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We use measurements of trace gases from the Microwave Limb Sounder and polar stratospheric clouds (PSCs) from the Cloud‐Aerosol Lidar with Orthogonal Polarization to investigate how the extraordinary stratospheric water vapor enhancement from the 2022 Hunga eruption affected polar processing during the 2023 Antarctic winter. Although the dynamical characteristics of the vortex itself were generally unexceptional, the excess moisture initially raised PSC formation threshold temperatures above typical values. Cold conditions, especially in early July, prompted ice PSC formation and unusually severe irreversible dehydration at higher levels (500–700 K), while atypical hydration occurred at lower levels (380–460 K). Heterogeneous chemical processing was more extensive, both vertically (up to 750–800 K) and temporally (earlier in the season), than in prior Antarctic winters. The resultant HCl depletion and ClO enhancement redefined their previously observed ranges at and above 600 K. Albeit unmatched in the satellite record, the early‐winter upper‐level chlorine activation was insufficient to induce substantial ozone loss. Chlorine activation, denitrification, and dehydration processes ran to completion by July/August, with trace gas evolution mostly following the climatological mean thereafter, but with chlorine deactivation starting slightly later than usual. While cumulative ozone losses at 410–550 K were relatively large, probably because of the delayed chlorine deactivation, they were not unprecedented. Thus, ozone depletion was unremarkable throughout the lower stratosphere. Although Hunga enhanced PSC formation and chemical processing in early winter, saturation of lower stratospheric denitrification, dehydration, and chlorine activation (as is typical in the Antarctic) prevented an exceptionally severe ozone hole in 2023. The 2022 eruption of the Hunga volcano produced an increase in stratospheric water vapor unmatched in the satellite record. Excess moisture pervaded the Antarctic stratosphere at the beginning of the 2023 Southern Hemisphere winter. Cold moist conditions prompted formation of ice clouds in the Antarctic stratosphere at higher altitudes than usual. Falling ice particles caused unusually severe permanent removal of water vapor from these levels and deposited it at lower altitudes. Chemical processing on polar stratospheric cloud particles occurred both earlier in the season and over a broader vertical domain than typical, resulting in exceptional early‐winter upper‐level conversion of chlorine from nonreactive into ozone‐destroying forms. Chemical processing ran its course by midwinter (i.e., July/August), with trace gases, including ozone, mostly following average behavior thereafter. Deactivation (reconversion of chlorine into nonreactive forms) started slightly later than in most years. While cumulative seasonal ozone losses were relatively large at some levels, probably because of the delayed chlorine deactivation, they were not unprecedented. Thus, although Hunga accelerated and expanded the vertical extent of chemical processing in early winter, that chemical processing reached completion as usually happens in Antarctic midwinter, preventing an extraordinarily severe ozone hole in 2023. Hydration from Hunga led to unusually early and vertically extensive polar stratospheric cloud activity in the 2023 Antarctic vortexAs a result, heterogeneous chlorine activation occurred weeks earlier and at higher altitudes and dehydration removed more H2O than typicalDespite exceptional early‐winter conditions, cumulative ozone losses were unremarkable because stratospheric chemical processing saturated Hydration from Hunga led to unusually early and vertically extensive polar stratospheric cloud activity in the 2023 Antarctic vortex As a result, heterogeneous chlorine activation occurred weeks earlier and at higher altitudes and dehydration removed more H2O than typical Despite exceptional early‐winter conditions, cumulative ozone losses were unremarkable because stratospheric chemical processing saturated

The January 2022 eruption of Hunga Tonga‐Hunga Ha'apai (HT‐HH) caused the largest enhancement in stratospheric aerosol loading in decades and produced an unprecedented enhancement in stratospheric water vapor, leading to strong stratospheric cooling that in turn induced changes in the large‐scale circulation. Here we use satellite measurements of gas‐phase constituents together with aerosol extinction to investigate the extent to which the thick aerosol, excess moisture, and strong cooling enabled heterogeneous chemical processing. In the southern tropics, unambiguous signatures of substantial chlorine and nitrogen repartitioning appear over a broad vertical domain almost immediately after the eruption, with depletion of N2O5, NOx, and HCl accompanied by enhancement of HNO3, ClO, and ClONO2. After initially rising steeply, HNO3and ClO plateau, maintaining fairly constant abundances for several months. These patterns are consistent with the saturation of N2O5hydrolysis, suggesting that this reaction is the primary mechanism for the observed composition changes. The southern midlatitudes and subtropics show similar but weaker enhancements in ClO and ClONO2. In those regions, however, effects of anomalous transport dominate the evolution of HNO3and HCl, obscuring the signs of heterogeneous processing. Perturbations in chlorine species are considerably weaker than those measured in the southern midlatitude stratosphere in 2020 following the Australian New Year's fires. The moderate HT‐HH‐induced enhancements in reactive chlorine seen throughout the southern middle and low‐latitude stratosphere, far smaller than those in typical winter polar vortices, do not lead to appreciable chemical ozone loss; rather, extrapolar lower‐stratospheric ozone remains primarily controlled by dynamical processes. The January 2022 eruption of the Hunga Tonga‐Hunga Ha'apai (HT‐HH) volcano caused the largest increase in stratospheric aerosol in decades and produced an unprecedented increase in stratospheric water vapor, which led to strong stratospheric cooling that in turn altered stratospheric winds. Here we use satellite measurements to investigate the extent to which the thick aerosol, excess moisture, and strong cooling enabled heterogeneous chemical processing. In the southern tropics, unambiguous signatures of substantial changes in several chlorine and nitrogen species appear over a broad vertical range almost immediately after the eruption, with decreases in some species mirrored by increases in others. The southern midlatitudes and subtropics show similar but weaker evidence of chemical processing. In those regions, however, effects of anomalous wind patterns obscure the signs of heterogeneous processing in some species. Perturbations in chlorine are considerably weaker than those measured in the southern midlatitude stratosphere following the Australian New Year's fires in 2020. The moderate HT‐HH‐induced enhancements in reactive chlorine seen throughout the southern middle and low‐latitude stratosphere, far smaller than those in typical winter polar vortices, do not lead to appreciable chemical ozone loss; rather, outside the polar regions, lower‐stratospheric ozone remains primarily controlled by dynamical processes. Satellite data show widespread stratospheric chlorine and nitrogen repartitioning in 2022 following the Hunga Tonga‐Hunga Ha'apai eruptionObserved composition changes are consistent with heterogeneous processing on volcanic sulfate aerosol, in particular the hydrolysis of N2O5Moderate enhancements in reactive chlorine in the southern mid‐ and low‐latitude stratosphere did not cause appreciable chemical ozone loss Satellite data show widespread stratospheric chlorine and nitrogen repartitioning in 2022 following the Hunga Tonga‐Hunga Ha'apai eruption Observed composition changes are consistent with heterogeneous processing on volcanic sulfate aerosol, in particular the hydrolysis of N2O5 Moderate enhancements in reactive chlorine in the southern mid‐ and low‐latitude stratosphere did not cause appreciable chemical ozone loss

We present a case study based on balloon‐borne ozone measurements during the SouthEast American Consortium for Intensive Ozonesonde Network Study in August–September 2013. Data from Socorro, NM (34°N, 107°W) show a layer of anomalously low ozone in the upper troposphere (UT) during 8–14 August. Back trajectories, UT jet analyses, and data from the Microwave Limb Sounder (MLS) on the Aura satellite indicate that this feature originated from the marine boundary layer in the eastern/central tropical Pacific, where several disturbances and one hurricane (Henriette) formed within an active region of the Intertropical Convergence Zone in early August 2013. The hurricane and nearby convection pumped boundary layer air with low ozone (20–30 ppbv) into the UT. This outflow was advected to North America 3–5 days later by a strong subtropical jet, forming a tongue of low ozone observed in MLS fields and a corresponding layer of low ozone in Socorro vertical profiles. Impacts of a tropical cyclone are seen in tropospheric ozone at midlatitudes

Following the 15 January 2022 Hunga Tonga‐Hunga Ha'apai eruption, several trace gases measured by the Aura Microwave Limb Sounder (MLS) displayed anomalous stratospheric values. Trajectories and radiance simulations confirm that the H2O, SO2, and HCl enhancements were injected by the eruption. In comparison with those from previous eruptions, the SO2and HCl mass injections were unexceptional, although they reached higher altitudes. In contrast, the H2O injection was unprecedented in both magnitude (far exceeding any previous values in the 17‐year MLS record) and altitude (penetrating into the mesosphere). We estimate the mass of H2O injected into the stratosphere to be 146 ± 5 Tg, or ∼10% of the stratospheric burden. It may take several years for the H2O plume to dissipate. This eruption could impact climate not through surface cooling due to sulfate aerosols, but rather through surface warming due to the radiative forcing from the excess stratospheric H2O. The violent Hunga Tonga‐Hunga Ha'apai eruption on 15 January 2022 not only injected ash into the stratosphere but also large amounts of water vapor, breaking all records for direct injection of water vapor, by a volcano or otherwise, in the satellite era. This is not surprising since the Hunga Tonga‐Hunga Ha'apai caldera was formerly situated 150 m below sea level. The massive blast injected water vapor up to altitudes as high as 53 km. Using measurements from the Microwave Limb Sounder on NASA's Aura satellite, we estimate that the excess water vapor is equivalent to around 10% of the amount of water vapor typically residing in the stratosphere. Unlike previous strong eruptions, this event may not cool the surface, but rather it could potentially warm the surface due to the excess water vapor. Following the Hunga Tonga‐Hunga Ha'apai eruption, the Aura Microwave Limb Sounder measured enhancements of stratospheric H2O, SO2, and HClThe mass of SO2and HCl injected is comparable to that from prior eruptions, whereas the magnitude of the H2O injection is unprecedentedExcess stratospheric H2O will persist for years, could affect stratospheric chemistry and dynamics, and may lead to surface warming Following the Hunga Tonga‐Hunga Ha'apai eruption, the Aura Microwave Limb Sounder measured enhancements of stratospheric H2O, SO2, and HCl The mass of SO2and HCl injected is comparable to that from prior eruptions, whereas the magnitude of the H2O injection is unprecedented Excess stratospheric H2O will persist for years, could affect stratospheric chemistry and dynamics, and may lead to surface warming

Stratospheric water vapour is a powerful greenhouse gas. The longest available record from balloon observations over Boulder, Colorado, USA shows increases in stratospheric water vapour concentrations that cannot be fully explained by observed changes in the main drivers, tropical tropopause temperatures and methane. Satellite observations could help resolve the issue, but constructing a reliable long-term data record from individual short satellite records is challenging. Here we present an approach to merge satellite data sets with the help of a chemistry–climate model nudged to observed meteorology. We use the models’ water vapour as a transfer function between data sets that overcomes issues arising from instrument drift and short overlap periods. In the lower stratosphere, our water vapour record extends back to 1988 and water vapour concentrations largely follow tropical tropopause temperatures. Lower and mid-stratospheric long-term trends are negative, and the trends from Boulder are shown not to be globally representative. In the upper stratosphere, our record extends back to 1986 and shows positive long-term trends. The altitudinal differences in the trends are explained by methane oxidation together with a strengthened lower-stratospheric and a weakened upper-stratospheric circulation inferred by this analysis. Our results call into question previous estimates of surface radiative forcing based on presumed global long-term increases in water vapour concentrations in the lower stratosphere.

The 2019/2020 Australian New Year's wildfires injected record amounts of smoke and biomass burning products into the lower stratosphere. The Aura Microwave Limb Sounder (MLS) tracked the evolution of distinct plumes of fire–influenced air as they rapidly spiraled up to the mid–stratosphere. In the months following the fires, smoke spread throughout the Southern Hemisphere (SH) stratosphere. We contrast the evolution of the SH midlatitude lower stratosphere in 2020 with the 17–year MLS record. Long after the coherent plumes dispersed, data from MLS and other satellite instruments show unprecedented persistent and pervasive depletion in HCl (50%–60% below climatology) and enhancements in ClO and ClONO2that were not transport related; peak anomalies occurred in mid–2020. We conclude that the observed perturbations likely arose from heterogeneous chlorine activation on widespread smoke particles. The sustained chlorine activation was far weaker than in typical winter polar vortices, inducing at most minor changes in ozone. In late December 2019/early January 2020, during the Black Summer fire season in Australia, an intense outbreak of a number of fire–induced and smoke–infused thunderstorms occurred, collectively known as the Australian New Year's event. Multiple plumes of polluted air were rapidly lofted from the surface and deposited in the lower stratosphere (above 14 km altitude). The largest of the plumes of biomass burning pollutants were tracked by the Microwave Limb Sounder (MLS) on NASA's Aura satellite for several months as they circled the globe. In addition to those distinct confined plumes, smoke from the fires permeated the Southern Hemisphere (SH) lower stratosphere. Well after the discrete plumes dispersed, MLS and other satellite instruments observed widespread and long–lasting perturbations in the composition of the SH midlatitude lower stratosphere. In particular, unprecedented depletion in the main stratospheric chlorine reservoir species (HCl) was accompanied by enhancements in another chlorine reservoir (ClONO2) as well as the ozone–destroying form of chlorine (ClO), with maximum departures from average measured in mid–2020. These anomalous signatures likely arose from chemical processing on smoke particles. The enhancement in the ozone–destroying form of chlorine, far weaker than that in winter polar regions, had only minor effects on ozone. Satellite data show strongly depleted HCl and enhanced ClO and ClONO2in the southern midlatitude lower stratosphere through much of 2020The unprecedented midlatitude chlorine repartitioning likely arose from heterogeneous activation on widespread Australian wildfire smokeThe sustained midlatitude chlorine activation was far weaker than in typical winter polar vortices, inducing at most minor changes in ozone Satellite data show strongly depleted HCl and enhanced ClO and ClONO2in the southern midlatitude lower stratosphere through much of 2020 The unprecedented midlatitude chlorine repartitioning likely arose from heterogeneous activation on widespread Australian wildfire smoke The sustained midlatitude chlorine activation was far weaker than in typical winter polar vortices, inducing at most minor changes in ozone

We have used a new, isentropic‐coordinate three‐dimensional chemical transport model to investigate the decay of ClO and evolution of other species in the Antarctic polar vortex during September 1992. The model simulations cover the same southern hemisphere period studied in a companion data paper by Santee et al.[this issue]. The model is initialized using the available data from the Microwave Limb Sounder (MLS) and Cryogenic Limb Array Etalon Spectrometer (CLAES) on the Upper Atmosphere Research Satellite (UARS). During the model initialization chemical inconsistencies in the UARS data became evident. Fields of odd nitrogen (NOy) derived from CLAES N2O underestimated the sum of the direct observations of the major NOyspecies. Results from the model integrations at 465 K and 585 K are sampled in the same way as the various UARS instruments and compared to the observations both directly and by considering average quantities in the inner and edge vortex regions. Sampling the observed species in the same way as the UARS instruments is important in removing any spurious trends due, for example, to changing solar zenith angle. While the model can reproduce the magnitude of the MLS ClO observations at 585 K, this is not possible at 465 K. The model partitions too much ClO into Cl2O2to reproduce the observed ClO which is around 2.0 parts per billion by volume (ppbv) averaged within the polar vortex. The model also underestimates CLAES ClONO2in the inner vortex at 465 K due to heterogeneous processing. The observations require that effectively all of the inorganic chlorine is in the form of ClO and ClONO2in the inner vortex at this altitude. In the basic model run, the decay of ClO produces ClONO2which is not observed by CLAES. Our results indicate the potential importance of the speculative reaction between OH and ClO producing HCl for the recovery of HCl in the Antarctic spring. By including this reaction, the decay of model ClO into HCl is enhanced, yielding better agreement with HCl data from the Halogen Occultation Experiment (HALOE) data. Similar results can also be obtained by including the reaction between HO2and ClO to produce HCl with a 3% channel. The model generally reproduces the observed O3destruction during September. The most significant discrepancy for O3is in the inner vortex at 465 K where the model underestimates the observed O3loss rate, especially when the effects of vertical motion are included.

Initial results of upper-tropospheric water vapor obtained from the Microwave Limb Sounder (MLS) on the Upper Atmosphere Research Satellite (UARS)are presented. MLS is less affected by clouds than infrared or visible techniques, and the UARSorbit provides daily humidity monitoring for approximately two-thirds of the earth. Best results are currently obtained when water vapor abundances are approximately 100–300 ppmv, corresponding to approximately 12-km height in the Tropics and 7 km at high latitudes. The observed latitude variation of water vapor at 215 hPa is in good agreement with the U.K. Universities's Global Atmospheric Modelling Project model. The ability to observe synoptic-scale features associated with tropopause height variations is clearly illustrated by comparison with the National Aeronautics and Space Administration Goddard Space Flight Center assimilation model. Humidity detrainment streams extending from tropical convective regions are also observed.

Recovery from enhanced chlorine conditions in the lower stratospheric polar regions of both hemispheres is investigated using data from the Upper Atmosphere Research Satellite (UARS). Microwave Limb Sounder (MLS) measurements of ClO within the polar vortices are used to infer ClOx(ClO + 2Cl2O2) abundances that are then correlated with simultaneous Cryogenic Limb Array Etalon Spectrometer (CLAES) measurements of ClONO2and Halogen Occultation Experiment (HALOE) measurements of HCl obtained starting within 5 days of the end of the MLS and CLAES high‐latitude observing periods in each hemisphere. Time series of vortex‐averaged mixing ratios are calculated on two potential temperature surfaces (585 K and 465 K) in the lower stratosphere for approximately month‐long intervals during late winter: August 17 – September 17, 1992, in the southern hemisphere and February 12 – March 16, 1993, in the northern hemisphere. The observed mixing ratios are adjusted for the effects of vertical transport using diabatic vertical velocities estimated from CLAES tracer data. In the northern hemisphere, the decrease in ClOxis balanced on both surfaces by an increase in ClONO2. In the southern hemisphere, continuing polar stratospheric cloud activity prevents ClO from undergoing sustained decline until about September 3. In contrast to the northern hemisphere, there is no significant chemical change in vortex‐averaged ClONO2at 465 K, and there is an apparent decrease in ClONO2at 585 K, even after the enhanced ClO abundances have started to recede. Results from the SLIMCAT chemical transport model [Chipperfield et al., this issue] initialized with UARS data and run with OH + ClO → HCl + O2as an 8% channel suggest that the primary recovery product in the south during this time period is not ClONO2, but HCl. HALOE HCl mixing ratios are extrapolated back to the time of the MLS and CLAES data. At 585 K, the chlorine budget can be made to balance by extrapolating HCl back to a value of 0.6 parts per billion by volume (ppbv) at the beginning of the study period; at 465 K, the contribution from extrapolated HCl is not sufficient to offset the loss in ClOx, and there is a slight imbalance between the decrease in reactive chlorine and the change in chlorine reservoirs. The difficulty in closing the chlorine budget in the southern hemisphere may arise from complications caused by ongoing activation, incomplete photochemical assumptions, and/or inadequate data quality.