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

Kunpeng Chen,Jun Zhao

doi:10.1038/s41598-020-61968-2

Abstract

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.

Highlights

Organic sulfate plays important roles in modulating properties of atmospheric aerosols
We proposed that possible pathways (R8–R11) exist for reactions involving sulfurous acid (H2SO3) with formaldehyde via similar mechanisms to sulfite ions, leading respectively to the formation of HMSA/HMHSi since H2SO3 is a weak acid and exists in a large quantity under aqueous environments
The properties in particular the pH values of atmospheric aqueous aerosols were significantly affected in the presence of organic sulfur-containing compounds

Summary

Introduction

Organic sulfate plays important roles in modulating properties of atmospheric aerosols. The HMS formation was thought to be the most important pathway for atmospheric organic sulfate through a five-member-ring transition state (Fig. S1b) by attacking the sulfur atom in bisulfite ions on the carbon atom in formaldehyde (R4). We propose an alternative pathway for the reaction between bisulfite ions and formaldehyde under aqueous environments, that is, the carbon atom in formaldehyde can be attacked by an oxygen atom instead of the sulfur atom in bisulfite ions (R5) This reaction proceeds via a proposed six-member-ring transition state (Fig. S1d), leading to formation of an HMS isomer, hydroxymethyl sulfite (HMSi, CH2(OH)OSO2−, Fig. S1c). We performed quantum-chemical modeling to kinetically and thermodynamically evaluate the formation of HMS/HMSA and HMSi/HMHSi. As water can transform protons and lead to hydrogen-shift in aqueous environment, the reactions R4/R5 become R6/R7 respectively by including water into the reaction systems The proton-dissociated equilibrium constants for the conjugate acids HMSA/HMHSi were calculated to evaluate their effects on pH values in atmospheric aqueous aerosols through our “in-house” kinetic model

Methods

Results

Conclusion