Articles | Volume 16, issue 17
Atmos. Chem. Phys., 16, 10793–10808, 2016
https://doi.org/10.5194/acp-16-10793-2016
Atmos. Chem. Phys., 16, 10793–10808, 2016
https://doi.org/10.5194/acp-16-10793-2016

Research article 31 Aug 2016

Research article | 31 Aug 2016

Impact of molecular structure on secondary organic aerosol formation from aromatic hydrocarbon photooxidation under low-NOx conditions

Lijie Li1,2, Ping Tang1,2, Shunsuke Nakao1,2,a, and David R. Cocker III1,2 Lijie Li et al.
  • 1University of California, Riverside, Department of Chemical and Environmental Engineering, Riverside, CA 92507, USA
  • 2College of Engineering – Center for Environmental Research and Technology (CE-CERT), Riverside, CA 92507, USA
  • acurrently at: Clarkson University, Department of Chemical and Biomolecular Engineering, Potsdam, NY 13699, USA

Abstract. The molecular structure of volatile organic compounds determines their oxidation pathway, directly impacting secondary organic aerosol (SOA) formation. This study comprehensively investigates the impact of molecular structure on SOA formation from the photooxidation of 12 different eight- to nine-carbon aromatic hydrocarbons under low-NOx conditions. The effects of the alkyl substitute number, location, carbon chain length and branching structure on the photooxidation of aromatic hydrocarbons are demonstrated by analyzing SOA yield, chemical composition and physical properties. Aromatic hydrocarbons, categorized into five groups, show a yield order of ortho (o-xylene and o-ethyltoluene) > one substitute (ethylbenzene, propylbenzene and isopropylbenzene) > meta (m-xylene and m-ethyltoluene) > three substitute (trimethylbenzenes) > para (p-xylene and p-ethyltoluene). SOA yields of aromatic hydrocarbon photooxidation do not monotonically decrease when increasing alkyl substitute number. The ortho position promotes SOA formation while the para position suppresses aromatic oxidation and SOA formation. Observed SOA chemical composition and volatility confirm that higher yield is associated with further oxidation. SOA chemical composition also suggests that aromatic oxidation increases with increasing alkyl substitute chain length and branching structure. Further, carbon dilution conjecture developed by Li et al. (2016) is extended in this study to serve as a standard method to determine the extent of oxidation of an alkyl-substituted aromatic hydrocarbon.

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This study comprehensively investigates the molecular structure impact on SOA formation during the photooxidation of aromatic hydrocarbons under low-NOx conditions by simultaneously analyzing SOA yield, chemical composition and physical properties. The result suggests that the substitute location, carbon chain length and branching structure exert greater impact on SOA formation than the substitute number. This study improve the understanding of SOA formation from anthropogenic sources.
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