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Methanesulphonic acid (MSA) and sulphate are the end products of two different oxidation pathways of atmospheric dimethylsulphide (DMS).Sulphate aerosols can act as cloud condensation nuclei that affect incoming solar radiation and thus climate.Stable isotope techniques can be used to define continental and biogenic emissions,determine the origin of sulphate in aerosol samples and provide insight into DMS oxidation.In 2003 Canadian SOLAS conducted three cruises to determine seasonal variations over North Atlantic Ocean.Size segregated aerosol filters,sulphur dioxide and atmospheric DMS were collected and analyzed.Isotope values along with sulphate,MSA and cation concentrations were analysed.Biological sulphate concentrations ranged up to 2.0 μg/m 3 in the summer with some MSA values,typically orders of magnitude less than sulphate,reached above 70 ng/m 3 in the more northern oceans.In the spring these values were 1.7μg/m 3 and 50ng/m 3 respectively.This is the first time subsequent seasonal measurements have been done for these measurements.A comparison of the results for the three seasons will be looked at along with the seasonal influences of the DMS oxidation pathways.