Abstract
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.
Highlights
Organic aerosols are critical to human health (Dockery et al, 1993; Krewski et al, 2003; Davidson et al, 2005), climate change (IPCC, 2007) and visibility (Pöschl, 2005; Seinfeld and Pandis, 2006)
Global anthropogenic secondary organic aerosol (SOA) sources are underestimated by current models (Henze et al, 2008; Matsui et al, 2009; Hallquist et al, 2009; Farina et al, 2010) and are more likely to increase due to the increase of known anthropogenic emissions (Heald et al, 2008)
H / C barely increases (1.33 to 1.34) from the propylbenzene precursor to its resulting SOA and there is even a decreasing trend from isopropylbenzene to its SOA. This indicates that a high H / C component loss reaction such as alkyl-part dissociation during photooxidation is an important reaction to SOA formation from aromatic hydrocarbons containing longer carbon chains
Summary
Organic aerosols are critical to human health (Dockery et al, 1993; Krewski et al, 2003; Davidson et al, 2005), climate change (IPCC, 2007) and visibility (Pöschl, 2005; Seinfeld and Pandis, 2006). Aromatic studies categorized SOA yield solely based on substitute number (Odum et al, 1997a, b) Those chamber experiments were conducted at highNOx conditions, which are well above levels present in the atmosphere. Ng et al (2007) show there is no significant yield difference between one-substitute (toluene) and two-substitute (m-xylene) aromatics in the absence of NOx. The current work focuses on molecular structure impact on SOA formation at more atmospherically relevant NOx and aerosol loadings. Few studies comprehensively consider the overall alkyl effect on SOA formation from aromatic hydrocarbons, including the substitute number, position, carbon chain length and branching structure, especially under low-NOx conditions. This work examines 12 aromatic hydrocarbons, all of which are isomers with eight or nine carbons, to investigate the impact of molecular structure on SOA formation from aromatic hydrocarbon photooxidation under low NOx (10– 138 ppb). Stitute dilution conjecture is further developed from methyl dilution theory (Li et al, 2016)
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