Particle diffusivities and deposition velocities over a horizontal smooth surface

Abstract

Particle effective eddy diffusion coefficients, ϵ/ν, [(inertial + diffusion)/viscous forces], were calculated from experimental deposition fluxes to a smooth horizontal surface. The ϵ/ν are a function of both particle inertia ( τ +), the distance from the surface ( y +), and the air friction velocity ( u ∗ ) which is contained in both dimensionless terms. These ϵ/ν are greater than the ϵ/ν for particle deposition along a vertical surface. Both correlations show that particle effective eddy diffusion coefficients are much greater than the eddy diffusion of air momentum for distances from the surface of y + < 20. For greater distances, particle concentration profiles at a distance of one cm were used to calculate ϵ/ν for y + values from 200 to 900. The results show data scatter, but can be represented by a constant ϵ/ν = 140 for y + > 350. These ϵ/ν are similar to those for air momentum transfer. The effective eddy diffusivities are combined with Brownian diffusion and gravity settling to predict particle deposition velocities for particle size from 10 −3 to 10 2 μm diameter for particle densities of 1 and 10 gm/cm 3. Each of these mechanisms of particle transport controls the predicted deposition in some range of particle size. The deposition velocities show minima of 1.8 × 10 −4 to 7.7 × 10 −3 cm/sec for particle sizes from 0.066 to 0.54 μm. The minima as well as all deposition velocities are equal to or much greater than predicted from gravitational settling.