The S isotope composition of methanesulfonic acid in Arctic aerosols

Methanesulfonic acid (MSA) in aerosols over the ocean is the main oxidation product of dimethyl sulfide (DMS). In addition, a small part of MSA in the atmosphere is derived from anthropogenic sources such as biomass burning which produces dimethyl disulfide (DMDS) that is oxidized to MSA. MSA and sulfate concentrations and the S isotope composition of MSA and sulfate for aerosol samples collected at Alert, Nunavut in the high Arctic from 1994 to 2002 are reported here. The S isotope composition of MSA and sulfate varies with season. Spring and summer samples have delta 34S values ranging from +12.5 to +16.6 per mill for MSA and from +9.2 to +12.7 per mill for sulfate. Samples from fall and winter have delta 34S that range from +8.2 to 10.5 per mill for MSA and from +4.2 to +7.9 per mill for sulfate. Delta 34S values for MSA are always higher (about 2 to 3 per mill) than those for the corresponding aerosol sulfate. The delta 34S value of MSA should be the same as that for DMS (+15.8 to 18.7 per mill), but measured delta 34S values for MSA were lower (+8.2 to 16.6 per mill). Differences in delta 34S values between MSA and sulfate may due be to sulfur (S) origin for each compound and/or isotope fractionation. The source of this discrepancy and its implications are examined.