Atmospheric Gas-Phase Formation of Methanesulfonic Acid.

Despite its impact on the climate, the mechanism of methanesulfonic acid (MSA) formation in the oxidation of dimethyl sulfide (DMS) remains unclear. The DMS + OH reaction is known to form methanesulfinic acid (MSIA), methane sulfenic acid (MSEA), the methylthio radical (CH 3 S), and hydroperoxymethyl thioformate (HPMTF). Among them, HPMTF reacts further to form SO 2 and OCS, while the other three form the CH 3 SO 2 radical. Based on theoretical calculations, we find that the CH 3 SO 2 radical can add O 2 to form CH 3 S(O) 2 OO, which can react further to form MSA. The branching ratio is highly temperature sensitive, and the MSA yield increases with decreasing temperature. In warmer regions, SO 2 is the dominant product of DMS oxidation, while in colder regions, large amounts of MSA can form. Global modeling indicates that the proposed temperature-sensitive MSA formation mechanism leads to a substantial increase in the simulated global atmospheric MSA formation and burden.