AAAR 31st Annual Conference
October 8-12, 2012
Hyatt Regency Minneapolis
Minneapolis, Minnesota, USA
Abstract View
Seasonal and Spatial Variations of Individual Organic Compounds of Coarse Particulate Matter in the Los Angeles Basin
KALAM CHEUNG, Michael Olson, Brandon Shelton, James Schauer, Constantinos Sioutas, University of Southern California
Abstract Number: 29 Working Group: Urban Aerosols
Abstract While the organic content of ultrafine and fine particulate matter (PM) is well studied, the organic composition of coarse particles has remained largely uncharacterized. To improve our understanding on the sources and formation mechanisms of coarse PM-bound organic species, coarse particles were collected once a week from April 2008 to March 2009 at 10 sampling sites in the Los Angeles Basin. Samples were composited into two seasons: summer (June 2008 to September 2008) and winter (November 2008 to February 2009), and were analyzed for the levels of individual organic constituents using gas chromatography-mass spectrometry. Our results reveal that the predominant organic constituents in the coarse size fraction, namely n-alkanoic acids and medium molecular weight (MW) n-alkanes (C25 to C35), were well correlated with crustal materials. Polycyclic aromatic hydrocarbons (PAHs) and hopanes (both in low concentrations), and high MW n-alkanes (C37 and C38), were associated with traffic-related emissions. In summertime, when onshore winds were strong, the downwind/rural sites had higher levels of PAHs, n-alkanes and n-alkanoic acids. At the urban sites, an opposite trend was observed - the levels of PAHs, n-alkanes and n-alkanoic acids were higher in winter, when the low wind speed limited long range transport of particles. The mass fraction of organic compounds to coarse PM was higher in winter, due to the lower concentrations of other coarse PM components (such as sea salt, secondary ions) and/or the increasing source strengths of organic compounds, the latter of which is supported by traffic-induced re-suspension of mineral and road dust in cooler months, when both wind speed and atmospheric dilution were low. Overall, our results suggest that soil and the associated biota represent the major source of organic constituents in coarse particles, with a more pronounced influence in the downwind semi-rural/rural areas in summer.