Field experiments on deposition of particles

The deposition velocities to grass of particles with diameters between 1 and 5 jum were also found to be proportional to w (Chamberlain, 1967), but for these particles impaction has much less effect. Particles smaller than about 5 jum diameter are unlikely to bounce off surface in normal ambient conditions. The presence of micro-roughness elements on surfaces increases vg for these particles, and Chamberlain (1967) found more deposition to real leaves than to smooth sticky artificial leaves when the particle size was less than 5 jum. 

In a wind tunnel, the direction and velocity of the airflow are predetermined, but in the field they are variable. This presents problems in the sampling of airborne particles, as discussed by May et al. (1976). If suck samples are used, the flow should be isokinetic, which requires that the sampler be aligned upwind and that the flow velocity into the sampler equals the wind speed. If these conditions are not met 

Plant pathologists need to study the dispersion of spores in the field. To avoid the need for a power supply, and the difficulties of arranging isokinetic samplers, passive samplers are often used (Gregory, 1973). Horizontal microscope slides are used to collect spores by sedimentation and vertical cylinders are used to collect them by impaction. The efficiency of captures by the cylinders depends on the wind speed, and requires calibration in the wind tunnel. 

The results of the field experiments are summarised in Table 6.4. The velocity of deposition of Lycopodium spores to dry grass and cereal crops was about twice the sedimentation velocity, so vv and vt were about equal. When the cereal crop was thoroughly wetted, v, was about twice vv. 

In the last two columns of Table 6.4 the turbulent deposition velocities of mass and momentum are shown. For droplets, vt and vm are approximately equal, indicating that capture by the surface is efficient, and aerodynamic resistance the limiting factor. 

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