simulation from other pollutants of interest. The method for

simulating primary emissions of fine particulates follows.

The MESOPUFF-II dispersion model includes the option for

modeling primary emissions of SO . Hence, since these are in =

4

particulate form, SO can be used as a surrogate for the =

4

emissions of other fine particulate species. If the model is

run simulating only SO emissions, with the chemistry options =

4

turned off, an estimate of the ambient concentrations of

primary particulate species can be obtained. The deposition

algorithms for particulate SO can be used for other fine =

4

particulates, as well. Since the MESOPUFF-II does not include

a provision for gravitational settling, ambient concentrations

of the course particle fraction (diameters > 2.5 μm) may be

overestimated over transport distances greater than 50 km.

6. SUMMARY

The IWAQM considered two Lagrangian puff models for use in

regional scale modeling analyses, the MESOPUFF-II and the ARM3.

Both models have known limitations. After comparison and

examination of the models, it was determined that the ARM3

contained a number of fundamental coding errors which rendered

its use suspect. Although the errors are correctable, they

invalidate previous evaluation analyses completed with the

ARM3. The MESOPUFF-II has been shown to be applicable, on a

theoretical basis, for long range transport; has been evaluated

with adequate data bases; and has been shown to not be biased

toward underestimation. Therefore, the IWAQM is recommending

the use of the MESOPUFF-II for long range transport analyses,

in the interim, until a more suitable model construct can be

developed.

It would be desirable to be able to recommend a single

modeling approach for estimating regional impacts on the NAAQS

and PSD increments, that is, have a single model which could

treat source-receptor distances of less than and greater than

50 km. Given the need for regulatory consistency, it was

determined that for calculating impacts on quantities regulated

under the Clean Air Act, techniques identified in the Guideline

on Air Quality Models (Revised) (EPA, 1986) should be used for

source-receptor distances of less than 50 km. The MESOPUFF-II

model should be run for source-receptor distances greater than

50 km and the results of these two techniques should be added

together.

 

 

 

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