Comparison of secondary organic aerosol (SOA) formation during o-, m-, and p-xylene photooxidation.

Despite extensive effort to characterize xylene-isomer-derived secondary organic aerosols (SOAs) over the past decade, differences in SOA composition among xylene isomers, and their relative contributions to SOA formation remain poorly understood. Herein, we reinvestigated the photooxidation of o-, m-, and p-xylene under two limiting NO conditions. Dicarbonyls, TBM (the acronym of C-trione, 2,3-butanedione, and 3-methyl-2-oxiranecarbaldehyde with the same [M+H]m/z value of 87), and highly oxidized species (HOS), based on the m/z 61 fragment, were determined to be the predominant SOA components arising from xylene photooxidation; however, their relative contributions to SOA formation appear to depend on the xylene substitution pattern. In the initial stages of the reaction, dicarbonyls present in the SOA from m- and p-xylene, and TBM in the SOA from o-xylene, were the main contributors to new particle formation (NPF). Based on their significant levels of formation, HOS and TBM were characterized to be critical components that enhance SOA growth. High NO levels were noted to inhibit the formation of C-trione and 2,3-butanedione in the SOA from m- and o-xylene, whereas the formation of 3-methyl-2-oxiranecarbaldehyde during p-xylene photooxidation was significantly promoted. These results reveal that the substitution pattern of the xylene isomer is a significant factor that determines these differences. In addition, decreases in the levels of dicarbonyls and TBM during NPF and the formation of HOS in the presence of high levels of NO may be important factors that lead to lower SOA yields compared to those obtained under low-NO conditions. This work contributes to a better understanding of the formation mechanism of xylene-derived SOAs.