Page 283 - Fundamentals of Air Pollution

IV. Modeling 239

Hazardous materials may enter the air pathway by evaporation from
leaking containers and release of these gases through fissures and spaces
between soil particles. Another pathway may release hazardous substances
to the air if they are water soluble. Then ground water passing leaking
containers may carry substances to or near the surface, where they may
be released to the air near the original source or at locations at significant
distances.

A. Estimates of Long-Term Impact

If the foregoing problems of emissions estimation can be overcome, or
if it is possible to make estimates of maximum possible and minimum
possible emissions, then it is quite easy to make estimates of resulting long-
term impact on the surrounding area. The representation of the emissions
may be through consideration of an area source or area sources; or if vent
pipes are releasing material or flaring the gases, point sources should be
used.
A single finite line source method is used to simulate area sources in the
long-term (seasons to years) model ISCLT (3). Although this method has
been criticized as frequently underestimating concentrations for receptors
that are quite close to the area source (within two or three side lengths
away), this model is usually used for these estimates. In addition to the
long-term estimate of emission rate for each constituent to be modeled, the
ISCLT model requires meteorological data in the form of a joint frequency
distribution of three parameters: wind direction (in 16 classes), wind speed
(in 6 classes), and Pasquill stability class (in 6 classes). As long as the
emissions can be considered relatively constant over the period of simula-
tion, the long-term estimates will represent mean concentrations over the
period represented by the meteorological data.

B. Estimates of Short-Term Impact during Remediation
If it is necessary to consider short-term (hours or days) impact, the model
PAL (4) will do a superior simulation of the area sources and a similar
simulation of any point sources as done by the ISCLT model.
In addition to short-term emission estimates, normally for hourly periods,
the meteorological data include hourly wind direction, wind speed, and
Pasquill stability class. Although of secondary importance, the hourly data
also include temperature (only important if buoyant plume rise needs to
be calculated from any sources) and mixing height.
The short-term model can then be used to estimate resulting concentra-
tions during specific periods or to estimate concentrations for suspected
adverse meteorological conditions, so that changes can be incorporated in