Page 398 - Fundamentals of Air Pollution 3E

352 21. Air Pollution Climatology

TABLE 21-5
Maximum Hours of Each Stability in One Direction

Nashville Knoxville
Number Total for Number Total for
Stability of hours direction Direction of hours direction Direction
(220 112
A 7 184 22 8
|364 |20
B 25 252 16 38 616 6
C 79 736 36 81 616 6
D 324 736 36 332 616 6
E 123 736 36 82 431 22
F 118 736 36 79 342 2.5
(G) 66 736 36 45 174 31

It is noted that backing the Knoxville frequencies by 60° would result in
the maxima occurring together. This is tabulated in the fifth column of
Table 21-6. The difference between Nashville and this artificial frequency
is then obtained (the sixth column). The sum of the absolute values of the
differences expressed as a percentage of the total is 30.2; the differences have
been reduced by more than half. This does not prove that the frequencies are
similar except that the winds at Knoxville are channeled by the major
topographical features in the vicinity, but it does imply that this is an
explanation.
Certainly the directional frequencies of these sites cannot be considered
as representative of each other. However, this comparison would seem
to indicate that the percentage of the stability categories may be more
conservative over distance than the direction frequencies.

III. FREQUENCY OF ATMOSPHERIC STAGNATIONS

At times when the surface pressure gradient is weak, resulting in light
winds in the atmosphere's lowest layers, and there is a closed high-pressure
system aloft, there is potential for the buildup of air pollutant concentra-
tions. This is especially true if the system is slow-moving so that light
winds remain in the same vicinity for several days. With light winds there
will be little dilution of pollutants at the source and not much advection
of the polluted air away from source areas.
Korshover (3) studied stagnating anticyclones in the eastern United States
over two periods totaling 30 years. He found that for stagnation to occur
for 4 days or longer, the high-pressure system had to have a warm core.
Korshover's criteria included a wind speed of 15 knots or less, no frontal