Sulphur isotopic composition of aerosols over the western North Atlantic Ocean

During spring (1998) and fall (1999) cruises of the Northern Oceans Dimethylsulphide (DMS) Emissions Model programme (NODEM II and III), size-segregated atmospheric aerosol samples were collected above the western North Atlantic Ocean. The soluble portion of the aerosols was analyzed for concentrations of sulphate, sodium, nitrate, and methane sulphonic acid (MSA). Sulphur isotopic compositions were measured on total soluble sulphur. The stable isotope data conform to a mixing model involving contributions from biogenic, anthropogenic, and sea salt sulphur sources. Biogenic sulphur fractions calculated using stable isotopic compositions range from 0.00 to 0.64 of the total sulphate, higher than most previously reported values. %MSA/NSS (non-sea-salt) SO42- ranges from 0 to 94. The data suggest that sulphates of clearly biogenic origin (MSA) are formed in areas where surface-ocean DMS concentrations are high. These sulphates contribute to aerosol mass in particles up to 3 mum in diameter. This seems to occur by condensation onto existing aerosols. Stable isotope data indicate that these existing aerosols can be of either biogenic or anthropogenic origin depending on the size fraction and geographic location. Stable isotope data also reveal other areas where sulphate aerosols of biogenic origin are produced, not associated with MSA.