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The 14‐month‐long (December 1999 to February 2001) Central California Regional PM 10 /PM 2.5 Air Quality Study (CRPAQS) consisted of acquiring speciated PM 2.5 measurements at 38 sites representing urban, rural, and boundary environments in the San Joaquin Valley air basin. The study's goal was to understand the development of widespread pollution episodes by examining the spatial variability of PM 2.5 , ammonium nitrate (NH 4 NO 3 ), and carbonaceous material on annual, seasonal, and episodic timescales. It was found that PM 2.5 and NH 4 NO 3 concentrations decrease rapidly as altitude increases, confirming that topography influences the ventilation and transport of pollutants. High PM 2.5 levels from November 2000 to January 2001 contributed to 50–75% of annual average concentrations. Contributions from organic matter differed substantially between urban and rural areas. Winter meteorology and intensive residential wood combustion are likely key factors for the winter‐nonwinter and urban‐rural contrasts that were observed. Short‐duration measurements during the intensive operating periods confirm the role of upper air currents on valley‐wide transport of NH 4 NO 3 . Zones of representation for PM 2.5 varied from 5 to 10 km for the urban Fresno and Bakersfield sites, and increased to 15–20 km for the boundary and rural sites. Secondary NH 4 NO 3 occurred region‐wide during winter, spreading over a much wider geographical zone than carbonaceous aerosol.
Chow et al. (Wed,) studied this question.