Inorganic Sulfur Species Formed upon Heterogeneous OH Oxidation of Organosulfates: A Case Study of Methyl Sulfate

Recent studies reveal that organosulfates at the particle surface can be oxidized by gas-phase OH radicals with significant rates. Inorganic sulfur species, such as the bisulfate ion (HSO4-) and sulfate ion (SO42-), can be formed upon these heterogeneous oxidation processes through the formation and subsequent reactions of sulfate radical anions (SO4·-) in the particle phase. However, the amount of inorganic sulfur species produced in these heterogeneous oxidation reactions is not known. We investigate the heterogeneous OH oxidation of sodium methyl sulfate (CH3SO4Na), the smallest organosulfate detected in atmospheric particles, using an oxidation flow reactor at a relative humidity of 75%. We quantify the kinetics by measuring the decay of CH3SO4Na and the amount of HSO4- and SO42- formed upon oxidation using ion chromatography. Kinetic measurements determine the heterogeneous OH reaction rate to be (5.72 ± 0.14) × 10-13 cm3 molecule-1 s-1, with an effective OH uptake coefficient, γeff, of 0.31 ± 0.06. The molar yield of inorganic sulfur species, defined as the total number of moles of HSO4- and SO42- formed per mole of CH3SO4Na consumed upon oxidation, is found to be significant and has an average value of 0.62 ± 0.18 upon oxidation. A kinetic model is developed to describe the kinetics and inorganic sulfur species formation upon oxidation. Model simulations suggest that CH3SO4Na tends to decompose rapidly into formaldehyde and SO4·-, and the reaction of SO4·- with CH3SO4Na plays a significant role in both governing the kinetics and the formation of inorganic sulfur species