NATURAL DEPOSITION 



Under a given set of conditions, p is assumed to depend only on concentra- 

 tion, though we do not yet know how it is affected by wind-speed, tur- 

 bulence and other factors. The 'velocity of deposition', introduced by 

 Chamberlain (1956), was defined as 



amount deposited per sq. cm. of surface per second 

 ^ volumetric concentration per cc. above surface 



If p is assumed to be independent of wind-speed, it will be apparent 

 that, in our notation, 



P = Vg/u. 



MEASUREMENT OF DEPOSITION COEFFICIENT, 'p'. 



The first attempt to evaluate p by Gregory (1945) was based on experi- 

 ments by Stepanov (1935), whose results were tested against Sutton's 

 (1932) eddy-diffusion theory. 



Sutton's formulae had been developed for calculating the concentra- 

 tion of a cloud of particles whose deposition was negligible — the number of 

 particles in the cloud, Q^q, remaining constant throughout the diffusion. 

 In our problem, although the effect of gravity on dispersion has been 

 neglected, the quantity of spores remaining in suspension is steadily 

 diminishing owing to a relatively large deposition from that part of the 

 cloud which is in contact with the ground, so that Q^^, the total quantity 

 remaining in the cloud when its centre has moved to a distance x, is less 

 than the original Q^q. It has been showTi* that Q^x"^ill decrease exponenti- 

 ally with increasing distance, according to the equation : 



,(l-*m) 



Clx=Q.oexp. 



2px* 



Virr) C(I - |m)_ 



Values of Q^^ ^^d d for two values of the parameter m have been 

 calculated, and it is now possible to test the theory against Stepanov's 

 results. In each of his experiments the total number of spores liberated 

 differed, so that the data had first to be put on a comparable basis by 

 equating the mean deposition, d, at 5 metres, to 100 per cent, and then 

 expressing the deposition observed at greater distances by relative 

 percentages. The logarithms of the observed relative depositions were 

 plotted against the logarithms of the distances in centimetres. The ex- 

 pected depositions when p = 0-05, 0-025, and zero, respectively, were also 

 plotted, and the line calculated for p = 0-05 was seen to approach most 

 clearly to the observed values (Gregory, 1945). A deposition coefficient of 

 p = 0-05 means that, in travelling across i sq. cm. of surface, the entire 

 cloud would deposit a quantity of spores approximately equivalent to 

 the number contained in a slice half-a-millimetre thick through the 

 axial plane of the cloud. This value of p was estimated from experiments 

 in winds of about i metre per sec. 



* By Margaret F. Gregory {see Appendix in Gregory, 1945). 



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