THE MICROBIOLOGY OF THE ATMOSPHERE 



workers have considered that these large estimates for deposition near the 

 source are incompatible with the facts about dispersal in the upper-air, 

 and over long distances, which are discussed in Chapters X and XIV. 

 This dilemma is considered further below (pp. i8o and 197). 



The effect of values of m lower than 1-75 is to increase deposition 

 near the source and to decrease it at greater distances. With liberation at 

 ground-level, the distance at which deposition per unit area is equal with 

 either m = 1-75 or 1-24, lies between x = 10 and x = 100 metres. For 

 other heights of liberation above ground-level, calculations are not yet 

 complete. 



Application of Gradients 



The results given graphically in Figs. 24 to 27 can be used in various 

 ways, some of which are illustrated in the following examples. 



(i) Deposition at a point downwind. Assuming that a million uredo- 

 spores are liberated at a point one metre above ground-level under 

 standard atmospheric conditions, what number would be deposited per 

 square centimetre of ground at a point 100 metres downwind? Choose 

 Fig. 25, because p = 0-05 is the relevant coefficient of deposition for 

 uredospores (rather than Figs. 26 and 27, which refer to smaller spores). 

 Choose the group of three lines marked 'd,,.' (deposition per square 

 centimetre downwind of a point source). Choose the middle of these three 

 lines, as m = 1-75 under standard conditions. Read-off the value of 

 'logarithm (deposit -^ Qp)' for the distance of 100 metres. This value 

 is approximately = ^-4. As in this example Qp = lo*', log Qp = 6-o, so 

 we now have log d^^ = 6-o + 5-2, therefore log d^^ = 1-4, and d,,, = 25 

 spores per square centimetre. 



But this calculation assumes no depletion of the cloud, and deposition 

 must now^ be allowed for by replacing Q^^ by Q^^- From Fig. 24, choose the 

 group of curves marked p = 0-05 and, as the height of liberation is i 

 metre, choose the line for h = i, and read off the value for Q^x/Q_o ^t 100 

 metres. This value is approximately 0-25, indicating that only about a 

 quarter of the spores liberated are still in suspension at the distance of 

 100 metres from the source. We therefore have the value of the corrected 

 d^ = 0-25 X 25 = 6. The answer to the problem is that we predict a 

 deposit of six uredospores per square centimetre under the conditions 

 postulated. 



(ii) Fitting theoretical curve to observed data can best be illustrated by a 

 published example that gives actual distances and percentages of leaves 

 infected. Relative distances are useless because the slope of the gradient- 

 line is characteristic of an absolute distance, and also with relative 

 percentages we cannot apply the multiple-infection transformation if the 

 data require it. 



176 



