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 NO x and VOC emissions from different source regions in the Houston‐Galveston‐Brazoria area and their interactions. First‐order responses of daytime ozone to Houston NO x emissions are typically positive but are negative in the core region, indicating a NO x ‐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 NO x emissions coincide with those of NO x 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 NO x emissions and strong interactions between the impacts of NO x and VOC emissions changes. Due to this intense nonlinearity, moderate perturbations (10–30%) in either NO x or VOC emissions inventories could flip whether Houston ozone is modeled to be more responsive to VOC control or NO x control. Thus the accuracy of emission inventories could strongly influence predictions of ozone response to emission reductions.