deliver NWM‐derived tropospheric delays to a broader community of space geodetic techniques. Plain Language Summary: Precise positioning with microwave‐based space geodetic techniques, such as Global Navigation Satellite Systems (GNSS), requires accurate modeling of the atmospheric refraction. Numerical Weather Models (NWM) can provide tropospheric delays with an accuracy of 1–2 cm in zenith direction and are therefore useful for improving the data analysis. However, due to the large data volume to handle, NWM‐based products are typically provided only for selected sites, or on a global grid referring to a specific height. We provide an efficient method to represent the vertical profile of tropospheric delay from the Earth surface up to 14 km altitude with a precision of 1–2 mm. The method is used to preserve the precision of NWM‐derived tropospheric delays at the altitudes using three to four coefficients per geographic location (longitude, latitude) at the ground. This paves the way of applying the NWM‐based accurate tropospheric delays in space geodetic data analysis, especially for global augmentations of real‐time GNSS, which play a critical role in the rapid characterization and early warning of geohazards such as earthquake and tsunami, as well as kinematic platforms of high altitudes. Key Points: New method for precise modeling of the zenith hydrostatic and wet delays from the Earth surface up to an altitude of 14 kmTropospheric delay vertical modeling precision of better than 3 mm is achieved on a global scaleThe method provides numerical weather model‐derived precise tropospheric augmentation correction for real‐time space geodetic techniques [ABSTRACT FROM AUTHOR]

This work applies a new remote sensing approach to indirectly probe the electric fields in the discharge region of fast breakdown (FB) processes during lightning initiation. A VHF‐UHF broadband (1–750 MHz) measurement system is used to record the radiation spectra of FB. The average spectrum over 4 FB events exhibits a −10 dB spectral cutoff at 430 MHz, which yields an electrical field of 1.96–2.02 times the conventional breakdown field at 8–9 km altitudes. This in turn implies a localized field enhancement inside the region dominating the VHF‐UHF emissions of >7 times the expected pre‐discharge background field for lightning initiation. Such a high field has implications for the dynamics of individual streamers and their impact on the immediate environment, and also constrains the geometry of parts of the FB structure, suggesting that streamers initiate continuously in the enhanced field ahead of a narrow (aspect ratio >4) and conducting region. Plain Language Summary: The electric field inside the region of lightning initiation is one of the most fundamental quantities in a lightning discharge, but it is nearly impossible to make any in situ measurement. Recent studies found that lightning can be initiated by a distinct process named fast breakdown (FB). Here, we apply a new remote sensing approach to indirectly probe the electric fields in the discharge zone of FB. A carefully calibrated broadband measurement system is used to record the radio signatures and power spectra of FB processes. We find that the electric field responsible for the FB streamer propagation is as high as 2.0 times the conventional breakdown field. This high field has implications for the immediate environment and dynamics of individual streamers, and also put constraints on the geometry of the fast breakdown structure. Key Points: Electric fields that drive fast breakdown (FB) processes are quantified using 1–750 MHz VHF‐UHF spectral measurementsFB spectra exhibit a −10 dB cutoff frequency of 430 MHz that implies an E‐field of 2.0 Ek driving streamer growth inside the FB processThis field is >7 times higher than the large‐scale field, implying that the FB process involves a narrow electrically conducting structure [ABSTRACT FROM AUTHOR]

We have compiled and analyzed a comprehensive data set of field observations of iodine speciation in marine aerosol. The soluble iodine content of fine aerosol (PM1) is dominated by soluble organic iodine (SOI; ∼50%) and iodide (∼30%), while the coarse fraction is dominated by iodate (∼50%), with nonnegligible amounts of iodide (∼20%). The SOI fraction shows an equatorial maximum and minima coinciding with the ocean "deserts," which suggests a link between soluble iodine speciation in aerosol and ocean productivity. Among the major aerosol ions, organic anions and non‐sea‐salt sulfate show positive correlations with SOI in PM1. Alkali cations are positively correlated to iodate and negatively correlated with SOI and iodide in coarse aerosol. These relationships suggest that under acidic conditions iodate is reduced to HOI, which reacts with organic matter to form SOI, a possible source of iodide. In less acidic sea‐salt or dust‐rich coarse aerosols, HOI oxidation to iodate and reaction with organic matter likely compete. Plain Language Summary: Iodine has a profound impact on tropospheric chemistry and plays a key role in in mammalian metabolism. Marine aerosol is an atmospheric iodine reservoir and its carrier to continental food chains. However, the chemistry behind the varying concentrations of iodine‐bearing species observed in aerosol is poorly understood, which hinders model predictions about iodine recycling to the gas phase and accumulation in aerosol. To shed some light on this problem, we have compiled and analyzed a global data set of field observations of iodine speciation in aerosol over the ocean between 1983 and 2018. Comparison of the spatial trends of the relative amounts of soluble iodine species in aerosol and ocean surface remote sensing variables reveals a link to ocean productivity, which points to an important role of organics emitted by the ocean and condensed on aerosol surfaces. Furthermore, the speciation of iodine differs between coarse and fine aerosol, which are originated, respectively, from condensation of gases derived from ocean surface emissions (sulfate aerosol) and aeolian processes (sea salt and dust) and, as a consequence of their different composition, have different pH. Overall, organic matter and acidity emerge from our study as the factors controlling iodine speciation in marine aerosol. Key Points: Iodide and SOI are more abundant in PM1, while the coarse fraction is dominated by iodate; iodide is only 30% lower in the coarse fractionIodine speciation in fine aerosol is related to ocean productivity and present a distinct latitudinal variationDifferent chemical regimes correlated with acidic and alkali ions prevail in fine and coarse aerosol speciation [ABSTRACT FROM AUTHOR]

Lightning plays a major role in tropospheric oxidation, and its role on modulating tropospheric chemistry was thought to be emissions of nitrogen oxides (NOx). Recent field and laboratory measurements demonstrate that lightning generates extremely large amounts of oxidants, including hydrogen oxides (HOx) and O3. Here, we implement these lightning‐produced oxidants in a global chemical transport model to examine its global impact on tropospheric composition. We find that lightning‐produced oxidants can increase global mass weighted OH by 0.3%–10% and affect CO, O3, and reactive nitrogen substantially, depending on the emission strength of oxidants from lightning. Our work highlights the importance and uncertainties of lightning‐produced oxidants, as well as the need for rethinking the role of lightning in tropospheric oxidation chemistry. Plain Language Summary: We examine the global impact of lighting‐produced oxidants on tropospheric composition with a global chemical transport model. The model results show that lightning‐produced oxidants can increase global mass weighted OH by 0.3%–10% and affect CO, O3, and reactive nitrogen substantially, depending on the emission strength of oxidants from lightning. Key Points: Lightning‐produced oxidants can significantly affect tropospheric compositionCurrent estimate of global impact is largely uncertain and requires future field measurements for better quantificationOur work suggests the need of revisiting current estimates of global lightning NOx emissions, with oxidants included [ABSTRACT FROM AUTHOR]

A particular strength of lightning remote sensing is the variety of lightning types observed, each with a unique occurrence context and characteristically different emission. Distinct energetic intracloud (EIC) lightning discharges—compact intracloud lightning discharges (CIDs) and energetic intracloud pulses (EIPs)—produce intense RF radiation, suggesting large currents inside the cloud, and they also have different production mechanisms and occurrence contexts. A Low‐Frequency (LF) lightning remote sensing instrument array was deployed during the RELAMPAGO field campaign in west central Argentina, designed to investigate convective storms that produce high‐impact weather. LF data from the campaign can provide a valuable data set for researching the lightning context of EICs in a variety of subtropical convective storms. This paper describes the production of an LF‐CID data set in RELAMPAGO and includes a preliminary analysis of CID prevalence. Geolocated lightning events and their corresponding observed waveforms from the RELAMPAGO LF data set are used in the classification of EICs. Height estimates based on skywave reflections are computed, where prefit residual data editing is used to improve robustness against outliers. Even if EIPs occurred within the network, given the low number of very high‐peak current events and receiver saturation, automatic classification of EIPs may not be feasible using this data. The classification of CIDs, on the other hand, is straightforward and their properties, for both positive and negative polarity, are investigated. A few RELAMPAGO case studies are also presented, where high variability of CID prevalence in ordinary storms and high‐altitude positive CIDs, possibly in overshooting tops, are observed. Key Points: Classification and height estimation of energetic intracloud lightning are investigated using RELAMPAGO Low‐Frequency (LF) lightning waveformsA small number of high‐peak current events and saturation of LF receivers hinder the observation of Energetic In‐Cloud Pulses in RELAMPAGOA catalog of RELAMPAGO Compact Intracloud Discharges is produced to be used in future study of their occurrence in different storm types [ABSTRACT FROM AUTHOR]

A one‐year's worth of near‐global marine shallow single‐layer cloud top radiative cooling (CTRC) is derived from a radiative transfer model with inputs from the satellite cloud retrievals and reanalysis sounding. The mean cloud top radiative flux divergence is 61 Wm−2, decomposed into the longwave and shortwave components of 73 and −11 W m−2, respectively. Equatorward of 30°N/S, the CTRC is largely a reflection of free‐atmospheric specific humidity distribution: a dry atmosphere enhances CTRC by reducing downward thermal radiation. Consequently, the cooling minimizes in the "wet" tropics and maximizes in the "dry" eastern subtropics. Poleward of 30°N/S, the CTRC decreases slightly due to the colder clouds that emit less effectively. The CTRC exhibits distinctive seasonal cycles with stronger cooling in the winter and has amplitudes of order 10–20 Wm−2 in stratocumulus‐rich regions. The datasets were used to train a machine‐learning model that substantially speeds up the retrieval. Plain Language Summary: Marine low‐lying clouds cool by emitting thermal radiation. The cooling is known as cloud top radiative cooling (CTRC). A change in CTRC can influence the properties of marine clouds via many avenues, ranging from altering the vertical motions of the clouds to changing the clouds' ability to reflect sunlight. Despite the importance of CTRC to the climate system, its climatological characteristics, namely how it varies with space and time, remain unknown. This work fills this knowledge gap. We generate the product of the CTRC over the global ocean using a novel satellite methodology developed in our previous work. Analyses of the data show that the spatial and temporal distributions of the CTRC are largely reflections of the atmospheric humidity: the drier the atmosphere, the stronger the cooling. As a result, the CTRC is weakest in the wet tropics and strongest in the dry eastern subtropical oceans, such as the west of California coast. We also use the CTRC data to train a machine‐learning algorithm that can substantially speed up the calculation of CTRC. Key Points: A one‐year's worth of global marine shallow single‐layer cloud top radiative cooling (CTRC) is derived from satellite and reanalysis dataSpatial and seasonal variations of CTRC are largely reflections of changes in free‐tropospheric humidity and cloud top temperatureA neural network model for the CTRC was trained, which substantially speeds up the retrieval while maintaining good accuracy [ABSTRACT FROM AUTHOR]

This study investigates the use of airborne in situ measurements to derive transit time distributions (TTD) from the boundary layer (BL) to the upper troposphere (UT) over the highly convective tropical western Pacific (TWP). The feasibility of this method is demonstrated using 42 volatile organic compounds (VOCs) measured during the Convective Transport of Active Species in the Tropics (CONTRAST) experiment. Two important approximations necessary for the application are the constant chemical lifetimes for each compound and the representation of the BL source by the local CONTRAST data. To characterize uncertainties associated with the first approximation, we quantify the changes in derived TTDs when chemical lifetimes are estimated using conditions of the BL, UT, and tropospheric average. With the support of a trajectory model study in a companion paper, we characterize the BL source region contributing to the transport to the sampled UT. In addition to the TTDs derived using a regional average, we analyze the potential information content in locally averaged measurements to represent the dynamical variability of the region. Around 150 TTDs, derived using measurements on a ∼100 km spatial scale, show a distribution of mean and mode transit times consistent with the wide range of convective conditions encountered during the campaign. Two extreme cases, with the shortest and the near‐longest TTD, are examined using the dynamical background of the measurements and back trajectory analyses. The result provides physical consistency supporting the hypothesis that sufficient information can be obtained from measurements to resolve dynamical variability of the region. Plain Language Summary: In this study, we use measurements of the atmospheric chemical composition to derive vertical transport timescales over the tropical western Pacific, where convective processes dominate. A large suite of trace gases with a wide range of chemical lifetimes are measured from a research aircraft. The trace gas measurements are used to construct "transit time distributions," which capture the relative contributions of various transport pathways to the chemical composition of the upper troposphere in the study region. Results highlight the wealth of information in atmospheric chemical composition measurements, which is important not only for understanding chemistry of the atmosphere but also for quantifying aspects of atmospheric dynamics and circulation. Key Points: The approximations and uncertainties in deriving airborne chemical observation‐based tropospheric transit time distributions are examinedThe necessary approximation of trace gas lifetimes is found to be the largest source of uncertaintyThe method successfully identified a range of dynamical transport variability from aircraft measurements [ABSTRACT FROM AUTHOR]

The hydroxyl radical (OH) is a powerful oxidant in the troposphere controlling the atmospheric lifetimes of many short‐lived climate forcers such as methane. In this study, the GFDL‐AM4.1 model is used to investigate the meteorological impacts on OH and the methane budget and lifetime over 1980–2017. Driven by meteorological reanalyses from the National Centers for Environmental Prediction and the Modern‐Era Retrospective analysis for Research and Applications (Version 2), our model gives an 11.2 Tg yr−1 difference in the derived methane emissions and 0.24 years difference in the estimated methane lifetime. The results suggest that meteorology affects the mean OH concentrations but not OH trend, while the latter shows strong correlation with reactive nitrogen emissions. Despite the relatively small difference in global tropospheric OH (∼2%) induced by different meteorological forcings, the difference is much larger regionally, leading to a difference of 8 Tg yr−1 in tropical methane emissions. Plain Language Summary: The hydroxyl radical (OH) is extremely reactive in the atmosphere and able to destroy many other chemicals, such as methane, a strong greenhouse gas that contributes significantly to global warming. Therefore, OH is very important for methane concentrations and lifetime. Changes in the meteorological features (e.g., temperature, wind patterns, and relative humidity) would affect OH in the atmosphere. In this study, we use a three‐dimensional numerical model to understand the meteorological impacts on OH and the resulting impacts on methane budget and lifetime over 1980–2017. With different meteorological datasets, we find there is a 2% difference in global mean tropospheric OH concentrations, with much larger differences over tropics. We calculate methane sources and loss due to OH and find an 11.2 Tg yr−1 difference in the global mean methane sources with 8 Tg yr−1 difference in the tropics, and 0.24 years difference in methane lifetime between the two meteorological datasets. Key Points: Hydroxyl radical (OH) abundances vary by up to 2% globally and by larger amounts regionally (>10%) when nudged with different meteorological reanalysesMeteorology‐driven differences in OH lead to differences of 11.2 Tg yr−1 in estimated methane emissions and 0.24 years in methane lifetimeMeteorology affects OH concentrations but not the OH trend, with the latter showing strong correlation with reactive nitrogen emissions [ABSTRACT FROM AUTHOR]

Hydrogen peroxide (H2O2) acts as a terminal sink for atmospheric HOx radicals (OH and HO2), playing a key role in tropospheric O3 formation. However, there are few field measurements of atmospheric H2O2 to assess its role in O3 formation, especially for the seriously polluted region of the North China Plain. In this study, H2O2 concentrations were measured at the foot of Mt. Tai from May to July 2018 and the summit of Mt. Tai from May to June 2019, with average values of 0.93 ± 1.01 and 2.05 ± 1.20 ppb, respectively. H2O2 exhibited a pronounced diurnal variation with a noon‐peak at the foot of Mt. Tai, which could be well reproduced by a gas‐phase box model with H2O2 dry deposition velocity of 3 cm s−1 included, indicating H2O2 was mainly photochemically produced. Modeling analysis showed H2O2 production at the foot was most sensitive to alkenes and aromatics, while the source and sink for H2O2 were dominated by HO2 recombination and dry deposition, respectively. Compared with the summer‐measurement in 2007, the remarkable elevation of H2O2 at the summit might be ascribed to volatile organic compounds (VOCs) increase and SO2 decline. Both H2O2‐O3 correlation and H2O2/NOz ratio suggested O3 formation at the foot of Mt. Tai was mainly VOC‐sensitive in the early morning and shifted to NOx‐sensitive thereafter. Therefore, reduction of VOCs emission especially for the reactive species of alkenes and aromatics in the morning as well as NOx emission around noontime will be effective for mitigating the serious O3 (as well as H2O2) pollution in Tai'an city. Key Points: Gas‐phase hydrogen peroxide (H2O2) at the summit of Mt. Tai evidently increased from 2007 to 2018, which might be ascribed to volatile organic compounds (VOCs) increase and SO2 declineObserved H2O2 at the foot of Mt. Tai could be well reproduced by a box model with H2O2 dry deposition velocity of 3 cm s−1 includedO3 formation in Tai'an city was mainly VOC‐sensitive in the early morning and shifted to NOx‐sensitive thereafter [ABSTRACT FROM AUTHOR]

Rationale: The triple oxygen isotopic composition (Δ17O) of tropospheric ozone (O3) is a useful tracer for identifying the source and is essential for clarifying the atmospheric chemistry of oxidants. However, the single nitrite-coated filter method is inaccurate owing to the nitrate blank produced through the reaction of nitrite and oxygen compounds other than O3. Methods: A multistep nitrite-coated filter-pack system is newly adopted to transfer the O-atoms in terminal positions of O3 to nitrite on each filter to determine the Δ17O of O3 in terminal positions (denoted as Δ17O(O3)term). The NO3- produced by this reaction is chemically converted into N2O, and continuous-flow isotope ratio mass spectrometry (CF-IRMS) is used to determine the oxygen isotopic compositions. Results: The reciprocal of the NO3- quantities on the nitrite-coated filters in each sample showed a strong linear relationship with Δ17O of NO3-. Using the linear relation, we corrected the changes in Δ17O of NO3- on the filters. We verified the accuracy of the new method through the measurement of artificial O3 with known Δ17O(O3)term value that had been determined from the changes in Δ17O of O2. The Δ17O(O3)term of tropospheric O3 was in agreement with previous studies. Conclusions: We accurately determined the d18O and Δ17O values of tropospheric O3 by blank correction using our new method. Measurements of Δ17O(O3)term of the ambient troposphere showed 1.1 ± 0.7? diurnal variations between daytime (higher) and nighttime (lower) due likely to the formation of the temperature inversion layer at night. [ABSTRACT FROM AUTHOR]