Impact of regional transport on the anthropogenic and biogenic secondary organic aerosols in the Los Angeles Basin

This manuscript explores the role of regional transport on anthropogenic and biogenic secondary organic carbon (SOC) concentrations in ambient fine particulate (PM2.5) organic carbon (OC) in the Los Angeles (LA) Basin. Daily organic molecular markers, water soluble organic carbon (WSOC), OC, and elemental carbon (EC) measurements from May 2009 through April 2010 at a central site in downtown LA, and results from a positive matrix factorization (PMF) analysis of these data, were used to understand the role of regional transport on SOC concentrations. A backward-trajectory analysis, coupled with the measurements and estimated source contributions, were used to evaluate the origins of SOC aerosols. Anthropogenic and biogenic SOC were identified in central LA over the study period, together contributing 40% of the annual average PM2.5 OC mass. There were distinct seasonal variations, with high contributions of anthropogenic SOC in summer, and high contributions of biogenic SOC in spring. The back-trajectory analysis, coupled with daily source contributions of SOC and organic compounds as indicators, allowed us to identify potential source locations and dominant meteorological conditions contributing to elevated SOC at the measurement site. The results show that air mass movements from the Pacific Ocean are associated with higher contributions of anthropogenic SOC to the PM2.5 OC in downtown LA, suggesting that the combination of local meteorological conditions and local anthropogenic emissions led to an increase in the anthropogenic SOC. In contrast, air masses passing over the Central Valley and forested areas where there are biogenic hydrocarbon emissions are closely associated with higher contributions of biogenic SOC in the region. The study emphasizes that higher anthropogenic SOC contributions are due to the combination of local emissions with humidity air from the ocean, and that higher biogenic SOC contributions are impacted by transport of pollutants from regions outside the LA Basin.