1NOAA ESRL Chemical Sciences Division, 325 Broadway, Boulder, CO 80305, USA
2CIRES, University of Colorado, Boulder, CO 80309, USA
3NOAA ESRL Global Monitoring Division, 325 Broadway, Boulder, CO 80305, USA
4Humboldt State Marine Lab, 570 Ewing St., Trinidad Head, CA, 95570, USA
Abstract. Ozone sondes launched from Trinidad Head, California provide a measure of background O3 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 O3 gradient and correlation analysis indicate that O3-rich air from above the marine boundary layer is transported to the surface. Surface O3 is found to increase proportionally to the transported background. At the surface site experiencing the highest O3 concentrations, the mean maximum daily 8-h average (MDA8) O3 on exceedance days (i.e. those days when MDA8 O3 exceeds 75 ppbv) is 20 ppbv higher than on non-exceedance days. The transported background O3, 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 O3 background is generally 15–35 ppbv, and that it is lower, rather than higher, during pollution episodes. The present work indicates that O3 transported on hemispheric scales substantially impacts air quality in some areas of the US.