In response to these mandates, EPA promulgated the regional haze rule (RHR) on July 1,

1999.65 Under section 51.308(d)(1) of this rule, States must “establish goals (expressed in

deciviews) that provide for reasonable progress towards achieving natural visibility conditions

for each Class I area within a State”. These reasonable progress goals must provide for an

improvement in visibility for the most impaired days over the period of the implementation plan

and ensure no degradation in visibility for the least impaired days over the same period.66

Reasonable Progress Goals (RPGs), measured in deciviews, are interim goals that represent

incremental visibility improvement over time toward the goal of natural background conditions

and are developed in consultation with other affected States and Federal Land Managers.67

In determining what would constitute reasonable progress, section 169A(g) of the Clean

Air Act requires States to consider the following four factors:

• The costs of compliance;

• The time necessary for compliance;

• The energy and non-air quality environmental impacts of compliance; and

• The remaining useful life of existing sources that contribute to visibility impairment.68

States must demonstrate in their SIPs how these factors are taken into consideration in selecting

the RPG for each Class I area in the State. More details on the setting of RPGs and the

appropriate use of the four factors is contained in “Guidance for Setting Reasonable Progress

Goals Under the Regional Haze Program” (U.S. EPA, 2007).

The regional haze rule also establishes an additional analytical requirement in the process

of establishing RPG. States are required to determine the rate of improvement in visibility

needed to reach natural conditions by 2064, and to set each RPG taking this “glidepath” into

account.69 The glidepath is the amount of visibility improvement needed in each review period

to stay on a linear path towards visibility improvement to natural conditions by 2064. The

glidepath represents a linear or uniform rate of progress. Therefore, the amount of visibility

improvement needed to stay on the glidepath is also referred to as “uniform rate of progress”.

This document uses the terms “glidepath” and “uniform rate of progress” interchangeably.


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