Identification and Tracking of Polluted Air Masses in the South-Central Coast Air Basin

Abstract

Canister samples of air taken during the South-Central Coast Cooperative Air Monitoring Program (SCCCAMP) 1985 field study program were analyzed for concentrations of over 50 hydrocarbons as well as chlorofluorocarbons (CFCs), carbon monoxide, hydrogen, and methane. Additional evidence of location and timing of airmass origin was obtained by utilizing long-lived halocarbons such as F-12 as “tracers of opportunity” in conjunction with known source profiles. Wind trajectories were developed from hourly gridded wind fields produced by a diagnostic wind model utilizing observed wind data. These wind trajectories were used to determine how pollutants from major source areas might be transported to sampling sites. Particulate lidar height–distance traverses were made from aircraft that provided a view of pollutant layering. Mixing height and vertical pollutant concentration distributions were obtained in order to determine if observed pollutant concentrations were consistent with the degree of stagnation present and hypothesized transport pathway. Analyses to track specific polluted air masses were conducted for the 13 September, 21 September, 23–24 September, and 2–3 October 1985 intensive study periods. The analyses find that elevated ozone concentrations during these periods are primarily attributed to transport and storage of ozone-enriched air from Los Angeles. During one type of episode (2–3 October) ozone and ozone precursors are stored near the surface over the Santa Barbara Channel overnight and transported into coastal areas on the following day. In another type of episode (23–24 September) ozone is transported into the study domain from the San Fernando Valley and Los Angeles via flow around the Santa Monica Hills. Transport of pollutant-enriched air takes place in a layer 200–500 m aloft, in many places overlaying cleaner marine-layer air. This advected ozone is mixed down to contribute to ground-level ozone concentrations over terrain where the marine layer does not intrude. Systematic transport between air basins calls for regional-scale photochemical modeling to accurately address issues concerning exceedances of the National Ambient Air Quality Standard (NAAQS) for ozone observed in Ventura and Santa Barbara counties.