Page 369 - Fundamentals of Air Pollution 3E

324 20. Air Pollution Modeling and Prediction

Fig. 20-1. Concentration of an air pollutant at the point of maximum ground-level concen-
tration as a function of wind speed and Pasquill stability category (A-F).

do not change rapidly with wind speed. For E and F stabilities the concentra-
1
tions are nearly the same (assuming that A0/Az is 0.02 K m" for E stability
and 0.035 for F stability), but are considerably less than for the unstable
and neutral cases.

II. MODELING TECHNIQUES
Gaussian techniques, discussed in Chapter 19, are reasonable for esti-
mates of concentrations of nonreative pollutants within 20 km of point
sources. It is preferable to utilize on-site wind fluctuation measurements
to estimate the horizontal and vertical spreading of a pollutant plume
released from a point source.
In addition to the Gaussian modeling techniques already discussed, four
other methods will be considered.

A. Box Model

Models which assume uniform mixing throughout the volume of a three-
dimensional box are useful for estimating concentrations, especially for
first approximations. For steady-state emission and atmospheric conditions,
with no upwind background concentrations, the concentration is given by

where x is the steady-state concentration, Ax is the distance over which
the emissions take place, q a is the area emission rate, z i is the mixing height,
and u is the mean wind speed through the vertical extent of the box.
When there is an upwind background concentration Xb and the mixing
height is rising with time into a layer aloft having an average concentration
of x a> the equation of continuity is