3 Questions to Save Time & Money in Dispersion Modeling

3QDispersionCoverPhoto-01Are you trying to demonstrate modeled compliance by increasing stack heights or installing additional emission control? If so, you are probably trying to overcome the flaws in AERMOD that result in predictions that may be 2 to 8 times higher than reality. Rather than finding solutions to overcome the flaws in the model or installing expensive ambient monitors to prove compliance, the first step should be to assess the building dimension inputs being used in the model (i.e., the BPIP output file). These 3 questions will help you determine if your results are being overstated due to exaggerated downwash effects:

  • Are the building lengths more than 4 times the heights for the controlling wind directions?
  • Are building widths more than 4 times the heights for the controlling wind directions?
  • Are the buildings porous or streamlined (e.g., tanks, silos) for the controlling wind directions?

If you answered “NO” to all three questions, then a modeled compliance solution may be your best approach. But if you answered “YES” to any of these questions , and if the stack height is less than three times the building height, then it is very likely that AERMOD is overpredicting by factors of 2 to 8. In these cases, more accurate building dimension inputs are needed to obtain lower and more accurate concentration estimates.

Some of the theoretical reasons why AERMOD overpredicts for the cases above are outlined in my 11th modeling conference presentation, “Building Downwash – Problems, Solutions and Next Generation“.

An expert/industry/EPA cooperative research project is needed to correct these problems, but until that happens, the only tool available to approximately fix the model is through the use of wind-tunnel-determined Equivalent Building Dimensions (EBD). Learn more about EBD in the video AERMOD Beyond the Box: How EBD Improves Accuracy

If you answered “YES” to any of the questions above then the building structures at the site are very long, very wide, porous and/or streamlined. Since AERMOD only works relatively well for short or narrow buildings positioned directly upwind or downwind of the building, the goal of the EBD study is to find a building that replicates the dispersion characteristics with the actual site structures in place. The photographs below show examples of the actual site dispersion compared to that with the EBD. In the lower figure, the trial that used no building in the test model agreed best with the site dispersion. The EBD values are then used in place of BPIP-generated building dimension inputs, thereby allowing for more accurate and lower concentration estimates. Once EBD are determined, an optimum modeled compliance solution can be found.

An EBD study is classified as a source characterization study by EPA, which only requires that a protocol be submitted and approved in advance by the appropriate agency before conducting the study. This is similar to any modeling protocol required for any permitting type study.

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