source (a recommended user-input value adopted in

this study). On the other extreme are MESOPUFF and

MESOPLUME, which specify 100 km as the transition

distance.

Therefore, it seems clear that the performance evaluations

were indeed carried out with the transition between distance

and time dependent formulations set at 10 km. Thus the IWAQM

is recommending that the model be run with the 10 km setting.

The previous protocol also recommended that the model be

run assuming no chemical conversion or deposition of pollutants

(either wet or dry). For the purposes of IWAQM, namely to

calculate visibility impacts, secondary pollutants, such as

SO , are the contributing pollutants. After reviewing the =

4

algorithms used in the MESOPUFF-II code for calculating the

chemical conversion and deposition, the IWAQM considered them

simple, but adequate. Therefore, the IWAQM is recommending

that they be used, recognizing the following limitations.

First, the treatment of the aqueous phase conversion of SO to 2

SO is likely to be greatly underestimated. Field measurements =

4

have indicated that when a plume passes through a nonprecipitating

cloud that the conversion of SO to SO can be as 2 4

=

high as 100% per hour. The assumed conversion of 3% per hour

is, therefore, expected to be an underestimate. The model,

however, does not adequately treat the occurrence of nonprecipitating

clouds. Therefore, the tendency will be to

underestimate SO formation when non-precipitating clouds would =

4

be present, with a commensurate overestimation of the primary

SO concentration. 2

Ultimately, when examining Air Quality Related Values

(AQRVs), one of the parameters of interest is frequently

deposition of SO and NO . Also, even when trying to estimate = 

4 3

impacts on visibility, an accurate assessment should include

the removal of these pollutants from the atmosphere.

 

 

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