                       Model Change Bulletin

                               MCB#4                       3/18/93

                       CTDMPLUS (dated 93076)


     This Model Change Bulletin is to notify users of changes
made to correct problems in CTDMPLUS (dated 91107).  The specific
changes, the problems addressed, and the consequences of the
changes are summarized in the following.  Reevaluation of the
revised model demonstrated that these modifications did not
affect design concentrations and, thus, should not have a
significant regulatory impact.

     These revisions constitute CTDMPLUS (dated 93076) and
completely replace CTDMPLUS (dated 91107).  With this bulletin,
the source codes and executables for CTDMPLUS (dated 93076) are
being made available for downloading from the Air Quality Models
area of the File Transfer Section of SCRAM.  CTDMPLUS (dated
91107) is obsolete and, with this bulletin, will no longer be
available on SCRAM.  All copies of such should be discarded.


     Users of CTDMPLUS are referred to a companion WordPerfect
5.1 document (CTPLSMOD.WPF) "Modifications to the CTDMPLUS and
CTSCREEN Models" for more detailed discussion of these problems
(see Documentation below).  All of the problems described in the
following have been corrected in CTDMPLUS (dated 93076).  The
subheadings in Section 1 below follow the outline of the
companion WordPerfect 5.1 document, CTPLSMOD.WPF.


1. PROBLEMS IN CTDMPLUS (dated 91107)


1.1  Wrap Calculations

     Two problems are related to situations where a source is
very close to the terrain feature:  1) model adjustment of WRAP
ellipse size to prevent a source from being located inside the
fitted ellipse (to prevent a mathematical error) and 2) a travel
time problem for receptors located on actual terrain contours
that 'jut out' in the upwind direction from the fitted ellipse. 
Both problems may inappropriately affect concentrations during
stable/neutral conditions.  The second problem, in particular,
may result in significant overestimates.


1.2  Convergence Problem

     The algorithm in CTDMPLUS for computing the position of the
streamline that passes through the source occasionally has
problems converging on a solution.  This results in the error
message "ENDLESS LOOP IN PATH" and a null concentration
calculation for the given simulation.  Calculations for other
simulations are not affected.  This error occurs only in
stable/neutral conditions.


1.3  Meteorological PROFILE file

     CTDMPLUS expects that the instrumentation levels for the
meteorological tower are entered in increasing order in the
PROFILE file.  The check for this was coded incorrectly.  In some
cases this may result in an abnormal termination with the error
message "PROFILE HEIGHT VALUE INCORRECT ..."; this may occur even
when the data are entered correctly.  This coding error does not
affect concentration estimates.


1.4  Limit on Convective Scaling Velocity

     CTDMPLUS places a lower limit on the convective scaling
velocity of 0.167 times the wind speed half-way between stack-top
and plume height.  In multiple stack situations this algorithm
may result in an incorrect scaling velocity.  This affects
CTDMPLUS calculations in convective conditions only.


1.5  Negative Emission Rates

     The model does not currently allow negative emission rates. 
This prevents the use of negative emission rates to estimate air
quality improvement when an existing source is retired.


1.6  Source Contribution Table

     The CTDMPLUS option for source contribution tables
potentially can result in an extremely large output file
(CTDM.OUT).  To avoid this, source contribution tables in
CTDMPLUS (dated 93076) are written to a separate file (ASCII or
Binary).

1.7  Special handling of receptors below stack top

     An additional switch (ISTKTP) has been added to the CTDM.IN
file.  Setting this switch to 1 will cause the contribution from
a particular source to be set to 0 for all receptors with
elevations below stack- top for that source.  This switch will
facilitate analysis of results for situations requiring both
simple and complex terrain modeling.


2.   Documentation

     Modifications to correct these problems have been completed
and are described in a companion WordPerfect 5.1 document
(CTPLSMOD.WPF) "Modifications to the CTDMPLUS and CTSCREEN
Models".  This document provides detailed discussion of these
problems, how they were resolved, the likely effect of the
modifications on modeled concentrations, and the results of a
model reevaluation.  It is available for downloading from SCRAM
as CTPLSMOD.ZIP.  Additional material is provided in the
WordPerfect 5.1 document (CTDMSUP.WPF) which describes
modifications to the CTDMPLUS user's guide.  The latter document
is available for downloading from SCRAM as CTDMSUP.ZIP.  Note
that the unziped files have the same name with a 'WPF' extension.


3.   CTDMPLUS Design Value Comparisons

     CTDMPLUS (dated 93076) was tested and reevaluated using the
Lovett, Westvaco, and Widows Creek data sets from previous
evaluations.  Convective and neutral/stable results are
summarized separately in the following.  For convective
conditions, the one modification which could affect
concentrations (section 1.4), was found to have no effect on the
top 25 1-hour concentration estimates for each of the three data
sets.  For neutral/stable conditions, the results were as
follows:  for the Lovett data base, there were no changes in the
top 25 concentration estimates; for the Westvaco data base, 14 of
the top 25 concentrations were affected - the maximum absolute
difference between CTDMPLUS (dated 93076) and CTDMPLUS (dated
91107) was less than 1 percent; for the Widows Creek data base,
23 of the top 25 concentrations were affected - the maximum
absolute difference was less than 5 percent.


4.   Contacts for Questions

     Questions related to the use of CTDMPLUS in specific
regulatory applications should be directed to the appropriate
regulatory agency or Regional Modeling Contact (see State/Local
Modeling Contacts in the Agency Communications Section of SCRAM).

     Generic questions related to the use and regulatory
applicability of the model should be directed to Desmond Bailey
via SCRAM EMAIL or phone (919) 541-5248.

     Questions or problems related to program execution should be
reported to Donna Schwede via SCRAM EMAIL or phone (919) 541-3255
or to Steve Perry phone (919) 541-1341.

     Users who would like to share comments, suggestions, or
helpful hints are encouraged to use the Model Conferencing area
on the SCRAM Bulletin Board (see Model Conferencing area in the
Public Communications Section of SCRAM).


     Technical correspondence should be directed to:

     Dr. Steven Perry or
     Ms. Donna Schwede
     Applied Modeling Research Branch
     ACMD/AREAL/USEPA
     MD-80
     Research Triangle Park, NC  27711

     Fax: (919) 541-1379

