Air-water gas exchange

The exchange of inert and sparingly soluble gases - including carbon dioxide, methane, and oxygen - between the atmosphere and oceans is controlled by a thin 20- to 200-um-thick boundary layer at the top of the ocean. The hydrodynamics in this layer are significantly different from boundary layers at rigid walls, since the orbital motion of the waves is of the same order as the velocities in the viscous boundary layer. Laboratory and field measurements show that wind waves significantly increase the gas transfer rate and that it is significantly influenced in this way by surfactants. Because of limited experimental techniques, the mechanisms for this enhancement and the structure of the turbulence in the boundary layer at a wavy water surface are still not known. A number of new imaging techniques are described that give direct insight into the processes and promise to trigger substantial theoretical progress in the near future.