Changes in Fine Particulate Matter Measurement Methods and Ambient Concentrations in California

Abstract:

Networks for measuring ambient fine particle (PM_2.5) mass concentrations in California have undergone major changes between 1988 and 2013, transitioning from 24-hour average filter-based sampling to beta attenuation monitors (BAM) that provide hourly measurements on a continuous basis. Over the last decade, between 2003 and 2013, the number of routine PM_2.5 measurement sites increased from 10 to 19 in the San Francisco Bay area, from 15 to 27 in the San Joaquin Valley, and from 17 to 22 in the Los Angeles area. The fraction of days with PM_2.5 data in each air basin also increased substantially, from 44-50% to 74-89% of site-days per year, due mainly to increased reliance on BAM in place of filter-based sampling.

This research explores variations of PM_2.5 on various time scales, using PM mass measurements from three major California air basins mentioned above. Regression analysis of collocated BAM and filter-based measurements of PM_2.5, shows BAM reads higher than the filter data by 3-6 µg/m³. 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, PM_2.5 concentrations are elevated in winter compared to summer, with basin-wide average increases of about a factor of two for both the San Francisco Bay area and the San Joaquin Valley. Winter season increases are prominent at night due to wood-burning and stagnant atmospheric conditions. The reverse is true at inland sites in southern California, which see reductions in PM_2.5 during the in winter. Weekend concentrations are slightly lower than on weekdays. Reductions of ~10% in weekend PM_2.5 relative to mid-week values are observed in the San Francisco Bay area and the San Joaquin Valley. Larger reductions of ~25% in PM_2.5 are observed on weekends at traffic-dominated sites such as West Oakland. Weekday-weekend differences in PM_2.5 mass have been difficult to observe in past studies because the changes are often relatively small, and because much of the filter-based sampling has been done on a once every third or sixth day schedule. In contrast, BAM provides daily data and therefore increased statistical power in detecting subtle effects such as weekday-weekend differences.