Organic Peroxides and Sulfur Dioxide in Aerosol: Source of Particulate Sulfate

Sulfur oxides (SO) are important atmospheric trace species in both gas and particulate phases, and sulfate is a major component of atmospheric aerosol. One potentially important source of particulate sulfate formation is the oxidation of dissolved SO₂ by organic peroxides, which comprises a major fraction of secondary organic aerosol (SOA). In this study, we investigated the reaction kinetics and mechanisms between SO₂ and condensed-phase peroxides. pH-dependent aqueous phase reaction rate constants between S(IV) and organic peroxide standards were measured. Highly oxygenated organic peroxides with O/C > 0.6 in α-pinene SOA react rapidly with S(IV) species in the aqueous phase. The reactions between organic peroxides and S(IV) yield both inorganic sulfate and organosulfates (OS), as observed by electrospray ionization ion mobility mass spectrometry. For the first time, ³⁴S-labeling experiments in this study revealed that dissolved SO₂ forms OS via direct reactions without forming inorganic sulfate as a reactive intermediate. Kinetics of OS formation was estimated semiquantitatively, and such reaction was found to account for 30-60% of sulfur reacted. The photochemical box model GAMMA was applied to assess the implications of the measured SO₂ consumption and OS formation rates. Our findings indicate that this novel pathway of SO₂-peroxide reaction is important for sulfate formation in submicron aerosol.