Air-water CO2 exchange can be chemically enhanced as a result of hydration reactions of the CO2 system. As I understand the original literature (e.g. Wanninkhof and Knox 1996), the air-water gas flux is enhanced for the case of CO2 influx from air to water, because under this condition incoming CO2 reacts to form bicarbonate and by that increases the near-surface CO2 concentration gradient. According to Kuss and Schneider (2004) the process is equivalent for both in- and eflux conditions. I have seen several studies published where the flux was corrected for chemical enhancement for the case of CO2 eflux from water to air. Why and how would the air-water CO2 flux be enhanced under eflux conditions?
The reason that chemical enhancement works in both directions is because all the chemical species in the carbonate system are in equilibrium; the reactions don’t happen in just one direction.
As you wrote, when CO2 is higher in the air than water, there will be CO2 diffusion into the water. This "new" CO2 perturbs the carbonate system, with some of the newly-added CO2 getting converted to bicarbonate in the boundary layer as the carbonate system moves toward a new equilibrium. This results in a steeper concentration gradient between air and water (vs. a hypothetical situation where only molecular diffusion of CO2 was happening) and can enhance the CO2 flux into the water.
When the concentration gradient is in the other direction (CO2 higher in water than air), CO2 will diffuse out of the water and into the air. As CO2 leaves the water, the carbonate system is again perturbed and the system will move toward a new equilibrium by converting some of the bicarbonate in the boundary layer to CO2. This again steepens the concentration gradient between water and air and can enhance the flux of CO2 out of the water. It's the same process you described in your question but in the other direction; CO2 is being produced from bicarbonate instead of being converted to bicarbonate.
My understanding is that chemical enhancement of CO2 fluxes is only significant in low-turbulence environments and in high pH waters (and is negligible at pH values typical of many freshwaters) so it may or may not be relevant in your system.
Thank you Scott C. Neubauer for your very clear answer! Your explanation makes sense to me, and I agree that chemical enhancement is only significant in low-turbulence / high pH environments.
Yes - Scott outlined this perfectly. More field validation is required to investigate whether the emperical models developed in the lab are tranferable to the real world though.