| 07 Jan 2026
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 the CH4/CO2 proxy ratio using the operational RemoTeC CH4 retrieval algorithm, an also examines persistence errors for a potential CO2 product. 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, the induced biases remain 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, localized outliers reaching up to 1.59 % for CH4, 1.55 % for CO2, 4.01 % for CO and 0.71 % for the proxy constitute a substantial fraction of the CO and the proxy performance targets, and exceed those defined for CH4 and CO2. Our findings suggest that targeted post-filtering, such as omitting coastal and land–water interface pixels, can effectively reduce the most significant outliers in the persistence bias, thus aligning the retrieval biases with the required mission performance.
This paper uses an OSSE to examine the effect of lagged SWIR detector response on Sentinel-5 retrievals of CH4, CO2, and CO. The work appears to have been carefully done and is clearly explained. But it is technical in nature, narrowly specific to Sentinel-5, not particularly original, and in the end the effect turns out to cause relatively small errors. It uses an OSSE formulation originally developed for MicroCarb that the MicroCarb team apparently did not bother to publish (it is cited as personal communication). I would expect the work described here to be in a Sentinel-5 ATBD, or as part of a broader Sentinel-5 publication, not as a stand-alone peer-reviewed publication. At the same time, I welcome information on Sentinel-5 in the peer-reviewed literature. I didn't know how to express this in the "For final publication..." buttons and the only reason I clicked on "major revisions" is for the editors to decide whether this paper passes the bar for AMT. It is marginal in my opinion. But if the editors decide that it passes the bar then only minor revisions are needed.
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