DIMETHYLSULFIDE MEASURED IN THE WESTERN PACIFIC AND THE SOUTHERN INDIAN-OCEAN

Dimethylsulfide was measured in the atmosphere and the ocean water from 28 December 1990 to 13 March 1991 in the western Pacific Ocean. Mean DMS concentration in the atmosphere was low west of 180-degrees longitude on the equator (11 pptv) as well as the temperate North Pacific Ocean (9 pptv). Atmospheric DMS concentration along 160-degrees-W was nearly uniform at about 42 pptv from 15-degrees-N to the equator. Sharp increases of atmospheric DMS concentration appeared south of the equator along 160-degrees-W and 180-degrees, and the atmospheric DMS concentration reached approximately 95 pptv around (5-degrees-S, 160-degrees-W). Concentrations of DMS in the surface seawater were also high south of the equator, and the maximum value was 1.85 nmol L-1 at (5-degrees-S, 180-degrees). Simultaneous measurement of atmospheric and oceanic DMS was made at an interval of 6 hours on the track from (6-degrees-S, 179-degrees-W) to (11-degrees-S, 175.5-degrees-W). While the concentration of DMS in the surface seawater was quite uniform at 1.26 nmol L-1, the atmospheric DMS concentration varied diurnally with a mean value of 76 pptv. Observed DMS concentrations in the atmosphere and the ocean, together with model simulations, demonstrate that the main processes controlling the concentration and diurnal variation of atmospheric DMS are the magnitude of DMS flux from the ocean to the atmosphere and the oxidation reaction with the hydroxyl radical (OH) in the daytime. The reaction with the nitrate radical (NO3) at night is also important, because it decreases the amplitude of the DMS diurnal variation. In addition, wind speed seems to be an important factor regulating the DMS flux. Another measurement of atmospheric DMS was conducted during 3 12 December 1990 in the southern Indian Ocean. North of 50-degrees-S, the atmospheric DMS concentration was approximately 60 pptv, but south of 50'S it drastically increased up to 460 pptv poleward. High DMS concentration in the ocean and low OH concentration due to stormy weather are considered as possible causes of the high atmospheric DMS concentration.