Model Change Bulletin
              
                                    MCB#1                         09/09/95

                             ISCST3 (dated 95250)
                             ISCLT3 (dated 95250)


     This Model Change Bulletin initiates the use of the Industrial Source
Complex Dispersion Model (ISC3).

     Changes from the ISCST2 (dated 93109) and ISCLT2 (dated 93109) include
(1) a new area source algorithm, (2) a revised dry deposition algorithm, (3) a
wet deposition algorithm, (4) COMPLEX1 algorithms for complex terrain
calculations, and (5) a pit retention algorithm.

     A detailed description of ISCST3 and ISCLT3 files and their contents is
contained in the README.TXT file, located with the *.EXEs.  The ISCEV3 model
has not been updated at this time.  It is currently being updated and a SCRAM
Alert will notify users of its availability.  Likewise, the ISC2VOL3.ZIP file
was not updated.  Due to budget restrictions, there are no imminent plans to
revise the programming guide (ISC2VOL3.ZIP).

     The following is a list of changes which have occurred between the Beta
Testing versions of ISCST3/ISCLT3 (dated 95200) and the current versions of
ISCST3/ISCLT3 (dated 95250):


ISCST3

  (1)  Fixed problem with summary tables for short term averages, which
occurred when requesting non-consecutive high values (e.g. OU RECTABLE FIRST
SECOND SIXTH),

  (2)  Fixed error for open pit sources with release heights above the base
of the pit, and

  (3)  Added fatal error message for open pit source with no particle
categories specified.


ISCLT3

  (1)  Same as items (2) and (3) above.




     This program will henceforth be identified by its name and a Julian date,
i.e.: ISCST3 (dated 95250).  Future changes to this model will be identified
by an updated Julian date found on the first line of the model source code
and followed by a reference to a SCRAM BBS Model Change Bulletin (MCB#n) which
will explain the nature and contents of the changes.  All Model Change
Bulletins for this program will always be available for back reference on
this BBS.  In the future, notification of changes (MCBs) to this prgram will be
made in the SCRAM ALERTS section of this BBS immediately following the LOGON
procedure.




            ******************************************************
            *                                                    *
            *      Amendment #1 to Model Change Bulletin #1      *
            *                      04/17/96                      *
            *                                                    *
            ******************************************************



The purpose of this amendment is to alert users of ISC3 that it is possible
to inadvertently use the model in a way that was not intended.  When a user
is estimating air quality credits (e.g., PSD increment expansion) due to
source shut down or emissions reductions, negative emissions can be input to
the model as part of the calculations.  While this approach is widely used in
flat terrain applications, serious reservations have been expressed regarding
its use with complex terrain screening techniques.  Due to the inherent
conservative nature of these screening techniques, excessive air quality
credits may be estimated.  The current version of ISC3 has the capability of
using complex terrain screening for estimating air quality credits.

Although the Guideline on Air Quality Models (Revised) does not specifically
provide guidance on using models to estimate air quality credits, there are
precedents for such usage.  The EPA Model Clearinghouse responded to this
issue in a November 6, 1992 memorandum to EPA Region I (see Model Clearinghouse
in Area H on the main SCRAM menu, then select Current Memos, file CFYM#77.TXT).
This was in response to a proposal to use a screening model to estimate air
quality or concentration credits in intermediate terrain due to emission
reductions.  Note that intermediate terrain is defined as the terrain between
stack height and plume height.  Also note that when screening modeling is
conducted in the intermediate terrain area to calculate concentration
estimates, the GAQM recommends that concentration estimates be calculated for
each receptor and hour with both a simple terrain model (with terrain cut-off
at stack top) and a complex terrain screening model.  The larger of the model
estimates would then be selected for that hour.

EPA Region X has presented a method that would allow ISC3 to be applied for
estimating air quality credits consistent with the November 6, 1992 Model
Clearinghouse memorandum.  This method consists of coding changes to the ISC3
which were developed by RTP Environmental Associates.  Since there may be
other cases where a similar modeling analysis for concentrations credits is
necessary, the coding changes submitted by Region X are available in Area F of
SCRAM BBS - Topics for Review/Comment.  Implementation of these coding changes
is the responsibility of the user and would require recompilation of the
program.  However, the Agency has not made a decision on the acceptability of
these changes for a formal modification of ISC3 and they should only be used
on a case-by-case basis with the concurrence of the appropriate regulatory
agencies.  Comments on the accuracy of the code and its compatibility with the
November 6, 1992 memorandum should be directed to Dennis Doll (US EPA)
via SCRAM email or INTERNET doll.dennis@epamail.epa.gov     .