Role of methyl group number on SOA formation from monocyclic aromatic hydrocarbons photooxidation under low-NOx conditions

摘要

Substitution of methyl groups onto the aromatic ring determines the secondaryorganic aerosol (SOA)formation from the monocyclic aromatic hydrocarbon precursor (SOA yield andchemical composition). This study links the number of methyl groups on thearomatic ring to SOA formation from monocyclic aromatic hydrocarbonsphotooxidation under low-NO conditions (HC/NO    10 ppbC : ppb).Monocyclic aromatic hydrocarbons with increasing numbers ofmethyl groups are systematically studied. SOA formation frompentamethylbenzene and hexamethylbenzene are reported for the first time. Adecreasing SOA yield with increasing number of methyl groups is observed.Linear trends are found in both vs.  and O / C vs. H / C forSOA from monocyclic aromatic hydrocarbons with zero to six methyl groups. AnSOA oxidation state predictive method based on benzene is used to examinethe effect of added methyl groups on aromatic oxidation under low-NOconditions. Further, the impact of methyl group number on density andvolatility of SOA from monocyclic aromatic hydrocarbons is explored.Finally, a mechanism for methyl group impact on SOA formation is suggested.Overall, this work suggests that, as more methyl groups are attached on thearomatic ring, SOA products from these monocyclic aromatic hydrocarbonsbecome less oxidized per mass/carbon on the basis of SOA yield or chemical composition.