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

• Published in: Atmospheric chemistry and physics 2024-06, Vol.24 (11), p.6719
• Main Authors: Nawaz, M. Omar, Johnson, Jeremiah, Yarwood, Greg, de Foy, Benjamin, Judd, Laura, Goldberg, Daniel L
• Format: Article
• Language: English
• Subjects: Numerical weather forecasting; Simulation methods; Tracers (Biology)
• ISSN: 1680-7316

Nitrogen dioxide (NO.sub.2) is a precursor of ozone (O.sub.3) and fine particulate matter (PM.sub.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.sub.2 through observations and simulations. The TRACER-AQ campaign, conducted in Houston, Texas, in September 2021, provided a unique opportunity to compare observed NO.sub.2 columns from ground-, airborne-, and satellite-based spectrometers. In this study, we investigate how these observational datasets compare and simulate column NO.sub.2 using WRF-CAMx with fine resolution (444 x 444 m.sup.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 (r.sup.2 = 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 (r.sup.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 with Pandora observations, the WRF-CAMx simulation had reduced correlation (r.sup.2 = 0.34) and a low bias (-21.2 %) over the entire study region. We find particularly poor agreement between simulated NO.sub.2 columns and GCAS-observed NO.sub.2 columns in downtown Houston, an area of high population and roadway densities. These findings point to a potential underestimate of NO.sub.x emissions (NO.sub.x = NO + NO.sub.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.