Highly nonlinear ozone formation in the Houston region and implications for emission controls

Abstract

Photochemical modeling with high‐order sensitivity analysis is applied to simulate the nonlinear responses of ozone to NOx and VOC emissions from different source regions in the Houston‐Galveston‐Brazoria area and their interactions. First‐order responses of daytime ozone to Houston NOx emissions are typically positive but are negative in the core region, indicating a NOx‐inhibited chemical regime there. Houston anthropogenic VOC emissions exert a spatially smaller impact on ozone but are important to high ozone concentrations in the core region. Highest ozone concentrations in the Houston region typically occur where the impacts of the Houston Ship Channel NOx emissions coincide with those of NOx emitted from the rest of the Houston region. Daytime ozone is found to exhibit a more nonlinear responsiveness to precursor emissions in Houston than has been reported in other regions, including a strongly concave response to local NOx emissions and strong interactions between the impacts of NOx and VOC emissions changes. Due to this intense nonlinearity, moderate perturbations (10–30%) in either NOx or VOC emissions inventories could flip whether Houston ozone is modeled to be more responsive to VOC control or NOx control. Thus the accuracy of emission inventories could strongly influence predictions of ozone response to emission reductions.