Organosulfates are important organosulfur compounds present in atmospheric
particles. While the abundance, composition, and formation mechanisms of
organosulfates have been extensively investigated, it remains unclear how
they transform and evolve throughout their atmospheric lifetime. To acquire a
fundamental understanding of how organosulfates chemically transform in the
atmosphere, this work investigates the heterogeneous OH radical-initiated
oxidation of sodium methyl sulfate (CH3SO4Na) droplets, the
smallest organosulfate detected in atmospheric particles, using an aerosol
flow tube reactor at a high relative humidity (RH) of 85 %. Aerosol mass
spectra measured by a soft atmospheric pressure ionization source (direct
analysis in real time, DART) coupled with a high-resolution mass spectrometer
showed that neither functionalization nor fragmentation products are
detected. Instead, the ion signal intensity of the bisulfate ion
(HSO4−) has been found to increase significantly after OH oxidation.
We postulate that sodium methyl sulfate tends to fragment into a formaldehyde
(CH2O) and a sulfate radical anion (SO4 ⋅ −) upon OH
oxidation. The formaldehyde is likely partitioned back to the gas phase due
to its high volatility. The sulfate radical anion, similar to OH radical, can
abstract a hydrogen atom from neighboring sodium methyl sulfate to form the
bisulfate ion, contributing to the secondary chemistry. Kinetic measurements
show that the heterogeneous OH reaction rate constant, k, is (3.79 ± 0.19) × 10−13 cm3 molecule−1 s−1 with an
effective OH uptake coefficient, γeff, of 0.17 ± 0.03. While
about 40 % of sodium methyl sulfate is being oxidized at the maximum OH
exposure (1.27 × 1012 molecule cm−3 s), only a 3 %
decrease in particle diameter is observed. This can be attributed to a small
fraction of particle mass lost via the formation and volatilization of
formaldehyde. Overall, we firstly demonstrate that the heterogeneous OH
oxidation of an organosulfate can lead to the formation of sulfate radical
anion and produce inorganic sulfate. Fragmentation processes and sulfate
radical anion chemistry play a key role in determining the compositional
evolution of sodium methyl sulfate during heterogeneous OH oxidation
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