Dry Deposition: Sc^−2/3 Revisited

Simulations of the processes contributing to the deposition of trace gases and small particles from the air to natural surfaces routinely describe the consequences of changing molecular diffusivity in terms of the Schmidt number, Sc ≡ ν/D, where ν is kinematic viscosity and D the molecular diffusivity of the constituent in question. Using well-verified results of pipe flow experiments, early workers proposed that the relevant property entering dry deposition and other models of similar kind should be Sc^−2/3 rather than Sc⁻¹ as would be expected from historic flat plate experiments. Upon reconsideration, it is now proposed that no universal power-law dependence on Sc can be expected; the corresponding role of molecular diffusivity is likely to be site-specific. Relevant experimental evidence remains elusive.