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  • Sentinel 5 Precursor (S5P) was launched on the 13th of October 2017 carrying the TROPOspheric Monitoring Instrument (TROPOMI). These data products provide geolocated total, tropospheric, or stratospheric Nitrogen dioxide concentrations. The TROPOMI NO2 data products pose an improvement over previous NO2 data sets, particularly in their unprecedented spatial resolution (7×3.5 km2), but also in the separation of the stratospheric and tropospheric contributions of the retrieved slant columns, and in the calculation of the air-mass factors used to convert slant to total columns. Nitrogen dioxide (NO2) and nitrogen oxide (NO) together are usually referred to as nitrogen oxides (NOx = NO + NO2). They are important trace gases in the Earth’s atmosphere, present in both the troposphere and the stratosphere. They enter the atmosphere as a result of anthropogenic activities (notably fossil fuel combustion and biomass burning) and natural processes (such as microbiological processes in soils, wildfires and lightning). During the daytime, i.e. in the presence of sunlight, a photochemical cycle involving ozone (O3) converts NO into NO2 (and vice versa) on a timescale of minutes, so that NO2 is a robust measure for concentrations of nitrogen oxides. Tropospheric and stratospheric concentrations of NO2 are monitored all over the world by a variety of instruments either ground-based, in-situ (balloon, aircraft), or satellite-based each with its own specific advantages.