An intercomparison of satellite, airborne, and ground-level observations with WRF-CAMx simulations of NO ₂ columns over Houston, TX during the September 2021 TRACER-AQ campaign

Nitrogen dioxide (NO ₂) is a precursor of ozone (O ₃) and fine particulate matter (PM _2.5) – two pollutants that are above regulatory guidelines in many cities. Bringing urban areas into compliance of these regulatory standards motivates an understanding of the distribution and sources of NO ₂ through observations and simulations. The TRACER-AQ campaign, conducted in Houston, TX in September 2021, provided a unique opportunity to compare observed NO ₂ columns from ground-, airborne-, and satellite-based spectrometers. In this study, we investigate how these observational datasets compare, and simulate column NO ₂ using WRF-CAMx with fine resolution (444 x 444 m ²) comparable to the airborne column measurements. We find that observations from the GEOCAPE Airborne Simulator (GCAS) instrument were strongly correlated (r ²=0.80) to observations from Pandora spectrometers with a negligible bias (NMB=0.1 %). Remote-sensing observations from the TROPOMI instrument were generally well correlated with Pandora observations (r ²=0.73) with a negative bias (NMB=-22.8 %). We intercompare different versions of TROPOMI data and find similar correlations across three versions but slightly different biases (from -22.8 % in v2.4.0 to -18.2 % in the NASA MINDS product). Compared to Pandora observations, the WRF-CAMx simulation had reduced correlation (r ²=0.34) and a low bias (-25.5 %) over the entire study region. We find particularly poor agreement between simulated NO ₂ columns and GCAS-observed NO ₂ columns in downtown Houston an area of high population and roadway densities. These findings point to a potential underestimate of vehicle NO _X emissions in the WRF-CAMx simulation driven by the Texas state inventory; and further investigation is recommended.