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Abstract. Nitrogen dioxide (NO2) is one of the main data products measured by the Tropospheric Monitoring Instrument (TROPOMI) on the Sentinel-5 Precursor(S5P) satellite, which combines a high signal-to-noise ratio with dailyglobal coverage and high spatial resolution.TROPOMI provides a valuable source of information to monitoremissions from local sources such as power plants, industry, cities,traffic and ships, and variability of these sources in time.Validation exercises of NO2 v1.2–v1.3 data, however, have revealed that TROPOMI's tropospheric vertical column densities (VCDs) are too low by up to 50 % over highly polluted areas. These findings are mainly attributed to biases in the cloud pressureretrieval, the surface albedo climatology and the low resolution of thea priori profiles derived from global simulations of the TM5-MP chemistrymodel. This study describes improvements in the TROPOMI NO2 retrievalleading to version v2.2, operational since 1 July 2021.Compared to v1.x, the main changes are the following. (1) The NO2-v2.2 data are based on version-2 level-1b (ir)radiance spectra with improved calibration, which results in a small and fairlyhomogeneous increase in the NO2 slant columns of 3 % to 4 %, most of which ends up as a small increase in the stratospheric columns. (2) The cloud pressures are derived with a new version of the FRESCO cloudretrieval already introduced in NO2-v1.4, which led to a lowering of the cloud pressure, resulting in larger tropospheric NO2 columnsover polluted scenes with a small but non-zero cloud coverage. (3) For cloud-free scenes a surface albedo correction is introduced based on the observed reflectance, which also leads to a general increase in thetropospheric NO2 columns over polluted scenes of order 15 %. (4) An outlier removal was implemented in the spectral fit, which increasesthe number of good-quality retrievals over the South Atlantic Anomaly region and over bright clouds where saturation may occur. (5) Snow/ice information is now obtained from ECMWF weather data, increasing the number of valid retrievals at high latitudes. On average the NO2-v2.2 data have tropospheric VCDs that are between 10 % and 40 % larger than the v1.x data, depending on the level of pollution and season; the largest impact is found at mid and high latitudes in wintertime.This has brought these tropospheric NO2 closer to Ozone Monitoring Instrument (OMI) observations. Ground-based validation shows on average an improvement of the negative bias of the stratospheric (from −6 % to −3 %), tropospheric (from −32 % to −23 %) and total (from −12 % to −5 %) columns.For individual measurement stations, however, the picture is morecomplex, in particular for the tropospheric and total columns.

atmospheric measurement techniques

Sentinel-5P TROPOMI NO&amp;lt;sub&amp;gt;2&amp;lt;/sub&amp;gt; retrieval: impact of version v2.2 improvements and comparisons with OMI and ground-based data