Determination of the S isotope composition of methanesulfonic acid.

Sulfur (S) isotopes have been used to apportion the amount of biogenic and anthropogenic sulfate in remote environments, an important parameter that is used to model the global radiation budget. A key assumption in the apportionment calculations is that there is little isotope selectivity as reduced compounds such as dimethyl sulfide (DMS) are oxidized. This paper describes a method to determine, for the first time, the S isotope composition of methanesulfonic acid (MSA), the product of DMS oxidation. The isotope composition of MSA was measured directly by EA-IRMS and was used as an isotope reference for the method. Synthetic mixtures approximating the conditions expected for aerosol MSA samples were prepared to test this method. First, MSA solutions were measured alone and then in combination with MSA and SO42-. In synthetic mixtures, SO42- was separated from MSA by precipitating it as BaSO4 prior to preparation of MSA for isotope analysis. The delta S-34 value for MSA solutions was -2.6 parts per thousand (SD +/- 0.4 parts per thousand), which is not different from the delta S-34 obtained from MSA filtrate after precipitating SO42- from the mixture (-2.7 +/- 0.3 parts per thousand). However, these values are offset from direct EA-IRMS analysis of MSA used as the isotope reference by -1.1 +/- 0.2 parts per thousand, and this must be accounted for in reporting MSA measurements. The S isotope measurements using this method approach a limiting value above 300 mu g of MSA. This is approximately equal to the MSA found in 20 000 m(3) of air, assuming ambient concentrations of similar to 15 ng m(-3). Three samples of MSA from the Pacific Ocean measured using this technique have an average delta S-34 value of +17.4 +/- 0.7 parts per thousand.