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 tau(aq) estimates the rate of this transfer; (2) Molecules must cross the droplet's interface; time scale tau(int) 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 tau(air) 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-tau(air), tau(int), and tau(aq) 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.