Impact of transported background ozone inflow on summertime air quality in a California ozone exceedance area
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 increases proportionally to the transported background. At the surface site experiencing the highest O3 concentrations, transported background O3 accounts for more than half (11 ppbv) of the 20 ppbv difference in the mean maximum 8-h average O3 between exceedance and non-exceedance days. This finding contrasts with model calculations that indicate the US policy relevant O3 background is generally 15–35 ppbv, and that it decreases during pollution episodes. The present work indicates that O3 transported on hemispheric scales substantially impacts air quality in some areas of the US.