Heterogeneous OH Oxidation of Isoprene Epoxydiol-Derived Organosulfates: Kinetics, Chemistry and Formation of Inorganic Sulfate

Abstract. Acid-catalyzed multiphase chemistry epoxydiols formed from isopreneoxidation yields the most abundant organosulfates (i.e., methyltetrolsulfates) detected in atmospheric fine aerosols in the boundary layer. Thispotentially determines the physicochemical properties fine aerosols inisoprene-rich regions. However, chemical stability these organosulfatesremains unclear. As a result, we investigate the heterogeneous oxidation ofaerosols consisting potassium 3-methyltetrol sulfate ester( C 5 H 11 SO 7 K ) by gas-phase hydroxyl (OH) radicals at a relativehumidity (RH) 70.8 %. Real-time molecular composition the aerosolsis obtained by using a Direct Analysis in Real Time (DART) ionization sourcecoupled to a high-resolution mass spectrometer. Aerosol mass spectra revealthat 3-methyltetrol sulfate ester can be detected as its anionic form( C 5 H 11 SO 7 - ) via direct ionization in the negativeionization mode. Kinetic measurements reveal that the effective heterogeneousOH rate constant is measured to be 4.74 ± 0.2 × 10 - 13  cm 3  molecule −1  s −1 with a chemical lifetime against OHoxidation 16.2±0.3  days, assuming an OH radical concentration of 1.5×10 6  molecules cm −3 . Comparison this lifetime withthose against other aerosol removal processes, such as dry and wetdeposition, suggests that 3-methyltetrol sulfate ester is likely to bechemically stable over atmospheric timescales. Aerosol mass spectra only showan increase in the intensity bisulfate ion ( HSO 4 - ) afteroxidation, suggesting the importance fragmentation processes. Overall,potassium 3-methyltetrol sulfate ester likely decomposes to form volatilefragmentation products and aqueous-phase sulfate radial anion( SO 4 ⚫ - ). SO 4 ⚫ - subsequently undergoesintermolecular hydrogen abstraction to form HSO 4 - . These processesappear to explain the compositional evolution 3-methyltetrol sulfate esterduring heterogeneous OH oxidation.