Determining the key sources of uncertainty in dimethyl sulfide and methanethiol oxidation under tropical, temperate, and polar marine conditions

Abstract. This study quantifies how uncertainties in the gas-phase rate constants used in the oxidation mechanisms of dimethyl sulfide (DMS) and methanethiol (CH₃SH) (both major natural sources of sulfur to the atmosphere), affect products such as methanesulfonic acid and sulfuric acid, which influence cloud formation and climate.

We updated our previously reported DMS oxidation mechanism and extended it to include 9 halogen, 71 aqueous, and 4 CH₃SH reactions. This updated mechanism was then run in box models covering temperate, tropical, and polar marine conditions based on field campaigns.

Constrained Monte Carlo sampling was employed to propagate the uncertainties in the mechanism. Uncertainties in the concentrations of the products were time-dependent and ranged from 10–200 % for most species, with OCS, methanesulfonic acid, and sulfuric acid having the largest uncertainties.

Sensitivity analysis using the EASI RBD-FAST algorithm was performed to identify which reactions and processes were the largest sources of uncertainty for the modelled oxidation products. Individually, reactions involving the formation and loss of CH₃SO₂O₂ were major contributors to the uncertainties in gas-phase methanesulfonic acid and sulfuric acid. Reactions of species with OH and rate constants based on structure-activity relationships were commonly found to significantly contribute to uncertainty in most of the DMS oxidation products studied. Large uncertainties associated with OCS were attributed to the photolysis of hydroperoxymethyl thioformate, which has not yet been studied experimentally or theoretically. We suggest that future work on DMS oxidation should prioritise these processes to reduce the uncertainty in the climate impact of marine sulfur species.