New observations of NO2 in the upper troposphere from TROPOMI
Abstract. Nitrogen oxides ( NO x ≡ NO + NO 2 ) 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 ∘ × 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. 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 free-tropospheric sites at Mauna Loa, Izana, and Altzomoni and MAX-DOAS and
Pandora tropospheric columns at Izana. 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  ×  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 due to altitude differences in TROPOMI and OMI cloud products and
 NO2 retrieval algorithms. 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.