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.
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 experimental investigation. With a continuous source emitting during a limited time, the area dose will be the same as if the same total quantity of particles, had been liberated in a number of successive instantaneous puffs arriving in a series of greatly fluctuating concentrations.
We can conveniently express the percentage efficiency of a trapping surface as This convention expresses the efficiency with which a surface clears the spore-cloud to a height of one centimetre above the surface. Deposition on a surface takes place in several ways, including impaction and sedi mentation, for sedimentation seldom acts alone.