recommended long range transport model. As mentioned

previously, this is not always the case. Therefore, it is

expected that application of the Level I techniques will need

to be reviewed by those with sound professional judgement.

Under light wind conditions or under recirculation conditions,

the MESOPUFF-II model may yield higher concentrations than

steady-state, Gaussian plume models. Therefore, if these

conditions occur, the Level I analysis should be adequately

assessed. These conditions are not necessarily generally

defined; the IWAQM is not attempting to define them further.

It is anticipated that as more experience is gained with the

Level I and II techniques, further resolution on when either

method is appropriate can be better elucidated.

Since many of Class I areas are located in complex

terrain, the issue of the appropriate methods for applying

Level I long range transport modeling techniques to these areas

arises. Each analysis will have unique characteristics which

must be evaluated on a case-by-case basis. In many cases,

however, some of the following considerations will apply. As

noted previously, models incorporating bivariate plume

dispersion parameters are suggested for use in a Level I

analysis. This suggestion is based on the presumption that

after a plume has travelled 50 km or farther, it will have been

affected by a variety of processes, land use, and terrain

intervening along the trajectory of the plume. Therefore, it

may not be appropriate to use a plume impaction model for these

circumstances. Similarly, under many conditions, when long

range transport is involved in moving pollutants to a Class I

area, the plumes will be traversing steadily rising terrain.

Under the constraints of existing, steady-state, Gaussian plume

models, receptor heights are either restricted to be no

greater than the height of the stack which emitted the

pollutants, or in the case of some of the plume impaction

models, the concentrations are considered to be zero if the



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