ATMOSPHERIC SULFUR CYCLING IN THE TROPICAL PACIFIC MARINE BOUNDARY-LAYER (12-DEGREES-S,135-DEGREES-W) - A COMPARISON OF FIELD DATA AND MODEL RESULTS .1. DIMETHYLSULFIDE

Shipboard measurements of atmospheric and seawater DMS were made at 12 degrees S, 135 degrees W for 6 days during March 1992. The mean seawa ter DMS concentration during this period was 4.1 +/- 0.45 nM (1 sigma, n = 260) and the mean atmospheric DMS mole fraction was 453 +/- 93 pm ol mol(-1) (1 sigma, n = 843). Consistent atmospheric diel cycles were observed, with a nighttime maximum and daytime minimum and an amplitu de of approximately 85 pmol mol(-1). Photochemical box model calculati ons were made to test the sensitivity of atmospheric DMS concentration s to the following parameters: 1) sea-to-air flux, 2) boundary layer h eight, 3) oxidation rate, and 4) vertical entrainment velocities. The observed relationship between the mean oceanic and atmospheric DMS lev els require the use of an air-sea exchange coefficient which is at the upper limit end of the range of commonly used parameterizations. The amplitude of the diel cycle in atmospheric DMS is significantly larger than that predicted by a photochemical model. This suggests that the sea-to-air DMS flux is higher than was previously thought, and the rat e of daytime oxidation of DMS is substantially underestimated by curre nt photochemical models of DMS oxidation.