Because of the high level of turbulence in the mine, the pollutant is initially mixed prior to exiting the pit.  Therefore some initial vertical dispersion is included to represent this in the effective area source.  Using the effective pit depth, de, as the representative dimension over which the pollutant is vertically mixed in the pit, the initial vertical dispersion value, σzo, is equal to de/4.3.  Note that 4.3×σzo represents about 90% of a Gaussian plume (in the vertical), so that the mixing in the pit is assumed to approximately equal the mixing in a plume.

 

     Therefore, for the effective area source representing the pit emissions, the initial dispersion is included with ambient dispersion as:

 

For receptors close to the pit, the initial dispersion value can be particularly important.

 

     Once the model has determined the characteristics of the effective area used to model pit emissions for a particular hour, the area source algorithm described in Section 1.2.3 is used to calculate the concentration or deposition flux values at the receptors being modeled.

 

 

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