Conceptual view of boundary layer concentration profiles. Resistance to gas transfer is concentrated in the diffusive sublayers. On the left (blue line) is the activity profile for an insoluble gas with the resistance in the aqueous-side diffusive sublayer. On the right (red line) is the profile for a soluble gas with resistance in the air-side diffusive sublayer. Soluble gases have an Ostwald solubility > 100. While an enriched layer of OM is suggested (in red) below the interface, it is unclear what effect this would have on the distribution of gas concentration in the SML, so the contours are left unchanged. C indicates gas concentration; H, Henry coefficient of solubility; subscript a, in air and subscript w, in seawater. Redrawn and reconceptualised from Wanninkhof et al. (2009) and from Jenkinson et al. (2018).