Molecular characterization of water-soluble organic compounds in PM2.5 using ultrahigh resolution mass spectrometry

Water-soluble organic compounds (WSOCs) are a complex mixture of organic components with a variety of chemicals structures that may have significant impacts on the formation process and health hazards of atmospheric fine particles. In this study, the molecular characteristics of WSOCs in PM_2.5 were investigated using ultrahigh resolution mass spectrometry. In total 7389 compounds in PM_2.5 water extracts were identified, including CHO^±, CHOS^±, CHON^±, CHONS^±, CH⁺, CHS⁺, CHN⁺ and CHNS⁺ species. CHO^± and CHON^± were the major components in PM_2.5 water extracts. S-containing compounds detected in both ionization modes were observed with distinct molecular characteristics. Selective partitioning of WSOCs between PM_2.5 water extracts and polydimethylsiloxane (PDMS, log D_PDMS = 0.51-3.87) coating phases was found, which was significantly correlated with molecular characteristic (i.e. double-bond equivalent, number of O and H atoms, O/C ratios, and aromaticity equivalent). The extent of accumulation for negatively charged compounds was generally lower, which related to the existence of polar functional groups, such as hydroxyl, carboxyl, nitrate, and sulfate, as observed by MS/MS fragmentation analysis.