Sep 09, 2020

While most air dispersion modeling analyses are focused on finding the magnitude and location of maximum ground level pollutant concentrations, the maximum concentration tells only part of the story. Is the maximum concentration a rare event, or do high concentrations occur frequently? Assessing the frequency with which high concentrations occur can be very important for certain types of modeling. For example, in assessing the odor impacts of a facility like a wastewater treatment plant, the key question may be how often do noticeable odors impact a given location, not what is the maximum odor that ever impacts a location. BREEZE AERMOD, CALPUFF, and 3D Analyst make it very easy to count the exceedances of a specified concentration threshold, and to plot those exceedances on a map. This can be a powerful tool to identify how frequently harmful or objectionable concentrations may occur, and what specific locations are most frequently impacted by those concentrations. This article will explain how to use BREEZE AERMOD and BREEZE 3D Analyst to produce an exceedance count plot that contains this information. Note that BREEZE CALPUFF can be used with BREEZE 3D Analyst to produce the same type of results.

  1. Set everything up normally in BREEZE AERMOD for a typical model run. NOTE: If you wish to do this procedure with BREEZE CALPUFF, skip from here to step 5. Exceedance Count Modeling Tip - 1

  2. In the Project tab, click Output to open the output options window:
    Exceedance Count Modeling Tip - 2

  3. In the Options tab of the Output Options window, check the “Post files” box. It is OK to select other options as well (for example, most users will need to select some Receptor table type output).

  4. Click on the Post Files tab and check the 1-hour box for each source group you wish to get an exceedance count plot for. In this example, only the “ALL” source group is going to be processed.
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  5. Run AERMOD normally (or run CALPUFF if you are using CALPUFF).

  6. Once the run is complete, open the results in BREEZE 3D Analyst.
    If you are using AERMOD, do this either by choosing the “Analyze model results with 3D Analyst” option, or by going to Project Tools-->3D Analyst in the Project tab. 
    Exceedance Count Modeling Tip - 4            Exceedance Count Modeling Tip - 5
    If you are using CALPUFF, simply open 3D Analyst from your desktop or Start menu, and then click the “b” button-->Open and select the .con file that contains your raw CALPUFF model results.

  7. If you see a Select Data window in 3D Analyst, choose “Post data”.
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  8. Once your file is open in 3D Analyst, click on the Data tab, and then select “Extract-->Exceedances”.
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  9. Under “Threshold values”, enter the concentration that you wish to count exceedances of. Optionally, you can change the date period that the exceedance count will cover, or you can apply a multiplier to your concentration data using the “Scaling (A)” field, or add in a background concentration using the “Translation (B)” field.
    In this example, we are counting the number of times that a threshold of 100 µg/m3 was exceeded during the period of January 1-5, 1995, and no multipliers or additive factors are applied.
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  10. Click the “Extract” button. You will be prompted to choose the location and filename you wish to save the exceedance results to. Then click “Save”. Exceedance Count Modeling Tip - 9

  11. When prompted to “Open new extract file in 3D Analyst”, choose Yes.
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  12. The exceedance count plot will open in 3D Analyst. However, you will probably want to change the color scale to be more appropriate for the range of exceedance values in the plot. To do this, click the “Contour levels” button in the top left corner of the program:
    Exceedance Count Modeling Tip - 11

  13. Enter the contour levels you wish to use, then click OK. Note that you can easily come back to change these later if you are not happy with your initial choices. In this case, the color scale is being set to show a range of exceedance counts from 1 exceedance hour up to 40 exceedance hours.
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  14. Finally, you may wish to change the Legend settings. Since an exceedance count is always a whole number, it makes sense to change the Legend from scientific to standard notation, and to not show any decimal places in the Legend. These settings can be changed by clicking on “Appearance”:
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    Then, under the “Legend” heading, changing “DecimalPlaces” to 0 and “Format” to “Standard”:
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  15. If you click on the Map tab, you will now be able to see a plot of the exceedance count at each gridpoint. If you click on the Table tab, you will see a list of each receptor and the exceedance count associated with that particular receptor.
    Exceedance Count Modeling Tip - 15
If you have questions when extracting exceedance counts, you can easily access the user manual by clicking the blue question mark icons in the interface. You can also contact the BREEZE Support Team via email at support@trinityconsultants.com or by phone at +1 (972) 661-8881 ext. 4, and it will be our pleasure to assist you.