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

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... he 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.