Surfactant effects on air-sea gas exchange under turbulent conditions

Abstract

In a series of laboratory gas exchange studies we found that under turbulent conditions additions of two synthetic surfactants (polyethylene oxide and oleyl alcohol) to distilled water and seawater led to reductions in oxygen evasion at the air-liquid interface. The oxygen exchange coefficient relative to that of a distilled water control asymptotically reached a lower limit of ≈50% as surfactant concentration was increased. For natural seawater samples, an asymptotic reduction in relative gas exchange rate was demonstrated for increasing amounts of surface-active material as determined from surface pressure-area isotherms. Possibly surfactants act to reduce gas exchange by creating surface pressure forces that oppose and reduce turbulent eddy velocities and, correspondingly, reduce surface renewal. However, the greatest reductions in the oxygen exchange coefficient occurred at initial surface pressure ( π i) less than 0.5 mN m −1. This result may have been due to the presence of soluble surfactants, which are known to be very effective in reducing gas exchange but which do not display concentration-dependent surface pressures. Based on results from an opportunistic sampling survey of marine waters and a cruise to the Sargasso Sea, a gradient in gas exchange reductions from 5–15% in oceanic waters to 50% in nearshore waters was found. However, reductions of 50% occured for surface film material obtained from the Sargasso Sea. Our results provide some measure of the potential for reduction of gas exchange at high turbulence in marine waters in the presence of natural surfactants.