A simple model of chemistry effects on the Air‐Sea CO₂ exchange coefficient

The transfer of CO₂ between air and water can be quantified by the use of a transfer or exchange coefficient. Here we have studied the effect on the exchange coefficient of CO₂ from inclusion of the carbonate buffer system in water. The methodology has been to solve the flux budget for the chemical reaction equations in the water column, combining the vertical flux in the air and the flux across the interface by the introduction of a so called resistance model for the whole system where the system have been simplified with respect to variation of certain parameters. The thermal effects on the flow and chemistry modelling have in the present study not been included, although this also plays a significant role on the air‐sea exchange. The model shows the effects on the air‐sea exchange coefficient from the carbonate chemistry in the water column are negligible if the measurements are taken at sufficient depth in the water. Hence, it is found that the skin‐layer or interface layer contains the largest resistance and therefore controls the flux.

It is also shown that measurement of the near‐ surface CO₂ gradients in ocean water is very difficult, as the gradients of CO₂ occur within the first meter from the water surface. In reality the equilibrium is reached within the first meter from the water surface. The results justify the use of standard methods to estimate the flux of CO₂ by measuring a concentration at typical larger than 3 meters depth and 10 meters above the surface.