Dry deposition of aerosol particles

The removal of aerosol particles under dry weather conditions is caused by turbulent diffusion and gravitational sedimentation, which transport particles to the Earth s surface, as well as by impaction on vegetation, buildings and other objects. Turbulent diffusion itself does not remove particles (Twomey, 1977). Soil and other surfaces are bordered by a thin laminar layer ( 1 mm thick) across which particles must be transported by other processes (e.g. phoretic forces, molecular diffusion, sedimentation). 

Disregarding impaction, dry removal of aerosol particles can be described formally by the following equation (e.g. Makhonyko, 1966)  

Impaction is a function of several variables. According to Ranz and Wong (1952) the impaction efficiency (defined in Subsection 4.1.2) depends on the parameter ijj  

The dry deposition of aerosol particles (D ) is generally measured by horizontal microscopic slide or so-called dustfall cans and jars. However, the results of such measurements, wide-spread in local pollution studies (Corn, 1976), have to be interpreted with caution because of the disturbance of the laminar and turbulent flow regime by the collector. Furthermore, the laminar layer covering the collector surface may be very different from that over soil and vegetation. In any case, if we also measure the particle concentration N, a parameter with the dimension of velocity can be defined (Junge, 1963)  

To estimate the significance of impaction by trees we refer to the results of Neuburger and his co-workers (see Hidy, 1973), based on particle concentration measurements carried out inside and outside a forest area. Neuburger s data show that more than 80 % of pollen (coarse particles) is removed by trees while the corresponding value for Aitken particles is 34 %. Further research is needed, however, to confirm these results. 

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