Estimation of gas phase, water phase transfer velocity from air-sea flux measurements of methanol and acetone during the HiWinGS cruise

The air-sea transfer of highly soluble gases, such as many organic compounds, is limited by the rate of transport in the gas phase. Large uncertainties remain with regard to the magnitude and scaling of the gas phase transfer velocity, in part due to a paucity of observations. Here we present recent air-sea flux measurements of methanol and acetone by the eddy covariance technique from HiWinGS (the High Wind Gas Exchange Study) cruise in 2013. We also measured the dissolved concentrations of these compounds, enabling a derivation of their transfer velocities. Due to their low sea surface saturations, both methanol and acetone were consistently deposited from the atmosphere to the surface ocean. The largest influxes occurred in regions of high atmospheric abundance and strong winds (up to 25 m s-1). While the highly soluble methanol is almost purely gas phase controlled, acetone is an order of magnitude less soluble and thus subject to both gas phase and water phase controls. By comparing the concurrent transfer velocities of methanol, acetone, and sensible heat, we are able to constrain the individual transfer velocity in the gas phase and water phase. The magnitude and scaling of the gas phase transfer velocity from the COARE model demonstrate reasonable agreement with our measurements. Our indirectly estimated water phase transfer velocity is largely consistent with previous observations of the shear-driven (i.e. non-bubble mediated) transfer of dimethyl sulfide. Comparisons of these results with other measurements of air-sea gas transfer during HiWinGS (e.g. CO2, DMS, hydrocarbons) in the near future will provide further insight into the physical processes controlling gas exchange from both sides of the interface.