Surface source function for sea‐salt aerosol and aerosol dry deposition to the ocean surface

Abstract

The dry deposition velocity for a uniform surface source of particles, such as sea‐salt aerosol, is shown to be fundamentally different than that for a source of particles from above or upwind. An expression for the deposition velocity for a uniform surface source of particles and an improved expression for the deposition velocity when the source is from above (upwind) is derived. The equilibrium method of deriving the sea‐salt source function from an aerosol concentration, measured at a reference height, and the deposition velocity, is shown to be of little value for particles smaller than about 5 to 10 μm in radius for two reasons: (1) The time to establish equilibrium between the source and loss by dry deposition is much longer than the typical lifetime of small particles determined by precipitation scavenging. (2) It is difficult, if not impossible, to correct for the effect of synoptic‐scale vertical velocities and the effect of mixing between the marine boundary layer and the free troposphere. A sea‐salt aerosol source function that combines the Monahan et al. [1986] formulation at radii smaller than about 10 μm with the modified Smith et al. [1993] formulation at radii larger than 10 μm is proposed.