THE UPPER-AIR SPORA 



would be A =^ xo^l'^s- Concentrations should give a straight line when 

 plotted against the logarithm of the height. 



The method of approach to the problem suffers from two defects in 

 practice. The coefficient for diffusion (Schmidt's 'A' or Taylor's 'K') is 

 not invariable with height, and it is doubtful whether a steady state is 

 ever reached with the great diurnal changes occurring when the source 

 consists of living organisms. 



C. G. Johnson & Penman (195 1) supposed that the vertical distribu- 

 tion of aphids at any one time is determined by the net effect of upward 

 transport by turbulence and downward transport by the combined action 

 of gravity and biological impulse — the mean clearance rate. If x is the 

 concentration at height z, and co is the 'mean clearance rate', they deduced 

 that a graph of log x against log z should yield a straight line. 



Attempts have been made to fit empirical curves to observational data 

 on vertical gradients. Wolfenbarger (1946, 1959) used regression equa- 

 tions of the t}'pe: Y = a + b log x + c/x. C. G. Johnson (1957) fitted 

 records of insect-trap catches with: f(z) = C(z + Ze)""^, where f(z) is 

 concentration at height z, C is a scale factor depending on population 

 size, A is an index of the diffusion process and the profile, and Ze is a 

 parameter whose significance probably depends on the rate of exchange 

 of insects between the air and the ground. 



Particles entering the air near ground-level become mixed throughout 

 the layer of frictional turbulence so long as the wind blows. Convection 

 provides a local intermittent mechanism which distributes spores from 

 the ground layer throughout the troposphere. Observed vertical concen- 

 tration gradients sometimes fit theoretical lines quite adequately, and they 

 may well describe long-term averages. But theoretical treatments of this 

 problem are often unsatisfactory, especially in failing to predict concen- 

 trations in the first few hundred feet. 



The ideal situation is seldom realized, because conditions change too 

 rapidly for a stable state to be attained. Wind velocity increases with 

 height, and layers of air at different heights in a vertical column at any 

 one time will previously have been over different places at different times. 

 The thickness of the turbulent layer of air is always changing. Biological 

 factors in a diurnal cycle put vastly differing numbers of organisms into 

 the air at different times, and vertical concentration is continually building- 

 up or decaying. Temperature inversions will affect vertical diffusion; 

 according to Jacobs (195 1), 'The presence of a stable layer at the surface 

 will prevent or retard the introduction of surface organisms into the upper 

 atmosphere but will, at the same time, maintain higher concentrations of 

 organisms in the surface layers ; the presence of a discontinuity surface 

 in the upper air will limit vertical transport in either direction, resulting 

 in the concentration of organisms above or below such a surface'. Con- 

 centration will often start to decrease from the active surface of a crop 

 and not from true ground-level. 



133 



