Characterization of Particulate Matter in California.

The size, composition, and concentration of particulate matter (PM) vary with location and time. Several monitoring/sampling programs are operated in California to characterize PM less than 2.5 and 10 µm in aerodynamic diameter (PM2.5 and PM10). This paper presents a broad summary of the spatial and temporal variations observed in ambient PM2.5 and PM10 concentrations in California. Many areas that have high PM10 concentrations also have relatively high PM2.5 concentrations, and data indicate that a significant portion of the PM10 air quality problem is caused by PM2.5. To develop effective plans for attaining the ambient PM standards, improved understanding of these unique problems is needed. Since 1989, pollution control efforts-whether specifically targeted for particulate matter or indirectly via controls on gaseous emissions-have caused annual average PM2.5 and PM10 concentrations to decline at most sites in California.