THE MICROBIOLOGY OF THE ATMOSPHERE 



The problem was next taken up experimentally by Chamberlain 

 (1956), who made Lycopodhim spores radioactive by steeping them in a 

 solution of iodine-131 in carbon tetrachloride. When dry, they were 

 liberated at a height of i metre above a grass field. The concentration of 

 the spore-cloud was measured 20 metres downwind by using sticky 

 cylinders 0-65 cm. in diameter at 30, 60, and 90 cm, above ground-level. 

 The wind-speed at each height was measured and, from the known 

 impaction efficiency of cylinders, the dosage at each height could be 

 estimated (dosage did not vary much with height at this distance). The 

 radioactivity of the grass turf just in front of each sampling position 

 was measured to give an estimate of deposition. 'Velocity of deposition', 

 Vg, and deposition coefficient, p, were then calculated from these two 

 measurements (Table XI), the results giving some support to the idea that 

 p is independent of wind-speed. 



TABLE XI 



DEPOSITION ON GRASS OF LyCOpodiuTll SPORES ACTIVATED WITH IODINE-131 



(data of Chamberlain's, 1956) AT 20 metres from point where liberated 



AT I metre above GROUND-LEVEL 



Further information was obtained by Gregory, Longhurst & Sreer- 

 amulu {unpublished) from experiments in 1956-57 at the Imperial College 

 Field Station, Ascot, England. Spores of Lycopodium^ and the much 

 smaller spores of a bracket fungus, Ganoderma applanatum, were liber- 

 ated a short distance above ground-level in a field of short, rough grass. 

 Cloud concentration and deposition, measured at a number of positions 

 simultaneously, enabled p or Vg to be estimated at various distances up 

 to 10 metres from the point source (Table XII). (Two additional recent 

 estimations at i metre from the source, with Lycopodium spores on a 

 smooth lawn at Rothamsted Experimental Station, gave p = 0-05 and 

 0-09, and Vg = 3'i and 5-3 cm. per sec, respectively.) A remarkable 

 phenomenon, evident from Table XII, is that both Vg and p vary with 

 distance, decreasing with distance from the source. 



Much more experimental work is needed before the relation between 

 X and d can be established, but meanwhile it appears that, at 10 metres or 

 so from the source near ground-level, Vg nearly equals the terminal 

 velocity of the particle, whereas closer to the source the value may be up 



78 



