Turbulent transport of atmospheric aerosols and formation of large-scale structures

Turbulent transport of aerosols and droplets in a random velocity field wit h a finite correlation time is studied. We derived a mean-field equation an d an equation for the second moment for a number density of aerosols. The f inite correlation time of random velocity field results in the appearance o f the high-order spatial derivatives in these equations. The finite correla tion time and compressibility of the velocity field can cause a depletion o f turbulent diffusion and a modification of an effective mean drift velocit y. The coefficient of turbulent diffusion in the vertical direction can be depleted by 25 % due to the finite correlation time of a turbulent velocity field. The latter result is in compliance with the known anisotropy of the coefficient of turbulent diffusion in the atmosphere. The effective mean d rift velocity is caused by a compressibility of particles velocity field an d results in formation of large-scale inhomogeneities in spatial distributi on of aerosols in the vicinity of the atmospheric temperature inversion. Re sults obtained by Saffman (1960) for the effect of molecular diffusivity in turbulent diffusion are generalized for the case of compressible and aniso tropic random velocity field. A mechanism of formation of small-scale inhom ogeneities in particles spatial distribution is also discussed. This mechan ism is associated with an excitation of a small-scale instability of the se cond moment of number density of particles. The obtained results are import ant in the analysis of various atmospheric phenomena, e.g., atmospheric aer osols, droplets and smog formation. (C) 2000 Elsevier Science Ltd. All righ ts reserved.