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 CTDMPLUS (dated 91107) is obsolete and, with this bulletin, will no longer be available on SCRAM.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