Vertical profiles of global tropospheric nitrogen dioxide (NO₂) obtained by cloud-slicing TROPOMI

Abstract. Routine observations of the vertical distribution of tropospheric nitrogen oxides (NO_x ≡ NO + NO₂) are severely lacking, despite the large influence of NO_x on climate, air quality, and atmospheric oxidants. Here we derive vertical profiles of global seasonal mean tropospheric NO₂ by applying the cloud-slicing method to TROPOspheric Monitoring Instrument (TROPOMI) columns of NO₂ retrieved above optically thick clouds. The resultant NO₂ are at a horizontal resolution of 1° × 1° for multiple years (June 2018 to May 2022) covering 5 layers in the upper (180–320 hPa and 320–450 hPa) and mid (450–600 hPa and 600–800 hPa) troposphere, and the marine boundary layer (800 hPa to the Earth’s surface). Terrestrial boundary layer NO₂ are obtained as the difference between TROPOMI tropospheric columns and the integrated column of cloud-sliced NO₂ in all layers above the boundary layer. Cloud-slicing NO₂ is typically 20–60 pptv throughout the free troposphere and spatial coverage ranges from > 60 % in the mid-troposphere to < 20 % in the upper troposphere and boundary layer. Our product is similar (within 10–15 pptv) to NO₂ data from NASA DC-8 aircraft campaigns (INTEX-A, ARCTAS, SEAC⁴RS, ATom) when both datasets are abundant and sampling coverage is commensurate, but such instances are rare. We use the cloud-sliced NO₂ to critique current knowledge of the vertical distribution of global NO₂, as simulated with the GEOS-Chem chemical transport model updated to include peroxypropionyl nitrate (PPN) and aerosol nitrate photolysis that liberate NO₂ in the lower and mid-troposphere for aerosol nitrate photolysis and upper troposphere for PPN. Multiyear GEOS-Chem and cloud-sliced means are compared to mitigate the influence of interannual variability. We find that for cloud-sliced NO₂ the interannual variability is ~10 pptv over remote areas and ~25 pptv over areas influenced by lightning and surface sources. The model consistently underestimates NO₂ across the remote marine troposphere by ~15 pptv. In the northern midlatitudes, GEOS-Chem overestimates mid-tropospheric NO₂ by 20–50 pptv, as NO_x production per lightning flash is parameterised to be almost double the rest of the world. There is a critical need for in-situ NO₂ measurements in the tropical terrestrial troposphere to evaluate cloud-sliced NO₂ there. The model and cloud-sliced NO₂ discrepancies identified here need to be investigated further to ensure confident use of models to understand and interpret factors affecting the global distribution of tropospheric NO_x, ozone and other oxidants.