To model 1-hour and annual concentrations of NO2 for the National Ambient Air Quality Standard (NAAQS), the U.S. EPA has approved a three-tiered approach as detailed in Section 5.2.4 of EPA’s Guideline on Air Quality Models, Appendix W to 40 CFR Part 511 (Appendix W). This approach, as shown below, is recommended to estimate the ambient concentrations of NO2:
- Tier 1: Assume complete conversion of all emitted NO to NO2
- Tier 2: Multiple Tier 1 results by a representative equilibrium NO2/NOX ratio
- Tier 3: Perform a detailed analysis on a case-by-case basis
In the past, most facilities only needed to use Tier 1 and 2 to demonstrate compliance with the NAAQS but with the current, more stringent 1-hour NO2 NAAQS, the use of Tier 3 has increased.
The refined methods allowed under Tier 3 are the Ozone-Limiting Method (OLM) and the Plume Volume Molar Ratio Method (PVMRM). The OLM compares the maximum NOX concentration and the ambient ozone concentration to identify the factor that limits the formation of NO2 as follows:
- If the ozone concentration is greater than the maximum NOX concentration, then total conversion of NOX to NO2 is assumed;
- If the maximum NOX concentration is greater than the ozone concentration, then the ambient ozone concentration limits the formation of NO2.
The PVMRM calculates the molar ratio of ozone to NOx at each receptor location using special dispersion rates. The ozone/NOx molar ratio is used together with the NOx concentration to determine the NO2 concentration, accounting for the possibility of overlapping plumes from different sources.
Both of these methods require the user to provide the background ozone concentration. This can take the form of a single concentration, or it can vary on a seasonal basis, with wind direction, etc. If hourly ozone concentrations are available, these can be used directly to provide greater accuracy and reduce conservatism. However, AERMOD requires the hourly background concentration data to be in a very precise text format. To assist modelers when using this option, BREEZE AERMOD includes the BREEZE Ozone File Editor. This editor allows users to create, view, and edit hourly ozone files for use in the OLM or PVMRM. To use this editor, users will need to follow these steps in BREEZE AERMOD:
- Open BREEZE AERMOD
- In the Project tab, select the Project Tools menu and from the dropdown menu, select Ozone File Editor. The following screen will appear:
- Select the desired year from the Default Year dropdown menu. Note: The year selected must be identical to that in the meteorological data file that you will be utilizing in your model run.
- Click the “Fill in date” button. The Year, Month, Day, Hour columns will be populated with the values for the entire year. The Concentration column will be set to zero by default.
- Set the Concentration field to the actual ozone concentration values for that particular hour. To ease the entry of these values, users may utilize the copy and paste functions by either using the shortcut keys (e.g., CTRL+C and CTRL+V) or by selecting Copy or Paste from the Edit dropdown menu.
- Save the ozone file by selecting File, then Save or Save As, and enter the file name.
This file can then be referenced in the NO2/ NOX Conversion tab of the Control options for use in BREEZE AERMOD. To reference the file, users will need to complete the steps below:
- Open the Control options menu
- Select NO2 as the Pollutant
- Select User defined under Templates, and unselect Use regulatory defaults
- Go to the NO2/ NOX Conversion tab
- Select Use Ozone Limiting Method (OLM) or Plume Volume Molar Ratio Method (PVMRM) from the Ozone options from the Ozone options
- Beside Method, select Background concentration file from the dropdown
- Reference the saved ozone file by clicking the “…” button in the Ozone file field and selecting the file that has just been saved
Utilizing the Ozone File Editor tool will save modelers time and effort when using the OLM or PVMRM methods. If you have questions when using this editor or referencing an ozone file, you may click Help to access the user guide, or you may contact the BREEZE Support Team by firstname.lastname@example.org or +1-972-661-8881.