Classification of Ozone Episodes in Central California

The Central California Ozone Study (CCOS) Meteorological Working Group was formed by the CCOS Technical Committee to support the CCOS planning effort sponsored by the California Air Resources Board. Two goals of the group are to: 1) identify meteorological scenarios of interest for ozone episodes in Central California, and 2) develop region-wide forecast indicators for the meteorological scenarios of interest.

To accomplish these goals, the Group has examined ozone episodes throughout an extended Central California region, from the Pacific ocean on the west to the foothills of the Siskiyou, Cascade and Sierra Mountains on the north and east and to the San Rafael and Tehachapi Mountains on the south. Previous classification schemes have focused on the EPA 1-hour ozone standard of 0.12 ppm. This effort includes consideration of the proposed 8-hour standard of 0.08 ppm and provides a comprehensive operational definition of an "ozone episode" throughout Central California. Temporal and spatial patterns of ozone concentrations are examined for three ozone seasons (May-October) during 1996-1998 and for nine ozone seasons during 1990-1998. Classification by inspection of Daily Weather Maps is compared to the objective classification techniques of cluster analysis, regression analysis, and descriptive statistics derived from meteorological and air quality parameters. Ozone episode scenarios of interest are defined, and forecast procedures are discussed. Transport and dispersion of pollutants is also addressed for each scenario.