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 (PM2.5), 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 PM2.5 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.