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.

In recent years, the need for high-quality long-term mesospheric ozone records has become increasingly evident, as they are essential for understanding chemical, dynamical, and radiative processes in the middle and upper atmosphere and their coupling with the lower layers. Here, we present a new merged dataset of ozone profiles in the middle atmosphere (METEOR-O3), created from several limb-viewing satellite instruments: HALOE, GOMOS, MIPAS, ACE-FTS, MLS, and SOFIE. The merged dataset covers the period from 1991 to 2023 and provides deseasonalized ozone anomalies in 10 degrees latitude bins between 80 degrees S and 80 degrees N, from approximately 22 to 100 km. The deseasonalized ozone anomalies are used for global and seasonal trend analysis. The results show positive upper stratospheric ozone trends in both hemispheres, with magnitudes of 1 %-2 % per decade between 35 and 45 km, indicating continued ozone recovery consistent with previous assessments. In contrast, mesospheric ozone (above similar to 60 km) exhibits negative trends of -1 % to -3 % per decade, with the strongest decreases of about -8 % to -12 % per decade between 80 and 90 km. Seasonal analyses confirm positive trends in the upper stratosphere across all seasons and persistent negative trends in the upper mesosphere, strongest at high latitudes above 80 km. The METEOR-O3 dataset provides the first global, long-term merged record suitable for detailed studies of mesospheric/lower thermospheric ozone variability and trend evaluation, providing valuable information for model validation and assessments of upper atmospheric changes.

Abstract. We develop and apply a novel method for estimating stratospheric mean age of air (AoA) from trace gases observed by the Atmospheric Chemistry Experiment – Fourier Transform Spectrometer (ACE-FTS). The method combines SF6 with five additional long-lived tracers, N2O, CH4, CFC-11, CFC-12, and HCFC-22, and is evaluated for ACE-FTS dataset versions 3.6 and 5.2. A proof of concept using the Chemical Lagrangian Model of the Stratosphere (CLaMS) shows that the multi-tracer approach reduces the uncertainty of zonal-mean AoA by about 50 % relative to conventional methods based only on SF6 and yields AoA estimates at individual profiles with an average uncertainty of about 0.3 years. Applied to ACE-FTS data, the multi-tracer method produces smoother AoA fields and strongly suppresses noise in sparsely sampled regions compared with the conventional convolution method. Comparison with in situ AoA estimates from SF6 and CO2 generally favors the multi-tracer product, although some discrepancies remain. The method provides a precise and spatially resolved satellite-based AoA product and a promising basis for future studies of long-term changes in stratospheric circulation. Comparison of ERA5 AoA with the new satellite product indicates a slow bias in the reanalysis. The long-term AoA trend pattern over 2004–2021 shows increasing age in the Northern Hemisphere stratosphere between about 18–28 km and decreasing age in the tropics and Southern Hemisphere subtropics, largely consistent with ERA5. In particular, AoA increases in the Northern relative to the Southern Hemisphere.

Climate models predict that the Brewer–Dobson circulation (BDC) will accelerate due to tropospheric warming, leading to a redistribution of trace gases and, consequently, to a change of the radiative properties of the atmosphere. Changes in the BDC are diagnosed by the so-called “age of air”, that is, the time since air in the stratosphere exited the troposphere. These changes can be derived from a long-term observation-based record of long-lived trace gases with increasing concentration in the troposphere, such as sulfur hexafluoride (SF6). The Atmospheric Chemistry Experiment Fourier Transform Spectrometer (ACE-FTS) provides the longest available continuous time series of vertically resolved SF6 measurements, spanning 2004 to the present. In this study, a new age-of-air product is derived from the ACE-FTS SF6 dataset. The ACE-FTS product is in good agreement with other observation-based age-of-air datasets and shows the expected global distribution of age-of-air values. Age of air from a chemistry–climate model is evaluated, and the linear trend of the observation-based age of air is calculated in 12 regions within the lower stratospheric midlatitudes (14–20 km, 40–70°) in each hemisphere. In 8 of 12 regions, there was not a statistically significant trend. The trends in the other regions, specifically 50–60 and 60–70° S at 17–20 km and 40–50° N at 14–17 and 17–20 km, are negative and significant to 2 standard deviations. This is therefore the first observation-based age-of-air trend study to suggest an acceleration of the shallow branch of the BDC, which transports air poleward in the lower stratosphere, in regions within both hemispheres.

According to satellite measurements from multiple instruments, water vapour (H2O) concentrations, in most regions of the stratosphere, have been increasing at a statistically significant rate of ∼1 %–5 % per decade since the early 2000s. Previous studies have estimated stratospheric H2O trends, but none have simultaneously quantified the contributions from all main sources (temperature variations in the tropical tropopause region, changes in the Brewer–Dobson circulation, and changes in methane (CH4) concentrations and oxidation) at all latitudes. Atmospheric Chemistry Experiment–Fourier Transform Spectrometer (ACE-FTS) measurements are used to estimate altitude-/latitude-dependent stratospheric H2O trends from 2004–2021 due to these sources. Results indicate that rising temperatures in the tropical tropopause region play a significant role in the increases, accounting for ∼1 %–4 % per decade in the tropical lower mid-stratosphere and in the mid-latitudes below ∼20 km. By regressing to ACE-FTS N2O concentrations, it is found that, in the lower mid-stratosphere, general circulation changes have led to both significant H2O increases and significant H2O decreases on the order of 1 %–2 % per decade depending on the altitude/latitude region. Making use of measured and modelled CH4 concentrations, the increase in H2O due to CH4 oxidation is calculated to be ∼1 %–2 % per decade above ∼30 km in the Northern Hemisphere and throughout the stratosphere in the Southern Hemisphere. After accounting for these sources, there are still regions of the mid-latitude lower mid-stratosphere that exhibit significant residual H2O trends increasing at 1 %–2 % per decade. Results indicate that these unaccounted-for increases could potentially be explained by increases in upper-tropospheric molecular hydrogen.

The Brewer-Dobson Circulation (BDC) is one of the main determinants of trace gas distributions in the atmosphere. Climate models predict that atmospheric warming will cause the BDC to accelerate, modifying where greenhouse gases are most active and impacting the radiative properties of the atmosphere, resulting in a feedback effect. This acceleration is difficult to verify with observations because the speed of the BDC cannot be measured directly. However, changes in stratospheric transport can be identified using the stratospheric age of air, defined as the time since an air parcel entered the stratosphere from the troposphere. A decrease in age of air at higher latitudes would suggest a reduction in transit times, signifying an acceleration of the BDC. Age of air can be calculated using long-lived “clock tracers” such as sulfur hexafluoride (SF6), an industrial gas that is produced in the troposphere, has a negligible seasonal cycle, and has no stratospheric sinks. Due to its small concentrations, measurements have been historically limited, but detecting changes in age of air derived from SF6 requires a long-term, and ideally consistent (i.e., measured by the same instrument), time series. The Atmospheric Chemistry Experiment Fourier Transform Spectrometer (ACE-FTS) provides the longest available vertically-resolved record of SF6, spanning 2004 to the present. This study presents a new age of air product derived from the ACE-FTS SF6 dataset using an updated version of the method used for the Michelson Interferometer for Passive Atmospheric Sounding (MIPAS) SF6 dataset, which spans the 2002-2012 period. In this presentation, the method for age of air calculation will be presented along with comparisons with other age of air profile datasets derived from MIPAS and balloon measurements. The long-term trend in age of air will be estimated using this new product with the goal of corroborating the predictions made by climate models.

Two decades of ACE-FTS, the Atmospheric Chemistry Experiment - Fourier Transform Spectrometer, version 5.2 (v5.2) ozone data (2004-2023) are evaluated with ozonesonde data from across the globe. The biases between the ACE-FTS and ozonesonde measurements are first estimated by analyzing coincident data pairs. A second approach is taken for the validation by comparing the ACE-FTS and ozonesonde monthly mean time series, with the former generated by sampling the ACE-FTS data within latitude/longitude boxes (i.e., +/- 5 degrees/+/- 30 degrees) surrounding the stations and calculating the monthly averages. The biases, correlations, variation patterns, and the mean states of the two time series are compared. The biases estimated in this way exhibit more consistent and smoother features than using the coincident pair method. The ACE-FTS and ozonesonde monthly mean time series are highly correlated and exhibit similar variation patterns in the lower stratosphere at all latitudes. The ACE-FTS instrument drifts for each station are assessed in terms of the long-term linear trends relative to ozonesondes, which, although highly stable, may have their own minor changes with time. The ACE-FTS ozone profiles exhibit in general high biases in the stratosphere for altitudes above similar to 20 km, increasing with altitude up to similar to 10 % at around 30 km. For altitudes between 20 km and the tropopause, biases of up to +/- 10 % are found, depending on altitude and latitude with the largest biases found in the tropics and southern mid-latitudes. The ACE-FTS instrument drifts are generally non-significant overall in the stratosphere with high variation between the stations. Averaging the individual station instrument drifts within several latitude bands results in small non-significant drifts of within +/- 1 %-2 % per decade in the northern mid-latitudes to high latitudes and the southern high latitudes. It also results in a positive but non-significant drift of up to 5 % per decade in the tropics and southern mid-latitudes, with overall uncertainties in this region ranging up to 5 %-10 % per decade (2 sigma level) in the low stratosphere. As part of this assessment, an analysis of ozonesonde measurement stability using ACE-FTS as a transfer standard is conducted and finds small step changes in ozonesonde response at some stations. These results are in general agreement with recent findings using other satellite data sources.

Abstract. Two decades of ACE-FTS version 5.2 (v5.2) ozone data (2004–2023) are evaluated with ozonesonde data from across the globe. The biases between the ACE-FTS and ozonesonde measurements are first estimated by analysing coincident data pairs. A second approach is taken for the validation by comparing the ACE-FTS and ozonesonde monthly mean time series, with the former generated by sampling the ACE-FTS data within latitude/longitude boxes (i.e., ±5°/±30°) surrounding the stations and calculating the monthly averages. The biases, correlations, variation patterns and the mean states of the two time series are compared. The biases estimated in this way exhibit more consistent and smoother features than using the coincident pair method. The ACE-FTS and ozonesonde monthly mean time series are highly correlated and exhibit similar variation patterns in the lower stratosphere at all latitudes. The ACE-FTS instrument drifts for each station are assessed in terms of the long-term linear trends relative to ozonesondes, which, although highly stable, may have their own minor changes with time. The ACE-FTS ozone profiles exhibit in general high biases in the stratosphere, increasing with altitude up to ~10 % at around 30 km, and have local maximum differences with ozonesonde profiles at the tropopause heights. The ACE-FTS instrument drifts are generally insignificant overall in the stratosphere with high variation between the stations. Averaging the individual station instrument drifts within several latitude bands results in small insignificant drifts of within ±1 % dec-1 in the northern mid- to high latitudes, and the southern high latitudes, and a small positive insignificant drift of 0–3 % dec-1 in the tropics and southern mid-latitudes with overall uncertainties at 2–3 % dec-1 (2σ level) in the low stratosphere. In the troposphere, the average ACE-FTS instrument drifts vary with altitude and exhibit large drifts between -10 and +10 % dec-1 with uncertainties of 10 % dec-1.

The Atmospheric Chemistry Experiment Fourier Transform Spectrometer (ACE-FTS) is currently providing the only measurements of vertically resolved chlorodifluoromethane (HCFC-22) from space. This study assesses the ACE-FTS HCFC-22 v5.2 product in the upper troposphere and lower stratosphere, as well as simulations of HCFC-22 from a 39-year specified dynamics run of the Canadian Middle Atmosphere Model (CMAM39) in the same region. In general, ACE-FTS HCFC-22 observations tend to agree with subsampled CMAM39 data to within ±5 %, except for between ∼ 15 and 25 km in the extratropical regions where ACE-FTS exhibits a negative bias of 5 %–30 % and near 6 km in the tropics where ACE-FTS exhibits a bias of −15 %. When comparing against correlative satellite, aircraft, and balloon data, ACE-FTS typically exhibits a low bias on the order of 0 %–10 % between ∼ 5 and 15 km and is within ±15 % between ∼ 15 and 25 km. ACE-FTS, CMAM39, and surface flask measurements from the NOAA Global Monitoring Laboratory's surface air-sampling network all exhibit consistent tropospheric HCFC-22 trends ranging between 6.8 and 7.8 ppt yr−1 (within 95 % confidence) for 2004–2012 and between 3.1 and 4.7 ppt yr−1 (within 95 % confidence) for 2012–2018. Interhemispheric differences (IHDs) of HCFC-22 were also derived using ACE-FTS, NOAA, and CMAM39 data, and all three yielded consistent and correlated (r≥0.42) IHD time series, with the results indicating that surface IHD values decreased at a rate of 2.2 ± 1.1 ppt per decade between 2004 and 2018.

In this paper, we present the updated SAGE-CCI-OMPS+ climate data record of monthly zonal mean ozone profiles. This dataset covers the stratosphere and combines measurements by nine limb and occultation satellite instruments – SAGE II (Stratospheric Aerosol and Gases Experiment II), OSIRIS (Optical Spectrograph and InfraRed Imaging System), MIPAS (Michelson Interferometer for Passive Atmospheric Sounding), SCIAMACHY (SCanning Imaging Spectrometer for Atmospheric CHartographY), GOMOS (Global Ozone Monitoring by Occultation of Stars), ACE-FTS (Atmospheric Chemistry Experiment Fourier Transform Spectrometer), OMPS-LP (Ozone Monitor Profiling Suite Limb Profiler), POAM (Polar Ozone and Aerosol Measurement) III, and SAGE III/ISS (Stratospheric Aerosol and Gases Experiment III on the International Space Station). Compared to the original version of the SAGE-CCI-OMPS dataset (Sofieva et al., 2017b), the update includes new versions of MIPAS, ACE-FTS, and OSIRIS datasets and introduces data from additional sensors (POAM III and SAGE III/ISS) and retrieval processors (OMPS-LP). In this paper, we show detailed intercomparisons of ozone profiles from different instruments and data versions, with a focus on the detection of possible drifts in the datasets. The SAGE-CCI-OMPS+ dataset has a better coverage of polar regions and of the upper troposphere and the lower stratosphere (UTLS) than the previous dataset. We also studied the influence of including new datasets on ozone trends, which are estimated using multiple linear regression. The changes in the merged dataset do not change the overall morphology of post-1997 ozone trends; statistically significant trends are observed in the upper stratosphere. The largest changes in ozone trends are observed in polar regions, especially in the Southern Hemisphere. The updated SAGE-CCI-OMPS+ dataset contains profiles of deseasonalized anomalies and ozone concentrations from 1984 to 2021, in 10∘ latitude bins from 90∘ S to 90∘ N and in the altitude range from 10 to 50 km. The dataset is open access and available at https://climate.esa.int/en/projects/ozone/data/ (last access: 9 March 2023) and at ftp://cci_web@ftp-ae.oma.be/esacci (ESA Climate Office; last access: 9 March 2023).

This paper will describe current validation studies and overall mission status for the Atmospheric Chemistry Experiment Fourier Transform Spectrometer (ACE-FTS). This will focus on validation of the newest data version and on long-term analysis.

<p>The Atmospheric Chemistry Experiment – Fourier Transform Spectrometer (ACE-FTS) is a high spectral resolution (0.02 cm<sup>-1</sup>) spectrometer on the SciSat satellite that has been taking solar occultation measurements of the Earth’s limb since February 2004. ACE-FTS measures vertical profiles of temperature/pressure and concentrations of over 60 trace gases, including ozone and water vapour, with a vertical resolution of ~2-6 km. From the beginning of the mission, ACE-FTS water vapour data exhibits a significant positive trend on the order of a few percent per decade throughout the stratosphere and mesosphere, in agreement with similar measurements from the Microwave Limb Sounder (MLS) and the Sounding of the Atmosphere using Broadband Emission Radiometry (SABER) instruments. This study will investigate the link between ACE-FTS water vapour trends and decadal variations in other atmospheric parameters, including local and tropopause temperatures, ozone and methane concentrations, and solar flux, and will discuss how ACE-FTS measurements of HDO can be used to continue measuring background water vapour trends post-eruption of the Hunga Tonga-Hunga Ha’apai volcano, which injected ~160 Tg of water vapour (more than 10% of the stratospheric water vapour budget) into the stratosphere.</p>

Trends in stratospheric trace gases like HCl, N2O, O3, and NOy show a hemispheric asymmetry over the last 2 decades, with trends having opposing signs in the Northern Hemisphere and Southern Hemisphere. Here we use N2O, a long-lived tracer with a tropospheric source, as a proxy for stratospheric circulation in the multiple linear regression model used to calculate stratospheric trace gas trends. This is done in an effort to isolate trends due to circulation changes from trends due to the chemical effects of ozone-depleting substances. Measurements from the Atmospheric Chemistry Experiment Fourier Transform Spectrometer (ACE-FTS) and the Optical Spectrograph and InfraRed Imager System (OSIRIS) are considered, along with model results from the Whole Atmosphere Community Climate Model (WACCM). Trends in HCl, O3, and NOy for 2004–2018 are examined. Using the N2O regression proxy, we show that observed HCl increases in the Northern Hemisphere are due to changes in the stratospheric circulation. We also show that negative O3 trends above 30 hPa in the Northern Hemisphere can be explained by a change in the circulation but that negative ozone trends at lower levels cannot. Trends in stratospheric NOy are found to be largely consistent with trends in N2O.

Abstract. The Atmospheric Chemistry Experiment Fourier Transform Spectrometer (ACE-FTS) is currently providing the only measurements of vertically resolved chlorodifluoromethane (HCFC-22) from space. This study assesses the ACE-FTS HCFC-22 v5.2 product in the upper troposphere – lower stratosphere, as well as the simulated concentrations of HCFC-22 from a 39-year specified dynamics run of the Canadian Middle Atmosphere Model (CMAM39) in the same region. In general, ACE-FTS HCFC-22 observations tend to agree with subsampled CMAM39 data to within ±5 %, except for between ~15 and 25 km in the extratropical regions where ACE-FTS exhibits a negative bias of 5–30 %, and near 6 km in the tropics where ACE FTS exhibits a bias of 15 %. When comparing against correlative satellite, aircraft, and balloon data, ACE-FTS typically exhibits a low bias on the order of 0–10 % between ~5–15 km and is within ±15 % between ~15–25 km. ACE-FTS, CMAM39, and surface flask measurements from the NOAA Global Monitoring Laboratory’s surface air-sampling network, all exhibit consistent tropospheric HCFC-22 trends ranging between 6.8 and 7.8 pptv/year (within 95 % confidence) for 2004–2012, and between 3.1 and 4.7 pptv/year (within 95 % confidence) for 2012–2018. Interhemispheric differences (IHD) of HCFC-22 concentrations were also derived using ACE-FTS, NOAA, and CMAM39 data, and all three yielded consistent and correlated (r≥0.42) IHD timeseries, with the results indicating that surface IHD values decreased at a rate of 2.2±1.1 pptv/decade between 2004 and 2018.

3.5/3.6(panel (a)) and version 4.1/4.2(panel (b)).

The v7.2 NO2 retrieval for the Optical Spectrograph and InfraRed Imager System (OSIRIS) was designed to improve sensitivity in the upper troposphere–lower stratosphere (UTLS) and to reduce an observed low bias in the previous version, v6.0. The details of this retrieval are described and then the data are compared to coincident NO2 profiles from the Atmospheric Chemistry Experiment–Fourier Transform Spectrometer (ACE-FTS) and the Stratospheric Aerosol and Gas Experiment III on the International Space Station (SAGE III/ISS). The PRATMO photochemical box model was used to account for differences in the measurement times of the instruments: all datasets were scaled to the same local solar time of 12:00 LST. Coincident ACE-FTS and OSIRIS NO2 measurements agree within 20 % throughout much of the stratosphere. Coincident SAGE III/ISS and OSIRIS NO2 measurements also agree within 20 %, with OSIRIS biased low at all altitudes and latitudes. The ACE-FTS, OSIRIS, and SAGE III-ISS NO2 monthly zonal mean data show very similar variability in time at most altitude and latitudes.