An Investigation of High-Ozone Episodes during the California Baseline Ozone Transport Study (CABOTS)

Because the EPA recently lowered the ambient air quality standard for the 8-hr average of ozone (O3) to70 ppbv, California must continue to achieve significant reductions in ozone precursor emissions and prepare for new State Implementation Plans (SIP) to demonstrate how ground-level ambient ozone will be reduced below the new health-based standard. Prior studies suggest that background levels of ozone traveling across the Pacific Ocean can significantly influence surface ozone throughout California, particularly during the spring. To better understand the contributions of the external natural and anthropogenic pollution sources as well as atmospheric processes for surface ozone concentrations in California during the spring and summer months, the California Baseline Ozone Transport Study (CABOTS) has been established.

For the first time, near daily ozonesondes were launched from spring to summer in Bodega Bay, California during the high ozone seasons from mid-May to mid-August to study the spatial and temporal variations of high ozone layers as well as the progression of background ozone entering the California coast from the Pacific Ocean. A second near daily ozonesonde site at Half Moon Bay, California was also established in July to study in more detail the latitudinal differences and in the onshore flow of ozone during the peak of the summer high ozone season.

Preliminary results from the ozonesonde data show numerous episodes of high ozone layers entering California from different sources, particularly a multi-day stratosphere-troposphere intrusion event from June 9-14 and the wildfire activity from the Soberanes Fire in Monterey County. Ozonesonde data during the stratospheric intrusion event show ozone layers as high as 110 ppb at 3 km. Measurements on late July, the ozonesonde data from Bodega Bay and Half Moon Bay captured a high ozone event related to an air mass downwind of the Soberanes Fire in Monterey County.

The temporal progression of ozonesonde measurements and subsequent analysis of these two cases will be discussed with a focus on the contribution of background ozone to surface ozone sites inland (e.g., urban) as well as likely origins of layers aloft. Comparisons of current ozonesondes versus prior ozonesonde studies of California will also be examined.