Page 351 - Fundamentals of Air Pollution 3E

306 19. Transport and Dispersion of Air Pollutants

which verifies that, to the nearest 20 m, the maximum is at 0.28 km. Note
that the concentration obtained from this equation is the same as that
obtained from the approximation equation for the maximum.
Buoyancy-induced dispersion, which is caused near the source due to
the rapid expansion of the plume during the rapid rise of the thermally
buoyant plume after its release from the point of discharge, should also be
included for buoyant releases (15). The effective vertical dispersion cr 2e is
found from

where AH, the plume rise, and cr 2 are evaluated at the distance x from the
source. Beyond the distance to the final rise, AH is a constant. At shorter
distances, it is evaluated for the gradually rising plume (see Chapter 20).
Since in the initial growth phases of a buoyant plume the plume is nearly
symmetrical about its centerline, the buoyancy-induced dispersion in the
crosswind (horizontal) direction is assumed to be equal to that in the verti-
cal. Thus, the effective horizontal dispersion cr ye is found from

The Gaussian plume equations are then used by substituting the value of
<r ye for cr y and <r ze for cr z.

IV. DISPERSION INSTRUMENTATION

A. Measurements near the Surface
Near-surface (within 10 m of the ground) meteorological instrumentation
always includes wind measurements and should include turbulence mea-
surements as well. Such measurements can be made at 10 m above ground
by using a guyed tower. A cup anemometer and wind vane (Fig. 19- 7),
or a vane with a propeller speed sensor mounted in front (Fig. 19-8), can
be the basic wind system. The wind sensor should have a threshold starting
1
1
speed of less than 0.5 m s" , an accuracy of 0.2 m s^ or 5%, and a distance
constant of less than 5 m for proper response. The primary quantity needed
is the hourly average wind speed. A representative value may be obtained
from values taken each minute, although values taken at intervals of
1-5 sec are better.
The vane can be used for both average wind direction and the fluctuation
statistic cr a, both determined over hourly intervals. The vane should have
a distance constant of less than 5 m and a damping ratio greater than or
equal to 0.4 to have a proper response. Relative accuracy should be 1° and
absolute accuracy should be 5°. In order to estimate cr a accurately, the
direction should be sampled at intervals of 1-5 sec. This can best be accom-