ATMOSPHERIC POLLUTION — FRENKIEL 



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Figure 7. — Hypothetical dispersion of smoke puffs released at two points of the Los Angeles 

 Basin at 7 a. m. and 8 a. m., respectively. 



magnitude and in direction for the whole urban area. However, in 

 some cases it may not be possible to define such a constant mean wind 

 velocity. Figure 7 illustrates how one may be able to construct a 

 mathematical model of atmospheric pollution when the "mean" wind 

 velocity cannot be considered as being the same for the whole area. 

 On this figure an outline of Los Angeles County is illustrated with 

 two trajectories determined from measured hourly wind streamlines. 

 Let us consider, for instance, the point marked with the number 7. 

 We assume that a puff of smoke has been emitted above this location 

 at 7 a. m. The puff is carried downstream with the general wind 

 velocity along the trajectory. At the same time, the turbulent wind 

 fluctuations disperse the smoke puff to sizes illustrated by the elliptical 

 figures for successive hours following the emission. A smoke puff 

 emitted at the same point but at another time may follow a different 

 trajectory and have a different rate of dispersion. By adding the 

 effects of a large number of similar smoke puffs emitted at the same 



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