ENV 6146:Atmospheric Dispersion Modeling
March, 2010
Presented by:
Adeeba Abdul Raheem
Web resources to download SCREEN3 and
Modeling Guides:
A: Software
EPA (Dos based program):
Http://www.Epa.Gov/ttn/scram/dispersion_screening.Htm
User friendly interface:
Http://www.Weblakes.Com/products/screen/index.Html
B:Guidance for Air Dispersion Modeling
http://www.epa.gov/ttn/scram/userg/screen/screen3d.pdf
http://www.valleyair.org/busind/pto/tox_resources/Mod
eling%20Guidance%20W_O%20Pic.pdf/
http://www.colorado.gov/airquality/permits/screen.pdf
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SCREEN3 Tutorial
 Introduction
 Overview
of SCREEN3
 Getting started
 Input data
 Different interfaces
 Model description
 Examples
 Conclusions
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 SCREEN3
is a steady-state Gaussian plume
model which uses worst-case meteorological
data to predict ambient pollutant
concentrations resulting from single continuous
emission sources
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 SCREEN3
is the current regulatory screening
model for air permitting applications.
 The
original SCREEN model was released by EPA
in 1988
 Based
on the same steady-state Gaussian plume
algorithms as ISC3
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SCREEN3 can perform all the single source short-term
calculations including:
Estimating max. ground-level concentrations
 Incorporating the effects of building downwash
 Estimating concentrations in the cavity
recirculation zone.
 Estimating concentrations due to inversion break-up
and shoreline fumigation.


Determining plume rise for flare releases.
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http://www.trainex.org/web_courses/subpart_x/Encyclopedia%20X%20pdf%20files/Environmental%20Assessment%20pdf%20files/AirDispersionEmissionModelingX.pdf
http://www.epa.gov/ttn/fera/data/risk/vol_1/chapter_09.pdf
 Convert
all lengths and distances to meters
 Convert temperatures to degrees Kelvin
 Identify building contributions to air dispersion
(stack emissions)
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To perform a modeling study using SCREEN3, data
for the following input requirements must be
supplied:
 Source Type (Point, Flare, Area or Volume)
 Physical Source and Emissions Characteristics
 Meteorology: SCREEN3 can consider all
conditions, or a specific stability class and wind
speed can be provided.
 Building Downwash: If this option is used then
building dimensions (height, length and
width)must be specified.
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 Full:
complete set of stability - wind speed
combinations examined for worst case scenario at
each downwind location
 Stability
class: worst case scenarios for
predetermined wind speeds
 Stability
class - wind speed combination:
calculations reported for only the combination
specified by user
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 Inversion
break-up - pollutant release into the
radiation inversion layer moves horizontally with
little dispersion due to the strong stability of the
inversion layer
 Shoreline
fumigation (sources within 3000 m of a
large body of water)
 If
a source with a tall stack (greater than 65m) is
located in a coastal region, then the effects of coastal
(or shoreline) fumigation may be significant
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Point sources are typically used when modeling
releases from sources like stacks and isolated vents.
Input requirements for point sources include:
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 Emission
rate (g/s)
 Stack Height (m)
 Shortest distance to property line
 Stack velocity (or volumetric airflow)
 Stack gas temperature (K)
 Stack Inside Diameter
 Building Height, Length, Width
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Area sources are used to model low level or ground
level releases where releases occur over an area
(e.g., landfills, storage piles, slag dumps, and lagoons).
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 Emission
Rate per unit area (g/(s-m2))
 Source Release Height
 Larger Side Length of Rectangular Area (m)
 Smaller Side Length of Rectangular Area (m)
 Receptor Height Above Ground (m or ft.): This
may be used to model impacts at “flagpole”
receptors. The default value is assumed to be 0.0 m
(i.e., ground-level receptors)
 Wind Direction
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Volume source is used to model releases from a
variety of industrial sources, such as building roof
monitors, fugitive leaks from an industrial facility,
multiple vents, and conveyor belts.
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 Emission
Rate in grams per second (g/s).
 Source Release Height above ground surface
 Initial Lateral Dimension(m)
 Initial Vertical Dimension(m)
 Receptor Height Above Ground [m or ft]
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Flare sources are used as control devices for a
variety of sources. SCREEN3 supports flares directly
through its flare source type.
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 Emission
Rate in grams per second (g/s).
 Flare Stack Height
 Total Heat Release Rate in calories per second
(cal/s) for the flare.
 Receptor Height Above Ground
 EPA’s SCREEN model assumes
stack gas exit velocity (Vs) = 20m/s,
stack gas exit temperature (Ts) of 1,273K
calculates an effective stack diameter based on the
heat release rate.
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
Buildings and other structures near a relatively short stack can
have a substantial effect on plume transport and dispersion, and on
the resulting ground-level concentrations that are observed.
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
Building downwash can occur when
HStack= HS< Hb + 1.5L
HStack= Height of Stack
Hb = Height of Building
L = lesser of Hb or PBW
PBW = Maximum
Projected Building Width

Screen model will do this calculation when the
building downwash option is used. If HS> Hb +
1.5L, then building downwash will not be shown in
SCREEN results
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 Building
Height(m)
 Minimum Horizontal Building Dimension(m)
 Maximum Horizontal Building Dimension (m)
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 An
alternative mixing height algorithm (Brode,
1991)
 Optional
input of an anemometer height in
place of the default height of 10 meters.
 An
alternative building cavity algorithm
(Schulman and Scire, 1993)
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
The alternative mixing height is determined by using the
maximum of a predetermined mixing height or a value
adjusted slightly higher than the plume height, whichever
is greater.
Selection of this algorithm results
in concentrations that are generally
more conservative than output
from the ISCST3 model.

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 The
optional input of an anemometer height in
place of the default height(10 m) affects the stack
top wind speeds for Choice of Meteorology
selections 1 and 2
 For
Choice of Meteorology selection 3, the user is
prompted to enter a 10 meter wind speed which is
unaffected by any optionally entered anemometer
height
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 The
published concentration results using this
algorithm model the sampled wind tunnel test
concentrations better than the regulatory algorithm
for the range selected.
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 The
complex terrain algorithms in SCREEN3 are for
point and flare sources, not area/volume sources.
 It
is usually recommended that the receptor height be
set to 0 meters (e.g., ground-level)
 In
simple terrain areas, SCREEN3 calculates 1-hour
concentration estimates. In complex terrain, the model
provides 24-hour concentration values.
(Conversion tables will be provided in the class)
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
Automated Distances Option:
It gives the user the option of using a preselected array of 50 distances
ranging from 100 m out to 50,000 m (50 km) using the following
increments

Discrete Distances Option:
The Discrete Distances option allows the user to find the maximum
impact at specific locations of interest, such as nearby residences,
hospitals, or schools.
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