Convective aerosol fluxes near the ground surface

Abstract

1. The asymptotics for mass concentration of finely dispersed aerosol and estimates of hydrodynamic parameters in the viscous thermal boundary layer near the ground surface were obtained on the basis of field measurements in the Caspian desert. The dynamic velocity and temperature drop in the thermal boundary layer are external parameters of the problem considered. The underlying surface, which can emit particles into the atmosphere under specific conditions, is one of the atmospheric aerosol sources. Experimental data and theoretical estimates show that separation of particles from the surface can be related to turbulent stresses induced by velocity shear in the boundary layer over the underlying surface. It occurs when dynamic velocity u ∗ reaches a critical value of ~20‐30 cm/s (see [1] and references in this publication). The dynamic velocity (friction velocity) u ∗ = 〈 u ' v ' 〉 1/2 proportional to turbulent velocity pulsations determines the thickness δ ∗ of the viscous boundary layer near the underlying (smooth) surface, δ ∗ ~ , ν ≈ 0.13 cm 2 /s is the kinematic viscosity of air. The value of δ ∗ is of the order of 100 µ m for the values of u ∗ given here. Particles with sizes exceeding this value can be separated from the boundary layer depending on the degree of cohesion with the underlying surface and their mass. A different situation is related to small sand and dust particles whose size D is significantly smaller than δ ∗ ( D ~ 0.1– 10 µ m). They are completely located in the viscous boundary layer, where turbulent stresses of the wind velocity decrease sharply and cannot overcome the cohesion force between particles. However, the experiν u * mental data provide evidence that particles with an even sum-micrometer size of the ground origin are found in the atmosphere. Several mechanisms exist that explain the appearance of small submicrometer sand and dust particles in the atmosphere. The most known among them are the saltation mechanism when particles of 100 µ m in size separated from the ground release smaller particles during the impact when they fall onto the surface; another mechanism is related to electrization of particles [2]. The former of these mechanisms is directly related to the influence of sufficiently strong turbulent velocity pulsations on the particles of medium size, which appear only when the mean wind velocity reaches sufficiently high values (~10 m/s) in the upper part of the surface boundary layer. However, the amount of small particles in the atmosphere and conditions of their appearance indicate that their ascent can also occur during period that can be characterized as calm weather and when the wind force is obviously insufficient for the formation of strong shear turbulence over the underlying surface [3].