# Air Plan Approval; ID; Regional Haze Plan for the Second Implementation Period > Briefs, arguments, decisions, and more. URL: https://www.frixlaw.com/law-library/documents/fr%3A2025-04906 ## Record - **Collection:** Federal Register - **Document type:** Proposed Rule - **Published:** March 24, 2025 - **Citation:** 90 FR 13516 ## Text ENVIRONMENTAL PROTECTION AGENCY 40 CFR Part 52 [EPA-R10-OAR 2024-0545; FRL-11879-01-R10] Air Plan Approval; ID; Regional Haze Plan for the Second Implementation Period AGENCY: Environmental Protection Agency (EPA). ACTION: Proposed rule. SUMMARY: The Environmental Protection Agency (EPA) is proposing to approve the Idaho regional haze State Implementation Plan (SIP) revision submitted on August 5, 2022, and supplemented on May 8, 2024. Idaho submitted the SIP revision to address the requirement to make reasonable progress toward the national goal of preventing any future, and remedying any existing, anthropogenic impairment of visibility in certain national parks and wilderness areas. DATES: Written comments must be received on or before April 23, 2025. ADDRESSES: Submit your comments, identified by Docket ID No. EPA-R10-OAR-2024-0545 at https://www.regulations.gov. For comments submitted at regulations.gov, follow the online instructions for submitting comments. Once submitted, comments may not be edited or removed from regulations.gov . For either manner of submission, the EPA may publish any comment received to its public docket. Do not submit electronically any information you consider to be confidential business information or other information the disclosure of which is restricted by statute. Multimedia submissions (audio, video, etc.) must be accompanied by a written comment. The written comment is considered the official comment and should include discussion of all points you wish to make. The EPA will generally not consider comments or comment contents located outside of the primary submission ( i.e., on the web, cloud, or other file sharing system). For additional submission methods, please contact the person identified in the FOR FURTHER INFORMATION CONTACT section. For the full EPA public comment policy, information about confidential business information or multimedia submissions, and general guidance on making effective comments, please visit https://www.epa.gov/dockets/commenting-epa-dockets. FOR FURTHER INFORMATION CONTACT: John Chi, EPA Region 10, 1200 Sixth Avenue, Suite 155, Seattle, WA 98101, at (206) 553-1185 or chi.john@epa.gov. SUPPLEMENTARY INFORMATION: Throughout this document, the use of “we” and “our” means “the EPA.” Table of Contents I. Background and Requirements for Regional Haze Plans A. Regional Haze Background B. Roles of Agencies in Addressing Regional Haze II. Requirements for Regional Haze Plans for the Second Implementation Period A. Identification of Class I Areas B. Calculations of Baseline, Current, and Natural Visibility Conditions; Progress to Date; and the Uniform Rate of Progress C. Long-Term Strategy for Regional Haze D. Reasonable Progress Goals E. Monitoring Strategy and Other State Implementation Plan Requirements F. Requirements for Periodic Reports Describing Progress Towards the Reasonable Progress Goals G. Requirements for State and Federal Land Manager Coordination III. The EPA's Evaluation of the Idaho Regional Haze SIP Revision for the Second Implementation Period A. Background on the Idaho First Implementation Period SIP Revision B. The Idaho Second Implementation Period SIP Revision and the EPA's Evaluation C. Identification of Class I Areas D. Calculations of Baseline, Current, and Natural Visibility Conditions; Progress to Date; and the Uniform Rate of Progress E. Long-Term Strategy for Regional Haze F. Reasonable Progress Goals G. Monitoring Strategy and Other Implementation Plan Requirements H. Requirements for Periodic Reports Describing Progress Towards the Reasonable Progress Goals I. Requirements for State and Federal Land Manager Coordination IV. Proposed Action V. Incorporation by Reference VI. Statutory and Executive Order Reviews I. Background and Requirements for Regional Haze Plans A. Regional Haze Background In the 1977 Clean Air Act Amendments, Congress created a program 1 to protect visibility in the nation's mandatory class I Federal areas, which include certain national parks and wilderness areas. 2 Congress established as a national goal the “prevention of any future, and the remedying of any existing, impairment of visibility in mandatory class I Federal areas which impairment results from manmade air pollution.” 3 Congress further directed the EPA to promulgate regulations to assure reasonable progress toward meeting this national goal. 4 1 Clean Air Act section 169A. 2 Areas statutorily designated as 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. Clean Air Act 162(a). There are 156 mandatory class I Federal areas. The list of areas to which the visibility protection program applies is set forth in 40 CFR part 81, subpart D. 3 Clean Air Act section 169A(a)(1). 4 Clean Air Act section 169A(a)(4). In 1990, Congress added section 169B to the Clean Air Act to further address visibility impairment, specifically, impairment from regional haze. The EPA subsequently promulgated the Regional Haze Rule on July 1, 1999 (64 FR 35714), codified at 40 CFR 51.308. 5 These regional haze regulations are a central component of the EPA's comprehensive visibility protection program for Class I areas. 5 In addition to the generally applicable regional haze provisions at 40 CFR 51.308, the EPA also promulgated regulations specific to addressing regional haze visibility impairment in Class I areas on the Colorado Plateau at 40 CFR 51.309. The latter regulations are applicable only for specific jurisdictions' regional haze plans submitted no later than December 17, 2007, and thus are not relevant here. Regional haze is visibility impairment that is produced by a multitude of anthropogenic sources and activities which are located across a broad geographic area and that emit pollutants that impair visibility. Visibility impairing pollutants include fine and coarse particulate matter (PM) ( e.g., sulfates, nitrates, organic carbon, elemental carbon, and soil dust) and their precursors ( e.g., sulfur dioxide (SO 2 ), nitrogen oxides (NO X ), and, in some cases, volatile organic compounds (VOC) and ammonia (NH 3 )). Fine particle precursors react in the atmosphere to form fine particulate matter (PM 2.5 ), which impairs visibility by scattering and absorbing light. Visibility impairment reduces the perception of clarity and color, as well as visible distance. 6 6 There are several ways to measure the amount of visibility impairment, i.e., haze. One such measurement is the deciview, which is the principal metric used by the Regional Haze Rule. Under many circumstances, a change in one deciview will be perceived by the human eye to be the same on both clear and hazy days. The deciview is unitless. It is proportional to the logarithm of the atmospheric extinction of light, which is the perceived dimming of light due to its being scattered and absorbed as it passes through the atmosphere. Atmospheric light extinction (b ext ) is a metric used to for expressing visibility and is measured in inverse megameters (Mm −1 ). To address regional haze visibility impairment, the 1999 Regional Haze Rule established an iterative planning process that requires both States in which Class I areas are located and States “the emissions from which may reasonably be anticipated to cause or contribute to any impairment of visibility” in a Class I area to periodically submit SIP revisions to address such impairment. 7 Under the Clean Air Act, each SIP revision must contain “a long-term (ten to fifteen years) strategy for making reasonable progress toward meeting the national goal”. 8 The initial round of SIP revisions also had to address the statutory requirement that certain older, larger sources of visibility impairing pollutants install and operate the best available retrofit technology (BART). 9 States' first regional haze SIPs were due by December 17, 2007, 10 with subsequent SIP revisions containing updated long-term strategies originally due July 31, 2018, and every ten years thereafter. 11 The EPA established in the 1999 Regional Haze Rule that all States either have Class I areas within their borders or “contain sources whose emissions are reasonably anticipated to contribute to regional haze in a Class I area”; therefore, all States must submit regional haze SIPs. 12 7 Clean Air Act section 169A(b)(2). See also 40 CFR 51.308(b), (f) (establishing submission dates for iterative regional haze SIP revisions (64 FR 35714, July 1, 1999, at page 35768). The Regional Haze Rule expresses the statutory requirement for states to submit plans addressing out-of-state Class I areas by providing that states must address visibility impairment “in each mandatory Class I Federal area located outside the State that may be affected by emissions from within the State.” 40 CFR 51.308(d), (f). 8 Clean Air Act section 169A(b)(2)(B). 9 Clean Air Act section 169A(b)(2)(A); 40 CFR 51.308(d), (e). 10 40 CFR 51.308(b). 11 64 FR 35714, July 1, 1999, at page 35768. 12 64 FR 35714, July 1, 1999, at page 35721. In addition to each of the fifty states, the EPA also concluded that the Virgin Islands and District of Columbia must also submit regional haze SIPs because they either contain a Class I area or contain sources whose emissions are reasonably anticipated to contribute regional haze in a Class I area. See 40 CFR 51.300(b), (d)(3). Much of the focus in the first implementation period of the regional haze program, which ran from 2007 through 2018, was on satisfying States' BART obligations. First implementation period SIPs were additionally required to contain long-term strategies for making reasonable progress toward the national visibility goal, of which BART is one component. The core required elements for the first implementation period SIPs (other than BART) are laid out in 40 CFR 51.308(d). On January 10, 2017, the EPA promulgated revisions to the Regional Haze Rule that apply for the second and subsequent implementation periods (82 FR 3078). The 2017 rulemaking made several changes to the requirements for regional haze SIPs to clarify States' obligations and streamline certain regional haze requirements. The revisions to the regional haze program for the second and subsequent implementation periods focused on the requirement that SIPs contain long-term strategies for making reasonable progress towards the national visibility goal. The reasonable progress requirements as revised in the 2017 rulemaking (referred to here as the 2017 Regional Haze Rule Revisions) are codified at 40 CFR 51.308(f). Among other changes, the 2017 Regional Haze Rule Revisions adjusted the deadline for States to submit their second implementation period SIPs from July 31, 2018, to July 31, 2021, clarified the order of analysis and the relationship between RPGs and the long-term strategy, and focused on making visibility improvements on the days with the most anthropogenic visibility impairment, as opposed to the days with the most visibility impairment overall. The EPA also revised requirements of the visibility protection program related to periodic progress reports and Federal Land Manager consultation. The specific requirements applicable to second implementation period regional haze SIP revisions are addressed in detail in the following paragraphs. B. Roles of Agencies in Addressing Regional Haze Because the air pollutants and pollution affecting visibility in Class I areas can be transported over long distances, successful implementation of the regional haze program requires long-term, regional coordination among multiple jurisdictions and agencies that have responsibility for Class I areas and the emissions that impact visibility in those areas. In order to address regional haze, States need to develop strategies in coordination with one another, considering the effect of emissions from one jurisdiction on the air quality in another. Five regional planning organizations (RPOs), 13 which include representation from State and tribal governments, the EPA, and Federal Land Managers, were developed in the lead-up to the first implementation period to address regional haze. Regional planning organizations evaluate technical information to better understand how emissions impact Class I areas across the country, pursue the development of regional strategies to reduce emissions of particulate matter and other pollutants leading to regional haze, and help States meet the consultation requirements of the Regional Haze Rule. 13 RPOs are sometimes also referred to as “multi-jurisdictional organizations,” or MJOs. For the purposes of this notice, the terms RPO and MJO are synonymous. 1. The Western Regional Air Partnership The Western Regional Air Partnership (WRAP) 14 is one of five regional air quality planning organizations across the United States. 15 The WRAP functions as a voluntary partnership of State, tribal, Federal, and local air agencies whose purpose is to understand current and evolving regional air quality issues in the west. There are 15 member States in the WRAP, including Idaho, in addition to 28 Tribes and 30 Local air agency members. 16 The WRAP Federal partners include the EPA, National Park Service, Fish and Wildlife Service, Forest Service, and Bureau of Land Management. 14 The WRAP website may be found at https://www.wrapair2.org/. 15 See https://www.epa.gov/visibility/visibility-regional-planning-organizations/ for information about the regional planning organizations, or RPOs, for visibility. 16 The WRAP membership list may be found at https://www.wrapair2.org/membership.aspx/. Based on emissions and monitoring data supplied by its membership, the WRAP produced technical tools to support regional modeling of visibility impacts at Class I areas across the west. 17 The “WRAP Technical Support System” consolidated air quality monitoring data, meteorological and receptor modeling data analyses, emissions inventories and projections, and gridded air quality/visibility regional modeling results. The WRAP Technical Support System is accessible by members and allows for the creation of maps, figures, and tables to export and use in developing regional haze SIP revisions, and maintains the original source data for verification and further analysis. 17 Technical information may be found at https://www.wrapair2.org/RHPWG.aspx/ and in the docket for this action. II. Requirements for Regional Haze Plans for the Second Implementation Period Under the Clean Air Act and the EPA's regulations, all 50 States, the District of Columbia, and the United States (U.S.) Virgin Islands are required to submit regional haze SIPs satisfying the applicable requirements for the second implementation period of the regional haze program by July 31, 2021. Each State's SIP must contain a long-term strategy for making reasonable progress toward meeting the national goal of remedying any existing and preventing any future anthropogenic visibility impairment in Class I areas. 18 To this end, 40 CFR 51.308(f) lays out the process by which States determine what constitutes their long-term strategies, with the order of the requirements in section 51.308(f)(1) through (3) generally mirroring the order of the steps in the reasonable progress analysis 19 and (f)(4) through (6) containing additional, related requirements. 18 Clean Air Act section 169A(b)(2)(B). 19 The EPA explained in the 2017 Regional Haze Rule Revisions that we were adopting new regulatory language in 40 CFR 51.308(f) that, unlike the structure in 51.308(d), “tracked the actual planning sequence.” (82 FR 3091, January 10, 2017). Broadly speaking, a State first must identify the Class I areas within the State and determine the Class I areas outside the State in which visibility may be affected by emissions from the State. These are the Class I areas that must be addressed in the State's long-term strategy. 20 For each Class I area within its borders, a State must then calculate the baseline (five-year average period of 2000-2004), current, and natural visibility conditions ( i.e., visibility conditions without anthropogenic visibility impairment) for that area, as well as the visibility improvement made to date and the “uniform rate of progress” (URP). The URP is the linear rate of progress needed to attain natural visibility conditions, assuming a starting point of baseline visibility conditions in 2004 and ending with natural conditions in 2064. This linear interpolation is used as a tracking metric to help states assess the amount of progress they are making towards the national visibility goal over time in each Class I area. 21 Each State having a Class I area and/or emissions that may affect visibility in a Class I area must then develop a long-term strategy that includes the enforceable emission limitations, compliance schedules, and other measures that are necessary to make reasonable progress in such areas. A reasonable progress determination is based on applying the four factors in Clean Air Act section 169A(g)(1) to sources of visibility-impairing pollutants that the State has selected to assess for controls for the second implementation period. 22 20 40 CFR 51.308(f), (f)(2). 21 40 CFR 51.308(f)(1). 22 40 CFR 51.308(f)(2). Additionally, as further explained below, the Regional Haze Rule at 40 CFR 51.308(f)(2)(iv) separately provides five “additional factors” 23 that States must consider in developing their long-term strategies. A State evaluates potential emission reduction measures for those selected sources and determines which are necessary to make reasonable progress. Those measures are then incorporated into the State's long-term strategy. After a State has developed its long-term strategy, it then establishes RPGs for each Class I area within its borders by modeling the visibility impacts of all reasonable progress controls at the end of the second implementation period, i.e., in 2028, as well as the impacts of other requirements of the Clean Air Act. The RPGs include reasonable progress controls not only for sources in the State in which the Class I area is located, but also for sources in other States that contribute to visibility impairment in that area. The RPGs are then compared to the baseline visibility conditions and the uniform rate of progress to ensure that progress is being made towards the statutory goal of preventing any future and remedying any existing anthropogenic visibility impairment in Class I areas. 24 23 The five “additional factors” for consideration in 40 CFR 51.308(f)(2)(iv) are distinct from the four factors listed in Clean Air Act section 169A(g)(1) and 40 CFR 51.308(f)(2)(i) that States must consider and apply to sources in determining reasonable progress. 24 40 CFR 51.308(f)(2)-(3). In addition to satisfying the requirements at 40 CFR 51.308(f) related to reasonable progress, the regional haze SIP revisions for the second implementation period must address the requirements in section 51.308(g)(1) through (5) pertaining to periodic reports describing progress towards the RPGs, 40 CFR 51.308(f)(5), as well as requirements for Federal Land Manager consultation that apply to all visibility protection SIPs and SIP revisions. 25 25 40 CFR 51.308(i). A State must submit its regional haze SIP and subsequent SIP revisions to the EPA according to the requirements applicable to all SIP revisions under the Clean Air Act and the EPA's regulations. 26 Upon EPA approval, a SIP is enforceable by the EPA and the public under the Clean Air Act. If the EPA finds that a State fails to make a required SIP revision, or if the EPA finds that a SIP is incomplete or disapproves the SIP, the EPA must promulgate a Federal implementation plan (FIP) that satisfies the applicable requirements. 27 26 See Clean Air Act section 169(b)(2); Clean Air Act section 110(a). 27 Clean Air Act section 110(c)(1). A. Identification of Class I Areas The first step in developing a regional haze SIP is for a State to determine which Class I areas, in addition to those within its borders, “may be affected” by emissions from within the State. In the 1999 Regional Haze Rule, the EPA determined that all States contribute to visibility impairment in at least one Class I area and explained that the statute and regulations lay out an “extremely low triggering threshold” for determining “whether States should be required to engage in air quality planning and analysis as a prerequisite to determining the need for control of emissions from sources within their State.” 28 28 64 FR 35714, July 1, 1999, at pages 35720-35722. A State must determine which Class I areas must be addressed by its SIP by evaluating the total emissions of visibility impairing pollutants from all sources within the State. The determination of which Class I areas may be affected by a State's emissions is subject to the requirement in 40 CFR 51.308(f)(2)(iii) to “document the technical basis, including modeling, monitoring, cost, engineering, and emissions information, on which the State is relying to determine the emission reduction measures that are necessary to make reasonable progress in each mandatory Class I Federal area it affects.” B. Calculations of Baseline, Current, and Natural Visibility Conditions; Progress to Date; and the Uniform Rate of Progress As part of assessing whether a SIP revision for the second implementation period is providing for reasonable progress towards the national visibility goal, the Regional Haze Rule contains requirements in section 51.308(f)(1) related to tracking visibility improvement over time. The requirements of this section apply only to States having Class I areas within their borders; the required calculations must be made for each such Class I area. The EPA's 2018 Visibility Tracking Guidance 29 provides recommendations to assist States in satisfying their obligations under section 51.308(f)(1); specifically, in developing information on baseline, current, and natural visibility conditions, and in making optional adjustments to the uniform rate of progress to account for the impacts of international anthropogenic emissions and prescribed fires. 30 29 The 2018 Visibility Tracking Guidance references and relies on parts of the 2003 Tracking Guidance: “Guidance for Tracking Progress Under the Regional Haze Rule,” which can be found at https://www.epa.gov/sites/default/files/2021-03/documents/tracking.pdf and in the docket for this action. 30 82 FR 3078, January 10, 2017, at pages 3103-05. The Regional Haze Rule requires tracking of visibility conditions on two sets of days: the clearest and the most impaired days. Visibility conditions for both sets of days are expressed as the average deciview index for the relevant five-year period (the period representing baseline or current visibility conditions). The Regional Haze Rule provides that the relevant sets of days for visibility tracking purposes are the 20% clearest (the 20% of monitored days in a calendar year with the lowest values of the deciview index) and 20% most impaired days (the 20% of monitored days in a calendar year with the highest amounts of anthropogenic visibility impairment). 31 A State must calculate visibility conditions for both the 20% clearest and 20% most impaired days for the baseline period of 2000-2004 and the most recent five-year period for which visibility monitoring data are available (representing current visibility conditions). 32 States must also calculate natural visibility conditions for the clearest and most impaired days 33 by estimating the conditions that would exist on those two sets of days absent anthropogenic visibility impairment. 34 Using all these data, States must then calculate, for each Class I area, the amount of progress made since the baseline period (2000-2004) and how much improvement is left to achieve in order to reach natural visibility conditions. 31 40 CFR 51.301. This notice also refers to the 20% clearest and 20% most anthropogenically impaired days as the “clearest” and “most impaired” or “most anthropogenically impaired” days, respectively. 32 40 CFR 51.308(f)(1)(i), (iii). 33 The Regional Haze Rule at 40 CFR 51.308(f)(1)(ii) contains an error related to the requirement for calculating two sets of natural conditions values. The rule says, “most impaired days or the clearest days” where it should say “most impaired days and clearest days.” This is an error that was intended to be corrected in the 2017 Regional Haze Rule Revisions but did not get corrected in the final rule language. This is supported by the preamble text on page 3098 in the document published at 82 FR 3078, January 10, 2017: “In the final version of 40 CFR 51.308(f)(1)(ii), an occurrence of “or” has been corrected to “and” to indicate that natural visibility conditions for both the most impaired days and the clearest days must be based on available monitoring information.” 34 40 CFR 51.308(f)(1)(ii). Using the data for the set of most impaired days only, States must plot a line between visibility conditions in the baseline period and natural visibility conditions for each Class I area to determine the uniform rate of progress—the amount of visibility improvement, measured in deciviews, that would need to be achieved during each implementation period in order to achieve natural visibility conditions by the end of 2064. The uniform rate of progress is used in later steps of the reasonable progress analysis for informational purposes and to provide a non-enforceable benchmark against which to assess a Class I area's rate of visibility improvement. Additionally, in the 2017 Regional Haze Rule Revisions, the EPA provided States the option of proposing to adjust the endpoint of the uniform rate of progress to account for impacts of anthropogenic sources outside the U.S. and/or impacts of certain types of wildland prescribed fires. These adjustments, which must be approved by the EPA, are intended to avoid any perception that States should compensate for impacts from international anthropogenic sources and to give States the flexibility to determine that limiting the use of wildland-prescribed fire is not necessary for reasonable progress. 35 35 82 FR 3078, January 10, 2017, at page 3107, footnote 116. The EPA's 2018 Visibility Tracking Guidance can be used to help satisfy the 40 CFR 51.308(f)(1) requirements, including in developing information on baseline, current, and natural visibility conditions, and in making optional adjustments to the uniform rate of progress. In addition, the 2020 Data Completeness Memo provides recommendations on the data completeness language referenced in section 51.308(f)(1)(i) and provides updated natural conditions estimates for each Class I area. C. Long-Term Strategy for Regional Haze The core component of a regional haze SIP revision is a long-term strategy that addresses regional haze in each Class I area within a State's borders and each Class I area that may be affected by emissions from the State. The long-term strategy “must include the enforceable emissions limitations, compliance schedules, and other measures that are necessary to make reasonable progress, as determined pursuant to (f)(2)(i) through (iv).” 36 The amount of progress that is “reasonable progress” is based on applying the four statutory factors in Clean Air Act section 169A(g)(1) in an evaluation of potential control options for sources of visibility impairing pollutants, which is referred to as a “four-factor” analysis. The outcome of that analysis is the emission reduction measures that a particular source or group of sources needs to implement in order to make reasonable progress towards the national visibility goal. 37 Emission reduction measures that are necessary to make reasonable progress may be either new, additional control measures for a source, or they may be the existing emission reduction measures that a source is already implementing. See 82 FR 3078, January 10, 2017, at pages 3092-93. Such measures must be represented by “enforceable emissions limitations, compliance schedules, and other measures” ( i.e., any additional compliance tools) in a State's long-term strategy in its SIP. 38 36 40 CFR 51.308(f)(2). 37 40 CFR 51.308(f)(2)(i). 38 40 CFR 51.308(f)(2). Section 51.308(f)(2)(i) provides the requirements for the four-factor analysis. The first step of this analysis entails selecting the sources to be evaluated for emission reduction measures; to this end, States should consider “major and minor stationary sources or groups of sources, mobile sources, and area sources” of visibility impairing pollutants for potential four-factor control analysis. 39 A threshold question at this step is which visibility impairing pollutants will be analyzed. 39 40 CFR 51.308(f)(2)(ii). While States have discretion to choose any source selection methodology that is reasonable, whatever choices they make should be reasonably explained. To this end, 40 CFR 51.308(f)(2)(i) requires that a State's SIP revision include “a description of the criteria it used to determine which sources or groups of sources it evaluated.” The technical basis for source selection, which may include methods for quantifying potential visibility impacts such as emissions divided by distance metrics, trajectory analyses, residence time analyses, and/or photochemical modeling, must also be appropriately documented, as required by 40 CFR 51.308(f)(2)(iii). Once a State has selected the set of sources, the next step is to determine the emissions reduction measures for those sources that are necessary to make reasonable progress for the second implementation period. 40 This is accomplished by considering the four factors—“the costs of compliance, the time necessary for compliance, and the energy and nonair quality environmental impacts of compliance, and the remaining useful life of any existing source subject to such requirements.” 41 The EPA has explained that the four-factor analysis is an assessment of potential emission reduction measures ( i.e., control options) for sources; “use of the terms `compliance' and `subject to such requirements' in section 169A(g)(1) strongly indicates that Congress intended the relevant determination to be the requirements with which sources would have to comply in order to satisfy the [Clean Air Act's] reasonable progress mandate.” 42 Thus, for each source it has selected for four-factor analysis, 43 a State should consider a “meaningful set” of technically feasible control options for reducing emissions of visibility impairing pollutants. 44 40 The Clean Air Act provides that, “[i]n determining reasonable progress there shall be taken into consideration” the four statutory factors. Clean Air Act section 169A(g)(1). However, in addition to four-factor analyses for selected sources, groups of sources, or source categories, a state may also consider additional emission reduction measures for inclusion in its long-term strategy, e.g., from other newly adopted, on-the-books, or on-the-way rules and measures for sources not selected for four-factor analysis for the second implementation period. 41 Clean Air Act 169A(g)(1). 42 82 FR 3078, January 10, 2017, at page 3091. 43 “Each source” or “particular source” is used here as shorthand. While a source-specific analysis is one way of applying the four factors, neither the statute nor the Regional Haze Rule requires states to evaluate individual sources. Rather, states have “the flexibility to conduct four-factor analyses for specific sources, groups of sources or even entire source categories, depending on state policy preferences and the specific circumstances of each state.” 82 FR 3078, January 10, 2017, at page 3088. However, not all approaches to grouping sources for four-factor analysis are necessarily reasonable; the reasonableness of grouping sources in any particular instance will depend on the circumstances and the manner in which grouping is conducted. If it is feasible to establish and enforce different requirements for sources or subgroups of sources, and if relevant factors can be quantified for those sources or subgroups, then states should make a separate reasonable progress determination for each source or subgroup. 2021 Clarifications Memo at pages 7-8. 44 82 FR 3078, January 10, 2017, at page 3088. After identifying a reasonable set of potential control options for the sources it has selected, a State then collects information on the four factors with regard to each option identified. The EPA has also explained that, in addition to the four statutory factors, States have flexibility under the Clean Air Act and Regional Haze Rule to reasonably consider visibility benefits as an additional factor alongside the four statutory factors. 45 Ultimately, while States have discretion to reasonably weigh the factors and to determine what level of control is needed, section 51.308(f)(2)(i) provides that a State “must include in its implementation plan a description of . . . how the four factors were taken into consideration in selecting the measure for inclusion in its long-term strategy.” 45 See, e.g., Responses to Comments on Protection of Visibility: Amendments to Requirements for State Plans; Proposed Rule (81 FR 26942, May 4, 2016) (December 2016), Docket Number EPA-HQ-OAR-2015-0531, U.S. Environmental Protection Agency at page 186; EPA 2019 Guidance at pages 36-37. As explained above, section 51.308(f)(2)(i) requires States to determine the emission reduction measures for sources that are necessary to make reasonable progress by considering the four factors. Pursuant to section 51.308(f)(2), measures that are necessary to make reasonable progress towards the national visibility goal must be included in a State's long-term strategy and in its SIP. 46 If the outcome of a four-factor analysis is that an emissions reduction measure is necessary to make reasonable progress towards remedying anthropogenic visibility impairment, that measure must be included in the SIP. 46 States may choose to, but are not required to, include measures in their long-term strategies beyond just the emission reduction measures that are necessary for reasonable progress. See 2021 Clarifications Memo at 16. For example, states with smoke management programs may choose to submit their smoke management plans to the EPA for inclusion in their SIPs but are not required to do so. See, e.g., 82 FR 3078, January 10, 2017, at pages 3108-3109, (requirement to consider smoke management practices and smoke management programs under 40 CFR 51.308(f)(2)(iv) does not require states to adopt such practices or programs into their SIPs, although they may elect to do so). As with source selection, the characterization of information on each of the factors is also subject to the documentation requirement in section 51.308(f)(2)(iii). The reasonable progress analysis, including source selection, information gathering, characterization of the four statutory factors (and potentially visibility), balancing of the four factors, and selection of the emission reduction measures that represent reasonable progress, is a technically complex exercise, but also a flexible one that provides States with bounded discretion to design and implement approaches appropriate to their circumstances. Given this flexibility, section 51.308(f)(2)(iii) plays an important function in requiring a State to document the technical basis for its decision making so that the public and the EPA can comprehend and evaluate the information and analysis the State relied upon to determine what emission reduction measures must be in place to make reasonable progress. The technical documentation must include the modeling, monitoring, cost, engineering, and emissions information on which the State relied to determine the measures necessary to make reasonable progress. This documentation requirement can be met through the provision of and reliance on technical analyses developed through a regional planning process, so long as that process and its output has been approved by all State participants. In addition to the explicit regulatory requirement to document the technical basis of their reasonable progress determinations, States are also subject to the general principle that those determinations must be reasonably moored to the statute. 47 That is, a State's decisions about the emission reduction measures that are necessary to make reasonable progress must be consistent with the statutory goal of remedying existing and preventing future visibility impairment. 47 See Arizona ex rel. Darwin v. U.S. EPA, 815 F.3d 519, 531 (9th Cir. 2016); Nebraska v. U.S. EPA, 812 F.3d 662, 668 (8th Cir. 2016); North Dakota v. EPA, 730 F.3d 750, 761 (8th Cir. 2013); Oklahoma v. EPA, 723 F.3d 1201, 1206, 1208-10 (10th Cir. 2013); cf. also National Parks Conservation Association v. EPA, 803 F.3d 151, 165 (3d Cir. 2015); Alaska Department of Environmental Conservation v. EPA, 540 U.S. 461, 485, 490 (2004). The four statutory factors (and potentially visibility) are used to determine what emission reduction measures for selected sources must be included in a State's long-term strategy for making reasonable progress. Additionally, the Regional Haze Rule at 40 CFR 51.308(f)(2)(iv) separately provides five “additional factors” 48 that States must consider in developing their long-term strategies: (1) Emission reductions due to ongoing air pollution control programs, including measures to address reasonably attributable visibility impairment; (2) measures to reduce the impacts of construction activities; (3) source retirement and replacement schedules; (4) basic smoke management practices for prescribed fire used for agricultural and wildland vegetation management purposes and smoke management programs; and (5) the anticipated net effect on visibility due to projected changes in point, area, and mobile source emissions over the period addressed by the long-term strategy. 48 The five “additional factors” for consideration in section 51.308(f)(2)(iv) are distinct from the four factors listed in Clean Air Act section 169A(g)(1) and 40 CFR 51.308(f)(2)(i) that states must consider and apply to sources in determining reasonable progress. Because the air pollution that causes regional haze crosses State boundaries, section 51.308(f)(2)(ii) requires a State to consult with other States that also have emissions that are reasonably anticipated to contribute to visibility impairment in a given Class I area. Consultation allows for each State that impacts visibility in an area to share whatever technical information, analyses, and control determinations may be necessary to develop coordinated emission management strategies. This coordination may be managed through inter- and intra-regional planning organization consultation and the development of regional emissions strategies; additional consultations between States outside of regional planning organization processes may also occur. If a State, pursuant to consultation, agrees that certain measures ( e.g., a certain emission limitation) are necessary to make reasonable progress at a Class I area, it must include those measures in its SIP. 49 Additionally, the Regional Haze Rule requires that States that contribute to visibility impairment at the same Class I area consider the emission reduction measures the other contributing States have identified as being necessary to make reasonable progress for their own sources. 50 If a State has been asked to consider or adopt certain emission reduction measures, but ultimately determines those measures are not necessary to make reasonable progress, that State must document in its SIP the actions taken to resolve the disagreement. 51 The EPA will consider the technical information and explanations presented by the submitting State and the State with which it disagrees when considering whether to approve the SIP revision. Under all circumstances, a State must document in its SIP revision all substantive consultations with other contributing States. 52 49 40 CFR 51.308(f)(2)(ii)(A). 50 40 CFR 51.308(f)(2)(ii)(B). 51 40 CFR 51.308(f)(2)(ii)(C). 52 40 CFR 51.308(f)(2)(ii)(C). D. Reasonable Progress Goals Reasonable progress goals (RPGs) “measure the progress that is projected to be achieved by the control measures States have determined are necessary to make reasonable progress based on a four-factor analysis.” 53 Their primary purpose is to assist the public and the EPA in assessing the reasonableness of States' long-term strategies for making reasonable progress towards the national visibility goal. 54 States in which Class I areas are located must establish two RPGs, both in deciviews—one representing visibility conditions on the clearest days and one representing visibility on the most anthropogenically impaired days—for each area within their borders. 55 The two RPGs are intended to reflect the projected impacts, on the two sets of days, of the emission reduction measures the State with the Class I area, as well as all other contributing States, have included in their long-term strategies for the second implementation period. The RPGs also account for the projected impacts of implementing other Clean Air Act requirements, including non-SIP based requirements. Because RPGs are the modeled result of the measures in States' long-term strategies (as well as other measures required under the Clean Air Act), they cannot be determined before States have conducted their four-factor analyses and determined the control measures that are necessary to make reasonable progress. 56 53 82 FR 3078, January 10, 2017, at page 3091. 54 40 CFR 51.308(f)(3)(iii)-(iv). 55 40 CFR 51.308(f)(3)(i). 56 82 FR 3078, January 10, 2017, at page 3092. For the second implementation period, the RPGs are set for 2028. RPGs are not enforceable targets, 40 CFR 51.308(f)(3)(iii). While States are not legally obligated to achieve the visibility conditions described in their RPGs, section 51.308(f)(3)(i) requires that “[t]he long-term strategy and the reasonable progress goals must provide for an improvement in visibility for the most impaired days since the baseline period and ensure no degradation in visibility for the clearest days since the baseline period.” Thus, States are required to have emission reduction measures in their long-term strategies that are projected to achieve visibility conditions on the most impaired days that are better than the baseline period and shows no degradation on the clearest days compared to the clearest days from the baseline period. The baseline period for the purpose of this comparison is the baseline visibility condition—the annual average visibility condition for the period 2000-2004. 57 57 40 CFR 51.308(f)(1)(i); 82 FR 2078, January 10, 2017, at pages 3097-98. So that RPGs may also serve as a metric for assessing the amount of progress a State is making towards the national visibility goal, the Regional Haze Rule requires States with Class I areas to compare the 2028 RPG for the most impaired days to the corresponding point on the uniform rate of progress line (representing visibility conditions in 2028 if visibility were to improve at a linear rate from conditions in the baseline period of 2000-2004 to natural visibility conditions in 2064). If the most impaired days RPG in 2028 is above the uniform rate of progress ( i.e., if visibility conditions are improving more slowly than the rate described by the uniform rate of progress), each State that contributes to visibility impairment in the Class I area must demonstrate, based on the four-factor analysis required under 40 CFR 51.308(f)(2)(i), that no additional emission reduction measures would be reasonable to include in its long-term strategy. 58 To this end, 40 CFR 51.308(f)(3)(ii) requires that each State contributing to visibility impairment in a Class I area that is projected to improve more slowly than the uniform rate of progress provide “a robust demonstration, including documenting the criteria used to determine which sources or groups [of] sources were evaluated and how the four factors required by paragraph (f)(2)(i) were taken into consideration in selecting the measures for inclusion in its long-term strategy.” 58 40 CFR 51.308(f)(3)(ii). E. Monitoring Strategy and Other State Implementation Plan Requirements Section 51.308(f)(6) requires States to have certain strategies and elements in place for assessing and reporting on visibility. Individual requirements under this subsection apply either to States with Class I areas within their borders, States with no Class I areas but that are reasonably anticipated to cause or contribute to visibility impairment in any Class I area, or both. A State with Class I areas within its borders must submit with its SIP revision a monitoring strategy for measuring, characterizing, and reporting regional haze visibility impairment that is representative of all Class I areas within the State. SIP revisions for such States must also provide for the establishment of any additional monitoring sites or equipment needed to assess visibility conditions in Class I areas, as well as reporting of all visibility monitoring data to the EPA at least annually. Compliance with the monitoring strategy requirement may be met through a State's participation in the Interagency Monitoring of Protected Visual Environments (IMPROVE) monitoring network, which is used to measure visibility impairment caused by air pollution at the 156 Class I areas covered by the visibility program. 59 The IMPROVE monitoring data is used to determine the 20% most anthropogenically impaired and 20% clearest sets of days every year at each Class I area and tracks visibility impairment over time. 59 40 CFR 51.308(f)(6), (f)(6)(i), (f)(6)(iv). All States' SIPs must provide for procedures by which monitoring data and other information are used to determine the contribution of emissions from within the State to regional haze visibility impairment in affected Class I areas. 60 Section 51.308(f)(6)(v) further requires that all States' SIPs provide for a Statewide inventory of emissions of pollutants that are reasonably anticipated to cause or contribute to visibility impairment in any Class I area; the inventory must include emissions for the most recent year for which data are available and estimates of future projected emissions. States must also include commitments to update their inventories periodically. The inventories themselves do not need to be included as elements in the SIP and are not subject to EPA review as part of the EPA's evaluation of a SIP revision. 60 40 CFR 51.308(f)(6)(ii), (iii). All States' SIPs must also provide for any other elements, including reporting, recordkeeping, and other measures, that are necessary for States to assess and report on visibility. 61 A State may note in its regional haze SIP that its compliance with the Air Emissions Reporting Rule in 40 CFR part 51, subpart A satisfies the requirement to provide for an emissions inventory for the most recent year for which data are available. To satisfy the requirement to provide estimates of future projected emissions, a State may explain in its SIP how projected emissions were developed for use in establishing RPGs for its own and nearby Class I areas. 61 40 CFR 51.308(f)(6)(vi). Separate from the requirements related to monitoring for regional haze purposes under 40 CFR 51.308(f)(6), the Regional Haze Rule also contains a requirement at section 51.308(f)(4) related to any additional monitoring that may be needed to address visibility impairment in Class I areas from a single source or a small group of sources. This is called “reasonably attributable visibility impairment.” 62 Under this provision, if the EPA or the Federal Land Manager of an affected Class I area has advised a State that additional monitoring is needed to assess reasonably attributable visibility impairment, the State must include in its SIP revision for the second implementation period an appropriate strategy for evaluating such impairment. 62 The EPA's visibility protection regulations define “reasonably attributable visibility impairment” as “visibility impairment that is caused by the emission of air pollutants from one, or a small number of sources.” 40 CFR 51.301. F. Requirements for Periodic Reports Describing Progress Towards the Reasonable Progress Goals Section 51.308(f)(5) requires a State's regional haze SIP revision to address the requirements of paragraphs 40 CFR 51.308(g)(1) through (5) so that the plan revision due in 2021 will serve also as a progress report addressing the period since submission of the progress report for the first implementation period. The regional haze progress report requirement is designed to inform the public and the EPA about a State's implementation of its existing long-term strategy and whether such implementation is in fact resulting in the expected visibility improvement. 63 To this end, every State's SIP revision for the second implementation period is required to describe the status of implementation of all measures included in the State's long-term strategy, including BART and reasonable progress emission reduction measures from the first implementation period, and the resulting emissions reductions. 64 63 81 FR 26942, May 4, 2016, at page 26950; 82 FR 3078, January 10, 2017, at page 3119. 64 40 CFR 51.308(g)(1) and (2). A core component of the progress report requirements is an assessment of changes in visibility conditions on the clearest and most impaired days. For second implementation period progress reports, section 51.308(g)(3) requires States with Class I areas within their borders to first determine current visibility conditions for each area on the most impaired and clearest days, 40 CFR 51.308(g)(3)(i)(B), and then to calculate the difference between those current conditions and baseline (2000-2004) visibility conditions in order to assess progress made to date. 65 States must also assess the changes in visibility impairment for the most impaired and clearest days since they submitted their first implementation period progress reports. 66 Since different States submitted their first implementation period progress reports at different times, the starting point for this assessment will vary. 65 40 CFR 51.308(g)(3)(ii)(B). 66 40 CFR 51.308(g)(3)(iii)(B), (f)(5). Similarly, States must provide analyses tracking the change in emissions of pollutants contributing to visibility impairment from all sources and activities within the State over the period since they submitted their first implementation period progress reports. 67 Changes in emissions should be identified by the type of source or activity. Section 51.308(g)(5) also addresses changes in emissions since the period addressed by the previous progress report and requires States' SIP revisions to include an assessment of any significant changes in anthropogenic emissions within or outside the State. This assessment must include an explanation of whether these changes in emissions were anticipated and whether they have limited or impeded progress in reducing emissions and improving visibility relative to what the State projected based on its long-term strategy for the first implementation period. 67 40 CFR 51.308(g)(4), (f)(5). G. Requirements for State and Federal Land Manager Coordination Clean Air Act section 169A(d) requires that before a State holds a public hearing on a proposed regional haze SIP revision, it must consult with the appropriate Federal Land Manager or Federal Land Managers; pursuant to that consultation, the State must include a summary of the Federal Land Managers' conclusions and recommendations in the notice to the public. Consistent with this statutory requirement, the Regional Haze Rule also requires that States “provide the [Federal Land Manager] with an opportunity for consultation, in person and at a point early enough in the State's policy analyses of its long-term strategy emission reduction obligation so that information and recommendations provided by the [Federal Land Manager] can meaningfully inform the State's decisions on the long-term strategy.” 68 Consultation that occurs 120 days prior to any public hearing or public comment opportunity will be deemed “early enough,” but the Regional Haze Rule provides that in any event the opportunity for consultation must be provided at least 60 days before a public hearing or comment opportunity. This consultation must include the opportunity for the Federal Land Managers to discuss their assessment of visibility impairment in any Class I area and their recommendations on the development and implementation of strategies to address such impairment. 69 68 40 CFR 51.308(i)(2). 69 Ibid. In order for the EPA to evaluate whether Federal Land Manager consultation meeting the requirements of the Regional Haze Rule has occurred, the SIP revision should include documentation of the timing and content of such consultation. The SIP revision submitted to the EPA must also describe how the State addressed any comments provided by the Federal Land Managers. 70 Finally, a SIP revision must provide procedures for continuing consultation between the State and Federal Land Managers regarding the State's visibility protection program, including development and review of SIP revisions, five-year progress reports, and the implementation of other programs having the potential to contribute to impairment of visibility in Class I areas. 71 70 40 CFR 51.308(i)(3). 71 40 CFR 51.308(i)(4). III. The EPA's Evaluation of the Idaho Regional Haze SIP Revision for the Second Implementation Period A. Background on the Idaho First Implementation Period SIP Revision Idaho submitted its regional haze plan for the first implementation period on October 25, 2010. 72 The Clean Air Act required that first implementation period plans include, among other things, a long-term strategy for making reasonable progress and best available retrofit technology (BART) requirements for certain older facilities, where applicable. 73 The EPA approved Idaho's first implementation period plan in two actions on June 22, 2011 (76 FR 36329), and November 8, 2012 (77 FR 66929). Subsequently, on June 29, 2012, Idaho submitted BART revisions that the EPA approved on April 28, 2014 (79 FR 23273). On June 28, 2016, the State submitted a five-year progress report, approved by the EPA on July 15, 2019 (84 FR 33697). 74 In the action to approve the progress report, the EPA determined that the Idaho regional haze plan for the first implementation period was adequate and required no substantive revision. 75 72 2008 through 2018. 73 The requirements for regional haze SIPs for the first implementation period are contained in Clean Air Act section 169A(b)(2)(B) and 40 CFR 51.308(d) and (e). See also 40 CFR 51.308(b). 74 For details, please see the progress report in the EPA's prior action at https://www.regulations.gov under docket number EPA-R10-OAR-2017-0571. 75 84 FR 33697, July 15, 2019, at page 33698. B. The Idaho Second Implementation Period SIP Revision and the EPA's Evaluation On August 5, 2022, Idaho submitted a regional haze plan for the second implementation period. 76 Idaho made the submission available for public comment from June 22, 2022, through July 21, 2022, and held a public hearing on July 21, 2022. 77 The State received and responded to public comments and included the comments and responses in the submission. 78 Later, on September 27, 2024, Idaho submitted an additional action to supplement the August 5, 2022, submission. Idaho made the supplement available for public comment from August 12, 2024, to September 11, 2024, and received no public comments. 79 76 2018 through 2028. 77 Idaho Regional Haze Plan State Implementation Plan for the 2nd Implementation Period (Idaho 2022 plan submission) at Appendix C. Consultation Dates and Appendix G. Public Comment Period. 78 Id. at Appendix H. DEQ Responses to Public Comments. 79 See Idaho supplemental submission dated September 27, 2024, at page 36 and Appendix G. Public Comment Period. The following sections of this preamble describe the Idaho 2022 plan submission and the Idaho 2024 supplemental submission (herein referred to as “the Idaho submissions” or “the submissions”) and detail the EPA's evaluation of the submission against the requirements of the Clean Air Act and Regional Haze Rule. The Idaho submission and the EPA's supporting documentation may be found in the docket for this action. C. Identification of Class I Areas Section 169A(b)(2) of the Clean Air Act requires each State in which any Class I area is located or “the emissions from which may reasonably be anticipated to cause or contribute to any impairment of visibility” in a Class I area to have a plan for making reasonable progress toward the national visibility goal. The Regional Haze Rule implements this statutory requirement at 40 CFR 51.308(f), which provides that each State's plan “must address regional haze in each mandatory Class I Federal area located within the State and in each mandatory Class I Federal area located outside the State that may be affected by emissions from within the State,” and (f)(2), which requires each State's plan to include a long-term strategy that addresses regional haze in such Class I areas. 1. Idaho Class I Areas There are five mandatory Class I areas, or portions of such areas, within Idaho. 80 Craters of the Moon National Monument and Preserve, Sawtooth Wilderness Area, and Selway-Bitterroot Wilderness Area lie completely within Idaho State borders. The Hells Canyon Wilderness Area is a shared Class I area with Oregon and Yellowstone National Park is a shared Class I area with Wyoming. In its submissions, Idaho addresses all regional haze requirements in the three Class I areas that lie completely within Idaho. 81 Idaho's submissions also include a long-term strategy that addresses visibility impairment in the Hells Canyon Wilderness and Yellowstone National Park. By agreement with Idaho, Oregon and Wyoming, respectively, address core regional haze requirements for these two Class I areas, including calculations of visibility conditions, long-term strategy, reasonable progress goals, and monitoring. 82 Finally, Idaho's submissions address regional haze visibility impairment in other Class I areas in neighboring States. 80 See 40 CFR 81.410. 81 Idaho 2022 plan submission, tables 23-28. 82 Id., pages 3-4. a. Craters of the Moon National Monument and Preserve The Craters of the Moon National Monument and Preserve is made up of 43,243 acres on the Snake River Plain in south-central Idaho. 83 It is managed by the National Park Service and contains more than 25 volcanic cones and 60 distinct lava flows that are part of the Great Rift volcanic zone that continues along the Snake River Plain. 84 83 Id., page 3. 84 Ibid. b. Hells Canyon Wilderness Area The Hells Canyon Wilderness Area, managed by the U.S. Forest Service, is located on the border between Oregon and Idaho. The Snake River divides the wilderness, with 131,133 acres in Oregon, and 83,811 acres in Idaho. 85 85 See 40 CFR 81.410. c. Sawtooth Wilderness Area The Sawtooth Wilderness Area is comprised of 216,383 acres in central Idaho managed by the U.S. Forest Service. 86 The wilderness area includes the Sawtooth Mountains, home to approximately 40 peaks over 10,000 feet. 87 86 Ibid. 87 Ibid. d. Selway-Bitterroot Wilderness Area The Selway-Bitterroot Wilderness Area is located in north Idaho and crosses the Idaho-Montana border. 88 The area, managed by the U.S. Forest Service, spans 1,240,700 acres of rough mountainous terrain, dense forests, mountain lakes, and the Selway River. 89 88 Ibid. 89 Ibid. e. Yellowstone National Park Yellowstone National Park, managed by the National Park Service, covers 2.2 million acres, primarily in Wyoming. 90 A small portion of the park is located in eastern Idaho. 90 Ibid. 2. Idaho Visibility Monitors Haze species are measured and analyzed via the Interagency Monitoring of Protected Visual Environments (IMPROVE) network. 91 Table 1 of this preamble lists the IMPROVE monitors representing visibility at Idaho Class I areas. 91 IMPROVE website at http://vista.cira.colostate.edu/Improve. Table 1—Monitors Representing Visibility at Idaho Class I Areas 92 Monitor ID Sponsor Class I area Years operated CRMO1 National Park Service Craters of the Moon National Monument and Preserve 2001-present. HECA1 U.S. Forest Service Hells Canyon Wilderness Area 2001-present. SAWT1 U.S. Forest Service Sawtooth Wilderness Area 2001-present. SULA1 U.S. Forest Service Selway-Bitterroot Wilderness Area 2001-present. YELL2 National Park Service Yellowstone National Park 1991-present. In the submissions, Idaho documented that the State had consulted with Montana, Nevada, Oregon, Utah, Washington, and Wyoming on potential interstate visibility impacts to shared Class I areas and Class I areas outside of Idaho. 93 The Idaho Department of Environmental Quality (Idaho DEQ) shared source selection and evaluation data, however, no other State requested Idaho undertake additional four-factor analyses on top of those already conducted by Idaho. 94 Idaho committed to continued consultation with states in the west on interstate visibility contributions. 95 92 Sources: Idaho 2022 plan submission at page 11 and Federal Land Manager Environmental Database at https://views.cira.colostate.edu/fed/ . 93 Idaho 2022 plan submission, pages 86-90. 94 Id., pages 89-90. 95 Id., page 96. D. Calculations of Baseline, Current, and Natural Visibility Conditions; Progress to Date; and the Uniform Rate of Progress Section 51.308(f)(1) requires States to determine the following for “each mandatory Class I Federal area located within the State”: baseline visibility conditions for the most impaired and clearest days, natural visibility conditions for the most impaired and clearest days, progress to date for the most impaired and clearest days, the differences between current visibility conditions and natural visibility conditions, and the uniform rate of progress. This section also provides the option for States to propose adjustments to the uniform rate of progress line for a Class I area to account for visibility impacts from anthropogenic sources outside the U.S. and/or the impacts from wildland prescribed fires that were conducted for certain, specified objectives. 96 96 40 CFR 51.308(f)(1)(vi)(B). 1. Idaho Visibility Conditions The Idaho submissions addressed baseline, current and natural visibility conditions and the uniform rate of progress for Craters of the Moon National Monument and Preserve, Sawtooth Wilderness Area, and Selway-Bitterroot Wilderness Area, as required by the 2017 Regional Haze Rule and the EPA's technical guidance on tracking visibility progress. 97 Table 2 of this preamble summarizes visibility progress on the clearest days. Table 3 of this preamble summarizes visibility progress on the most impaired days, including adjustments to each Class I area's uniform rate of progress (URP) and natural conditions endpoint that the EPA modeled to account for certain international anthropogenic emissions and wildland prescribed fires. 98 97 EPA Technical Guidance on Tracking Visibility Progress for the Second Implementation Period of the Regional Haze Program, December 2018. Idaho defers to Oregon and Wyoming to provide this information for Hells Canyon Wilderness Area and Yellowstone National Park. See 89 FR 13622, February 23, 2024, at page 13636; 89 FR 95121, December 2, 2024, at page 95125. 98 Technical Support Document for the EPA's 2028 Updated Regional Haze Modeling, September 19, 2019. 99 Source: Idaho 2022 plan submission, table 6, page 12. 100 Sources: Idaho 2022 plan submission, table 4, page 11, and Technical Support Document for the EPA's 2028 Updated Regional Haze Modeling, September 19, 2019. Table 2—Clearest Days Visibility Conditions at Idaho Class I Areas in Deciviews 99 Monitor ID Class I area Baseline 2000-2004 Current 2014-2018 Natural 2064 Progress to date a Current minus Natural b CRMO1 Craters of the Moon National Monument and Preserve 4.31 2.68 1.73 1.63 0.95 SAWT1 Sawtooth Wilderness Area 4.00 2.58 1.51 1.42 1.07 SULA1 Selway-Bitterroot Wilderness Area 2.57 1.60 1.12 0.97 0.48 a Progress to date is the difference between the baseline and current conditions. A positive value indicates that visibility has improved. b A positive value indicates that current visibility has not reached natural conditions. Table 3—Most Impaired Days Visibility Conditions at Idaho Class I Areas in Deciviews 100 Monitor ID Class I Area Baseline 2000-2004 Current 2014-2018 Un-adjusted URP 2028 EPA- adjusted URP 2028 Natural 2064 Progress to date Current minus Natural EPA- adjusted Natural 2064 CRMO1 Craters of the Moon National Monument and Preserve 11.91 8.50 9.13 10.17 4.97 3.41 3.53 7.56 SAWT1 Sawtooth Wilderness Area 9.61 8.61 7.64 8.33 4.67 1 3.91 6.41 SULA1 Selway-Bitterroot Wilderness Area 10.06 8.37 8.23 9.07 5.48 1.69 2.92 7.58 The data in Tables 2 and 3 of this preamble indicate that current visibility has improved since the baseline period for both the clearest and most impaired days for each Class I area. In addition, Idaho included both the URP and an adjusted URP. Idaho relied upon the WRAP regional scale modeling using CAMx 2028OTBa2 H-L SA to adjust the URP. 101 The model projected international emissions and prescribed fire contributions, which the WRAP then used to adjust the natural visibility conditions in 2064. 102 The EPA proposes to determine that Idaho used scientifically valid data and methods for estimating the impacts of international emissions and wildland prescribed fire in the three Class I areas. 103 The EPA proposes to find that the Idaho submissions meet the requirements of 40 CFR 51.308(f)(1) to calculate baseline, current, and natural visibility conditions; progress to date; and the uniform rate of progress, including an adjusted URP, for the second implementation period. 101 Idaho 2022 plan submission, page 92. 102 Id. 103 40 CFR 51.308(f)(1)(vi)(B). E. Long-Term Strategy for Regional Haze Each State having a Class I area within its borders or emissions that may affect visibility in a Class I area must develop a long-term strategy for making reasonable progress towards the national visibility goal. 104 As explained in the background discussion in section I. of this preamble, reasonable progress is achieved when all States contributing to visibility impairment in a Class I area are implementing the measures determined—through application of the four statutory factors to sources of visibility impairing pollutants—to be necessary to make reasonable progress. 105 Each state's long-term strategy must include the enforceable emission limitations, compliance schedules, and other measures that are necessary to make reasonable progress. 106 After considering the four statutory factors, all measures that are determined to be necessary to make reasonable progress must be in the long-term strategy. In developing its long-term strategy, a State must also consider five additional factors. 107 As part of its reasonable progress determinations, the State must describe the criteria used to determine which sources or group of sources were evaluated ( i.e., subjected to four-factor analysis) for the second implementation period and how the four factors were taken into consideration in selecting the emission reduction measures for inclusion in the long-term strategy. 108 104 Clean Air Act section 169A(b)(2)(B). 105 40 CFR 51.308(f)(2)(i). 106 40 CFR 51.308(f)(2). 107 40 CFR 51.308(f)(2)(iv). 108 40 CFR 51.308(f)(2)(iii). States may rely on technical information developed by the regional planning organizations of which they are members to select sources for four-factor analysis and to conduct that analysis, as well as to satisfy the documentation requirements under section 51.308(f). Where a regional planning organization has performed source selection and/or four-factor analyses (or considered the five additional factors in section 51.308(f)(2)(iv)) for its member States, those States may rely on the regional planning organization's analyses for the purpose of satisfying the requirements of section 51.308(f)(2)(i) so long as the States have a reasonable basis to do so and all State participants in the regional planning organization process have approved the technical analyses. 109 States may also satisfy the requirement of section 51.308(f)(2)(ii) to engage in interstate consultation with other States that have emissions that are reasonably anticipated to contribute to visibility impairment in a given Class I area under the auspices of intra- and inter-regional planning organization engagement. 109 40 CFR 51.308(f)(3)(iii). The following paragraphs describe how the Idaho submissions addressed the requirements of 40 CFR 51.308(f)(2) and summarizes the EPA's evaluation of Idaho's submissions. 1. Pollutants Impacting Visibility at Idaho Class I Areas Idaho evaluated the haze composition at each of the IMPROVE monitors representing visibility at the Craters of the Moon National Monument and Preserve (CRMO1), Sawtooth Wilderness Area (SAWT1), and Selway-Bitterroot Wilderness Area (SULA1). In the submissions, Idaho illustrated that ammonium nitrate contributed the most to total light extinction at the CRMO1 monitor, followed by ammonium sulfate for each year from 2001 through 2018. 110 Idaho determined that the most impaired days at CRMO1 occurred mainly in the fall and winter and that nitrate and sulfate contributed the most to light extinction on these fall and winter most impaired days. 111 110 Idaho 2022 plan submission, pages 13-16. 111 Id., page 15. The Idaho submissions documented that organic carbon contributed the most to total light extinction at the SAWT1 monitor followed by ammonium sulfate for each year from 2001 through 2018. 112 Idaho determined that the anthropogenic contributions of ammonium nitrate were smaller at SAWT1, and that the anthropogenic fractions of organic carbon, elemental carbon and sulfate light extinction were the predominant contributors to annual haze at the monitor. 113 112 Id., pages 16-19. 113 Ibid. With respect to the SULA1 monitor, Idaho stated in the submissions that average aerosol light extinction on the most impaired days was largely from organic carbon and ammonium sulfate (47% and 25%, respectively), however lower levels of coarse mass, elemental carbon, ammonium nitrate, and fine soil were also present. 114 Idaho stated that the most impaired days occurred in the spring, summer, and fall. 115 According to Idaho, during these months, organic carbon made up the largest proportion of visibility impairing pollutants. 116 114 Id., pages 19-22. 115 Ibid. 116 Id. A review of IMPROVE data confirms the State's analysis of average haze composition at Idaho IMPROVE monitors and supports the State's decision to evaluate NO X , SO 2 , and PM 10 contributions to haze. 117 Importantly, Idaho evaluated specific pollutant emissions on a unit-by-unit basis for each source as described in the following paragraphs of this preamble. 117 See “Haze Composition at Idaho Class I Areas.xls” in the docket for this action. Annual average extinction composition for the years 2001 through 2022 for CRMO1, SAWT1, and SULA1. Data pulled from FED AQRV Visibility Tools. Federal Land Manager Environmental Database (FED); CSU and the Cooperative Institute for Research in the Atmosphere (CIRA). 2. Idaho Source Selection According to the State's submissions, Idaho used the source selection methodology developed by the WRAP for western States. 118 The WRAP's approach used the Q/d method, where Q is the sum of visibility impairing pollutants (NO X , SO 2 and PM 10 ), and d is the distance (kilometers) to the boundary of the nearest Class I area. The Idaho DEQ screened sources as described in the following steps: 119 118 See the WRAP Technical Support System (TSS) at www.wrapair2.org . 119 Idaho 2022 plan submission, page 54. 1. Identify those facilities with total facility-wide emissions of visibility impairing pollutants (NO X , SO 2 and PM 10 ) greater than 25 tons per year (tpy) based on 2014 National Emissions Inventory (NEI) data. 2. Calculate the distance from each facility identified in Step 1 to the nearest Class I area boundary (including those in other States) in kilometers (km). Facilities greater than 400 km from the nearest Class I area were considered to have minimal impact on visibility and were excluded. 3. Identify those facilities with a Q/d greater than the State-defined threshold. Idaho used a Q/d threshold of 2.0 because the State estimated that the threshold captured 70% to 80% of emissions from Idaho facilities. 4. Refine the Q/d analysis using more recent 2017 NEI data to screen out sources that have a Q/d less than the State-defined threshold for 2017 emissions. Idaho's initial source screening used 2014 emissions inventory data to identify 14 facilities in Idaho with Q/d greater than 2.0. 120 Refining the Q/d analysis using 2017 emissions inventory data screened out three additional facilities from the original 14 (Idaho Forest Group LLC-Riley Creek-Moyie Springs, Plummer Forest Group, Inc-Post Falls, and Rexburg Facility of Basic American Foods). 121 Idaho also screened out a facility outside of the State's regulatory purview (Boise Airport) and screened out a facility near Sawtooth Wilderness Area (Northwest Pipeline—Mountain Home) because the facility primarily emitted NO X . Idaho stated this was appropriate because anthropogenic contributions to NO X at SAWT1 were found to be negligible. 122 This screening process yielded nine Idaho facilities with Q/d greater than 2.0. 120 Idaho 2022 plan submission, page 55. See table 22 as updated by Idaho 2024 supplemental submission. 121 Id., page 55. 122 Id., page 56. See also figure 11. Idaho also used the WRAP weighted emissions potential (WEP) to confirm the selected sources. 123 According to Idaho's submissions, the WEP is a screening tool used to identified those sources contributing to visibility impairment in the 2014-2018 period and still operating in 2028 that have the potential to contribute to haze formation at Class I areas. 124 The rank point analysis consists of facility-level 2028 emissions for NO X or SO 2 sources overlaid with the corresponding extinction-weighted residence time for ammonium nitrate or ammonium sulfate. 125 123 Id., pages 61-62. 124 Id. 125 Id. Idaho also identified 27 Class I areas in five neighboring states (Montana, Nevada, Oregon, Washington, Wyoming) that could potentially be affected by emissions from sources within Idaho. However, applying the same source screening analysis yielded no additional Idaho facilities beyond the nine already selected for four-factor analysis. 126 Table 4 of this preamble lists the final nine selected sources. 126 Id., tables 24-28. 127 Source: table 22 of Idaho 2022 plan submission, as corrected by Idaho 2024 supplemental submission. Table 4—Idaho Selected Sources 127 Facility Nearest Class I area Distance (km) 2017 (tpy) 2017 Q/d P4 Production LLC (TV Facility) (P4) Grand Teton National Park 111.9 2,938.4 26.3 Clearwater Paper Corp-Pulp and Paper and Consumer Products (Clearwater Paper) Hells Canyon Wilderness 70.9 1,554 21.9 The Amalgamated Sugar Company LLC-Twin Falls (TASCO-Twin Falls) Jarbidge Wilderness 95.6 1,420 14.8 J.R. Simplot Company-Don Siding Pocatello (Simplot) Craters of the Moon Wilderness 86.1 876.3 10.2 The Amalgamated Sugar Company LLC-Paul (TASCO-Paul) Craters of the Moon Wilderness 78.0 577 7.3 Northwest Pipeline LLC-Soda Springs (NWP) Grand Teton National Park 122.2 579.8 4.7 ITAFOS Conda LLC (ITAFOS) Grand Teton National Park 104.0 477.7 4.6 The Amalgamated Sugar Company LLC-Nampa (TASCO-Nampa) Sawtooth Wilderness 114.6 590.9 5.1 Tamarack Mill, LLC Dba Evergreen Forest and Tamarack Energy Partnership (Tamarack Mills) Hells Canyon Wilderness 25.5 69.1 2.7 3. Emissions Units and Pollutants After selecting the nine sources, Idaho used the following steps to identify specific emissions units at each source: (1) Exclude processes or emissions units that emitted less than 20 tons per year of NO X , SO 2 , and PM 10 combined (based on 2014 and/or 2017 NEI data); (2) Identify those processes and emissions units where the summed emissions make up 70% or more of the total facility-wide emissions; (3) Identify the pollutant(s) of concern for the nearest Class I area for each facility, using the IMPROVE monitoring data and WEP ranking. 128 Table 5 of this preamble shows the emissions units and pollutants Idaho selected for review. 128 Idaho 2022 plan submission, appendix D. 129 Source: Idaho 2022 plan submission, page 64. See table 31 as corrected by Idaho 2024 supplemental submission. Table 5—Idaho Emissions Units and Pollutant Selected for Four-Factor Analysis 129 Facility Emissions unit Pollutants Clearwater Paper No. 4 Power Boiler NO X , SO 2 . Clearwater Paper No. 4 Recovery Furnace NO X , PM 10 . Clearwater Paper No. 5 Recovery Furnace NO X , PM 10 . ITAFOS East Sulfuric Acid Plant SO 2 . NWP-Soda Springs RICE 4 (TCVA-16) NO X . NWP-Soda Springs RICE 1-3 (TLA-6 Engines) NO X . P4 Nodulizing Kiln NO X , SO 2 , PM 10 . Simplot No. 300 Sulfuric Acid Plant SO 2 , PM 10 . Simplot No. 400 Sulfuric Acid Plant NO X , SO 2 . Tamarack Mills Riley Cogeneration Boiler NO X , PM 10 . TASCO-Nampa Riley Boiler NO X , SO 2 , PM 10 . TASCO-Paul B&W Boiler NO X . TASCO-Paul Rentech Boiler NO X . TASCO-Paul North and South Pulp Dryers NO X , SO 2 , PM 10 . TASCO-Twin Falls Foster Wheeler Boiler NO X , SO 2 , PM 10 . TASCO-Twin Falls B&W Boiler NO X , SO 2 , PM 10 . Based on a review of the information provided in the submission, we propose to determine that the Idaho source, unit, and pollutant selection methodology used for the regional haze second implementation period satisfies the requirement in 40 CFR 51.308(f)(2)(i) that the State include in its SIP a description of the criteria it used to determine which sources it evaluated. 4. Idaho Control Analyses and Determinations In developing its regional haze second implementation period plan submission, Idaho established a cost threshold of $6,100 per ton pollutant removed by adjusting the $5,000 per ton BART cost-effectiveness threshold (used during the first implementation period) for inflation. 130 The EPA did not establish a cost-effectiveness threshold for the second implementation period. Rather, the EPA's 2019 Guidance on Regional Haze State Implementation Plans for the Second Implementation Period (EPA 2019 Guidance) clarified that States have the flexibility to decide a reasonable approach to evaluating costs. 131 130 Idaho 2022 plan submission, pages 64 and 65. 131 Guidance on Regional Haze State Implementation Plans for the Second Implementation Period. The EPA Office of Air Quality Planning and Standards, Research Triangle Park (August 20, 2019), page 38 (EPA 2019 Guidance), available in the docket for this action and at https://www.epa.gov/visibility/guidance-regional-haze-state-implementation-plans-second-implementation-period . Table 6 of this preamble lists the control technologies, fuel specifications, and emission limits that Idaho determined are necessary for reasonable progress in the second implementation period, and the associated permit conditions that make the controls enforceable as a practical matter, including compliance schedules, monitoring, recordkeeping and reporting requirements. Table 6—Idaho Regional Haze Requirements 132 Facility Emissions unit Requirement Mechanism Clearwater Paper No. 4 Power Boiler 5.4 SO 2 emissions not to exceed 0.80 lb/MMBtu (30-day average) 5.5 NO X emissions not to exceed 0.2 lb/MMBtu (3-hr rolling average) when burning wood waste/gas and 0.3 lb/MMBtu (3-hr rolling average) when burning wood waste/gas 5.6 NO X emissions not to exceed 0.20 lb/MMBtu (3-hr rolling average) when burning gaseous fossil fuel and 0.3 lb/MMBtu (3-hr rolling average) when burning liquid fossil fuel, liquid fossil fuel/wood, or gaseous fossil fuel/wood 5.7 SO 2 emissions not to exceed 100 tons per any consecutive 12-month period Permit T1-2020.0024 issued March 30, 2023; conditions 5.4, 5.5, 5.6, 5.7, 5.10 through 5.15, 26.22, and 26.23. Clearwater Paper No. 4 Recovery Furnace 8.1 PM emissions not to exceed 0.040 gr/dscf at 8% oxygen using ESP Permit T1-2020.0024 issued March 30, 2023; conditions 7.1, 7.4, 7.9, 7.10, 8.1, 8.6, 26.22, 26.23, 26.26, 26.27, 26.28, and 26.29. Clearwater Paper No. 5 Recovery Furnace 9.1 PM emissions not to exceed 0.044 gr/dscf at 8% oxygen using ESP 9.2 PM emissions not to exceed 58 lb/hr or 0.03 gr/dscf Permit T1-2020.0024 issued November 26, 2021; conditions 7.1, 7.4, 7.9, 7.10, 9.1, 9.2, 9.6, 9.11, 26.22, 26.23, 26.26, 26.27, 26.28, and 26.29. 9.6 NO X emissions not to exceed 160 lb/hr, 700 tons/year, or 100 ppm on a dry basis at 8% oxygen ITAFOS East Sulfuric Acid Plant 5.1 SO 2 emissions not to exceed 258 lb/hr and 735.5 tpy Permit T1-2016.0015 issued March 2, 2022; conditions 5.1, 5.4, 5.5, 5.11, 16.22, and 16.23. NWP-Soda Springs RICE 1-3 (Clark TLA-6 Engines) RICE 4 (Clark TCVA-16) Replace the four existing RICE engines with two gas-fired turbines by July 31, 2031 Compliance Agreement Schedule Case No. E-2023.0011 dated September 1, 2023. P4 Nodulizing Kiln PM 10 emissions not to exceed 30.0 lb/hr SO 2 emissions not to exceed 143 lb/hr Permit T1-2020.0029 issued December 23, 2021; conditions 4.2, 4.4, 4.5, 4.6, 4.7, 4.19, 4.20, 4.21, 13.22, and 13.33. P4 Nodulizing Kiln Conduct NO X emissions testing and establish NO X emission limit Compliance Agreement Schedule Case No. E-2023.0013 dated November 27, 2023. P4 Cooler Spray Tower 4.2 PM 10 emissions not to exceed 27.0 lb/hr 4.2 SO 2 emissions not to exceed 177 lb/hr Permit T1-2020.0029 issued December 23, 2021; conditions 4.2, 4.4, 4.5, 4.6, 4.7, 4.19, 4.20, 4.21, 13.22, and 13.33. Simplot No. 300 Sulfuric Acid Plant 15.9 PM 10 emissions not to exceed 11.4 lb/hr based on 24-hour average and 49.8 tpy based on any consecutive 12-month period using mist eliminators and wet scrubbers (Related Consent Agreement in Portneuf Valley PM 10 SIP) Permit T1-2017.0024 issued March 29, 2023; conditions 15.9, 15.10, 15.11, 15.19, 15.20, 15.21, 15.22, 15.25, 15.27, 16.19, 18.22, and 18.23. 15.10 SO 2 emissions not to exceed 2.5 lb/ton of 100% sulfuric acid produced on a rolling 3-hour average basis, except during periods of startup, shutdown, or malfunction 15.10 SO 2 emissions not to exceed 1.5 lb/ton 100% sulfuric acid produced on a rolling 365-day average basis including periods of startup, shutdown, or malfunction 15.11 SO 2 emissions not to exceed 4.0 lb/ton of 100% sulfuric acid produced (Portneuf Valley PM 10 SIP) 15.11 SO 2 emissions not to exceed 170 lb/hr calculated as a 3-hr rolling average and 750 tpy based on any consecutive 12-month period (Portneuf Valley PM 10 SIP) 15.11 SO 2 emissions not to exceed 28 lb/ton of 100% sulfuric acid produced in accordance with IDAPA 58.01.01.846 (Portneuf Valley PM 10 SIP) Simplot No. 400 Sulfuric Acid Plant 16.6 NO X emissions not to exceed 10.1 lb/hr (24-hour average) (Portneuf Valley PM 10 SIP) 16.6 NO X emissions not to exceed 42.1 tpy based on any consecutive 12-month period (Portneuf Valley PM 10 SIP) 16.9 SO 2 emissions not to exceed 2.5 lb/ton of 100% sulfuric acid produced on a rolling 3-hour average basis, except during periods of startup, shutdown, or malfunction 16.9 SO 2 emissions not to exceed 1.6 lb/ton 100% sulfuric acid produced on a rolling 365-day average basis including periods of startup, shutdown, or malfunction 16.10 SO 2 emissions not to exceed 4 lb/ton of 100% sulfuric acid produced and 999 lb per each running three-hour period (Portneuf Valley PM 10 SIP) Permit T1-2017.0024 issued March 29, 2023; conditions 16.6, 16.9, 16.10, 16.19, 16.20, 16.21, 16.22, 16.26, 16.27, 18.22, and 18.23. Permit T1-9507-114-1 issued April 5, 2004 (incorporated by reference into the Idaho SIP at 40 CFR 52.670(d)); conditions. Tamarack Mills Riley Cogeneration Boiler 5.2 PM 2.5 /PM 10 emissions not to exceed 18.00 lb/hr 5.2 NO X emissions not to exceed 22.44 lb/hr 5.3 Particulate matter emissions not to exceed 0.080 gr/dscf at 8% oxygen 5.5 Fire wood waste exclusively, as defined Permit T1-2019-0024 issued October 17, 2022; conditions 5.2, 5.3, 5.5, 5.8, 5.17, 10.22, and 10.23. TASCO-Nampa Riley Boiler 4.8 Fire exclusively on natural gas and no longer fire coal by July 1, 2027 Permit P-2018.0011 issued February 15, 2023; condition 4.8. TASCO-Paul B&W Boiler NO X emissions not to exceed 132.0 tpy Combust natural gas only. Operate up to two of the three boilers simultaneously except during startup and shutdown when the three boilers may be partially operated Operation of the three boilers shall not exceed 40,000,000 therms (for all boilers combined) for the campaign year as defined Permit T1-2019-0020 issued November 5, 2021; conditions 4.4, 4.5, 4.6, 4.7, 4.10, 11.22, and 11.23. TASCO-Paul Rentech Boiler 4.3 NO X emissions not to exceed 0.10 lb/MMBtu (30-day average) 4.4 NO X emissions not to exceed 132.0 tpy. 4.5 Combust natural gas only. 4.6 Operate up to two of the three boilers simultaneously except during startup and shutdown when the three boilers may be partially operated 4.7 Operation of the three boilers shall not exceed 40,000,000 therms (for all boilers combined) for the campaign year as defined 4.9 Maximum heat input capacity shall not exceed 385 MMBtu/hr Permit T1-2019-0020 issued November 5, 2021; conditions 4.3, 4.4, 4.5, 4.6, 4.7, 4.9, 4.10, 4.11, 4.12, 4.15, 4.16, 4.18, 11.22, and 11.23. TASCO-Twin Falls Foster Wheeler Boiler 4.9 On and after January 1, 2023, fuel exclusively by natural gas Permit T1-2016.0017, issued on January 21, 2022; condition 4.9. TASCO-Twin Falls B&W Boiler 5.2 Only combust natural gas as fuel Permit T1-2016.0017, issued on January 21, 2022; condition 5.2. The following paragraphs of this preamble describe the Idaho control analyses and determinations and summarize the EPA's review by facility. For the reasons set forth in the following paragraphs, the EPA is proposing to approve Idaho's 2022 and 2024 SIP submissions as meeting the requirement in 40 CFR 51.308(f)(2)(i) that the State submit a long-term strategy that includes the enforceable emissions limitations, compliance schedules, and other measures that are necessary for reasonable progress based on an evaluation of the four statutory factors. 132 Idaho 2022 plan submission as updated by Idaho 2024 supplemental submission. See tables 37a, 37b, 38, 39, 40, 41a, 41b, 42. a. Clearwater Paper (Idaho DEQ Facility ID 069-00001) i. Background Clearwater Paper is a large kraft pulp mill located in Lewiston, Idaho. The mill converts chipped wood and sawdust into bleached pulp through a series of digestion, washing, screening, delignification, and bleaching operations. In the two recovery furnaces, the bleached pulp is formed, dried, treated, and sized to produce paperboard or consumer products. 133 Both recovery furnaces fire black liquor and natural gas and are equipped with electrostatic precipitators (ESPs) to control particulate matter. 134 133 Idaho 2022 plan submission, Appendix B. Four-Factor Analyses and Reviews. Clearwater Paper Corp.—Pulp and Paperboard Division. 134 Idaho 2022 plan submission, pages 80 and 81. Power for the facility is produced by three boilers that combust natural gas and fuel oil, in addition to a fourth high-pressure, high-temperature boiler that combusts cellulosic biomass (hog fuel, bark, lumber, chips sawdust, sander dust, wood pallets, clean wood), dewatered pulp and paper sludge, natural gas, and fuel oil. 135 135 Idaho 2022 plan submission, Appendix B. Four-Factor Analyses and Reviews. Clearwater Paper Corp.—Pulp and Paperboard Division. ii. Idaho Control Determination Clearwater Paper: No. 4 and No. 5 Recovery Furnaces Idaho conducted a review of NO X and PM 10 retrofit control options for the No. 4 and 5 recovery furnaces. For NO X , Idaho determined that it would not be technically feasible to retrofit the No. 4 and No. 5 recovery furnaces with low NO X burners, ultra low NO X burners (ULNB), flue gas recirculation, overfire air, selective non-catalytic reduction (SNCR), selective catalytic reduction (SCR), or low-temperature oxidation (LoTOx) technologies. Among other reasons, Idaho argued that those technologies have not been utilized on recovery furnaces that burn black liquor solids. 136 The facility stated that a quarternary air system has been implemented at just one similar facility in the U.S., where it was installed to comply with lowest achievable emission rate (LAER) requirements under Clean Air Act title I, part D (with an associated NO X emissions limit of 85 parts per million by volume, dry (ppmvd) at 8% oxygen). 137 Because the No. 4 recovery furnace was previously found to be emitting NO X at an even lower rate (75 ppmvd at 8% oxygen), Idaho determined that it was reasonable to conclude that installation of a quaternary air system would not reduce NO X emissions from the No. 4 recovery furnace. 138 136 Ibid. 137 Ibid. 138 Id. at page 10. Furthermore, Idaho stated that the No. 5 recovery furnace is already subject to major source pre-construction permitting limits for NO X (160 pounds per hour or 700 tons per year or 100 ppm) as set forth in the facility's operating permit and that NO X emissions have remained constant since 2014. 139 Idaho therefore determined that the NO X emission limits established through the PSD process constituted existing effective controls for the No. 5 recovery furnace. 139 Idaho 2022 plan submission, table 37; page 81. For PM 10 , Idaho stated that the No. 4 and No. 5 recovery furnaces are subject to National Emission Standards for Hazardous Air Pollutants (NESHAP) for Chemical Recovery Combustion Sources at Kraft, Soda, Sulfite, and Stand-Alone Semichemical Pulp Mills at 40 CFR part 63, subpart MM. 140 The NESHAP requires the use of electrostatic precipitators (ESPs) to comply with Maximum Available Control Technology (MACT) limits of 0.044 and 0.030 grains per dry standard cubic foot (gr/dscf) corrected to 8% oxygen, respectively. 141 Idaho determined that these requirements constituted existing effective controls for PM 10 . 140 Idaho 2022 plan submission, Appendix B. Four-Factor Analyses and Reviews. Clearwater Paper Corp.—Pulp and Paperboard Division. 141 Ibid. Clearwater Paper: No. 4 Power Boiler Idaho noted that the No. 4 power boiler was retrofitted with an overfire air system in 2016 and is currently subject to the NO X emission limits in the New Source Performance Standards (NSPS) for Fossil-Fuel-Fired Steam Generators in 40 CFR part 60, subpart D, specifically 0.20 lb/MMBtu NO X when firing natural gas and 0.30 lb/MMBtu NO X when firing wood or fuel oil. 142 Idaho evaluated additional retrofit NO X and SO 2 controls for the No. 4 power boiler under the four statutory factors. 143 For NO X , Idaho assessed the feasibility and costs of retrofitting the boiler with additional NO X controls, including LNB, ULNB, SNCR, SCR, and LoTOx. 144 Idaho determined that ULNB and flue gas recirculation were technologically infeasible. For the remaining, feasible controls, Idaho concluded that the cost to install any one of these systems would exceed the State's established cost-effectiveness threshold. 145 142 Idaho 2022 plan submission, page 80. 143 Id., page 2. 144 Id., page 3. 145 Idaho 2022 plan submission, Appendix B. Four-Factor Analyses and Reviews. Clearwater Paper Corp.—Pulp and Paperboard Division. Thus, the State concluded that the existing overfire air system and current permitted NO X limits for the No. 4 power boiler were necessary for reasonable progress. For SO 2 , the State identified retrofitting the No. 4 power boiler with a wet scrubber, lime spray dryer and baghouse, circulating dry scrubber, and reducing the sulfur content of the fuel as potential SO 2 controls. Idaho determined that reducing the sulfur content of fuel fired in the No. 4 power boiler was not feasible, most notably because the sulfur content of the hog fuel fired in the boiler is variable and difficult to control. 146 The State determined that retrofitting the No. 4 power boiler with a wet scrubber, lime spray dryer and baghouse, or circulating dry scrubber were each technically feasible SO 2 control options, however, Idaho estimated the cost of compliance for each of these technically feasible SO 2 control options would exceed the State's established cost-effectiveness threshold. 147 Idaho therefore determined that the NSPS requirements for Fossil-Fuel-Fired Steam Generators in 40 CFR part 60, subpart D, specifically, limiting SO 2 emissions to 0.80 lb/MMBtu and particulate matter emissions to 0.10 lb/MMBtu, constituted existing effective controls. 148 146 Id., page 10. 147 Id., page 5-6. 148 Idaho 2022 plan submission, page 80. We note that as part of the September 27, 2024, supplement, Idaho obtained and submitted additional information from the facility assessing fuel usage and limits for the No. 4 power boiler. 149 The facility stated that to meet existing permitted NO X and SO 2 limits, fuel oil is restricted to approximately 4-5% of annual MMBtu consumption. Upon review of the supplemental facility information, Idaho determined that it is not feasible to switch to low-sulfur fuel oil, because the use of fuel oil is limited. 150 149 Idaho 2022 plan submission, Appendix B. as supplemented by Idaho 2024 supplemental submission, Appendix F. Federal Land Managers Consultation Comments and DEQ Responses (Append), page 35. 150 Id. The State also considered the time necessary for installing the retrofit controls, energy and non-air quality environmental impacts of the controls, and remaining useful life of control technologies. 151 Idaho estimated that each of the technologically feasible NO X and SO 2 controls would take 32 months to implement. Idaho also noted that operation of the NO X and SO 2 controls would increase energy demand at the facility. 152 Idaho also indicated that a wet scrubber would increase the amount of water used, and LoTOx would increase the amount of nitrates in the facility's wastewater. Regarding remaining useful life of the controls, Idaho indicated the controls would have a lifetime of 20 years. 153 151 Idaho 2022 plan submission, Appendix B. Four-Factor Analyses and Reviews. Clearwater Paper Corp.—Pulp and Paperboard Division, pages 7-9. 152 Id. 153 Id. Idaho submitted the permit conditions that implement the existing NO X and SO 2 limits along with the associated monitoring, recordkeeping, and reporting requirements and compliance schedule for incorporation by reference into the Idaho SIP at 40 CFR 52.670(d). 154 See Table 6 of this preamble. 154 Each control measure necessary for reasonable progress is to be submitted in a form that is enforceable as a practical matter. The practically-enforceable provisions are then incorporated by reference into the CFR to be made enforceable by the EPA and citizens. See 57 FR 13497, April 16, 1992, at page 13567 (explaining principles, including enforceability and accountability, to which SIPs and implementing instruments must adhere to help assure that planned emission reductions will be achieved); and 77 FR 74355, December 14, 2012, at page 74365 (State's SIP must contain monitoring, recordkeeping, and reporting components necessary to make regional haze-related emission limitations enforceable). iii. EPA Evaluation Clearwater Paper: No. 4 and No. 5 Recovery Furnaces For PM 10 , we concur with Idaho's determination that the existing ESPs and associated emission limits to meet MACT requirements constitute existing effective controls. 155 As stated in the EPA 2019 Guidance on page 24, for a unit that complies with MACT, it is unlikely that an analysis of control measures would conclude that even more stringent control of PM is necessary to make reasonable progress. 155 EPA 2019 Guidance, pages 23 and 24. For NO X , the EPA does not agree with the State's finding that selective catalytic reduction (SCR), or low-temperature oxidation (LoTOx) technologies would not be technically feasible because they had not been used on the sources in question. In fact, the EPA has frequently found that controls which have been demonstrated on one type of source are feasible on another, related source. 156 Nevertheless, the EPA agrees with Idaho's ultimate conclusion that additional controls are not necessary in this case because the current NO X emission rate for the No. 4 recovery furnace (75 ppmvd at 8% oxygen) appears commensurate with LAER for recovery furnaces. Finally, we note that the No. 5 recovery furnace is subject to PSD BACT limits. 157 156 See 89 FR 67341, August 20, 2024, at page 67360. 157 Idaho 2022 plan submission, page 80. Therefore, we agree with Idaho's determination that the existing NO X controls on the No. 4 and No. 5 recovery furnaces are necessary for reasonable progress. Accordingly, we propose to find that the permit conditions submitted by Idaho for the No. 4 and No. 5 recovery furnaces are sufficient to make the above-described PM 10 and NO X requirements enforceable as a practical matter. 158 We propose to approve and incorporate by reference the permit conditions that implement the requirements and associated monitoring, recordkeeping and reporting requirements and compliance schedules specified in Table 6 of this preamble into the Idaho SIP at 40 CFR 52.670(d). 158 Idaho 2024 supplemental submission, Appendix J. Redacted Permits and Attachments for Regional Haze (New), 1. Clearwater Paper Corp.—Pulp and Paperboard Division Redacted Permits. Clearwater Paper: No. 4 Power Boiler We have determined that Idaho adequately considered the four statutory factors when determining the NO X and SO 2 controls necessary for the No. 4 power boiler. Idaho identified and evaluated a reasonable set of potential controls: three SO 2 controls and five NO X controls, and Idaho adequately estimated the cost-effectiveness of each of the feasible controls, using vendor quotes or the EPA's Control Cost Manual to estimate the cost-effectiveness of controls. 159 159 Idaho 2024 supplemental submission, appendix H, DEQ Responses to Public Comments (Replace), page 41. After reviewing additional information submitted on fuel usage and associated limits for SO 2 , we concur with Idaho's decision that it is not feasible to require the facility to fire lower sulfur fuel oil in the No. 4 power boiler at this time. Information in the September 27, 2024, supplemental submission stated that the No. 4 power boiler fires hog fuel and natural gas primarily, and while being permitted to fire higher sulfur fuel oil, the facility must limit the amount of fuel oil fired due to operational requirements and to ensure compliance with the current 100 ton per year SO 2 emission limit. 160 The oil emissions are limited by the existing NO X permit limit of 0.3 lb/MMBtu or 842 tpy for oil/wood and the existing SO 2 permit limit of 0.80 lb/MMBtu or 100 tons per any consecutive 12-month period. 161 Additionally, there are several monitoring, recordkeeping, and reporting requirements in the existing permit that will ensure compliance with the existing NO X and SO 2 emission limits. 160 Idaho 2024 supplemental submission, appendix B, Clearwater power boiler fuel oil analysis. 161 Ibid. The EPA concurs with Idaho's finding that the existing NO X and SO 2 emission limits established pursuant to the NSPS requirements for Fossil-Fuel-Fired Steam Generators in 40 CFR part 60, subpart D are necessary for reasonable progress. We also find that the submitted permit conditions for the Clearwater Paper No. 4 Power Boiler are sufficient to make the existing NO X and SO 2 requirements enforceable as a practical matter. We propose to approve and incorporate by reference the permit conditions that implement the existing requirements and associated monitoring, recordkeeping and reporting requirements and compliance schedules specified in Table 6 of this preamble into the Idaho SIP at 40 CFR 52.670(d). b. ITAFOS (Idaho DEQ Facility ID 029-00003) i. Background ITAFOS Conda LLC produces fertilizer in Soda Springs, Idaho. The East Sulfuric Acid Plant is a sulfur burning, dual-contact, dual-absorption plant that produces sulfuric acid and steam for use in other facility processes. 162 The plant combusts elemental sulfur in air to produce sulfur dioxide which is then passed through a series of four catalyst beds to convert the sulfur dioxide into sulfur trioxide. The primary pollutant emitted from this process is SO 2 . 163 The gas exiting the plant stack is continuously monitored for SO 2 . 164 162 Idaho 2022 plan submission, appendix B, ITAFOS Four-Factor Analysis Review, page 1. 163 Id. 164 Id., pages 1 and 2. ii. Idaho Control Determination ITAFOS: East Sulfuric Acid Plant Idaho evaluated retrofit SO 2 controls for the East Sulfuric Acid Plant using the four statutory factors. 165 In its initial 2022 submission, Idaho submitted evaluations of five retrofit SO 2 controls: wet flue gas desulfurization (WFGD), hydrogen peroxide scrubber, dry sorbent injection (DSI), spray dry absorber (SDA), and circulating dry scrubber (CDS). Idaho's 2022 submission includes an evaluation of the technological feasibility of the controls, cost-effectiveness of the controls, time necessary for compliance, energy and non-air quality environmental impacts, and remaining useful life of the retrofit controls. 166 Idaho determined that SDA and CDS were not technologically feasible because the temperature of the exhaust gas in the East Sulfuric Acid Plant is too low for the controls to effectively remove SO 2 . 167 165 Id., page 2. 166 Id., appendix B, ITAFOS Four-Factor Analysis Review, pages 3-6. 167 Id, page 3. In its 2022 submission, Idaho determined that WFGD, hydrogen peroxide scrubbers, and DSI were technically feasible options for SO 2 retrofit controls. 168 Based on information obtained from the company, Idaho calculated the cost-effectiveness of the three technologically feasible controls. According to Idaho, WFGD was cost effective at $4,100 per ton, hydrogen peroxide scrubbers at $4,777 per ton, and DSI at $4,121 per ton. 169 168 Id., page 3. See table 2. 169 Id., pages 5 and 6. Idaho updated its evaluations of the three retrofit controls in its September 27, 2024, supplemental submission. 170 Idaho submitted additional information obtained from the facility that impacted the technologically feasibility and cost of certain retrofit controls. For DSI, Idaho determined that the following factors rendered it technologically infeasible: (1) physical constraints that would impact the ability to install add-on DSI control equipment in the immediate vicinity to the East Sulfuric Acid Plant stack; (2) concerns about how the sorbent used in the control equipment could impact the existing chemical process; and (3) added costs that Idaho did not consider in its 2022 submission, including ancillary equipment needed to support WFGD control technology. 171 170 Idaho 2024 supplemental submission, appendix B Four Factor Analysis Reviews (Append). 171 Ibid. The revised cost estimates found that WFGD retrofit technology would cost $6,270 per ton, hydrogen peroxide scrubbers would cost $7,120 per ton, and DSI would cost $6,210 per ton. 172 All of these estimates were above the State-established cost-effectiveness threshold. Idaho also included an additional updated cost calculation for WFGD that further considered site-specific considerations. 173 According to this update, WFGD had a cost-effectiveness of $7,976. 174 Idaho ultimately determined that it would not require SO 2 retrofit control technology to be installed and that the inherent plant design (dual absorption contact process, vertical tube mist eliminator, and cesium catalyst in the fourth bed of the converter) and compliance with the NSPS standard for sulfur dioxide and acid mist (40 CFR part 60, subpart H) were necessary for reasonable progress. 175 Specifically, the current operating permit requires, among other things, that the owner or operator shall not cause to be discharged into the atmosphere from the East Sulfuric Acid Plant any gases which contain sulfur dioxide in excess of 2 kg per metric ton of acid produced (4 pounds per ton), the production being expressed as 100% sulfuric acid, in accordance with 40 CFR 60.82(a) (condition 5.7). 176 172 Id., pages 5 and 6. 173 Id., pages 7-8. 174 Id., page 10. 175 Ibid. 176 Ibid. As part of the Idaho 2024 supplemental submission, Idaho submitted the permit conditions that implement the existing SO 2 requirements and associated monitoring, recordkeeping and reporting requirements and compliance schedule for incorporation by reference into the Idaho SIP at 40 CFR 52.670(d). 177 177 Each control measure necessary for reasonable progress is to be submitted in a form that is enforceable as a practical matter. The practically enforceable provisions are then incorporated by reference into the CFR to be made enforceable by the EPA and citizens. See 57 FR 13497, April 16, 1992, at page 13567 (explaining principles, including enforceability and accountability, to which SIPs and implementing instruments must adhere to help assure that planned emission reductions will be achieved); and 77 FR 74355, December 14, 2012, at page 74365 (State's SIP must contain monitoring, recordkeeping, and reporting components necessary to make regional haze-related emission limitations enforceable). iii. EPA Evaluation ITAFOS: East Sulfuric Acid Plant The EPA reviewed Idaho's evaluation of SO 2 controls at the ITAFOS East Sulfuric Acid Plant in the states 2022 and 2024 submissions and has determined that the State selected potential retrofit controls, evaluated the technological and economic feasibility of the retrofit controls, and adequately considered each of the statutory factors when determining the controls necessary for reasonable progress. 178 178 EPA 2019 Guidance, page 37 (“We anticipate that the outcome of the decision-making process by a state regarding a control measure may most often depend on how the state assesses the balance between the cost of compliance and the visibility benefits, with the other three statutory factors either being subsumed into the cost of compliance or not being major considerations.”). Regarding technological feasibility, Idaho provided a valid basis to determine CDS and ammonia packed-bed scrubber were not feasible. For DSI and WFGD, the EPA does not agree that the factors Idaho cites render these options technologically infeasible. DSI and WFGD are common retrofit SO 2 controls that have proven effective in multiple applications. The need to construct baghouses, absorbing towers, and extended ductwork is not uncommon. These are factors the vendor should take into consideration in designing the system for a particular application. The EPA does recognize, however, that these same factors necessarily impact the cost of the controls and may impact the control efficiency. With respect to cost calculations, the EPA recommended in the EPA 2019 Guidance that States follow the EPA's Control Cost Manual recommendations to ensure consistent cost calculations across controls and sources. 179 The EPA also recommended that States explain any deviations or alternative approaches. 180 Finally, the Control Cost Manual provides for generic cost estimates using a consistent methodology, but recommends States obtain facility-specific vendor cost quotes when practical. 181 179 EPA 2019 Guidance, page 32. 180 Id. 181 Id. In evaluating the cost of WFGD, a hydrogen peroxide scrubber, and DSI, Idaho obtained cost information from equipment vendors. 182 Idaho conducted subsequent evaluations of its initial cost estimates to ensure the cost estimates took into consideration all the ancillary equipment necessary and site specific complexities. Idaho adequately explained its cost calculation methodology, its use of the Control Cost Manual, and its rationale for adjusting initial vendor estimates based on site-specific information. Therefore, based on the State's consideration of the four statutory factors, we agree with Idaho's determinations that additional SO 2 controls on the East Sulfuric Acid Plant are not necessary for reasonable progress. 182 Idaho 2024 supplemental submission, appendix B Four Factor Analysis and Review (Append), page 5-10. We propose to approve and incorporate by reference the permit conditions that implement the existing SO 2 requirements and associated monitoring, recordkeeping and reporting requirements and compliance schedules specified in Table 6 of this preamble into the Idaho SIP at 40 CFR 52.670(d). c. NWP-Soda Springs (Idaho DEQ Facility ID 007-00008) i. Background Northwest Pipeline—Soda Springs (NWP) is a natural gas compressor station located near Soda Springs, Idaho. The compressor station operates remotely and is used to compress and transmit natural gas along the transmission pipeline. 183 The facility has four natural gas-fired lean-burn reciprocating internal-combustion engines (RICE) (three TLA-6 IC engines and one TCVA-16 IC engine) that utilize air/fuel ratio controls and ignition timing delay to control NO X emissions. 184 183 Idaho 2022 plan submission, page 66. 184 Ibid. ii. Idaho Control Determination NWP—Soda Springs: RICE Engines Idaho evaluated the RICE engines for NO X controls. 185 The facility identified seven available retrofit NO X control technologies for the four RICE engines: air/fuel ratio controls, ignition timing delay, SCR, SNCR, NSCR, electrification, and low emission combustion retrofit (LEC). 186 Upon review, the facility concluded that LEC was the only technically feasible retrofit technology available and developed cost estimates. 187 Idaho estimated that the LEC retrofit would reduce NO X emissions by 87%. 188 The Idaho DEQ reviewed the facility's cost estimates for LEC, adjusted certain aspects, including the interest rate used and equipment life, and concluded such a retrofit would cost $10,656 per ton removed for the TCVA-16 IC engine and $24,874 per ton removed for the TLA-6 IC engines, exceeding the State-established cost-effectiveness threshold. 189 185 Idaho 2022 plan submission, appendix B, Four-Factor Analyses Reviews, 4 Northwest Pipeline. 186 Ibid. 187 Ibid. 188 Ibid. 189 Ibid. Idaho also evaluated the time necessary for compliance, the energy and non-air quality environmental impacts, and remaining useful life of an LEC retrofit. 190 Idaho estimated that such a retrofit would take 12 to 18 months to design and install. Idaho also indicated that the LEC retrofit would increase electricity consumption. Idaho estimated that the remaining lives of the engines were 20 years. 190 Ibid. Based on its review of the four factors, Idaho determined that the LEC retrofit was not cost-effective. However, after the initial 2022 submission, Idaho entered into a compliance agreement schedule with the facility to replace the four RICE engines with two gas-fired turbines by July of 2031. 191 All four RICE engines will be removed and replaced with two gas-fired turbines, specifically a Solar Centaur 40-4700S 15 ppm NO X unit and a Solar Taurus 70-10802S 9 ppm NO X unit. 192 Idaho determined that the replacements would achieve a 98% reduction in NO X —based on potential to emit. 193 191 Idaho 2024 supplemental submission, appendix J. Redacted Permits and Attachments for Regional Haze (New), Northwest Pipeline, LLC, CAS dated September 1, 2023. 192 Ibid. 193 Idaho 2024 supplemental submission, page 10. Idaho estimates the total reduction of NO X PTE upon completion of the equipment upgrade project will be 1687.17 tpy. Idaho determined the engine replacements were necessary for reasonable progress and as part of the September 27, 2024, supplemental submission, Idaho included the compliance agreement schedule for incorporation by reference into the Idaho SIP at 40 CFR 52.670(d). 194 See Table 6 of this preamble for details. 194 Each control measure necessary for reasonable progress is to be submitted in a form that is enforceable as a practical matter. The practically enforceable provisions are then incorporated by reference into the CFR to be made enforceable by the EPA and citizens. See 57 FR 13497, April 16, 1992, at page 13567 (explaining principles, including enforceability and accountability, to which SIPs and implementing instruments must adhere to help assure that planned emission reductions will be achieved); and 77 FR 74355, December 14, 2012, at page 74365 (State's SIP must contain monitoring, recordkeeping, and reporting components necessary to make regional haze-related emission limitations enforceable). iii. EPA Evaluation The EPA concurs that Idaho adequately considered the four statutory factors in determining the control necessary for reasonable progress at the NWP-Soda Springs facility. Accordingly, the EPA concurs with Idaho's determination that the requirement to remove the four RICE engines and replace them with two gas-fired turbines by July 31, 2031, is necessary for reasonable progress. We propose to approve and incorporate by reference the submitted compliance agreement schedule specified in Table 6 of this preamble into the Idaho SIP at 40 CFR 52.670(d). d. P4 Productions LLC (P4) (Idaho DEQ Facility ID 029-00001) i. Background P4 Production LLC (P4) owns and operates an elemental phosphorus manufacturing facility located in Soda Springs, Idaho, where phosphate ore is nodulized in a rotary kiln. 195 Emissions from the nodulizing kiln are controlled by a dust knockout chamber, spray tower, four parallel cyclonic separator pairs, four parallel Hydro-Sonic scrubbers and demisters, and a lime concentrated dual alkali SO 2 scrubbing system. 196 195 Idaho 2022 plan submission, page 70. 196 Id., appendix B. Four-Factor Analyses and Reviews, P4 Production LLC. ii. Idaho Control Determination P4: Nodulizing Kiln Idaho selected the nodulizing kiln for four-factor analysis for NO X , PM 10 , and SO 2 . For NO X , the facility identified the following potential retrofit technologies: good combustion practices, low NO X burners, SCR, and SNCR. 197 However, all were eliminated by the facility as technically infeasible. P4's primary rationale was the temperature demands for sintering phosphate ore are inconsistent with the temperature needs for the controls and that high particulate loading would fowl the catalyst. 198 197 Ibid. 198 Ibid. Idaho concurred that no technically feasible control technologies were available. The nodulizing kiln is not subject to any existing NO X controls or limits. Thus, Idaho did not determine that existing NO X controls are necessary for reasonable progress. However, to establish a NO X limit for the nodulizing kiln, the Idaho DEQ entered into a compliance agreement schedule (CAS) with the facility to establish a NO X emission limit for the nodulizing kiln. 199 The CAS requires the facility to submit a performance test protocol for approval by the Idaho DEQ, conduct testing over 12 months, submit a NO X emissions test report for approval by the Idaho DEQ, and submit a permit application to include a new NO X emission limit. 200 199 Idaho 2024 supplemental submission, page 11-12. 200 Idaho 2024 supplemental submission, Appendix J. Redacted Permits and Attachments for Regional Haze (New), 5. P4 Production LLC Redacted Permit and Compliance Agreement Schedule. For PM 10 , the facility reviewed four retrofit control technologies: good combustion practices, ESP, fabric filters, and wet scrubbers. Of these alternatives, wet scrubbers and wet ESPs were identified as technically feasible. 201 P4 Production already employs a Venturi wet scrubber system to control PM 10 emissions from the nodulizing kiln. 202 Idaho estimated that the existing wet scrubber system achieves 95% PM 10 control and concluded that it is the most effective control for PM 10 . 203 Idaho determined that the current Venturi wet scrubber system constituted existing effective controls for the nodulizing kiln. In the 2024 submission, Idaho included the permit conditions establishing PM 10 emissions limits reflecting operation of the Venturi wet scrubber system. 204 201 Idaho 2022 plan submission, appendix B, Four-Factor Analyses and Reviews, P4 Production LLC. 202 The kiln also include a dust knockout chamber, a spray tower, four parallel Hydro-Sonic systems, eight parallel cyclonic separator, and four mist eliminators that each provide PM 10 control. 203 Idaho 2022 plan submission, Appendix B. Four-Factor Analyses and Reviews, P4 Production LLC. 204 Idaho 2024 supplemental submission, Appendix J. Redacted Permits and Attachments for Regional Haze (New), 6. P4 Production LLC Redacted Permit. For SO 2 , the facility currently employs a lime concentrate dual alkali (LCDA) system that achieves 97% SO 2 emissions reductions. 205 Idaho identified process controls and flue gas desulfurization (FGD) as potential retrofit controls, however the Idaho ultimately determined these were either technically infeasible or would not achieve greater emissions reductions than the existing LCDA system. Thus, Idaho determined that the existing LCDA system constituted existing effective controls for SO 2 . In the 2024 submission, Idaho included permit conditions establishing SO 2 emissions limits reflecting operation of the LCDA system. 206 205 Id. 206 Idaho 2024 supplemental submission, Appendix J. Redacted Permits and Attachments for Regional Haze (New), 6. P4 Production LLC Redacted Permit. iii. EPA Evaluation P4: Nodulizing Kiln Idaho adequately considered the four statutory factors in determining the controls necessary for reasonable progress at P4 and adequately determined that there are no additional NO X controls that are feasible. Given that there is no current limit on NO X emissions from the nodulizing kiln, the EPA agrees that existing NO X controls are not necessary for reasonable progress. The CAS will assist Idaho is establishing a NO X emissions limit and thus: (1) help prevent future visibility impairment; and (2) assist the State in future regional haze planning efforts. The CAS includes a detailed timeline for testing, developing, and implementing a NO X emission limit along with agreed upon methods, with associated monitoring and recordkeeping requirements. Therefore, the EPA is proposing to approve the CAS and incorporate it into Idaho's SIP as a SIP strengthening measure. The EPA concurs with Idaho's determination that no new SO 2 controls are reasonable and the current LCDA system and associated SO 2 emission limit (143 lb/hr) are necessa [Text truncated at 120,000 characters. The full text is on the page linked above.] --- Source: Frix Law Library, https://www.frixlaw.com/law-library/documents/fr%3A2025-04906. Public record. Not legal advice.