Estimation of bubble-mediated air–sea gas exchange from concurrent DMS and CO<sub>2</sub> transfer velocities at intermediate–high wind speeds

Abstract

Simultaneous air–sea fluxes and concentration differences of dimethylsulfide (DMS) and carbon dioxide (CO₂) were measured during a summertime North Atlantic cruise in 2011. This data set reveals significant differences between the gas transfer velocities of these two gases (Δ<i>k</i>_w) over a range of wind speeds up to 21 m s⁻¹. These differences occur at and above the approximate wind speed threshold when waves begin breaking. Whitecap fraction (a proxy for bubbles) was also measured and has a positive relationship with Δ<i>k</i>_w, consistent with enhanced bubble-mediated transfer of the less soluble CO₂ relative to that of the more soluble DMS. However, the correlation of Δ<i>k</i>_w with whitecap fraction is no stronger than with wind speed. Models used to estimate bubble-mediated transfer from in situ whitecap fraction underpredict the observations, particularly at intermediate wind speeds. Examining the differences between gas transfer velocities of gases with different solubilities is a useful way to detect the impact of bubble-mediated exchange. More simultaneous gas transfer measurements of different solubility gases across a wide range of oceanic conditions are needed to understand the factors controlling the magnitude and scaling of bubble-mediated gas exchange.