additional uncertainty into the model results (compared to modeling a full year). The conclusion

from the study was that at least 14 days are needed to represent each quarter. This assumes that

the model results are being used in a relative sense. The study also found that nitrate is the most

difficult of the species to adequately represent with a reduced number of modeling days.

Based on current information, for annual average PM2.5, we recommend modeling a full

year (or more). Reduced modeling of 15 or more days per quarter100 may be acceptable in some

cases, but areas where nitrate is an important component of PM2.5 should consider modeling a

full year (or at least the time periods when nitrates are high). Modeling less than a full year (e.g.

one month per quarter) can be useful for initial strategy development and sensitivity analyses. In

this way, information can be learned from partial year runs and can be supplemented with full

year model runs later in the process. This may save resources and allow more model runs to be

completed (relatively) quickly.

14.1.4 Regional Haze

The goals for regional haze focus on the 20% of days with best and worst visibility.

Sampling in Class I areas occurs once every three days. Thus, each year there will be about 24

“worst” days and 24 “best” days to choose from. Since the base period against which reasonable

progress is to be gauged is 5 years long, there could be as many as 120 “best” and “worst” days

to choose among for modeling.


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