Formation of Organosulfur Compounds from Aqueous Phase Reactions of S(IV) with Methacrolein and Methyl Vinyl Ketone in the Presence of Transition Metal Ions

The catalytic oxidation of S(IV) species by transition metal ions (TMIs) is a significant pathway for aqueous sulfate formation. Sulfoxy radicals, such as SO3-, SO4-, and SO5- radicals, are major intermediates in this catalytic reaction. These radicals can also react with organic compounds to produce organosulfur compounds, which are important tracers of secondary organic aerosol (SOA) formation. However, the mechanism for the formation of organosulfur compounds via this pathway is not well understood. Here, we investigate the formation of organosulfur compounds from the aqueous reaction of isoprene oxidation products, in this case, methacrolein (MACR) and methyl vinyl ketone (MVK), with sulfite/bisulfite in the presence of different TMIs. In particular, we show here how this chemistry is influenced by the presence of Fe3+ and Mn2+ both separately and together. Fe3+ and Mn2+ have distinctly different effects on the mechanism for organosulfur formation. Both C-2-C-4 organosulfur compounds and their oligomers are formed in the presence of Fe-3(+), whereas only C-2-C-4 organosulfur compounds are observed in the presence of Mn2+. Fe3+ dominates the formation of organosulfur compounds when both Fe3+ and Mn2+ are present, as indicated by the significant increase of oligomers observed even at low ratios of Fe3+ to Mn2+ (1:25). Furthermore, the quantification of organosulfur compounds shows that organosulfur compound formation through TMI-catalyzed reactions makes a significant contribution to the fate of S(IV) in the aqueous phase. The importance of these reactions in the atmosphere is discussed.