Chemical characterization of outdoor PM2.5 and gas-phase compounds in Mira Loma, California

Outdoor fine particulate matter (PM2.5) and gas-phase compounds were investigated in Mira Loma, a semi-rural community in southern California, between September 2001 and January 2002. Outdoor PM2.5 concentrations for the study duration averaged 41.8 μg m−3, with nitrate and organic carbon being the major contributors ( 26% each). Higher PM2.5 concentrations were observed during the first half of the study relative to the second half, corresponding with actinic flux calculations. Gas-phase ammonia concentrations were 6–7 times greater than nitric acid concentrations, while formaldehyde and acetaldehyde concentrations were consistent with Air Resources Board results. Use of a back trajectory model together with PM2.5 chemical composition data showed distinct differences in nitrate and trace element concentrations for back trajectories originating from Los Angeles and Orange counties versus those originating from Nevada. Source apportionment using a chemical mass balance model helped demonstrate that the total contribution of secondary aerosol (56%) was greater than the total primary vehicular exhaust contribution to PM2.5 by a factor of 5. These results highlight the importance of region-wide secondary sources to the local air quality in this semi-rural area.