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Abstract. Acid-catalyzed multiphase chemistry of 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 of fine aerosols inisoprene-rich regions. However, chemical stability of these organosulfatesremains unclear. As a result, we investigate the heterogeneous oxidation ofaerosols consisting of potassium 3-methyltetrol sulfate ester(C5H11SO7K) by gas-phase hydroxyl (OH) radicals at a relativehumidity (RH) of 70.8 %. Real-time molecular composition of 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(C5H11SO7-) 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 cm3 molecule−1 s−1 with a chemical lifetime against OHoxidation of 16.2±0.3 days, assuming an OH radical concentration of1.5×106 molecules cm−3. Comparison of 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 of bisulfate ion (HSO4-) afteroxidation, suggesting the importance of fragmentation processes. Overall,potassium 3-methyltetrol sulfate ester likely decomposes to form volatilefragmentation products and aqueous-phase sulfate radial anion(SO4⚫-). SO4⚫- subsequently undergoesintermolecular hydrogen abstraction to form HSO4-. These processesappear to explain the compositional evolution of 3-methyltetrol sulfate esterduring heterogeneous OH oxidation.