Particle-Bound Highly Oxidized Organic Molecules Derived from Aromatic Hydrocarbons in an Urban Atmosphere

Previous laboratory and theoretical studies have demonstrated that gas-phase oxidation of aromatic hydrocarbons can produce highly oxidized organic molecules (HOMs) via autoxidation, yet our molecular-level understanding of aromatics-derived HOMs existing stably in the particulate phase in ambient air remains unclear. In this study, filter-based ambient fine particle samples were collected in the Pearl River Delta region in south China and particle-bound HOMs derived from gas-phase autoxidation of aromatic hydrocarbons were assessed by using a Fourier transform ion cyclotron resonance mass spectrometer. The results revealed that aromatics-derived HOMs that survived in fine particles were more diverse than those observed in the laboratory or predicted by theory. O6-7 class species were the most abundant species (75-90% signal intensity) among the aromaticsderived CHO-HOMs. The aromatics-derived CHON-HOM monomers accounted for 7-10% of all observed CHON species. The observed aromatics-derived CHO- and CHON-HOMs accounted for 1.7-5.1 parts per thousand and 0.2-0.6 parts per thousand, respectively, of the total particulate organic aerosol mass. The semiquantified concentrations of aromatics-derived HOMs were comparable to those of aromatics-derived SOA estimated by the tracer method and increased significantly with relative humidity. The results suggest a substantial contribution of aromatics-derived HOMs in aromatics-derived SOA in an urban atmosphere, particularly during haze days with higher relative humidity.