Temporal variations of PM2.5, PM10, and gaseous precursors during the 1995integrated monitoring study in central California

The spatial and temporal distributions of particle mass and its chemical co nstituents are essential for understanding the source-receptor relationship s as well as the chemical, physical, and meteorological processes that resu lt in elevated particulate concentrations in California's San Joaquin Valle y (SJV). Fine particulate matter (PM2.5), coarse particulate matter (PM10), and aerosol precursor gases were sampled on a 3-hr time base at two urban (BakersfieId and Fresno) and two non-urban (Kern Wildlife Refuge and Chowch illa) core sites in the SJV during the winter of 1995-1996. Day-to-day variations of PM2.5 and PM10 and their chemical constituents wer e influenced by the synoptic-scale meteorology and were coherent among the four core sites. Under non-rainy conditions, similar diurnal variations of PM2.5 and coarse aerosol were found at the two urban sites, with concentrat ions peaking during the nighttime hours. Conversely, PM2.5 and coarse aeros ol peaked during the morning and afternoon hours at the two nonurban sites. Under rainy and foggy conditions, these diurnal patterns were absent or gr eatly suppressed. In the urban areas, elevated concentrations of primary pollutants (e.g., or ganic and elemental carbons) during the late afternoon and nighttime hours reflected the impact from residential wood combustion and motor vehicle exh aust. During the daytime, these concentrations decreased as the mixed layer deepened. Increases of secondary nitrate and sulfate concentrations were f ound during the daylight hours as a result of photochemical reactions. At t he non-urban sites, the same increases in secondary aerosol concentrations occurred during the daylight hours but with a discernable lag time. Concent rations of the primary pollutants also increased at the non-urban sites dur ing the daytime. These observations are attributed to mixing aloft of prima ry aerosols and secondary precursor gases in urban areas followed by rapid transport aloft to non-urban areas coupled with photochemical conversion.