DEPOSITION PROCESSES 



Microscopic observation of deposits showed that, as was expected, 

 when the glass slide was horizontal the stagnation zone was on the edge, 

 and when the slide was vertical this zone was in the middle of the slide. 

 At intermediate presentation angles, seen most clearly at angles near 90°, 

 the stagnation zone shifted, at 115° occupying zone B, and reaching zone 

 A at about 120° at the higher wind-speeds. At lower wind-speeds, orien- 

 tation was less definite. 



Deposition on 9 cm. Diameter Petri Dish 



The Petri dish trap, extensively used in aerobiological mould surveys, 

 was tested horizontally after pouring with 1 5 cc. of 2 per cent water agar 

 (tests showed that deposition and retention on this medium were similar 

 to those on glycerine jelly). Mean deposition efficiencies (per cent A.D.) 

 for I cm. square zones on the agar surface are given by Gregory & Sted- 

 man (1953). At all wind-speeds, narrow edge drifts occurred behind the 

 rim of the leading edge and in front of the rim of the trailing edge. 

 Efficiency at 0-5 metres per sec. was low, but at i-i and 1-7 metres per sec. 

 efficiency was high, apparently because of the large contribution to the 

 total made by the front and back edge drifts. At 3-2 metres per sec. and 

 above, efficiency fell off substantially below expectation — apparently 

 because the sampling surface was almost entirely shadowed by the i cm. 

 high rim of the dish. Efficiencies were somewhat higher at 9-5 metres 

 per sec. 



Effects produced by the rim of the Petri dish were nearly eliminated 

 by placing the dish at the bottom of a metal cylinder 13 cm. deep and 

 11-5 cm. in diameter, sunk below a horizontal flat surface consisting of a 

 square cardboard platform cutting the central axis of the wind-tunnel. 

 The cardboard fitted flush with the mouth of the cylinder and extended 

 1 1 cm. up- and downwind. 



The Petri dish was also tested in a vertical position. At 9-4 and 5-5 

 metres per sec, the deposit was four times as great in the central zone of 

 2*5 cm. radius as it was in the peripheral centimetre around the rim. At, 

 and below, 3-2 metres per sec. the difference was reversed, with nearly 

 75 per cent more spores just insids the rim of the dish than elsewhere. 



Retention and Blow-off From Clean Surfaces 



Experiments showed that there is no appreciable loss of Lycopodium 

 spores from the deposit on the surface of a slide with a sticky coating of 

 glycerine jelly at any of the wind-speeds tested. Blow-off from a non- 

 sticky glass surface, however, depended on the wind-speed and the angle 

 of incidence of the wind. Clean microscope slides were placed in a spore- 

 cloud at 0-5 metres per sec. to obtain a deposit, and were then placed 



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