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within the stable layer. The height of the mixing layer generally ranges between 1,000 ft and
4,000 ft depending upon weather conditions and time of day. The boundary between the two
layers is evidenced by the bottom of cumulus clouds.
The presence of a stable layer above the mixing layer has the effect of restricting vertical
diffusion of pollutants. This “lidding” effect requires a modification of the simple Gaussian
approximation for it to remain accurate at distances greater than several kilometers downwind of
the emission source. For applications such as airports, however, where the major pollutant of
concern (CO) is not likely to be transported at high concentration very far from the source,
mixing depth effects on downwind pollutant concentrations may be ignored without too much
loss of accuracy.
I2.3 Topography
Many dispersion models require topographic information for the area being modeled. Digitized
topographic information for all sites in the U.S. is available from the United States Geological
Survey and other sources, but this data must be properly formatted and aligned with the grid used
to determine the layout of the site.
2 Meteorologists divide the sky into eight sections to determine the degree of cloud cover. If three or fewer
sections contain clouds, the sky is considered clear: if four or more section have clouds, the sky is
considered cloudy.
3 Category D should be used for overcast conditions during day or night.
4 Source: Turner's "Workbook of Atmospheric Dispersion Estimates," in EPA's APTI Course 423:
Dispersion of Air Pollution — Theory and Model Application, Selected Readings Packet, p. 1-6.
I-13
The simple Gaussian approximation given above is not reliable in areas of complex terrain (i.e.,
areas in which the terrain rises above the effective emission height), and may not be valid in
intermediate terrain (i.e., areas in which the terrain rises above the stack height but not the
effective emission height). Models have been developed for use in complex and intermediate
terrain, but the focus of these models is stationary point source emissions rather than mobile
sources, which are the greatest emission sources at airports and air bases.
Terrain in the vicinity of airports is usually quite flat because of the requirement for a level
runway, approach, and climb-out area. Dispersion models can take advantage of this property of
airport locations to make the simplifying assumption that the terrain is flat. This assumption
allows the model to use the Gaussian approximation with no modifications that would increase
the computational requirement.
I2.4 Receptors
Receptors are defined by the user as those areas in which pollutant concentrations in air are to be
calculated. If an overall view of pollutant concentration on and off the site is desired, then a grid
of receptors should be defined. For many applications, however, only those locations defined as
“sensitive” (i.e., where the public is likely to come into contact with emissions) may be modeled
in order to reduce the computational requirement. For a complex emissions scenario such as an
airport, reducing the number of receptors may be necessary because each receptor defined may
add hours to the computation time.
I-14
Appendix J: State Indirect Source Review
Regulations
J-2
J-3
Appendix J: State Indirect Source Review Regulations
CONTENTS
J1. OVERVIEW ________________________________________________________5
LIST OF TABLES
Table J-1: State Threshold Criteria for Indirect Source Review __________________________________6
J-4
J-5
Appendix J: State Indirect Source Review Regulations
J1. OVERVIEW
Several states have Indirect Source Review (ISR) regulations. ISR regulations establish threshold
levels above which an air quality review is required. ISR thresholds vary by state, and in a few
states, may only apply to a small localized area. For those ISR regulations applicable to airports
and air bases, the thresholds usually are based on parking lot capacity, highway (annual daily or
hourly) traffic volume, and/or airport annual passengers or aircraft operations. Airports and air
bases below the threshold levels would be exempt from the review.
Table J-1 lists ISR thresholds for states that have ISR regulations at the time of this writing.
Individual states should be contacted regarding the specifics of their ISR regulations.
J-6
Table J-1: State Threshold Criteria for Indirect Source Review1
State Parking Lots Highways2 Airports
California
(North Coast
Air Basin3)
1,000 new spaces or increase
of 1,000 new spaces
20,000 ADT 50,000 annual aircraft operations
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