THE MICROBIOLOGY OF THE ATMOSPHERE 



may be said that impaction efficiency on a microscope slide held at right- 

 angles to the wind is low, but might be as high as 25 per cent with grass 

 pollen or rust uredospores in winds of 9 metres per sec. 



EFFECT OF THICKNESS OF SLIDE 



Deposition on a horizontal or inclined surface is evidently complex 

 and may be disturbed by edge shadowing. Tests were therefore made with 

 both thicker and thinner slides, and with thick plates, 10 cm. wide, 

 having a double-bevelled edge. 



With a horizontal plate 6-4 mm. thick, the edge-effect was present, 

 and at medium wind-speeds edge-shadow became pronounced. At i-i 

 metres per sec. nearly the whole surface was in the shadow of the leading 

 edge, and at 3-2 and 5-5 metres per sec. there was almost no deposit on the 

 slide. At 9*5 metres per sec, however, there was some turbulent deposition 

 on both upper and lower surfaces, both in the turbulent and streamlined 

 wind-tunnel. 



When the leading edge of the plate, 6-4 mm. thick, was sharpened with 

 a double bevel to form a 45° edge facing the wind (as used by Landahl 

 & Herrmann, 1949), a very different effect was observed. In a wind of 0-5 

 metres per sec. there was a very even deposit over the whole upper 

 surface, but at i • i metres per sec. an edge-shadow developed and spread 

 across the whole surface as the wind-speed increased, until at 5-7 metre 

 per sec. deposition was negligible. There was no recovery by turbulent 

 deposition at higher wind-speeds, and at 9-5 metres per sec. the thick, 

 bevelled trap under-estimated spore concentration by a factor of 200 

 times. 



Thin horizontal surfaces, on the contrary, gave efficiencies much 

 nearer to the expected values for gravity sedimentation. However, edge- 

 shadow and turbulent deposition occurred — even with an edge o-oi6 cm. 

 thick (a microscope cover-glass). A double-edged 'wafer' safety razor- 

 blade gave uniform deposits on the upper surface, which were close to 

 expected values— except at 9-5 metres per sec, where the deposit was 

 three times that expected. 



ORIENTATION OF SPORES 



Lycopodiuni spores showed different orientations in different parts of 

 the deposit. Gregory (1951) stated that, on the stagnation zone upwind of 

 a vertical cylinder, the spores lie with the rounded distal surface uppermost, 

 and that spores settling in air under the influence of gravity come to rest 

 in the same position. Further observation shows that this contention was 

 incorrect, and that in the stagnation zone or its equivalent the spore lies 

 with the rounded distal surface touching the sticky cylinder. Evidently the 

 spore becomes orientated with the point trailing as it moves through the 

 air. Orientation with the point upwind is therefore characteristic of 

 Lycopodiuni in the stagnation zone. 



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