Airborne measurements of dimethylsulfide, sulfur dioxide, and aerosol ions over the southern ocean south of australia

Vertical distributions of dimethylsulfide (DMS), sulfur dioxide (SO2), aerosol methanesulfonate (MSA), non-sea-salt sulfate (nss-SO2-4), 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-SO2-4 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-SO2-4 profiles were mostly parallel to those of MSA, except when air was advected partially from continental areas (Africa, Australia). In contrast to SO2, nss-SO2-4 values decreased significantly in the middle FT. NH+4/nss-SO2-4 mole ratios indicate that most non-sea-salt sulfate particles in the ML were neutralized by ammonium.

Duke Scholars

Author William Chameides Earth and Climate Sciences