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

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.