In February 2022, the Canadian-led Atmospheric Chemistry Experiment (ACE) mission completes its eighteenth year of measurements on board the SCISAT satellite. The long lifetime of ACE provides a valuable time series of composition measurements that contribute to our understanding of ozone recovery, climate change and pollutant emissions. These profiles of atmospheric trace gases and aerosols provide altitude-resolved data that are necessary for understanding processes that occur at specific altitudes or over limited vertical length scales.

The SCISAT/ACE mission uses infrared and UV-visible spectroscopy to make its solar occultation measurements. The primary instrument on-board, the ACE Fourier Transform Spectrometer (ACE-FTS) is a high-resolution (0.02 cm-1) FTS operating between 750 and 4400 cm-1. It also contains two filtered imagers (0.525 and 1.02 microns) to measure atmospheric extinction due to clouds and aerosols. The second instrument is a dual UV-visible-NIR spectrophotometer called ACE-MAESTRO (Measurements of Aerosol Extinction in the Stratosphere and Troposphere Retrieved by Occultation) which was designed to extend the ACE wavelength coverage to the 280-1030 nm spectral region. From these measurements, altitude profiles of atmospheric trace gas species, aerosol extinction, temperature and pressure are retrieved. The 650 km altitude, 74 degree circular orbit of SCISAT provides global measurement coverage over a latitude range of ~85 ºN to ~85 ºS with a focus on the Arctic and Antarctic regions.

The ACE data set can be combined with other data sets to provide the climate data records required for long term monitoring of ozone and related species and for initialization and testing of chemistry-climate models. In order to do this, it is essential to quantify the biases between the different instruments and investigate their changes over the operational time period. Validation and comparison studies are a necessary component of this data assessment process. Highlights of validation and science results from the ACE mission will be presented in this paper along with mission and instrument status.

Over roughly the last two decades, aerosol amounts in the stratosphere have been highly variable due to perturbations from volcanic eruptions and pyro-cumulous forest fire events. This has been especially true in the last few years with volcanic eruptions and forest fire events spanning the tropics to high latitudes creating a complex evolution of aerosol distributions. A large fraction of the aerosol from these events resides in the lowermost stratosphere, which occurs at mid and high latitudes between the tropopause (near 8-10 km altitude) and approximately 15 km, and is a challenging region to measure with satellite instrumentation. However, with the installation of SAGE III on ISS in 2017, and the ongoing operations of both OSIRIS on the Odin satellite (an ESA Third-Party Mission), OMPS-LP on the NASA SNPP satellite, and NASA CALIPSO, we have a unique opportunity to study these complex aerosol events with the perspectives provided by the combined solar occulation, limb scatter, and lidar observations. This work explores the impact of three recent aerosol events using this combined set of observations with a focus on the lowermost stratosphere: Ambae volcanic eruption in 2018, Raikoke volcanic eruption in 2019, and the Australian fire event in 2020. Several aspects are addressed including biases between instruments, sensitivity to clouds and saturation, zonal variability, and how these factors depend on viewing geometry, aerosol loading conditions, and source.

In our study we present an update of global and regional total ozone trends and their seasonal dependence derived from the GOME-type Total Ozone Essential Climate Variable (GTO-ECV) data record that covers the past 26 years (1995-2021). GTO-ECV combines observations from a series of six nadir-viewing satellite sensors of the GOME-type that measure in the ultraviolet and visible spectral range. The excellent inter-sensor consistency, achieved by the common retrieval algorithm GODFIT_v4, is a good prerequisite for merging the individual datasets. GTO-ECV provides monthly mean total ozone at a remarkable spatial resolution of 1°x1°. It has been developed by the German Aerospace Center (DLR) and as part of the European Space Agency’s Climate Change Initiative (ESA-CCI) ozone project since 2010. In the framework of ESA-CCI+ it has been further improved and expanded with new satellite sensors including TROPOMI/Sentinel-5P and GOME-2/MetOp-C. In the framework of the EU/ECMWF Copernicus Climate Change Service (C3S/2) ozone projects it is extended in time on a quasi-operational basis.
Investigating the long-term evolution of total ozone is of major interest due to its role as a key constituent in the Earth’s atmosphere. Although the success of the Montreal Protocol and its subsequent amendments is undisputed, and apparent signs of recovery have already been detected, open questions remain, in particular with regard to hemispheric differences, regional dependence of the trends and their respective uncertainties, as well as the impact of climate change and possible variations and long-term changes in atmospheric dynamics. We report on the spatial as well as the seasonal distribution of the trends and the correlation with the explanatory variables (e.g., QBO, ENSO, solar cycle or Arctic/Antarctic oscillation) used in the standard multiple linear least-squares regression model. In the middle latitudes of the Southern Hemisphere we found significant positive trends of about ~1%/decade. In the Northern Hemisphere we found a pronounced longitudinal structure in the trend pattern with significant positive trends over the North Atlantic region and non-significant negative trends elsewhere, except for Eurasia. Regarding the seasonal variation of the trends we found different patterns depending on latitude that point to dynamic effects in addition to effects related to decreasing amounts of Ozone Depleting Substances.

In this work, we analysed in depth the trends in stratospheric ozone profiles. For the analyses of the seasonal dependence of ozone trends, we used four long-term merged data sets, SAGE-CCI-OMPS, SAGE-OSIRIS-OMPS, GOZCARDS, and SWOOSH, which provide more than 30 years of monthly zonal mean ozone profiles in the stratosphere. We focused on trends between 2000 and 2020. All merged datasets show similar results, although some discrepancies are observed. In the upper stratosphere, the trends are positive throughout all seasons and the majority of latitudes. The largest upper-stratospheric ozone trends are observed during local winter (up to 6% per decade) and equinox (up to 3% per decade) at mid-latitudes. In the equatorial region, we found a very strong seasonal dependence of ozone trends at all altitudes: the trends vary from positive to negative, with the sign of transition depending on altitude and season.

For analyses of regional trends in stratospheric ozone profiles, we used a recently created MErged GRIdded Dataset of Ozone Profiles (MEGRIDOP) with a resolved longitudinal structure. MEGRIDOP is derived from data from six limb and occultation satellite instruments: GOMOS, SCIAMACHY and MIPAS on Envisat, OSIRIS on Odin, OMPS on Suomi-NPP, and MLS on Aura. It covers the period from late 2001 until the end of 2020. We found that stratospheric ozone trends exhibit longitudinal structures at Northern Hemisphere middle and high latitudes, with enhanced trends over Scandinavia and the Atlantic region. This agrees well with previous analyses and might be due to changes in dynamical processes related to the Brewer–Dobson circulation.

From merged long-term total ozone datasets a recovery of about +0.5%/decade in annual mean zonal mean total ozone was found in the extratropics through 2020. The five merged datasets used are SBUV COH (NOAA) and SBUV MOD (NASA), both based upon ozone profile data from the series of BUV, SBUV, SBUV/2, and OMPS-NP satellite instruments spanning the years from 1970 to 2020, ESA CCI GTO-ECV and University of Bremen GSG (both 1995-2020) based upon the series of GOME, SCIAMACHY, GOME-2, OMI (CCI only), and TROPOMI (CCI only), and WOUDC zonal mean data (1964-2020) derived from ground based Brewer, Dobson, SAOZ spectrometers, and Russian filter instruments.

The extratropical recovery trends are consistent with the continuous decline in stratospheric halogen loading since about the middle 1990s as a consequence of the Montreal Protocol and its Amendments on phasing out ozone depleting substances. Nevertheless, the recovery trends in the northern hemisphere have been compensated by changes in atmospheric dynamics leading to stable ozone levels since about 2000 (apart from year-to-year variability). Currently, the near-global (60°S-60°N) average total ozone level is about 2.5% below the 1964-1980 mean.

Using the TOMCAT global 3-D chemistry transport model (CTM), forced by ERA5 reanalysies, we compare trends from the merged data sets with model simulations to check on the consistency between both and to elucidate further on the impact from long-term changes in atmospheric circulation and transport on global total ozone changes.

A recovery of winter polar ozone has been so far confirmed above Antarctica during the onset of the ozone hole in September, but not in later months and in the Arctic. The model data will be used to investigate trends in the polar region in more detail, in particular in those months where no wintertime observations from the UV satellites are available due to polar night conditions.

The Gimballed Limb Observer of Air Radiance in the Atmosphere (GLORIA) is an airborne IR limb imager capable of acquiring both 2-D data sets (curtains along the flight path) and, when the carrier aircraft is flying around the observed air mass, spatially highly resolved 3-D tomographic data (temperature and trace gas concentrations). Here we present a case study of a Rossby wave (RW) breaking event observed during two subsequent flights two days apart as part of the Wave Driven Isentropic Exchange (WISE) campaign in October 2017. RW breaking is known to steepen tracer gradients and facilitate stratosphere-troposphere exchange (STE). Our measurements reveal complex spatial structures in stratospheric tracers (ozone and nitric acid) with multiple vertically stacked filaments. Backward trajectory analysis is used to demonstrate that these features are related to several previous Rossby wave breaking events and that the small-scale structure of the UTLS in the Rossby wave breaking region, which is otherwise very hard to observe, can be understood as stirring and mixing of air masses of tropospheric and stratospheric origin. The measurements showed signatures of enhanced mixing between stratospheric and tropospheric air near the polar jet with some transport of water vapour into the stratosphere. Some of the air masses seen in 3-D data were encountered again two days later, stretched to very thin filament (horizontal thickness down to 30 km at some altitudes) rich in stratospheric tracers. This repeated measurement allowed us to directly observe and analyse the progress of mixing processes in a thin filament over two days.

GLORIA is the airborne demonstrator of the satellite based infrared limb imager CAIRT proposed as the Earth Explorer 11 candidate. The high vertical and horizontal resolution proposed for the CAIRT instrument as well as its ability to observe a large 3-D volume in a single pass would result in a high probability to observe air masses repeatedly, as was demonstrated with GLORIA 3-D data in this study. The global data sets that CAIRT would produce would then be very valuable for mixing characterisation.