algorithms to treat such effects, the model has been found to

have a number of coding errors which render its use suspect.

One of the rationales for limiting the interim modeling

choices to either the MESOPUFF-II or the ARM3 was their

performance relative to other similar models. After the coding

error, described above, was discovered in the current version

of the ARM3 code, an examination was made of the originally

released version of the code. The code was found to contain

the same fundamental error found in the current version. Thus,

the evaluations of the ARM3 were carried out using only the

surface wind fields. For the case of the Oklahoma data, this

is not necessarily a major problem, since it was based on a

surface tracer release. However, the Savannah River evaluation

involved an elevated release into a wind level above the

surface level. If the surface level and the upper level were

very similar, the ARM3 evaluation results may be valid, but

unless the corrected model is re-run with the Savannah River

data, confidence should not be placed in the Savannah River

comparisons.

4. REGULATORY CONSIDERATIONS FOR THE USE

OF LONG RANGE TRANSPORT MODELS

The Guideline explicitly identifies certain steady-state,

Gaussian plume models which are preferred for calculating

concentrations of inert pollutants for source-receptor

distances of less than 50 km. The MESOPUFF-II model has been

shown to replicate steady-state, Gaussian plume results, under

many conditions, when it is run with steady-state meteorology

and Pasquill-Gifford dispersion coefficients (Scire et al.,

1984a). When MESOPUFF-II is run in an operational mode,

however, the results could be quite different, since the

meteorology will be time varying and the simulated pollutant

transport can be quite different than the steady-state

approximation. Having two acceptable techniques (i.e.,

MESOPUFF II and a Guideline model) which can give different

results for the same application is untenable for regulatory

purposes. Therefore, while it would be desirable to run only

one model, it will be necessary to run both a preferred,

Guideline model and the recommended long range transport model

for many permitting situations.

 

 

 

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