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Chemical Transformation of Methanesulfonic Acid and Sodium Methanesulfonate through Heterogeneous OH Oxidation

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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 Chan
Methanesulfonic 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.