Airborne measurements of dimethylsulfide, sulfur dioxide, and aerosol ions over the Southern Ocean South of Australia

• Published in: Journal of Atmospheric Chemistry
• Main Authors: Berresheim, H., Andreae, M. O., Ayers, G. P., Gillett, R. W., Merrill, J. T., Davis, V. J., Chameides, W. L.
• Format: Text
• Language: unknown
• Published: DigitalCommons@URI 1990
• Subjects: dimethylsulfide; marine aerosol; Marine atmosphere; methanesulfonate; non-sea-salt sulfate; Southern Ocean; sulfur dioxide; vertical distributions; Austral
• Online Access: https://digitalcommons.uri.edu/gsofacpubs/1951; https://doi.org/10.1007/BF00053868

Vertical distributions of dimethylsulfide (DMS), sulfur dioxide (SO2), aerosol methane-sulfonate (MSA), non-sea-salt sulfate (nss-SO42-), and other aerosol ions were measured in maritime air west of Tasmania (Australia) during December 1986. A few cloudwater and rainwater samples were also collected and analyzed for major anions and cations. DMS concentrations in the mixed layer (ML) were typically between 15-60 ppt (parts per trillion, 10-12; 24 ppt=1 nmol m-3 (20°C, 1013 hPa)) and decreased in the free troposphere (FT) to about <1-2.4 ppt at 3 km. One profile study showed elevated DMS concentrations at cloud level consistent with turbulent transport ('cloud pumping') of air below convective cloud cells. In another case, a diel variation of DMS was observed in the ML. Our data suggest that meteorological rather than photochemical processes were responsible for this behavior. Based on model calculations we estimate a DMS lifetime in the ML of 0.9 days and a DMS sea-to-air flux of 2-3 μmol m-2 d-1. These estimates pertain to early austral summer conditions and southern mid-ocean latitudes. Typical MSA concentrations were 11 ppt in the ML and 4.7-6.8 ppt in the FT. Sulfur-dioxide values were almost constant in the ML and the lower FT within a range of 4-22 ppt between individual flight days. A strong increase of the SO2 concentration in the middle FT (5.3 km) was observed. We estimate the residence time of SO2 in the ML to be about 1 day. Aqueous-phase oxidation in clouds is probably the major removal process for SO2. The corresponding removal rate is estimated to be a factor of 3 larger than the rate of homogeneous oxidation of SO2 by OH. Model calculations suggest that roughly two-thirds of DMS in the ML are converted to SO2 and one-third to MSA. On the other hand, MSA/nss-SO42- mole ratios were significantly higher compared to values previously reported for other ocean areas suggesting a relatively higher production of MSA from DMS oxidation over the Southern Ocean. Nss-SO42- profiles were mostly parallel to ...