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Download fileChemical Transformation of Methanesulfonic Acid and Sodium Methanesulfonate through Heterogeneous OH Oxidation
journal contribution
posted on 2018-07-17, 00:00 authored by Kai Chung Kwong, Man Mei Chim, Erik Hans Hoffmann, Andreas Tilgner, Hartmut Herrmann, James F. Davies, Kevin R. Wilson, Man Nin ChanMethanesulfonic acid (CH3SO3H, MSA) is one
of the major organosulfur acids formed from the photochemical oxidation
of dimethyl sulfide (DMS) produced by oceanic phytoplankton. MSA can
react with metal halides (e.g., sodium chloride) in ambient aerosols
to form methanesulfonate salts (e.g., sodium methanesulfonate, CH3SO3Na). While the formation processes of MSA and
its salts are reasonably well understood, their subsequent chemical
transformations in the atmosphere are not fully resolved. MSA and
its salts accumulate near the aerosol surface due to their surface
activities, which make them available to heterogeneous oxidation at
the gas–aerosol interface by oxidants such as hydroxyl (OH)
radicals. In this work, the compositional changes of aerosol comprised
of MSA and its sodium salt (CH3SO3Na) are measured
following heterogeneous OH oxidation. An aerosol flow tube reactor
is coupled with a soft atmospheric pressure ionization source (Direct
Analysis in Real Time, DART) and a high-resolution mass spectrometer
at a relative humidity (RH) of 90%. DART-aerosol mass spectra reveal
that MSA and CH3SO3Na can be detected as methanesulfonate
ion (CH3SO3–) with minimal
fragmentation in the negative ionization mode. Kinetic measurements
show that OH oxidation with MSA and CH3SO3Na
has an effective OH uptake coefficient of 0.45 ± 0.14 and 0.20
± 0.06, respectively, revealing that MSA reacts with OH radical
faster than its sodium salt. One possibility for the difference in
reactivity of these two compounds is that CH3SO3Na is more hygroscopic than MSA. The increase in the coverage of
water molecules at the surface of CH3SO3Na might
reduce the reactive collision probability between CH3SO3– and OH radicals, resulting in a smaller
reaction rate. MSA and CH3SO3Na dissociate to
form CH3SO3–, which tends
to fragment into formaldehyde (HCHO) and a sulfite radical (SO3•–) upon oxidation. Formaldehyde
partitions back to the gas phase owing to its high volatility, and
SO3•– can initiate a series of
chain reactions involving various inorganic sulfur radicals and ions
in the aerosol phase. Overall, the fragmentation and SO3•–-initiated chemistry are the major processes
controlling the chemical evolution of MSA and its sodium salt aerosols
during heterogeneous OH oxidation.
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