Factors To be Taken into account in dispersion modeling:
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Continuous Vs. “Puffs”
Pollutant concentrations
Pollutant temperature at release
Pollutant velocity at release
Wind speed (at point of release)
Wind direction
Surface roughness (season dependant – state of CT asks different parameters be used for snow
vs. non-snow)
Pasquill Stability Class
o Mechanical turbulence
 Structures
 Vegetation
 Wind shear
o Buoyant turbulence (heating or cooling or air near earths surface – creates heat fluxes
towards/away from surface. Strongly releated to time of day.)
Meteorological conditions
Scope (will a facility violate air quality at property boundary, or plume of radiation from Japan to
the United States)
Stack height
Averaging time
Stack tip downwash (pollutants in bottom of plume caught in low pressure created by stack)
Area terrain
Gaussian Modeling Assumptions:
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Continuous emissions
Conservation of mass
Steady state conditions
Dispersion well represented by Gaussian distribution
Gaussian models - Not applicable:
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Convective situations (vertical temperature gradient)
Ground level releases(turbulence not homogeneous due to presence of surface)
Typical Dispersion Modeling Programs for Compliance:
Typical inputs: stack height, stack top diameter, exit velocity, stack temperature, Pasquill stability class,
wind direction, wind speed, temperature, mixing height
Steps:
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Loop 1
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Loop 2
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Loop 3
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Downwind distance/crosswind distance of the receptor from the source determined
Pollutant concentration at receptor
Plume rise/effective plume height for the meteorological condition for the hour
Loop 2 executed for each hour taking appropriate meteorological conditions for each
hour (meteorological data can be run from a different model)
Typical outputs: highest and second highest concentrations for each receptor for averaging times
(including 1 hour, 8 hour, 24 hour). Also average concentrations over a period of simulation (usually
year) in tabular form
Programs available:
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AERMOD – recommended by EPA for compliance modeling purposes
AERSCREEN – screening version of AERMOD recommended by EPA. Gives absolute worst case
scenario concentrations assuming worst case meteorological conditions. You can use
AERSCREEN to get out of AERMOD modeling requirements.
SCREEN3 – screening model recommended by CT DEEP. (Note –recommended by CT DEEP in
guidance issued in 2009, prior to EPAs 2011 approval of AERSCREEN as an approved modeling
program)
Note – AERMOD, AERSCREEN, and SCREEN3 use the Gaussian distribution as a basis for
calculations
Other models:
L1023 – available with Atmospheric Dispersion Estimates by D. Bruce Turner as an introduction to air
dispersion modeling
GaussianPlume.m – can provide visual 3-D plume