The spatial and temporal distributions of particle mass and its chemical constituents are essential for understanding the source-receptor relationships as well as the chemical, physical, and meteorological processes that result in elevated particulate concentrations in California's San Joaquin Valley (SJV). Fine particulate matter (PM 2.5 ), coarse particulate matter (PM 10 ), and aerosol precursor gases were sampled on a 3-hr time base at two urban (Bakersfield and Fresno) and two non-urban (Kern Wildlife Refuge and Chowchilla) core sites in the SJV during the winter of 1995-1996. Day-to-day variations of PM 2.5 and PM 10 and their chemical constituents were influenced by the synopticscale meteorology and were coherent among the four core IMPLICATIONS Concentrations of fine particulates (PM 2.5 ) in California's SJV during winter result from a complex interaction of source emissions, chemical transformation, and meteorological factors. While concentrations of primary pollutants like elemental and organic carbons are higher in urban than in rural areas, vertical mixing and transport aloft significantly impact their concentrations in rural areas. Concentrations of ammonium nitrate, a significant component of PM 2.5 , and ammonium sulfate, are much more spatially homogeneous. These compounds, produced from gaseous oxides of sulfur and nitrogen, are transported from urban areas, photochemically oxidized, and combined with ammonia produced in outlying areas. An understanding of the mechanisms responsible for elevated PM 2.5 concentrations will be necessary for attaining new federal compliance standards for PM 2.5 in the SJV.sites. Under non-rainy conditions, similar diurnal variations of PM 2.5 and coarse aerosol were found at the two urban sites, with concentrations peaking during the nighttime hours. Conversely, PM 2.5 and coarse aerosol peaked during the morning and afternoon hours at the two nonurban sites. Under rainy and foggy conditions, these diurnal patterns were absent or greatly suppressed.In the urban areas, elevated concentrations of primary pollutants (e.g., organic and elemental carbons) during the late afternoon and nighttime hours reflected the impact from residential wood combustion and motor vehicle exhaust. During the daytime, these concentrations decreased as the mixed layer deepened. Increases of secondary nitrate and sulfate concentrations were found during the daylight hours as a result of photochemical reactions. At the non-urban sites, the same increases in secondary aerosol concentrations occurred during the daylight hours but with a discernable lag time. Concentrations of the primary pollutants also increased at the non-urban sites during the daytime. These observations are attributed to mixing aloft of primary aerosols and secondary precursor gases in urban areas followed by rapid transport aloft to non-urban areas coupled with photochemical conversion.

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