3a. performance evaluations of the model in similar

circumstances have shown that the model is not

biased toward underestimates, or

3b. after consultation with the EPA Regional Office,

a second model is selected as a baseline or

reference point for performance and the interim

procedures are then used to demonstrate that the

proposed model performs better than the

reference model.

One such situation is long range transport, that is,

transport of pollution beyond distances of 50 km. Therefore,

in order for any Phase 1 recommendation to be viable, from a

regulatory point of view, it will need to satisfy criteria 1.,

2., and 3a. above. It is recognized that justification of an

approach under 3b. is beyond the scope of most projects.

The processes which become important in the transport of

pollution over long distances include the spatial and temporal

variability of the winds which transport and disperse air

pollutants in the presence of various terrain and water

features, the chemical transformation of the pollutants as they

travel, and the deposition of the pollutants along the way.

There are existing long range transport models available which

meet some, but not all of these needs and some which meet these

needs, but have not been sufficiently tested.

One of the primary goals of the IWAQM is to evaluate

existing modeling codes and either recommend one as an accepted

approach or combine the better elements of several of the

existing codes, creating a new modeling construct. Either of

the above approaches will require full testing and evaluation.

Creating a model with all of the desired features and testing

it and evaluating it requires time. There is, however, an

immediate need for assessing the impacts of long range

transport of pollutants into Class I areas. Therefore, the

IWAQM decided to review a limited set of existing long range



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