Theoretical investigation of a potentially important formation pathway of organosulfate in atmospheric aqueous aerosols

Organic sulfate plays important roles in modulating properties of atmospheric aerosols. Recent studies showed that organic sulfate was likably interpreted as inorganic sulfate in field measurements using advanced instruments such as Aerosol Mass Spectrometer and the major contributor to organic sulfate was thought to be hydroxymethanesulfonate (HMS). This study proposed that besides HMS, its isomer hydroxymethyl sulfite (HMSi), which has not been identified in atmospheric aerosols, can emerge as the product of aqueous reactions between sulfur dioxide and formaldehyde. Results from quantum chemical modeling showed that formation of HMS and HMSi was several orders of magnitude faster than that of their corresponding conjugate acids, HMSA and HMHSi. In addition, water involvement can largely accelerate respectively the formation rate of HMS/HMSA and HMSi, but decelerate that of HMHSi, demonstrating the non-negligible role of water in the formation process. Furthermore, our kinetic model implemented with the calculated parameters indicates that HMSi/HMHSi but not HMS/HMSA can significantly alter the pH values of atmospheric aqueous aerosols and HMHSi is the most abundant species among HMS/HMSA and HMSi/HMHSi. Therefore, the newly-discovered pathway via HMSi/HMHSi formation should be of great concern and its kinetic parameters should be implemented in future models of atmospheric chemistry.