VI 



DEPOSITION PROCESSES 



We have now considered airborne micro-organisms as diffusing clouds. 

 Before we can discuss processes by which they are deposited in the com- 

 plex outdoor environment, we must deal with deposition processes under 

 simplified, ideal conditions. The word 'deposition' is used in a general 

 sense to include all processes by which airborne particles are transferred 

 from aerial suspension to the surface of a liquid or solid. One form of 

 deposition, the impaction of droplets or particles on surfaces, has been 

 extensively studied both theoretically and in wind-tunnel experiments. 

 It is highly relevant to the problems of spore deposition in nature and of 

 sampling techniques which form the topics of Chapters VII and VIII. 



1^ • 1^^ 



u 



> 



X 



%■ 



Fig. 8.— Diagram showing relation between concentration (x = number of spores per unit 

 volume); wind-speed = u; area dose (A.D. = number of spores passing through frame of unit 

 area); and trap dose (T.D. = number of spores deposited on unit area of surface). 



The relation between concentration of the spore-cloud, x, and deposi- 

 tion on the surface (T.D. = trap dose) over which the spore-cloud travels, 

 is illustrated in Fig. 8, together with the concept of 'area dose' (A.D. = 

 the number of particles flowing through an imaginary frame of unit 

 area cross-section at right-angles to the direction of the wind). Concen- 

 tration of the cloud (x = number of spores per cubic metre) is the more 

 fundamental measurement, and the one of greatest interest to the allergist, 

 whose patients inhale volumes of air. The trap dose, which measures 

 deposition on a surface, is of more interest to plant pathologists, plant 

 breeders, and pollen analysts. The area dose is a useful concept in passing 

 from the one measurement to the other. For a given concentration of 

 particles per unit volume of air, the area dose must increase with wind- 

 speed, but whether the trap dose will also be affected is a matter for 



58 



