States are asked to calculate the reduction in deciviews (on the worst visibility days) by

an amount which is proportional to the number of years between the base period and the first

review period vs. the number of years between the base period and 2064. This establishes the

glidepath or uniform rate of progress. For example, if a Class I area’s mean worst visibility

during the 2000-2004 base period were 35 deciviews and natural background were estimated to

be 8 deciviews, the glidepath for 2018 would be to reduce visibility impairment on the worst

days by 6.3 deciviews (i.e., [35 -8][(2018-2004)/(2064-2004)]). More detailed examples of this

calculation are contained in (U.S. EPA, 2003b).

The goal of the regional haze program is to return to natural conditions by 2064, and

States are required to demonstrate, by the end of the first planning period (2018), reasonable

progress toward meeting that goal. The U.S. EPA has developed guidance on how to track

visibility for the regional haze rule (U.S. EPA, 2003b). In Section 6.3, we describe how the

ambient trends in regional haze will be determined. The modeling assessment for uniform rate

of progress needs to provide a good predictor for what the trend in observed air quality will be

in the future. This is most likely to happen if we reproduce the calculations which will be

applied to the ambient observations as closely as practical. In Section 6.4, we present and

recommend a modeling analysis which closely replicates the ambient data analysis. Section 6.5

addresses how to develop key inputs for the recommended modeling assessment of uniform rate

of progress.


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