Seasonal and spatial variations of individual organic compounds of coarse particulate matter in the Los Angeles Basin

Abstract

To study the organic composition of ambient coarse particulate matter (CPM; 2.5–10 μm), coarse particles were collected one day a week from April 2008 to March 2009 at 10 sampling sites in the Los Angeles Basin. Samples were compiled into summer (June 2008 to September 2008) and winter (November 2008 to February 2009) composites, and were subsequently analyzed for individual organic constituents using gas chromatography-mass spectrometry. n-alkanoic acids and medium molecular weight (MW) n-alkanes (C25 to C35) – the major constituents in the coarse size fraction – showed good associations with crustal materials. Polycyclic aromatic hydrocarbons (PAHs) and hopanes (both in low concentrations), as well as high MW n-alkanes (C37 and C38), were associated with traffic-related emissions. In the summer, when prevailing onshore winds were strong, the downwind/rural sites had higher concentrations of PAHs, n-alkanes and n-alkanoic acids. An opposite trend was observed at the urban sites, where the levels of PAHs, n-alkanes and n-alkanoic acids were higher in the winter, when the low wind speed limited long-range atmospheric transport. In general, the contribution of organic compounds to CPM mass was higher during wintertime, due to a reduction in the fraction of other CPM components (sea salt, secondary ions, etc.) and/or the increase in source strengths of organic compounds. The latter is consistent with the traffic-induced re-suspension of mineral and road dust, as previously observed in this basin. Overall, our results suggest that emissions from natural sources (soil and associated biota) constitute the majority of the organic content in coarse particles, with a more pronounced influence in the semi-rural/rural areas in Riverside/Lancaster compared with urban Los Angeles in the summer.