Dry deposition of airborne lead

Airborne lead is subject to dry deposition by two major mechanisms. The first is that of gravitational settling, and this is significant for all particles of 10 fim, but is rapid only for the very large particles ( 50 fim) [2], The second mechanism, which applies to particles of all sizes but is of greatest importance for the smaller particles, is deposition onto surfaces due to impaction, and for the extremely small particles due also to Brownian diffusion. 

The magnitude of particle deposition due to gravitational settling from the plume of an elevated point source, such as smelter stack, is a function of the atmospheric concentration and the settling velocity, itself determined by Stoke s Law. A description of the means of estimating deposition by this mechanism 

Deposition of vehicle exhaust lead aerosol to plant and soil surfaces by diffusion and impaction mechanisms has been studied by Little and Wiffen [3]. These workers found that rough or hairy leaf surfaces were up to eight times more efficient at collection of lead than were smooth surfaces. The velocity of deposition, Fg, of lead aerosol was determined and some results are shown in Fig. 4.1. 

Fresh exhaust aerosols showed higher values of Vg than aged aggregated aerosols, and this was attributed to the higher diffusivity of the finer primary exhaust particles [3]. 

Deposition velocities to grass and soil surfaces were also measured (Table 4.1). Grass is clearly a rather efficient aerosol collector more efficient than soil. 

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