When amending the Clean Air Act in 1977, Congress added provisions focused on protecting the quality of clean air areas, and especially of national parks and other important national sites. Codifying regulations developed by EPA in 1974 and 1975, the Prevention of Significant Deterioration (PSD) program focuses on preventing further deterioration of air quality in pristine areas of the country by specifying how much increase in pollution levels is permitted. Mandatory class I areas—those areas that receive the maximum amount of protection—include most national parks, national wilderness areas, and national memorial parks, currently 156 areas. PSD regulations apply to emissions of sulfur dioxide (SO 2 ), particulates (PM), and nitrogen oxides (NOx) from new and modified sources of air pollution. Along with the PSD program for new sources, the Congress also added a new Section 169A, setting "as a national goal the prevention of any future, and the remedying of any existing , impairment to visibility in mandatory class I Federal areas...." PSD and Section 169A act in tandem, with PSD controlling new sources of impairment and Section 169A reducing emissions from existing sources of impairment. Under PSD, major new or modified sources in PSD areas must undergo preconstruction review and must install "best available control technology" (BACT); more stringent controls can be required if modeling indicates that BACT is insufficient to avoid violating an allowable PSD increment or the National Ambient Air Quality Standard itself. Under Section 169A, 26 categories of major stationary sources of pollution in existence on the date of enactment (1977), but not more than 15 years old as of that date, must install "best available retrofit technology" (BART) if the state determines the source may reasonably be anticipated to cause or contribute to any impairment of visibility in a class I area. Included in the list are electric generating units (EGUs). Implementing these provisions protecting visibility has not been easy, particularly Section 169A respecting existing sources. First, EPA had to define what visibility was. In general, visibility impairment from human activities manifests itself in two ways: (1) plume blight, where a clearly identifiable plume of smoke emanates from one or more sources; and (2) regional haze, where a uniform reduction in visual range occurs, or a layered discoloration by hovering bands of air tinged brown, yellow, or red. Second, EPA had to promulgate regulations within 24 months of enactment to assure that State Implementation Plans (SIPs) required (1) reasonable progress toward meeting the national goal mentioned earlier, and (2) compliance with several very specific provisions, including the Best Available Retrofit Technology (BART) requirements for existing sources. EPA promulgated rules in 1980 to address visibility impairment that was "reasonably attributable" to a single source or small group of sources (i.e., plume blight). As with many air pollution regulations, these visibility regulations are implemented by states through SIPs. In general, the 36 states with mandatory class I areas were required to revise their SIPs to assure reasonable progress toward the national visibility goal. The major elements of the regulation were (1) identifying existing sources causing visibility impairment and creating procedures for determining which existing stationary sources should be subject to BART requirements; (2) assessing potential adverse impacts from proposed new sources (or modified old sources) and recommending remedial actions via the New Source Review (NSR) process and the PSD program; (3) developing a 10-15 year long-term strategy to make "reasonable progress" toward the visibility goal; and (4) conducting visibility monitoring in mandatory class I areas. As noted, these regulations deal with plume blight only—regional haze reduction was explicitly delayed until some future date. This lack of aggressive implementation of Section 169A extended to the implementation of the 1980 regulations as well. After 35 of 36 states missed the September 1981 deadline for final visibility plans, the Environmental Defense Fund sued the EPA in 1982 to implement the plume blight regulations. The suit was settled in 1984 with the EPA developing a phased-in schedule for compliance with a December 1986 deadline for states to revise their SIPs to include controls on existing sources that hinder visibility goals. This sequential implementation of plume blight regulations actually extended through 1989. So far, the only BART installation to occur under the 1980 regulations has been the installation of sulfur dioxide scrubbers at the Navajo Generating Station in Arizona in 1991. EPA's lack of initiative on visibility during the 1980s prompted the Congress to revisit the issue in the 1990 amendments to the Clean Air Act. Those actions included a new Title IV, controlling precursors of acid rain and regional haze, and a new Section 169B. In some ways, Section 169B was a triggering mechanism to force EPA to move on Section 169A with respect to regional haze. Specifically, the 1990 Amendments required EPA to establish a Grand Canyon Visibility Transport Commission (GCVTC) within 12 months of enactment (and other commissions upon its own discretion or petition from at least two states). Commissions were required to assess the scientific, technical, and other data available on visibility impairment from potential or projected emissions growth in the region. Based on those data, the commissions were to issue reports within four years to EPA recommending what measures, if any, should be taken to remedy such impairment. Within 18 months of receiving a commission's report, EPA was to carry out its responsibilities under Section 169A, including criteria for measuring "reasonable progress" toward the national goal. Finally, states affected by any regulations promulgated under Section 169A were required to revise their SIPs within 12 months of such promulgation. In 1991, a Visibility Transport Commission for the region affecting visibility in Grand Canyon National Park was established. In June 1996, this commission (consisting of the governors of Arizona, California, Colorado, Nevada, New Mexico, Oregon, Utah, and Wyoming, and the leaders of five Indian tribes) approved a set of recommendations for improving western vistas. There were nine primary recommendations, including increased energy conservation, use of renewable energy, and emission reductions from stationary sources. The commission's Baseline Forecast anticipated that current regulatory programs would reduce emissions of sulfur dioxide from stationary sources (power plants, smelters, and other industrial sources) 13% by the year 2000, although additional measures under consideration might reduce emissions 20%-30%. In light of this uncertainty about the effects of current programs and the fact that emissions were projected to decline in the short term without additional regulation, the commission agreed to set only regional targets for sulfur dioxide emissions in the year 2000. The ultimate targets would be in the range of 50%-70% reduction by the year 2040, but "interim targets may also be needed to ensure steady and continuing emission reductions and to promote investment in pollution prevention." If the targets are exceeded, this would trigger a regulatory program, probably including a regional cap on emissions, with market-based trading. The 1990 Clean Air Act Amendments required the EPA Administrator to take action under Section 169A within 18 months of receipt of a commission report. The proposed rule appeared in the Federal Register on July 31, 1997. The final regional haze rule was published on July 1, 1999. The regional haze program represents a nationwide effort to protect 156 PSD class I areas from visibility impairment from manmade air pollution. All 50 states are included under the program—including those that do not have any class I areas within their boundaries—since pollution causing haze can travel beyond a state's boundaries and contribute to impaired visibility in a class I area located elsewhere. The rule encourages regional approaches. Indeed, the final rule includes special provisions (Section 309 program) that permit the former member-states of the Grand Canyon Visibility Transport Commission to implement their specific recommendations within the framework of the national regional haze program (Section 308 program). States are required under Section 169A to develop SIPs that ensure reasonable progress toward the national goal. Under Section 308 of the rule, SIPs must contain the following: Reasonable progress goals. States must establish goals expressed in deciviews that provide for reasonable progress toward achieving natural visibility conditions in class I areas by 2064. Calculations of baseline and natural visibility conditions. States must determine baseline conditions expressed in deciviews for the most impaired and least impaired days during 2000-2004. Long-term planning. States must submit a long-term strategy to address regional haze for each class I area within the state or affected by emissions within the state. The strategy must include compliance schedules, enforceable emission limitations, and other measures necessary to achieve reasonable progress goals. Monitor strategy. States must submit with the SIP a strategy for measuring, characterizing, and reporting regional haze. Best Available Retrofit Technology (BART). States must submit a BART implementation plan, including emission limitations and compliance schedules for each BART-eligible source that "may reasonably be anticipated" to contribute to visibility impairment in a class I area. States may choose to use a trading program or other alternative, if that alternative will achieve greater reasonable progress to natural visibility conditions than BART. Tracking Progress. SIPs must include several provisions to ensure the adequacy of the SIP. In particular, the SIP must include requirements for submitting SIP revisions to EPA every 10 years, beginning in 2018. Progress reports tracking the state's reasonable progress efforts are due every five years. Reports must include a determination of the adequacy of the state's SIP. An alternative program is provided in Section 309 as an option for nine former members of the Grand Canyon Visibility Transport Commission (GCVTC). Five states chose to meet the EPA deadline for inclusion under this option. Based on the commission's 1996 report, Section 309 allows states to choose to follow the commission's recommendations for reducing visibility impairment in the 16 class I areas in Colorado rather than the Section 308 program, up to the year 2018.