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1.2.4 The Short‑Term Open Pit Source Model

     The ISC open pit source model is used to estimate impacts for particulate emissions originating from a below-grade open pit, such as a surface coal mine or a stone quarry.  The ISC models allow the open pit source to be characterized by a rectangular shape with an aspect ratio (length/width) of up to 10 to 1.  The rectangular pit may also be rotated relative to a north-south and east-west orientation.  Since the open pit model does not apply to receptors located within the boundary of the pit, the concentration at those receptors will be set to zero by the ISC models.

     The model accounts for partial retention of emissions within the pit by calculating an escape fraction for each particle size category.  The variations in escape fractions across particle sizes result in a modified distribution of mass escaping from the pit.  Fluid modeling has shown that within-pit emissions have a tendency to escape from the upwind side of the pit.  The open pit algorithm simulates the escaping pit emissions by using an effective rectangular area source using the ISC area source algorithm described in Section 1.2.3.  The shape, size and location of the effective area source varies with the wind direction and the relative depth of the pit.  Because the shape and location of the effective area source varies with wind direction, a single open pit source should not be subdivided into multiple pit sources.

     The escape fraction for each particle size catagory, εi, is calculated as follows:

 

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