Building Downwash Option:
The building downwash option is only applicable to Point and Flare source types. Downwash occurs when the aerodynamic turbulence induced by nearby buildings cause a pollutant emitted from an elevated source to be mixed rapidly toward the ground (downwash), resulting in higher ground-level concentrations.
Complex Terrain Option:
The complex terrain option is only applicable to Point and Flare source types.
The complex terrain option of Screen calculates maximum 24-hour concentrations for terrain elevations exceeding stack height.
Simple Terrain Option:
When modeling simple terrain, you are given the option of modeling either:
Simple Flat Terrain: where terrain heights are assumed not to exceed stack base elevation. If this option is used, terrain height is considered to be 0.0 m.
Simple Elevated Terrain (Not Available for Area Sources): where terrain heights exceed stack base but are below stack height. If this option is used, the terrain height above stack base should be specified. If terrain heights above physical stack height are entered, Screen View gives a warning message and the user has to reenter a valid height value.
To model simple elevated or simple flat terrain, you can choose either or both of the distance options (Automated Distances or Discrete Distances).
Automated Distances Option:
The Automated Distances option gives the user the option of using a preselected array of 50 distances ranging from 100 m out to 50,000 m (50 km) using the following increments.
For the Automated Distances option, the Screen model calculates the maximum concentration across a range of meteorological conditions for the minimum distance given. Screen then computes the concentration for each distance in the array larger than the minimum and less than or equal to the maximum. Screen also uses an iteration routine to determine the maximum value associated with that distance to the nearest meter.
Discrete Distances Option:
The Discrete Distances option allows the user to find the maximum impact at specific locations of interest, such as nearby residences, hospitals, or schools, and may also be used for long-range transport estimates out to 100 km. Any number of distances (equal or greater than 1.0 m) can be input and the maximum concentration for each distance will be calculated.
Choice of Meteorology :
Three choices of meteorology are available when modeling with Simple Terrain screening:
Full Meteorology (All Stability Classes and Wind Speeds): Screen will examine all six stability classes (five for urban sources) and their associated wind speeds. Screen examines a range of stability classes and wind speeds to identify the "worst case" meteorological conditions, i.e., the combination of wind speed and stability that results in the maximum ground level concentrations.
Input Single Stability Class: You have to select the stability class to be used (A through F) from the stability list. Screen will then examine a range of wind speeds for that stability class only.
Input Single Stability Class and Wind Speed: You have to select the stability class and input the 10-meter wind speed to be used. Screen will examine only that particular stability class and wind speed. To help you on the choice of the wind speed, Screen View gives the wind speed range for each stability class, which is printed at the bottom of the wind speed text box.
Generally the Full Meteorology option should be selected. The other two options were originally included in the Screen model for testing purposes only, but may be useful when particular meteorological conditions are of concern.
Fumigation occurs when a plume that was originally emitted into a stable layer is mixed rapidly to ground level when unstable air below the plume reaches plume level.
The Fumigation option is only available for Point and Flare source types if :
Screen View Graphic Results
The Rural dispersion coefficient was selected (for rural inland sites), and
The Stack Height >= 10 meters (see Note 1 in case of Flare type source)
After you run your project, Screen View generates a series of plots (distance versus concentration) using the output file generated by EPA's Screen3 model. If your project ran successfully, you can have access to the "Graphic Results" Window and view these plots.
In the "Graphic Results" Window, a list of the available plots can be found on the drop-down list box located below the toolbar menu. Many options are available to the user to enhance the appearance of the plots generated from the Screen3 output file. You can have access to these options through the Options Buttons located on the right-hand-side of the "Graphic Results" Window.
Screen View System Requirements
At least 20 MB of available hard disk space and 16 MB of RAM
Windows versions 95/98/2000/NT