Perspectives on Estimating the Spray-Mediated Flux of Gases across the Air-Sea Interface

Sea spray effectively increases the surface area of the ocean and, therefore, should influence the exchange of any material transferred across the air-sea interface. For 30 years, ocean-atmosphere scientists have been analyzing the role of ocean bubbles in air-sea gas transfer because bubbles likewise provide extra contact between air and water. Yet, to date, no one has similarly evaluated how spray droplets mediate air-sea gas transfer.

In this presentation, I will outline a microphysical model to use for quantifying the spray-mediated transfer of gases across the air-sea interface. That model currently includes coupled equations for predicting the temperature, size, and salinity of individual spray droplets that, at formation, range in radius from 0.5 to 500 μm. To this system of equations, I will add a third microphysics-based equation that models the time-dependent concentration of an arbitrary gas in a spray droplet as functions of environmental conditions and the droplet's evolution in temperature, size, and salinity. The predicted change in the gas concentration in the droplet as it evolves thereby quantifies how efficient it is in exchanging gas with the atmosphere. To get the net effect of all spray droplets, I sum the individual contributions over the known rate at which droplets of a given radius are produced at the sea surface. The upshot of my message is that environmentally important gases can cross the air-sea interface by three routes: right at the interface, where the rate is controlled by molecular processes in the water; via bubbles in whitecaps, where the rate is still controlled by molecular processes in the water; and through the spray droplets produced by whitecaps and breaking waves, for which faster microphysical processes in the air around the droplets set the exchange rate.