Atmospheric reaction of methanethiol with hydroxyl radicals and chlorine atoms: implications for the atmospheric sulfur cycle and HONO formation

Abstract

Despite the importance of the oxidation of reduced sulfur compounds in the biogeochemical sulfur cycle and climate, the mechanism of the transformation of methanethiol (CH₃SH), the second highest gas-phase marine sulfur emission besides dimethyl sulfide, remains weakly explored. This work presents an investigation of the reaction of CH₃SH with hydroxyl radicals (˙OH) and chlorine atoms (Cl˙) using the DLPNO-CCSD(T)/aug-cc-pVTZ//M06-2X/aug-cc-pV(T+d)Z method, both in the absence and presence of water. The results indicate that the hydrogen abstraction by ˙OH/Cl˙ from the –SH group is more favored than from the –CH₃ group, leading to the formation of CH₃S˙ and ˙CH₂SH radicals, respectively, and HCl. While HCl is known to participate in tropospheric physicochemical processes, the fates of CH₃S˙ and ˙CH₂SH radicals were further examined. Their reactions with O₂ lead to and H₂CS formation while reactions with NO₂ indicate fast formation of HONO and H₂CS. Moreover, the CH₃S˙ reaction with O₂ also forms though at a relatively lower rate than The overall rate coefficients for CH₃SH + ˙OH and CH₃SH + Cl˙ reactions at 298 K were determined to be 1.69 × 10⁻¹¹ and 7.49 × 10⁻¹⁰ cm³ per molecule per s, respectively, and were found to exhibit a negative temperature dependency in the temperature range 260–360 K. Besides its contribution to atmospheric sulfate, this study highlights the potential influence of CH₃SH on the atmospheric oxidative capacity through HONO and formation, and the necessity to fully assess its chemistry and the contribution thereof to atmospheric chemistry.