Atmospheric Gas-Phase Formation of Methanesulfonic Acid
Jing Chen, Joseph R. Lane, Kelvin H. Bates, Henrik G. Kjaergaard
Abstract
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.