THE MICROBIOLOGY OF THE ATMOSPHERE 



per sec. Colwell sampled simultaneously with vacuum cleaners and Petri 

 dishes on the ground, and estimated the number of pollen grains with the 

 help of a Geiger-Miiller counter. Maximum deposition in this experiment 

 was obtained at 5-8 metres horizontal distance from the source. 

 For Sutton's parameters, the values adopted here are : 



Cx = Cy = 0-21 (metres)^, Q = 0-12 (metres)*, and m = 1-75. 



As a measure of deposition, p has been chosen in preference to Chamber- 

 lain's Vg/u, because the wind-speed under which deposition occurred is 

 usually not known in the open air. However, the curves, which are calcu- 

 lated for p = 0-05, o-oi, and o-ooi in Figs. 25, 26, and 27, respectively, 

 can be used with Chamberlain's velocity of deposition if the wind-speed 

 is known, because provisionally it is taken that p = Vg/u. 



More field experiments are needed before we can decide whether 

 deposition over rough ground depends on time or on distance — a contrast 

 analagous to the rival diffusion theories of W. Schmidt and Sutton. From 

 Chapter VII it seems that p depends on the terminal velocity of the par- 

 ticle, and numerically it has a value of about one-fiftieth to one-hundredth 

 of the terminal velocit)' expressed in centimetres per second. No allowance 

 has been made in these calculations for the hitherto unexplained larger 

 values of p which have been observed within a few metres of a source 

 near ground-level. 



Some plant pathogens have large, readily-impacting spores. When 

 liberated at, or near, ground-level, although a small proportion may travel 

 vast distances, most of these spores probably do not move very far from 

 the source. Fig. 24 suggests that for 'impactor' spores with a deposition 

 coefficient of p == 0-05, and liberated at 10 cm. above ground-level, 

 94 per cent would be deposited within 100 metres of the source.* But 

 'penetrators', with p = o-ooi, would be deposited only to the extent of 

 6 per cent within 100 metres under similar conditions. 



In spite of neglecting terminal velocity during the diffusion process — 

 a feature criticized by Schrodter(i96o) — this method predicts a much more 

 rapid loss of material from the cloud than does Rombakis's method: 

 perhaps because the ground-level concentration of the cloud is restored 

 by downward diffusion much more efficiently than by sedimentation. 

 On our theory, 50 per cent of large spores liberated at 10 cm. above 

 ground would be deposited within 10 metres, but, according to 

 Schrodter's calculations, this fraction would travel at least 200 metres 

 (Table XXVII). Such rapid decrease in the value of Q^^ near a ground 

 source emitting large spores is supported by experimental evidence which 

 has already been summarized in Table XIV (Chapter VII). Yet some 



* My mistake (Gregory, 1952; see correction, 1958) in stating that 99-9 per cent of 

 those liberated at ground-level would be deposited within 100 metres, was due to 

 misreading Qo = 10'^ as 10^" in my own table (Gregory, 1945, Table 21) — not to the 

 misunderstanding suggested by Schrodter (i960, p. 178). 



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