for “best” visibility days. As with “worst” days, the preferred approach is to model an entire

year and select an RRF value for concentrations averaged over the 20% “best” visibility days for

each Class I area. If this is not feasible or only a limited number of Class I areas are to be

considered in an analysis, States may review when “best” visibility days are observed to occur in

the base period. Model > ~ 10 days with observed “best” visibility and average predicted

baseline and future concentrations for each PM component. The RRF values are the ratios of the

future to baseline modeled averages. The appropriate RRF values should then be used in concert

with each observed “best” day to estimate future concentrations for each component on each

identified “best” day.

Alternative RRF calculations. The default glidepath analysis is relatively simple in that

a single mean RRF is calculated for each PM component (separate RRFs on worst and best

days). A series of tests with more complicated methods has shown that 1) the difference

between various versions of the test are usually small and 2) each of the alternative tests has

limitations in its applicability (Environ, 2005). Possible variations include the use of day

specific RRFs, season (or quarter) specific RRFs, or climatological based RRFs. In some cases,

these more complicated techniques may provide different answers, but sometimes not. There are

specific limitations noted with each of these alternatives. We have chosen to keep the single

mean RRF test as the default recommendation. States are encouraged to explore other methods

for estimating RRFs if it is felt that the default recommendation is too simplistic to accurately

capture the change in future visibility at any particular Class I area. The SIP demonstration

should use the most appropriate method of projecting future concentrations for the characteristics

of each Class I area84. Alternative methods should be discussed in the modeling protocol and

discussed with the appropriate EPA Regional Office and FLMs.



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