10th International Aerosol Conference September 2 - September 7, 2018 America's Center Convention Complex St. Louis, Missouri, USA
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Chemical Characterization of Secondary Organic Aerosol (SOA) in a Transitional Season of Biogenic VOC Emission
YUNLE CHEN, Theodora Nah, David Tanner, Masayuki Takeuchi, Hongyu Guo, Amy P. Sullivan, Lu Xu, Rodney J. Weber, Greg Huey, Nga Lee Ng, Georgia Institute of Technology
Abstract Number: 1107 Working Group: Source Apportionment
Abstract The formation and evolution of SOA was investigated at Yorkville, a rural site 55km NW of Atlanta, in late summer (mid-August ~ mid-October, 2016), a period when the biogenic volatile organic compounds (VOC) emissions were gradually decreasing. This is a rural site in the Southeastern Aerosol Characterization (SEARCH) network, situated in a mixed forest–agricultural area and is characterized by intense agricultural emissions. A High-Resolution Time-of-Flight Mass Spectrometer (HR-ToF-AMS) was deployed to characterize non-refractory submicron particles. Co-located instruments (CIMS, FIGAERO-CIMS, GC-MS, PILS-IC, etc.) were used to comprehensively characterize both gas- and particle-phase composition.
Unconstrained Positive Matrix Factorization (PMF) analysis and Multilinear Engine (ME-2) analysis with constrained profiles are applied to identify and quantify OA sources. Prior measurements at the same site suggest that biogenic SOA is a predominant source of total OA at the Yorkville site, where contribution from isoprene-derived OA (Isoprene-OA) to total OA is 36% in summer (July), but nearly zero in winter (December). In this study, the continuously decreasing isoprene emission throughout the measurement period poses a challenge to resolving isoprene-derived OA using PMF. With constrained Isoprene-OA profile (from Centreville measurements during 2013 summer, resolved by PMF), ME-2 analysis successfully resolved four factors, including Isoprene-OA, less-oxidized oxygenated OA (LO-OOA), more-oxidized oxygenated OA (MO-OOA), and a possible sesquiterpene-derived OA (Sesquiterpene-OA).
With isoprene emission decreasing and NO concentration relatively constant, we observed a clear shift of isoprene oxidation pathways from “low-NOx” chemistry to “high-NOx” chemistry, as NO / isoprene ratio increased. Our measurements of gas-phase organic acids (e.g. formic acid) show good correlation with isoprene-OA, suggesting isoprene aerosol formation processes are possibly an important source for atmospheric organic acids, which is consistent with recent studies that isoprene photooxidation is an important contributor to atmospheric organic acid budget. Meanwhile, we observed a good correlation between MO-OOA factor with the sum of particle-phase organic acids (C2 – C5 acids, Pearson’s R2 = 0.83), suggesting that the ubiquitous MO-OOA factor could be composed of particulate organic acids. The LO-OOA factor and Sesquiterpene-OA factor show correlation with particulate C10 and C15 organic nitrates, indicating possible aerosol sources from nitrate radical chemistry and/or photooxidation in the presence of NOx.