Transport of lycopodium spores and other small particles to rough surfaces
Author: Chamberlain, AC
Proceedings of the Royal Society of London. Series A, Mathematical and physical sciences 296:45-70.
HERO ID: 3221921
Measurements have been made in the field and in a wind tunnel of the transport of Lycopodium spores . . .
Measurements have been made in the field and in a wind tunnel of the transport of Lycopodium spores to grass and other surfaces, and wind tunnel experiments also have been done with aerosols of various smaller particle sizes. The spores and other particles were made radioactive to enable the deposition of small numbers on rough surfaces to be detected. In principle the experiments in the wind tunnel were similar to those previously done with gases (Chamberlam 1966), but the mechanisms by which particles and gases are transported across the boundary layer are different. The velocity of deposition v$_g$ of the particle to the surface is equal to the terminal velocity v$_s$ if the wind speed is very small, but at higher speeds deposition by impaction on roughness elements becomes progressively more important. If the roughness elements are of a form which gives good impaction efficiency, and have a sticky surface, v$_g$ is determined by the rate of eddy diffusion in the turbulent boundary layer above the surface, and may equal or even exceed the analogous velocity of deposition of momentum. The effect of surface texture and stickiness was investigated by comparing the catch of particles on segments of real leaves with the catch on similarity shaped segments of PVC treated with adhesive. Stickiness is important in determining v$_g$ for particles of about 10 $\mu$m diameter upwards, but not for smaller particles. In the field experiments, the use of radioactive tagging enabled the presence of a few Lycopodium spores in several grams of grass or soil to be detected, and the deposition could be measured at ranges up to 100 m from the source. At low wind speeds, v$_g$ was only a little greater than v$_s$ but at higher speeds the contribution of impaction became evident. A particularly high value was obtained when the grass was wet after recent rain. The field results with Lycopodium give a ratio of velocity of deposition to wind speed of 0$\cdot$01, and this value is used to calculate the percentage of large spores or pollen grains which will travel various distances in normal meteorological conditions. It is found that the median range is about 1 km if the particles are liberated at a height of 50 cm, but 10 km if the height is 10 m. The relative importance of direct deposition to the ground and washout by ram of the air spora is considered, and is shown to depend on the effective height of the cloud of particles. For an effective height of 500 m, derived from vertical profiles of concentration observed from aircraft, it is calculated that about 25% of the total deposition of pollen grains may be in rain.