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

Mechanisms for high offshore ozone (O-3) events in the Houston area have not been systematically examined due to limited O-3 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 O-3. The model can capture the spatiotemporal variability in the daytime (10:00-18:00 central daylight time, CDT) O-3 for the three boats (R > 0.7) but tends to overestimate O-3 by similar to 10 ppb on clean days and underestimate O-3 by similar to 3 ppb during high-O-3 events. The process analysis tool in CAMx identifies O-3 chemistry as the major process leading to high-O-3 concentrations. The region-wide increase in the long-lived volatile organic compounds (VOCs) through advection transits O-3 formation to be more sensitive to NOx, leading to more O-3 production under a NOx-limited regime. In addition, the VOC-limited O-3 formation is also boosted along western Galveston Bay and the Gulf Coast under high-NOx conditions brought by the northeasterly winds from the Houston Ship Channel. Two case studies illustrate that high offshore O-3 events can develop under both large- and mesoscale 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.