.454/B-95-004
United States
Environmental Protection
Agency
Office of Air Quality
Planning and Standards
Research Triangle Park. NC 27711
EPA-454/B-95-004
September 1995
Air
~* EPA SCREENS User's Guide
ENVIRONMENTAL
v PROTECTION
} AGEfvSCY
DALLAS, TEXAS
LIBRARY
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EPA-454/B-95-004
SCREEN3 Model User's Guide
U.S. ENVIRONMENTAL PROTECTION AGENCY
Office of Air Quality Planning and Standards
Emissions, Monitoring, and Analysis Division
Research Triangle Park, North Carolina 27711
i
J September 1995
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DISCLAIMER
The information in this document has been reviewed in its
entirety by the U.S. Environmental Protection Agency (EPA), and
approved for publication as an EPA document. Any mention of
trade names, products, or services does not convey, and should
not be interpreted as conveying official EPA approval,
endorsement, or recommendation.
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PREFACE
The SCREEN3 Model User's Guide is an update to Appendix A of
"Screening Procedures for Estimating the Air Quality Impact of
Stationary Sources" (EPA, 1988), which was later revised and
published as a separate document (EPA, 1995a). The SCREENS model
includes several modifications and enhancements to the original
SCREEN model, including updates to the code to ensure consistency
with the dispersion algorithms in the Industrial Source Complex
(ISC3) model (EPA, 1995b). Also, three new non-regulatory
options were added to the code.
Although attempts are made to thoroughly check computer
programs with a wide variety of input data, errors are
occasionally found. Any suspected errors and technical questions
regarding the use of the SCREENS model should be directed to
Chief, Air Quality Modeling Group, OAQPS/EMAD, MD-14, Research
Triangle Park, NC 27711. Copies of the SCREENS model may be
obtained from the National Technical Information Service (NTIS),
U.S. Department of Commerce, 5285 Port Royal Road, Springfield,
VA 22161, telephone (703) 487-4650, or may be downloaded from the
Support Center for Regulatory Air Models (SCRAM) Bulletin Board
System (BBS). The SCRAM BBS may be accessed at (919) 541-5742.
Questions related to connecting to SCRAM should be directed to
the TTN Helpline at (919) 541-5384.
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ACKNOWLEDGEMENTS
• This report has been funded by the United States
Environmental Protection Agency (EPA) under contract 68D00124 to
Pacific Environmental Services, Inc. (PES).
Mr. Roger W. Erode, Pacific Environmental Services, Inc.
(PES), is the principal contributor to the SCREENS Model User's
Guide. In addition, this document was reviewed and commented
upon by Mr. Dennis G. Atkinson (EPA, OAQPS), Mr. James L. Dicke
(EPA, OAQPS), and Mr. John S. Irwin (EPA, OAQPS). Revisions to
the original SCREENS User's Guide were reviewed and commented
upon by Dennis G. Atkinson (EPA, OAQPS), the SCREENS Work
Assignment Manager and Mr. Peter A. Eckhoff (EPA, OAQPS).
IV
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CONTENTS
PREFACE iii
ACKNOWLEDGEMENTS iv
FIGURES vi
TABLES vii
1. INTRODUCTION 1
1.1 Overview of User's Guide 1
1.2 Purpose of SCREEN 1
1.3 What is needed in order to use SCREEN? 1
1.4 What will SCREEN do? 2
1.5 What will SCREEN not do? 2
1.6 How will SCREEN results compare to hand
calculations from the document? 3
1.7 How does SCREEN differ from PTPLU, PTMAX and PTDIS?
4
1.8 What changes have been incorporated into SCREEN? . . 4
1.9 What constitutes regulatory default in SCREEN? .... 5
2 . TUTORIAL 7
2.1 What is needed? 7
2.2 Setup on the PC 7
2.3 Executing the Model 7
2.4 Point Source Example 8
2.5 Flare Release Example 15
2.6 Area Source Example 16
2.7 Volume Source Example 17
2.8 Non-Regulatory Options 18
3. TECHNICAL DESCRIPTION 35
3.1 Basic Concepts of Dispersion Modeling 35
3.2 Worst Case Meteorological Conditions 36
3.3 Plume Rise for Point Sources 38
3.4 Dispersion Parameters 39
3.5 Buoyancy Induced Dispersion 39
3.6 Building Downwash 39
3.7 Fumigation 41
3.8 Complex Terrain 24-hour Screen 44
4. NOTE TO PROGRAMMERS 49
5. REFERENCES 51
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FIGURES
Figure Page
1. Point Source Options in SCREEN 19
2. SCREEN Point Source Example for Complex Terrain .... 20
3. SCREEN Point Source Example with Building Downwash ... 21
4. Flow Chart of Inputs and Outputs for SCREEN Point
Source 24
5. SCREEN Flare Release Example 26
6. Flow Chart of Inputs and Outputs for SCREEN Flare
Release 28
7. SCREEN Area Source Example 30
8. Flow Chart of Inputs and Outputs for SCREEN Area Source
32
9. SCREEN Volume Source Example 33
10. Flow Chart of Inputs and Outputs for SCREEN Volume
Source 34
VI
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TABLES
Page
1. Summary of Suggested Procedures for Estimating Initial
Lateral Dimensions (oyo) and Initial Vertical
Dimensions (a2C) for Volume Sources 18
2. Wind Speed and Stability Class Combinations Used by the
SCREEN Model 37
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1. INTRODUCTION
1.1 Overview of User's Guide
It will be easier to understand this user's guide and the
SCREEN model if you are already familiar with the "Screening
Procedures for Estimating the Air Quality Impact of Stationary
Sources" (EPA, 1995a).
This introduction should answer most of your general
questions about what the SCREEN model can (and cannot) do, and
explain its relationship to the Screening Procedures Document
(SPD) above.
Section 2 provides several examples of how to run the SCREEN
model and will also help the novice user get started. The point
source example provides the most detailed description and should
be read before the other examples. If you are already familiar
with personal computers and with the screening procedures, you
probably will not have much trouble simply running SCREEN and
"experimenting" with it. It runs interactively, and the prompts
should be self explanatory.
Section 3 provides background technical information as a
reference for those who want to know more about how SCREEN makes
certain calculations. The discussion in Section 3 is intended to
be as brief as possible, with reference to other documents for
more detailed descriptions.
1.2 Purpose of SCREEN
The SCREEN model was developed to provide an easy-to-use
method of obtaining pollutant concentration estimates based on
the screening procedures document. By taking advantage of the
rapid growth in the availability and use of personal computers
(PCs), the SCREEN model makes screening calculations accessible
to a wide range of users.
1.3 What is needed in order to use SCREEN?
SCREEN will run on an IBM-PC compatible personal computer
with at least 256K of RAM. You will need at least one 5 1/4 inch
double-sided, double-density (360K) or a 5 1/4 inch high density
(1.2MB) disk drive. The program will run with or without a math
coprocessor chip. Execution time will be greatly enhanced with a
math coprocessor chip present (about a factor of 5 in computer
time) and will also benefit from the use of a hard disk drive.
SCREEN will write a date and time to the output file, provided
that a real time clock is available.
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1.4 What will SCREEN do?
SCREEN runs interactively on the PC, meaning that the
program asks the user a series of questions in order to obtain
the necessary input data, and to determine which options to
exercise. SCREEN can perform all of the single source,
short-term calculations in the screening procedures document,
including estimating maximum ground-level concentrations and the
distance to the maximum (Step 4 of Section 4.2, SPD),
incorporating the effects of building downwash on the maximum
concentrations for both the near wake and far wake regions
(Section 4.5.1), estimating concentrations in the cavity
recirculation zone (Section 4.5.1), estimating concentrations due
to inversion break-up and shoreline fumigation (Section 4.5.3),
and determining plume rise for flare releases (Step l of Section
4.2). The model can incorporate the effects of simple elevated
terrain on maximum concentrations (Section 4.2), and can also
estimate 24-hour average concentrations due to plume impaction in
complex terrain using the VALLEY model 24-hour screening
procedure (Section 4.5.2). Simple area sources can be modeled
with SCREEN using a numerical integration approach. The SCREEN
model can also be used to model the effects of simple volume
sources using a virtual point source procedure. The area and
volume source algorithms are described in Volume II of the ISC
model user's guide (EPA, I995b). The SCREEN model can also
calculate the maximum concentration at any number of
user-specified distances in flat or elevated simple terrain
(Section 4.3), including distances out to 100km for long-range
transport (Section 4.5.6).
1.5 What will SCREEN not do?
SCREEN can not explicitly determine maximum impacts from
multiple sources, except for the procedure to handle multiple
nearby stacks by merging emissions into a single "representative"
stack (Section 2.2). The user is directed to the MPTER (Pierce
and Turner, 1980) or ISC (EPA, 1995b) models on EPA's Support
Center for Regulatory Air Models (SCRAM) Bulletin Board System
(BBS) to model short-term impacts for multiple sources. With the
exception of the 24-hour estimate for complex terrain impacts,
the results from SCREEN are estimated maximum 1-hour
concentrations. To handle longer period averages, the screening
procedures document contains recommended adjustment factors to
estimate concentrations out to 24 hours from the maximum 1-hour
value (Section 4.2, Step 5). For seasonal or annual averages,
Section 4.4 of the screening procedures document contains a
procedure using hand calculations, but the use of ISCLT (EPA,
1995b) or another long-term model on the SCRAM BBS is
recommended.
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1.6 How will SCREEN results compare to hand calculations from the
document?
The SCREEN model is based on the same modeling assumptions
that are incorporated into the screening procedures and
nomographs, and for many sources the results will be very
comparable, with estimated maximum concentrations differing by
less than about 5 percent across a range of source
characteristics. However, there are a few differences of which
the user should be aware. For some sources, particularly taller
sources with greater buoyancy, the differences in estimated
concentrations will be larger, with the hand calculation
exceeding the SCREEN model result by as much as 25 percent. These
differences are described in more detail below.
The SCREEN model can provide estimated concentrations for
distances less than 100 meters (down to one meter as in other
regulatory models), whereas the nomographs used in the hand
calculations are limited to distances greater than or equal to
100 meters. The SCREEN model is also not limited to plume
heights of 300 meters, whereas the nomographs are. In both
cases, caution should be used in interpreting results that are
outside the range of the nomographs.
In addition, SCREEN examines a full range of meteorological
conditions, including all stability classes and wind speeds (see
Section 3) to find maximum impacts, whereas to keep the hand
calculations tractable only a subset of meteorological conditions
(stability classes A, C, and E or F) likely to contribute to the
maximum concentration are examined. The use of a full set of
meteorological conditions is required in SCREEN because maximum
concentrations are also given as a function of distance, and
because A, C, and E or F stability may not be controlling for
sources with building downwash (not included in the hand
calculations). SCREEN explicitly calculates the effects of
multiple reflections of the plume off the elevated inversion and
off the ground when calculating concentrations under limited
mixing conditions. To account for these reflections, the hand
calculation screening procedure (Procedure (a) of Step 4 in
Section 4.2, SPD) increases the calculated maximum concentrations
for A stability by a factor ranging from 1.0 to 2.0. The factor
is intended to be a conservative estimate of the increase due to
limited mixing, and may be slightly higher (about 5 to 10
percent) than the increase obtained from SCREEN using the
multiple reflections, depending on the source. Also, SCREEN
handles the near neutral/high wind speed case [Procedure (b)] by
examining a range of wind speeds for stability class C and
selecting the maximum. In contrast, the hand calculations are
based on the maximum concentration estimated using stability
class C with a calculated critical wind speed and a 10 meter wind
speed of 10 m/s. This difference should result in differences in
maximum concentrations of less than about 5 percent for those
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sources where the near neutral/high wind speed case is
controlling.
The SCREEN model results also include the effects of
buoyancy-induced dispersion (BID), which are not accounted for by
the hand calculations (except for fumigation). The inclusion of
BID in SCREEN may either increase or decrease the estimated
concentrations, depending on the source and distance. For
sources with plume heights below the 300 meter limit of the hand
calculations, the effect of BID on estimated maximum
concentrations will usually be less than about ± 10 percent. For
elevated sources with relatively large buoyancy, the inclusion of
BID may be expected to decrease the estimated maximum
concentration by as much as 25 percent.
1.7 How does SCREEN differ from PTPLU, PTMAX and PTDIS?
The PT-series of models have been used in the past to obtain
results for certain screening procedures in Volume 10R (EPA,
1977). The SCREEN model is designed specifically as a
computerized implementation of the revised screening procedures,
and is much more complete than the earlier models, as described
above. The SCREEN model also requires less manual
"postprocessing" than the earlier models by listing the maximum
concentrations in the output. However, many of the algorithms in
SCREEN are the same as those contained in PTPLU-2.0 (Pierce,
1986). For the same source parameters and for given
meteorological conditions, the two models will give comparable
results. SCREEN also incorporates the option to estimate
concentrations at discrete user-specified distances, which was
available with PTDIS, but is not included in PTPLU.
1.8 What changes have been,,, incorporated into SCREEN?
The SCREENS model (dated 95250) includes one major revision
to the previous version of SCREEN (dated 92245). The finite line
segment algorithm for modeling area sources has been replaced
with a numerical integration algorithm based on the ISCST (EPA,
1995b) model. The new algorithm allows the user to model
rectangular area sources with aspect ratios (length/width) of up
to 10:1. The new algorithm also provides estimates of
concentration within the area source itself and also includes
three non-regulatory options.
Three new non-regulatory optional features have been added
to this model. The first feature is the inclusion of an
alternative mixing height algorithm (Erode, 1991). 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. Both the mixing
height and adjustment values to the plume height are based on
stability class. Selection of this algorithm results in
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concentrations that are generally more conservative than output
from the ISCST3 model.
i
The second feature allows the optional input of an
anemometer height in place of the default height of 10 meters.
This 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 entered a 10 meter wind speed which is
unaffected by any optionally entered anemometer height.
The third feature is the inclusion of an alternative
building cavity algorithm (Schulman and Scire, 1993). The
published concentration results using this algorithm model the
sampled wind tunnel test concentrations better than the
regulatory algorithm for the range selected.
The options are activated by adding flags and a value to the
line in the input file containing the source type input.
1.9 What constitutes the regulatory default in SCREEN?
Regulatory default consists of: l) entering the appropriate
input source characteristics, 2) selecting the appropriate
regulatory options (see Figure 1 ), and 3) then using the
recommended SCREEN defaults. Discussion of the SCREEN inputs,
regulatory options, and defaults can be found in Section 2 of
this document and throughout Section 4 of the Screening
Procedures document (See References). Regulatory default does
not include the use of any of the three new non-regulatory
options mentioned in Section 1.8.
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2. TUTORIAL
2.1 What is needed?
• IBM-PC compatible with at least 256K bytes of RAM, and a 5
1/4 inch double-sided, double-density or high density disk
drive.
• Diskette provided with SCREEN software (or files downloaded
from the SCRAM BBS).
• Hard or floppy disk drive (minimum of 1 MB memory
available).
• Math coprocessor chip (optional but recommended).
• Blank diskette for use in making a backup copy of software.
2.2 Setup on the PC
Using the DISKCOPY command of DOS (Disk Operating System) or
similar routine, make a backup copy of the SCREEN software. Store
the original SCREEN software diskette in a safe location. The
DISKCOPY command will also format the blank disk if needed.
The following set-up instructions assume that the user has a
system with a hard disk drive and the "pkunzip" decompression
program resident on the hard disk drive. The "pkunzip" program
can be obtained via the Support Center for Regulatory Air Models
(SCRAM) Bulletin Board System (BBS) by accessing the
archivers/dearchivers option under system utilities on the top
menu.
Insert the SCREEN diskette in floppy drive A: and enter the
following command at the DOS prompt from drive C: (either from
the root directory or a subdirectory):
PKUNZIP A:SCREENS
This command will decompress the six files from the SCREEN
diskette and place them on the hard disk. The hard disk will now
contain the executable file of SCREEN, called SCREEN3.EXE, as
well as the FORTRAN source files, SCREENS.FOR and MAIN.INC, an
example input file, EXAMPLE.DAT, an associated output file
EXAMPLE.OUT, and this document, the SCREENS Model User's Guide
(in WordPerfect 5.1 format), SCREENS.WPF.
2.3 Executing the Model
The SCREEN model is written as an interactive program for
the PC, as described earlier. Therefore, SCREEN is normally
executed by simply typing SCREEN from any drive and directory
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that contains the SCREEN3.EXE file, and responding to the prompts
provided by the program. However, a mechanism has been provided
to accommodate the fact that for some applications of SCREEN the
user might want to perform several runs for the same source
changing only one or a few input parameters. This mechanism
takes advantage of the fact that the Disk Operating System (DOS)
on PCs allows for the redirection of input that is normally
provided via the keyboard to be read from a file instead. As an
example, to run the sample problem provided on the disk one would
type:
SCREEN3
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identify the source type, and should enter 'P' or 'p1 for a point
source (the model will identify either upper or lower case
letters and will repeat the prompt until a valid response is
given).
For a point source, the user will be asked to provide the
following inputs:
Point Source Inputs
Emission rate (g/s)
Stack height (m)
Stack inside diameter (m)
Stack gas exit velocity (m/s) or
flow rate (ftj/min or nr/s)
Stack gas temperature (K)
Ambient temperature (K) (use default of 293K if
not known)
Receptor height above ground (may be used to
define flagpole receptors) (m)
Urban/rural option (U = urban, R = rural)
The SCREEN model uses free format to read the numerical
input data, with the exception of the exit velocity/flow rate
option. The default choice for this input is stack gas exit
velocity, which SCREEN will read as free format. However, if the
user precedes the input with the characters VF= in columns 1-3,
then SCREEN will interpret the input as flow rate in actual cubic
feet per minute (ACFM). Alternatively, if the user inputs the
characters VM= in columns 1-3, then SCREEN will interpret the
input as flow rate in nr/s. The user can input either upper or
lower case characters for VF and VM. The flow rate values are
then converted to exit velocity in m/s for use in the plume rise
equations, based on the diameter of the stack.
SCREEN allows for the selection of urban or rural dispersion
coefficients. The urban dispersion option is selected by
entering a 'U' (lower or upper case) in column 1, while the rural
dispersion option is selected by entering an 'R' (upper or lower
case) in column 1. For compatibility with the previous version
of the model, SCREEN also allows for an input of '!' to select
the urban option, or a '2' to select the rural option.
Determination of the applicability of urban or rural dispersion
is based upon land use or population density. In general, if 50
percent or more of an area 3 km around the source satisfies the
urban criteria (Auer, 1978), the site is deemed in an urban
setting. Of the two methods, the land use procedure is
considered more definitive. For more detailed guidance on land
use classification for urban and rural, refer to Section 8.2.8 of
the "Guideline On Air Quality Models (Revised)" and Supplement A
(EPA, 1987a).
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Figure 1 presents the order of regulatory options within the
SCREEN model for point sources and is annotated with the
corresponding sections from the screening procedures document. In
order to obtain results from SCREEN corresponding to the
procedures in Step 4 of Section 4.2, the user should select the
full meteorology option, the automated distance array option,
and, if applicable for the source, the simple elevated terrain
option. The simple elevated terrain option would be used if the
terrain rises above the stack base elevation but is less than the
height of the physical stack. These, as well as the other
options in Figure 1, are explained in more detail below. A
flagpole receptor is defined as any receptor which is located
above local ground level, e.g., to represent the roof or balcony
of a building.
2.4.1 Building Downwash Option
There are two downwash options available with this model, a
regulatory and a non-regulatory option. Both are discussed
below.
2.4.1.1 Regulatory Building Downwash Option
Following the basic input of source characteristics, a
SCREEN prompt asks if building downwash is to be considered, and
if so, prompts for building height, minimum horizontal dimension,
and maximum horizontal dimension, in meters, are presented. The
downwash screening procedure assumes that the building can be
approximated by a simple rectangular box. Wake effects are
included in any calculations made using the automated distance
array or discrete distance options (described below). Cavity
calculations are made for two building orientations - first with
the minimum horizontal building dimension alongwind, and second
with the maximum horizontal dimension alongwind. The cavity
calculations are summarized at the end of the distance-dependent
calculations. Refer to Section 3.6 for more details on the
building downwash cavity and wake screening procedure.
2.4.1.2 Non-Regulatory Building Downwash Option
A Schulman-Scire Building Downwash/Cavity option can be
selected along with two other non-regulatory options by entering
the appropriate flag, SS, on the line containing the source type
input. The program will later ask for the building height,
minimum horizontal dimension, and maximum horizontal dimension in
meters as is done for the regulatory cavity option. However, for
this option only, the program will ask for the position of the
source on the building with respect to the two building
orientations mentioned in 2.4.1.1. The response will need to be
in the form of a ratio of the stack distance from a building
centerline drawn perpendicular to the wind over the horizontal
dimension of the side of the building which is parallel to the
10
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wind. The program will show a figure on how to calculate the
correct ratio for a particular orientation.
2.4.2 Complex Terrain Option
The complex terrain option of SCREEN allows the user to
estimate impacts for cases where terrain elevations exceed stack
height. If the user elects this option, then SCREEN will
calculate and print out a final stable plume height and distance
to final rise for the VALLEY model 24-hour screening technique.
This technique assumes stability class F (E for urban) and a
stack height wind speed of 2.5 m/s. For complex terrain, maximum
impacts are expected to occur for plume impaction on the elevated
terrain under stable conditions. The user is therefore
instructed to enter minimum distances and terrain heights for
which impaction is likely, given the plume height calculated, and
taking into account complex terrain closer than the distance to
final rise. If the plume is at or below the terrain height for
the distance entered, then SCREEN will make a 24-hour
concentration estimate using the VALLEY screening technique. If
the terrain is above stack height but below plume centerline
height for the distance entered, then SCREEN will make a VALLEY
24-hour estimate (assuming E or F and 2.5 m/s), and also estimate
the maximum concentration across a full range of meteorological
conditions using simple terrain procedures with terrain "chopped
off" at physical stack height. The higher of the two estimates
is selected as controlling for that distance and terrain height
(both estimates are printed out for comparison). The simple
terrain estimate is adjusted to represent a 24-hour average by
multiplying by a factor of 0.4, while the VALLEY 24-hour estimate
incorporates the 0.25 factor used in the VALLEY model.
Calculations continue for each terrain height/distance
combination entered until a terrain height of zero is entered.
The user will then have the option to continue with simple
terrain calculations or to exit the program. It should be noted
that SCREEN will not consider building downwash effects in either
the VALLEY or the simple terrain component of the complex terrain
screening procedure, even if the building downwash option is
selected. SCREEN also uses a receptor height above ground of
0.0m (i.e. no flagpole receptors) in the complex terrain option
even if a non-zero value is entered. The original receptor
height is saved for later calculations. Refer to Section 3 for
more details on the complex terrain screening procedure.
2.4.3 Simple Elevated or Flat Terrain Option
The user is given the option in SCREEN of modeling either
simple elevated terrain, where terrain heights exceed stack base
but are below stack height, or simple flat terrain, where terrain
heights are assumed not to exceed stack base elevation. If the
user elects not to use the option for simple terrain screening
11
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with terrain above stack base, then flat terrain is assumed and
tthe terrain height is assigned a value of zero. If the simple
'elevated terrain option is used, SCREEN will prompt the user to
enter a terrain height above stack base. If terrain heights
above physical stack height are entered by the user for this
option, they are chopped off at the physical stack height.
The simple elevated terrain screening procedure assumes that
the plume elevation above sea level is not affected by the
elevated terrain. Concentration estimates are made by reducing
the calculated plume height by the user-supplied terrain height
above stack base. Neither the plume height nor terrain height
are allowed to go below zero. The user can model simple elevated
terrain using either or both of the distance options described
below, i.e., the automated distance array or the discrete
distance option. When the simple elevated terrain calculations
for each distance option are completed, the user will have the
option of continuing simple terrain calculations for that option
with a new terrain height. (For flat terrain the user will not be
given the option to continue with a new terrain height). For
conservatism and to discourage the user from modeling terrain
heights that decrease with distance, the new terrain height for
the automated distances cannot be lower than the previous height
for that run. The user is still given considerable flexibility
to model the effects of elevated terrain below stack height
across a wide range of situations.
For relatively uniform elevated terrain, or as a "first cut"
conservative estimate of terrain effects, the user should input
the maximum terrain elevation (above stack base) within 50 km of
the source, and exercise the automated distance array option out
to 50 km. For isolated terrain features a separate calculation
can be made using the discrete distance option for the distance
to the terrain feature, with the terrain height input as the
maximum height of the feature above stack base. Where terrain
heights vary with distance from the source, then the SCREEN model
can be run on each of several concentric rings using the minimum
and maximum distance inputs of the automated distance option to
define each ring, and using the maximum terrain elevation above
stack base within each ring for terrain height input. As noted
above, the terrain heights are not allowed to decrease with
distance in SCREEN. If terrain decreasing with distance (in all
directions) can be justified for a particular source, then the
distance rings would have to be modeled using separate SCREEN
runs, and the results combined. The overall maximum
concentration would then be the controlling value. The optimum
ring sizes will depend on how the terrain heights vary with
distance, but as a "first cut" it is suggested that ring sizes of
about 5 km be used (i.e., 0-5km, 5-10km, etc.). The application
of SCREEN to evaluating the effects of elevated terrain should be
done in consultation with the permitting agency.
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2.4.4 Choice of Meteorology
For simple elevated or flat terrain screening, the user will
be given the option of selecting from three choices of
meteorology: (1) full meteorology (all stability classes and wind
speeds); (2) specifying a single stability class; or (3)
specifying a single stability class and wind speed. Generally,
the full meteorology option should be selected. The other two
options were originally included for testing purposes only, but
may be useful when particular meteorological conditions are of
concern. Refer to Section 3 for more details on the
determination of worst case meteorological conditions by SCREEN.
2.4.5 Automated Distance Array Option
The automated distance array option of SCREEN gives the user
the option of using a pre-selected array of 50 distances ranging
from 100m out to 50 km. Increments of 100m are used out to
3,000m, with 500m increments from 3,000m to 10 km, 5 km
increments from 10 km to 30 km, and 10 km increments out to 50
km. When using the automated distance array, SCREEN prompts the
user for a minimum and maximum distance to use, which should be
input in free format, i.e., separated by a comma or a space.
SCREEN then calculates the maximum concentration across a range
of meteorological conditions for the minimum distance given (> 1
meter), and then for each distance in the array larger than the
minimum and less than or equal to the maximum. Thus, the user
can input the minimum site boundary distance as the minimum
distance for calculation and obtain a concentration estimate at
the site boundary and beyond, while ignoring distances less than
the site boundary.
If the automated distance array is used, then the SCREEN
model will use an iteration routine to determine the maximum
value and associated distance to the nearest meter. If the
minimum and maximum distances entered do not encompass the true
maximum concentration, then the maximum value calculated by
SCREEN may not be the true maximum. Therefore, it is recommended
that the maximum distance be set sufficiently large initially to
ensure that the maximum concentration is found. This distance
will depend on the source, and some "trial and error" may be
necessary, however, the user can input a distance of 50,000m to
examine the entire array. The iteration routine stops after 50
iterations and prints out a message if the maximum is not found.
Also, since there may be several local maxima in the
concentration distribution associated with different wind speeds,
it is possible that SCREEN will not identify the overall maximum
in its iteration. This is not likely to be a frequent
occurrence, but will be more likely for stability classes C and D
due to the larger number of wind speeds examined.
2.4.6 Discrete Distance Option
13
-------
The discrete distance option of SCREEN allows the user to
input specific distances. Any number of distances (> 1 meter)
can be input by the user and the maximum concentration for each
distance will be calculated. The user will always be given this
option whether or not the automated distance array option is
used. The option is terminated by entering a distance of zero
(0). SCREEN will accept distances out to 100 km for long-range
transport estimates with the discrete distance option. However,
for distances greater than 50 km, SCREEN sets the minimum 10
meter wind speed at 2 m/s to avoid unrealistic transport times.
2.4.7 Fumigation Option
Once the distance-dependent calculations are completed,
SCREEN will give the user the option of estimating maximum
concentrations and distance to the maximum associated with
inversion break-up fumigation, and shoreline fumigation. The
option for fumigation calculations is applicable only for rural
inland sites with stack heights greater than or equal to 10
meters (within 3,000m onshore from a large body of water.) The
fumigation algorithm also ignores any potential effects of
elevated terrain.
Once all calculations are completed, SCREEN summarizes the
maximum concentrations for each of the calculation procedures
considered. Before execution is stopped, whether it is after
complex terrain calculations are completed or at the end of the
simple terrain calculations, the user is given the option of
printing a hardcopy of the results. Whether or not a hardcopy is
printed, the results of the session, including all input data and
concentration estimates, are stored in a file called SCREEN.OUT.
This file is opened by the model each time it is run. If a file
named SCREEN.OUT already exists, then its contents will be
overwritten and lost. Thus, if you wish to save results of a
particular run, then change the name of the output file using the
DOS RENAME command, e.g., type 'REN SCREEN.OUT SAMPLE1.OUT', or
print the file using the option at the end of the program. If
SCREEN.OUT is later printed using the DOS PRINT command, the
FORTRAN carriage controls will not be observed. (Instructions
are included in Section 4 for simple modifications to the SCREEN
code that allow the user to specify an output filename for each
run.)
Figure 2 shows an example using the complex terrain screen
only. Figure 3 shows an example for an urban point source which
uses the building downwash option. In the DWASH column of the
output, 'NO' indicates that no downwash is included, 'HS1 means
that Huber-Snyder downwash is included, 'SS' means that
Schulman-Scire downwash is included, and 'NA' means that downwash
is not applicable since the downwind distance is less than 3L, .
A blank in the DWASH column means that no calculation was made
for that distance because the concentration was so small.
14
-------
Figure 4 presents a flow chart of all the inputs and various
options of SCREEN for point sources. Also illustrated are all of
the outputs from SCREEN. If a cell on the flow chart does not
contain the words "Enter" or "Print out", then it is an internal
test or process of the program, and is included to show the flow
of the program.
2.5 Flare Release Example
By answering 'F' or 'f to the question on source type the
user selects the flare release option. This option is similar to
the point source described above except for the inputs needed to
calculate plume rise. The inputs for flare releases are as
follows:
Flare Release Inputs
Emission rate (g/s)
Flare stack height (m)
Total heat release rate (cal/s)
Receptor height above ground (m)
Urban/rural option (U = urban, R = rural)
The SCREEN model calculates plume rise for flares based on
an effective buoyancy flux parameter. An ambient temperature of
293K is assumed in this calculation and therefore none is input
by the user. It is assumed that 55 percent of the total heat is
lost due to radiation. Plume rise is calculated from the top of
the flame, assuming that the flame is bent 45 degrees from the
vertical. SCREEN calculates and prints out the effective release
height for the flare. SCREEN provides the same options for
flares as described earlier for point sources, including building
downwash, complex and/or simple terrain, fumigation, and the
automated and/or discrete distances. The order of these options
and the user prompts are the same as described for the point
source example.
While building downwash is included as an option for flare
releases, it should be noted that SCREEN assumes an effective
stack gas exit velocity (v,) of 20 m/s and an effective stack gas
exit temperature (T,) of 1,273K, and calculates an effective
stack diameter based on the heat release rate. These effective
stack parameters are somewhat arbitrary, but the resulting
buoyancy flux estimate is expected to give reasonable final plume
rise estimates for flares. However, since building downwash
estimates depend on transitional momentum plume rise and
transitional buoyant plume rise calculations, the selection of
effective stack parameters could influence the estimates.
Therefore, building downwash estimates should be used with extra
caution for flare releases. If more realistic stack parameters
can be determined, then the estimate could alternatively be made
15
-------
with the point source option of SCREEN. In doing so, care should
be taken to account for the vertical height of the flame in
specifying the release height (see Section 3). Figure 5 shows an
example for a flare release, and Figure 6 shows a flow chart of
the flare release inputs, options, and output.
2.6 Area Source Example
The third source type option in SCREEN is for area sources,
which is selected by entering 'A1 or 'a' for source type. The
area source algorithm in SCREEN is based on a numerical
integration approach, and allows for the area source to be
approximated by a rectangular area, The inputs requested for
area sources are as follows:
Area Source Inputs
Emission rate [g/(s-m2)]
Source release height (m)
Length of larger side of the rectangular area (m)
Length of smaller side of the rectangular area (m)
Receptor height above ground (m)
Urban/rural option (U = urban, R = rural)
Wind direction search option (if no, specify
desired angle)
Note that the emission rate for area sources is input as an
emission rate per unit area in units of g/(s-nr). These units
are consistent with the ISCST model.
Since the concentration at a particular distance downwind
from a rectangular area is dependent on the orientation of the
area relative to the wind direction, the SCREEN model provides
the user with two options for treating wind direction. The first
option, which should be used for most applications of SCREEN and
is the regulatory default, is for the model to search through a
range of wind directions to find the maximum concentration. The
range of directions used in the search is determined from a set
of look-up tables based on the aspect ratio of the area source,
the stability category, and the downwind distance. The SCREEN
model also provides the user an option to specify a wind
direction orientation relative to the long axis of the
rectangular area. The second option may be used to estimate the
concentration at a particular receptor location relative to the
area. The output table for area sources includes the wind
direction associated with the maximum concentration at each
distance.
The user has the same options for handling distances and the
same choices of meteorology as described above for point sources,
16
-------
but no complex terrain, elevated simple terrain, building
downwash, or fumigation calculations are made for area sources.
Distances are measured from the center of the rectangular area.
Since the numerical integration algorithm can estimate
concentrations within the area source, the user can enter any
value for the minimum distance. Figure 7 shows an example of
SCREEN for an area source, using both the automated and discrete
distance options. Figure 8 provides a flow chart of. inputs,
options, and outputs for area sources.
2.7 Volume Source Example
The fourth source type option in SCREEN is for volume
sources, which is selected by entering 'V or 'v1 for source
type. The volume source algorithm is based on a virtual point
source approach, and may be used for non-buoyant sources whose
emissions occupy some initial volume. The inputs requested for
volume sources are as follows:
Volume Source Inputs
Emission rate (g/s)
Source release height (m)
Initial lateral dimension of volume (m)
Initial vertical dimension of volume (m)
Receptor height above ground (m)
Urban/rural option (U = urban, R = rural)
The user must determine the initial dimensions of the volume
source plume before exercising the SCREEN model volume source.
Table 1 provides guidance on determining these inputs. Since the
volume source algorithm cannot estimate concentrations within the
volume source, the model will give a concentration of zero for
distances (measured from the center of the volume) of less than
2.15 o .. Figure 9 shows an example of SCREEN for a volume
source, and Figure 10 provides a flow chart of inputs, options,
and outputs for volume sources.
17
-------
TABLE 1.
SUMMARY OF SUGGESTED PROCEDURES FOR ESTIMATING
INITIAL LATERAL DIMENSIONS (avo) AND
INITIAL VERTICAL DIMENSIONS (a.J FOR VOLUME SOURCES
Description of Source
Initial Dimension
(a) Initial Lateral Dimensions (ovc)
Single Volume Source
oyc
length of side divided
by 4.3
(b) Initial Vertical Dimensions (azc!
Surface-Based Source (he - 0)
Elevated Source (h~ > 0) on or
Adjacent to a Building
Elevated Source (he > 0) not on
or Adjacent to a Building
ozo = vertical dimension of
source divided by 2.15
azc = building height divided
by 2.15
azc = vertical dimension of
source divided by 4.3
2.8 Non-regulatory Options
On the same source type input line, the program allows the
input of three additional input, N, nn.n, and SS. Where 'nn.n'
represents a numerical anemometer height such as 7.5 meters.
These input, when entered, cause the program to use the non-
regulatory Erode 2 Mixing Height (1991) option (N), a user-
specified anemometer height (nn.n), and/or a non-regulatory
building downwash/cavity option (Schulman and Scire, 1993) (SS, in
SCREEN printout). While additional input is required for the
Schulman-Scire Building Downwash/Cavity option, as was discussed
in Section 2.4.1.2, no additional input data are required for the
other two options.
18
-------
Order of Options
in SCREEN
' Input Source
Characteristics
Building
Downwash
i Option
i
Complex
Terrain
Option
Simple Elevated
or Flat Terrain
Option"
Choice
of
Meteorology"
Automated
Distance Array
Option"
Discrete
Distance
Option"
Fumigation
Option
(Rural Only)
Corresponding Section in
Screening Procedures Document
Section 4 5.1
Section 4.5 2
Section 4 2
Section 4 2, Step 4
Section 4 2, Step 4
Section 4 3
Section 456
Section 4 5.3
*These options also apply to
Area Sources, Section 454
for Distances < 50km
for Distances > 50km
Figure 1 Point Source Options in SCREEN
19
-------
*** SCREENS MODEL RUN ***
*** VERSION DATED 95250 ***
POINT SOURCE EXAMPLE WITH COMPLEX TERRAIN
COMPLEX TERRAIN INPUTS:
SOURCE TYPE
EMISSION RATE (G/S)
STACK HT (M)
STACK DIAMETER (M)
STACK VELOCITY (M/S)
STACK GAS TEMP (K)
AMBIENT AIR TEMP (K)
RECEPTOR HEIGHT (M)
URBAN/RURAL OPTION
POINT
100.000
100.0000
2.5000
25.0000
450.0000
293.0000
.0000
RURAL
09/07/95
12:00:00
BUOY. FLUX = 133.643 M**4/S**3; MOM. FLUX = 635.851 M**4/S**2.
FINAL STABLE PLUME HEIGHT (M) = 192.9
DISTANCE TO FINAL RISE (M) = 151.3
TERR
HT
(M)
150.
200.
200.
200.
DIST
(M)
1000.
2000.
5000.
10000.
MAX 24 -HR
CONC
(UG/M**3)
243.4
284.3
91.39
37.36
•VALLEY 24-HR CALCS*
PLUME HT
CONC
(UG/M**3)
243.4
284.3
91.39
37.36
ABOVE STK
BASE (M)
192.9
192.9
192.9
192.9
**SIMPLE
CONC
(UG/M**3)
161.1
.0000
.0000
.0000
TERRAIN 24- HR CALCS**
PLUME HT
ABOVE STK
HGT (M)
32.9
.0
.0
.0
U10M USTK
SC
4
0
0
0
(M/S)
15.0 21
.0
.0
.0
.2
.0
.0
.0
SUMMARY OF SCREEN MODEL RESULTS
CALCULATION
PROCEDURE
MAX CONC
(UG/M**3)
DIST TO
MAX (M)
TERRAIN
HT (M)
COMPLEX TERRAIN
284.3
2000.
200. (24-HR CONC)
REMEMBER TO INCLUDE BACKGROUND CONCENTRATIONS **
Figure 2. SCREEN Point Source Example for Complex Terrain
20
-------
09/07/95
12:00:00
*** SCREENS MODEL RUN ***
*** VERSION DATED 95250 ***
POINT SOURCE EXAMPLE WITH BUILDING DOWNUASH
SIMPLE TERRAIN INPUTS:
SOURCE TYPE = POINT
EMISSION RATE (G/S) = 100.000
STACK HEIGHT (M) = 100.0000
STK INSIDE DIAM (M) = 2.0000
STK EXIT VELOCITY (M/S)= 15.0000
STK GAS EXIT TEMP (K) = 450.0000
AMBIENT AIR TEMP (K) = 293.0000
RECEPTOR HEIGHT (M) = .0000
URBAN/RURAL OPTION = URBAN
BUILDING HEIGHT (M) = 80.0000
MIN HOR1Z BLDG DIM (M) = 80.0000
MAX HORIZ BLDG DIM (M) = 100.0000
BUOY. FLUX = 51.319 M**4/S**3; MOM. FLUX = 146.500 M**4/S**2.
*** FULL METEOROLOGY ***
SCREEN AUTOMATED DISTANCES
TERRAIN HEIGHT OF
0. M ABOVE STACK BASE USED FOR FOLLOWING DISTANCES ***
DIST
(M)
100.
200.
300.
400.
500.
600.
700.
800.
900.
1000.
CONC
(UG/M**3)
.0000
.0000
631.6
517.4
494.6
578.0
638.4
715.3
699.4
681.9
STAB
0
0
1
1
6
6
6
6
6
6
U10M
(M/S)
.0
.0
1.5
1.5
1.0
1.0
1.0
1.0
1.0
1.0
USTK
(M/S)
.0
.0
2.1
2.1
2.0
2.0
2.0
2.0
2.0
2.0
MIX HT
(M)
480
480
10000
10000
10000
10000
10000
10000
.0
.0
.0
.0
.0
.0
.0
.0
.0
.0
PLUME
HT (M)
125
140
113
113
113
113
113
113
.00
.00
.11
.59
.08
.08
.08
.08
.08
.08
SIGMA
Y (M)
.00
.00
90.71
118.85
50.21
59.27
68.06
76.59
84.89
92.97
SIGMA
Z (M)
.00
.00
82.09
113.59
50.05
54.62
59.18
65.44
68.33
71.13
DUASH
NA
NA
SS
ss
SS
ss
ss
ss
ss
ss
MAXIMUM 1-HR CONCENTRATION AT OR BEYOND 100. M:
800. 715.3 6 1.0 2.0 10000.0 113.08
DUASH= MEANS NO CALC MADE (CONC = 0.0)
DWASH=NO MEANS NO BUILDING DOWNUASH USED
DWASH=HS MEANS HUBER-SNYDER DOWNWASH USED
DWASH=SS MEANS SCHULMAN-SCIRE DOWNWASH USED
DWASH=NA MEANS DOWNWASH NOT APPLICABLE, X<3*LB
76.59 65.44
SS
Figure 3. SCREEN Point Source Example with Building Downwash (Page 1 of 2)
21
-------
*** CAVITY CALCULATION
CONC CUG/M**3)
CRIT WS 310M (M/S) =
CRIT WS 3 HS (M/S) =
DILUTION WS (M/S) =
CAVITY HT (M) =
CAVITY LENGTH (M) =
ALONGWIND DIM (M) =
. 1 *** *** CAVITY CALCULATION - 2 ***
3168. CONC (UG/M**3) = 1691.
3.32 CRIT WS 310M (M/S) = 7.77
5.26 CRIT WS a HS (M/S) = 12.32
2.63 DILUTION WS (M/S) = 6.16
114.88 CAVITY HT (M) = 105.20
142.41 CAVITY LENGTH (M) = 101.30
80.00 ALONGWIND DIM (M) = 100.00
***************************************
*** SUMMARY OF SCREEN MODEL RESULTS ***
CALCULATION
PROCEDURE
MAX CONC D1ST TO
(UG/M**3) MAX (M)
TERRAIN
HT CM)
SIMPLE TERRAIN 715.3
BUILDING CAVITY-1 3168.
BUILDING CAVITY-2 1691.
800.
142.
101.
0.
(DIST = CAVITY LENGTH)
(DIST = CAVITY LENGTH)
***************************************************
** REMEMBER TO INCLUDE BACKGROUND CONCENTRATIONS **
Figure 3. SCREEN Point Source Example with Building Downwash (Page 2 of 2)
22
-------
This page is intentionally left blank,
23
-------
Figure 4. Flow Chart of Inputs and Outputs for SCREEN Point Source (Page 1 of 2)
24
-------
Vs. r. / Vrj-j-/ /
w .£ .... / w ...... / 7"-«
/ " / /•F- / 71
/ tmr Tm» / / / / M«
. V" ./ h**» «•"• / ^/ e»r /- / M-W1*
'^^^^^W lock MM j^^^^^i DWttfri •«* l^^^^i CM""M*"
/ W / / IOK* )•) / / M lp.t«.l
/ / / / / c""
r M*t* ^V / PFI« ou MU /
rr-» )^—y -;: /
r «f u / / npi«a*>» /
V X / c" ' °" /
/ - - / / - - /
/ HnMurr / / Dwant* /
I f^—~m—J . /
/ "* / 7 *** W*t* /
/ •"
-------
09/07/95
12:00:00
*** SCREENS MODEL RUN ***
*** VERSION DATED 95250 ***
FLARE RELEASE EXAMPLE
SIMPLE TERRAIN INPUTS:
SOURCE TYPE = FLARE
EMISSION RATE (G/S) = 1000.00
FLARE STACK HEIGHT (M) = 100.0000
TOT HEAT RLS (CAL/S) = .100000E+08
RECEPTOR HEIGHT (M) = .0000
URBAN/RURAL OPTION = RURAL
EFF RELEASE HEIGHT (M) = 110.1150
BUILDING HEIGHT (M) = .0000
MIN HORIZ BLDG DIM (M) = .0000
MAX HORIZ BLDG DIM (M) = .0000
BUOY. FLUX = 165.803 M**4/S**3; MOM. FLUX = 101.103 M**4/S**2.
*** FULL METEOROLOGY ***
*** SCREEN AUTOMATED DISTANCES ***
TERRAIN HEIGHT OF
0. M ABOVE STACK BASE USED FOR FOLLOWING DISTANCES ***
DIST
(M)
250.
300.
400.
500.
600.
700.
800.
900.
1000.
1100.
1200.
1300.
1400.
1500.
1600.
1700.
1800.
1900.
2000.
CONC
(UG/M**3) STAB
.7733E-04 5
.2501E-03 1
1.283 1
66.54 1
407.0 1
741.2 1
944.9 1
1303. 1
1449. 1
1448. 1
1387. 1
1315. 1
1248. 1
1187. 1
1132. 1
1082. 1
1036. 1
993.9 1
957.5 1
U10M
(M/S)
1
3
3
3
3
3
1
1
1
1
1
1
1
1
1
1
1
1
1
.0
.0
.0
.0
.0
.0
.5
.5
.5
.5
.5
.5
.5
.5
.5
.5
.5
.5
.0
USTK
(M/S)
MIX HT
(M)
2.3 10000.0
3.5
3.5
3.5
3.5
3.5
1.8
1.8
1.8
1.8
1.8
1.8
1.8
1.8
1.8
1.8
1.8
1.8
1.2
960.0
960.0
960.0
960.0
960.0
579.5
579.5
579.5
579.5
579.5
579.5
579.5
579.5
579.5
579.5
579.5
579.5
813.6
PLUME
HT (M)
233
344
344
344
344
344
578
578
578
578
578
578
578
578
578
578
578
578
812
.54
.28
.28
.28
.28
.28
.45
.45
.45
.45
.45
.45
.45
.45
.45
.45
.45
.45
.62
SIGMA
Y (M)
38.05
78.46
100.36
121.51
142.09
162.21
210.37
231.47
247.92
263.50
279.21
295.03
310.90
326.80
342.72
358.64
374.55
390.43
432.95
SIGMA
Z (M) DWASH
36.
57.
80.
113.
161.
220.
308.
386.
473.
571.
680.
802.
934.
1078.
1234.
1401.
1580.
1770.
1978.
05
07
87
75
96
50
17
36
16
19
86
07
77
93
58
74
46
78
42
NO
NO
NO
NO
NO
NO
NO
NO
NO
NO
NO
NO
NO
NO
NO
NO
NO
NO
NO
MAXIMUM 1-HR CONCENTRATION AT OR BEYOND 250. M:
1046. 1461. 1 1.5 1.8 579.5 578.45
DWASH= MEANS NO CALC MADE (CONC = 0.0)
DWASH=NO MEANS NO BUILDING DOWNWASH USED
DWASH=HS MEANS HUBER-SNYDER DOWNWASH USED
DWASH=SS MEANS SCHULMAN-SCIRE DOWNWASH USED
DWASH=NA MEANS DOWNWASH NOT APPLICABLE, X<3*LB
254.91 515.82
NO
Figure 5. SCREEN Flare Release Example (Page 1 of 2)
26
-------
***************************************
*** SUMMARY OF SCREEN MODEL RESULTS ***
CALCULATION
PROCEDURE
MAX CONC
(UG/M**3)
DIST TO
MAX (M)
TERRAIN
HT (M)
SIMPLE TERRAIN 1461. 1046. 0.
***************************************************
** REMEMBER TO INCLUDE BACKGROUND CONCENTRATIONS **
***************************************************
Figure 5. SCREEN Flare Release Example (Page 2 of 2)
27
-------
Figure 6. Flow Chart of Inputs and Outputs for SCREEN Flare Release (Page 1 of 2)
28
-------
/ in* TmM / / EMI Mh> im / I fm
i-^--y r .r /—J t-i" /—f ~.^
7 " / 7 ^ / r
,:z \>.. /'^ Tr. 7 7 .- / / —-
inen? J W tat* »»i« / """ "W two™, «ir /* " ^ ctmt«»«»otv
r.;/ / - / / ";: "? / /S-
X^\ FJ
.^-*^rr-7 ^)^-y Frr
X / Ic-• °-
/i- L-/.==.-/
/ -IW, / /. •«..* /
/ - ' - / / " /
Figure 6. Flow Chart of Inputs and Outputs for SCREEN Flare Release (Page 2 of 2)
29
-------
09/07/95
12:00:00
***« SCREENS MODEL RUN ***
*** VERSION DATED 95250 ***
AREA SOURCE EXAMPLE
SIMPLE TERRAIN INPUTS:
SOURCE TYPE = AREA
EMISSION RATE (G/(S-M**2» = .250000E-02
SOURCE HEIGHT (M) = 5.0000
LENGTH OF LARGER SIDE (M) = 200.0000
LENGTH OF SMALLER SIDE (M) = 200.0000
RECEPTOR HEIGHT (M) = .0000
URBAN/RURAL OPTION = URBAN
MODEL ESTIMATES DIRECTION TO MAX CONCENTRATION
BUOY. FLUX = .000 M**4/S**3; MOM. FLUX = .000 M**4/S**2.
*** FULL METEOROLOGY ***
*** SCREEN AUTOMATED DISTANCES ***
*** TERRAIN HEIGHT OF 0. M ABOVE STACK BASE USED FOR FOLLOWING DISTANCES ***
DIST
(M)
150.
200.
300.
400.
500.
600.
700.
800.
900.
1000.
CONC
(UG/M**3)
.4067E+05
.3784E+05
.2430E+05
.1755E+05
.1356E+05
.1091E+05
9028.
7629.
6559.
5718.
STAB
5
5
5
5
5
5
5
5
5
5
U10M
CM/S)
1.0
1.0
1.0
1.0
1.0
1.0
1.0
1.0
1.0
1.0
USTK MIX HT
(M/S) (M)
1.0 10000.0
1.0 10000.0
1.0 10000.0
1.0 10000.0
1.0 10000.0
1.0 10000.0
1.0 10000.0
1.0 10000.0
1.0 10000.0
1.0 10000.0
PLUME
HT (M)
5.00
5.00
5.00
5.00
5.00
5.00
5.00
5.00
5.00
5.00
MAX DIR
(DEC)
43.
45.
45.
45.
45.
45.
45.
45.
44.
45.
MAXIMUM 1-HR CONCENTRATION AT OR BEYOND 150. M:
168. .4178E+05 5 1.0 1.0 10000.0 5.00 45.
Figure 7. SCREEN Area Source Example (Page 1 of 2)
30
-------
*** SCREEN DISCRETE DISTANCES ***
*** TERRAIN HEIGHT OF 0. M ABOVE STACK BASE USED FOR FOLLOWING DISTANCES ***
DIST CONC U10M USTK MIX HT PLUME MAX DIR
(M) (UG/M**3) STAB (M/S) (M/S) (M) HT (M) (DEC)
5000. 718.1 5 1.0 1.0 10000.0 5.00 38.
10000. 321.3 5 1.0 1.0 10000.0 5.00 1.
20000. 150.4 5 1.0 1.0 10000.0 5.00 31.
50000. 71.25 4 1.0 1.0 320.0 5.00 11.
*** SUMMARY OF SCREEN MODEL RESULTS ***
***************************************
CALCULATION MAX CONC DIST TO TERRAIN
PROCEDURE (UG/M'3) MAX (M) HT (M)
SIMPLE TERRAIN .4178E+05 168. 0.
** REMEMBER TO INCLUDE BACKGROUND CONCENTRATIONS **
Figure 7. SCREEN Area Source Example (Page 2 of 2)
31
-------
(Typ* SCREENS \
U> START I
Figure 8. Flow Chart of Inputs and Outputs for SCREEN Area Source
32
-------
09/07/95
13:00:00
*** SCREEN3 MODEL RUN ***
*** VERSION DATED 95250 ***
VOLUME SOURCE EXAMPLE
SIMPLE TERRAIN INPUTS:
SOURCE TYPE = VOLUME
EMISSION RATE (G/S) = 1.00000
SOURCE HEIGHT (M) = 10.0000
INIT. LATERAL DIMEN (M) = 50.0000
INIT. VERTICAL DIMEN (M) = 20.0000
RECEPTOR HEIGHT (M) = .0000
URBAN/RURAL OPTION = RURAL
BUOY. FLUX = .000 M**4/S**3; MOM. FLUX = .000 M**4/S**2.
*** FULL METEOROLOGY ***
*** SCREEN AUTOMATED DISTANCES ***
**********************************
*** TERRAIN HEIGHT OF 0. M ABOVE STACK BASE USED FOR FOLLOWING DISTANCES
DIST
(M)
100.
200.
300.
400.
500.
600.
700.
800.
900.
1000.
CON
(UG/M
.OOC
239.
224.
209.
195.
183.
173.
163.
154.
146.
1C
l**3)
10
5
1
1
7
8
0
2
4
3
STAB
0
6
6
6
6
6
6
6
6
6
U10M
(M/S)
.0
1.0
1.0
1.0
1.0
1.0
1.0
1.0
1.0
1.0
USTK
(M/S)
.0
1.0
1.0
1.0
1.0
1.0
1.0
1.0
1.0
1.0
MIX
(M
10000
10000
10000
10000
10000
10000
10000
10000
10000
HT
)
.0
.0
.0
.0
.0
.0
.0
.0
.0
.0
PLUME
HT (M)
.00
10.00
10.00
10.00
10.00
10.00
10.00
10.00
10.00
10.00
SIGMA
Y (M)
.00
55.68
58.61
61.51
64.41
67.28
70.15
73.00
75.84
78.66
SIGMA
Z (M)
.00
21.40
21.82
22.40
22.96
23.52
24.06
24.60
25.12
25.64
DWASH
NO
NO
NO
NO
NO
NO
NO
NO
NO
MAXIMUM 1-HR CONCENTRATION AT OR BEYOND 100. M:
109. 257.5 6 1.0 1.0 10000.0 10.00 53.04 20.78 NO
DWASH= MEANS NO CALC MADE (CONC = 0.0)
DWASH=NO MEANS NO BUILDING DOWNWASH USED
DWASH=HS MEANS HUBER-SNYDER DOUNWASH USED
DWASH=SS MEANS SCHULMAN-SCIRE DOWNWASH USED
DWASH=NA MEANS DOWNWASH NOT APPLICABLE, X<3*LB
*** SUMMARY OF SCREEN MODEL RESULTS ***
CALCULATION
PROCEDURE
MAX CONC
(UG/M**3)
DIST TO
MAX (M)
TERRAIN
HT (M)
SIMPLE TERRAIN 257.5 109. 0.
REMEMBER TO INCLUDE BACKGROUND CONCENTRATIONS **
Figure 9. SCREEN Volume Source Example
33
-------
(•V* •cwtw* A
V tTART I
/tmt fctUWt. /
*21 • ^J» /
Cf«' Mill
~. ~ / /
Mtfnm / I H»
c.»mw / / »***•*,
K K»*cM*« / / ••««
D"~ / /
Figure 10. Flow Chart of Inputs and Outputs for SCREEN Volume Source
34
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3. TECHNICAL DESCRIPTION
• Most of the techniques used in the SCREEN model are based on
assumptions and methods common to other EPA dispersion models.
For the sake of brevity, lengthy technical descriptions that are
available elsewhere are not duplicated here. This discussion
will concentrate on how those methods are incorporated into
SCREEN and on describing those techniques that are unique to
SCREEN.
3.1 Basic Concepts of Dispersion Modeling
SCREEN uses a Gaussian plume model that incorporates source -
related factors and meteorological factors to estimate pollutant
concentration from continuous sources. It is assumed that the
pollutant does not undergo any chemical reactions, and that no
other removal processes, such as wet or dry deposition, act on
the plume during its transport from the source. The Gaussian
model equations and the interactions of the source -related and
meteorological factors are described in Volume II of the ISC
user's guide (EPA, 1995b) , and in the Workbook of Atmospheric
Dispersion Estimates (Turner, 1970} .
The basic equation for determining ground- level
concentrations under the plume centerline is:
X = Q/(2nuso..oz) {exp[-M( (zr-he)/o.) 2]
-fc( (2r+he)/o.) 2]
-fc{ (zr-lu-2NzJ/o.) 2]
-M( (zr+h6-2Nz.) /o:) 2]
-fc( (zr-he+2Nz,)/a,) 2]
-H( (z;+he+2NzJ/a:) 2] ] }
where:
X = concentration (g/m )
Q = emission rate (g/s)
n = 3.141593
us = stack height wind speed (m/s)
o. = lateral dispersion parameter (m)
a: = vertical dispersion parameter (m)
z, = receptor height above ground (m)
h? = plume centerline height (m)
z_ = mixing height (m)
k = summation limit for multiple reflections of plume
off of the ground and elevated inversion, usually
35
-------
Note that for stable conditions and/or mixing heights greater
than or equal to 10,000m, unlimited mixing is assumed and the
summation term is assumed to be zero.
Equation 1 is used to model the plume impacts from point
sources, flare releases, and volume releases in SCREEN. The
SCREEN volume source option uses a virtual point source approach,
as described in Volume II (Section 1.2.2) of the ISC model user's
guide (EPA, I995b). The user inputs the initial lateral and
vertical dimensions of the volume source, as described in Section
2.7 above.
The SCREEN model uses a numerical integration algorithm for
modeling impacts from area sources, as described in Volume II
(Section 1.2.3) of the ISC model user's guide (EPA, I995b). The
area source is assumed to be a rectangular shape, and the model
can be used to estimate concentrations within the area.
3.2 Worst Case Meteorological Conditions
SCREEN examines a range of stability classes and wind speeds
to identify the "worst case" meteorological conditions, i.e., the
combination of wind speed and stability that results in the
maximum ground level concentrations. The wind speed and
stability class combinations used by SCREEN are given in ^able
2. The 10-meter wind speeds given in Table 2 are adjusted to
stack height by SCREEN using the wind profile power law exponents
given in Table 3-1 of the screening procedures document. For
release heights of less than 10 meters, the wind speeds listed in
Table 2 are used without adjustment. For distances greater than
50 km (available with the discrete distance option), SCREEN sets
2 m/s as the lower limit for the 10-meter wind speed to avoid
unrealistic transport times. Table 2 includes some cases that may
not be considered standard stability class/wind speed
combinations, namely E with winds less than 2 m/s, and F with
winds greater than 3 m/s. The combinations of E and winds of 1 -
1.5 m/s are often excluded because the algorithm developed by
Turner (1964) to determine stability class from routine National
Weather Service (NWS) observations excludes cases of E stability
for wind speeds less than 4 knots (2 m/s). These combinations
are included in SCREEN because they are valid combinations that
could appear in a data set using on-site meteorological data with
another stability class method. A wind speed of 6 knots (the
highest speed for F stability in Turner's scheme) measured at a
typical NWS anemometer height of 20 feet (6.1 meters) corresponds
to a 10 meter wind speed of 4 m/s under F stability. Therefore
the combination of F and 4 m/s has been included.
36
-------
Stabi lity
Class
A
B
C
D
E
F
Table 2. Wind Speed and Stability Class Combinations
Used by the SCREEN Model
10-m Wind Speed
(m/s)
1 1.5 2 2.5 3 3.5 4 4.5 5 8 10 15 20
******
The user has three choices of meteorological data to
examine. The first choice, which should be used in most
applications, is to use "Full Meteorology" which examines all six
stability classes (five for urban sources) and their associated
wind speeds. Using full meteorology with the automated distance
array (described in Section 2), SCREEN prints out the maximum
concentration for each distance, and the overall maximum and
associated distance. The overall maximum concentration from
SCREEN represents the controlling 1-hour value corresponding to
the result from Procedures (a) - (c) in Step 4 of Section 4.2.
Full meteorology is used instead of the A, C, and E or F subset
used by the hand calculations because SCREEN provides maximum
concentrations as a function of distance, and stability classes
A, C and E or F may not be controlling for all distances. The
use of A, C, and E or F may also not give the maximum
concentration when building downwash is considered. The second
choice is to input a single stability class (1 = A, 2 = B, ..., 6
= F). SCREEN will examine a range of wind speeds for that
stability class only. Using this option the user is able to
determine the maximum concentrations associated with each of the
individual procedures, (a) - (c), in Step 4 of Section 4.2. The
third choice is to specify a single stability class and wind
speed. The last two choices were originally put into SCREEN to
facilitate testing only, but they may be useful if particular
meteorological conditions are of concern. However, they are not
recommended for routine uses of SCREEN.
The mixing height used in SCREEN for neutral and unstable
conditions (classes A-D) is based on an estimate of the
mechanically driven mixing height. The mechanical mixing height,
z, (m), is calculated (Randerson, 1984) as
37
-------
zn = 0.3 u*/f (2)
where: u* = friction velocity (m/s)
f = Coriolis parameter (9.374 x 1CT! s"1 at 40°
latitude)
Using a log-linear profile of the wind speed, and assuming a
surface roughness length of about 0.3m, u* is estimated from the
10-meter wind speed, u:c, as
u* = 0.1 u-, (3)
Substituting for u* in Equation 2 we have
z^ = 320 u:c. (4)
The mechanical mixing height is taken to be the minimum daytime
mixing height. To be conservative for limited mixing
calculations, if the value of zm from Equation 3 is less than the
plume height, he, then the mixing height used in calculating the
concentration is set equal to he + 1. For stable conditions, the
mixing height is set equal to 10,000m to represent unlimited
mixing.
3.3 Plume Rise for Point Sources
The use of the methods of Briggs to estimate plume rise are
discussed in detail in Section 1.1.4 of Volume II of the ISC
user's guide (EPA, 1995b). These methods are also incorporated
in the SCREEN model.
Stack tip downwash is estimated following Briggs (1973, p.4)
for all sources except those employing the Schulman-Scire
downwash algorithm. Buoyancy flux for non-flare point sources is
calculated from
FC = gvsds2 (T£-Ta) / (4TS) , (5)
which is described in Section 4 of the screening procedures
document and is equivalent to Briggs' (1975, p. 63) Equation 12.
Buoyancy flux for flare releases is estimated from
Fc = 1.66 x 10"5 x H, (6)
where H is the total heat release rate of the flare (cal/s). This
formula was derived from Equation 4.20 of Briggs (1969), assuming
Ta = 293K, p = 1205 g/m, CF = 0.24 cal/gK, and that the sensible
heat release rate, QM = ((T.45) H. The sensible heat rate is
based on the assumption that 55 percent of the total heat
released is lost due to radiation (Leahey and Davies, 1984). The
buoyancy flux for flares is calculated in SCREEN by assuming
38
-------
effective stack parameters of v5 = 20 m/s, Ts = 1,273K, and
solving for an effective stack diameter, ds = 9.88 x 10~4 (Q-) "•".
The momentum flux, which is used in estimating plume rise
for building downwash effects, is calculated from,
F, = v3*ds'Ta/(4Ts) . (7)
The ISC user's guide (EPA, I995b) describes the equations
used to estimate buoyant plume rise and momentum plume rise for
both unstable/neutral and stable conditions. Also described are
transitional plume rise and how to estimate the distance to final
rise. Final plume rise is used in SCREEN for all cases with the
exception of the complex terrain screening procedure and for
building downwash effects.
The buoyant line source plume rise formulas that are used
for the Schulman-Scire downwash scheme are described in Section
1.1.4.11 of Volume II of the ISC user's guide (EPA, 1995b).
These formulas apply to sources where h, < Hb + 0.5LD. For
sources subject to downwash but not meeting this criterion, the
downwash algorithms of Huber and Snyder (EPA, 1995b) are used,
which employ the Briggs plume rise formulas referenced above.
3.4 Dispersion Parameters
The formulas used for calculating vertical (oz) and lateral
(a ) dispersion parameters for rural and urban sites are
described in Section 1.1.5 of Volume II of the ISC user's guide
(EPA, 1995b).
3.5 Buoyancy Induced Dispersion
Throughout the SCREEN model, with the exception of the
Schulman-Scire downwash algorithm, the dispersion parameters, o.
and o-, are adjusted to account for the effects of buoyancy
induced dispersion as follows:
o,, = (c. 2 + (Ah/3.5) *)z'- (8)
o:e = (Or2 + (Ah/3.5) 2):';
where Ah is the distance-dependent plume rise. (Note that for
inversion break-up and shoreline fumigation, distances are always
beyond the distance to final rise, and therefore Ah = final plume
rise) .
3.6 Building Downwash
3.6.1 Cavity Recirculation Region
The cavity calculations are a revision of the procedure
described in the Regional Workshops on Air Quality Modeling
39
-------
Summary Report, Appendix C (EPA, 1983), and are based largely on
results published by Hosker (1984).
If non-zero building dimensions are input to SCREEN for
either point or flare releases, then cavity calculations will be
made as follows. The cavity height, hc (m) , is estimated based
on the following equation from Hosker (1984):
tu = hD (1.0 + 1.6 exp (-1.3L/hb) ) , (9)
where: h_ = building height (m)
L = alongwind dimension of the building (m) .
Using the plume height based on momentum rise at two building
heights downwind, including stack tip downwash, a critical (i.e.,
minimum) stack height wind speed is calculated that will just put
the plume into the cavity (defined by plume centerline height =
cavity height). The critical wind speed is then adjusted from
stack height to 10-meter using a power law with an exponent of
0.2 to represent neutral conditions (no attempt is made to
differentiate between urban or rural sites or different stability
classes). If the critical wind speed (adjusted to 10-meters) is
less than or equal to 20 m/s, then a cavity concentration is
calculated, otherwise the cavity concentration is assumed to be
zero. Concentrations within the cavity, Xc, are estimated by the
following approximation (Hosker, 1984):
X: = Q/U.5 Ac u) (10)
where: Q = emission rate (g/s)
A.: = H.-W = cross-sectional area of the building normal
to the wind (m2)
W = crosswind dimension of the building (m)
u = wind speed (m/s).
For u, a value of one-half the stack height critical wind speed
is used, but not greater than 10 m/s and not less than 1 m/s.
Thus, the calculation of Xc is linked to the determination of a
critical wind speed. The concentration, Xc/ is assumed to be
uniform within the cavity.
The cavity length, x,, measured from the lee side of the
building, is estimated by the following (Hosker, 1984):
(l) for short buildings (L/hE < 2) ,
x, = (A) (W) (11)
1.0 + B(W/hD)
(2) for long buildings (L/hc > 2) ,
xr = 1.75 (W) (12)
40
-------
1.0 + 0.25(W/hr)
where: ht = building height (m)
L = alongwind building dimension (m)
W = crosswind building dimension (m)
A = -2.0 + 3.7 (L/hb)"-'3, and
B = -0.15 + 0.305 (L/hcr1/3.
The equations above for cavity height, concentration and
cavity length are all sensitive to building orientation through
the terms L, W and AF. Therefore, the entire cavity procedure is
performed for two orientations, first with the minimum horizontal
dimension alongwind and second with the maximum horizontal
dimension alongwind. For screening purposes, this is thought to
give reasonable bounds on the cavity estimates. The first case
will maximize the cavity height, and therefore minimize the
critical wind speed. However, the Ap term will also be larger and
will tend to reduce concentrations. The highest concentration
that potentially effects ambient air should be used as the
controlling value for the cavity procedure.
3.6.2 Wake Region
The calculations for the building wake region are based on
the ISC model (EPA, I995b). The wake effects are divided into
two regions, one referred to as the "near wake" extending from
3Lr to 10L, (L, is the lesser of the building height, h,, and
maximum projected width), and the other as the "far wake" for
distances greater than lOLr. For the SCREEN model, the maximum
projected width is calculated from the input minimum and maximum
horizontal dimensions as (L2 + W2)"~. The remainder of the
building wake calculations in SCREEN are based on the ISC user's
guide (EPA, 1995b).
It should be noted that, unlike the cavity calculation, the
comparison of plume height (due to momentum rise at two building
heights) to wake height to determine if wake effects apply does
not include stack tip downwash. This is done for consistency
with the ISC model.
3.7 Fumigation
3.7.1 Inversion Break-up Fumigation
The inversion break-up screening calculations are based on
procedures described in the Workbook of Atmospheric Dispersion
Estimates (Turner, 1970). The distance to maximum fumigation is
based on an estimate of the time required for the mixing layer to
develop from the top of the stack to the top of the plume, using
Equation 5.5 of Turner (1970):
x~l3> = u t.
= (u p, cP/R) (A6/Az) (h, - h£) [(h, + hs)/2] (13)
41
-------
where:
>
x^j. = downwind distance to maximum concentration (m)
t,, = time required for mixing layer to develop from top of
stack to top of plume(s)
u = wind speed (2.5 m/s assumed)
pa = ambient air density (1205 g/m3 at 20°C)
cc = specific heat of the air at constant pressure (0.24
cal/gK)
R = net rate of sensible heating of an air column by
solar radiation (about 67 cal/m2/s)
A0/Az = vertical potential temperature gradient (assume 0.035
K/m for F stability)
h = height of the top of the plume (m) = he + 2o:e (he is
the plume centerline height)
h, = physical stack height (m).
oze = vertical dispersion parameter incorporating buoyancy
induced dispersion (m)
The values of u and AS/Az are based on assumed conditions of
stability class F and stack height wind speed of 2.5 m/s for the
stable layer above the inversion. The value of h_ incorporates
the effect of buoyancy induced dispersion on a., however, elevated
terrain effects are ignored. The equation above is solved by
iteration, starting from an initial guess of xra>; = 5,000m.
The maximum ground-level concentration due to inversion
break-up fumigation, X,, is calculated from Equation 5.2 of Turner
(1970) .
Xf = Q/[ (2n) :'-u(Gy5 + h6/8) (he + 20^) ] (14)
where Q is the emission rate (g/s), and other terms are defined
above. The dispersion parameters, o,e and o=,., incorporate the
effects of buoyancy induced dispersion. If the distance to the
maximum fumigation is less than 2000m, then SCREEN sets Xf = 0
since for such short distances the fumigation concentration is not
likely to exceed the unstable/limited mixing concentration
estimated by the simple terrain screening procedure.
3.7.2 Shoreline Fumigation
For rural sources within 3000m of a large body of water,
maximum shoreline fumigation concentrations can be estimated by
SCREEN. A stable onshore flow is assumed with stability class F
(A6/Az = 0.035 K/m) and stack height wind speed of 2.5 m/s.
Similar to the inversion break-up fumigation case, the maximum
ground-level shoreline fumigation concentration is assumed to
occur where the top of the stable plume intersects the top of the
well-mixed thermal internal boundary layer (TIBL).
An evaluation of coastal fumigation models (EPA, 1987b) has
shown that the TIBL height as a function of distance inland is
42
-------
well-represented in rural areas with relatively flat terrain by an
equation of the form:
hT = A [x]°'£ (15)
where: hT = height of the TIBL (m)
A = TIBL factor containing physics needed for TIBL
parameterization (including heat flux) (in1)
x = inland distance from shoreline (m).
Studies (e.g. Misra and Onlock, 1982) have shown that the TIBL
factor, A, ranges from about 2 to 6. For screening purposes, A is
conservatively set equal to 6, since this will minimize the
distance to plume/TIBL intersection, and therefore tend to
maximize the concentration estimate.
As with the inversion break-up case, the distance to maximum
ground-level concentration is determined by iteration. The
equation used for the shoreline fumigation case is:
x^, = [(h6 + 2a:e)/6] 2 - xs (16)
where:
x.ax = downwind distance to maximum concentration (m)
xs = shortest distance from source to shoreline (m)
h£ = plume centerline height (m)
o:e = vertical dispersion parameter incorporating buoyancy
induced dispersion (m)
Plume height is based on the assumed F stability and 2.5 m/s wind
speed, and the dispersion parameter (o.e) incorporates the effects
of buoyancy induced dispersion. If x_5> is less than 200m, then no
shoreline fumigation calculation is made, since the plume may
still be influenced by transitional rise and its interaction with
the TIBL is more difficult to model.
The maximum ground-level concentration due to shoreline
fumigation, X-, is also calculated from Turner's (1970) Equation
5.2:
Xf = Q/ [ (2n):-Eu(ay,+he/8) (he+2oze) ] (14)
with o,e and o-e incorporating the effects of buoyancy induced
dispersion.
Even though the calculation of x^ above accounts for the
distance from the source to the shoreline in x5, extra caution
should be used in interpreting results as the value of x«
increases. The use of A=6 in Equations 15 and 16 may not be
conservative in these cases since there will be an increased
chance that the plume will be calculated as being below the TIBL
height, and therefore no fumigation concentration estimated.
Whereas a smaller value of A could put the plume above the TIBL
43
-------
with a potentially high fumigation concentration. Also, this
screening procedure considers only TIBLs that begin formation at
the shoreline, and neglects TIBLs that begin to form offshore.
3.8 Complex Terrain 24-hour Screen
The SCREEN model also contains the option to calculate
maximum 24-hour concentrations for terrain elevations above stack
height. A final plume height and distance to final rise are
calculated based on the VALLEY model screening technique (Burt,
1977) assuming conditions of F stability (E for urban) and a stack
height wind speed of 2.5 m/s. Stack tip downwash is incorporated
in the plume rise calculation.
The user then inputs a terrain height and a distance (m) for
the nearest terrain feature likely to experience plume impaction,
taking into account complex terrain closer than the distance to
final rise. If the plume height is at or below the terrain height
for the distance entered, then SCREEN will make a 24-hour average
concentration estimate using the VALLEY screening technique. If
the terrain is above stack height but below plume centerline
height, then SCREEN will make a VALLEY 24-hour estimate (assuming
F or E and 2.5 m/s), and also estimate the maximum concentration
across a full range of meteorological conditions using simple
terrain procedures with terrain "chopped off" at physical stack
height, and select the higher estimate. Calculations continue
until a terrain height of zero is entered. For the VALLEY model
concentration SCREEN will calculate a sector-averaged ground-level
concentration with the plume centerline height (hj as the larger
of 10.Om or the difference between plume height and terrain
height. The equation used is
X = 2.032 Q exp [-0.5 (he/oze) '] . (17)
O-e U X
Note that for screening purposes, concentrations are not
attenuated for terrain heights above plume height. The dispersion
parameter, o^, incorporates the effects of buoyancy induced
dispersion (BID). For the simple terrain calculation SCREEN
examines concentrations for the full range of meteorological
conditions and selects the highest ground level concentration.
Plume heights are reduced by the chopped off terrain height for
the simple terrain calculation. To adjust the concentrations to
24-hour averages, the VALLEY screening value is multiplied by
0.25, as done in the VALLEY model, and the simple terrain value is
multiplied by the 0.4 factor used in Step 5 of Section 4.2.
3.9 Non-regulatory Options
3.9.1 Erode 2 Mixing Height Option
The Erode 2 Mixing Height (Erode, 1991) option calculates a
mixing height that is calculated based on the calculated plume
44
-------
height, the anemometer height wind speed and a stability-dependent
factor which is compared to a stability-dependent minimum mixing
height. The algorithm is expressed as:
ZI = MAX (ZInlnf HE* (1.0 + ZIfact * U:0)
where ZI^ is 300m for A, 100m for B, and 30m for both C and D
stabilities, and ZIfact is 0.01 for A, 0.02 for B, 0.03 for C, and
0.04 for D stability. Erode found that the results of using this
algorithm appear to provide a fairly consistent level of
conservatism relative to the ISCST model.
3.9.2 Variable Anemometer Height Option
The anemometer height is used in adjusting the wind speed to
stack height wind speed for cavity calculations based on the
following power law function:
UO = UOTEN*(AMAX1(10,HS)/ZREF)**0.20
Ul = U1TEN*(AMAX1(10,HS)/ZREF)**0.20
where: UOTEN - initial wind speed value set to 20 m/s.
U1TEN - initial wind speed value set to 1 m/s.
HS - stack height
ZREF - anemometer height
UOTEN is adjusted downward in speed and U1TEN is adjusted upward
in speed in an iterative process until the minimum wind speed, UC,
that will entrain the plume into a building's cavity is found.
The critical wind speed is then adjusted to the anemometer height,
using the reverse of the power law above, as follows:
UC10M = UC * (ZREF/AMAX1(10,HS))**0.20
where: UC10M - represents the critical wind speed at
anemometer height, ZREF.
The variables HANE and ZREF are used interchangeably.
3.9.3 Schulman-Scire Building Downwash/Cavity Option
A non-regulatory building downwash/cavity algorithm (Schulman
and Scire,1993) has been added as a non-regulatory option. This
option is based on the diffusing plume approach with fractional
capture of the plume by the near-wake recirculation cavity.
Extensive parameterization is used to define a building
length scale, roof recirculation cavity, maximum height of the
roof cavity, and the length of the downwind recirculation cavity
(as measured from the lee face of the building).
45
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A building length scale for flow and diffusion is defined as:
R = BS exp(2/3) * BL exp(l/3)
where: BS is the smaller of the building height and
projected width for the minimum side orientation
BL is the larger of the building height and projected
width for the maximum side orientation.
The length of the roof recirculation cavity is estimated as:
LC = 0.9 * R
The roof cavity will reattach to the roof if LC < L where L
is the downwind length of the roof.
The maximum height of the roof cavity is defined as:
HC=0.22*R atx=0.5*R
where x is the downwind distance.
The program uses two algorithms to determine the height and
width of the downwind recirculation cavity or near-wake. If the
roof cavity reattaches to the roof, the height and width are:
HR = H where H is the building height
WR = W where W is the projected width normal to the wind.
If the roof cavity does not reattach, the height and width are:
HR = H + HC
WR = 0.6 * H + 1.1*W
and measured from the lee face of the building.
The length of the recirculation region is calculated using
the formula:
LR = 1.8W/ [ (L/H)C':' * (1.0 + 0.24W/H)]
with the restriction that L/H is set equal to 0.3 if L/H < 0.3,
and L/H is set equal to 3.0 if L/H > 3.0.
The ground level concentration in the recirculation region is
calculated assuming the mass fraction of the plume, below HR at
the downwind end of the region, is captured into the region. The
calculation assumes a Gaussian distribution of the vertical mass
of the plume at that point using the following formula:
a. = 0.21R0'25 xc'"5
46
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The cavity concentration, C, is then calculated as a fraction
of the plume content using the following empirical formula:
C =
* BC Q/(B;W;A:
where: fc is the mass fraction of the plume captured in the
recirculation region
B: is an empirical constant approximately equal to 16
wc is the stack exit speed
Ac is the stack exit face area
Up is the upwind wind speed at roof level
s" is the "stretched string" distance between the
stack base and the receptor.
The position of the stack on the roof is taken into
consideration. A ratio is calculated based on the distance of the
stack from a centerline of the building perpendicular to the wind
flow for each of two orientations divided by the along wind flow
length of the building. Below is an example where the along wind
flow length is HW and the distance of the stack from the
centerline is "x" ; producing a ratio of .4. Note that the ratio
is always a positive number. Ratios greater than .5 indicate that
the stack is not on the roof .
v-
HW
(x/HW = .4)-->
HL
+ •
5
.5
HL
(x/HL = .15)---
. S
HW
+ •
,5
- +
.5
47
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48
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4. NOTE TO PROGRAMMERS
The SCREEN model executable provided on SCRAM was compiled
using the Microsoft FORTRAN Compiler, Version 5.1. It was
compiled with the emulator library, meaning that the executable
file (SCREEN3.EXE) will run with or without a math coprocessor
chip. A minimum of 256 KB of RAM is required to execute the
model. Provided in a compressed file on SCRAM are the executable
file, SCREEN3.EXE; the FORTRAN source code files, SCREEN3A.FOR,
SCREEN3B.FOR, SCREEN3C.FOR, MAIN.INC, and DEPVAR.INC; a sample
input file, EXAMPLE.DAT; an associated output file, EXAMPLE.OUT;
and this document, the SCREEN3 Model User's Guide (in WordPerfect
5.1 format), SCREEN3R.WPF. Also included is a READ.ME file with
instructions on extracting SCREEN.
The SCREEN model provided was compiled with the following
Microsoft FORTRAN compile command:
FL /FPi /Gt 7FeSCREEN3.EXE SCREEN3A.FOR SCREEN3B.FOR SCREEN3C.FOR
where the /FPi compile option specifies the emulator library and
causes floating point operations to be processed using in-line
instructions rather than library CALLs (used for faster
execution), the /Gt option specifies the data threshold for
storing data in a new segment, and the /FeSCREEN3.EXE option
specifies the name of the executable file. SCREEN3 uses the
FORTRAN default unit number of 5 (five) for reading input from the
keyboard and 6 (six) for writing to the screen. The unit number
for the disk output file, SCREEN.OUT, is set internally to 9, and
the unit number for writing inputs to the data file, SCREEN.DAT,
is set to 7. These unit numbers are assigned to the variables IRD,
IPRT, IOUT, and IDAT, respectively, and are initialized in BLOCK
DATA at the end of the SCREEN3.FOR source file. The Microsoft
version of SCREEN also uses the GETDAT and GETTIM system routines
for retrieving the date and time. These routines require the
variables to be INTEGER*2, and they may differ on other compilers.
The following simple change can be made to the SCREEN source
file, SCREEN3A.FOR, in order to create a version that will accept
a user-specified output filename, instead of automatically writing
to the file SCREEN.OUT. An ASCII text editor or a wordprocessor
that has an ASCII or nondocument mode may be used to edit the
source file. Delete the letter C from Column 1 on lines 262 to
265. They should read as follows:
WRITE(IPRT,*) ' '
94 WRITE(IPRT,*) 'ENTER NAME FOR OUTPUT FILE'
READ(IRD,95) OUTFIL
95 FORMAT(A12)
49
-------
With this change, if the user-specified filename already exists,
it will be overwritten. If desired, the OPEN statement on line
267 may also be changed to read as follows:
OPEN(IOUT,FILE=OUTFIL,STATUS='NEW',ERR=94)
With this additional change, the program will continue to prompt
for the input filename until a filename that doesn't already exist
is entered by the user. Before recompiling, make any other
changes that may be necessary for the particular compiler being
used. It should be noted that without optimization, the source
file may be too large to compile as a single unit. In this case,
the SCREEN3A.FOR and SCREEN3B.FOR files may need to be split up
into separate modules that can be compiled separately and then
linked together.
The SCREEN model code has also been successfully compiled
with the Lahey F77/EM-32 Fortran compiler, with the following
compile commands:
F77L3 SCREEN3A.FOR /NO /NW /D1LAHEY
F77L3 SCREEN3B.FOR /NO /NW
where the /NO option suppresses the printing of compile options,
/NW suppresses certain warning messages, and /D1LAHEY defines
LAHEY for implementing the conditional compile block of Lahey-
specific statements for retrieving the system date and time for
the output file. Follow the instructions with the Lahey compiler
for linking the model to create an executable file.
50
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5. REFERENCES
Auer, Jr., A.H., 1978. Correlation of Land Use and Cover with
Meteorological Anomalies. Journal of Applied Meteorology,
17(5): 636-643.
Briggs, G.A., 1969. Plume Rise. USAEC Critical Review Series,
TID-25075, National Technical Information Service,
Springfield, Virginia 22151.
Briggs, G.A., 1973. Diffusion Estimation for Small Emissions.
NOAA ATDL, Contribution File No. 79 (Draft). Oak Ridge, TN.
Briggs, G.A., 1975. Plume Rise Predictions. In: Lectures on Air
Pollution and Environmental Impact Analysis. Haugen, D.A.
(ed.), American Meteorological Society, Boston, MA, pp.
59-111.
Erode, R.W., 1991. A Comparison of SCREEN Model Dispersion
Estimates with Estimates From a Refined Dispersion Model.
Seventh Joint Conference on Applications of Air Pollution
Meteorology with A&WMA.. 93-96.
Burt, E.W., 1977. Valley Model User's Guide. EPA-450/2-77-018.
U.S. Environmental Protection Agency, Research Triangle
Park, NC.
Environmental Protection Agency, 1977. Guidelines for Air
Quality Maintenance Planning and Analysis, Volume 10
(Revised): Procedures for Evaluating Air Quality Impact of
New Stationary Sources. EPA-450/4-77-001 (OAQPS Number 1.2-
029R), Research Triangle Park, NC.
Environmental Protection Agency, 1983. Regional Workshops on Air
Quality Modeling: A Summary Report - Addendum.
EPA-450/4-82-015. U.S. Environmental Protection Agency,
Research Triangle Park, NC.
Environmental Protection Agency, 1987a. Guideline On Air Quality
Models (Revised) and Supplement A. EPA-450/2-78-027R. U.S.
Environmental Protection Agency, Research Triangle Park, NC.
Environmental Protection Agency, 1987b. Analysis and Evaluation
of Statistical Coastal Fumigation Models. EPA-450/4-87-002.
U.S. Environmental Protection Agency, Research Triangle
Park, NC.
Environmental Protection Agency, 1988. Screening Procedures for
Estimating the Air Quality Impact of Stationary Sources -
Draft for Public Comment. EPA-450/4-88-010. U.S.
Environmental Protection Agency, Research Triangle Park, NC.
51
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Environmental Protection Agency, 1995a: Screening Procedures for
Estimating the Air Quality Impact of Stationary Sources,
Revised. EPA-450/R-92-019. U.S. Environmental Protection
Agency, Research Triangle Park, NC.
Environmental Protection Agency, I995b. Industrial Source
Complex (ISC3) Dispersion Model User's Guide. EPA-454/B-95-
003b. U.S. Environmental Protection Agency, Research
Triangle Park, NC.
Hosker, R.P., 1984. Flow and Diffusion Near Obstacles. In:
Atmospheric Science and Power Production. Randerson, D.
(ed.), DOE/TIC-27601, U.S. Department of Energy, Washington,
D.C.
Leahey, D.M. and M.J.E. Davies, 1984. Observations of Plume Rise
from Sour Gas Flares. Atmospheric Environment, 18, 917-922.
Misra, P.K. and S. Onlock, 1982. Modelling Continuous Fumigation
of Nanticoke Generating Station Plume. Atmospheric
Environment. 16, 479-482.
Pierce, T.E., D.B. Turner, J.A. Catalano, and F.V. Hale, 1982.
PTPLU - A Single Source Gaussian Dispersion Algorithm User's
Guide. EPA-600/8-82-014. U.S. Environmental Protection
Agency, Research Triangle Park, NC.
Pierce, T.E., 1986. Addendum to PTPLU - A Single Source Gaussian
Dispersion Algorithm. EPA/600/8-86-042. U.S. Environmental
Protection Agency, Research Triangle Park, NC. (Available
only from NTIS. NTIS Accession Number PB87-145 363.)
Pierce, T.E. and D.B. Turner, 1980. User's Guide for MPTER - A
Multiple Point Gaussian Dispersion Algorithm With Optional
Terrain Adjustment. EPA-600/8-80-016. U.S. Environmental
Protection Agency, Research Triangle Park, NC.
Randerson, D., 1984. Atmospheric Boundary Layer. In: Atmospheric
Science and Power Production. Randerson, D. (ed.),
DOE/TIC-27601, U.S. Department of Energy, Washington, D.C.
Schulman,L.L. and Scire, J.S., 1993. Building Downwash Screening
Modeling for the Downwind Recirculation Cavity. Air and
Waste.. August. 1122-1127.
Turner, D. B., 1964. A Diffusion Model for an Urban Area.
Journal of Applied Meteorology. 3_, 83-91.
Turner, D.B., 1970. Workbook of Atmospheric Dispersion
Estimates. Revised, Sixth printing, Jan. 1973. Office of
Air Programs Publication No. AP-26.
52
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TECHNICAL REPORT DATA
(Please read Instructions on reverse before completing)
1. REPORT NO.
EPA-454/B-95-004
3. RECIPIENT'S ACCESSION NO.
4. TITLE AND SUBTITLE
SCREENS User's Guide
5. REPORT DATE
September 1995
6. PERFORMING ORGANIZATION CODE
7. AUTHOR(S)
8. PERFORMING ORGANIZATION REPORT NO.
9. PERFORMING ORGANIZATION NAME AND ADDRESS
Pacific Environmental Services, Inc.
5001 South Miami Boulevard
P.O. Box 12077
Research Triangle Park, NC 27709-2077
10. PROGRAM ELEMENT NO.
11. CONTRACT/GRANT NO.
68D30032
12. SPONSORING AGENCY NAME AND ADDRESS
13. TYPE OF REPORT AND PERIOD COVERED
U.S. Environmental Protection Agency
Office of Air Quality Planning and Standards
Emissions, Monitoring, and Analysis Division
Research Triangle Park, NC 27711
14 SPONSORING AGENCY CODE
15. SUPPLEMENTARY NOTES
This document supercedes/replaces EPA-45Q/4-92-006.
16 ABSTRACT
This document presents user instructions on the use of the SCREENS screening model. SCREENS is a
PC-driven, Gaussian-plume atmospheric dispersion model which calculates maximum 1-hour, downwind
concentrations of non-reactive pollutants. The primary revision between SCREEN2 and SCREENS is the
incorporation of the numerical integration algorithm for area sources. The SCREENS model serves as a
regulatory screening model to determine compliance with the National Ambient Air Quality Standard
(NAAQS) or other air quality standards.
17.
KEY WORDS AND DOCUMENT ANALYSIS
DESCRIPTORS
b. IDENTIFIERS/OPEN ENDED TERMS
c. COSATI Field/Group
Air Pollution
Turbulent Diffusion
Meteorology
Air Dispersion Models
Computer model
Industrial Sources
Deposition
Downwash
Dispersion
18. DISTRIBUTION STATEMENT
Release Unlimited
19. SECURITY CLASS (Report)
Unclassified
21 NO. OF PAGES
55
20. SECURITY CLASS (Page)
Unclassified
22 PRICE
EPA Form 2220-1 (Rev. 4-77)
PREVIOUS EDITION IS OBSOLETE
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