Changes in fine particulate matter measurement methods and ambient concentrations in California

The fine particle (PM2.5) air pollution monitoring network in California underwent major changes between 1988 and 2013, adding continuous beta attenuation monitors (BAM) to the network. This paper analyzes temporal patterns using PM2.5 mass measurements from three major air basins. Regressions of BAM against filter measurements generally show BAM reads higher by 3–6 μg/m3 for annual averages. Most monitoring sites show clear downward trends in PM2.5 concentrations over time, especially in the Los Angeles area where concentrations have decreased by more than 50% since the 1990s. In most cases, PM2.5 concentrations are elevated in winter compared to summer, with basin-wide average increases of 80 ± 25% for the San Francisco Bay area and 123 ± 28% for the San Joaquin Valley. The differences are more prominent at night due to wood-burning and stagnant atmospheric conditions. The reverse is true at inland sites in southern California, which show average reductions of 46 ± 12% in winter. Weekend concentrations are slightly lower than on weekdays. Average weekend reductions in PM2.5 relative to mid-week values are 8.9 ± 5.9% for the San Francisco Bay area and 8.0 ± 3.0% for the San Joaquin Valley. Up to 25% reductions in PM2.5 are observed on weekends at traffic-dominated sites such as West Oakland.