Shipboard measurements of dimethyl sulfide and SO2 southwest of Tasmania during the First Aerosol Characterization Experiment (ACE 1)

Measurements of seawater dimethylsulfide (DMS), atmospheric dimethylsulfide, and sulfur dioxide (SO2) were made on board the R/V Discoverer in the Southern Ocean, southeast of Australia, as part of the First Aerosol Characterization Experiment (ACE 1). The measurements covered a latitude range of 40 degrees S-55 degrees S during November-December 1995. Seawater DMS concentrations ranged from 0.4 to 6.8 nM, with a mean of 1.7 +/- 1.1 nM (1 sigma). The highest DMS concentrations were found in subtropical convergence zone waters north of 44 degrees S, and the lowest were found in polar waters south of 49 degrees S. In general, seawater DMS concentrations increased during the course of the study, presumably due to the onset of austral spring warming. Atmospheric DMS concentrations ranged from 24 to 350 parts per trillion by volume (pptv), with a mean of 112 +/- 61 pptv(1 sigma). Atmospheric SO2 was predominantly of marine origin with occasional anthropogenic input,;as evidenced by correlation with elevated Rn-222 and air mass trajectories. Concentrations ranged from 3 to 1000 pptv with a mean of 48.8 +/- 149 pptv (1 sigma) and a median 15.8 pptv. The mean SO2 concentration observed in undisturbed marine air was 11.9 +/- 7.6 pptv (1 sigma), and the mean DMS to SO2 ratio in these conditions was 13 +/- 9 (1 sigma). Diurnal variations in SO2 were observed, with a daytime maximum and early morning minimum in agreement with model simulations of DMS oxidation in the marine boundary layer. Steady state calculations and photochemical box model simulations suggest that the DMS to SO2 conversion efficiency in this region is 30-50%. Comparison of these results with results from warmer regions suggests that the DMS to SO2 conversion efficiency has a positive temperature dependence.