6.6 Discussion

The comparison of calculations of the two- and three-dimensional versions of the RPM indicate that the choice of model resolution is dependent on the output metric(s) considered. The two case studies presented show that the two-dimensional version is preferable to the three-dimensional version when the average plume concentrations are calculated, since both the two- and three-dimensional versions give close estimates. However, when the output metrics require a higher resolution (for example, the ground level ozone concentrations are required when calculating human exposure to ozone), the three dimensional version is to be preferred. Further, the uncertainty associated with the calculation of average plume concentrations is relatively small.

When evaluating the uncertainties associated with model formulation, the approach suggested here, construction of a hierarchy of models with increasing detail, is recommended. The comparison of the model results can provide insight into what level of detail is sufficient to produce results similar to more detailed models. Such knowledge is very useful in building an ``optimal model'', one that produces outputs similar to more detailed models, but requires much less detailed inputs and much fewer computational resources. Further, results from models with varying detail provide an estimate of the range of model calculations, thus helping in characterizing the uncertainty associated with model formulation and detail.