United States
Environmental Protection
Atmospheric Sciences Research
Research Triangle Park NC 27711
Research and Development
EPA/600/S8-85/029 Jan. 1986
Project  Summary
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
  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).

  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
• 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
• choice of one of seven vertical disper-
  sion parameter schemes; and
• optional output of concentration versus
  stability histograms at user-specified

  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,
   • 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

   Conclusions and
    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
Center for Environmental Research
Cincinnati OH 45268
Official Business
Penalty for Private Use $300

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