Understanding offshore high-ozone events during TRACER-AQ 2021 in Houston: Insights from WRF-CAMx photochemical modeling

Abstract. Mechanisms for high offshore ozone (O₃) events in the Houston area have not been systematically examined due to limited O₃ measurements over water. In this study, we used the datasets collected by three boats deployed in Galveston Bay and the Gulf of Mexico during the Tracking Aerosol Convection Interactions ExpeRiment/Air Quality (TRACER-AQ) field campaign period (September 2021) in combination with the Weather Research and Forecasting (WRF) coupled Comprehensive Air quality Model with Extensions (CAMx) modeling system (WRF-CAMx) to investigate the reasons for high offshore O₃. The model can capture the spatiotemporal variability of daytime (10:00–18:00) O₃ for the three boats (R > 0.7) but tends to overestimate O₃ by ~10 ppb on clean days and underestimate O₃ by ~3 ppb during high-O₃ events. The process analysis tool in CAMx identifies O₃ chemistry as the major process leading to high O₃ concentrations. The region-wide increase of long-lived VOCs through advection not only leads to more O₃ production under a NO_x-limited regime but also fosters VOC-limited O₃ formation along western Galveston Bay and the Gulf coast under high-NO_x conditions brought by the northeasterly winds from the Houston Ship Channel. Two case studies illustrate that high offshore O₃ events can develop under both large- and meso-scale circulations, indicating both the regional and local emissions need to be stringently controlled. Wind conditions are demonstrated to be important meteorological factors in such events, so they must be well represented in photochemical models to forecast air quality over the urban coastal regions accurately.