Ozone formation along the California-Mexican border region during Cal-Mex 2010 field campaign

The purpose of this study is to evaluate the ozone (O₃) formation along the California-Mexico borderregion using the WRF-CHEM model in association with the Cal-Mex 2010 field campaign. Four two-day episodes in 2010 are chosen based on plume transport patterns: 1) May 15-16 (plume north), 2) May29-30(plume southwest), 3) June 4-5 (plume east), and 4) June 13-14 (plume southeast). Generally, the predicted O₃ spatial patterns and temporal variations agree well with the observations at the ambient monitoring sites in the San Diego-Tijuana region, but in the Calexico-Mexicali region, the model frequently underestimates the observation. In the San Diego-Tijuana region, the morning anthropogenic precursor emissions in the urbanized coastal plain are carried inland and mixed with the local biogenic emissions during transport, causing the high O₃ level over the mountain region. Biogenic emissions enhance the O₃ concentrations by up to 40ppb over the mountain region in the afternoon. The factor separation approach is used to evaluate the contributions of trans-boundary transport of emissions from California and Baja California to the O₃ level in the California-Mexico border region. The Baja California emissions play a minor role in the O₃ formation in the San Diego region and do not seem to contribute to the O₃ exceedances in the region, but have large potential to cause O₃ exceedances in the Calexico region. The California emissions canconsiderably enhance the O₃ level in the Tijuana region. Generally, the California emissions play amoreimportant role than the Baja California emissions on O₃ formation in the border region (within 40km totheCalifornia-Mexico border). On average, the O₃ concentrations in the border region are decreased by 2-4ppbin the afternoon due to the interactions of emissions from California and Baja California. Further studies need to be conducted to improve the sea breeze simulations in the border region for evaluating O₃ formation.