The hydroxyl radical (OH) is chemically coupled to other atmospheric constituents including water vapor, NOx, ozone, CO, and methane that provide the sources and sinks of OH. These species have longer lifetimes than OH itself and consequently undergo atmospheric transport, allowing dynamics to indirectly affect OH. We investigated whether a combination of meteorological variables and idealized tracers can predict the OH distributions for 40 degrees S-40 degrees N simulated by multiple models. We find that they can explain 70% or more of the variance in July spatial anomalies in OH with the zonal mean removed at 400 hPa, and 59% or more for tropospheric column OH (tcolOH). We find two constituents observed from space, water vapor and NO2, can together serve as proxies for much of the 40 degrees S-40 degrees N spatial variability in OH at 400 hPa, especially over the ocean. Multiple linear regression (MLR) on water vapor and NO2 columns versus tcolOH results in r 2 > 0.5 for the interannual variability in January tcolOH over more than half of the 40 degrees S-40 degrees N domain in most models. These results highlight the value of satellite observations of water vapor and NO2 for constraining simulated OH variability. However, the relative sensitivity of OH to each of these two variables differs between models. Consequently, understanding individual models' relative sensitivities can help maximize the value of these observational constraints. The results of our proof-of-concept study are encouraging and justify additional research to fully explore the potential of other satellite-observable variables for the development of process-based diagnostics and constraining the spatiotemporal variations of tropospheric OH.

We evaluate the climate response of the NASA Goddard Earth Observing System (GEOS) coupled Atmosphere-Ocean General Circulation Model (AOGCM) to idealized quadrupling of CO2 concentrations. A pre-industrial simulation and an abrupt 4 & times; CO2 simulation are conducted with the GEOS AOGCM following the Coupled Model Intercomparison Project (CMIP) approach. The 4 & times; CO2 run shows expected tropospheric and stratospheric climate response features, including Arctic amplification, intensification of the hydrologic cycle, Hadley Cell expansion, a poleward shift of the Southern Hemisphere jet stream, acceleration of the Brewer-Dobson circulation, and a weaker Quasi-Biennial Oscillation. At the ocean surface, we see a complete disappearance of Arctic sea ice in July-December, and in the Southern Hemisphere, there is a poleward shift of the surface wind stress that drives the Southern Ocean meridional overturning circulation poleward. In the ocean, there is enhanced heat content but reduced heat export to the high northern latitudes coincident with a slowing of the Atlantic Meridional Overturning Circulation. The Equilibrium Climate Sensitivity (ECS) of the GEOS AOGCM is estimated to be 2.55 K, in the lowest quartile of the CMIP5 and CMIP6 ranges. The low ECS is due to a strong negative feedback and a weak effective radiative forcing.

Stratospheric temperature, ozone, and constituent observations show significant perturbations in the 1-2 years following the January 2022 Hunga volcanic eruption. This study uses GSFC2D model simulations forced with satellite-based Hunga aerosol and water vapor anomalies to investigate the resulting impacts in the Southern Hemisphere stratosphere (tropics to midlatitudes) during the 2 & frac14; years following the eruption. The relative impacts of the volcanic forcings are quantified and compared with dynamical changes caused by the quasi-biennial oscillation (QBO), which exerts a substantial influence. Largest volcanic-driven chemical and radiative changes occur during 2022 and gradually diminish thereafter. Significant model enhancements in HNO3 and ClO, and corresponding reductions in NO2 and HCl, are driven by the Hunga aerosol via heterogeneous reactions, and the Hunga water vapor perturbation via gas-phase reactions. However, for model ozone, the net volcanic chemical impacts are small. For total column ozone, the aerosol-induced effects are mostly negative with maximum changes of -3 DU, while the water vapor-induced impacts are mostly positive with maximum changes of similar to+1 DU. Simulated total ozone anomalies driven by transport are significantly larger (+/- 5-12 DU), consistent with previous studies. Model temperature anomalies are driven mainly by a combination of the QBO (+/- 1-2 K) and radiative cooling of the Hunga H2O plume (-2-3 K). The Hunga aerosol had a minor impact on stratospheric temperature, causing a net warming of at most +0.5 K.

Previous studies on net ozone production rates (PO3) and their sensitivities to precursors relied on limited in-situ data, often coarse and uncertain chemical transport models (CTMs), and ozone indicators like the formaldehyde-to-nitrogen dioxide ratio (FNR). However, FNR fails to fully capture PO3's complex relationships with pollution, light, and water vapor. To address this, we refine the satellite-based PO3 product from Souri et al. (2025) with key advancements: (i) a deep neural network to parametrize high-dimensional non-linear ozone chemistry without the need for empirical linearization of atmospheric conditions, (ii) incorporation of water vapor, (iii) improved error characterization, and (iv) the application of a finer CTM to dynamically convert column retrievals into near-surface mixing ratios. Our PO3 sensitivity maps surpass traditional FNR-based assessments by quantifying sensitivity magnitudes - factoring in photolysis rates and water vapor - with greater spatial information. Our new product provides daily near-clear sky PO3 and sensitivity maps using bias-corrected OMI (2005-2019, 0.25 degrees x 0.25 degrees) and TROPOMI (2018-2023, 0.1 degrees x 0.1 degrees), with values aligning within 10 %. High PO3 rates (> 8 ppbv h(-1)) appear in urban and biomass-burning regions under strong photochemical activity, including during a heatwave in the northeastern U.S. Photolysis rates are the dominant factor dictating the seasonality of PO3 magnitudes and sensitivities. The stability and long-term records of OMI retrievals (2005-2019) enable us to provide the first global maps of PO3 linear trends showing a surge of > 30 % over China, the Middle East, and India, while a reduction in the eastern U.S., southern Europe, and several regions in Africa.

The unprecedented water vapor amount (WV, 150–160 Tg) injected by the 2022 eruption of Hunga Tonga–Hunga Haʻapai not only directly cooled the stratosphere, but also facilitated the formation and growth of sulfate particles, indirectly heating it. Here, we developed analytical models constrained by satellite observations to quantify these contrasting roles of WV in stratospheric temperature perturbations. Our analysis revealed that condensation and nucleation processes facilitated by abundant WV accounted for 90

The 2022 Hunga volcanic eruption injected a significant amount of water vapor and a moderate amount of sulfur dioxide into the stratosphere, causing observable responses in the climate system. We have developed a model-observation comparison project to investigate the evolution of volcanic water and aerosols and their impacts on atmospheric dynamics, chemistry, and climate, using several state-of-the-art chemistry climate models. The project goals are (1) to evaluate the current chemistry-climate models to quantify their performance in comparison to observations and (2) to understand atmospheric responses in the Earth system after this exceptional event and investigate the potential impacts in the projected future. To achieve these goals, we designed specific experiments for direct comparisons to observations, for example from balloons and the Microwave Limb Sounder satellite instrument. Experiment 1 consists of two sets of free-running ensemble experiments from 2022 to 2031: one with fixed sea-surface temperatures and sea ice and one with coupled ocean. These experiments will help to understand the long-term evolution of water vapor and aerosols; quantify HTHH effects on stratospheric and mesospheric temperatures, dynamics, and transport; understand the impact of dynamic changes on ozone chemistry; quantify the net radiative forcings; and evaluate any surface climate impact. Experiment 2 is a nudged-run experiment from 2022 to 2023 using observed meteorology. To allow participation of more climate models with varying complexities of aerosol simulation, we include two sets of simulations in Experiment 2: Experiment 2a is designed for models with internally generated aerosol, while Experiment 2b is designed for models using prescribed aerosol surface area density. This experiment will help to analyze H2O and aerosol evolution, quantify the net radiative forcings, understand the impacts on mid-latitude and polar O3 chemistry, and allow close comparisons with observations.

The accurate representation of tropospheric hydroxyl radical (TOH) is crucial for reasonably modeling methane concentrations — a potent greenhouse gas. We use an improved parameterization of TOH using an interpretable and agile machine learning module named ECCOH (pronounced "echo") in NASA's GEOS global model to unravel the intricacies of TOH to its key inputs. However, the accuracy of this model is hampered by the accurate representation of its critical inputs. Fortunately, retrieving trace gases like nitrogen dioxide (NO2) and formaldehyde (HCHO) from space-borne sensors, like the Aura Ozone Monitoring Instrument (OMI), has seen remarkable progress. Consequently, we leverage these observations to assess how they can effectively alleviate some biases in TOH and can help better reproduce its long-term trends. In contrast to the earlier investigations, the refined representation of TOH archives a finer spatial resolution (1x1 degrees), and it is more up to date (2005-2019), allowing for elucidating the impact of recent emission regulations, such as those imposed in China, on TOH. OMI NO2 yields valuable insights over biomass-burning areas in Eastern Europe and central Africa, where our prior emission estimates possess significant biases, mitigating regional TOH biases up to 20%. Oceanic HCHO concentrations, serving as a proxy for TOH due to the predominant chemical pathway of VOC oxidation through OH, are only moderately altered by OMI HCHO, attributed to low signal-to-noise ratios and satisfactory representation of HCHO in the a priori simulations. Ultimately, we disentangle the convoluted map of TOH linear trends by isolating five pivotal inputs to the TOH parameterization, including stratospheric ozone, tropospheric ozone, water vapor, HCHO, and NO2. Our results demonstrate that these five parameters can collectively explain 65% of the variability in TOH trends alone. With the deployment of new satellites with enhanced sensor configurations and better temporal resolutions, our mission at NASA is to exploit those observations to improve the representation of many variables highly linked to TOH.

The hydroxyl radical (OH) plays a central role in tropospheric chemistry, as well as influencing the lifetimes of some greenhouse gases. Because of limitations in our ability to observe OH, we have historically relied on indirect methods to constrain its concentrations, trends, and variations but only as annual global or annual semi-hemispheric averages. Recent methods demonstrated the feasibility of indirectly constraining tropospheric OH on finer spatio-temporal scales using satellite observations as proxies for the photochemical drivers of OH (e.g., nitrogen dioxide, formaldehyde, isoprene, water vapor, ozone). We found that there are currently reasonable satellite proxies to constrain up to about 75 % of the global sources of tropospheric OH and up to about 50 % of the global sinks. With additional research and investment in observing various volatile organic compounds, there is potential to constrain an additional 10 % of the global sources and 30 % of the global sinks. We propose steps forward for the development of a comprehensive space-based observing strategy, which will improve our ability to indirectly constrain OH on much finer spatio-temporal scales than previously achieved. We discuss the strengths and limitations of such an observing strategy and potential improvements to current satellite instrument observing capabilities that would enable better constraint of OH. Suborbital observations (i.e., data collected from non-satellite platforms such as aircraft, balloons, and buildings) are required to collect information difficult to obtain from space and for validation of satellite-based OH estimates; therefore, they should be an integral part of a comprehensive observing strategy.

The tropospheric hydroxyl (TOH) radical is a key player in regulating oxidation of various compounds in Earth's atmosphere. Despite its pivotal role, the spatiotemporal distributions of OH are poorly constrained. Past modeling studies suggest that the main drivers of OH, including NO2, tropospheric ozone (TO3), and H2O(v), have increased TOH globally. However, these findings often offer a global average and may not include more recent changes in diverse compounds emitted on various spatiotemporal scales. Here, we aim to deepen our understanding of global TOH trends for more recent years (2005–2019) at 1×1°. To achieve this, we use satellite observations of HCHO and NO2 to constrain simulated TOH using a technique based on a Bayesian data fusion method, alongside a machine learning module named the Efficient CH4-CO-OH (ECCOH) configuration, which is integrated into NASA's Goddard Earth Observing System (GEOS) global model. This innovative module helps efficiently predict the convoluted response of TOH to its drivers and proxies in a statistical way. Aura Ozone Monitoring Instrument (OMI) NO2 observations suggest that the simulation has high biases for biomass burning activities in Africa and eastern Europe, resulting in a regional overestimation of up to 20 % in TOH. OMI HCHO primarily impacts the oceans, where TOH linearly correlates with this proxy. Five key parameters, i.e., TO3, H2O(v), NO2, HCHO, and stratospheric ozone, can collectively explain 65 % of the variance in TOH trends. The overall trend of TOH influenced by NO2 remains positive, but it varies greatly because of the differences in the signs of anthropogenic emissions. Over the oceans, TOH trends are primarily positive in the Northern Hemisphere, resulting from the upward trends in HCHO, TO3, and H2O(v). Using the present framework, we can tap the power of satellites to quickly gain a deeper understanding of simulated TOH trends and biases.

Observations of March 2024 Arctic (63 degrees N-90 degrees N) total column ozone set a record high of 477 Dobson Units (DU) against the 1979-2023 satellite era time series. It was about 60 DU higher than average and 6 DU higher than the previous March 1979 471 DU record. Daily Arctic ozone was above average for every day in March 2024, and set record highs from 11-26 March 2024. Microwave Limb Sounder data show this record ozone anomaly was concentrated in the lower stratosphere (10-30 km). These record values developed over the 2023-2024 winter and can be associated with vertically propagating planetary-scale wave events that caused significant stratospheric warmings. These wave events forced poleward and downward ozone advection into the lower stratosphere, leading to record column ozone levels. The above average levels persisted through August 2024 and across the northern hemisphere. Man-made chlorofluorocarbons (CFCs) depleted the Earth ozone layer. The 1987 Montreal Protocol curbed CFC growth, but because CFCs have multi-decadal lifetimes, Arctic ozone is not expected to recover back to 1980 levels until similar to 2045. Current high CFC levels combined with persistent and cold polar vortices led to severe Arctic ozone spring depletion in 1997, 2011, and 2020. Contrary to expectations, March 2024 Arctic ozone showed a record high level, dramatically contrasting against the severe depletion events. Meteorological and ozone profile information show that the exceptional 2024 ozone was mainly found in the lowermost Arctic stratosphere, in association with record high lowermost stratospheric temperatures. The ozone levels incrementally increased during the 2023-2024 winter because of large-scale weather systems that propagated from the troposphere into the stratosphere. Collectively, these weather systems also were at a record level, moving higher ozone concentrations from the mid-latitudes and upper stratospheric into the Arctic region. This record high ozone would likely have not occurred if CFC levels had not begun slowly declining in response to the Montreal Protocol. Given the absence of high Arctic ozone since the 1970s, the March 2024 record high should be considered a positive harbinger of the future Arctic ozone layer. Arctic total column ozone in March 2024 set a record high for the 1979-present period Polar lower stratosphere temperatures also set a record high in March 2024 in the MERRA-2 reanalysis data A record amount of Rossby waves propagating upward from the troposphere caused the record total ozone and lower stratospheric temperature

Satellite observations and ground-based measurements have indicated a high variability in the aerosol optical depth (AOD) in the Middle East region in recent decades. In the period that extends from 2003 to 2012, observations show a positive AOD trend of 0.01–0.04 per year or a total increase of 0.1–0.4 per decade. This study aimed to investigate if the observed trend was also captured by the NASA Goddard Earth Observing System (GEOS) model. To this end, we examined changes in the simulated dust emissions and dust AOD during this period. To understand the factors driving the increase in AOD in this region we also examined meteorological and surface parameters important for dust emissions, such as wind fields and soil moisture. Two GEOS model simulations were used in this study: the Modern-Era Retrospective analysis for Research and Applications, Version 2 (MERRA-2) reanalysis (with meteorological and aerosol AOD data assimilated) and MERRA-2 Global Modeling Initiative (GMI) Replay (with meteorology constrained by the MERRA-2 reanalysis but without aerosol assimilation). We did not find notable changes in the modeled 10 m wind speed and soil moisture. However, analysis of Moderate Resolution Imaging Spectroradiometer (MODIS) normalized difference vegetation index (NDVI) data did show an average decrease of 8 % per year in the region encompassing Syria and Iraq, which prompted us to quantify the effects of vegetation on dust emissions and AOD in the Middle East region. This was done by performing a sensitivity experiment in which we enhanced dust emissions in grid cells where the NDVI decreased. The simulation results supported our hypothesis that the loss of vegetation cover and the associated increase in dust emissions over Syria and Iraq can partially explain the increase in AOD downwind. The model simulations indicated dust emissions need to be 10-fold larger in those grid cells in order to reproduce the observed AOD and trend in the model.

Selected output in NetCDF format from simulations performed with the GEOSCCM global climate model.

Large volcanic eruptions are known to influence the climate through a variety of mechanisms including aerosol-forced cooling and warming via emitted CO2. The January 2022 Hunga shallow underwater eruption caused an increase in stratospheric water vapor, and demonstrated how the associated positive radiative forcing can be an important component of an eruption's climate forcing. We present interactive stratospheric aerosol model simulations of super-volcanic eruptions with a range of SO2 emissions that can produce climate warming through feedback effects produced by a large igneous province (or "flood basalt") mid-latitude super-eruption using Goddard Earth Observing System Chemistry Climate Model climate model simulations. The model experiments suggest total SO2 emissions greater than or similar to 4,000 Tg/4 Gt generate a multi-year period of sustained aerosol absorptive local-heating of the upper troposphere and lower stratosphere and hence produce net climate warming after strong initial cooling. The eruptions produce stratospheric water vapor increases of factors of 8-600. The initiation of these feedbacks within the simulations suggest they could occur for individual stratovolcano eruptions of the scale of the Toba or Tambora eruptions. We note the sensitivity of our results to volcanic sulfate aerosol microphysics and model chemistry. Volcanic eruptions are important drivers of climate change throughout Earth's history. Using climate model experiments, we examine how massive volcanic eruptions can produce warmer climate conditions through an increase in stratospheric water vapor. We find that there is a range of volcanic sulfur dioxide emissions that can produce climate warming, but that the stratospheric water vapor increase occurs in all of our model experiments. Stratospheric water vapor increases by a factor of 8-600 for volcanic SO2 emissions of 1,875-60,000 Tg Volcanic SO2 emissions greater than or similar to 4,000 Tg can produce net climate warming through feedbacks Absorptive-heating driven increases to stratospheric water vapor likely becomes the dominant forcing for volcanic super-eruptions

The January 2022 eruption of the Hunga Tonga-Hunga Ha'apai underwater volcano injected a large amount of water vapor into the mid-stratosphere. This study uses model simulations to investigate the resulting stratospheric impacts out to 2031. Maximum radiatively-driven model temperature changes occur in the Southern Hemisphere (SH) subtropics in April-May 2022, with warming of similar to 1 K in the lower stratosphere and cooling of 3 K in the mid-stratosphere. The radiative cooling combined with adiabatic cooling driven by the quasi-biennial oscillation meridional circulation explains the near-record cold anomaly observed in the SH subtropical mid-stratosphere. Projected ozone responses maximize in 2023-2024 as the water vapor plume is transported globally throughout the stratosphere and mesosphere. The excess H2O increases the OH radical, causing a negative global ozone response (2%-10%) in the upper stratosphere and mesosphere due to increased odd hydrogen-ozone loss, and a small positive ozone response (0.5%-1%) in the mid-stratosphere due to interference of the NOx catalytic loss cycle by the additional OH. In the lower stratosphere, the excess H2O is projected to increase polar stratospheric clouds and springtime halogen-ozone loss, enhancing the Antarctic ozone hole by 25-30 DU in 2023. Arctic impact is small, with maximum additional ozone loss of 4-5 DU projected in spring 2024. These responses diminish after 2024 to be quite small by 2031, as the excess H2O is removed from the stratosphere with a 2.5-year e-folding time. Given the year-to-year variability of the stratosphere, the magnitudes of these ozone responses may be below the threshold of detectability in observations.

We examine the distribution of aerosol optical depth (AOD) across 27,707 northern hemisphere (NH) midlatitude cyclones for 2005–2018 using retrievals from the Moderate Resolution Spectroradiometer (MODIS) sensor on the Aqua satellite. Cyclone‐centered composites show AOD enhancements of 20%–45% relative to background conditions in the warm conveyor belt (WCB) airstream. Fine mode AOD accounts for 68% of this enhancement annually. Relative to background conditions, coarse mode AOD is enhanced by more than a factor of two near the center of the composite cyclone, co‐located with high surface wind speeds. Within the WCB, MODIS AOD maximizes in spring, with a secondary maximum in summer. Cyclone‐centered composites of AOD from the Modern Era Retrospective analysis for Research and Applications, version 2 Global Modeling Initiative (M2GMI) simulation reproduce the magnitude and seasonality of the MODIS AOD composites and enhancements. M2GMI simulations show that the AOD enhancement in the WCB is dominated by sulfate (37%) and organic aerosol (25%), with dust and sea salt each accounting for 15%. MODIS and M2GMI AOD are 60% larger in North Pacific WCBs compared to North Atlantic WCBs and show a strong relationship with anthropogenic pollution. We infer that NH midlatitude cyclones account for 355 Tg yr −1 of sea salt aerosol emissions annually, or 60% of the 30–80°N total. We find that deposition within WCBs is responsible for up to 35% of the total aerosol deposition over the NH ocean basins. Furthermore, the cloudy environment of WCBs leads to efficient secondary sulfate production.

Atmospheric nitrogen oxides (NOx) primarily from fuel combustion have recognized acute and chronic health and environmental effects. Machine learning (ML) methods have significantly enhanced our capacity to predict NOx concentrations at ground-level with high spatiotemporal resolution but may suffer from high estimation bias since they lack physical and chemical knowledge about air pollution dynamics. Chemical transport models (CTMs) leverage this knowledge; however, accurate predictions of ground-level concentrations typically necessitate extensive post-calibration. Here, we present a physics-informed deep learning framework that encodes advection-diffusion mechanisms and fluid dynamics constraints to jointly predict NO2 and NOx and reduce ML model bias by 21-42%. Our approach captures fine-scale transport of NO2 and NOx, generates robust spatial extrapolation, and provides explicit uncertainty estimation. The framework fuses knowledge-driven physicochemical principles of CTMs with the predictive power of ML for air quality exposure, health, and policy applications. Our approach offers significant improvements over purely data-driven ML methods and has unprecedented bias reduction in joint NO2 and NOx prediction.

Abstract The unprecedented amount (150–160 Tg) of water vapor (WV) from 2022 Hunga Tonga–Hunga Haʻapai (HT) eruption could cool the stratosphere and influence stratospheric sulfate particles formation and growth. However, it is still unclear that how much contribution from each of these diverse roles of WV to the stratospheric evolution and which role is dominant. Here, constrained by satellite observations, we develop analytical models to quantify the direct contribution of WV cooling and indirect contribution of WV affecting sulfate particles properties to stratospheric temperature modulation. For the first time, we reveal that the condensation and nucleation processes, promoted by abundant WV, contribute ~ 90% to the particle radius growth from ~ 0.2 µm to 0.35–0.45 µm after HT, accounting for observed strong aerosol extinction. This rapid growth rate is comparable to that in the first two months after the 1991 Mt. Pinatubo eruption, which emitted similar WV but ~ 80 times more sulfur dioxide. This disparity leads to stronger WV cooling than aerosols warming in the lower and middle stratosphere after HT, resulting in the strongest mid-latitude cooling since Pinatubo eruption of -8~-4 K for 4–7 months, opposite to the stratospheric warming dominated by volcanic aerosols often expected after volcanic eruptions.