Air quality dispersion models can also be divided into three categories based on their application to terrain features (Wilson 1993) which address the relationship between receptor elevations and the top of the stack. Early models were developed without any terrain consideration (i.e., they were flat). In simple terrain models, receptors are located below the stack top, while in complex terrain models, receptors are located at or above the stack top.
More recently, model developers have focused attention on elevations which are located between the stack top and the final height of the plume rise. These elevations are classified as intermediate terrain (see Figure 5.8.15), and calculations within the intermediate region can be evaluated by either simple or complex models. The model predicting the highest concentration for a receptor is conservatively selected for that point. Thus, intermediate terrain is an overlapping region for model predictions.
A model, CTDMPLUS, is included in the EPA's Appendix A to address this concern. The model can predict concentrations at the stack top or above for stability conditions and has been approved for intermediate and complex terrain applications (U.S. EPA 1989).
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