The Influence of Meteorology on the Air Quality in the San Luis Obispo County-Southwestern San Joaquin Valley Region for 3 6 August 1990

Abstract

The large volume of data measured during the 1990 summer San Joaquin Valley Air Quality Study/Atmospheric Utility Signatures, Predictions, and Experiments (SJVAQS/AUSPEX) provides a unique opportunity to examine the influence of meteorology on air quality for a variety of regions in central California. This paper provides a qualitative analysis of surface and upper-level meteorological and air quality data measured during 3?6 August 1990 in San Luis Obispo County (SLOC) and the southwestern side of the San Joaquin Valley (SJV). During this 4-day period, daytime and nighttime atmospheric mechanisms helped to transport ozone into layers aloft over the SJV. Air flowing out of the SJV in the afternoon transported elevated layers of ozone into SLOC. The daily onshore flow from the west opposed this outflow of air from the SJV. The onshore flow prevented the transported ozone from the SJV from reaching the surface and allowed some ventilation into the southwestern side of the SJV. However, on 5 August 1990, a strong ridge of high pressure over the western United States helped to weaken onshore flow and allowed outflow from the SJV to penetrate much further to the coast. These changes in the synoptic-scale meteorology increased transport of polluted air into the region and decreased overall circulations at the surface. As a result, ozone levels exceeded the California state standard for ozone (>90 ppb) at two remote sites in the SJV on 5 August, and at one site in the SLOC on 6 August 1990. This paper discusses the synoptic meteorology and the surface and upper-level meteorological and air quality data. Also, it is revealed that ozone transport and other atmospheric processes that influence surface air quality caused the ozone exceedances.