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used instead of manually following the Compilation of Air Pollutant Emission Factors
methodology because the program is much easier to use and employs the same EPArecommended
methodology for calculating storage tank emissions.
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WIND - EPA has developed the WIND computer model for estimating the wind erosion
emissions from material piles based on the methodologies described in Section 13.2.5 of Volume
I of Compilation of Air Pollutant Emission Factors. WIND calculates total particulate emissions
from a single storage pile per inputs provided by the user.
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4. DISPERSION ASSESSMENT
4.1 Introduction
The intent of a dispersion analysis is to assess the air pollutant concentrations at or near the
airport or air base resulting from the emissions inventoried in the emission assessment. These
pollutant concentrations are calculated to determine whether emissions from the site result in
unacceptably high air pollution levels downwind. This section provides a discussion of the
dispersion assessment.
Dispersion modeling has become an important part of the air permitting process. Under CAA, the
proposed installation or modification of a major stationary emission source now requires
dispersion modeling to show the effects of the proposed action on a community’s air quality. The
pollutant concentrations computed by a dispersion model are compared with the NAAQS or
other relevant air quality standards to determine whether or not a source of emissions is likely to
result in unacceptably high pollutant levels.
The dispersion model develops a mathematical approximation of future pollution levels resulting
from aircraft or airport / air base actions. The input parameters may include source emissions,
meteorological conditions, topography, etc. The meteorological parameters are used to find the
direction of pollutant transport, the receptors which will be affected and the most probable and
worst pollutant concentrations which can be expected at these receptors.
Several key pollutants are commonly considered in dispersion modeling at airports and air bases
including CO, PM-10, NOx, and possibly SO2 and HC.
CO emissions at airports and air bases arise from aircraft, ground vehicles and stationary
combustion sources. Although these sources are widespread, ambient CO concentrations may be
high in locations where vehicles slow down and idle, such as roadway intersections. Dispersion
analysis of areas surrounding each intersection of concern, both on and off the airport or air base,
is commonly performed to determine whether an increase in vehicular traffic can result in
congestion that produces locally high concentrations of CO that violate the NAAQS. Such
dispersion analysis of intersections is performed using computer models that take into account
the number and type of vehicles, their operating mode, their movement, and the length of delay
(Reference 74).
Particulate emissions arise from aircraft, ground vehicles, stationary combustion sources and
fugitive dust sources such as sand piles. However, these particulate emissions are rarely at levels
that would approach the NAAQS unless special circumstances, such as a high background
particulate level, were present. In such cases, PM-10 can be included in the dispersion model.
A NAAQS exists for NO2, which is the primary component of NOx emitted from combustion
sources. The emissions inventory described in Section Three of this document provides
emissions of NOx that may be included in the dispersion model. Significant NOx sources at
airports and air bases are: aircraft, ground vehicles and some stationary combustion sources.
Because minor components of NOx (e.g., NO and N2O) are fairly rapidly converted to NO2 in the
atmosphere, NOx emissions are reported on the basis of the molecular weight of NO2 (Reference
71), and may be assumed to be entirely composed of NO2 unless more detailed test data is
available. In most cases, NO2 levels above the air quality standards are not expected to result
from airport emissions.
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Sulfur dioxide emissions at airports and air bases come from the low levels of sulfur in jet fuel,
aviation gasoline, diesel and stationary combustion source fuels. However the SO2 levels
produced are very low and are not likely to result in violations of the NAAQS in the area
surrounding the airport or air base. SO2 may be included in the dispersion modeling if necessary.
There is no NAAQS for hydrocarbons, as a result HC is not commonly included in site-specific
dispersion modeling. However, HC and NOx in the atmosphere are precursors to the formation of
ozone, which does have an NAAQS standard. Ozone is typically not included in the airport or air
base dispersion models because its formation in the atmosphere is difficult to model on a local
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