THE MICROBIOLOGY OF THE ATMOSPHERE 



Spores under test are injected or otherwise liberated, usually on the 

 tunnel axis near the constriction. Spore trapping equipment, plants, etc., 

 can be inserted farther downwind through removable panels in the walls 

 of the working section. Wind-speeds of from 0-5 to nearly 10 metres 

 per second are obtained by changing pulleys on the belt drive between the 

 constant-speed electric motor and the fan, and by inserting screens across 

 the tunnel to increase its resistance. The lower wind-speeds are best 

 obtained by increasing the resistance of the tunnel, rather than by slowing 

 the fan, because outdoor wind movement disturbs the flow less when the 

 tunnel resistance is high than when it is low. 



Most tests of spore dispersal or deposition in the wind-tunnel involve 

 knowledge of the time-mean spore concentration of the air. The Cascade 

 Impactor (K. R. May, 1945; see also Chapter VIII), operated isokineti- 

 cally (i.e. with the orifice facing the wind and with suction adjusted to 

 draw air in through the orifice at the same speed as the wind) is taken as 

 standard to estimate the mean number of particles per cubic metre of air 

 during the period of the experiment. With this information, and knowing 

 the wind-speed, we can calculate the area dose (A.D. = x^t). The trap- 

 ping efficiency of any surface exposed to the spore-cloud in the wind- 

 tunnel can then be determined by estimating the number collected per 

 square centimetre of trap surface, and expressing it as per cent of the area 

 dose. 



Liberation of one million Lycopodium* spores at a point on the central 

 axis of the tunnel produced a conical cloud. At a sampling point on the 

 central axis of the tunnel, 1-4 metres downwind, the area dose was about 

 4,500 spores per sq. cm. under turbulent condition at wind-speeds of 

 from 5-75 to 9*7 metre per sec. Under streamline conditions the dispersal 

 cone was visibly narrower and the area dose nearly double at these wind- 

 speeds. At i-i metre per sec, however, streamline conditions gave a much 

 lower area dose because the cloud was displaced downwards under 

 gravity. On the whole, efficiency of impaction was not much affected by 

 whether the flow was turbulent or streamlined. 



IMPACTION ON CYLINDERS 



The efficiency of impaction on vertical sticky cylinders is increased by : 

 (i) increasing wind-speed; (2) increasing the mass of the particle (by 

 increase in size or density); and (3) decreasing the diameter of the cylinder 

 (except that large particles, such as Lycopodium spores, tend to blow off 

 narrow cylinders at high wind-speeds, whereas small spores, such as those 

 of Ustilago perennans, do not). As will be shown later, similar relations 

 hold for impaction on surfaces of other shapes, such as spheres, disks, 



* Spores of Lycopodium clavatiim are convenient for experimental work. They can 

 be bought easily; they separate readily from one another when blown into the air. 

 They are not smooth spheres, but their mean diameter is about 32 /x, density 1-175, 

 and terminal velocity estimated as 1-76 to 2-14 cm. per sec; the number per gram is 

 9-39 to 9-4 X 10'. 



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