The following describes the specification of the location, dimensions and adjusted emissions for the effective area source used for modeling open pit emissions. Consider an arbitrary rectangular-shaped pit with an arbitrary wind direction as shown in Figure 1-10. The steps that the model uses for determining the effective area source are as follows:
1.Determine the upwind sides of the pit based on the wind direction.
2.Compute the along wind length of the pit (l) based on the wind direction and the pit geometry . l varies between the lengths of the two sides of the rectangular pit as follows:
where L is the long axis and W is the short axis of the pit, and θ is the wind direction relative to the long axis (L) of the pit (therefore θ varies between 0° and 90°). Note that with this formulation and a square pit, the value of l will remain constant with wind direction at l = L = W. The along wind dimension, l, is the scaling factor used to normalize the depth of the pit.
3.The user specifies the average height of emissions from the floor of the pit (H) and the pit volume (V). The effective pit depth (de) and the relative pit depth (Dr) are then calculated as follows:
4.Based on observations and measurements in a wind tunnel study (Perry, et al., 1994), it is clear that the emissions within the pit are not uniformly released from the pit opening. Rather, the emissions show a tendency to be emitted primarily from an upwind sub-area of the pit opening. Therefore an effective area source (with Ae being the fractional size relative to the entire pit opening) is used to simulate the pit emissions. Ae represents a single area source whose dimensions and location depend on the effective depth of the pit and the wind direction. Based on wind tunnel results, if Dr³0.2, then the effective area is about 8% of the total opening of the mine (i.e. Ae=0.08). If Dr<0.2, then the fractional area increases as follows:
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