An assessment of the significance of sulphate sources over the Atlantic Ocean based on sulphur isotope data

The results of fine mode (<1 μm) aerosol sulphate, MSA and major ion composition and isotopic analyses are presented for aerosol samples collected over the Atlantic Ocean. The results are used to apportion the measured sulphate into sea-salt sulphate, anthropogenic sulphate and marine biogenic (dimethyl sulphide DMS) sulphate sources. The contribution of volcanic sulphur is argued to be very small within this data set. The results confirm other studies showing that DMS makes a major contribution to the sulphate burden over the remote ocean. DMS can be oxidised to form SO2 or methane sulphonic acid (MSA). Using our estimates of marine biogenic sulphate and measurements of MSA concentrations we confirm that the ratio of MSA/sulphate from DMS oxidation increases systematically from low to high latitudes, consistent with the laboratory measurements, inferences from field studies and modelling predictions of others.