A particle dry‐deposition parameterization scheme for use in tracer transport models

Abstract

A particle dry‐deposition parameterization scheme for use in tracer transport models is presented. The scheme produces particle deposition velocities relative to the bottom transport model level expressed in terms of wind speed, temperature, and air density at the bottom model level; surface momentum drag coefficient; particle size and density; characteristics of the surface roughness elements; and several parameters derived for different surface types. A two‐layer boundary layer model is employed to calculate the surface deposition velocities. Formulas for the particle deposition velocity are derived for four surface types: smooth surfaces, surfaces with bluff roughness elements, ocean surfaces, and vegetative canopies. The following processes are included in the model: turbulent transport through the boundary layer, gravitational settling, deposition by Brownian diffusion, interception and impaction on the surface roughness elements, diffusiophoresis on water surfaces, and, in limited cases, particle bounce‐off. Electrical and thermophoretic effects are ignored as well as particle blow‐off. As an illustrative example, the scheme is applied to a general circulation model to calculate the deposition velocities of ambient tropospheric aerosols. The calculated deposition velocities show a strong dependence upon the wind field in the bottom model level, the particle size distribution, and, to a lesser extent, the surface type. Particle mass deposition velocities of continental aerosols are found to lie in the range 0.003–0.036 cm/s for the accumulation mode and in the range 0.5–2.5 cm/s for the coarse mode.