DEPOSITION GRADIENTS AND ISOLATION 



Another set, Figs. 25-27, gives values of D, d, d,,., dj,,,, and d^^ on a 

 logarithmic scale for sources of various geometrical form, assuming no 

 loss from the cloud by deposition. To predict a gradient, the deposition 

 values are first read-off for various distances from the appropriate 

 line on Figs. 25 to 27, and allowance can then be made for loss from the 

 cloud at each distance by reading-off Q^^ as a fraction of Q^„ in Fig. 24. 



X metres 



100 1.000 



loooo 1000,00 



Fig. 24.— Fraction of spore-cloud remaining airborne (allowing for loss of spores from 

 spore-cloud by deposition to ground), expressed as Q.x/Q.o- Calculated from Chamberlain 

 (1956): for ?n = 1-75; Cz = 0-21 (metres)J; height of source above ground, h = o-o, o-i, i, 

 10, and ICO metres; distances from source, x = i metre to 100 km. 



CALCULATION OF Q_x 



Except in experimental spore-liberation tests, the quantity liberated, 

 Q_o, is usually unknown. However, cloud concentration and deposition 

 are proportional to Qp so, although the height of the gradient line will 

 depend on the source-strength, its slope will be unaffected by the value of 



Elevation of source has been allowed for by using the equation number 

 52 of Chamberlain (1956), to calculate values of Q^^ for heights h = o, 

 o-i, I, 10 and 100 metres. 



Fig. 24 shows that elevating the source decreases deposition on 

 ground near the source, and in using Figs. 25-27 for elevated sources 

 it is important to neglect those parts of the curves before deposition 

 starts. This decreased deposition near an elevated source was confirmed 

 experimentally by Colwell (1951), who liberated P-32 radioactive Phius 

 pollen at 3-5 metres above ground-level into a wind averaging 8-i metres 



171 



