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provided in Compilation of Air Pollutant Emission Factors if sampling is not possible; however,
threshold friction velocity is not given for either sand or salt. A default figure of 1.02 m/s,
measured for overburden material at a coal mine, may be used if no other source of information
is available.
H3.5.4.7 Surface Roughness Height of Stored Material (cm)
The surface roughness height is a measure of the resistance to wind flow near the surface of the
material caused by unevenness or roughness of the material. For some materials, default
parameters based on material type are provided in Section 13.2.5 of Volume I of Compilation of
Air Pollutant Emission Factors. As with threshold friction velocity, surface roughness height is
not explicitly given for sand or salt. A default figure of 0.3 cm may be used, based on a measured
value for overburden material at a coal mine.
H-30
H3.5.4.8 Wind Speed
Required information is the “fastest mile” of wind recorded daily for the time period being
investigated. The fastest mile is the highest measured wind speed, expressed in miles per hour, at
which air is measured by an anemometer to travel one mile. This data is available in electronic
form in the Local Climatological Data Summaries (LCD’s), provided by the National Climatic
Data Center (Reference 29) for most airport weather stations.
Appendix I: Dispersion Methodology
I-2
I-3
Appendix I: Dispersion Methodology
CONTENTS
I1 BACKGROUND______________________________________________________5
I2 DATA INPUTS_______________________________________________________8
I2.1 Emission Sources ________________________________________________________ 8
I2.1.1 Source Type _________________________________________________________________9
I2.1.2 Emissions of Each Pollutant _____________________________________________________9
I2.1.3 Location and Height of Emission Sources __________________________________________9
I2.1.4 Stack Characteristics __________________________________________________________9
I2.1.5 Surface Roughness Length _____________________________________________________10
I2.2 Meteorology ___________________________________________________________ 10
I2.2.1 Wind Speed and Direction _____________________________________________________11
I2.2.2 Atmospheric Stability_________________________________________________________11
I2.2.3 Mixing Layer Height _________________________________________________________12
I2.3 Topography ___________________________________________________________ 12
I2.4 Receptors _____________________________________________________________ 13
LIST OF TABLES
Table I-1: Surface Roughness Lengths for Various Land Uses __________________________________10
Table I-2: Key To P-G Stability Categories _________________________________________________12
LIST OF FIGURES
Figure I-1: Inputs and Outputs From Dispersion Modeling _____________________________________5
Figure I-2: Coordinate System Showing Gaussian Distributions in the Horizontal and Vertical Planes ___7
LIST OF EQUATIONS
Equation I-1: Gaussian Approximation _____________________________________________________6
Equation I-2: Gaussian Approximation (Receptors at Ground Level) ______________________________7
I-4
I-5
Appendix I: Dispersion Methodology
I1 BACKGROUND
The methodology used in dispersion modeling is quite different from the emissions inventory
methodologies given in this document. Whereas the latter achieve their results using information
specific to the sources, dispersion models provide a consideration of the context of the emissions,
including the atmosphere, topography, and location of sensitive areas, as illustrated in Figure I-1.
Accordingly, the computational requirements for dispersion modeling are much greater than for
emission inventory modeling. In practice, however, computer-based dispersion models act as a
sort of “black box”; as long as data inputs are properly specified, the dispersion model performs
the necessary calculations and produces a summary of pollutant concentrations at each receptor.
However, the user of a dispersion model should have an understanding of the basic modeling
concepts and limitations prior to using the model in regulatory applications.
This section provides an overview of the procedures in the dispersion modeling process as well
as a discussion of the mathematics of dispersion modeling.
Emission Source Information Meteorology
Topography Receptors
Atmospheric Concentration
at Receptors
Figure I-1: Inputs and Outputs From Dispersion Modeling
The first step in dispersion modeling is to compile detailed information on the source or group of
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