Vertical profiles of global tropospheric nitrogen dioxide (NO₂) obtained by cloud slicing the TROPOspheric Monitoring Instrument (TROPOMI)

Abstract

Routine observations of the vertical distribution of tropospheric nitrogen oxides (NOx ≡ NO + NO2) are severely lacking, despite the large influence of NOx 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 is provided at a horizontal resolution of 1° × 1° for multiple years (June 2018 to May 2022), covering five layers of the troposphere: two layers in the upper troposphere (180–320 hPa and 320–450 hPa), two layers in the middle troposphere (450–600 hPa and 600–800 hPa), and the marine boundary layer (800 hPa to the Earth's surface). NO2 in the terrestrial boundary layer is obtained as the difference between TROPOMI tropospheric columns and the integrated column of cloud-sliced NO2 in all layers above the boundary layer. Cloud-sliced NO2 typically ranges from 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. When both datasets are abundant and sampling coverage is commensurate, our product is similar (within 10–15 pptv) to NO2 data from NASA DC-8 aircraft campaigns. However, such instances are rare. We use cloud-sliced NO2 to critique current knowledge of the vertical distribution of global NO2, as simulated by the GEOS-Chem chemical transport model, which has been updated to include peroxypropionyl nitrate (PPN) and aerosol nitrate photolysis, liberating NO2 in the lower troposphere and mid-troposphere for aerosol nitrate photolysis and in the 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, 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. At the northern midlatitudes, GEOS-Chem overestimates mid-tropospheric NO2 by 20–50 pptv as NOx production per lightning flash is parameterised to be almost double that of 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 NOx, ozone, and other oxidants.