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Federal Register / Vol. 86, No. 137 / Wednesday, July 21, 2021 / Proposed Rules
I. Background A. The Regional Haze Program Regional haze is visibility impairment that occurs over a wide geographic area primarily from the pollution of fine particulates PM2.5 1 emitted into the air from a variety of sources. These fine particulates which cause haze consist of sulfates SO42-, nitrates NO3-, organic carbon OC, elemental carbon EC, and soil dust.2 PM2.5 precursors consist of sulfur dioxide SO2, nitrogen oxides NOX, ammonia NH3, and volatile organic compounds VOCs. Airborne PM2.5 can scatter and absorb the incident light and, therefore, lead to atmospheric opacity and horizontal visibility degradation which limits visual distance and reduces color, clarity, and contrast of view. PM2.5 can cause serious adverse health effects and mortality in humans. It also contributes to environmental effects such as acid deposition and eutrophication.
Emissions that affect visibility include a wide variety of natural and man-made sources. Natural sources can include windblown dust from dust storms and soot from wildfires. Man-made sources can include major and minor stationary sources, mobile sources, and area sources. Reducing PM2.5 and its precursor gases in the atmosphere is an effective method of improving visibility.
Data from the existing visibility monitoring network, Interagency Monitoring of Protected Visual Environments IMPROVE, shows that visibility impairment caused by air pollution occurs virtually all of the time at most national parks and wilderness areas. In 1999, the average visual range 3
in many Class I areas i.e., national parks and memorial parks, wilderness
areas, and international parks meeting certain size criteria in the western United States was 100150 kilometers km, or about one-half to two-thirds of the visual range that would exist under estimated natural conditions.4 In most of the eastern Class I areas of the United States, the average visual range was less than 30 km, or about one-fifth of the visual range that would exist under estimated natural conditions. CAA
programs have reduced emissions of some haze-causing pollution, lessening some visibility impairment, and resulting in partially improved average visual ranges.5
In section 169A of the 1977 CAA
Amendments, Congress created a program for protecting visibility in the nations national parks and wilderness areas. This section of the CAA
establishes as a national goal the prevention of any future, and the remedying of any existing, visibility impairment in mandatory Class I
Federal areas where impairment results from manmade air pollution.6 Congress added section 169B to the CAA in 1990
that added visibility protection provisions, and the EPA promulgated final regulations addressing regional haze as part of the 1999 Regional Haze Rule, which was most recently updated in 2017.7 The Regional Haze Rule revised the existing 1980 visibility regulations and established a more comprehensive visibility protection program for Class I areas. The requirements for regional haze, found at 40 CFR 51.308 and 51.309, are included in the EPAs broader visibility protection regulations at 40 CFR 51.300
through 51.309. The regional haze regulations require states to demonstrate 4 64

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1 Fine
particles are less than or equal to 2.5
microns mm in diameter and usually form secondary in nature indirectly from other sources.
Particles less than or equal to 10 mm in diameter are referred to as PM10. Particles greater than PM2.5
but less than PM10 are referred to as coarse mass.
Coarse mass can contribute to regional haze as well and is made up of primary particles directly emitted into the air. Fine particles tend to be man-made, while coarse particles tend to originate from natural events like wildfires and dust storms. Coarse mass settles out from the air more rapidly than fine particles and usually will be found relatively close to emission sources. Fine particles can be transported long distances by wind and can be found in the air thousands of miles from where they were formed.
2 Organic carbon OC can be emitted directly as particles or formed through reactions involving gaseous emissions. Elemental carbon EC, in contrast to organic carbon, is exclusively of primary origin and emitted by the incomplete combustion of carbon-based fuels. Elemental carbon particles are especially prevalent in diesel exhaust and smoke from wild and prescribed fires.
3 Visual range is the greatest distance, in km or miles, at which a dark object can be viewed against the sky by a typical observer.

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FR 35715 July 1, 1999.
interactive story map depicting efforts and recent progress by EPA and states to improve visibility at national parks and wilderness areas may be visited at: http arcg.is/29tAbS3.
6 Mandatory Class I Federal areas consist of national parks exceeding 6,000 acres, wilderness areas and national memorial parks exceeding 5,000
acres, and all international parks that were in existence on August 7, 1977. The EPA, in consultation with the Department of Interior, promulgated a list of 156 areas where visibility was identified as an important value. The extent of a mandatory Class I area includes subsequent changes in boundaries, such as park expansions. Although states and tribes may designate additional areas as Class I, the requirements of the visibility program set forth in the CAA applies only to mandatory Class I Federal areas. Each mandatory Class I
Federal area is the responsibility of a Federal Land Manager. When the term Class I area is used in this action, it means mandatory Class I Federal areas. See 44 FR 69122, November 30, 1979 and CAA Sections 162a, 169A, and 302i.
7 See the July 1, 1999 Regional Haze Rule final action 64 FR 35714, as amended on July 6, 2005
70 FR 39156, October 13, 2006 71 FR 60631, June 7, 2012 77 FR 33656 and on January 10, 2017 82
FR 3079.

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reasonable progress toward meeting the national goal of a return to natural visibility conditions for mandatory Class I Federal areas both within and outside states by 2064. The CAA
requirement in section 169Ab2 to submit a regional haze SIP applies to all fifty states, the District of Columbia, and the Virgin Islands. States were required to submit the first implementation plan addressing visibility impairment caused by regional haze no later than December 17, 2007.8
Section 169Ab2A of the CAA
directs states to evaluate the use of Best Available Retrofit Technology BART
controls at certain categories of existing major stationary sources 9 built between 1962 and 1977. These large, often under-controlled, older stationary sources are required to procure, install, and operate BART controls to address visibility impacts from them. Under the Regional Haze Rule, any of these BARTeligible sources 10 that are reasonably anticipated to cause or contribute to visibility impairment in a Class I area are determined to be subject-to-BART.11
States are directed to conduct BART
determinations for each source classified as subject-to-BART. 40 CFR
51.308e1iiA requires states or EPA in the case of a FIP to identify the level of control representing BART after considering the five statutory factors set out in CAA section 169Ag2. States must establish emission limits, a schedule of compliance, and other measures consistent with the BART
determination process for each source subject-to-BART. In lieu of requiring source-specific BART controls, states also have the flexibility to adopt alternative measures, as long as the alternative provides greater reasonable progress toward improving visibility 8 See 40 CFR 51.308b. The EPAs regional haze regulations require subsequent updates to the regional haze SIPs. 40 CFR 51.308gi.
9 See 42 U.S.C. 7491g7 listing the set of major stationary sources potentially subject-toBART.
10 See 40 CFR 51 Appendix Y, II. How to Identify BART-eligible Sources.
11 Under the BART Guidelines, states may select a visibility impact threshold, measured in deciviews dv, below which a BART-eligible source would not be expected to cause or contribute to visibility impairment in any Class I area. The state must document this threshold in the SIP and state the basis for its selection of that value. Any source with visibility impacts that model above the threshold value would be subject to a BART
determination review. The BART Guidelines acknowledge varying circumstances affecting different Class I areas. States should consider the number of emission sources affecting the Class I
areas at issue and the magnitude of the individual sources impacts. Any visibility impact threshold set by the state should not be higher than 0.5 dv.
See 40 CFR 51, Appendix Y, section III.A.1.

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