High-Resolution WRF-LES-Chem Simulations to Investigate Ozone Formation Regimes in Houston

Despite decades of ongoing mitigation efforts, ozone (O₃) levels remain persistently high in Houston, TX. For a high O₃ episode observed during the NASA Tracking Aerosol Convection Interactions ExpeRiment-Air Quality (TRACER-AQ) campaign, we use a high-resolution large-eddy simulation (LES) within the Weather Research and Forecasting model coupled with Chemistry (WRF-LES-Chem) to investigate temporal and spatial variations in O₃ formation regimes over the region. By leveraging improved simulations of O₃ and its precursors by LES, compared to the mesoscale WRF model, we derive and compare two O₃ sensitivity indicators: the formaldehyde-to-nitrogen dioxide ratio (FNR) and the ratio of radical loss via NO_X reactions to total primary radical production (L_N/Q). Specifically, we use L_N/Q to inform the threshold for FNR, the latter being a more commonly used and accessible indicator, although it is subject to significant uncertainties. We demonstrate that O₃ production in the Houston urban area transitions from a nearly homogeneous early morning VOC-limited regime to a NO_X-limited regime by midday. Using the L_N/Q indicator, we identify that a range of 0.6 < FNR < 1.8 falls in the transition zone of O₃ formation regime. The high-resolution modeling of O₃ formation and the FNR range developed in this LES study offers valuable insight for assessing future air quality and improving the understanding of atmospheric chemistry that underpins pollution control in Houston.