Impact of transported background ozone inflow on summertime air quality in a California ozone exceedance area

Ozone sondes launched from Trinidad Head, California provide a measure of background O(3) transported ashore, and allow an evaluation of the impact of this transport on air quality in California's Northern Sacramento Valley. A strong summertime vertical O(3) gradient and correlation analysis indicate that O(3)-rich air from above the marine boundary layer is transported to the surface. Surface O(3) is found to increase proportionally to the transported background. At the surface site experiencing the highest O(3) concentrations, the mean maximum daily 8-h average (MDA8) O(3) on exceedance days (i.e. those days when MDA8 O(3) exceeds 75 ppbv) is 20 ppbv higher than on non-exceedance days. The transported background O(3), as measured 22 h earlier by the Trinidad Head sondes, accounts for more than half (11 ppbv) of this difference. This finding contrasts with conclusions from model calculations that indicate the US policy relevant O(3) background is generally 15-35 ppbv, and that it is lower, rather than higher, during pollution episodes. The present work indicates that O(3) transported on hemispheric scales substantially impacts air quality in some areas of the US.