THE MICROBIOLOGY OF THE ATMOSPHERE 



rate of i mm. per hour would deplete an aerosol of Lycopodium spores 

 according to the equation 



X(t) = Xoexp. (-3-3 X lo-h). 



This implies that after one hour the concentration will be reduced to 

 one third of its initial value. Chamberlain applies the concept of Vg to 

 rain wash-out and shows that, the greater the height of the spore-cloud 

 through which rain falls, the greater the rate of wash-out. The study was 

 continued experimentally at Harwell by F. G. May (1958), who found 

 good agreement with Chamberlain's theoretical values when tested with 

 Lycopodium spores marked with iodine-131. When testing thundery 

 showers, May also obtained good agreement with values calculated by 

 Langmuir (1948) and by Mason (1957) — but the predominantly smaller 

 and more variable droplets of frontal rain removed particles faster than 

 had been predicted by theory. McCully et al. (1956) reported highly 

 efficient removal of fluorescent dusts by rain from the air in both labora- 

 tory and field tests. 



May (1958) points out that, in addition to removal by impaction, 

 raindrops can act as electrostatic collectors by means of two effects — • 

 the coulombic attraction between oppositely-charged spores and rain- 

 drops, and by induced attraction. He estimated that capture by coulombic 

 attraction in thunder rain might equal capture by induced attraction, and 

 that both together could amount to 20 per cent of the total wash-out. 

 On the other hand. May suggests that for frontal rain coulombic collec- 

 tion can be neglected. 



Relative Importance of Deposition Mechanisms 



With Qp spores liberated into the air from a source, the number of 

 spores remaining in suspension will be reduced during travel in the wind 

 by the action of various deposition processes and, at a distance x from 

 the source, the number remaining in suspension will be Q^^ (Chapter 

 XIII). 



The relative importance of the deposition processes can now be seen 

 to differ in different positions. Close to a source liberating spores near 

 ground-level, the mechanisms of impaction and boundary-layer exchange 

 will be the dominant fiictors in depleting the cloud. Near to the source, 

 rain-wash will have a relatively slight effect, because the height and breadth 

 of the cloud are small. But, as the cloud diffuses to its maximum height, 

 rain will have an increasingly large effect. With the still smaller particles 

 of radioactive dust, pushed into the stratosphere by megaton bombs and 

 falling back into the troposphere, rain-wash appears to be the most 

 important mechanism of removal. The level of strontium-90 in soil at 

 Antofagasta, Chile, in rain-free desert, is only i per cent of that general 

 for places in the same latitude with normal rainfall (Libby, 1956). 



