Abstract Tropospheric Emissions: Monitoring of Pollution (TEMPO) provides valuable hourly measurements of airborne pollutants over North America. Data quality over snowy surfaces may degrade due to difficulty in accurately characterizing surface reflectivity and cloud properties, as snow and cloud are both highly reflective and therefore challenging to distinguish optically. Cloud properties for satellite‐based trace gas retrievals are commonly extracted from measurements of oxygen, through either direct absorption (A band, B band) or collisional absorption (oxygen dimer). Here we investigate a combination of the two methods, with potential for extracting additional cloud information over challenging scenes due to the different responses of the two methods to various atmospheric conditions. A combined retrieval is successfully applied to simplified simulated data, extracting cloud information under conditions which would cause either retrieval to fail on its own. A second technique is demonstrated on the simulated data which attempts to flag partially cloudy scenes in the absence of external surface reflectivity information. The individual retrievals are applied to TEMPO data and compared in order to investigate their compatibility for potential use in combined retrievals. Variability in effective cloud fraction is found to be on the order of 0.1, and a bias in optical centroid pressure (representing the altitude of the cloud) is observed between methods, up to 50 hPa for fully overcast scenes and higher for partially cloud scenes. Further investigation is required to determine if these discrepancies are due to retrieval error, instrument calibration, or physical differences between spectral bands.

The 2022 Hunga eruption significantly perturbed the stratosphere by injecting substantial water vapor and , drastically changing the Aerosol Optical Depth (AOD) and particle size. Post-eruption, satellite limb-scattering retrievals of aerosol extinction from Ozone Mapping and Profiler Suite Limb Profiler (OMPS-LP) and Optical Spectrograph and InfraRed Imager System (OSIRIS) diverged from Stratospheric Aerosol and Gas Experiment (SAGE) on the International Space Station (SAGE III/ISS) solar occultation measurements. We demonstrate that this discrepancy stems from the fixed aerosol particle size assumptions inherent to the limb sensor's retrieval algorithms, which are different than the large particle sizes observed following the eruption. Using particle size distribution parameters derived from SAGE III/ISS measurements as input to the OMPS-LP and OSIRIS retrievals, we effectively eliminated the bias in retrieved extinction and AOD compared to SAGE III/ISS. This consistency across the three data sets provides an improved understanding of aerosol distributions in the highly perturbed stratosphere.

Tropical upwelling constitutes the ascending branch of the global mean stratospheric circulation and governs the thermal and chemical properties of the tropical stratosphere. A lack of direct observations and a spread in upwelling structure across the modern reanalysis creates difficulties in determining variability and long-term changes of tropical upwelling. We have derived time series of effective vertical transport in the tropical lower and middle stratosphere from MLS and SWOOSH water vapour for 2005-2020 and 1995-2020. Our calculated upwelling is found to be in the range of 0.21-0.33 mms-1 for 73-28 hPa in very good agreement with reanalysis vertical velocities (ERA5, JRA-3Q, MERRA-2) and other observation-based estimates (ANCISTRUS).We show that interannual variations of upwelling in the middle stratosphere are dominated by the QBO signal, which explains a large fraction of the upwelling anomalies. In the lower stratosphere, tropospheric modes of variability also play a role with the QBO and ENSO being equally important for explaining interannual variability. Individual peaks of strongly enhanced upwelling in the lower stratosphere in 2000/01 and 2011/12 cannot be explained by QBO or ENSO variability and coincide with known drops in water vapour and cold point temperatures. We use independent observational data to show that tropical upwelling is anticorrelated with long-lived stratospheric tracers such as ozone as expected, lending confidence to the derived values. A reduction in variability is observed for 2016-2020 in our calculated upwelling and observed ozone, which is consistent with the disruption to regular QBO variability over this period.

Abstract. The Hunga underwater volcanic eruption in January 2022 injected 150 Tg of water vapour (H2O) into the stratosphere, increasing the total stratospheric H2O mass by 10 %. This study investigates the transport of the Hunga H2O within, and out of, the stratosphere from 2022–2025, using H2O observations from the Microwave Limb Sounder (MLS) and the Atmospheric Chemistry Experiment – Fourier Transform Spectrometer (ACE-FTS), along with model simulations from the Whole Atmosphere Community Climate Model (WACCM) and the FLEXible PARTicle dispersion model (FLEXPART). The Hunga H2O is isolated by using the tropical cold point temperature to account for H2O that entered the stratosphere through the tropical tropopause, rather than via the eruption. The resulting residuals show the detailed evolution of the Hunga H2O over time while it moves to higher latitudes and lower altitudes. Including the lower stratosphere, approximately two-thirds of the Hunga H2O remains in the stratosphere in late 2025. There is good agreement between the observed and modelled H2O above 20 km. However, in the lower stratosphere observations show substantially more H2O compared to the model during 2024 and 2025, suggesting that the modelled transport across the lower stratosphere is too fast. Observations show excess H2O in the tropics in 2023 and 2024, suggesting evidence of recirculation from SH mid-latitudes back to the tropics. This mixing signature is found in WACCM simulations and confirmed in FLEXPART transport calculations. Including the lower stratosphere in the calculation increases the decay time scale of the Hunga stratospheric water vapor by 1–2 years.

The Tropospheric Emissions: Monitoring of Pollution (TEMPO) satellite provides hourly air quality measurements over North America. This study evaluates TEMPO observations over snow-covered surfaces, highlighting its ability to capture sharp spatial and temporal gradients in vertical column densities and surface concentrations, key parameters for assessing air quality and public health impacts. Data from the 2024 Study of Winter Air Pollution in Toronto, including in situ and mobile Multi-Axis Differential Optical Absorption Spectroscopy measurements, are used to assess TEMPO's precision and accuracy. Additional evaluations are performed at Pandora sites across North America to examine wintertime performance. Comparisons show strong correlations between TEMPO and surface observations, with significant improvements in bias after applying corrections to air mass factors, cloud fraction, and surface albedo (from -32% to -9% over snow).

Abstract. Atmospheric aerosols play a crucial role in Earth’s climate system, yet their spatio-temporal distribution, particularly in the free troposphere (FT) and upper troposphere–lower stratosphere (UTLS), remains poorly constrained, a major source of uncertainty in estimates of aerosol radiative forcing. To address this, we perform ECHAM/MESSy Atmospheric Chemistry (EMAC) model simulations with a newly developed setup, bridging the tropospheric and stratospheric regimes. Model output is evaluated against a comprehensive suite of observations of aerosol mass, number concentrations, and optical properties, showing good agreement across vertical layers and most geographical regions. The evaluated simulations provide a unified description of global distributions of key aerosol species, their composition, and number concentrations from the Earth's surface to the stratosphere. Simulated aerosol mass exhibits a global minimum between 400 and 200 hPa, marking the transition between FT and UTLS, with particle numbers peaking at similar altitudes or slightly higher in the tropics. Primary particles contribute less than 3.5 % to aerosol mass in the stratospheric overworld up to 10 hPa, substantially less than suggested by previous modelling studies and in closer agreement with recent observations. Stratospheric aerosol mass is dominated by sulfate, with a notable contribution (~15 %) from secondary organic aerosol throughout the global lower stratosphere. This work provides new constraints on aerosol distributions in the FT and UTLS, which remain underrepresented in global modelling studies, and enables future research on aerosol-climate interactions in this critical atmospheric regime.

The eruption of the Hunga volcano on January 15, 2022, was unprecedented in the satellite record because of the ~150 Tg of water injected in the stratosphere, paired to a relatively low (~0.5 Tg) sulfur dioxide injection. The uniqueness of this eruption provides an opportunity to evaluate chemistry-climate models over a new range of conditions, different from the sulfur rich eruptions on which they have generally been tested. We describe coordinated Hunga simulations from ten chemistry climate models with prognostic aerosol modules and show how the presence of the volcanic water vapor led to larger particles than would occur in a water-poor eruption. This has the effect of rapidly increasing the stratospheric aerosol optical depth in the first month and accelerating the settling of the volcanic aerosols in the following months. While the models are able to reproduce the observed evolution of the water vapor eruption plume and the distribution of volcanic aerosols. they fail to simulate the aerosol optical depth. Most of the difference between models and observations, and among models themselves, can be traced to the aerosol microphysics, which is highly dependent on the parameterizations made by each model.

Tha Canadian High altitude Aerosol Water vapour and Cloud (HAWC) mission is built upon innovative Canadian optical remote sensing technology and will provide important information related to water vapour, aerosols and cloud microphysics in the upper troposphere and lower stratopshere. HAWC is made up of three Canadian passive optical instruments, the Spatial Hetrodyne Observations of Water (SHOW), the Aerosol Limb Imager (ALI) and the Thin Ice Cloud and Far infraRed Emissions (TICFIRE) where the first two look at scattered sunlight in the atmospheric limb from low earth orbit and the latter instrument measures thermal emission in the nadir, also from low earth orbit. This presentation will outline the technology, the measurements and the expected scientific return of the HAWC mission that is expected to launch in the first part of the next decade.

Aerosols play an important role in Earth’s radiative balance, however, considerable uncertainty remains on their overall climate impact. The Aerosol Limb Imager (ALI) is a satellite instrument that will provide high-sensitivity measurements of aerosols in the upper troposphere and stratosphere. ALI will fly on the Canadian High-Altitude Aerosols, Water Vapour and Clouds (HAWC) satellite. We investigate the synergy between ALI and ground-based remote-sensing measurements from The Canadian Micro-Pulse Lidar Network (MPLCAN) to determine the possibility and advantages of a merger data product. The ALI retrieved quantities are not directly comparable to the MPL attenuated backscatter measurements, so assumptions are made about the constituents and optical properties of the atmosphere to compare them. We show that ALI’s novel particle size retrieval provides a method for comparison with lidar that is applicable to any backscatter lidar measurements where the lidar constant is well known, including networks such as NASA’s MPLNET and EUMETNET’s E-PROFILE network. Simulated coincident measurements from ALI and MPLCAN are performed for two aerosol cases: background stratospheric aerosols and a wildfire smoke layer. It is determined that MPLCAN has the potential to validate ALI measurements from 5-10km and extend the vertical coverage of measurements to near the surface. ALI can currently retrieve extinction for specific cases of light wildfire smoke, therefore MPLCAN and ALI could track a wildfire event simultaneously.

Aerosols in the upper troposphere play an important role in Earth's radiative balance and atmospheric composition. Satellite observations show recurring enhancements of aerosol extinction coefficient (AEC) in the upper troposphere and near the tropopause over the Asian summer monsoon (ASM) anticyclone (ASMA) region during July-August. However, substantial uncertainties remain regarding the roles of ASM dynamics, climate variability, and surface emissions in shaping upper tropospheric aerosols, as well as global model performance in this region. We present results from an AeroCom-coordinated multi-model study addressing these issues with nine global models covering the period 2000-2018. Large inter-model spread is found in non-volcanic AEC over the ASMA region, with coefficients of variation of 64 %-86 %. Diagnostics using standardized tracers show that approximately half of this spread arises from differences in transport and wet removal processes, with discrepancies in wet scavenging contributing roughly eight times more to the inter-model variance than transport. The multi-model ensemble simulates a significant increase in non-volcanic AEC in ASMA over the two-decade period at similar to 1.2 % yr-1, primarily driven by rising anthropogenic emissions in Asia. In contrast, interannual fluctuations are modulated by climate variability, represented by Multivariate ENSO Index. Comparison with satellite-retrieved AEC also reveals persistent model deficiencies, especially in representing volcanic aerosols. These findings highlight the importance of improving the aerosol wet scavenging schemes and provide a benchmark for future coordinated aerosol modeling and evaluation.

The stratopause, the boundary between the stratosphere and the mesosphere, is projected to cool and drop in response to anthropogenic greenhouse gas (GHG) emissions. A lack of long-term observations with high vertical resolution at the stratopause has made it difficult to quantify trends in this region. We use observations from the Optical Spectrograph and InfraRed Imager System (OSIRIS) and the Sounding of the Atmosphere using Broadband Emission Radiometry (SABER) instrument to assess the annual and inter-annual variability and to quantify trends in the stratopause temperature and height. The SABER and OSIRIS observations at the stratopause are highly correlated, and both show that the stratopause cooled by similar to 0.5-1 K per decade during 2005-2021. The observations also suggest that the tropical stratopause moved lower during this time period by 300-475 m per decade. The observational stratopause trends are consistent with trends from chemistry climate models simulations.

Geostationary measurements of trace gases provide valuable air quality data at unprecedented temporal scales. At high latitudes challenges begin to arise, such as lines of sight that stray from nadir, and (during winter) limited sunlight and pervasive snow cover. Motivated by the desire to fully take advantage of TEMPO (Tropospheric Emissions: Monitoring of Pollution) measurements over Canada, we investigate one of these issues: snow. A key challenge with measurements over snowy scenes is the similar reflectivity of snow and clouds. Trace gas algorithms rely on the contrast between surface and cloud reflectivities to estimate an effective cloud fraction which is necessary to characterize the light path for cloudy scenes. This snow-cloud ambiguity ultimately compromises the data quality, denying the opportunity to capitalize on the potential increase in surface sensitivity offered by the high reflectivity of snow. Here we present an algorithm that simultaneously uses O2-O2 and oxygen B-band absorption to extract cloud data for trace gas retrievals while reducing dependency on the surface-cloud reflectivity contrast.

The Aerosol Limb Imager (ALI) is designed to measure stratospheric aerosol by imaging limb-scattered sunlight. Each image taken by ALI is spectrally filtered at a tunable wavelength and refined to consist of either horizontally or vertically polarized light. Novel to limb imaging, these polarized observations of ALI provide a means to isolate tangent altitudes which have signal contaminated by clouds as identified by the depolarization of the scattered radiance. This avoids the ambiguity caused by clouds to be interpreted as aerosol in a retrieval. We present a polarized aerosol retrieval methodology which retrieves vertically resolved aerosol number density and median radius of a unimodal log-normal distribution, in addition to a scalar (uniform in altitude) width of the log-normal distribution. We explore the cloud discrimination and aerosol retrieval of ALI in simulation as validation of the efficacy and the limits to the technique. We then apply the retrieval to three example sets of observations taken from the most recent high-altitude balloon flight of ALI. One set provides a nominal exemplar, while the other two represent more difficult retrieval conditions of an increasingly polarized atmosphere. We compare the aerosol extinction of ALI in all three exemplar cases to the best coincident extinctions of three space based instruments: the Stratospheric Aerosol and Gas Experiment (SAGE III/ISS), the Ozone Mapping and Profiler Suite Limb Profiler (OMPS-LP), and the Optical Spectrograph and InfraRed Imaging System (OSIRIS). We provide discussion on the agreement of all three cases against the comparison instruments with respect to the efficacy of our approach. However, we find the retrieved aerosol extinction of ALI in the nominal case (considering the limitations of ballooning) is in good agreement (a median absolute percent difference < 29 %) to the extinction reported by SAGE III/ISS, OMPS-LP, and OSIRIS while also yielding aerosol particle size information. In the case of SAGE III/ISS, the comparison with ALI results is extended into the aerosol size parameters. In a nominal case SAGE III/ISS and ALI aerosol effective radius agree within uncertainty for the majority of altitudes.

The Hunga Tonga–Hunga Ha'apai Model–Observation Comparison (HTHH–MOC) project aims to comprehensively investigate the evolution of volcanic water vapor and sulfur emissions and their subsequent atmospheric impacts and underlying response mechanisms using state-of-the-art global climate models. This study evaluates multi-model ensemble simulations participating in the HTHH–MOC free-run experiment with climate projections for 10 years (2022–2032). Model results are evaluated against satellite observations to assess their ability to reproduce the observed evolution of stratospheric water vapor, aerosols, temperature, and ozone from 2022 to 2024. The participating models accurately capture the observed distribution patterns and associated upper atmospheric responses, providing confidence for their future projections. Model simulations suggest that the Hunga eruption-induced stratospheric water vapor anomaly lasts 4–7 years, with a water vapor e-folding time of 31–43 months. This prolonged water vapor perturbation leads to significant stratospheric and mesospheric cooling, resulting in significant ozone loss in the upper stratosphere and lower mesosphere for 7–10 years. Comparisons between simulations with both SO2 and H2O emissions and those with H2O-only emissions indicate that the pronounced dipole response with upper-stratospheric cooling and lower-stratospheric warming is driven by the combined effects of SO2 and H2O injections. These results highlight the prolonged atmospheric impacts of the Hunga eruption and the potential critical role of stratospheric water vapor in modulating long-term atmospheric chemistry and dynamics.

The Measurement of Aerosol Extinction in the Stratosphere and Troposphere Retrieved by Occultation (MAESTRO) instrument on the SCISAT satellite provides aerosol extinction measurements in multiple solar wavelength bands. In this study, we evaluate the quality and utility of MAESTRO version 3.13 stratospheric aerosol extinction retrievals, from February 2004–February 2021, through comparison with measurements from other satellite instruments. Despite significant scatter in the MAESTRO data, we find that gridded median MAESTRO aerosol extinctions and stratospheric aerosol optical depth (SAOD) values are generally in good agreement with those from other instruments during volcanically quiescent periods. After volcanic eruptions and wildfire injections, gridded median MAESTRO extinction and SAOD are well correlated with other measurement sets but generally biased low by 40 %–80 %. The Ångström exponent (AE), which can provide information on aerosol particle size, is derived from the MAESTRO spectral extinction measurements in the lowermost stratosphere, showing perturbations after volcanic eruptions qualitatively similar to those from the Stratospheric Aerosol and Gas Experiment on the International Space Station (SAGE III/ISS) for the eruptions of Ambae (2018) and Ulawun (2019). Differences in AE anomalies after the 2019 extratropical Raikoke eruption may be due to the different spatiotemporal sampling of the two instruments. Furthermore, we introduce a method to adjust MAESTRO extinction data based on comparison with extinction measurements from SAGE III/ISS during the period from June 2017–February 2021, resulting in improved comparison during volcanically active periods. Our work suggests that empirical bias correction may enhance the utility of MAESTRO aerosol extinction data, which can make it a useful complement to existing satellite records, especially when multi-wavelength solar occultation data from other instruments are unavailable.

Over the past few decades, a reduction in chlorinated long-lived ozone-depleting substance emissions due to the regulations imposed by the Montreal Protocol has led to a global decrease in stratospheric chlorine. At the same time, emissions of chlorinated Very Short-Lived Substances, which are unregulated, have increased. Here we show that observed changes of inorganic stratospheric chlorine are inconsistent with changes in the tropospheric abundances of long-lived ozone-depleting substances. Satellite observations of stratospheric chlorine species from the Atmospheric Chemistry Experiment-Fourier Transform Spectrometer during 2004-2020 reveal that the observed decrease in inorganic stratospheric chlorine is 25%-30% smaller than expected based on trends of long-lived ozone-depleting substances alone. At mid-latitudes in the lower stratosphere, this can be explained by the chlorinated Very Short-Lived Substances increase, which offsets the long-term reduction of stratospheric chlorine by up to 30%.

The High-Altitude Aerosols, Water Vapor, and Clouds (HAWC) mission is an observing system, with a planned launch around 2031, that is being developed by the Canadian Space Agency to provide collocated global measurements of aerosols, water vapor, and thin ice clouds in the upper troposphere and lower stratosphere with vertical coverage that extends into the troposphere in the polar regions. The mission is the Canadian contribution to NASA's Atmospheric Observing System (AOS), a satellite constellation will include multiple instruments to monitor aerosols, clouds, and precipitation as part of Earth System Observatory (ESO). The HAWC mission includes three innovative Canadian instruments: the Aerosol Limb Imager (ALI), the Thin Ice Clouds and Far Infrared Emissions (TICFIRE) instrument, and the Spatial Heterodyne Observations of Water (SHOW) instrument. ALI and SHOW will provide limb profiles of aerosol and water vapor with high spatial resolution (vertical and along track) and high sensitivity to fine aerosols and dry conditions. TICFIRE will provide nadir measurements of infrared radiation, thin ice cloud content, and cloud microphysical properties. These coordinated measurements will help build a more comprehensive picture of high-altitude aerosols, clouds, and water vapor in the atmosphere. In this paper, we present the HAWC concept and discuss the primary science objectives and requirements of the mission. The instrument payloads and data products are introduced, and synergies between HAWC and other AOS instruments are identified.

Abstract. Tropical upwelling constitutes the ascending branch of the global mean stratospheric circulation and governs the thermal and chemical properties of the tropical stratosphere. A lack of direct observations and a spread in upwelling structure across the modern reanalysis creates difficulties in determining variability and long-term changes of tropical upwelling. We have derived time series of effective vertical transport in the tropical lower and middle stratosphere from MLS and SWOOSH water vapour for 2005–2020 and 1995–2020. Our calculated upwelling is found to be in the range of 0.21–0.33 mm/s for 73–28 hPa in very good agreement with reanalysis vertical velocities (ERA5, JRA-3Q, MERRA-2) and other observation-based estimates (ANCISTRUS). We show that interannual variations of upwelling in the middle stratosphere are dominated by the QBO signal, which explains a large fraction of the upwelling anomalies. In the lower stratosphere, tropospheric modes of variability also play a role with the QBO and ENSO being equally important for explaining interannual variability. Individual peaks of strongly enhanced upwelling in the lower stratosphere in 2000/2001 and 2011/2012 cannot be explained by QBO or ENSO variability and coincide with known drops in water vapour and cold point temperatures. We use independent observational data to show that tropical upwelling is anticorrelated with long-lived tracers such as ozone as expected, lending confidence to the derived values. A reduction in variability is observed for 2016–2020 in both our calculated upwelling and observed ozone, which is consistent with the disruption to regular QBO variability over this period.

Stratospheric aerosols play a key role in atmospheric chemistry and climate. Their particle size is a crucial factor controlling the microphysical, radiative, and chemical aerosol processes in the stratosphere. Despite its importance, available observations on aerosol particle size are rather sparse. This limits our understanding and knowledge about the mechanisms and importance of chemical and climate aerosol feedbacks. The retrieval described by Malinina et al. (2018) provides the stratospheric particle size distribution (PSD) from SCIAMACHY (SCanning Imaging Absorption spectroMeter for Atmospheric CHartographY) limb observations in the tropics. This algorithm has now been improved and extended to work on the entire globe. Two PSD parameters of a unimodal lognormal PSD, the median radius and the geometric standard deviation, are retrieved between 18 and 35 km altitude from SCIAMACHY limb observations by a multiwavelength nonlinear regularized inversion. The approach assumes an aerosol particle number density profile that does not change during the retrieval. The effective Lambertian surface albedo pre-retrieved from coinciding SCIAMACHY nadir observations is integrated into the retrieval algorithm to mitigate the influence of the surface albedo on the retrieval results. The extinction coefficient and the effective radius are calculated from the PSD parameters. The aerosol characteristics from SCIAMACHY are compared with in situ balloon-borne measurements from Laramie, Wyoming, and retrievals from the satellite instruments of the Stratospheric Aerosol and Gas Experiment series (SAGE II and SAGE III) and Optical Spectrograph and InfraRed Imager System (OSIRIS). In the Northern Hemisphere, the median radius differs by less than 27 % and the geometric standard deviation by less than 11 % from both balloon-borne and SAGE III data. Differences are mainly attributed to errors in the assumed a priori number density profile. Globally, the SCIAMACHY extinction coefficient at 750 nm deviates by less than 35 % from SAGE II, SAGE III, and OSIRIS data. The effective radii from SCIAMACHY, balloon-borne measurements, and SAGE III agree within about 18 %, while the effective radius based on SAGE II measurements is systematically larger. The novel data set containing the PSD parameters, the effective radius, and the aerosol extinction coefficients at 525, 750, and 1020 nm from SCIAMACHY observations is publicly available.