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Federal Register / Vol. 86, No. 53 / Monday, March 22, 2021 / Rules and Regulations
Islands. States were required to submit the first implementation plan addressing visibility impairment caused by regional haze no later than December 17, 2007.8

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C. BART Requirements Section 169Ab2A of the CAA
directs states to evaluate the use of BART controls at certain categories of existing major stationary sources built between 1962 and 1977.9 Under 40 CFR
51.308e1ii, any BART-eligible source 10 that is reasonably anticipated to cause or contribute to visibility impairment in a Class I area is classified as subject-to-BART.11 States are directed to conduct BART determinations to address visibility impacts for each source classified as subject-to-BART.
These large, often under-controlled, older stationary sources are then required to procure, install, and operate the BART controls established in these determinations to reduce visibility impairment. The determinations must be based on an analysis of the best system of continuous emission control technology available and associated emission reductions achievable. States are required to identify the level of control representing BART after considering the five statutory factors set out in CAA section 169Ag2 for the potential BART controls.12 States must establish emission limits, a schedule of 8 See 40 CFR 51.308b. Also, under 40 CFR
51.308fi, the EPA requires subsequent updates to the regional haze SIPs for each implementation period. The next update for the second implementation period is due by July 31, 2021.
9 See 42 U.S.C. 7491g7, which lists the 26
source categories of major stationary sources potentially subject-to-BART.
10 BART-eligible sources are those sources that fall within one of 26 source categories that began operation on or after August 7, 1962, and were in existence on August 7, 1977, with potential emissions greater than 250 tons per year tpy. See 40 CFR 51 Appendix Y, section II.
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 specify 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 part 51, Appendix Y, section III.A.1.
12 The five statutory factors in determining BART
controls are: 1 Costs of compliance, 2 the energy and non-air quality environmental impacts, 3 any existing control technology present at the source, 4 the remaining useful life of the source, and 5
the degree of improvement in visibility which may reasonably be anticipated to result from the use of such technology.

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compliance, and other measures consistent with the BART determination process for each source subject-toBART.
D. BART Alternative Requirements A State may opt to implement or require participation in an emissions trading program or other alternative measure rather than require sources subject-to-BART to install, operate, and maintain BART. Such an emissions trading program or other alternative measure must achieve greater reasonable progress than would be achieved through the installation and operation of BART. In order to demonstrate that the alternative program achieves greater reasonable progress than source-specific BART, a state must demonstrate that its SIP
meets the requirements in 40 CFR
51.308e2i to iv. Among other things, the state must conduct an analysis of BART and the associated reductions for each source subject-toBART covered by the alternative program, and compare the reductions and visibility improvements of the alternative program to what would have been achieved by BART.
Pursuant to 40 CFR 51.308e2iE, the state must provide a determination under 40 CFR 51.308e3 or otherwise based on the clear weight of evidence that the alternative measure achieves greater reasonable progress than BART.
40 CFR 51.308e3 provides two specific tests applicable under specific circumstances for determining whether the alternative measure achieves greater reasonable progress than BART. Under the first test, if the distribution of emissions is not substantially different than under BART, and the alternative measure results in greater emission reductions, then the alternative measure may be deemed to achieve greater reasonable progress. Under the second test, if the distribution of emissions is significantly different, then the State must conduct dispersion modeling to determine the difference in visibility between BART and the alternative measure for each impacted Class I area, for the twenty percent best and worst days. The modeling would demonstrate greater reasonable progress if both of the following two criteria are met: i Visibility does not decline in any Class I area, and ii there is an overall improvement in visibility, determined by comparing the average difference between BART and the alternative over all affected Class I areas.
Alternatively, under 40 CFR
51.308e2iE, states may show based on the clear weight of evidence that the alternative achieves greater
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reasonable progress than would be achieved through the installation and operation of BART at the covered sources. As stated in the EPAs revisions to the Regional Haze Rule governing alternatives to source-specific BART
determinations, weight of evidence demonstrations attempt to make use of all available information and data which can inform a decision while recognizing the relative strengths and weaknesses of that information in arriving at the soundest decision possible.13 This array of information and other relevant data must be of sufficient quality to inform the comparison of visibility impacts between BART and the alternative. A
weight of evidence comparison may be warranted when there is confidence that the difference in visibility impacts between BART and the alternative scenarios are expected to be large enough to show that an alternative is better than BART. The EPA will carefully consider this evidence in evaluating any SIPs submitted by States employing such an approach.
Finally, under 40 CFR 51.308e2iii and iv, all emission reductions for the alternative program must take place during the period of the first long-term strategy for regional haze, and all the emission reductions resulting from the alternative program must be surplus to those reductions resulting from measures adopted to meet requirements of the CAA as of the baseline date of the SIP.
E. Long-Term Strategy and Reasonable Progress Requirements In addition to BART requirements, 40
CFR 51.308d3i to iv requires each state to include in its SIP a long-term strategy for the planning period that addresses regional haze visibility impairment for each Class I area located within the state and outside the state that may be affected by emissions generated from within the state. The long-term strategy is the vehicle for ensuring continuing reasonable progress toward achieving natural visibility conditions. It is a compilation of all control measures in the SIP that a state will use during the implementation period to meet the applicable reasonable progress goals RPGs established under 40 CFR 51.308d1 for each Class I
13 See 71 FR 60612, 60622 October 13, 2006.
Factors which can be used in a weight of evidence determination in this context may include, but not be limited to, future projected emissions levels under the alternative as compared to under BART;
future projected visibility conditions under the two scenarios; the geographic distribution of sources likely to reduce or increase emissions under the alternative as compared to BART sources;
monitoring data and emissions inventories; and sensitivity analyses of any models used.

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