Airborne measurements of dimethylsulfide, sulfur dioxide, and aerosol ions over the Southern Ocean South of Australia
Date of Original Version
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 those of MSA, except when air was advected partially from continental areas (Africa, Australia). In contrast to SO2, nss-SO42- values decreased significantly in the middle FT. NH4+/nss-SO42- mole ratios indicate that most non-sea-salt sulfate particles in the ML were neutralized by ammonium. © 1990 Kluwer Academic Publishers.
Publication Title, e.g., Journal
Journal of Atmospheric Chemistry
Berresheim, H., M. O. Andreae, G. P. Ayers, R. W. Gillett, J. T. Merrill, V. J. Davis, and W. L. Chameides. "Airborne measurements of dimethylsulfide, sulfur dioxide, and aerosol ions over the Southern Ocean South of Australia." Journal of Atmospheric Chemistry 10, 3 (1990). doi: 10.1007/BF00053868.