Heterogeneous OH oxidation of isoprene-epoxydiol-derived organosulfates: kinetics, chemistry and formation of inorganic sulfate

Abstract

Acid-catalyzed multiphase chemistry of epoxydiols formed from isoprene oxidation yields the most abundant organosulfates (i.e., methyltetrol sulfates) detected in atmospheric fine aerosols in the boundary layer. This potentially determines the physicochemical properties of fine aerosols in isoprene-rich regions. However, chemical stability of these organosulfates remains unclear. As a result, we investigate the heterogeneous oxidation of aerosols consisting of potassium 3-methyltetrol sulfate ester (C5H11SO7K) by gas-phase hydroxyl (OH) radicals at a relative humidity (RH) of 70.8 %. Real-time molecular composition of the aerosols is obtained by using a Direct Analysis in Real Time (DART) ionization source coupled to a high-resolution mass spectrometer. Aerosol mass spectra reveal that 3-methyltetrol sulfate ester can be detected as its anionic form (C5H11SO7-) via direct ionization in the negative ionization mode. Kinetic measurements reveal that the effective heterogeneous OH rate constant is measured to be 4.74±0.2×10-13 cm3 molecule−1 s−1 with a chemical lifetime against OH oxidation of 16.2±0.3 days, assuming an OH radical concentration of 1.5×106 molecules cm−3. Comparison of this lifetime with those against other aerosol removal processes, such as dry and wet deposition, suggests that 3-methyltetrol sulfate ester is likely to be chemically stable over atmospheric timescales. Aerosol mass spectra only show an increase in the intensity of bisulfate ion (HSO4-) after oxidation, suggesting the importance of fragmentation processes. Overall, potassium 3-methyltetrol sulfate ester likely decomposes to form volatile fragmentation products and aqueous-phase sulfate radial anion (SO4⚫-). SO4⚫- subsequently undergoes intermolecular hydrogen abstraction to form HSO4-. These processes appear to explain the compositional evolution of 3-methyltetrol sulfate ester during heterogeneous OH oxidation.