First estimate of NO2 in the upper troposphere from TROPOMI

Nitrogen oxides (NO _x = NO + NO₂) in the upper troposphere (~10-12 km) are effective at producing ozone in the upper troposphere where ozone is a potent greenhouse gas. Observations of NO_x in the upper troposphere are limited in time to a few intensive research aircraft campaigns and in space to commercial aircraft campaigns. There are satellite-derived observations of NO₂ in the upper troposphere from the Ozone Monitoring Instrument (OMI), but these are at very coarse resolutions (seasonal, > 2,000 km). The high-resolution Sentinel-5P/TROPOMI instrument offers higher spatial resolution and better cloud-resolving capability than OMI. Here we use synthetic columns of NO₂ from the GEOS-Chem chemical transport model to assess feasibility of deriving NO₂ in the upper troposphere using partial columns of NO₂ above cloudy scenes (the so-called cloud-slicing technique). The model is also used to quantify errors induced by uncertainties in cloud-top height and to determine whether NO₂ over cloudy scenes is representative of all-sky conditions (the "truth"). We find that the cloud-slicing approach is spatially consistent (R =0.5) with the "truth", but with a small (10 pptv) bias in background NO₂. Cloud-slicing is then applied to TROPOMI total columns of NO₂ to derive near-global observations of NO₂ in the upper troposphere and assessed against the existing OMI products and aircraft observations from NASA DC8 aircraft campaigns.