FIG. 5.8.5 Effective stack height (H), with dispersion beginning at a theoretical point above the stack. (Adapted with permission from American Society for Mechanical Engineers [ASME] Air Pollution Control Div., 1973, Recommended guide for the prediction of the dispersion of airborne effluents, 2d ed., New York: ASME.)
in describing a plume as it disperses, they increase with time and distance traveled. The rates of growth for oy and oz depend upon meteorological conditions. Sigma y is generally larger than oz, since no stratification obstacles are in the y (horizontal) direction. Sigma y and oz are strongly influenced by heat convection and mechanical turbulence, and, as these forces become more pronounced, the sigmas increase more quickly.
As a standard deviation, oy and oz characterize the broadness or sharpness of the normal distribution of pollutants within the plume. As both sigmas increase, the concentration value of a pollutant at the plume centerline decreases. However, the total amount of the pollutant in the plume remains the same; it is merely spread out over a wider range and thus, the concentration changes. Approximately two thirds of the plume is found between plus or minus one sigma, while 95% of the plume is found between plus or minus 2 sigma as shown in Figure 5.8.2. The plume edge is considered to be that concentration that is one-tenth the concentration of the centerline.
Sigma y and oz are generally determined from equations derived using empirical data obtained by Briggs (1969) and McElroy and Pooler (1968) and research performed by Bowne (1974). Figures 5.8.6 and 5.8.7 show plots of these curves. These plots were developed from the observed dispersion of a tracer gas over open, level ter-
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