Model Change Bulletin
              
                                 MCB#2                 8/14/89

                          RTDM32 (dated 89226)


     This Model Change Bulletin is formal notification of a change to the
RTDM3.2 Model.  The updated code is dated 89226 and further identified by
MCB#2.

     In June, the developer of  RTDM reported a coding error in RTDM3.2
that will give erroneous results in some model runs.  The error occurs in
the calculation of the partial reflection factor such that, in a few cases,
the concentration of a plume can increase as it moves downwind.  This gives
concentrations that can be too high--by a factor of two in the most extreme
cases.  The developer also proposed a correction to the code.  We concur
with both the assessment and correction proposed by the developer.

     The correction requires that the following statement be inserted after
statement VRT02660 in subroutine VERT:

                              K = K + 1

     THIS CORRECTION HAS BEEN MADE TO THE SOURCE CODE WHICH IS AVAILABLE ON
THE SCRAM BBS.

                              *  *  *

     In most cases,  this change will probably  make no difference in the
results.  When there is a change, the revised model will always produce lower
concentration estimates.  In some cases, the values can be lower by as much
as a factor of two.

     Three test cases were run to give a first indication of the kinds of
differences to expect.  These involved a low (35 meter), a medium (100 meter),
and a high (200 meter) stack.  These are the same test cases that have been
previously used to  evaluate the effects  of changes to  RTDM.  The only
differences in the results produced by the corrected model are for the low
(35 meter) stack.  The 1-hour high and high second high concentrations, and
the 3-hour high and high second high concentrations decreased by 27, 26, 20,
and 2 percent, respectively.  Both 24-hour concentrations remained unchanged.
All values for the medium (100 meter) and tall (200 meter) stacks remained
unchanged.

                             *** NOTE ***

     With no further changes to the RTDM3.2 Model code, it will compile using
the Microsoft 4.1 compiler and execute on a PC.  The same holds true for the
preprocessor program, PRERTDM and the post processor program, POSTRTDM.