In this work, sulphur hexafluoride (SF6), which is often used as a tracer for stratospheric transport due to its inertness in the stratosphere and nearly linear growth rate in the troposphere, is included in the chemistry transport model (CTM) of the Belgian Assimilation System for Chemical ObsErvations (BASCOE). Sink and recovery reactions for this species are implemented in the model, which has a top in the mesosphere at 0.01 hPa. The simulated SF6 distributions are compared with MIPAS and ACE-FTS observations and the global atmospheric lifetime is computed from CTM runs driven by three recent meteorological reanalyses: ERA5, MERRA2 and JRA-3Q. The results show that BASCOE SF6 profiles are generally within 10 % of the satellite observations below 10 hPa, although discrepancies increase at higher altitudes. The global atmospheric lifetime is used as an additional diagnostic for the implementation of the chemistry in the mesosphere, where satellite measurements are unavailable. The derived SF6 lifetimes are 2646 years with ERA5, 1909 years with MERRA2 and 2147 years with JRA-3Q, in accordance with recent literature. Due to the large spread of published lifetimes for SF6, the study is extended to N2O, CH4, CFC-11, CFC-12 and HCFC-22, to validate the SF6 results. The lifetimes for these species are in agreement with previously reported values, and their spread between simulations is smaller compared to SF6. This analysis highlights the sensitivity of SF6 to the input reanalysis data sets and thus to differences in dynamics.

The planned deployment of satellite mega-constellations will substantially increase the flux of anthropogenic space debris re-entering Earth’s atmosphere. A large fraction of this material is composed of aluminum, which will ablate during re-entry and form aluminum oxide (Al2O3) containing aerosols in the mesosphere and lower thermosphere. These particles represent a new, human-made metal aerosol source that may interact with natural meteoric smoke and potentially impact upper-and middle- atmospheric chemistry, radiative balance, polar mesospheric cloud, polar stratospheric cloud as well as stratospheric aerosol formation. However, observational constraints on the abundance and vertical distribution of such aluminum-bearing aerosols are currently very limited.Aluminum oxide exhibits characteristic spectral features in the mid-infrared, allowing detection via remote sensing spectroscopic measurements. In contrast to techniques based on scattering in the visible wavelength range, mid-infrared spectroscopic detection is independent of particle size as long as the particle radius remains small compared to the wavelength. This makes it particularly suited to constraining nanometer- to sub-micrometer-sized aluminum oxide aerosols expected from debris ablation. Moreover, spectrally resolved infrared limb measurements enable the quantification of total aerosol volume (and thus mass) profiles, providing a direct link between observed aerosol burdens and modeled debris input fluxes.In this work, we quantitatively assess the capability of a space-borne infrared limb-imaging instrument to detect and characterize aluminum oxide aerosols from re-entering space debris. We perform end-to-end simulations of atmospheric radiances and instrument response in the mid-infrared, incorporating realistic Al2O3 optical properties and assumed vertical profiles derived from debris model scenarios associated with upcoming mega-constellations. Radiative transfer calculations are used to compute infrared limb-emission spectra and sensitivities, which are then passed through an instrument simulator system representative of the CAIRT (Changing-Atmosphere Infra-Red Tomography) limb-imaging mission concept, studied as an EE11 candidate for ESA’s Earth Explorer program.We demonstrate that the characteristic mid-infrared absorption features of aluminum oxide remain detectable at realistic noise levels for CAIRT-like performance, over a range of plausible aerosol loads. Sensitivity analyses show that vertical profiles of total Al2O3 aerosol volume can be retrieved, even when particle sizes and shapes are not well constrained. Our results indicate that a CAIRT-type infrared limb-sounding mission could provide the first global, vertically resolved observational constraints on aluminum oxide aerosols from space debris.

The Changing Atmosphere Infra-Red Tomography Explorer (CAIRT) is a satellite mission concept that was proposed within ESA's Earth Explorer 11 (EE11) programme. Although it was not selected for implementation in the EE11 competition, the CAIRT concept was assessed as scientifically and technically mature and could be implemented under another ESA framework or through international collaboration. The mission is conceived to achieve a step change in our understanding of the coupling between atmospheric circulation, composition, and regional climate. The CAIRT concept is based on limb infrared tomography of the atmosphere from the troposphere to the lower thermosphere (between similar to 5 and similar to 115 km altitude) with a similar to 400 km-wide swath, providing a three-dimensional view of atmospheric structure at unprecedented scales.This paper investigates the capability of CAIRT to resolve gradients of ozone (O3) and water vapour (H2O) in the upper troposphere and lower stratosphere (UTLS) using an Observing System Simulation Experiment (OSSE). In this experiment, a reference atmosphere - the nature run in OSSE terminology - is built from the Copernicus Atmosphere Monitoring Service (CAMS) control simulation between October 2021 and March 2022. The nature run is used to synthesise CAIRT observations of O3 and H2O, employing a CAIRT orbit simulator and a simulator that accounts for CAIRT's expected measurement errors, vertical and along-track vertical resolution. Simulated CAIRT observations are then assimilated with the Belgian Assimilation System for Chemical ObsErvations (BASCOE) to produce analyses of O3 and H2O (the assimilation run). To quantify the added value of CAIRT data, a BASCOE control run without CAIRT assimilation is also performed. For comparison, simulated Aura Microwave Limb Sounder (MLS) O3 and H2O observations are also assimilated, allowing the performance of CAIRT to be evaluated relative to this reference instrument.The results show that (1) CAIRT O3 observations are able to constrain BASCOE analyses down to 7 km altitude - several kilometres lower (and thus better) than MLS; and (2) H2O observations can constrain BASCOE down to the tropopause region, with slightly better performance than MLS at high latitudes. Additional sensitivity tests assess the impact of CAIRT systematic errors, cloud coverage, and reduced swath width.

Following the increase of greenhouse gas emissions, atmospheric models predict a strengthening of the middle atmospheric Brewer-Dobson circulation (BDC). Changes in the BDC, inferred from age of air (AoA) trends, can influence UTLS exchange processes, including stratosphere–troposphere transport of ozone. While models predict an acceleration of the BDC (i.e. a decrease of AoA), in-situ balloon observations suggest the opposite, although not significantly, given the limited number of observations and the substantial uncertainties (Garny et al., 2024a). Additionally, meteorological reanalyses disagree on the sign and magnitude of AoA trends, despite providing an optimized estimate of atmospheric circulation constrained by observations.The Changing Atmosphere Infrared Tomography explorer (CAIRT) was proposed for ESA’s Earth Explorer 11 to address these inconsistencies. CAIRT was foreseen to achieve a precision of 0.5 years on the age of air, a requirement to assess long-term trends. This contribution aims to evaluate the capability of CAIRT to achieve this precision. Synthetic CAIRT profiles of six long-lived species (SF6, CH4, N2O, CFC11, CFC12 and HCFC22) are simulated by the Belgian Assimilation System for Chemical ObsErvations (BASCOE) chemistry transport model, considering CAIRT’s expected measurement errors and spatial resolution. CAIRT AoA observations, derived from the six long-lived species using the method of Voet et al. (2025), are compared to clock tracer AoA, simulated by the BASCOE model, to evaluate the agreement. The analysis is repeated three times by driving the model with the meteorological reanalyses MERRA2, ERA5, and JRA-3Q, respectively, to check if CAIRT precision would be sufficient to evaluate meteorological reanalyses.

Abstract. Using a model-based pseudo-reality (PR) future scenario and realistic pseudo-observations (POs) representative for a proposed satellite mission concept (CAIRT – the Changing-Atmosphere Infra-Red Tomography explorer), we analyse the capabilities of a high-spectral-resolution limb-emission sounding techniques to detect and quantitatively monitor very weak stratospheric aerosol injections (SAI) geoengineering interventions, in terms of the injected sulphur dioxide (SO2) and its evolution into sulphate aerosols (SA). Our results suggest that this technique would detect SO2 injections, at the horizontal and vertical scale, within hours to a few days since the SAI deployment. This concept would quantify the SO2 injected mass, even for injections of the order of magnitude of a few to some tens of tonnes of SO2, characteristic of "near-term" to "mid-term" experiments feasible even unilaterally/illegally, or in the context of small-scale outdoor experiments, with presently existing technology and at relatively low cost. In addition, our results suggest that this concept would be able to track the temporal evolution of the subsequently formed SA, as it spreads zonally and then toward higher latitudes through meridional dispersion, and to monitor changes in its vertical distribution over time through processes like self-lofting of the resulting SA plume. Existing satellite instruments, e.g. based on limb scattering, solar occultation, nadir observations and space LiDARs, do not have the capability to carry out such strategical observations of SAI. Our results stress the importance of increasing our global observational capabilities with high-spectral-resolution limb-emission satellite instruments, a capability not available at present and not foreseen in the near future.

Abstract. The Changing Atmosphere Infra-Red Tomography Explorer (CAIRT) is a satellite mission concept that was proposed within ESA's Earth Explorer 11 (EE11) programme. Although it was not selected for implementation in the EE11 competition, the CAIRT concept was assessed as scientifically and technically mature and could be implemented under another ESA framework or through international collaboration. The mission is conceived to achieve a step change in our understanding of the coupling between atmospheric circulation, composition, and regional climate. The CAIRT concept is based on limb infrared tomography of the atmosphere from the troposphere to the lower thermosphere (between ∼ 5 and ∼ 115 km altitude) with a ∼ 400 km-wide swath, providing a three-dimensional view of atmospheric structure at unprecedented scales. This paper investigates the capability of CAIRT to resolve gradients of ozone (O3) and water vapour (H2O) in the upper troposphere and lower stratosphere (UTLS) using an Observing System Simulation Experiment (OSSE). In this experiment, a reference atmosphere – the nature run in OSSE terminology – is built from the Copernicus Atmosphere Monitoring Service (CAMS) control simulation between October 2021 and March 2022. The nature run is used to synthesise CAIRT observations of O3 and H2O, employing a CAIRT orbit simulator and a simulator that accounts for CAIRT's expected measurement errors, vertical and along-track vertical resolution. Simulated CAIRT observations are then assimilated with the Belgian Assimilation System for Chemical ObsErvations (BASCOE) to produce analyses of O3 and H2O (the assimilation run). To quantify the added value of CAIRT data, a BASCOE control run without CAIRT assimilation is also performed. For comparison, simulated Aura Microwave Limb Sounder (MLS) O3 and H2O observations are also assimilated, allowing the performance of CAIRT to be evaluated relative to this reference instrument. The results show that (1) CAIRT O3 observations are able to constrain BASCOE analyses down to 7 km altitude – several kilometres lower (and thus better) than MLS; and (2) H2O observations can constrain BASCOE down to the tropopause region, with slightly better performance than MLS at high latitudes. Additional sensitivity tests assess the impact of CAIRT systematic errors, cloud coverage, and reduced swath width.

Abstract The Earth’s middle atmosphere spans the deep region from the upper troposphere/lower stratosphere at around 10 km altitude to the mesosphere/lower thermosphere at around 100 km altitude. It is being increasingly recognized for its role in driving extreme surface weather and regional climate change. Climate models predict large ongoing and future changes in the middle atmosphere composition and circulation. However, the observations needed to detect, attribute and understand these changes and their impacts, to test predictions, and thereby to improve our models, are lacking. Here we show the capacity of infrared limb-imaging tomography to provide the needed observations. This evaluation is based on studies performed within a recent satellite mission concept – the Changing-Atmosphere Infrared Tomography Explorer, CAIRT. Observing thermal infrared emissions simultaneously from the middle troposphere at about 4 km up to the lower thermosphere at about 115 km altitude this technique provides observations of temperature and an extensive range of trace gases with unprecedented spatial resolution of about 50 by 50 km horizontally and about 1 km vertically. We show how these observations would (a) help to quantify the changing atmospheric circulation, (b) allow characterization and quantification of the gravity waves that are critical in driving this circulation, (c) reveal how variability in solar radiation and energetic particles propagate downward to affect regional climate at the surface, (d) detect how volcanic eruptions and wildfires impact the middle atmosphere and climate, and (e) resolve how stratosphere-troposphere exchange affects ozone and water vapor in the crucial and climate-relevant tropopause region.

Abstract. Stratospheric aerosol injection (SAI) has been proposed as a potential method to counteract anthropogenic greenhouse gas–driven global warming but it may perturb the stratospheric ozone layer. Here, we use existing SAI scenario simulations as pseudo-reality (PR) input to assess how a future mid-infrared limb-emission sounding observing system would characterise its modelled ozone response. Our PR scenarios were generated using CESM2(WACCM6) simulations following the SSP5-34-OS overshoot pathway, with and without SAI. Pseudo-observations (PO) of a future instrument, modelled around the satellite mission concept CAIRT (the Changing-Atmosphere Infra-Red Tomography explorer), were generated using mission performance simulators, providing full error propagation and spatial smoothing characteristics. Our results demonstrate that a CAIRT-like mission can monitor and quantitatively characterise global, regional and seasonal ozone impacts associated with this SAI scenario. The parent PR fields used here contain a modelled SAI-induced ozone response, including: a) a pronounced additional depletion of total column ozone in the southern hemispheric high latitudes, with austral springtime reductions exceeding 20 DU between 2033–2062, consistent with enhanced heterogeneous halogen activation on sulphate aerosols, b) a delay in Antarctic ozone recovery, and c) a moderate ozone increase in winter and spring at northern hemispheric mid and high latitudes, associated with altered transport and weakened subtropical jets. All these different decadal impacts of SAI interventions are observable with CAIRT PO and are fully distinguishable from a baseline non-SAI scenario. These findings highlight the importance of advanced satellite observations, which are not available nowadays, to monitor and evaluate these impacts.

Contrary to the Antarctic, where ozone recovery has been observed for about a decade, the detection of positive ozone trends in the Arctic remains challenging due to higher natural variability of ozone in that region.Using a merging of long-term ozone data from Fourier transform infrared spectrometers, ozonesondes, and Dobson and Brewer spectrophotometers, we present regional long-term trends (2000-2024) for total, stratospheric and tropospheric ozone. First, ground-based measurements are cross-compared to two satellite data sets (MEGRIDOP and IASI-CDR). This enables the detection of drifts in ground-based data sets we further exclude from our study. We then use a representativeness study based on CAMS re-analysis data to define regions for which representative trends with reduced uncertainties are obtained by combining data sets from different instruments and stations. Annual and seasonal trends are calculated using a multiple linear regression technique involving a set of proxies that represent physical processes influencing the natural ozone variability.Annual trends indicate increasing total ozone over the Arctic, and are statistically significant over Canada and Reykjavik (+2.1 % per decade) and North-West Europe (Harestua and Lerwick, +0.7 % per decade). Ozone recovery is also observed over Canada in the mid-stratosphere (+2.0 % per decade) and over the North Pole region (Canada and Ny-& Aring;lesund) in the upper stratosphere (+2.1 % per decade to +3.8 % per decade). By analyzing the sensitivity of the ozone trends to the proxies, we observe a slow down of the expected ozone recovery, especially in the lower stratosphere, due to stratospheric cooling (-0.6 % per decade) and to the increase of volume of polar stratospheric clouds (-0.8 % per decade).

Abstract. Large positive anomalies of nitrous oxide (N2O) were observed in the northern hemisphere lower stratosphere in the late 2018/2019 boreal winter. Thanks to its long lifetime in the lower stratosphere, N2O is a robust tracer for stratospheric transport. This study investigates the magnitude, vertical structure, and dynamical origin of the late 2018/2019 boreal winter N2O anomaly using multiple chemical transport model simulations, chemical reanalyses, a specified-dynamics chemistry-climate model, and merged satellite observations. All datasets consistently show pronounced N2O positive anomalies in February 2019 in the northern mid-latitudes at 50 hPa. The N2O Transformed Eulerian Mean budget indicates that the N2O anomalies are primarily driven by enhanced meridional residual advection, which is in turn determined by enhanced planetary wave forcing. The extratropical effects of the quasi-biennial oscillation (QBO) determine these transport anomalies: the poleward QBO secondary circulation was unusually strong in the late 2018/2019 boreal winter, and a marked northward displacement of the zero wind line induced the enhanced planetary wave forcing. The combined strengthening of both the northward QBO secondary circulation and the planetary wave forcing led to unusually strong poleward advection in the northern lower stratosphere, which ultimately built up the N2O positive anomalies. These results indicate the value of long-lived stratospheric tracers such as N2O for diagnosing dynamically driven extreme events and for disentangling the contributions of different transport processes. We highlight the importance of the QBO teleconnections, particularly as a warming climate may change the frequency and intensity of extreme events thereby impacting these teleconnections.

The Changing-Atmosphere Infra-Red Tomography Explorer (CAIRT) is currently in Phase A as one of two final candidates for ESA’s Earth Explorer 11. As a Fourier transform infrared limb imager, CAIRT will observe simultaneously from the middle troposphere to the lower thermosphere at high spectral resolution and with unprecedented horizontal and vertical resolution. With this, CAIRT will provide critical information on (a) atmospheric gravity waves, circulation and mixing, (b) coupling with the upper atmosphere, solar variability and space weather and, (c) aerosols and pollutants in the upper troposphere and lower stratosphere. In this presentation we will give an overview of CAIRT’s science goals and the expected mission performance, based on latest results from feasibility studies performed during Phase 0.

Polar winter descent of reactive nitrogen (NOy) produced by energetic particle precipitation (EPP) in the mesosphere and lower thermosphere affects polar stratospheric ozone by catalytic reactions. This, in turn, may have implications for regional climate via radiative and dynamical feedbacks. NOy observations taken by the MIPAS/Envisat instrument during 2002--2012 have provided observational constraints on the solar-activity modulated variability of stratospheric EPP-NOy amounts. These constraints have allowed to formulate a chemical upper boundary condition for climate models in the context of solar forcing recommendations for CMIP6. Recently, a reprocessed MIPAS version 8 dataset has been released. Compared to the previous version, we assess what impact the changes in this new data version have on the EPP-NOy quantification, and on the formulation of chemical upper boundary conditions for climate models. The Earth Explorer 11 candidate “Changing Atmosphere Infra-Red Tomography” (CAIRT) will observe the altitude region from about 5 km to 115 km with an across-track resolution of 30 to 50 km within a 500 km wide field of view. This instrument will provide NOy and dynamical tracer observations from the upper troposphere to the lower thermosphere with unprecedented spatial resolution. Given that neither MIPAS nor any of the current instruments observes the lower thermosphere at this spatial resolution, we will assess the potential of this mission to advance our understanding of the EPP-climate link in the future.

ECMWF’s Integrated Forecasting System, extended with modules for atmospheric composition (IFS-COMPO) is used to provide global forecasts and analyses of the atmospheric composition in the framework of the Copernicus Atmosphere Monitoring Service (CAMS). In the last years intensive work has been dedicated to an improved representation of stratospheric composition in IFS-COMPO. Progresses concern both chemistry through the implementation and use of BASCOE in IFS-COMPO since cycle 48R1, and the extension of IFS-AER to also represent stratospheric sulfate and the associated processes, planned for cycle 49R1. In cycle 49R1, IFS-COMPO will have the capacity to forecast different aerosol parameters of importance for the stratospheric heterogeneous chemistry, such as the surface area density (SAD). The BASCOE module considers full stratospheric chemistry including heterogeneous reactions on the surfaces of polar stratospheric clouds (PSC) that control the extent and depth of ozone depletion events. To date BASCOE makes use of a combination of fixed information (particle number concentration, modal radius and the standard deviation of the aerosol particle size distribution), using a prescribed monthly SAD dataset to describe the aerosol contribution to stratospheric heterogeneous chemical processes. Here we present a further coupling of IFS-AER with BASCOE whereby online aerosol information from IFS-AER is used as an input to the BASCOE heterogeneous chemistry routines. Two possible coupling mechanisms have been implemented and tested for different test cases representative for high volcanic load (Pinatubo period), moderate volcanic load (2008-2009 period), and low, “background” aerosol load (1997-1998 period). Making use of different reference datasets like GloSSAC (for aerosols) and the BRAM-MLS reanalysis (for ozone and ozone-depleting chemical species), we evaluate the impact of these new coupling mechanisms between IFS-AER and BASCOE on aerosol transport and microphysics during the considered periods, and on simulated stratospheric atmospheric composition, focusing on several ozone-depletion events (“ozone holes”) in both Antarctic and Arctic winter.

The daily analyses and forecasts of atmospheric composition delivered by the Copernicus Atmosphere Monitoring Service (CAMS) are produced by the ECMWF Integrated Forecasting System configured for COMPOsition (IFS-COMPO). In 2023 this system was upgraded to Cy48 which solves explicitly for stratospheric chemistry through a module extracted from the Belgian Assimilation System for Chemical ObsErvations (BASCOE). In 2024 the system was further upgraded to Cy49 which improves the representation of stratospheric composition with an adjusted parameterization of Polar Stratospheric Clouds (PSC), updated chemical rates for heterogeneous chemistry, and the implementation of missing processes to simulate an accurate distribution of sulfate aerosols in the stratosphere. Here we report on these improvements and evaluate the resulting stratospheric composition in chemical forecast mode, where the model is constrained by assimilation of meteorological observations but not by assimilation of composition observations. These evaluations comprise 13 gas-phase species and sulfate aerosols in three case studies: a global-scale assessment during a quiescent period (July 2023 to May 2024) in the context of the operational upgrade of the CAMS system; the evolution of key tracers related to polar ozone depletion during the winter and spring seasons across several years; and the evolution of stratospheric aerosols over the three years following the June 1991 Mount Pinatubo eruption. The model captures the rapid increase of the sulfate burden after the Pinatubo eruption, with the peak of stratospheric sulfate burden timed correctly, gradual recovery, and expected vertical profiles for quiescent periods. A scorecard assessment of chemical forecasts in the stratosphere of IFS-COMPO Cy49 highlights good performance for O3, CH4, N2O, and H2O and adequate performance for HCl, ClO, BrO and BrONO2 in the polar lower stratosphere. The model performance is poorer for HNO3, N2O5, NO2 and ClONO2, highlighting the need to improve the representation of heterogeneous chemistry, particularly the interactivity between aerosols and gas-phase composition, and refine the parameterization of PSC to better capture their impact on gas-phase composition. Overestimations of CH4 and N2O in the upper stratosphere are potentially related to the Brewer–Dobson Circulation, and long-standing biases of NO2 and O3 in the upper stratosphere remain unresolved. Despite these points for further development, IFS-COMPO will be a useful tool for studies of the couplings between stratospheric aerosols and gas-phase chemistry. The current cycle paves the way for assimilating stratospheric composition observations beyond ozone.

The Copernicus Atmosphere Monitoring Service (CAMS) provides daily analyses and forecasts of the composition of the atmosphere, including the reactive gases such as O3, CO, NO2, HCHO and SO2; aerosol species; and greenhouse gases. The global CAMS analysis system (IFS-COMPO) is based on the ECMWF Integrated Forecasting System (IFS) for numerical weather prediction (NWP) and assimilates a large number of composition satellite products on top of the meteorological observations ingested in IFS. The CAMS system receives regular upgrades, following the upgrades of IFS. The last upgrade, Cy48R1, operational since 27 June 2023, was major with a large number of code changes, both for IFS-COMPO and for NWP. The main IFS-COMPO innovations include the introduction of full stratospheric chemistry; a major update of the emissions; a major update of the aerosol model, including the representation of secondary organic aerosol; several updates of the dust life cycle and optics; updates to the inorganic chemistry in the troposphere; and the assimilation of Visible Infrared Imaging Radiometer Suite (VIIRS) aerosol optical depth (AOD) and TROPOspheric Monitoring Instrument (TROPOMI) CO. The CAMS Cy48R1 upgrade was validated using a large number of independent measurement datasets, including surface in situ, surface remote sensing, routine aircraft, and balloon and satellite observations. In this paper we present the validation results for Cy48R1 by comparing them with the skill of the previous operational system (Cy47R3), with the independent observations as reference, for the period October 2022 to June 2023, during which daily forecasts from both cycles are available. Major improvements in skill are found for the ozone profile in the lower–middle stratosphere and for stratospheric NO2 due to the inclusion of full stratospheric chemistry. Stratospheric trace gases compare well with the Atmospheric Chemistry Experiment Fourier Transform Spectrometer (ACE-FTS) observations between 10 and 200 hPa, with larger deviations between 1 and 10 hPa. The impact of the updated emissions is especially visible over East Asia and is beneficial for the trace gases O3, NO2 and SO2. The CO column assimilation is now anchored by the Infrared Atmospheric Sounding Interferometer (IASI) instead of the Measurements Of Pollution in The Troposphere (MOPITT) instrument, which is beneficial for most of the CO comparisons, and the assimilation of TROPOMI CO data improves the model CO field in the troposphere. In general the aerosol optical depth has improved globally, but the dust evaluation shows more mixed results. The results of the 47 comparisons are summarised in a scorecard, which shows that 83 % of the evaluation datasets show a neutral or improved performance of Cy48R1 compared to the previous operational CAMS system, while 17 % indicate a (slight) degradation. This demonstrates the overall success of this upgrade.

<p>Sulfur hexafluoride (SF<sub>6</sub>) is a greenhouse gas that is emitted at the surface because of its use as an insulator in electrical transmission equipment and electronic devices. Since its quasi-linear emission growth and its very long lifetime, SF<sub>6</sub> can be used as a tracer for the Age of Air (AoA) to diagnose changes in the Brewer Dobson Circulation (BDC). The chemistry of SF<sub>6</sub> has been implemented in the Chemistry Transport Model (CTM) of the Belgian Assimilation System for Chemical ObsErvations (BASCOE). Reaction rates were taken from previous studies while an electron density has been taken from WACCM-X-SD simulations.<br />In this contribution, BASCOE-CTM simulations driven by ERA5 and MERRA2 will be discussed considering SF<sub>6</sub> with and without mesospheric sinks (i.e. passive SF<sub>6</sub> in the latter case). During the course of the simulations, the computed mixing ratios have also been saved in the space of MIPAS observations to analyse the impact of the MIPAS sampling in its AoA derivation.</p>

<p>ALTIUS (Atmospheric Limb Tracker for the Investigation of the Upcoming Stratosphere) is an atmospheric limb mission being implemented in ESA's Earth Watch programme and planned for launch in early 2026. The primary objective of the mission is to measure high-resolution stratospheric ozone concentration profiles. Secondary objectives are the retrievals of stratospheric aerosols particle density, NO<sub>2</sub>, water vapor and other minor species concentrations.</p> <p>This innovative instrument consists of three spectral high-resolution imagers: UV (250-355 nm), VIS (440-675 nm) and NIR (600-1040 nm) channels. The UV channel uses a stack of four Fabry-Pérot interferometers, while the VIS and NIR channels each rely on an AOTF (Acousto-Optical Tunable Filter). Each channel can image scenes independently of the others at given wavelengths and with a moderate spectral resolution, and high spatial sampling. The agility of ALTIUS allows for series of observations at desired wavelengths carefully chosen to retrieve the vertical profiles of species of interest.</p> <p>The instrument will perform measurements in different geometries to maximize global coverage: observing limb-scattered solar light in the dayside, solar occultations at the terminator, and stellar, lunar, and planetary occultations in the nightside.</p> <p>The status of the ALTIUS mission will be presented as well as the foreseen quality of the Level-1 observations.  The quality of the retrieved profile densities will be discussed with a particular focus on the high vertical resolution that can be achieved using this instrument. The added-value of the native imaging capabilities of ALTIUS in terms of observations, and in-flight calibrations, will be highlighted.</p>