United States
Environmental Protection
Agency
Atmospheric Sciences Research
Laboratory
Research Triangle Park NC 27711
Research and Development
EPA/600/S8-85/029 Jan. 1986
Project Summary
CDM-2.0—Climatological
Dispersion Model User's Guide
John S. Irwin, Thomas Chico, and Joseph Catalano
The Climatological Dispersion Model—
Version 2.0 (CDM-2.0) determines
long-term (seasonal or annual) quasi-
stable pollutant concentrations in rural
or urban settings using average emis-
sion rates from point and area sources
and a joint frequency distribution of
wind direction, wind speed, and stabil-
ity. The Gaussian plume hypothesis
forms the basis for the calculations.
Contributions from area sources are
calculated assuming the narrow plume
hypothesis and involve an upwind inte-
gration over the area sources. Compu-
tations can be made for up to 200 point
sources and 2500 area sources at an
unlimited number of receptor locations.
The number of point and area sources
can be easily modified within the code.
CDM-2.0 is an enhanced version of
CDM and includes the following op-
tions: 16 or 36 wind-direction sectors,
initial plume dispersion, buoyancy-in-
duced dispersion, stack-tip downwash,
and gradual (transitional) plume rise.
The user has a choice of seven vertical
dispersion parameter schemes. Option-
al output includes point and area con-
centration roses and histograms of
pollutant concentration by stability
class.
This Project Summary was developed
by EPA's Atmospheric Sciences Re-
search Laboratory, Research Triangle
Park. NC, to announce key findings of
the research project that is fully docu-
mented in a separate report of the same
title (see Project Report ordering infor-
mation at back).
Introduction
The Climatological Dispersion Model
User's Guide—Version 2.0 is an upgraded
version of the CDM that was released in
1973. CDM-2.0 is a long-term (seasonal
or annual) algorithm for evaluating the
effects of multiple point and area sources
in the near-field (within 25 km). The
modeling region should consist of rel-
atively flat terrain. The model includes
the following computation features in
common with CDM:
• handles up to 200 point sources and
2500 area sources,
• considers an unlimited number of
receptors,
• optionally uses Holland's equation
(1953) for limiting plume rise.
The number of sources can be modified
by a global change within the code.
Optional output features common to both
CDM and CDM-2.0 are point and area
concentration roses at user-specified
receptors. The user can reduce output
volume by only listing concentration
results and not echoing the input data.
Results and Discussion
Modeling features added to CDM-2.0
include the following:
• optional initial dispersion, buoyancy-
induced dispersion, stack-tip down-
wash, and gradual plume rise;
• choice of joint frequency function
based on 16 or 36 wind direction
sectors;
• choice of one of seven vertical disper-
sion parameter schemes; and
• optional output of concentration versus
stability histograms at user-specified
receptors.
The seven vertical dispersion param-
eter schemes are as follows:
-------
• Briggs-rural,
• Briggs-urban,
• Brookhaven National Laboratory,
• Klug,
• St. Louis,
• PGCDM, and
• PGSIG.
PGCDM are the Pasquill-Gifford curves
as-employed in the version, of COM
released in 1973. PGSIG are the Pasquill-
Gifford curves as currently employed in
the models approved for use for regulatory
applications. The plume rise algorithm
has been modified to handle rise during
stable conditions and to consider mo-
mentum-dominated plumes.
Modeling limitations are as follows:
• source emissions and meteorology
should be uncorrelated,
• variation in emission rates between
adjacent area sources is assumed to
be negligible,
• terrain should be flat to gently rolling,
and
• chemical reactions or removal mech-
anisms other than those that can be
handled as a simple exponential decay
are not considered.
It is assumed that one wind vector and
one stability category are representative
of the area being modeled at any given
time.
Conclusions and
Recommendations
CDM-2.0 is an enhanced version of the
Climatological Dispersion Model (Version
80247) released in 1973. This new
version provides greater flexibility for the
user in the characterizations of the dis-
persion and plume rise. The model can be
adapted for estimation of annual or
seasonal pollutant concentrations in both
urban and rural situations. For rural
applications, the area-source algorithm
(based on the narrow-plume hypothesis)
should be employed with caution. A|
default option has been incorporated in
the model to preselect several of the
modeling choices. The default selections
are consistent with current regulatory
guidance. It is intended that the default
selections be revised and updated when
and if regulatory guidance changes.
The EPA author. John S. Irwin. is with Atmospheric Sciences Research
Laboratory, Research Triangle Park. NC 27711; and Thomas Chico and Joseph
Catalano are with Aerocomp, Inc., Costa Mesa, CA 92626.
D. Bruce Turner is the EPA Project Officer (see below).
The complete report, entitled "COM-2.0—Climatological Dispersion Model User's
Guide, "(Order No. PB 86-136 546/AS; Cost: $ 16.95, subject to change) will be
available only from:
National Technical Information Service
5285 Port Royal Road
Springfield, VA 22161
Telephone: 703-487-4650
The EPA Project Officer can be contacted at:
Atmospheric Sciences Research Laboratory
U.S. Environmental Protection Agency
Research Triangle Park, NC 27711
United States
Environmental Protection
Agency
Center for Environmental Research
Information
Cincinnati OH 45268
Official Business
Penalty for Private Use $300
EPA/600/S8-85/029
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