This document focuses on the modeling analysis required to assess future visibility

improvement relative to the glidepath or uniform rate of progress (for each Class I area). As

mentioned previously, this document does not address the process of setting and evaluating

reasonable progress goals. The actual determination of reasonable progress is a multi-step

process which looks at the uniform rate of progress, the predicted rate of progress, and the four

factor test. In the end, the reasonable progress goal will be determined by the states for each

Class I area by considering the factors and the modeling analysis. The results of the modeling

analyses will be one of several components in the determination of reasonable progress.

6.2 Uniform Rate of Progress

Regional haze is calculated by estimating light scattering and absorption by components

of PM2.5. Baseline conditions represent the visibility conditions which exist for the best and

worst days at the time the regional haze program is established for each Class I area. Once

established, the baseline represents the starting point from which the uniform rate of progress

will be measured. The RHR also requires an estimate of “natural conditions” for each Class I

area which represents the visibility conditions that would exist in the absence of man-made


As explained in the RHR, the baseline for each Class I area is the average visibility (in

deciviews) for the 20% most impaired days, or “worst days”, and for the 20% least impaired

days, or “best days,” for the years 2000 through 200470.


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