Impact of Sentinel-5 SWIR detector persistence on trace gas retrievals

Abstract. Launched in August 2025, the Sentinel-5 (S5) mission aims to enhance greenhouse gas monitoring by working alongside existing Copernicus satellites, such as the Sentinel-5 Precursor. S5 is equipped with shortwave-infrared (SWIR) grating spectrometers that provide operational daily retrievals of CH4 and CO, while the coverage of the SWIR region also makes S5 measurements sensitive to atmospheric CO2. One particular challenge for S5 is the persistence effect in its SWIR mercury-cadmium-telluride (MCT) detectors, which can introduce scene-dependent radiometric biases when the signal at a detector pixel changes between consecutive readouts, as occurs in scenes with along-track brightness variations. This study quantifies persistence-induced biases for CH4, CO, and a potential CO2 product from full-physics retrievals, as well as for the CH4/CO2 proxy ratio, whose relative error is representative of proxy CH4 retrievals, using the operational RemoTeC CH4 retrieval algorithm. We simulate realistic scenes over regions like the Nile Delta, California’s Central Valley, and the Lusatian lignite district, which exhibit the spatial radiance variability relevant for persistence effects. Across these scenes, persistence-induced biases are generally small, with typical amplitudes below 0.12 % for CH4, 0.10 % for CO2, 0.34 % for CO, and 0.06 % for the CH4/CO2 proxy. However, larger deviations occur in high-contrast scenes, particularly at coasts and land–water transitions, where localized errors can reach up to 1.59 % for CH4, 1.55 % for CO2, 4.01 % for CO, and 0.71 % for the CH4/CO2 proxy ratio. These values represent a substantial fraction of the performance targets for CO and the proxy, and exceed those for CH4 and CO2. This indicates that, while persistence is not the dominant error source under most conditions, it can still produce non-negligible biases in spatially structured scenes, which may be misinterpreted as localized enhancements. Therefore, such effects should be mitigated, for example through targeted quality control or filtering of affected pixels.