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1.2.3 The Short‑Term Area Source Model

     The ISC Short Term area source model is based on a numerical integration over the area in the upwind and crosswind directions of the Gaussian point source plume formula given in Equation (1‑1).  Individual area sources may be represented as rectangles with aspect ratios (length/width) of up to 10 to 1.  In addition, the rectangles may be rotated relative to a north-south and east-west orientation.  As shown by Figure 1‑9, the effects of an irregularly shaped area can be simulated by dividing the area source into multiple areas.  Note that the size and shape of the individual area sources in Figure 1‑9 varies; the only requirement is that each area source must be a rectangle.  As a result, an irregular area source can be represented by a smaller number of area sources than if each area had to be a square shape.  Because of the flexibility in specifying elongated area sources with the Short Term model, up to an aspect ratio of about 10 to 1, the ISCST area source algorithm may also be useful for modeling certain types of line sources.

     The ground‑level concentration at a receptor located downwind of all or a portion of the source area is given by a double integral in the upwind (x) and crosswind (y) directions as:

where:

  QA

=  area source emission rate (mass per unit area per unit time

   K

=  units scaling coefficient (Equation (1‑1))

   V

 

=  vertical term (see Section 1.1.6)

 

   D

 

=  decay term as a function of x (see Section 1.1.7)

 

 

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