Chow et al. PM-16 Journal of the Air & Waste Management Association Volume 49 September 1999 ISSN 1047-3289 J. Air & Waste Manage. Assoc. 49 :PM-16-24 Copyright 1999 Air & Waste Management Association TECHNICAL PAPER Temporal Variations of PM 2.5 , PM 10 , and Gaseous Precursors during the 1995 Integrated Monitoring Study in Central California Judith C. Chow, John G. Watson, and Douglas H. Lowenthal Desert Research Institute, Reno, Nevada Richard Hackney and Karen Magliano California Air Resources Board, Sacramento, California Donald Lehrman and Ted Smith Technical and Business Systems Inc., Santa Rosa, California ABSTRACT The spatial and temporal distributions of particle mass and its chemical constituents are essential for understand- ing the source-receptor relationships as well as the chemi- cal, 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 synoptic- scale 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 meteoro- logical factors. While concentrations of primary pollutants like elemental and organic carbons are higher in urban than in rural areas, vertical mixing and transport aloft sig- nificantly impact their concentrations in rural areas. Con- centrations 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 am- monia produced in outlying areas. An understanding of the mechanisms responsible for elevated PM 2.5 concen- trations will be necessary for attaining new federal compli- ance standards for PM 2.5 in the SJV. sites. Under non-rainy conditions, similar diurnal varia- tions of PM 2.5 and coarse aerosol were found at the two urban sites, with concentrations peaking during the night- time hours. Conversely, PM 2.5 and coarse aerosol peaked during the morning and afternoon hours at the two non- urban sites. Under rainy and foggy conditions, these di- urnal patterns were absent or greatly suppressed. In the urban areas, elevated concentrations of pri- mary pollutants (e.g., organic and elemental carbons) dur- ing the late afternoon and nighttime hours reflected the impact from residential wood combustion and motor ve- hicle exhaust. During the daytime, these concentrations decreased as the mixed layer deepened. Increases of sec- ondary 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. Concen- trations 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 sec- ondary precursor gases in urban areas followed by rapid transport aloft to non-urban areas coupled with photo- chemical conversion. INTRODUCTION The California Regional PM 10 /PM 2.5 Air Quality Study (CRPAQS) is a multiyear effort to understand the causes of elevated suspended particulate concentrations and to evalu- ate ways to reduce them in central California. 1 The San Joaquin Valley (SJV) in central California frequently experi- ences elevated PM 10 (particles with aerodynamic diameters