Again, if this occurs, States should work with their Regional Office and FLM to

determine the best way to calculate visibility on the best and worst days. Potential options are to

use data from another nearby IMPROVE site, use nearby data from a different ambient data

network, or interpolate ambient data to the site. Another alternative is to estimate the 20% best

and worst days from the model outputs83.

Using a constant sample of days to estimate baseline and future visibility. For a

typical Class I area, there will be about 120 days per year having measurements needed to

estimate (dv)baseline with Equation (6.1). Thus, there should be about 24 “worst” and 24 “best”

visibility days for each of the 5 years in the base period. It is conceivable that the identity of

these “worst” and “best” days could change if emissions were altered to reflect net effects of

controls and growth. The recommended test described in Section 6.4 assumes that the identity of

the “worst” and “best” days remains unchanged. This is done primarily to avoid having to

perform iterative analyses to identify future worst and best visibility days and to keep the test

relatively simple and more readily understood. This assumption could cause improvement in

visibility to be overestimated for the “worst” days and could also cause the test to overestimate

the difficulty in preventing deterioration of visibility on the “best” days. However, for the

reasons described below, we do not believe the effects of this assumption are substantial.

It is unlikely that there would be any wholesale change in the identity of “worst” or

“best” days with future vs. current emissions. Analyses performed by Meyer, et al. (1997) have

shown that the predicted ranked severity of high ozone days is largely unaffected by simulated

controls and growth (i.e., highest days tend to remain the highest days after the effects of growth

and controls are simulated). There is no reason to expect a different outcome for other

secondary pollutants. If there are differences, we would expect these to occur near the

borderline between the “worst” days and more moderate days.



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