Model Change Bulletin MCB #6 TSCREEN (dated 94133) This Model Change Bulletin documents the updated TSCREEN (dated 94133) and initiates its use by the modeling community. The TSCREEN (dated 92334) model is now obsolete and should be replaced by this update. 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 94133) 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 SCREEN model used in the previous version of TSCREEN has been replaced with the current version of the SCREEN2 model (dated 92245) available from the SCRAM BBS. This version of the SCREEN2 model is consistent with the ISCST2 model (dated 93109), and uses a finite line segment algorithm for modeling area sources, instead of a virtual point source algorithm as used in the previous version. For the new SCREEN2 model, area source distances are measured from the center of the area source, instead of from the downwind edge. The SCREEN2 model also includes other modifications relative to the previous TSCREEN model in order to maintain consistency with the ISCST2 model. 2. Two new Superfund scenarios have been added -- Soil Vapor Extraction and Thermal Incineration. These are available as scenarios 4.2 and 4.4, respectively from the Superfund Release Type menu. Both make use of the SCREEN2 model point source option, and are based on the "Air/ Superfund National Technical Guidance Study Series: Models for Estimating Air Emission Rates from Superfund Remedial Actions," EPA-451/R-93-001, U.S. Environmental Protection Agency, March 1993. 3. Based on extensive diagnostic testing of the TSCREEN model, several bugs have been identified and corrected. These are briefly summarized below: a. A correction was made in the order of inputs read by the Britter- McQuaid model. The order of the Latent Heat of Vaporization and the Gas Heat Capacity variables was reversed in the Britter-McQuaid model in order to be consistent with the order on the output from the model and the order of variables sent to the model by TSCREEN. b. The calculation of the variables QC and MO, which are passed to the RVD model by TSCREEN, was corrected. c. The equation for calculating the variable BETA was corrected (by removing a factor of 2) in scenarios 2.3, 2.4, 2.5, 2.6, and 3.8. d. The density ratio check was corrected in scenarios 2.5 and 2.6. e. The calculation of the variable TS was corrected to use MAX instead of MIN in scenarios 3.8 and 3.9. f. The confidence interval for calculation of annual averages was corrected from +/- 0.2 to +/- 0.02. g. The liquid density values in the Chemical Database were corrected. The previous values were inconsistent with the stated units of kg/m^3. Additional data fields in the data base were also filled in. h. A warning message was added for cases when the ratio of release density to air density is between 1.00 and 1.02. The model currently treats such cases as passive releases, but application of other criteria, such as a Richardson number check, may indicate denser- than-air effects for certain release characteristics and meteorological conditions. Due to these revisions in the model, user's comparing this version of the TSCREEN model to that illustrated in the workbook will note that: a) the new Superfund scenarios (item 3 above) are not described; b) the changes to TSCREEN identified above have not been incorporated; c) several TSCREEN menus and screens, as well as estimated ambient air impacts are different than those shown in the workbook. The following is a summary of the differences that a user may encounter: 1. For scenarios involving continuous stack releases in which TSCREEN uses the SCREEN2 model, the user may find minor differences due to changes in the SCREEN2 model. 2. For scenarios involving continuous releases from ground level area sources in which TSCREEN uses the SCREEN2 model, the user will find major differences due to the new area source algorithm used in SCREEN2. 3. For scenarios involving instantaneous releases in which TSCREEN uses the PUFF model, the user will find no differences. 4. For scenarios involving pressurized releases in which TSCREEN uses the RVD model, the user will find a few differences due to corrections explained in (3)(b) above. 5. For scenarios involving liquids spills from which denser-than-air gases are released and TSCREEN uses the Britter and McQuaid model, the user may find minor differences due to changes explained in (3)(a) above. The TSCREEN model user's guide has been revised. The "User's Guide to TSCREEN, a Model for Screening Toxic Air Pollutant Concentrations (Revised)," EPA-454/B-94-023 can be downloaded from the documentation section. It can also be purchased from NTIS. This user's guide contains information, especially useful for first time users, on how to operate the model. 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 92334) have the following options for downloading the new TSCREEN model: 1. For simplicity, they may download all four zip files, copy them to a new subdirectory, and unzip them using PKUNZIP. 2. Since the contents of TSCREEN1.ZIP and TSCREEN2.ZIP have not changed, users of the previous version may want to download only the TSCREEN3.ZIP and TSCREEN4.ZIP files, copy them to the subdirectory where the previous version is located, and unzip them to replace the old files. They must replace all of the old files, including the OLDRUNS.DBF file that saves data from previous scenarios, since the database structure has changed. The user may note that the date and size of the TSCREEN1.ZIP and TSCREEN2.ZIP files has changed. This is because they were re-zipped with the new version of PKZIP. 3. Previous users may also download TSCREEN3.ZIP and TSCREEN4.ZIP, copy them to a new subdirectory, and then copy the TSCREEN1.ZIP and TSCREEN2.ZIP files previously downloaded to the same directory, before unzipping the files. In this way the two versions will be in separate directories, and they should then delete the previous version. New users should download all four zipped files, copy them to 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. Some users may also want the model source code, which is contained in the zipped file called TSCRCODE.ZIP.