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

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.