It is likely that numerous Class I areas will be considered simultaneously in a modeled

uniform rate of progress assessment. Thus, the preferred approach for regional haze-related

model applications is to simulate an entire, representative year (i.e., one whose mean derived

deciview values for “20% worst” and “20% best” days approximates mean values for deciviews

averaged over 5 years for the best and worst days). States can then base the RRF values on the

best and worst days in each Class I area for the modeled year (the ~24 best and worst days from

the modeling year). For areas included in modeling for the annual PM2.5 NAAQS, particularly if

that modeling simulates an entire representative year (or more), the modeling for PM2.5 should

also provide adequate results to analyze for regional haze.

In some cases, the meteorology for a particular year may be representative of the five

year period, but due to large emissions changes during the period, a single year of ambient

visibility is not representative of the five year mean. One solution is to compile the emissions

inventory using five year average emissions for the base period. Because the base year and

future year visibility represents a five year period, it is appropriate to average both base and

future year emissions in a similar manner.

100The recommendation is for 15 or more total days per quarter, not 15 or more days with

monitored data.


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