New observations of NO2 in the upper troposphere from TROPOMI

Nitrogen oxides (NOx = NOC+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 NO₂in the UT (450 180 hPa) at 1° ×1° by applying cloud-slicing to partial columns of NO₂from TROPOMI. This follows refinement of the cloud-slicing algorithm with synthetic partial columns from the GEOS-Chem chemical transport model. TROPOMI, prior to cloud-slicing, is corrected for a 13% underestimate in stratospheric NO₂variance and a 50%overestimate in free-tropospheric NO₂determined by comparison to Pandora total columns at high-altitude free-tropospheric sites at Mauna Loa, Izana, and Altzomoni and MAX-DOAS and Pandora tropospheric columns at Iza a. Two cloudsliced seasonal mean UT NO₂products 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 NO₂typically 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 ROCINNCAL, due to its greater abundance of optically thick clouds and wider cloud-top altitude range. TROPOMI UT NO₂seasonal means are spatially consistent (R D 0:6 0.8) with an existing coarser spatial resolution (5° latitude×8° longitude) UT NO₂product from the Ozone Monitoring Instrument (OMI). UT NO₂from TROPOMI is 12 26 pptv more than that from OMI due to increase in NO₂with altitude from the OMI pressure ceiling (280 hPa) to that for TROPOMI (180 hPa), but possibly also due to altitude differences in TROPOMI and OMI cloud products and NO₂retrieval algorithms. The TROPOMI UT NO₂product 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.