An intercomparison of satellite, airborne, and ground-level observations with WRF–CAMx simulations of NO2 columns over Houston, Texas, 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, Texas, 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 × 444 m 2) comparable to the airborne column measurements. We compare WRF-simulated meteorology to ground-level monitors and find good agreement. We find that observations from the GEOstationary Coastal and Air Pollution Events (GEO-CAPE) Airborne Simulator (GCAS) instrument were strongly correlated (r2 = 0.79) to observations from Pandora spectrometers with a slight high bias (normalized mean bias (NMB) = 3.4 %). Remote sensing observations from the TROPOspheric Monitoring Instrument (TROPOMI) were generally well correlated with Pandora observations (r2 = 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 with Pandora observations, the WRF–CAMx simulation had reduced correlation (r2 = 0.34) and a low bias (- 21.2 %) 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 NO x emissions (NO x = NO + NO 2) from sources associated with the urban core of Houston, such as mobile sources, in the WRF–CAMx simulation driven by the Texas state inventory, and further investigation is recommended. [ABSTRACT FROM AUTHOR]