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.</p> <p>This paper investigates the capability of CAIRT to resolve vertical gradients of ozone (O<sub>3</sub>) and water vapour (H<sub>2</sub>O) 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 O<sub>3</sub> and H<sub>2</sub>O, 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 O<sub>3</sub> and H<sub>2</sub>O (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) O<sub>3</sub> and H<sub>2</sub>O observations are also assimilated, allowing the performance of CAIRT to be evaluated relative to this reference instrument.</p> <p>The results show that CAIRT (1) O<sub>3</sub> observations are able to constrain BASCOE analyses down to 7 km altitude – several kilometres lower (and thus better) than MLS – and (2) H<sub>2</sub>O 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.