Heterogeneous Hydroxyl Radical Oxidation of Isoprene-Epoxydiol-Derived Methyltetrol Sulfates: Plausible Formation Mechanisms of Previously Unexplained Organosulfates in Ambient Fine Aerosols

Methyltetrol sulfates are unique tracers for secondary organic aerosols (SOA) formed from acid-driven multiphase chemistry of isoprene-derived epoxydiols. 2-Methyltetrol sulfate diastereomers (2-MTSs) are the dominant isomers and single most-abundant SOA tracers in atmospheric fine particulate matter (PM_2.5), but their atmospheric sinks remain unknown. We investigated the oxidative aging of authentic 2-MTS aerosols by gas-phase hydroxyl radicals (^•OH) at a relative humidity of 61 ± 1%. The effective rate constant for this heterogeneous reaction was determined as 4.9 ± 0.6 × 10^–13 cm³ molecules^–1 s^–1, corresponding to an atmospheric lifetime of 16 ± 2 days (assuming an ^•OH concentration of 1.5 × 10⁶ molecules cm^–3). Chemical changes to 2-MTSs were monitored by hydrophilic interaction liquid chromatography interfaced to electrospray ionization high-resolution quadrupole time-of-flight mass spectrometry (HILIC/ESI-HR-QTOFMS). Plausible reaction mechanisms are proposed for previously unknown OSs detected in atmospheric PM_2.5 at mass-to-charge ratios (m/z) of 139 (C₂H₃O₅S^–), 155 (C₂H₃O₆S^–), 169 (C₃H₅O₆S^–), 171 (C₃H₇O₆S^–), 185 (C₃H₅O₇S^–), 199 (C₄H₇O₇S^–), 211 (C₅H₇O₇S^–), 213 (C₅H₉O₇S^–), 227 (C₅H₇O₈S^–), 229 (C₅H₉O₈S^–), and 231 (C₅H₁₁O₈S^–). Heterogeneous ^•OH oxidation of 2-MTSs redistributes the particulate sulfur speciation into more oxygenated/functionalized OSs, likely modifying the aerosol physicochemical properties of SOA containing 2-MTSs.