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approximation is a general-purpose dispersion equation that has been modified for use on mobile
source emissions as well as stack emissions.
I2 DATA INPUTS
This section outlines various data inputs required for a dispersion modeling run. These include
characteristics of each emission source, meteorological parameters such as wind speed and
direction, local topography, and receptor locations.
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I2.1 Emission Sources
Dispersion models require several pieces of information about each emission source being
included in the model, including:
·  Source type.
·  Emissions of each pollutant for each time period being investigated.
Depending on the source type, some additional information about the sources is likely to be
required to model dispersion of pollutant emissions. These additional information requirements
include:
·  Source layout, including location and height.
·  For stack emissions, parameters including stack gas temperature, exit velocity, and inside
stack diameter.
·  For mobile sources, surface roughness length of surrounding area.
The data requirements and likely sources of information are listed below.
I2.1.1 Source Type
Dispersion models for complex sites such as airports provide categories for each of the source
types likely to be found on the site. These categories are necessary because of the differences in
the way that the sources are represented in the dispersion model. For example, stacks are
modeled as stationary point sources with a rising plume whereas roadways may be modeled as a
line source with no plume rise but with consideration of the effect of surface roughness on
dispersion. Therefore, specification of the source category type tells the computer how to model
a given source and which additional pieces of information are required.
I2.1.2 Emissions of Each Pollutant
The main goal of the source inventory is to estimate the emissions of pollutants from each
source. The pollutants of concern at airports are generally CO, NOx, SO2, PM-10, and HC. For
some sources, such as aircraft engines, emission factors are available for all of these. Other
sources emit only one pollutant. For example, particulate matter is the only air pollution problem
associated with sand or salt piles. The emissions inventory typically provides an average rate of
emissions of each pollutant for each source.
I2.1.3 Location and Height of Emission Sources
The physical layout of the emission sources is required by a dispersion model because the goal is
to provide a pollutant concentration in air that varies with location. To this end, all sources
should be located on a master grid that is used for the dispersion model. The height of the
emission point is also required for each source, as it is used in the Gaussian equation to calculate
ground-level concentrations downwind. Estimates of source location and height should be readily
available from the airport operator or base operations section.
I2.1.4 Stack Characteristics
Stack emissions, which make up a very small percentage of the overall emissions at airports,
require an estimate of plume rise before the Gaussian approximation may be applied. Plume rise
is a result of thermal buoyancy of the stack gas and the vertical momentum of the gas as it leaves
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the stack. Three parameters are required to calculate plume rise that are not otherwise collected
in the emissions inventory: stack gas temperature, vertical velocity of the gas exiting the stack,
and the inside diameter of the stack. These may be obtained by direct sampling of the stack.
I2.1.5 Surface Roughness Length
The surface roughness length of the area surrounding the roadway is often a required input
parameter in the dispersion modeling of emissions from mobile source. The surface roughness
length, in meters or centimeters, is a measure of the near-surface wind resistance. Table I-1
provides typical surface roughness lengths for a variety of land uses.
Surface Type Surface Roughness Length (cm)
Smooth desert
Grass (4 cm)
Grass (5-6 cm)
Alfalfa (15.2 cm)
Grass (60-70 cm)
Wheat (60 cm)
Corn (220 cm)
Citrus Orchard
Fir Forest
City Land Use:
- Apartment residential
- Central business district
- Office
- Park
- Single family residential
0.03
0.14
0.75
2.72
11.40
22.00
74.00
198.00
283.00
370.00
321.00
175.00
127.00
108.00
Table I-1: Surface Roughness Lengths for Various Land Uses1
I2.2 Meteorology
Dispersion of pollutants in the atmosphere is largely dependent upon meteorological conditions,
　
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