NATURAL DEPOSITION 



and the values are therefore underestimates; they were made without 

 extrapolation, except that, where the number of radii on each side of the 

 axis of dispersion was unequal, a symmetrical dispersion was assumed. 

 With liberation at 0-25 m. above ground-level, from 14 to 24 per cent of 

 the spores liberated were deposited within 10 metres' radius in winds of 

 little over i m. per sec. 



Deposition Mechanisms Outdoors 



boundary-layer exchange 



Observations on deposition out-of-doors are still meagre, but values 

 for Vg obtained with plant spores of two widely different sizes and fall 

 velocities, Lycopodium clavatum (1-75 cm. per sec.) and Ganodenna 

 applanatum (o-i8 cm. per sec), suggest the following picture of deposition. 



Deposition on the ground tends to remove spores from the base of the 

 cloud. The concentration of the cloud near ground-level is then restored 

 by diffusion — either horizontal diffusion from a near-by source, or down- 

 ward vertical diffusion from a reservoir of particles overhead, when the 

 atmosphere itself is acting as a source. Diffusion thus brings particles 

 do\NTi to the boundary layer, and here they settle-out mainly under 

 gravity. This is why, when Vg is estimated from x as measured just above 

 ground-level, it numerically approaches the Stokesian terminal velocity. 

 This process which has been called 'boundary layer exchange', continually 

 replenishes the ground layer of still air in which even minute spores can 

 sediment-out. 



The demarcation between the laminar surface layer and the turbulent 

 wind-stream is not sharp. From time to time, spore-bearing eddies break 

 into the laminar layer which is being cleared by sedimentation, removing 

 spore-free air and leaving, in exchange, small volumes of spore-laden 

 air. These spores sediment at terminal velocit}' under the influence of 

 gravity and are soon out of reach of further eddies. In boundary-layer 

 exchange, turbulence has a great effect in bringing spores down to the 

 layer where they can slowly sediment under the influence of gravity. 



sedimentation 



As we have seen, the effect of gravity is usually negligible on vertical 

 distribution in the atmosphere. Still air, as T\Tidall (1881) found long 

 ago, soon becomes purified because microbes settle out from it under 

 gravity' — a process which we call sedimentation. Out-of-doors the air 

 is almost never still — except within about a millimetre of the surface. 

 In the turbulent air-layer above the ground, the effects of gravity are 

 slight and difficult to demonstrate; but Rempe (1937) demonstrated 

 gravitational effects with tree pollen near Gottingen {see above ^ p. 21). 

 Under stable conditions on clear nights, when air at ground-level is 

 cooled by radiation, the laminar layer may extend to a height of several 

 F 81 



