As part of the BREEZE AERMOD Parallel development process, Trinity staff performed hundreds of AERMOD model runs using both the AERMOD Fortran executable available from the U.S. EPA SCRAM Web site (http://www.epa.gov/scram001/dispersion_prefrec.htm) and the BREEZE AERMOD Parallel Fortran executable developed by Trinity Consultants, Inc.  The U.S. EPA version number for both of these executables was 07026.

Upon communication regarding the availability of BREEZE AERMOD Parallel, staff from several state regulatory agencies indicated that they would defer to EPA’s Office of Air Quality Planning and Standards (OAQPS) to evaluate the BREEZE AERMOD Parallel model. Trinity management and developers from Trinity’s Project HPCM team teleconferenced with OAQPS on October 10, 2007 to clarify the requirements of EPA regarding the demonstration of equivalency between the open source version of the EPA model and the BREEZE Parallel version.  OAQPS concluded that it was the responsibility of the Regional Modelers to determine if BREEZE AERMOD Parallel (or any other commercial product) is valid for regulatory use.  To support this conclusion, EPA’s Guideline on Air Quality Models, Appendix W, Section 3.1.2(b) addresses the status of models where changes to the source code are made that do not affect the concentration estimates (as is the case with BREEZE AERMOD Parallel), stating, “when any changes are made, the Regional Administrator should require a test case example to demonstrate that the concentration estimates are not affected.”

Trinity has made available to all U.S. state and federal regulatory modelers the BREEZE AERMOD Parallel modeling application to assist in their equivalency demonstration.  In addition, Trinity has made available, as requested by OAQPS, model runs completed with the SCRAM version of AERMOD and BREEZE AERMOD Parallel.  Model scenarios include the test cases available for download from the AERMOD model page at the EPA’s SCRAM website (http://www.epa.gov/scram001/dispersion_prefrec.htm) and those described in this memo: http://www.epa.gov/scram001/7thconf/aermod/evalreadme.txt.

An equivalency document is also provided listing the differences between the model output list file generated by the SCRAM version as compared to the BREEZE Parallel version.  This document demonstrates that the only differences in the two results sets were the time stamps at the top of each output page.  Modelers who have the BREEZE AERMOD Parallel software can perform their own equivalency tests on these scenarios and any other scenario to replicate the equivalency demonstration.

It should be noted that the model scenarios available on the SCRAM website were not fully compatible with the 07026 version of EPA’s AERMOD model.  Trinity made the necessary modifications to the provided meteorological data (e.g., changed the version number from 02222 to 06341) so that it would run with EPA’s AERMOD Fortran executable.

The results of the file comparison are available at
http://www.breeze-software.com/downloads/aermodequiv/differences.txt.

The following zipped data sets contain the following files:

• .SFC and .PFL model ready meteorological data files. The .SFC file was modified by changing the version number from 02222 to 06341 in the .SFC file to make it compatible with AERMOD (i.e., allow AERMOD to process it without error).
• AERMOD input files for both the EPA SCRAM version of AERMOD and the BREEZE Parallel version of AERMOD. Each AERMOD input file has an .ami file extension. The BREEZE AERMOD Parallel input files have the designator “_MPI” preceding the .ami file extension.
• AERMOD output files for both the EPA SCRAM version of AERMOD and the BREEZE Parallel version of AERMOD. Each AERMOD output file has an .aml file extension. The BREEZE AERMOD Parallel output files have the designator “_MPI” preceding the .aml file extension.