S5P TROPOMI NO 2 slant column retrieval: method, stability, uncertainties and comparisons with OMI

Abstract. The Tropospheric Monitoring Instrument (TROPOMI), aboard the Sentinel-5 Precursor (S5P) satellite, launched on 13 Oct. 2017, provides measurements of atmospheric trace gases and of cloud and aerosol properties on an unprecedented spatial resolution of approximately 7 × 3.5 km2 (approx. 5.5 × 3.5 km2 as of 6 Aug. 2019), achieving near-global coverage in one day. The retrieval of nitrogen dioxide (NO2) concentrations is a 3-step procedure: slant column density (SCD) retrieval, separation of the SCD in its stratospheric and tropospheric components, and conversion of these into vertical column densities. This study focusses on the TROPOMI NO2 SCD retrieval: the retrieval method used, the stability of the SCDs and the SCD uncertainties, and a comparison against OMI NO2 SCDs. The statistical uncertainty, based on the spatial variability of the SCDs over a remote Pacific Ocean sector, is 8.63 μmol/m2 for all pixels (9.45 μmol/m2 for cloud-free pixels), which is very stable over time and some 30 % less than the long-term average over OMI/QA4ECV data (since the pixel size reduction TROPOMI uncertainties are ∼ 10 % larger). The SCD uncertainty reported by the DOAS fit is about 10 % larger than the statistical uncertainty, while for OMI/QA4ECV the DOAS uncertainty is some 20 % larger than its statistical uncertainty. Comparison of the SCDs themselves over the Pacific Ocean, averaged over one month, shows that TROPOMI is about 5 % higher than OMI/QA4ECV, which seems to be due mainly to the use of the so-called intensity offset correction in OMI/QA4ECV but not in TROPOMI: turning that correction off means about 5 % higher SCDs. The row-to-row variation in the SCDs of TROPOMI, the stripe amplitude, is 2.14 μmol/m2, while for OMI/QA4ECV it is ∼ 2 (∼ 5) larger in 2005 (2018), still a so-called stripe correction of this non-physical across-track variation is useful for TROPOMI data. In short, TROPOMI shows a superior performance compared against OMI/QA4ECV and operates as anticipated from instrument specifications. The TROPOMI data used in this study covers 30 April 2018 up to 31 Oct. 2019.