Primary and secondary carbonaceous species in the atmosphere of Western Riverside County, California

Elemental carbon (EC), organic carbon (OC) and PM2.5 mass concentrations were measured from September 2001 through January 2002 in Mira Loma, CA. EC and OC were analyzed using the NIOSH (National Institute of Occupational Safety and Health) 5040 thermal/optical transmittance method. OC concentrations in Mira Loma were found to be higher than those of other urban sites in the South Coast Air Basin (SoCAB), while EC concentrations were comparable to or lower than those of other SoCAB sites. Overall, OC and EC concentrations accounted for 26% and 5% of the total PM2.5, respectively. OC/EC ratios ranged from 1.6 to 12.8 with an average of 5.2. These values were higher than those observed at other urban sites in the United States by a factor of 2. A stronger correlation between suspended OC and EC concentrations was noted in months with lower photochemical activity (December and January, r = 0.82) than in months with greater photochemical activity (September and October, r = 0.64). The elevated levels of OC, OC/EC ratios, and the seasonal difference in correlation between OC and EC concentrations were attributed in part to significant secondary organic aerosol formation. The fraction of total organic carbon that was secondary organic carbon (SOC) was estimated using the OC/EC minimum ratio method and Chemical Mass Balance (CMB) modeling. Based on the OC/EC minimum ratio method, the contribution of SOC to the total organic carbon tended to be higher during the months with greater photochemical activity (63%) than those with lower photochemical activity (44%). Based on CMB modeling, SOC contributed to 14% of the total PM2.5 mass and 57% of the total organic carbon during the study period. Overall, these findings suggest that photochemical activity can appreciably affect total PM2.5 mass concentrations in Mira Loma, and that measures to control emissions of SOC precursors incorporated as part of a region-wide air quality management plan could lead to a perceptible drop in total PM2.5 mass concentrations in this area. (C) 2003 Elsevier Ltd. All rights reserved.