Characterization of the UV radiometric calibration for the TROPOMI operational ozone profile retrieval algorithm

The European Space Agency (ESA) Sentinel-5 Precursor (S5P) is a low Earth orbit polar satellite carrying the single payload instrument TROPOspheric Monitoring Instrument (TROPOMI). Since its launch on 13 October 2017, the S5P mission has been acquiring almost 7 years of nadir ozone profile data, retrieved from the UV spectral bands 1–2 (270–330 nm). The retrieval algorithm of the ozone profile can be strongly affected by systematic effects in the measured radiance, and absolute calibration of the input spectra is necessary. In this study, we characterize the TROPOMI bands 1–2 radiometric bias in comparison with simulations obtained with the Determining Instrument Specifications and Analysing Methods for Atmospheric Retrieval (DISAMAR) radiative transfer model. To account for these systematic effects, a radiometric correction (soft-calibration) is applied to the input measurements, which results in the reduction of the spectral reflectance fit residuals of 20–30 %, improved precision of the integrated total and tropospheric ozone columns of 10–15 % and a reduction of along-track orbit artifacts. Together with the analysis of the in-flight calibration measurements, the soft-calibration correction spectra provide also useful insights for the improvement of the instrument radiometric calibration. Therefore, bands 1–2 measurements have been reprocessed specifically for this study, with improvements regarding the detector straylight and background signal correction algorithms, to investigate the impact of the updates into the TROPOMI bands 1–2 radiometric bias. The new soft-calibration correction spectra, obtained with the reprocessed bands 1–2 measurements, show significantly reduced magnitude (around 15–20 %, especially in band 1) and also less across-track position and spectral/temporal biases. The new soft-calibration will be part of ESA’s next official ozone profile algorithm version 2.9.0, coordinated with L1b update to processor version 3.0.0, and used for the second TROPOMI mission reprocessing.