High-resolution large-eddy simulation to understand ozone formation and atmospheric oxidation capacity in Houston, Texas

Abstract. Highly reactive volatile organic compounds (HRVOCs) from mobile and petrochemical sources are important players in atmospheric photochemistry that contribute to the formation of ozone (O₃). In a typical elevated O₃ episode, we applied a high-resolution large eddy simulation (LES), coupled with the Weather Research and Forecasting model with chemistry (WRF-LES-Chem) to understand the mechanism of high O₃ production over the Houston area. Our modeling was constrained and evaluated using field measurements from the NASA Tracking Aerosol Convection Interactions ExpeRiment – Air Quality (TRACER-AQ) project, Texas Commission on Environmental Quality (TCEQ), and vertical column density observations from Pandora spectrometers. The modeling results show enhanced performance in the LES domain, compared to the mesoscale models in simulating key chemicals. O₃ sensitivity in the Houston urban area demonstrates a nearly homogenous early morning VOC-limited regime and transits to a noontime NO_X-limited regime. As the day progresses into the afternoon, the atmospheric oxidative capacity (AOC) increases with major contribution from hydroxyl (OH) radical (90 %). High concentrations of alkenes also increased O₃ (8–10 %) contribution to AOC in the late afternoon. The OH reactivity (K_OH) is dominated by isoprene (35.76 %), carbon monoxide (CO; 12.98 %), formaldehyde (HCHO; 12.21 %), and alkanes with C > 3 (6.29 %), thus accelerating the production of hydroperoxyl (HO₂) and peroxy (RO₂) radicals. The concentrations of short-lived VOCs such as HCHO and acetaldehyde from the oxidation of HRVOCs, increased in the afternoon, which elevated O₃ production rates under a NO_X-limited regime. The oxidation of isoprene also accelerated the production of HCHO and contributed to the production of HO₂ radicals, thus leading to a high O₃ production rate. This study suggests the possible impacts of NO_X-O₃-VOC sensitivity on O₃ production rates in polluted urban areas with high emission of HRVOCs, and also provides insights on radical chemistry that drives the photochemical processes of O₃ formation. Ultimately, the study underlines the need to control anthropogenic emissions such as alkenes and HCHO and also highlights the role of naturally emitted isoprene species in elevated urban O₃ levels.