Visibility Protection: Regional Haze State Plan Requirements Rule Revision

Visibility Protection: Regional Haze State Plan Requirements Rule Revision

SUPPLEMENTARY INFORMATION:

Table of Contents

I. Public Participation

Written Comments

Submit your comments, identified by Docket ID No. EPA-HQ-OAR-2025-1477, at https://www.regulations.gov (our preferred method), or the other methods identified in the ADDRESSES section. Once submitted, comments cannot be edited or removed from the docket. The EPA may publish any comment received to its public docket. The EPA requests that reviewers and commenters number their responses, for example, if responding to Topic 1, Question 1.a., please use the Topic and Question within a header before providing a response. Do not submit to EPA's docket at https://www.regulations.gov any information you consider to be Confidential Business Information (CBI), Proprietary Business Information (PBI), or other information whose disclosure 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). Please visit https://www.epa.gov/dockets/commenting-epa-dockets for additional submission methods; the full EPA public comment policy; information about CBI, PBI, or multimedia submissions; and general guidance on making effective comments.

II. General Information

A. Preamble Glossary of Terms and Acronyms

The following are abbreviations of terms used in this document.

ANPRM Advance notice of proposed rulemaking

NH ₃  Ammonia

BACT Best available control technology

BART Best available retrofit technology

CAA Clean Air Act

CBI Confidential business information

CFR Code of Federal Regulations

Class I areas Class I Federal areas

EPA Environmental Protection Agency

FIP Federal implementation plan

FLM Federal land manager

LAER Lowest achievable emissions rate

NAAQS National Ambient Air Quality Standards

NO _X  Nitrogen oxide

OMB Office of Management and Budget

PM Particulate matter

PM _2.5  Particulate matter equal to or less than 2.5 microns in diameter (fine particulate matter)

PM ₁₀  Particulate matter equal to or less than 10 microns in diameter

PSD Prevention of significant deterioration

PBI Proprietary business information

RACT Reasonable available control technology

RAVI Reasonably attributable visibility impairment

RPG Reasonable progress goal

RHR Regional Haze Rule

SIP State implementation plan

SO ₂  Sulfur dioxide

URP Uniform rate of progress

U.S. United States

VOC Volatile organic compound

B. How is this Federal Register document organized?

The information presented in this document is organized as follows:

I. Public Participation

II. General Information

A. Preamble Glossary of Terms and Acronyms

B. How is this Federal Register document organized?

C. Executive Summary

D. What is the purpose of this ANPRM?

E. Does this action apply to me?

III. What is the background for the EPA's proposed action?

A. Regional Haze

B. Requirements for Regional Haze SIPs for the First Planning Period

C. Requirements for Regional Haze SIPs for the Second Planning Period

D. EPA's 2024 Non-Regulatory Docket

IV. Request for Comments and Feedback

A. Overview and Introduction

B. Topic 1: Development and Implementation of a Reasonable Progress Metric and Consideration of the Four Statutory Factors

C. Topic 2: Development of Criteria Used To Determine When a SIP Revision Is Necessary

D. Topic 3: Determining SIP Content Requirements

V. Request for Comment and Additional Information

VI. What are the next steps EPA will take?

VII. Statutory and Executive Orders Reviews

C. Executive Summary

The Regional Haze program, established under Clean Air Act sections 169A and 169B, pertains to addressing visibility impairment in the 156 mandatory class I Federal areas, which includes specific national parks and wilderness areas. The program targets visibility impairment caused by manmade air pollution, primarily from industrial sources, vehicles, and other human activities. Emissions of pollutants such as sulfur dioxide (SO ₂ ), nitrogen oxides (NO _X ), and particulate matter contribute significantly to Regional Haze. The goals of the program are to prevent future, and remedy existing, impairment of visibility in identified Class I areas from manmade air pollution. A key statutory component of the program is the requirement for states to develop state implementation plans (SIPs), which outline strategies for achieving reasonable progress toward the national visibility goal articulated under CAA section 169A(a)(1). ⁽¹⁾ States are also tasked with monitoring visibility conditions and reporting progress to the EPA, including tracking emissions reductions and visibility improvements at Class I areas. ⁽²⁾

Throughout the implementation of the second planning period, we received feedback from different stakeholder groups regarding the unclear and resource intensive requirements of the Regional Haze program. For example, some stakeholders (including various state air agencies and regional planning organizations) commented that the process of developing a Haze SIP revision is burdensome to both the states and the EPA and that the EPA should provide regulatory clarity regarding states' SIP revision obligations. ⁽³⁾ In response to this feedback, on March 12, 2025, the EPA announced that a priority would be restructuring the Regional Haze program. ⁽⁴⁾ Consistent with this announcement, the EPA is reviewing its regulations implementing the Regional Haze program to ensure the regulations fulfill Congressional intent, are based on current scientific information, and reflect recent improvements in air quality at the 156 Class I areas. ⁽⁵⁾

D. What is the purpose of this ANPRM?

The EPA last revised the RHR in 2017 to clarify the relationship between long-term strategies and reasonable progress goals (RPGs) in SIPs and the long-term strategy obligation of all states; clarify and modify the requirements for periodic comprehensive revisions of SIPs; modify the set of days used to track progress towards natural visibility conditions to account for events such as wildfires; provide states with additional flexibility to account for impacts on visibility from anthropogenic sources outside the United States (U.S.) and from certain types of prescribed fires; modify certain requirements related to the timing and form of progress reports; and update, simplify, and extend to all states the provisions for reasonably attributable visibility impairment, while revoking most existing reasonably attributable visibility impairment Federal implementation plans (FIPs). ⁽⁶⁾ In the same action, the EPA also finalized an extension to the due date for second planning period SIP revisions from 2018 to 2021. ⁽⁷⁾ The EPA also proposed to extend the deadline for third planning period SIP revisions from 2028 to 2031, but has not yet finalized this proposal. ⁽⁸⁾

The current RHR requirements governing the second planning period are contained under 40 CFR 51.308(f), (g), (h), and (i). However, based on SIP development and processing experiences during implementation of the Regional Haze program's second planning period (2018 to 2028), the EPA has identified a need to streamline and clarify the program's requirements for the third planning period (2028 to 2038), and onward. Further, commenters expressed concerns regarding what constitutes an approvable SIP revision under the current RHR in the second and subsequent planning periods. ⁽⁹⁾

Therefore, the EPA is now seeking comment and input in restructuring existing regulations in a manner consistent with applicable requirements in CAA sections 169A and 169B pertaining to the protection of visibility at the 156 Class I areas addressed under the Regional Haze program. The EPA has identified several topics that are particularly relevant to the forthcoming RHR revisions and is soliciting feedback on ways to streamline and clarify certain requirements governing the Regional Haze program going forward. The EPA is issuing this ANPRM as an efficient means for gaining the information needed to inform EPA's decision-making, and to potentially aid in the development of proposed revisions to the RHR. The EPA encourages the public to participate in the regulatory process and provide specific suggestions regarding potential regulatory changes. Following the public comment period associated with this ANPRM, the Agency will move forward with fundamentally revising the Regional Haze program.

E. Does this action apply to me?

Entities that may be interested in this ANPRM include state, local, and Tribal governments, as well as Federal Land Managers (FLMs) responsible for protection of visibility in mandatory Federal Class I areas. This ANPRM may also be of interest to owners and operators of sources that emit particulate matter equal to or less than 10 microns in diameter (PM ₁₀ ), particulate matter equal to or less than 2.5 microns in diameter (PM _2.5 or fine PM), SO ₂ , NO _X , volatile organic compounds (VOC), ammonia (NH ₃ ), and other pollutants that may cause or contribute to visibility impairment. Others potentially interested in this ANPRM may include members of the general public who live, work, or recreate near or in mandatory Class I areas affected by visibility impairment. Additionally, members of the general public may be interested in this ANPRM because emissions sources that contribute to visibility impairment in Class I areas also may contribute to air pollution in other areas.

III. What is the background for the EPA's proposed action?

A. Regional Haze

Regional haze is visibility impairment that is produced by a multitude of sources and activities that are located across a broad geographic area and directly emit PM ₁₀ , PM _2.5 ( e.g., sulfates, nitrates, organic carbon, elemental carbon, and soil dust) and/or their precursors ( e.g., SO ₂ , NO _X , and, in some cases, NH ₃ and VOC). Fine particle precursors react in the atmosphere to form PM _2.5 , which impairs visibility by scattering and absorbing light. This light scattering and absorbing reduces the clarity, color, and visible distance that one can see. Particulate matter can also cause serious health effects in humans and contribute to environmental effects such as acid deposition and eutrophication.

B. Requirements for Regional Haze SIPs for the First Planning Period

Pursuant to a CAA directive to issue regulations, the EPA first promulgated a rule to address regional haze in 1999, which established the regulatory requirements for the first planning period Haze SIPs. ⁽¹⁰⁾ The 1999 RHR established a visibility protection program for Class I areas consistent with CAA section 169A. The requirements for the 1999 RHR and first planning period SIPs are found at 40 CFR 51.308(d) and (e), and 40 CFR 51.309. The initial Haze SIPs under the 1999 RHR were due to the EPA no later than December 17, 2007. ⁽¹¹⁾ Under 40 CFR 51.308(e), and the CAA, states were required to submit SIPs evaluating the use of the best available retrofit technology (BART) at certain larger, often uncontrolled, older stationary sources in order to address visibility impairment from these sources. ⁽¹²⁾ In addition to the BART requirements, the 1999 RHR also required states under 40 CFR 51.308(d) to establish two distinct RPGs for the most impaired and least impaired visibility days for each Class I area and a long-term strategy for making progress towards achieving the national visibility goal.

Since the RHR was finalized in 1999, Class I areas in all regions of the contiguous U.S. have experienced measurable improvements in visibility impairment. ^(13 14) Over the 2000-2019 period, there was an observed improvement in regional average visibility impairment at Class I areas, ranging from 0.5%/year to as much as 2.5%/year. ⁽¹⁵⁾ These visibility improvements were greatest in the eastern U.S., driven by strong decreases in sulfate impairment.

C. Requirements for Regional Haze SIPs for the Second Planning Period

In 2017, the EPA revised the Regional Haze Rule (2017 RHR) to clarify states' obligations and streamline certain Regional Haze requirements for the second planning period. ⁽¹⁶⁾ Whereas the 1999 RHR set the requirements for the first planning period, the 2017 RHR rule revisions contained requirements for the second planning period (and onward) relating to the requirement for SIPs to contain long-term strategies for making reasonable progress towards the national visibility goal. The requirements for the 2017 RHR are codified at 40 CFR 51.308(f), (g), (h), and (i). Among other changes, the 2017 RHR adjusted the deadline for states to submit their second planning period SIPs, clarified the order of analysis and the relationship between the RPGs and the long-term strategy, and focused on making visibility improvements on the days with the most manmade (or anthropogenic) visibility impairment, as opposed to the days with the most visibility impairment overall. In 2017, the EPA also revised requirements related to periodic progress reports and FLM consultation.

Currently, 40 CFR 51.308(f) requires states to submit periodic comprehensive revisions of implementation plans (referred to in this document as periodic comprehensive SIP revisions) addressing regional haze visibility impairment by no later than July 31, 2021, July 31, 2028, and every 10 years thereafter. All 50 states, the District of Columbia, and the U.S. Virgin Islands are required to submit SIPs satisfying the applicable requirements of the 2017 RHR. Each 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. To this end, 40 CFR 51.308(f) lays out the process by which states determine what constitutes their long-term strategies, with the requirements in 40 CFR 51.308(f)(1) through (3) establishing the process for evaluating previous and current visibility conditions at Class I areas, the development of a state's long-term strategy, and the establishment of Class I areas' RPGs. ⁽¹⁷⁾ Additionally, related requirements for SIP development are located at 40 CFR 51.308(f)(4) through (6). In addition to satisfying the requirements at 40 CFR 51.308(f) related to reasonable progress, SIP revisions must address the requirements in 40 CFR 51.308(g)(1) through (5) pertaining to periodic reports describing progress towards the RPGs, as well as requirements for FLM consultation in 40 CFR 51.308(i) that apply to all visibility protection SIPs and SIP revisions.

For additional background on the EPA's Regional Haze program and the 2017 RHR revisions, please refer to Section III: Overview of Visibility Protection Statutory Authority, Regulation, and Implementation of “Protection of Visibility: Amendments to Requirements for State Plans” of the 2017 RHR. ⁽¹⁸⁾

D. EPA's 2024 Non-Regulatory Docket

In Spring 2024, the EPA opened a non-regulatory docket (EPA-HQ-OAR-2023-0262-0001) to solicit feedback on a specific list of topics related to how the EPA could improve the implementation of the RHR in potential future rule revisions. The docket was open for public comment from March 28, 2024, to December 31, 2024, and the EPA received 34 comments. Copies of the comments received and the EPA's webinar presentation materials (docket ID: EPA-HQ-OAR-2023-0262-0002) are available at regulations.gov .

In preparing this ANPRM, the EPA reviewed the feedback received on the 2024 non-regulatory docket as well as comments received on individual second planning period SIP actions. In reviewing this feedback, the EPA observed concerns with the trajectory of the Regional Haze program, implementation difficulties with the program, and suggestions for changes to the current regulatory structure of the program. With this information, the EPA developed a set of updated questions regarding potential revisions to the regulatory framework of the Regional Haze program. Specifically, the EPA is issuing this ANPRM to solicit input on more specific and larger scale restructuring concepts that are intended to respond to the feedback received in the past several years.

A key goal of the forthcoming RHR revisions is to ensure clarity regarding what is needed to develop a fully approvable Regional Haze SIP revision, consistent with CAA requirements. The EPA is issuing this ANPRM with the intent of ensuring that any potential revisions align with the statutory goal of ensuring reasonable progress towards natural visibility conditions, while also providing the public the opportunity to submit additional ideas and reactions to the EPA in advance of our forthcoming rulemaking.

IV. Request for Comments and Feedback

A. Overview and Introduction

The EPA is requesting feedback on a restructuring of the Regional Haze program. To help guide feedback, the EPA is including background and an overview of priority topics in this ANPRM, including questions relating to how key aspects of the program could be implemented in future planning periods. Notably, the questions the EPA is highlighting, as well as the corresponding example solutions, do not represent the full universe of topics that could be addressed in a future rulemaking. Further, these questions should not be perceived as identifying the EPA's position on a given topic. Rather, they are intended to help reviewers consider different or new approaches for the Regional Haze program. To that end, this ANPRM focuses on three key topic areas that would serve to outline how the EPA might restructure the Regional Haze program. These topic areas are: (1) development/use of a reasonable progress metrics and consideration of the four statutory reasonable progress factors in CAA section 169A(g)(1), (2) development of SIP obligation criteria ( i.e., criteria used to determine when a SIP revision is required), and (3) determining SIP requirements for states that are required to submit a SIP revision.

In identifying these key topic areas, the EPA observes that a restructuring of the program would likely necessarily address these topic areas, which are foundational parts of the current Regional Haze program. The EPA observes that a program informed by current visibility conditions at Class I areas in determining when SIP revisions are required, as well as the content that SIP revisions must include, is aligned with at least some of the feedback received by the public. For example, rather than requiring every state (and territory) to submit a SIP every planning period, a targeted, data-driven approach that determines when SIP revisions are appropriate could be a way to manage the program moving forward in light of the progress to date in improving visibility conditions at the 156 Class I areas addressed under the Regional Haze program. The topic areas, questions, and concepts identified in this ANPRM are intended to support consideration of a programmatic restructuring based on a fundamental concept of a program that is data driven and recognizes both the current status of remaining visibility impairment at mandatory Class I areas and the measured improvement in visibility over the past 25 years of implementing the Regional Haze program.

Feedback on the Regional Haze program need not be limited to the material covered in this ANPRM and the three key topic areas. The EPA has provided an initial set of questions and issues to facilitate feedback. However, input is welcome on all aspects of the Regional Haze program and applicable requirements under the CAA. The EPA encourages reviewers and commenters to think broadly in their feedback and not limit feedback to specific requirements or aspects of the current 2017 RHR. In submitting comments in response to this ANPRM, the EPA encourages commenters to provide specific suggestions on program restructuring and revisions along with a legal rationale and policy objective. The EPA requests that reviewers and commenters number their responses, for example, if responding to Topic 1, Question 1.a., please use the Topic and Question within a header before providing a response. Finally, in providing feedback on the questions discussed below, the EPA welcomes commenters, where relevant, to provide redline-strikeout edits to the current regulatory text of 40 CFR 51.308(f), (g), (h), and (i) demonstrating how the EPA might incorporate commenters' suggested changes. Alternatively, where commenters foresee a need for new regulatory text to incorporate revisions to the Regional Haze program, commenters are encouraged to provide potential new regulatory text and an explanation of how commenters would implement the described changes.

B. Topic 1: Development and Implementation of a Reasonable Progress Metric and Consideration of the Four Statutory Factors

In the 2017 RHR, the EPA interpreted CAA section 169A(b)(2) to require states to substantively evaluate and determine potential emissions reductions by considering the four statutory factors in CAA section 169A(g)(1) after a state identified and selected sources that contribute to visibility impairment at Class I areas. ^(19 20) The EPA received feedback in its 2024 non-regulatory docket that the Agency should consider developing an objective and numerically-based reasonable progress metric (frequently referred to as a “safe harbor” in the comments received) that informs which, if any, additional measures may be necessary to make reasonable progress. Commenters also suggested that the reasonable progress metric could potentially be used to determine when a SIP revision is required. Comments to the 2024 non-regulatory docket also suggested that so long as reasonable progress towards the national goal continues to be made at Class I areas, states should not need to develop a SIP submission assessing additional measures that may be necessary to achieve reasonable progress. Therefore, by utilizing the concept of a “safe harbor” the EPA could develop an objective, numerical metric to inform how much progress a Class I area must make towards the national goal at any specific point in time. If the metric is met (visibility impairment is at or below the numerical metric at a certain point in time), the Class I area would be making reasonable progress towards the national goal.

This approach would be aligned with the CAA's direction in section 169B(e)(1) to include “criteria for measuring reasonable progress towards the national goal.” A reasonable progress metric would provide an objective way to determine the progress of the program and provide certainty to states regarding the amount of visibility improvement that is needed to meet the requirements of the Regional Haze program at specific points in time, and if/when further analysis of emissions control measures is needed. To the extent such a metric is used as the exclusive method for determining whether a Class I area is making reasonable progress, the EPA anticipates a need to explain the relationship between the metric and consideration of the four statutory factors. The CAA does not specify how or when the four statutory factors must be taken into consideration when evaluating the measures necessary for reasonable progress. ⁽²¹⁾ These criteria and/or metrics would also establish a framework that specifies when additional analysis is necessary to ensure that “reasonable progress” is being made, thereby dictating which specific actions (such as selecting sources for consideration of emissions control measures) a state must take during each planning period.

The EPA is considering whether to propose revising the rule to include a reasonable progress metric that would serve to identify when reasonable progress is being made towards the national visibility goal under CAA section 169A(a)(1). This concept would be aligned with stakeholder feedback that any metric used in this program should be a definitive metric that indicates if or when states have specific obligations to consider additional measures as may be necessary to make reasonable progress at one or more Class I areas. In order to explore these concepts further, the EPA solicits additional feedback on this idea. To assist in development of feedback, the EPA encourages consideration of the following questions.

1. Are there alternative approaches through which the EPA and/or states can meet the CAA section 169A(g)(1) requirement to consider the four factors in determining reasonable progress? Currently, this is achieved by requiring all states contributing to visibility impairment at a Class I area to evaluate and determine the emissions reduction measures that are necessary to make reasonable progress by considering the four statutory factors on a set of sources or group of sources identified at the state's discretion. Potential alternative approaches may include:

a. The EPA could develop a reasonable progress metric, consistent with CAA section 169B(e)(1), considering the four factors. If a Class I area does not achieve reasonable progress with measures already in the regulatory portion of the SIP for a particular time period, the rule could establish a process by which states would conduct more detailed analyses. These analyses would be consistent with CAA section 169A(g)(1) and would be used to identify additional controls or demonstrate that no additional controls are reasonable. For examples of what form the reasonable progress metric could take, please see Question 2 of Topic 1.

i. How could the EPA take the four factors under CAA section 169A(g)(1) into account when developing a reasonable progress metric? For example, the EPA could anticipate current measures to be considered into the reasonable progress metric. Here, control measures already in place may have been developed through requirements such as reasonable available control technology (RACT), best available control technology (BACT), or lowest achievable emissions rate (LAER), which have similar considerations to those of the four statutory factors.

b. The EPA could develop a reasonable progress metric, consistent with CAA section 169B(e)(1). If a Class I area does not achieve reasonable progress with existing measures previously incorporated into the SIP, states would need to further consider the four factors to either identify necessary controls or demonstrate that the EPA reasonable progress metric is too ambitious. For example, so long as the applicable Class I area(s) continue to make reasonable progress consistent with the metric, the EPA could determine that no additional consideration of the four factors is necessary to make reasonable progress at that specific point in time. In that case, states' existing, previously incorporated SIP measures would be all that is needed to make reasonable progress. In this format, the four factors serve as a “backstop” to ensure the Regional Haze program requirements are not overly burdensome or costly.

i. In this scenario, how must the EPA take the four factors under CAA section 169A(g)(1) into account when developing a reasonable progress metric?

c. Another potential approach could be for the EPA to complete a more comprehensive analysis of the projected visibility impacts of current measures, as well as potentially available additional measures, at Class I areas. In this analysis, the EPA would consider the four factors and identify potential available emissions reductions, calculate a projection of emissions to a future year ( i.e., project emissions), and conduct photochemical modeling to assess expected improvement in visibility impairment. The visibility improvements projected from the future year modeling would become the reasonable progress target that each Class I area must meet. Commenters are welcome to suggest inputs for this potential approach.

2. What form could a reasonable progress metric take? The EPA encourages commenters to provide feedback on how and when the four statutory factors would be taken into account within a reasonable progress metric, and who ( e.g., the EPA, states) should complete the analytical work needed to determine a reasonable progress metric for each Class I area. Potential approaches include:

a. Keep the current approach (perhaps with some minor adjustments). In this scenario, the currently defined 2017 RHR uniform rate of progress (URP) framework would apply (with adjustments for international anthropogenic and prescribed fires, ⁽²²⁾ but no change to the currently calculated 2064 end date). ⁽²³⁾ Being at or below the URP line indicates the reasonable progress requirement has been met, so long as the state has adequately considered the four statutory factors in developing its SIP submission. This scenario would rely on states to perform the four factor analysis on a set of selected sources, much like the second planning period analysis. ⁽²⁴⁾ If this approach were retained, restructuring could focus in on other aspects of the rule such as how a SIP is developed and when it is required.

b. Revise the technical considerations that were the basis of the URP framework. Potential revisions could include, but are not limited to, the following ideas (noting that some of these ideas are not mutually exclusive).

i. The EPA could change the end date to a year other than 2064, presumably a later year. This would change the reasonable progress requirement for any particular year and provide a longer glidepath (and changing the angle of the glidepath), with less progress needed over time to stay below the metric.

ii. The EPA could recalculate the URP every planning period (adjusting for international anthropogenic impairment and international and U.S. prescribed fire), which would be intended to ensure continuous visibility improvement based on current visibility conditions at Class I areas at the end of a planning period. Such a regularly occurring adjustment would ensure that progress is being made in each planning period at each Class I area. More progress would be required (steeper slope) for areas that are above the current URP, and less progress (gentler slope) would be required for areas that are below the current URP.

iii. Develop a completely new concept such as a percent improvement per planning period metric that is not based on the current URP. In this type of scenario, reasonable progress would be defined as achieving “X%” of remaining visibility improvement (with adjusted natural conditions or other end goal as the end point) per planning period that could be based on a required fixed percent progress every planning period, or photochemical modeling of future available emissions reductions, or other technical analyses.

3. Should the EPA revise the rule to include a concept akin to a “safe harbor” and what methods should the EPA use to track visibility conditions and determine reasonable progress? While stakeholders have long requested a “safe harbor” concept, the EPA is now soliciting specific information from the public on how CAA requirements can be embedded in such considerations. In this scenario, it is possible that the EPA's reasonable progress metric could serve as a regulatory “safe harbor” to better inform when a SIP revision is necessary. However, the EPA recognizes that data is needed to track visibility conditions at Class I areas to inform any kind of regulatory “safe harbor” implemented under the RHR. Potential approaches to track visibility conditions at Class I areas include:

a. Using the ambient data collected through the IMPROVE network.

i. How should the EPA balance the accuracy of ambient data, the associated time delay in collecting the data, and the time it takes between ambient data collection and the SIP revision development process?

b. Modeled estimates of U.S. anthropogenic impairment, tracked over time through periodic updated modeling.

c. A combination of ambient data and future projections, which is the current method employed under the 2017 RHR.

4. Are there recommended alternative metrics to the 20% clearest days and 20% most impaired days to track visibility impairment? Potential alternative approaches include:

a. Annual average of ambient visibility impairment (rather than only considering the most impaired and clearest days).

b. A different distribution of days ( e.g., the middle quintile—40th to 60th percentile—of deciviews). ⁽²⁵⁾

5. Should the EPA continue to track visibility impairment using IMPROVE ambient data in deciviews? Potential alternative approaches to track impairment include:

a. Only extinction values ( e.g., inverse megameters).

b. Trends in anthropogenic emissions of visibility impairing precursors.

C. Topic 2: Development of Criteria Used To Determine When a SIP Revision Is Necessary

The EPA received feedback in the 2024 non-regulatory docket that the effort states undertake in preparing a SIP should be commensurate with visibility improvements to date, as well as the resulting obligation for further visibility improvement, at impacted Class I areas. ⁽²⁶⁾ Likewise, air agency and industry stakeholders indicated that there may be situations where additional evaluation or implementation of further emissions controls are not necessary where Class I areas have made “enough” reasonable progress for the planning period at issue or where Class I areas are dominated by natural sources of visibility impairment ( e.g., wildfires or biogenics). ⁽²⁷⁾ In this scenario, stakeholders suggested that states should not be required to submit a SIP revision if reasonable progress is being made for that planning period. Implementation of such an approach would likely require significant changes and restructuring of the Regional Haze program. Concepts that would support such an approach are described below along with a broad solicitation for comment on these and any other concepts throughout this topic.

For example, the EPA could develop SIP obligation criteria that, when applied, would give states definitive information about whether or not a SIP revision is required. Further, such criteria would also inform the content of any SIP that might be required to be submitted. More specifically, in order to function this way, these criteria would need to identify Class I areas where sufficient visibility progress is being made at that specific point in time.

For this analysis, criteria could include (but are not limited to) consideration of a Class I area that is sufficiently “close to” natural visibility conditions and/or Class I areas that are below a reasonable progress metric for a particular time period. Further, the EPA could also identify states whose sources do not, or no longer, meaningfully contribute to visibility impairment in Class I areas for that point in time. With these two key technical pieces of information, the rule structure could essentially inform that if a state's contributions are so small as to not cause or contribute to visibility impairment at one or more Class I areas, the state is relieved of its obligations to conduct additional analysis of emissions control measures and revise its SIP for a specific planning period or point in time, so long as its current SIP-approved long-term strategy for addressing anthropogenic impairment is sufficient. This approach would also need to ensure that the statutory requirement for preventing future visibility impairment is also addressed.

The EPA also recognizes that until Class I areas meet the national goal under CAA section 169A(a)(1), some level of continued future planning is necessary in order to make reasonable progress and comply with the statute. Under CAA section 169A(a)(1), Congress established the national goal of “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.” Notably, this section of the CAA calls for “remedying of any existing impairment of visibility . . . which impairment results from manmade air pollution.” As visibility conditions at Class I areas continue to improve from reductions in anthropogenic impairment and get “close to” natural visibility conditions, the EPA observes that visibility impairment could reach a level below which it is not practical or feasible to further control. This could be viewed as a “ de minimis ” level of visibility impairment. A revised RHR could recognize this reality (where it exists) and seek to establish a “preservation” category for Class I areas where the EPA would determine that because a Class I area was so close to achieving natural conditions, additional measures would be unlikely to result in practical or feasible reductions in visibility impairing pollutants, including any perceptible improvement in visibility conditions. Therefore, just as the reasonable progress metric discussed in Topic 1 could identify when a Class I area has achieved reasonable progress for a specific point in time, Topic 2's “preservation” category could be used to identify Class I areas where anthropogenic visibility impairment is sufficiently minimal, suggesting that these areas have effectively achieved the national goal of the Regional Haze program, as outlined in CAA section 169A(a)(1). The 2017 RHR does not account for this, and thus this portion of the ANPRM is intended to solicit comment and identify potential approaches to address this fact.

For example, the EPA could establish a “preservation” category of Class I areas that are at or near achieving the national visibility goal. In the BART Guidelines, the EPA has generally identified a one deciview change as a small but noticeable change in visibility impairment. ⁽²⁸⁾ Additionally, for the purpose of identifying BART-eligible sources that caused or contributed to any impairment of visibility in a Class I area, the BART Guidelines identified 0.5 deciviews as a contribution to visibility impairment and one deciview as causing visibility impairment. ⁽²⁹⁾ Potentially informed by those concepts, the EPA could identify criteria based on deciview differences from natural conditions for when a Class I area could be in a “preservation” status. If a Class I area were to be placed into “preservation” status, nothing more would be needed to address impairment at that Class I area for an identified time period and/or planning period. However, this would not mean that the Regional Haze requirements have been fully met into perpetuity or that the respective Class I area(s) have reached natural conditions. At present, there are numerous remaining sources of anthropogenic emissions that contribute to visibility impairment, and such emissions may increase or change in scope over time. Therefore, this approach would still require a periodic evaluation of some sort (even if no SIP revision is ultimately required). The CAA also does not require the national goal (as articulated under CAA section 169A(a)(1)) to be achieved by a certain date. ⁽³⁰⁾ Therefore, “preservation” status for a Class I area could be considered as a temporary status (for the current planning period or point in time), and the EPA (and/or the state) would continue to track emissions and ambient data to ensure visibility has not degraded at those Class I areas. Under this concept, the EPA would specify options for remedying an increase in anthropogenic impairment if visibility were to degrade. Such options might include the trigger for a SIP revision, and/or parameters for the EPA to consider exercising its SIP call authority, under CAA section 110(k)(5), for certain states to evaluate emissions reduction measures through consideration of the four statutory factors.

Notably, the EPA does not intend for the potential establishment of a “preservation” category to affect the determination that visibility is an important value at the Class I area(s). Rather, it would serve as a regulatory tool for the EPA and states to track visibility improvement towards the national visibility goal and ensure states' SIP obligations reflect current visibility conditions at Class I areas. The statutory and regulatory Regional Haze requirements would remain for any state that may reasonably be anticipated to cause or contribute to any visibility impairment at any Class I area.

This topic identifies potential new regulatory approaches and associated criteria that may be applied to determine which mandatory Class I areas are currently making “sufficient reasonable progress” and/or are “close to” achieving natural conditions such that consideration of further emissions measures would not be necessary during a specific planning period or at a given point in time. To inform the EPA's decision on this issue, the EPA solicits feedback on the following questions:

6. Does the national visibility goal articulated under CAA section 169A(a)(1)  ⁽³¹⁾ require Class I areas to be at natural visibility conditions ( i.e., elimination of all U.S. anthropogenic visibility impairment) or does the goal refer to something less stringent than natural visibility conditions ( e.g., achieving a level of impairment that is consistent with no perceptible U.S. anthropogenic impairment)?

7. The national goal articulated under CAA section 169A(a)(1) requires both “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.” Congress adopted the visibility program in CAA section 169A to address existing visibility impairment and the Prevention of Significant Deterioration (PSD) program (CAA section 165) was intended to address (among other things) the prevention of future visibility impairment. ⁽³²⁾

a. What is necessary to address future anthropogenic visibility impairment? For example, is the PSD program sufficient to address the prevention of any future anthropogenic visibility impairment?

8. Should the EPA develop a numerical threshold to identify when Class I areas have achieved the national visibility goal? Potential approaches include but are not limited to:

a. Total estimated anthropogenic impairment of 0 deciviews (ambient data and/or model-based).

b. Total estimated anthropogenic impairment of 1 deciview or some other indicator of perceptible impairment (ambient data and/or model-based).

c. Estimated U.S. anthropogenic impairment of 0 deciviews (model-based).

d. Estimated U.S. anthropogenic impairment of

9. What types of criteria could the EPA describe to identify Class I areas where sufficient visibility progress is being made during a planning period such that states contributing to those areas would not have any SIP revision, or substantive SIP revision obligations related to those Class I areas ( i.e., not account for those areas in their SIP demonstration for that specific point in time)? Potential approaches include:

a. The EPA could determine that a Class I area is achieving reasonable progress based on reasonable progress metric discussed under Topic 1 of this ANPRM.

i. Compare recent ambient data or projected visibility to an identified reasonable progress metric to determine if criteria apply to that Class I area.

b. The EPA could develop a “preservation” category that would be defined as a Class I area being at or near natural visibility conditions.

ii. In establishing a “preservation” category, the EPA could compare recent ambient data or projected visibility data to estimated (adjusted) natural conditions to determine if the identified criteria apply to that Class I area. The EPA could strictly compare or establish a threshold that defines “close to” natural conditions.

c. The EPA could determine that states with “very small” anthropogenic contributions to any Class I areas meet the statutory and regulatory Regional Haze requirements and no new SIP revision would be required unless visibility in those Class I areas degrades or is projected to degrade.

iii. This would require the EPA to establish a de minimis contribution threshold. Considering the statutory language in CAA section 169A(b)(2), which states “the emissions from which may reasonably be anticipated to cause or contribute to any impairment of visibility in any such area,” how might the EPA establish and justify a threshold for emissions that cause or contribute to “any” visibility impairment at one or more Class I areas?

iv. In developing such an approach, are there lessons learned from other programmatic areas of the CAA where thresholds are used to identify SIP requirements ( e.g., PSD, interstate transport and national ambient air quality standards (NAAQS) planning, etc.)? The EPA solicits comments on the functionality of such approaches and implementation experiences associated with those programs and ways in which such programs might inform a similar style program in the Regional Haze context.

10. What technical analyses and data are needed to inform implementation of potential criteria; who is responsible for developing and analyzing such data; and can commenters identify updated available information from literature and/or recent studies? Potential approaches:

a. Updated estimates of natural conditions and international/prescribed fire adjustments.

b. Ambient data and/or photochemical modeling of visibility impairment at Class I areas.

c. Reduced form tools based on photochemical modeling, similar to those the EPA has developed for other CAA programs, such as PSD permitting. ⁽³³⁾

11. The EPA observes significant differences across the U.S. in visibility improvement made since the baseline period (2000-2004) and in existing impairment. For example, while the eastern states have made considerable progress towards reducing visibility impairing pollutants, Class I areas in the Eastern U.S. generally remain more impaired than western Class I areas. Given the significant difference in visibility conditions and progress across Class I areas ( e.g., East versus West), how can the EPA ensure reasonable progress is being made at all Class I areas?

D. Topic 3: Determining SIP Content Requirements

The EPA anticipates that even with a significant restructuring of the Regional Haze program some states (now and/or in the future) would still be required to submit a full Haze SIP revision. Many air agency comments to the 2024 non-regulatory docket expressed frustration with the workload necessary to achieve a fully approvable Haze SIP revision, as well as concerns with the lack of clarity associated with the 2017 RHR's regulatory and administrative requirements. ⁽³⁴⁾ Therefore, where the EPA determines a SIP revision to address visibility impairment at one or more Class I areas is necessary, the EPA recognizes a need to revise the Regional Haze program to ensure states have a clear understanding and pathway for achieving a fully approvable Haze SIP revision.

In response, the EPA is soliciting more targeted feedback to identify specific revisions that would serve to streamline the perceived or actual SIP development burdens on states when a SIP revision is required. To inform the EPA's decision on what regulatory changes would best support states when preparing a fully approvable Haze SIP revision, the EPA requests feedback on the following questions:

12. Should the EPA maintain the current approach under 40 CFR 51.308(f) to have “planning periods” every 10 years? Potential alternative approaches include:

a. Extend the 10-year planning periods to 15-year planning periods. CAA section 169A(b)(2)(B) states that all states must submit a SIP containing a 10-to-15-year long strategy for making reasonable progress towards the national goal articulated under CAA section 169A(a)(1). Under the 1999 and 2017 RHRs, the EPA has established that states must submit periodic comprehensive SIP revisions containing a 10-year long-term strategy for addressing anthropogenic impairment over the course of successive 10-year planning periods. ⁽³⁵⁾ However, the RHR could be revised so that planning periods occur in 15-year increments, as permitted by the statute. Under this scenario, states' long-term strategies would cover the 15-year period leading up to the next SIP revision compliance deadline for the next planning period.

b. Shift to requiring SIP revisions on an “as needed” basis. As mentioned under the previous bullet (1a), CAA section 169A(b)(2)(B) calls for states to submit a SIP containing “a long-term (ten to fifteen years) strategy for making reasonable progress toward meeting the national goal.” Furthermore, CAA section 169A(b)(2) requires each implementation plan “to contain such emission limits, schedules of compliance, other measures as may be necessary to make reasonable progress toward meeting the national goal.” If the measures incorporated into the states' long-term strategy continue to make reasonable progress towards the national goal, states would not be required to submit a SIP revision. Instead, states would be required to update their long-term strategies when sufficient reasonable progress is not being made towards the national goal, thereby fulfilling Congress's mandate for long-term strategies to contain “the measures as may be necessary” to achieve the national goal and potentially fulfill the statutory requirement to have a 10-to-15-year long-term strategy under CAA section 169A(b)(2)(B). In this approach, and if the EPA were to implement one or more of the metrics and criteria discussed under Topics 1 and 2 to determine when Haze SIP revisions are necessary, the EPA could issue a SIP call informed by the EPA's current understanding of visibility conditions at the 156 Class I areas. The RHR could also be revised to include a mechanism for the EPA to periodically report on visibility conditions at Class I areas to inform this decision, consistent with CAA section 169B(b).

13. The 2017 RHR allows states to include the impacts of other CAA regulatory programs when developing their Regional Haze SIPs ( e.g., NAAQS implementation). However, there is some ambiguity to what extent states must make these other CAA regulatory programs federally enforceable within the Regional Haze SIP ( i.e., the long-term strategy for Regional Haze). Therefore, how or when should states consider and/or rely upon emissions reductions from other CAA regulatory programs for Regional Haze purposes?

14. To what extent should states be required to incorporate sources' current emissions measures into their Regional Haze SIP revisions, consistent with the requirements of CAA section 169A(b)(2), in order to obtain “credit” for such reductions as part of their Regional Haze SIP and reasonable progress requirements?

a. What are potential pathways for making existing measures ( e.g., permit limitations, statewide emissions management strategies, source-specific consent agreements) federally enforceable in a SIP such that they can be relied upon for the reasonable progress determination under the Regional Haze program?

15. The purpose of the Regional Haze program, as outlined in CAA section 169A(a)(1), is to remedy any existing and prevent any future visibility impairment. How should visibility be considered as a regulatory factor to ensure Regional Haze SIP revisions are evaluated based on visibility improvement at Class I areas?

16. What would the benefits or drawbacks from removing states' requirements under the 2017 RHR to submit a 5-year progress report between SIP revision submittals under 40 CFR 51.308(g)?

17. In what way should the EPA consider revising the Reasonably Attributable Visibility Impairment (RAVI) provisions under 40 CFR 51.302 to ensure CAA objectives are met? Examples of potential revisions are:

a. Removing the RAVI provisions entirely from the RHR at 40 CFR 51.302, 40 CFR 51.304, and 40 CFR 51.305.

b. Restructuring RAVI by revising the process of FLMs certifying a RAVI for a source (or sources), and what happens after a RAVI is identified.

18. The EPA has observed in its implementation of the second planning period that there is disagreement between states and FLMs on the implementation of FLM consultation requirements. The CAA provides for consultation with FLMs ( see CAA section 169A(d)). The EPA also recognizes the unique and important role served by the FLMs as it pertains to mandatory Class I areas. The EPA solicits feedback on specific revisions to FLM consultation provisions in 40 CFR 51.308(i), consistent with the CAA, that ensures adequate FLM consultation but does not unnecessarily delay or cause undue burden on states and others engaged in the Regional Haze process. For example, in some instances, the EPA observed that FLMs received portions of draft SIPs prior to a public comment period on a SIP submittal at the state level. In other instances, there were disagreements about whether the “consultation” met the statutory and regulatory requirements.

a. The EPA solicits specific feedback regarding the level of consultation and materials that are needed to fulfill the statutory obligations under CAA section 169A(d). Similarly, the EPA solicits feedback regarding challenges states faced in submitting materials to FLMs to fulfill the consultation requirements. Examples of feedback the EPA would find most helpful include, but are not limited to:

i. In order to meet the statutory “consultation” requirements, which SIP materials/content, if any, must be offered to the FLMs during their opportunity for consultation?

ii. How can the EPA establish regulatory guidelines to clarify when a Haze SIP revision must undergo FLM consultation? For example, does a Haze SIP revision that addressed minor edits ( e.g., spelling or citation correction or revisions that are administrative in nature that do not modify SIP requirements) need to undergo FLM consultation? Or is FLM consultation only required when a state is proposing to substantively revise its long-term strategy or underlying analysis?

b. The 2017 RHR requires states to provide FLMs a minimum of 60 days to review Haze SIPs. ⁽³⁶⁾ How much time should states need to provide the FLMs during the opportunity for consultation?

c. How far in advance of the state public comment process should FLM consultation occur?

19. The 2017 RHR currently includes an interstate consultation process; however, the CAA itself does not mandate such a consultation. ⁽³⁷⁾ Throughout implementation of the program, the EPA observes that this provision brought states together to discuss impairment at Class I areas in ways that all parties could find beneficial. However, the interstate consultation process requires states to allocate additional resources and extend the SIP development timeline in a way that may not always result in a productive consultation. Given this context, the EPA solicits feedback regarding how the EPA could revise or clarify the interstate consultation process (40 CFR 51.308(f)(2)(ii)) states must undergo before submitting a SIP revision to the EPA.

a. Furthermore, what role should the regional planning organizations play in interstate consultation and overall SIP development?

V. Request for Comment and Additional Information

The EPA is seeking comment on all questions and topics described in this ANPRM and welcomes submission of any other information, including information which may not be specifically mentioned in this document. The EPA requests that commenters make specific recommendations and include supporting documentation where appropriate. In addition, the EPA is seeking comment on how the agency could consider the valuation of potential benefits from reducing regional haze. Please identify any relevant peer reviewed studies and the appropriateness of applying those studies within the context of potential regional haze regulatory changes. Instructions for providing written comments are provided under ADDRESSES , including how to submit any comments that contain CBI.

VI. What are the next steps EPA will take?

The EPA intends to use the information submitted in response to this ANPRM to inform a forthcoming proposed rulemaking to revise the RHR.

VII. Statutory and Executive Orders Reviews

Under Executive Order 12866, entitled Regulatory Planning and Review (58 FR 51735, October 4, 1993), this is a “significant regulatory action”. Accordingly, the EPA submitted this action to the Office of Management and Budget (OMB) for review under Executive Order 12866 and any changes made in response to Executive Order 12866 review have been documented in the docket for this action. Because this action does not propose or impose any requirements, other statutory and executive order reviews that apply to rulemaking do not apply. Should the EPA subsequently determine to pursue a rulemaking, the EPA will address the statutes and executive orders as applicable to that rulemaking.

Additional information about statutes and executive orders can be found at https://www.epa.gov/laws-regulations/laws-and-executive-orders.

⁽¹⁾  “Congress hereby declares 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.”

⁽²⁾  See 40 CFR 51.308(f)(6).

⁽³⁾  For example, see the following comments submitted to the 2024-nonregulatory docket (EPA-HQ-OAR-2023-0262) by SESARM/VISTAS, the Alaska Department of Environmental Quality, CenSARA, Minnesota Pollution Control Agency, and California Air Resources Board.

⁽⁴⁾ See https://www.epa.gov/newsreleases/epa-launches-biggest-deregulatory-action-us-history.

⁽⁵⁾ See https://www.epa.gov/newsreleases/administrator-zeldin-begins-restructuring-regional-haze-program.

⁽⁶⁾ See “ Protection of Visibility: Amendments to Requirements for State Plans ”. 82 FR 3078 (January 10, 2017).

⁽⁷⁾  Id.

⁽⁸⁾ See 89 FR 104471 (December 23, 2024).

⁽⁹⁾ See comments in EPA's non-regulatory docket (EPA-HQ-OAR-2023-0262).

⁽¹⁰⁾ See 64 FR 35714 (July 1, 1999).

⁽¹¹⁾ See 70 FR 39104 (July 6, 2005).

⁽¹²⁾  The set of “major stationary sources” potentially subject-to-BART is listed in CAA section 169A(g)(7).

⁽¹³⁾  The observed improvement was smaller in the Class I areas in Alaska and Hawaii, with an observed increase in visibility impairment in the Virgin Islands.

⁽¹⁴⁾ See 64 FR 35714 (July 1, 1999).

⁽¹⁵⁾ See Figure 7.9.5, IMPROVE Spatial and Seasonal Patterns and Temporal Variability of Haze and Its Constituents in the United States, Report VI, 2023.

⁽¹⁶⁾ See 82 FR 3078 (January 10, 2017).

⁽¹⁷⁾  We note that RPGs are a regulatory construct that the EPA developed to address the statutory mandate in CAA section 169B(e)(1), which required our regulations to include “criteria for measuring `reasonable progress' toward the national goal.” The RPGs are different than the statutory requirement under CAA section 169A(a)(4) to make reasonable progress towards the national visibility goal under CAA section 169A(a)(1). In the current regulatory construct, RPGs measure the progress that is projected to be achieved by the control measures a state has determined are necessary to make reasonable progress. 40 CFR 51.308(f)(3)(ii). However, consistent with both the 1999 RHR and 2017 RHR, the RPGs are unenforceable, though they create a benchmark that allows for analytical comparisons to the uniform rate of progress (URP) and mid-implementation-period course corrections if necessary. 82 FR 3078, 3091-3092 (January 10, 2017).

⁽¹⁸⁾ See https://www.federalregister.gov/d/2017-00268/p-94 .

⁽¹⁹⁾ See 82 FR 3078, January 10, 2017.

⁽²⁰⁾  CAA section 169A(g)(1) states “in determining reasonable progress there shall be taken into consideration 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.”

⁽²¹⁾ Id.

⁽²²⁾  Any reasonable progress metric that relies on natural conditions as an endpoint, and/or is adjusted for international anthropogenic and prescribed fire contributions should use revised estimates based on updated photochemical modeling or a combination of photochemical modeling and observational data.

⁽²³⁾  Note that the URP's 2064 end date does not represent the end date of the Regional Haze program. Rather, it purely serves as an end point for calculating a “glidepath” towards natural conditions over a 60-year time frame.

⁽²⁴⁾ See 90 FR 16478, April 18, 2025 and 90 FR 29737, July 7, 2025.

⁽²⁵⁾  A “deciview” is a unit of measurement for quantifying in a standard manner human perceptions of visibility. The deciview index is derived from calculated or measured light extinction, such that uniform increments of the index correspond to uniform incremental changes in perception across the entire range of conditions, from pristine to very obscured.

⁽²⁶⁾ See comments received in the EPA's 2024 non-regulatory docket (EPA-HQ-OAR-2023-0262).

⁽²⁷⁾  Id.

⁽²⁸⁾ See 64 FR 35725-35727, July 1, 1999.

⁽²⁹⁾ See 70 FR 39104, July 6, 2005.

⁽³⁰⁾ See CAA section 169A(f), which states: “. . . the meeting of the national goal specified in subsection (a)(1) of this section by any specific date or dates shall not be considered a `nondiscretionary duty' of the Administrator.”

⁽³¹⁾  “Congress hereby declares 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.”

⁽³²⁾  The 1977 House Conference Report states: “A major concern which prompted the House to adopt the visibility protection provision was the need to remedy existing pollution in Federal mandatory class I areas from existing sources. Issues with respect to visibility as an air quality value in application to new sources are to be resolved within the procedures for prevention of significant deterioration.” See Legislative History of the Clean Air Act Amendments of 1977 Public Law 95-95 91 Stat. 685 (1977).

⁽³³⁾  For an example of the methodologies behind reduced form tools, please see the Modeled Emission Rates Precursors (MERPs) Guidance: https://www.epa.gov/nsr/guidance-development-modeled-emission-rates-precursors-merps-tier-1-demonstration-tool-ozone .

⁽³⁴⁾ See comments in EPA's 2024 non-regulatory docket (EPA-HQ-OAR-2023-0262).

⁽³⁵⁾ See 82 FR 3078, January 10, 2017.

⁽³⁶⁾ See 40 CFR 51.308(i)(2).

⁽³⁷⁾ See 40 CFR 51.308(f)(2)(ii).