Abstract. This study presents an updated assessment of stratospheric and total column ozone trends over the 2000–2024 period using six merged limb-profile datasets and six merged total ozone datasets. Long-term changes were quantified using a multiple linear regression framework that accounts for dynamical and chemical variability. In addition to standard regressors (solar cycle, QBO, ENSO, stratospheric aerosol optical depth), we include Arctic and Antarctic Oscillation indices and the eddy heat flux in each hemisphere as proxies for dynamic variability. Volcanic (and wildfire) aerosol forcing is represented by separate proxies for three periods dominated by the major volcanic events of El Chichón, Pinatubo, and post-2000 volcanic eruptions, including Hunga-Tonga. These period-specific proxies are employed to better account for varying dynamical ozone responses that largely depend on the season and location of the eruptions. All profile datasets consistently show positive trends in the upper stratosphere, with the strongest ozone recovery in southern mid-latitudes, in agreement with other studies. In the lower stratosphere, trends remain weak, spatially heterogeneous, and predominantly negative. A comparison of stratospheric column trends derived from profile data with total ozone trends shows close agreement across latitude bands. Within the trend uncertainties, total column trends since 2000 are largely driven by stratospheric ozone changes, while tropospheric contributions to zonal-mean total ozone trends (the difference between total and stratospheric column trends) appear negligible. The extended regression framework improves the representation of recent dynamical variability and provides an updated perspective on stratospheric ozone recovery through 2024.

We use the CALIOP (Cloud-Aerosol Lidar with Orthogonal Polarization) instrument to determine the microphysical properties of the stratospheric aerosol plume after the Hunga eruption in 2022, the largest so far after the Pinatubo in 1991. In the early stages, low depolarization (<2%) is found everywhere except in patches of high depolarization (up to 35 %) detected within the plumes of sulfur compounds up to 3 d after the eruption. As standard CALIOP L2 products are not operational in the case of the Hunga aerosol plume, we implement an iterative method of successive approximations to retrieve extinction profiles, by estimating the aerosol optical depth (AOD) and then the lidar ratio (LR). The AOD of the plume at 532 nm is between 0.5 and 1.25 on the first four days, then decreases rapidly and stabilizes at 0.047 +/- 0.011 for March 2022. The LR is initially between 60 and 80 sr, consistent with the early growth of sulfate aerosol particles, and then decreases to 48 +/- 6sr between late January and late March 2022. Results are compared and validated with the solar occultation instrument SAGE III (Stratospheric Aerosol and Gas Experiment) on board the International Space Station (ISS) and Mie calculations. A comparison with limb-viewing instruments highlights significant quantitative disagreements in extinction and AOD estimates, which we attribute, in part, to the unusual size distribution of the aerosols within the Hunga plume.

Stratospheric aerosol injections have been proposed to mitigate the effects of global warming. The injection of sulphur dioxide into the stratosphere is one possible idea. However, depending on the latitude, high emission rates can lead to very low transmissions from the perspective of a typical satellite solar occultation instrument, leading to the so-called zero transmission problem. Consequently, it is highly unlikely that a physically meaningful retrieval of the stratospheric aerosol extinction profiles is possible, depending on the latitude and wavelength. The current study analyses, using MAECHAM5-HAM and SCIATRAN, continuous injections of 30 Tg S yr(-1) as a hypothetical largescale stratospheric aerosol injection scenario. For this purpose, sulphur dioxide was continuously injected at an altitude of 60 hPa (approximate to 19 km) into one grid box (2.8 degrees & times; 2.8 degrees) centred on the Equator at 121 degrees E. Specifically, it is investigated which wavelengths, depending on the latitude, are necessary for plausible aerosol extinction profile retrievals. While a wavelength of 520 nm is insufficient for the retrieval for 5 degrees N, the opposite can be concluded for 75 degrees N and 75 degrees S. For the latitudes 45 degrees N and 45 degrees S, a wavelength of at least 1543 nm is necessary. In contrast, 1900 nm is sufficient for 15 degrees N and 15 degrees S, as well as 5 degrees N. Simulation results for an emission rate of 10 Tg S yr(-1 )show that a minimum wavelength of 1543 nm is already sufficient for 5 degrees N. The results also emphasize that encountering the zero transmission problem at shorter wavelengths does not render solar occultation measurements impossible, it requires appropriate wavelength selection based on aerosol loading. Consistent with expectations, a longer wavelength is required for the latitude range of and near the injection. These findings are therefore also relevant for satellite solar occultation measurements after major volcanic eruptions.

Abstract. Stratospheric aerosol injections (SAI) have been proposed as a potential climate intervention to mitigate some effects of global warming. This method involves the idea of injecting sulphur dioxide into the stratosphere. Other ideas include the injection of solid particles, like alumina and calcite, as these particles absorb less terrestrial infrared radiation and scatter solar radiation more efficiently. The aim of the study is to investigate the detectability of the continuous injection of 5 Tg yr-1 of alumina and calcite with typical satellite solar occultation instruments using SOCOL-AERv2 (SOlar Climate Ozone Links-Atmospheric and Environmental Research Incorporation version 2) model simulation results and the SCIATRAN radiative transfer model. The results demonstrate that, under the assumptions made, it is possible to detect the injection of solid particles into the stratosphere and that the corresponding SAI signals can be distinguished from natural variability under near-background conditions, which is essential for the observational verification of potential SAI perturbations.

We present updated evaluation of stratospheric ozone profile trends in the 60 degrees S-60 degrees N latitude range using long-term ground-based and satellite climate data records, as well as simulations by chemistry-climate models. The trends are evaluated using the LOTUS (Long-term Ozone Trends and Uncertainties in the Stratosphere) regression model.Analyses of satellite data confirm the statistically significant positive ozone trends in the period 2000-2024 in the upper stratosphere of similar to 1-3 % per decade, with larger trends at mid-latitudes compared to the tropics. The trends are slightly positive or close to zero in the middle stratosphere, and mostly negative, -1 to -2 % per decade, in the lower stratosphere, but they are not statistically significant. The morphology and magnitude of ozone trends are similar to previous analyses (2000-2020 trends).Ozone trends in 2000-2024 predicted by chemistry-climate model simulations are in good agreement with combined satellite trends. In the upper stratosphere, models predict a slightly stronger ozone recovery than observations. In the lower stratosphere, both models and satellite observations report negative trends in the tropics, while modelled ozone trends are slightly positive at mid-latitudes.Ozone profile trends over several stations estimated from ground-based records capture the same overall vertical pattern of ozone trends as merged gridded satellite datasets.Analyses of regional ozone profile trends in 2003-2024 using merged satellite datasets confirmed the previous observations of a longitudinal structure in ozone trends in the NH mid-latitude stratosphere, with positive trends over Scandinavia and negative trends over Siberia. However, the magnitude of this dipole-like structure is reduced compared to previous analyses.

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.

Abstract. Volcanic stratospheric aerosols represent a key forcing for climate simulations in Phase 7 of the Coupled Model Intercomparison Project (CMIP7). The CMIP7 stratospheric aerosol forcing datasets consist of: i) a volcanic sulfur emission inventory; and ii) stratospheric aerosol optical properties, derived from emissions using a reduced-complexity volcanic aerosol model for 1750–1978, and from satellite observations for 1979–2023. Here, we compare the CMIP7 and CMIP6 stratospheric aerosol forcing datasets over the historical period, and evaluate them against an extensive compilation of observational datasets, including lunar eclipse, stellar extinction, pyrheliometers, lidar, and satellite datasets. CMIP6 and CMIP7 show reasonable agreement (< 20 % difference) on peak stratospheric aerosol optical depth (SAOD) for most large-magnitude eruptions including Krakatau (1883), Katmai (1912), Agung (1963), El Chichón (1982), and Pinatubo (1991). For small-to-moderate eruptions, SAOD is likely underestimated in CMIP6 during periods when no such eruptions are included, such as 1940–1960, while CMIP7 includes more pre-satellite small-to-moderate eruptions, as supported by lunar eclipse data. Observational datasets indicate that SAOD perturbations for some moderate-magnitude eruptions, including the 1902, 1921, 1928, 1929, and 1974 eruptions, are captured in CMIP6, but likely underestimated in CMIP7. In addition, independent lidar and stellar extinction measurements show that both CMIP6 and CMIP7 underestimate SAOD over the Northern Hemisphere after the 1982 El Chichón eruption due to observational gaps in satellite instruments. Our findings highlight the potential value of combining CMIP6's use of available observational constraints with CMIP7's extensive emission inventory to refine stratospheric aerosol forcing in future CMIP phases.

Abstract. A variety of chemical and dynamical processes in the troposphere and stratosphere affect tropical total column ozone (TCO), the net effect of which may cause changes in surface UV radiation and impact human and ecosystem health. We use dynamical linear modeling to estimate tropical trends in TCO and partial column ozone (PCO) in the troposphere and three stratospheric layers to assess agreement between satellite observational composites and chemistry–climate model (CCM) simulations from two multi-model experiments (CCMI-1 and CCMI-2022). While both model experiments show tropical TCO increases over 2000–2021, CCMI–2022 trends (+2.5 DU) agree slightly better with observations than CCMI-1 (+1.6 DU). However, this overall agreement obscures multiple systematic differences in PCO trends between the models and observations across atmospheric layers. For example, since 2000 tropical tropospheric PCO increased significantly in CCMI-2022 (+1.5 DU) but not in CCMI–1 (+0.3 DU), largely explaining the difference in TCO trends. Also, despite nearly identical stratospheric PCO trends, CCMI-2022 trends are slightly more negative in the lower stratospheric (by ~0.5 DU), compensated by more positive middle/upper stratosphere trends compared to CCMI-1. Crucially, substantial differences exist across observational PCO trends, particularly in the troposphere and middle/upper stratosphere, and these disagreements limit the ability to evaluate CCM fidelity. Furthermore, while the inter-model correlation between late and early 21st century trends is suggestive of a potential emergent constraint on future ozone trends, the spread in observational trends precludes its observational implementation.

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.

Abstract. Retrievals of stratospheric ozone profiles from space-borne limb-scatter measurements in the UV-Visible spectral range were found to produce a bias if the effective reflectance of the underlying scene has a strong gradient along the instrument line of sight. This effect is particularly pronounced when the underlying scene between the sub-satellite and the tangent point passes from desert to ocean or from cloudy to cloud-free conditions, especially over the ocean. For stratospheric ozone columns retrieved from Ozone Mapper and Profiler Suite Limb Profiler (OMPS-LP) observations, regional biases of up to 7–10 DU were detected. When applying the limb-nadir matching technique, i.e. combining stratospheric and total ozone column information to obtain the residual tropospheric column, these underlying scene related biases amount to about 10–30 % of typical tropospheric ozone column values. In this manuscript we present an improved retrieval algorithm, which substantially mitigates the observed biases in the stratospheric and tropospheric ozone. The algorithm employs an extended version of the SCIATRAN radiative transfer model (v4.8) capable of accounting for variations of the effective reflectance of the underlying scene along the instrument line of sight. VIIRS observations are used to prescribe the variation of the effective reflectance of the underlying scene along the instrument line of sight in the radiative transfer model. The manuscript describes the implementation strategy, discusses the results in terms of stratospheric ozone changes with respect to the standard version and presents updated OMPS-LP/OMPS-NM and new OMPS-LP/TROPOMI tropospheric ozone column data sets along with the validation of the results with respect to correlative observations. Any retrieval of observations from solar limb sounding spectrometers is potentially affected by this issue and should take it into account.

When the polar vortex meanders and shifts towards the equator, air masses from the tropics and subtropics can be transported towards the pole in so-called tropical-subtropical streamers. These large-scale structures are areas of low potential vorticity and high pressure, containing dry air with high ozone mixing ratios. The presence of these streamers can also be seen in changes in stratospheric optical properties. Satellite instruments such as OMPS-LP measuring the limb scattering of these aerosols are capable of observing an increase in the aerosol extinction coefficient in the mid-stratosphere at the edge of the vortex. The high spatial sampling of the limb instrument ensures that the trajectory of the streamer can be accurately monitored. Following a displacement and deformation of the vortex, aerosol transport to high latitudes occurred in the Northern Hemisphere in spring 2017. The additional stratospheric aerosol mass of around 1000 t at an altitude of 24–38 km remained at middle and high latitudes for just under a month in that year. This aerosol mass increase resulted in an estimated 70 % rise in the total mass within this altitude range at high latitudes. Frozen-in anticyclones, in which low latitude air is trapped in the circulation at high latitudes after the end of the polar vortex, can also be observed in the aerosol extinction coefficient. The observation of a particularly long-lived anticyclone in 2005, which is visible in the extinction coefficient, is presented. This is the first study documenting streamer events and frozen-in anticyclones in stratospheric aerosols.

Solar radiation management is a method in the field of geoengineering that aims to modify the Earth's shortwave radiation budget. One idea is to inject sulfur dioxide or sulfuric acid into the stratosphere, where sulfate aerosols are then formed. Such experiments can probably be observed, for example, with satellite occultation instruments like SAGE III/ISS. The aim of the current study is to analyse, using MAECHAM-HAM simulations and retrievals with the radiative transfer program SCIATRAN, whether it is possible to detect the formed stratospheric aerosols from emissions of 1 and 2 Tg S yr-1 (sulfur per year) with the currently active satellite occultation instruments, taking into account an error estimate that is as realistic as possible. If these smaller amounts of sulfur are detectable, larger amounts will also be detectable. The calculations show that, considering the natural variability and the assumptions made here, the stratospheric aerosols formed from emissions of 1 and 2 TgSyr-1 in the quasi-steady-state phase can be detected, which is not the case in the first month of the 2-year initial phase.

About 10% of the total amount of ozone resides in the troposphere, which acts as a potent greenhouse gas. Anthropogenic emissions and biomass burning are the main sources of ozone in the troposphere, and overexposure to this pollutant causes health problems and damages vegetation. A combination of space-borne limb and nadir measurements in the UV-visible spectral range (so-called limb-nadir matching, LNM) provides valuable information on tropospheric ozone. This study uses data from the SCanning Imaging Absorption spectroMeter for Atmospheric CartograpHY (SCIAMACHY) (2002-2012) and Ozone Mapping and Profiler Suite on board of Suomi National Polar-Orbiting Partnership (OMPS/NPP, since 2012). Both instruments observe the atmosphere in both limb and nadir geometry. Tropospheric ozone columns are retrieved globally by subtracting the stratospheric ozone column calculated from limb observations from the total ozone column derived from the nadir measurements. Tropospheric ozone retrievals use different upper altitude limits to calculate the tropospheric ozone column. In the case of the LNM technique, the upper limit is defined by the thermal and/or dynamical tropopause. The Convective Clouds Differential technique (CCD) calculates the tropospheric ozone column up to 270 hPa. Phase II of the Tropospheric Ozone Assessment Report (TOAR-II) uses different pressure levels for different latitudes as an upper limit for the tropospheric column. After updating and improving the SCIAMACHY-LNM and the OMPS/NPP-LNM datasets, we obtained a long-term dataset of tropospheric ozone (2002-2023) by merging them. Here, we present this new long-term LNM tropospheric ozone column dataset, which has been converted to the different definitions of column heights as prescribed in TOAR II. The datasets are validated using ozonesondes, and the results for the different column definitions are evaluated and discussed.

The first Tropospheric Ozone Assessment Report (TOAR, 2014–2019) encountered several observational challenges that limited the confidence in estimates of the burden, short-term variability, and long-term changes of ozone in the free troposphere. One of these challenges is the difficulty to interpret the consistency of satellite measurements obtained with different techniques from multiple sensors, leading to differences in spatiotemporal sampling, vertical smoothing, a-priori information, and uncertainty characterisation. This motivated the Committee on Earth Observation Satellites (CEOS) to initiate a coordinated activity VC-20-01 on improving the assessment and harmonisation of tropospheric ozone measured from space. Here, we report on work that contributes to this CEOS activity, as well as to the ongoing second TOAR assessment (TOAR-II, 2020–2025). Our objective is to harmonise the spatiotemporal perspective of (sixteen) satellite ozone data records, thereby accounting as much as possible for differences in vertical smoothing and sampling. Four harmonisation methods are presented to achieve this goal: two for ozone profiles obtained from nadir sounders (UV-visible, IR, and combined UV-IR), and two for tropospheric ozone column products derived by one of the residual methods (Convective Cloud Differential or Limb–Nadir Matching). We discuss to what extent harmonisation may affect assessments of the spatial distribution, seasonal cycle, and long-term changes in free tropospheric ozone, and we anchor the harmonised profile data to ozonesonde measurements recently homogenised as part of TOAR-II. We find that approaches that use global ozone fields as a transfer standard (here the Copernicus Atmosphere Monitoring Service ReAnalysis, CAMSRA) to constrain the harmonisation generally lead to the largest reduction of the inter-product dispersion (IPD) between satellite datasets. These harmonisation efforts, however, only partially account for the observed discrepancies between the satellite datasets, with a reduction of about 10 %–40 % of the IPD upon harmonisation, depending on the products involved and with strong spatiotemporal dependences. This work therefore provides evidence that it is not only the differences in spatiotemporal smoothing and sampling, but rather the differences in measurement uncertainty that pose the main challenge to the assessment of the spatial distribution and temporal evolution of free tropospheric ozone from satellite observations.

Abstract. The cryosphere plays a crucial role in global climate change. To accurately quantify impacts of typical cryospheric surface types, such as snow, ice, and melt ponds on the radiative processes both in the atmosphere and at the surface, new developments in the radiative transfer modeling are necessary. This paper summarizes recent developments in the coupled atmosphere-snow(water)-ice-water radiative transfer model SCIATRAN, which are essential for cryospheric science applications. Novel implementations include a polarized treatment of the coupled ocean-atmosphere, support for multi-layer ice with an ice crust, a flexible interface for incorporating diverse total suspended matter, and an improved cloud parameter input for mixed clouds. We also introduce new surface reflection models and expanded databases of inherent optical properties for snow and ice. Furthermore, it includes selected verification and validation results obtained by comparing SCIATRAN simulations with benchmark data and with measurements from various campaigns. The SCIATRAN software package is freely distributed via the homepage of the Institute of Environmental Physics (IUP), University of Bremen: https://www.iup.uni-bremen.de/sciatran/.

This paper presents an intercomparison between existing tropospheric ozone column (TrOC) datasets obtained using combined limb and nadir observations, i.e., exploiting collocated stratospheric profile and total column information retrieved from limb and nadir satellite observations, respectively. In particular, seven datasets have been considered, covering the past 2 decades and consisting of monthly-averaged time series with nearly global coverage. We perform a comparison in terms of climatology and seasonality, investigate the tropopause height used for the construction of each dataset and the related biases, and finally discuss long-term TrOC drifts and trends. The overall goal of the study is to assess the consistency between the datasets and explore possible strategies to reconcile the differences between them. Despite uncertainties associated with the limb–nadir residual methodology and large biases between the mean values of the considered datasets, we identify an overall agreement of TrOC distribution patterns. The different tropopause height definitions used to construct the datasets did not show a relevant role in explaining the biases between them. We demonstrate that a thorough investigation of the drifts with respect to ground-based observations is needed to evaluate TrOC trends from satellite data and that long-term trends in specific regions can be consistently detected, e.g., a positive trend of up to 1.5 DU per decade over China for the 2005–2021 period.