Page 383 - Fundamentals of Air Pollution 3E

338 20. Air Pollution Modeling and Prediction

E. Utilization of Models
For the air quality manager to place model estimates in the proper per-
spective to aid in making decisions, it is becoming increasingly important
to place error bounds about model estimates. In order to do this effectively/
a history of model performance under circumstances similar to those of
common model use must be established for the various models. It is antici-
pated that performance standards will eventually be set for models.

F. Modeling to Meet Regulatory Requirements
In the United States if the anticipated air pollution impact is sufficiently
large, modeling has been a requirement for new sources in order to obtain
a permit to construct. The modeling is conducted following guidance issued
by the U.S. Environmental Protection Agency (56, 57). The meeting of
all requirements is examined on a pollutant-by-pollutant basis. Using the
assumptions of a design that will meet all emission requirements, the impact
of the new source, which includes all new sources and changes to existing
sources at this facility, is modeled to determine pollutant impact. This is
usually done using a screening-type model such as SCREEN (58). The
impacts are compared to the modeling significance levels for this pollutant
for various averaging times. These levels are generally about 1/50 of the
National Air Quality Standards. If the impact is less than the significance
level, the permit can usually be obtained without additional modeling. If
the impact is larger than the significance level, a radius is defined which
is the greatest distance to the point at which the impact falls to the signifi-
cance level. Using this radius, a circle is defined which is the area of
significance for this new facility. All sources (not only this facility, but all
others) emitting this pollutant are modeled to compare anticipated impact
with the National Ambient Air Quality Standards and with the Prevention
of Significant Deterioration increments.
The implementation of the Clean Air Act Amendments of 1990 (CAAA90)
will require not only permitting for new sources but also permits for existing
facilities. There is also a requirement for reexamination of these permits at
intervals not longer than 5 years. The permit programs are set up and
administered by the states under the review and guidance of EPA. These
programs include the collection of permit fees sufficient to support the
program. Implementation of CAAA90 will require the application of Maxi-
mum Achievable Control Technology (MACT) on an industry-by-industry
basis as specified by EPA. Following the use of MACT, a calculation will
be made of the residual risk due to the remaining pollutant emissions.
This will be accomplished using air quality dispersion modeling that is
interpreted using risk factors derived from health information with the
inclusion of appropriate safety factors.