THE MICROBIOLOGY OF THE ATMOSPHERE 



In July and August, while bacterial concentrations of 55,000 per cubic 

 metre were current in the air of the Rue de Rivoli in Paris, and 7,600 per 

 cubic metre in the Pare Montsouris, Miquel found only eight bacteria 

 (and numerous moulds) per cubic metre at a height of i metre above the 

 surface of a field at Lake Thun (570 m. above sea-level), and none at 

 stations between 2,000 metres altitude and the summit of the Eiger at 

 nearly 4,000 metres. Miquel attributed this purity to the effect of reduced 

 atmospheric pressure doubling the volume of air and diluting its dust 

 load, to the rarefied air less easily holding particles in suspension, and to 

 the absence of local sources of contamination — especially in the regions 

 of perpetual snow. In similar volumetric data from the Dauphine Alps 

 recorded by Bonnier et al. (191 1), bacteria decreased with height more 

 rapidly than moulds. 



No one has yet compared concentrations at different heights above 

 ground-level over plateaux with those over mountains, or over flat and 

 convex surfaces at the same altitude. 



The purity of the air in regions of perpetual snow is understandable, 

 but it is surprising that air at one or two metres above ground-level in 

 mountain valleys should also contain so few microbes. Geiger (1950) 

 envisages mountain slopes as covered with a skin of air having the usual 

 characteristics of air near the ground but easily removed by wind and 

 convection — except where protected by vegetation. Convex surfaces 

 generally have a more extreme climate than flat surfaces, and concave 

 surfaces are still more equable. 



THE ROLE OF TURBULENCE 



The role of turbulence in diffusing spore-clouds vertically was first 

 emphasized by Schmidt (191 8, 1925), although the theory was developed 

 for heat transfer by Taylor (191 5) with information derived from tem- 

 perature records over the Great Banks of Newfoundland. Schmidt 

 argued that, when a stable state of diffusion by eddies has been reached, 

 the number of particles falling under the influence of gravity across any 

 horizontal boundary is compensated for by the number of particles 

 moved upwards by diffusion, and so the concentration of particles in the 

 air should decrease exponentially with increasing height according to the 

 equation 



VsZ 



X = Xo exp. - -j- 



where xo = concentration at height z = 0, 

 Vs = terminal velocity of fall, 



A = Schmidt's 'Austausch' or intermixing coefficient which is 

 assumed to be invariable with height. 



The total spore content of the column of air standing above i sq. cm. 



132 



