Direct sensitivity analysis of ozone formation and transport in California's San Joaquin Valley

Spatial and temporal variations of ozone-limiting reagents and the importance of local vs upwind emission sources in the San Joaquin Valley of central California during a five-day episode are studied by first- and second- order sensitivity analysis with the Decoupled Direct Method. Despite considerable spatial variations, nitrogen oxides (NOx) emission reductions are overall more effective than volatile organic compound (VOC) control for attaining the 8-hr ozone standard in this region, in contrast to the VOC control that works better for attaining the prior 1-hr ozone standard. Inter-basin source contributions of NOx emissions are limited to the northern part of the SJV, while anthropogenic VOC (AVOC) emissions influence ozone formation in the SJV further downwind. Among model input parameters studied here, uncertainties in emissions of NOx and AVOC, and the rate coefficient of the OH+NO2 termination reaction, are found to have the greatest effects on first-order ozone responses to changes in NOx emissions. Uncertainties in biogenic VOC emissions only have a modest effect because they are generally not collocated with anthropogenic sources.