Spray-Mediated Air-Sea Gas Exchange: The Governing Time Scales

It is not known whether sea spray droplets can act as agents that influence air-sea gas exchange. We begin to address that question here by evaluating the time scales that govern spray-mediated air-sea gas transfer. To move between the interior of a spray droplet and the atmospheric gas reservoir, gas molecules must complete three distinct steps: (1) Gas molecules must mix between the interior surface and the deep interior of the aqueous solution droplet; time scale τaq estimates the rate of this transfer; (2) Molecules must cross the droplet’s interface; time scale τint parameterizes this transfer; and (3) The molecules must transit a “jump” layer between a spray droplet’s exterior surface and the atmospheric gas reservoir; time scale τair dictates the rate of this transfer. The same steps, in reverse order, pertain to gas molecules moving from an atmospheric reservoir to a drop’s interior. For the six most plentiful gases, excluding water vapor, in the atmosphere—helium, neon, argon, oxygen, nitrogen, and carbon dioxide—τair, τint, and τaq are shorter than the time scales that quantify the rate at which a newly formed spray droplet’s temperature, radius, and salinity evolve. We therefore conclude that, following the assumptions herein, a model for spray-mediated air-sea gas exchange can assume that the gas concentration in spray droplets is always in instantaneous equilibrium with the local atmospheric gas concentration.