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scale, and because the effects of elevated ozone concentrations are generally felt on a regional
rather than a local level.
In summary, dispersion modeling at airports is usually concerned with calculating local CO
levels, and may be expanded to include PM-10, NOx, SO2 and HC if circumstances warrant their
inclusion.
Depending upon the goals of the dispersion modeling effort, one of two types of models may be
selected: screening models or refined models. Screening models use simplified emissions and
meteorological inputs to provide a snapshot of the likely worst-case air quality scenario. Refined
models require detailed input on emissions and climate to provide air quality estimates for a large
number of time periods, typically each hour of a given year. Refined models are often required
by regulatory agencies because they capture a wide range of meteorological and operating
conditions. Many computer models used for dispersion calculations can operate in both screening
and refined mode so only a single model is needed to perform both types of calculations.
For a detailed discussion of the dispersion methodology, inputs, and data sources, please see
Appendix I.
4.2 Dispersion Inputs
Inputs required for a dispersion modeling run include various characteristics of each emission
source, meteorological parameters, local topography and receptor locations.
4.2.1 Emissions Sources
Dispersion models require several pieces of information about each emission source being
included in the model. The output of a detailed emissions inventory should provide source type
and emissions of each pollutant for each time period being investigated.
4.2.2 Meteorology
Dispersion of pollutants in the atmosphere is largely dependent upon meteorological conditions
such as wind speed and direction, atmospheric stability and mixing depth. Wind speed and
direction are the most important parameters in the modeling of dispersion of pollutants in air.
Atmospheric stability is related to the turbulence of the atmosphere, and is determined by a
combination of wind speed, cloud cover and solar radiation. In unstable atmospheric conditions,
high turbulence and associated vertical mixing produce a peak ground-level pollutant
concentration near the emission source. Whereas in a stable atmospheric conditions, a low level
of vertical mixing results in low ground-level steady-state concentrations near the source. In most
cases, the most unstable atmospheric conditions occur during daylight hours, with low wind
speeds and high solar radiation. The most stable atmospheric conditions occur at night, during
times of low wind speeds and clear skies. Finally, the mixing layer height
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has the effect of restricting vertical diffusion of pollutants. Generally, the height of the mixing
layer ranges between 1,000 ft and 4,000 ft.
4.2.3 Topography
The terrain in the vicinity of airports and air bases is usually quite flat because of the requirement
for a level runway, approach and climbout area. Dispersion models can take advantage of this
property of airport and air base locations to make the simplifying assumption that the terrain is
flat. This assumption allows the model to use the Gaussian approximation without modifications
that would increase the computational requirement.
Some sources, such as training fires, stacks, and painting operations, produce emission sin the
form of a buoyant plume. These can have an impact on air quality much farther downwind than
the flat area surrounding an airport. In such cases, topography may play a role in altering the
downwind dispersion of the plume and may be included in the dispersion analysis. Dispersion
models designed for use in complex terrain (i.e., terrain that rises above the level of the plume) or
intermediate terrain (i.e., terrain that rises above the stack height but not above the plume
elevation) are available. For these models, an additional input of a digitized terrain grid file is
necessary.
4.2.4 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 site is desired, then a grid of
receptors should be defined. For many applications, however, only those location defined as
“sensitive” (e.g. 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 computational time.
4.3 Available Models
The following are EPA-preferred models, which are those models that do not require a rigorous
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