Transport of small particles by Brownian diffusion

Experiments on transfer of submicrometre radioactive particles to smooth surfaces (Wells Chamberlain, 1967 Chamberlain et al., 1984) have shown that the dependency of vg on D213 holds over many orders of magnitude of D. This means that the transport by Brownian diffusion becomes progressively less effective as the particle size increases. For example a particle of 0.1 pm diameter has a diffusivity of 6.8 x 10 10 m2 s 1, a factor 1.2 x 104 smaller than that of I2 vapour. Since D does not depend on the particle density, it is appropriate to discuss transport by Brownian motion in terms of the particle diameter. The aerodynamic diameter, dA, is equal to dppp2 where pp is the particle density in c.g.s. units (g cm-3) not SI units (kg m-3), and is the appropriate parameter for particles with dp 1 pm, for which impaction and sedimentation are the mechanisms of deposition. 

Shoots of nettle, white poplar and beech were placed in the tunnel which was operated at a wind speed of 2.5 m s 1. Beech leaves are smooth, whereas those of white poplar and nettle have hairs. The velocities of deposition to the leaves of the three species are shown in Table 6.1. As expected from the lower diffusivity of the larger particles, vg was less for the 0.2- m than for the 0.03-,am particles. Deposition to 

The characteristic lengths of the leaves were 23 mm (nettle), 28 mm (white poplar), and 27 mm (beech). Using these values of L, the wind speed (2.5 ms-1) and the diffusivities as given in Table 6.1, the values of Sh can be calculated and the results are plotted in Fig. 6.1. Also shown are results of Belot (1975), who exposed oak leaves and pine needles to an aerosol of 0.17 m diameter uranine particles in a wind tunnel, and used the fluorescence of the dye to measure the deposition. The points from Belot s experiments and those of Little Wiffen he fairly close to the theoretical line, with the exception of the results of 0.2- m lead aerosol to nettle and white poplar. These latter show enhanced deposition, probably due to impaction on, or interception by, the hairs on the leaves. 

It has been suggested that the planting of trees alongside motorways might reduce the dispersion over from motor exhausts. Little Wiffen showed that the measured vg values, when related to the foliar surface of a belt of trees, would not be enough to reduce the amount of lead dispersed significantly. 

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