Targeting Houston’s ozone: How sector-specific emissions drive spatiotemporal sensitivity

Abstract. The Higher-Order Decoupled Direct Method (HDDM) is applied to a 2019 regulatory photochemical transport modeling episode to evaluate tropospheric ozone (O₃) responses to source-specific nitrogen oxide (NOx) and volatile organic compound (VOC) emissions in Houston, Texas. The Houston airshed contains a unique mix of industrial and urban emissions that challenges air quality models in predicting O₃, adding uncertainty in their use of determining effective regulatory policy. The regulatory air quality model evaluated in this study predicts that high O₃ plumes are VOC-limited and NOx-inhibited in the morning, before transitioning to a NOx-limited regime midday and throughout the afternoon. Mean O₃ HDDM sensitivities in high O₃ plumes were highest for several source groups, including non-electric generating unit (EGU) VOC (2.65 ppb), non-EGU NOx (2.41 ppb), marine NOx (2.25 ppb), on-road NOx (1.48 ppb), and area source VOC (1.38 ppb). In predictions of the maximum daily 8-hour average (MDA8) O₃ concentrations, there were strong interactions between industrial and urban emissions in O₃ plumes formed to the southwest of Houston. The importance of nonlinear chemistry varies by source, with the model predicting a strong contribution from second-order and cross-sensitivities for non-EGU and marine sources. The substantial influence of both NOx and VOC sources on O₃ sensitivity highlights the need for improved emissions inventories, and more detailed sensitivity analysis.