meteorological data sets for each end of the Class I area,

particularly since the results will be combined with a model

which uses spatially and temporally varying wind fields. The

use of multiple meteorological data sets in a steady-state,

Gaussian plume modeling analysis is not the recommended

approach in the Guideline; therefore, this is a matter for the

appropriate regulatory agencies and should only be considered

on a case-by-case basis in consultation with the appropriate

EPA regional office.

As noted previously, it is recommended that both the long

range transport model and the steady-state Gaussian model be

run with five years of meteorological input data. The

exception to this is when the source being permitted is within

50 km of the Class I area and has collected at least one year

of on-site meteorological data. If a cumulative impact

analysis, which includes sources beyond 50 km, is required, the

meteorological period of analysis for both analyses should

correspond to the period of on-site meteorological data. If,

however, the source being permitted is beyond 50 km from the

affected Class I area, then it is recommended that the full

five years of data should be run for all analyses.

Combining the results of steady-state Gaussian models with

a Lagrangian puff model, such as the MESOPUFF-II, will produce

some contrived results. The steady-state model assumes

instantaneous transport, whereas the Lagrangian puff model

simulates the actual transport time. It may be physically

impossible for the emissions from a source modeled with the

steady-state model to reach a receptor at the predicted time,

given the wind speed. However, steady-state models do not

generally accurately predict the time and location of maximum

concentrations, but are routinely applied as if they do provide

such information. Therefore, while combining the results of

two fundamentally different modeling systems is somewhat

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