Dissolved Gases

The stagnant film model describes the net gas flux between atmosphere and water as a function of differences in partial pressure and thus does not describe a system at equilibrium. As compared to other models, it takes the thickness z (in μm magnitude) of the boundary layer into account. The boundary layer is a laminar layer separating the turbulent bulk phases of water and atmosphere, thus is located directly at the phase boundary. We assume, that across this layer, gas exchange between atmosphere and liquid phase is only driven via molecular diffusion DG. Therefore, the previous deduction, that the gas exchange velocity Eg is determining the overall net flux of gas in or out of the liquid phase, can now be replaced with the term Eg = Dg/z. The thickness of the boundary layer is primarily wind driven and becomes smaller with higher windspeeds. Increasing winds, besides enhancing surface area, therefore increase the net flux of gases between atmosphere and water as the term Dg/z becomes bigger, as z becomes smaller.

F = (Dg/z) * (Gsw – Gsw-int) Whereas G: concentration of gas in the seawater (sw) and directly at the seawater interface (sw-int.) keep in mind: The differences in concentrations G are the main driver for gas exchange

Gas exchange coefficients are usually determined by tracer experiments using e.g.: 226Ra -> 222Rn. The differences of the equilibrium observed in the mixed layer, as compared to waters below the mixed layer can be attributed to the degassing of Rn (previously Ra, not a gas). This can be extrapolated to other gases and conditions.