Ionic Strength Effect on Photochemistry of Fluorene and Dimethylsulfoxide at the Air–Sea Interface: Alternative Formation Pathway of Organic Sulfur Compounds in a Marine Atmosphere

The photochemical transformation processes occurring at the sea surface lead to the formation of low volatility organic compounds that contribute to new particle formation in the marine boundary layer. Here, we suggest an alternative formation pathway of organosulfur and oxygenated aliphatic compounds, initiated by sunlight-activated fluorene (FL) and dimethylsulfoxide (DMSO) in the presence of the halide ions Cl^–, Br^–, and I^–. In particular, we observe a prompt formation of four typical precursors of secondary organic aerosols, methanesulfonic acid (CH₃SO₃H), methanesulfinic acid (CH₃SO₂H), hydroxymethanesulfonic acid (CH₄O₄S, MSAOH), and 2-hydroxyethenesulfonic acid (C₂H₅O₄SH), upon light irradiation of FL/DMSO. The photochemical modeling shows that in the summer period, MSAOH would be formed in just a few hours under fair-weather conditions, which makes the photosensitized chemistry of FL toward DMSO potentially significant. We also show that the sunlight-initiated degradation of FL is considerably faster in the presence of halide ions, compared to its photochemical degradation in the dilute aqueous phase. Therefore, the photochemical degradation of FL and potentially other polycyclic aromatic hydrocarbons would be faster in high ionic strength aerosol particles, compared to analogous diluted aqueous-phase photochemical processes occurring in, for example, clouds. This could considerably affect the absorbing properties of the aerosols, which should be considered in future modeling studies.