Page 91 - Fundamentals of Air Pollution 3E

62 5. The Philosophy of Air Pollution Control

for application on very short notice to prevent an impending episode from
becoming a disaster. Since an episode usually varies from a minimum of
about 36 hr to a maximum of 3 or 4 days, temporary controls on emissions
much more severe than are called for by the long-term strategic control
scenario must be implemented rapidly and maintained for the duration of
the episode. After the weather conditions that gave rise to the episode
have passed, these temporary episode controls can be relaxed and controls
can revert to those required for long-term strategic control.
The mechanisms by which a jurisdiction develops its air pollution control
strategies and episode control tactics are outlined in Fig. 5-1. Most of the
boxes in the figure have already been discussed—sources, pollutant emit-
ted, transport and diffusion, atmospheric chemistry, pollutant half-life, air
quality, and air pollution effects. To complete an analysis of the elements
of the air pollution system, it is necessary to explain the several boxes not
yet discussed.

II. EPISODE CONTROL

The distinguishing feature of an air pollution episode is its persistence
for several days, allowing continued buildup of pollution levels. Consider
the situation of the air pollution control officer who is expected to decide
when to use the stringent control restrictions required by the episode control
tactics scenario (Fig. 5-2 and Table 5-1). If these restrictions are imposed
and the episode does not mature, i.e., the weather improves and blows
away the pollution without allowing it to accumulate for another 24 hours
or more, the officer will have required for naught a very large expenditure
by the community and a serious disruption of the community's normal
activities. Also, part of the officer's credibility in the community will be
destroyed. If this happens more than once, the officer will be accused of
crying wolf, and when a real episode occurs the warnings will be unheeded.
If, however, the reverse situation occurs—i.e., the restrictions are not in-
voked and an episode does occur—there can be illness or possibly deaths
in the community that could have been averted.
In deciding whether or not to initiate episode emergency plans, the
control officer cannot rely solely on measurements from air quality monitor-
ing stations, because even if pollutant concentrations rise toward acute
levels over the preceding hours, these readings give no information on
whether they will rise or fall during the succeeding hours.
The only way to avert this dilemma is for the community to develop
and utilize its capability of forecasting the advent and persistence of the
stagnation conditions during which an episode occurs and its capability of
computing pollution concentration buildup under stagnation conditions.
The details of how these forecasts and computations are made are discussed