New Observations of Upper Tropospheric NO₂ from TROPOMI

Abstract

Nitrogen oxides (NOₓ ≡ NO + NO₂) in the NOₓ-limited upper troposphere (UT) are long-lived and so have a large influence on the oxidizing capacity of the troposphere and formation of the greenhouse gas ozone. Models misrepresent NOₓ in the UT and observations to address deficiencies in models are sparse. Here we obtain a year of near-global seasonal mean mixing ratios of NO₂ in the UT (450–180 hPa) at 1 ° x 1° by applying cloud-slicing to partial columns of NO₂ from TROPOMI. This follows refinement of the cloud-slicing algorithm with synthetic partial columns from the GEOS-Chem chemical transport model. We find that synthetic cloud-sliced UT NO₂ are spatially consistent (R = 0.64) with UT NO₂ calculated across the same cloud pressure range and scenes as are cloud-sliced (“true” UT NO₂), but the cloud-sliced UT NO₂ is 11–22 % more than the "true" all-sky seasonal mean. The largest contributors to differences between synthetic cloud-sliced and “true” UT NO₂ are target resolution of the cloud-sliced product and uniformity of overlying stratospheric NO₂. TROPOMI, prior to cloud-slicing, is corrected for a 13 % underestimate in stratospheric NO₂ variance and a 50 % overestimate in free tropospheric NO₂ determined by comparison to Pandora total columns at high-altitude sites in Mauna Loa, Izaña and Altzomoni, and MAX-DOAS and Pandora tropospheric columns at Izaña. Two cloud-sliced seasonal mean UT NO₂ products for June 2019 to May 2020 are retrieved from corrected TROPOMI total columns using distinct TROPOMI cloud products that assume clouds are reflective boundaries (FRESCO-S) or water droplet layers (ROCINN-CAL). TROPOMI UT NO₂ typically ranges from 20-30 pptv over remote oceans to > 80 pptv over locations with intense seasonal lightning. Spatial coverage is mostly in the tropics and subtropics with FRESCO-S and extends to the midlatitudes and polar regions with ROCINN-CAL, due to its greater abundance of optically thick clouds and wider cloud top altitude range. TROPOMI UT NO₂ seasonal means are spatially consistent (R = 0.6–0.8) with an existing coarser spatial resolution (5° latitude x 8° longitude) UT NO₂ product from the Ozone Monitoring Instrument (OMI). UT NO₂ from TROPOMI is 12–26 pptv more than that from OMI due to increase in NO₂ with altitude from the OMI pressure ceiling (280 hPa) to that for TROPOMI (180 hPa), but possibly also systematic altitude differences between the TROPOMI and OMI cloud products. The TROPOMI UT NO₂ product offers potential to evaluate and improve representation of UT NOₓ in models and supplement aircraft observations that are sporadic and susceptible to large biases in the UT.