THE MICROBIOLOGY OF THE ATMOSPHERE 



Russia. Such wind-operated aeroconiscopes or aeroscopes* are essentially 

 qualitative and there is no information on their efficiency. Finding an 

 organism means that it was certainly present in the air, but gives no 

 information about its numbers. When wind-speed is low, failure to find 

 an organism does not necessarily mean that it was not present. 



FORCED AIR-FLOW IMPACTORS 



Samplers through which air is drawn by pumps, fans, or aspirators, 

 can be made relatively independent of changes in wind-speed and differ- 

 ences in particle size, and they can accordingly give a volumetric reading 

 under field conditions. 



For sampling clouds of inorganic particles, which are often present 

 in high concentration in closed spaces, various forms of koniometer have 

 been developed (e.g. the Kotze koniometer, the Owen jet dust-counter, 

 and the Aitken nucleus counter). None of these is well suited to the 

 requirements of aerobiology; because of the small volume of air sampled, 

 they are suitable only for high concentrations, and their aerodynamic 

 features make isokinetic operation impossible (also, aggregates are shat- 

 tered by a high speed of impact). Their efficiency has been tested by 

 C. N. Davies et al. (195 1), and their advantages and drawbacks are well 

 summarized by Green & Lane (1957). As was emphasized by K. R. May 

 (1945), a true sample of moving air 'is only obtained by isokinetic samp- 

 ling, that is, when the sampling tube, with a feathered leading edge, 

 faces upstream with suction applied at such a rate that the air velocities 

 in the tube and main airstream are equal'. Many of the devices extensively 

 used in biological work are satisfactory for sampling bacterial aerosols 

 consisting of single bacterial cells or mould spores in still air, but fall 

 far short of the ideal isokinetic sampling when used for bacteria carried 

 on 'rafts', and larger fungus spores or pollen grains in moving air. 



The main errors in impactor traps operated by suction are: (i) col- 

 lection errors due to failure of the spore to enter the orifice (these errors 

 may be reduced by isokinetic sampling with the orifice facing the wind, 

 see Watson, 1954); and (2) retention errors due to failure to deposit spores 

 in the correct place, either because they are lost on the walls of the trap 

 or because they pass through the trap. Splash-borne spores, travelling 

 mostly in droplets of 1 50-600 ^i diameter, are not yet satisfactorily collected 

 by any existing equipment. 



(i). Sieving filters. Drawing air through a filter with pores too small 

 for the organisms under investigation to pass through, is a simple tech- 

 nique which is, however, relatively little used as it is difficult to get a 

 sufficiently high rate of air-flow through small pores. Filter-paper 

 has been used by Frey & Keitt (1925) for Ventiiria spores, by Chamberlain 

 (1956) in wind-tunnel tests with Lycopodimn., by Gordon & Cupp (1953) 

 for Histoplasnia, and for pollen collection on Atlantic liners by Erdtman 

 (1937). Membrane or 'Multipore' filters of re-precipitated cellulose can 



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