Upper tropospheric NO2 retrieved from TROPOMI using cloud-slicing

Abstract:

Nitrogen oxides in the NO_x-limited upper troposphere (UT) have a large influence on the oxidizing capacity of the troposphere and formation of the greenhouse gas ozone. Models misrepresent NO_x in the UT and observations to address deficiencies in models are sparse. Here we obtain one year of near-global seasonal mean mixing ratios of NO₂ in the UT (450-180 hPa) by applying cloud-slicing to TROPOMI partial columns of NO₂. This follows refinement of the cloud-slicing algorithm by application to synthetic partial columns from the GEOS-Chem model and bias correction of TROPOMI compared to ground-based observations from Pandora and MAX-DOAS instruments at high-altitude sites. TROPOMI underestimates stratospheric NO₂ variance by 14%, overestimates the stratospheric column by 7 x 10¹⁴ molecules cm^-2 and overestimates the free tropospheric column by a factor of 2. Two cloud-sliced UT NO₂ products are retrieved using distinct cloud products, so-called FRESCO-S and ROCINN-CAL. These yield consistent UT NO₂, but with greater spatial coverage using ROCINN-CAL, due to its greater abundance of optically thick clouds and greater variability in cloud top heights at the midlatitudes and poles. The TROPOMI UT NO₂ products are spatially consistent with UT NO₂ from the existing Ozone Monitoring Instrument (OMI) product. A positive bias in the TROPOMI product of ~10 pptv is expected, due to finer resolution of TROPOMI pixels and the UT NO₂ product than OMI, and greater vertical extend of TROPOMI (up to 180 hPa) than OMI (280 hPa) over a pressure range in which NO₂ increases with altitude. The TROPOMI UT NO₂ product offers potential to evaluate and improve representation of UT NO_x in global chemical transport models and supplement in situ NO₂ observations from aircraft that are susceptible to large biases in the UT.