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