Assessment of dimethylsulfide sea‐air exchange rate

Abstract

The concentrations of dimethylsulfide (DMS) in surface seawater and in the atmosphere at three heights above sea level (1, 6, and 20 m) were measured in June 1992 in the Atlantic Ocean south of the Azores Islands (34–37°N, 23–26°W) at stations of the “Surface of Oceans: Fluxes and Interactions With the Atmosphere” cruise (SOFIA). Simultaneous meteorological and micrometeorologicai measurements were also made. Sea surface temperature and wind speed were used to estimate sea‐air exchange coefficients, according to two parameterizations of the piston velocity Kw reported by Liss and Merlivat [1986] and by Smethie et al. [1985]. The Liss and Merlivat's [1986] parameterization leads to values of Kw smaller by 40% on average than Smethie et al.'s [1985] parameterization, the difference depending on wind speed. Kw values were used to calculate the DMS flux from an air‐sea exchange model, with DMS concentrations measured in and above the seawater. Friction velocity, air temperature and heat flux were used to compute the eddy diffusion coefficient Kz as a function of altitude. In the cases where the DMS concentration decreased between 1 and 6 m, DMS flux in the 1–6 m layer was calculated from Kz and DMS atmospheric gradients according to the gradient‐transfer approach. DMS flux values obtained from the sea‐air exchange model and from the gradient‐transfer approach appear to be significantly correlated (R = 0.7, n = 15). The slopes of regressions suggest that the Liss and Merlivat's [1986] and Smethie et al.'s [1985] parameterizations of Kw both lead to DMS sea‐air fluxes lower than that calculated by the gradient‐transfer approach by a factor of 1.6 to 1.9 ± 0.5 and 1.1 to 1.4 ± 0.4, respectively.