Model Change Bulletin

                                 MCB #7

                          TSCREEN (dated 95260)

This Model Change Bulletin documents the revisions to the TSCREEN
model (dated 95260).  TSCREEN, a model for screening toxic air
pollutant concentrations, is based on the release scenarios and
methods described in "Workbook of Screening Techniques for
Assessing Impacts of Toxic Air Pollutants," EPA-454/R-92-024.  In
order to continue providing improved modeling techniques, the new
TSCREEN model (dated 95260) contains several changes from the
workbook that are described below.  These changes will be
incorporated into the workbook at a later date.

The changes to the TSCREEN model include the following:

   1. The SCREEN2 model used in the previous version of TSCREEN
has been replaced with the current version of the SCREEN3 model
(dated 95250) available from the SCRAM BBS.  This version of the
SCREEN3 model is consistent with the ISCST3 model (dated 95250).
 
   2. For scenarios that utilize the area source option of the
SCREEN3 model, the new area source algorithm is now based on a
double integration of the Gaussian plume kernel for area sources
instead of the finite line segment algorithm as used in the
previous version.  Area source distances continue to be measured
from the center of the area source.  The capability of making
concentration estimates within the area source is now possible.
The option to use a revised (draft) version of the SCREEN2 model
(dated 94133) that incorporates a numerical integration algorithm
for modeling area sources is thus not necessary any longer and is
removed. 

Due to these revisions in the model, ambient air impacts for
those scenarios that utilize the area source algorithm are
different than those obtained from the previous version of
TSCREEN, or shown in the current version of the workbook.

The TSCREEN model has been zipped into 4 files for convenient
downloading. These are TSCREEN1.ZIP; TSCREEN2.ZIP; TSCREEN3.ZIP;
and TSCREEN4.ZIP.  These contain the executable files that are
necessary to run the model.  Users of the previous version of the
TSCREEN model (dated 94133) need only download the files
TSCREEN3.ZIP and TSCREEN4.ZIP, since the information in files
TSCREEN1.ZIP and TSCREEN2.ZIP has not changed.  Some users may
want the model source code as well, and this file is called
TSCRCODE.ZIP.

After you download the 4 zipped files, copy them on a
subdirectory, unzip them using the PKUNZIP utility program, and
then type TSCREEN.  The program pop-up menus will appear, and you
are on your way.

If you should have questions or comments about the TSCREEN model,
contact Joe Touma at (919) 541-5381.