Abstract. As of the early 2000s, multiple contemporaneous global and vertically resolved nitrous oxide (N2O) datasets have become available from a suite of limb-viewing satellite instruments. Together, these datasets provide a 23-year and counting continuous N2O record. Due to its long lifetime, N2O is a valuable tracer of atmospheric transport, and a reliable long-term record therefore allows the examination of changes in stratospheric circulation as a result of climate change. In order to combine results from different satellite datasets for these purposes, it is necessary to account for their biases, which can be determined through evaluation studies. Here, N2O measurements in the upper troposphere and lower-to-middle stratosphere (5–40 km) are evaluated from five different satellite limb sounders: ACE-FTS on SCISAT, HIRDLS and MLS on Aura, MIPAS on Envisat, and SMR on Odin. This is done by comparing colocated satellite measurements with each other to determine the instruments’ relative biases and by comparing each dataset with independent reference data from balloon-borne instruments. ACE-FTS (v5.2), the two MIPAS datasets (V8), and SMR (v3.0.0) agree within ±5–10 %, but HIRDLS (v07) has variable performance depending on the region and time of year. MLS (v5) has a persistent deviation resulting in a low bias of up to 30 % in the lower stratosphere at 24 km. This bias is substantially reduced in v6. These findings are corroborated by the comparisons of each satellite instrument with balloon-borne data. This study lays the groundwork for creating a merged and bias-corrected N2O time series spanning from 2002 to the present using ACE-FTS, MIPAS, SMR, and possibly the MLS v6 dataset.

The January 2022 eruption of the undersea Hunga volcano injected an unprecedented amount of water vapor directly into the stratosphere. In this talk, we will use measurements of gas-phase constituents from Aura MLS (Microwave Limb Sounder) and polar stratospheric clouds (PSCs) from CALIOP (Cloud-Aerosol Lidar with Orthogonal Polarization) on CALIPSO (Cloud-Aerosol Lidar and Infrared Pathfinder Satellite Observations) together with meteorological reanalyses to investigate how the extraordinary stratospheric hydration and accompanying anomalies in stratospheric temperature and circulation from Hunga affected chemical processing and ozone destruction in the polar lower stratosphere. We will focus on the Antarctic ozone hole season of 2023, when the excess moisture led to unusually early and vertically extensive PSC activity and heterogeneous chlorine activation (i.e., depleted HCl and enhanced ClO) in early winter. Although 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 saturated in midwinter, with trace gas evolution essentially following the climatological mean thereafter. Thus, despite the exceptional early-winter conditions, cumulative ozone losses in the 2023 austral spring were mostly unremarkable because stratospheric chemical processing saturated, as typically happens in the Antarctic. We will also discuss the 2022 Antarctic winter, when the Hunga plume was effectively excluded from the southern polar region by the strong transport barrier at the edge of the vortex. As a result, Hunga had little effect on either the vortex itself or the chemical processing and ozone loss that took place within it during the 2022 Antarctic winter/spring. Finally, we will touch briefly on the influence of Hunga on the 2023/2024 Arctic winter that will have just concluded.

The eruption of Hunga volcano on 15 January 2022 was an exceptional event in the satellite era. Record-breaking heights of the volcanic plume were reported, a large amount of water was injected into the stratosphere and a broad spectrum of atmospheric waves were detected. Here, we use satellite measurements to show that a transient ring of small ice particles (~2 μm) formed around the plume. We hypothesize that the ice ring was generated by the passage of an atmospheric wave triggered by a pressure pulse at the surface corresponding to a violent explosion that occurred during the 15 January 2022 eruption sequence. The passage of the atmospheric wave produced a transient rarefaction in the upper troposphere-lower stratosphere, which in turn led to oscillations in ambient temperature. Due to the supersaturated state of the atmosphere with respect to ice, ice particles formed in the wake of the radially propagating atmospheric wave, allowing an exceptional opportunity to study ice particle growth via vapour deposition. This atmospheric phenomenon serves as an important natural experiment that reveals the time scale on which ice particles nucleate and grow given an abrupt perturbation in ambient temperature.

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 eruption of Hunga Tonga in January 2022 injected a large amount of water into the stratosphere. Satellite measurements from Aura Microwave Limb Sounder (MLS) show that this water vapor (H2O) has now spread throughout the stratosphere and into the lower mesosphere, resulting in an increase of >1 ppmv throughout most of this region. Measurements from three ground-based Water Vapor Millimeter Wave Spectrometer (WVMS) instruments and MLS are in good agreement, and show that in 2023 there was more H2O in the lower mesosphere than at any time since the WVMS measurements began in the 1990’s. At Table Mountain, California all WVMS H2O measurements at 54 km since June 2023, and all of the measurements from Mauna Loa, Hawaii, since the resumption of measurements in September 2023, show larger mixing ratios than any previous measurements. At 70 km several recent ~1 week WVMS retrievals in the last few months show the largest anomalies ever measured. The MLS measurements show that maximum H2O anomalies have occurred throughout almost all of the stratosphere and lower mesosphere since the eruption. As of November 2023, almost all of the ~140 Tg of water originally injected into the stratosphere by the Hunga Tonga eruption remains in the middle atmosphere at pressures below 83 hPa (altitudes above ~17 km). The eruption occurred during a period when stratospheric H2O was already slightly elevated above the 2004-2021 MLS average, and the November 2023 anomaly of ~160 Tg represents ~15% of the total mass of H2O in this region.

The 2022 Hunga eruption led to extraordinary water vapor enhancement throughout the stratospheric vortex at the beginning of the 2023 Antarctic winter. Although the dynamical characteristics of the vortex itself were generally unexceptional, the excess moisture initially raised the threshold temperatures for the formation of polar stratospheric clouds (PSCs) above typical values over a broad vertical domain. Low temperatures, especially during an early-July cold spell, prompted ice PSC formation and unusually severe irreversible dehydration at higher levels (500–700 K), while atypical rehydration 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 both 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 saturated in midwinter, with trace gas evolution essentially following the climatological mean thereafter. Chlorine deactivation started slightly later than in most years. 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 hastened the onset of and increased the vertical extent of PSC formation and chlorine activation in early winter, saturation of lower stratospheric chemical processing (as is typical in the Antarctic) prevented an exceptionally severe ozone hole in 2023.

The 2022 Hunga eruption caused unprecedented stratospheric hydration. Aura Microwave Limb Sounder (MLS) measurements show that the stratospheric water vapor mass remains essentially unchanged as of early 2024 and that the Hunga hydration occurred atop a robust (possibly accelerating) moistening trend in the stratosphere. Enhanced by the excess Hunga water vapor, dehydration via polar stratospheric cloud (PSC) sedimentation in the 2023 Antarctic vortex exceeded climatological values by ~50%. Simple projections, based solely on Antarctic dehydration, illustrate that the timing of the return to humidity levels that would have been expected absent the Hunga hydration depends on the ongoing stratospheric water vapor trend. For strong moistening, the influx of water entering the stratosphere could offset the enhanced PSC dehydration, resulting in a new, more humid ‘equilibrium’ stratospheric state. With the Hunga hydration compounding an underlying moistening trend, the stratosphere could remain anomalously humid for an extended period.

The eruption of the Hunga Tonga undersea volcano in January 2022 injected water vapor to altitudes as high as 53 km, but also an unprecedented and much larger amount of water vapor into the stratosphere. Several months after the eruption, measurements from the Aura Microwave Limb Sounder (MLS) and from three ground‐based Water Vapor Millimeter Wave Spectrometer instruments began to measure record‐high amounts of water vapor in the mesosphere over a wide range of latitudes. While there are indications that some of this mesospheric increase in water vapor was probably caused by the Hunga Tonga eruption, variations in water vapor mixing ratios also depend on dynamical factors. The phase of the QBO in 2015 was similar to that in 2022, and we make use of this similarity in order to better understand what role dynamics played in establishing the unusually large 2022 water vapor mixing ratios, both in the upper and lower mesosphere.

Abstract We use Aura Microwave Limb Sounder (MLS) trace gas measurements to investigate whether water vapor (H2O) injected into the stratosphere by the Hunga Tonga‐Hunga Ha'apai (HTHH) eruption affected the 2022 Antarctic stratospheric vortex. Other MLS‐measured long‐lived species are used to distinguish high HTHH H2O from that descending in the vortex from the upper‐stratospheric H2O peak. HTHH H2O reached high southern latitudes in June–July but was effectively excluded from the vortex by the strong transport barrier at its edge. MLS H2O, nitric acid, chlorine species, and ozone within the 2022 Antarctic polar vortex were near average; the vortex was large, strong, and long‐lived, but not exceptionally so. There is thus no clear evidence of HTHH influence on the 2022 Antarctic vortex or its composition. Substantial impacts on the stratospheric polar vortices are expected in succeeding years since the H2O injected by HTHH has spread globally.

The eruption of Hunga Tonga in January 2022 injected an amount of water vapor into the stratosphere that is unprecedented in the satellite era. In the ensuing months Aura Microwave Limb Sounder measurements showed that this plume of water vapor spread from its original injection site at 20.5 degrees S to Mauna Loa, Hawaii at 19.5 degrees N, where an increase was observed in April by the ground-based Water Vapor Millimeter-wave Spectrometer instruments. Interannual variations in water vapor occur over Mauna Loa due to both dynamical variations in the tropical stratosphere and variations in the amount of water vapor crossing the tropical tropopause, and we place the observed stratospheric water vapor increase from Hunga Tonga into context of these other variations that have been observed since 2013.

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 N 2 O 5 , NO x , and HCl accompanied by enhancement of HNO 3 , ClO, and ClONO 2 . After initially rising steeply, HNO 3 and ClO plateau, maintaining fairly constant abundances for several months. These patterns are consistent with the saturation of N 2 O 5 hydrolysis, 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 ClONO 2 . In those regions, however, effects of anomalous transport dominate the evolution of HNO 3 and 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 version 10 (v10) Atmospheric Carbon Observations from Space (ACOS) Level 2 full-physics (L2FP) retrieval algorithm has been applied to multiyear records of observations from NASA's Orbiting Carbon Observatory 2 and 3 sensors (OCO-2 and OCO-3, respectively) to provide estimates of the carbon dioxide (CO2) column-averaged dry-air mole fraction (XCO2). In this study, a number of improvements to the ACOS v10 L2FP algorithm are described. The post-processing quality filtering and bias correction of the XCO2 estimates against multiple truth proxies are also discussed. The OCO v10 data volumes and XCO2 estimates from the two sensors for the time period of August 2019 through February 2022 are compared, highlighting differences in spatiotemporal sampling but demonstrating broad agreement between the two sensors where they overlap in time and space. A number of evaluation sources applied to both sensors suggest they are broadly similar in data and error characteristics. Mean OCO-3 differences relative to collocated OCO-2 data are approximately 0.2 and −0.3 ppm for land and ocean observations, respectively. Comparison of XCO2 estimates to collocated Total Carbon Column Observing Network (TCCON) measurements shows root mean squared errors (RMSEs) of approximately 0.8 and 0.9 ppm for OCO-2 and OCO-3, respectively. An evaluation against XCO2 fields derived from atmospheric inversion systems that assimilated only near-surface CO2 observations, i.e., did not assimilate satellite CO2 measurements, yielded RMSEs of 1.0 and 1.1 ppm for OCO-2 and OCO-3, respectively. Evaluation of uncertainties in XCO2 over small areas, as well as XCO2 biases across land–ocean crossings, also indicates similar behavior in the error characteristics of both sensors. Taken together, these results demonstrate a broad consistency of OCO-2 and OCO-3 XCO2 measurements, suggesting they may be used together for scientific analyses.

A new algorithm to derive near-real-time (NRT) data products for the Aura Microwave Limb Sounder (MLS) is presented. The old approach was based on a simplified optimal estimation retrieval algorithm (OE-NRT) to reduce computational demands and latency. This paper describes the setup, training, and evaluation of a redesigned approach based on artificial neural networks (ANN-NRT), which is trained on >17 years of MLS radiance observations and composition profile retrievals. Comparisons of joint histograms and performance metrics derived between the two NRT results and the operational MLS products demonstrate a noticeable statistical improvement from ANN-NRT. This new approach results in higher correlation coefficients, in addition to lower root-mean-square deviations and biases at almost all retrieval levels compared to OE-NRT. The exceptions are pressure levels with concentrations close to 0 ppbv (parts per billion by volume), where the ANN models fail to establish a functional relationship and tend to predict 0. Depending on the application, this behavior might be advantageous. While the developed models can take advantage of the extended MLS data record, this study demonstrates that training ANN-NRT on just a single year of MLS observations is sufficient to improve upon OE-NRT. This confirms the potential of applying machine learning to the NRT efforts of other current and future mission concepts.

We present ground‐based microwave measurements of mesospheric water vapor made by the Water Vapor Millimeter‐wave Spectrometer (WVMS) instruments since the early 1990s from sites in California, Hawaii, and New Zealand. These measurements are compared with coincident measurements from the Halogen Occultation Experiment, the Aura Microwave Limb Sounder, and Sounding of the Atmosphere using Broadband Emission Radiometry; all of which combine to cover the entire time period of the ground‐based measurements. Comparisons are presented both on ∼weekly timescales in order to better identify discontinuities in the relative differences and on annual timescales in order to better study geophysical variations. The WVMS retrievals shown here are available on the Network for the Detection of Atmospheric Composition Change database. The range of WVMS trends and the differences from the satellite trends, with the latter varying over a range of ∼3%/decade, provide an estimate of how accurately it would be possible to determine multidecadal trends using ground‐based microwave instruments in a postsatellite era. This uncertainty is comparable to the trend in mesospheric water vapor that is expected to have occurred since the early 1990s.

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 ClONO2 that 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.

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 SO2 and 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.

Hunga Tonga-Hunga Haʻapai, a submarine volcano in the South Pacific, reached an eruption climax on 15 January 2022. The blast sent a plume of ash well into the stratosphere, triggered tsunami alerts across the world, and caused ionospheric disturbances. A few hours after the violent eruption, the Microwave Limb Sounder (MLS) measured enhanced values of water vapor at altitudes as high as 50 km - near the stratopause. On the following days, as the plume dispersed, several MLS chemical species, including H2O and SO2, displayed elevated values, far exceeding any previous values in the 18-year record. In this presentation we discuss the validity of these measurements, the stratospheric evolution of the SO2 and H2O plumes, and, lastly, the implications of the large-scale hydration of the stratosphere by the eruption.

The exceptionally strong and long‐lived Arctic stratospheric polar vortex in 2019/2020 resulted in large transport anomalies throughout the fall‐winter‐spring period from vortex development to breakup. These anomalies are studied using Aura MLS N 2 O, H 2 O, and CO long‐lived trace gas data, ACE‐FTS CH 4 data, and meteorological and trace gas fields from reanalyses. Anomalies are strongest throughout the winter in the lower through the middle stratosphere (from about 500K through 700K), with record low (high) departures from climatology in N 2 O and CH 4 (H 2 O). CO shows extreme high anomalies in midwinter through spring down to about 550K. Descent rates, vortex confinement, and trace gas distributions in the preceding months indicate that early winter anomalies in N 2 O and H 2 O arose primarily from entrainment of air with already‐anomalous values into the vortex as it developed in fall 2019 followed by descent of those anomalies to lower levels within the vortex. Trace gas anomalies in midwinter through the late vortex breakup in spring 2020 arose primarily from inhibition of mixing between vortex and extravortex air because of the exceptionally strong and persistent vortex. Persistent strong N 2 O and H 2 O gradients across the vortex edge demonstrate that air within the vortex and its remnants remained very strongly confined through late April (mid‐May) in the middle (lower) stratosphere. These results are important for understanding the evolution of trace gas distributions, which affects both polar chemical processing and radiative processes related to climate.

Hunga Tonga-Hunga Haʻapai, a submarine volcano in the South Pacific, reached an eruption climax on 15 January 2022. The blast sent a plume of ash well into the stratosphere, triggered tsunami alerts across the world, and caused ionospheric disturbances. A few hours after the violent eruption, the Microwave Limb Sounder (MLS) measured enhanced values of water vapor at altitudes as high as 50 km - near the stratopause. On the following days, as the plume dispersed, several MLS chemical species, including H2O and SO2, displayed elevated values, far exceeding any previous values in the 18-year record. In this presentation we discuss the validity of these measurements, the stratospheric evolution of the SO2 and H2O plumes, and, lastly, the implications of the large-scale hydration of the stratosphere by the eruption.