Modeling Tip: How to Set Up Variable Density Receptor Grid in AERMOD
BREEZE is the market-leading air dispersion modeling software used by environmental professionals around the world
Receptors are specific locations, set up in air quality dispersion models, where ambient concentration estimates or deposition values are needed. If the model is being used to conduct an air quality analysis for regulatory purposes, it is important to identify the extent and frequency of the highest concentrations for the averaging time. Because the model run time increases with the number of receptors added, however, finding the highest concentrations can be a tricky process. If too many receptors are added, then the model could take a long time to run, but if too few receptors are added, then the highest concentrations may be missed altogether. To provide assistance, there are tools available within BREEZE AERMOD that ease the process of identifying the highest concentrations while also minimizing the model runtime.
In a typical air quality dispersion modeling analysis, ground-level concentrations are generally calculated using four types of receptor grids that vary in spacing: fenceline, fine, medium, and coarse. A dense receptor grid (i.e., Boundary Receptors) is placed around the facility's boundary to capture all high concentrations along the fenceline. Then, as the distance from the site increases, the receptor grid density decreases. By using this approach, a denser receptor grid is placed closer to the source(s) where the highest concentrations are likely to be, and a less dense grid is further from the source(s) to account for any high concentrations that may be located further away from the site. As such, the total number of receptors in the model run is reduced thereby minimizing the model runtime while the location of the highest concentrations can still be identified.
BREEZE AERMOD offers users with a wealth of options and ways for creating model objects and for entering their parameters. To simplify the process of adding these variable density grids to an AERMOD scenario, the Variable Density Object Array tool may be used to facilitate the process of varying the spacing with just a few simple clicks of the mouse.
Follow these steps to quickly and easily set up a variable density receptor grid in BREEZE AERMOD:
1. Navigate to the Map tab and select Variable Density option in the Object Arrays dropdown menu.
2. To draw the Variable Density grid, place the mouse at the center on the fine grid, and hold and drag the left mouse button until the desired receptor size is reached. The drawing does not have to be accurate as the desired receptor size can be modified in the Object Array dialog box.
3. In the Object Array form:
- The default Configuration will be Variable Density as this was selected from the Object Arrays dropdown menu.
- Ensure the Discrete Receptor option is selected from the Object type dropdown menu.
- The displayed Center X and Center Y coordinates represent the location selected when drawing the grid. These can be modified in the form to the desired parameters.
- The default shape is a Square array, however, a Circular array may be selected instead from the Shape dropdown menu. Enter the Start, End and Spacing parameters to attain the desired grid, where:
- Start is the starting distance from the Center X and Y Parameters, and where the model where the model objects will begin to appear;
- End is the ending distance from the Center X and Y Parameters, and where the model objects will stop appearing; and
- Spacing is the spacing between the model objects within the Start and End distances.
The resulting grid represents the Variable Density Grid, similar to the image shown below. The Zoom To option in the Map Tab can be used to automatically zoom out to the extents of individual or all the model objects.
Example: With the parameters entered below, the Object Array dialog box will first generate the first discrete receptor grid with 500m spacing from 2000m from the center to 5500m from the center. It then generates the second discrete receptor grid with 1000m spacing from 5500m from the center to 10500m from the center.