THE MICROBIOLOGY OF THE ATMOSPHERE 



German forests in spring during a series of aeroplane flights by day and 

 by night (Rempe, 1937). In general, with light to moderate windy weather 

 and with cumulus clouds at about 2,000 metres, the pollen concentration 

 decreased only slightly up to 1,000 metres, and the maximum number of 

 grains might occur as high as 200 or even 500 metres. This was regarded 

 as a sign of a complete inversion of air masses. A similar distribution also 

 occurred under high pressure conditions without clouds but with strong- 

 thermal turbulence. By way of contrast, conditions associated with a 

 stratified cloud-layer and high wind velocities showed a marked decrease 

 of pollen with height. 



In night flights, the maximum number of grains was often reached at a 

 height of about 200 metres, i.e. above the temperature inversion which 

 often develops at night. At night the numbers trapped usually decreased 

 with increasing height much more than by day. The total numbers trapped 

 at all heights were also fewer by night than by day. From Rempe's data 

 the mean numbers of pollen grains trapped per 1-275 ^^- ^^i. of trap 

 surface per 20 minutes for all flights which extended up to 1,500 metres 

 were : 



Altitude (metres) : 10-40 200 500 1,000 1,500 



Day flights 904 849 852 581 267 



Night flights 577 560 283 85 45 



These records include pollen grains of various species; but, taking 

 Vs = 3 cm. per sec. as a moderate value for the speed of fall of pollen 

 grains, it can be shown that, for altitudes above the zone affected by 

 strong thermal turbulence and temperature inversions, Schmidt's in- 

 terchange coefficient A = 2-6 X 10^ for day flights, and r6 x 10^ for 

 night flights. This provides further evidence of the appropriateness of 

 considering the spore or pollen cloud as a suspension in air. Although at 

 heights of about 1,000 metres and upwards the average distribution 

 agreed well with that expected from terminal velocity balanced by eddy 

 diflfusion, near ground-level the suspension tended to become more 

 uniform than predicted, owing to the intermittent stirring of the lower 

 layers by strong mechanical and diurnal thermal turbulence. 



(iii) Sampling the upper air over the United States. In the upper 

 convective layer. Walker (1935) exposed Petri dishes of blood agar and, 

 after sampling an estimated 2,400 cubic metres of air, he concluded 

 that the atmosphere in that layer was sterile (two cultures of Staphylo- 

 coccus aureus were reasonably enough ignored as contaminants). However, 

 Proctor & Parker (1942) suggested that Walker's agar surfaces may have 

 been frozen and non-adhesive, because their own researches at the 

 Massachusetts Institute of Technology showed that the upper air over the 

 United States was far from sterile. 



In trapping from aeroplanes. Proctor & Parker used filters of lens 

 paper supported on wire gauze in brass tubes connected with the free 



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