Jul 13, 2020

The AERMOD model assumes emissions are emitted continuously, which is true in some cases but not in others. For example, a source may emit 100% of the source emission rate input in AERMOD during the Spring and Fall, 75% of the emission rate during the Summer, and 125% of the emission rate during the Winter. Similarly, the emission rate may vary during different times of the day and different days of the week. To account for these situations where releases are not continuous 24 hours a day, 7 days a week so that modelers can produce the most realistic modeling results, modelers have the ability in AERMOD to vary the emissions by:

  • Season
  • Month
  • Hour of day
  • Wind speed
  • Season and hour of day
  • Season, hour of day, and day of week
  • Season, hour of day, and the seven days of the week
  • Month, hour of day, and day of week
  • Month, hour of day, and the seven days of the week
  • Hour of day, and day of week
  • Hour of day, and seven days of week

When using a variable emission rate in AERMOD, the AERMOD model will multiply emission rate factors with the specified source's emission rate. (Note: These emission factors are not necessarily regulatory default options and should be used only with approval from a reviewing regulatory authority. The use of these options for a permitting analysis would typically require enforceable permit conditions limiting operations to the conditions modeled.)

The steps below will walk you through how to set up a variable emission rate in BREEZE AERMOD. (Modelers can also use hourly emissions files in the model. BREEZE AERMOD has an Hourly Emission File Editor that can be used to create hourly emission files. Click here to read the modeling tip about using this Editor.)

  1. In BREEZE AERMOD, you'll first need to add the Source. You can do this using any of the methods below: 
    • In the Project Tab, click Sources. (You can also access this Sources form in the other tabs by clicking Objects in the Model section and then Sources.) Enter the source data.BREEZE AERMOD - Sources
    • In the Data Tab, click Edit Objects, select the appropriate source type from the data tree on the left, and then click Add Objects. Enter the appropriate number and click Ok. Enter the source data. When finished entering the source data, click Apply Changes.
      BREEZE AERMOD - Add Objects and Apply Changes     BREEZE AERMOD - Edit Objects
    • In the Map Tab, click Sources in the Drawing, Selection, and Editing Tools section and select the appropriate source to add. You'll be prompted to place the source on the screen in the appropriate location. (You can edit the location afterward as needed in the Source window that will pop up.) Enter the source data.
      BREEZE AERMOD - Map - Sources
  2. Once the source has been added, you can specify a variable emission rate. To do this, you'll need to open the Source form, which you can do in the Project, Data, or Map tab:
    • In the Project tab, simply click the same Sources button that you did in order to add the source.
    • In the Data tab, double click on the row containing the source data.
    • In the Map tab, double click on the source itself.
  3. Once in the Source form, click the check box beside “Use variable emission rates” in the Source options. After checking this box, a Variable Emission Rates tab will appear:
    BREEZE AERMOD - Use variable emission rates
  4. Click on the Variable Emission Rates tab and then specify:
    • Which option you want to use as the temporal variation for this source;
    • Whether the source is in the Northern Hemisphere or not;
    • The appropriate emission factor(s) based on the option selected.
  5. Once this data has been entered, click Ok. You have just added a variable emission rate(s) for that source.
    BREEZE AERMOD - Variable Emissions Rates

These steps will need to be completed for every source. Or, you can apply a variable emission rate to an entire group of sources. To do this, please follow the steps below.

  1. After the sources have been added, open the Source form. You can do this by:
    • Clicking Source in the Model Options section of the Project tab;
      BREEZE AERMOD - Project - Source
    • Clicking Options in the Model section of the Data, Map, 3D, or Report tab and selecting Source from the dropdown.
      BREEZE AERMOD - Options - Source
  2. Once in the Source Options form, click on the Source Tools tab.
  3. In the Tool section, ensure “Apply variable emission rates to a group of sources” is selected.
  4. Select the appropriate sources that you want to apply the variable emission rates to, and then set the variable emission rate factors.
  5. Click Apply Factors to Selected Source(s) and then OK. You have now assigned variable emission rates to a selected group of sources.
    BREEZE AERMOD - Source Tools

The example above demonstrates a case where two point sources emitted 100% of the source emission rate during the Spring and Fall, 75% of the emission rate during the Summer, and 125% of the emission rate during the Winter.

If you have questions when setting up variable emission rates, 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.