SUPPLEMENTARY INFORMATION:

Throughout this proposed rule, the use of “we,” “us,” or “our” is intended to refer to the EPA. We use multiple abbreviations and terms in this proposed rule. While this list may not be exhaustive, for ease of reading and for reference purposes, the EPA defines the following terms and acronyms here:

AQS Air Quality System

CAA Clean Air Act

CBI Confidential Business Information

CFR Code of Federal Regulations

CO Carbon Monoxide

DV Design Value

EPA Environmental Protection Agency

FR Federal Register

LMP Limited Maintenance Plan

MOVES EPA's MOtor Vehicle Emission Simulator

NAAQS National Ambient Air Quality Standard

NEI National Emissions Inventory

NO_X  Nitrogen Oxides

OKI Ohio-Kentucky-Indiana

PM₁₀  Particulate Matter with an aerodynamic diameter less than 10 microns

ppm Parts per million

RACT Reasonably Available Control Technology

SIP State Implementation Plan

tpsd tons per summer day

VOC Volatile Organic Compounds

Table of Contents

I. Executive Summary

II. Background

III. Kentucky's SIP Submittal

IV. The EPA's Evaluation of Kentucky's SIP Submittal

A. Attainment Emissions Inventory

B. Maintenance Demonstration

C. Monitoring Network and Verification of Continued Attainment

D. Contingency Plan

V. Transportation Conformity

VI. Proposed Action

VII. Statutory and Executive Order Reviews

I. Executive Summary

In accordance with the Clean Air Act (CAA or Act), the EPA is proposing to approve the Northern Kentucky Area's second 10-year LMP for the 1997 8-hour ozone NAAQS, adopted by the Cabinet on July 23, 2024, and submitted to the Agency as a revision to the Kentucky SIP on July 23, 2024.

On April 30, 2004, the tri-state Cincinnati-Hamilton Area, which is composed of Boone, Campbell, and Kenton Counties in Kentucky; Butler, Clermont, Clinton, Hamilton, and Warren Counties in Ohio; and a portion of Dearborn County in Indiana, was designated as nonattainment for the 1997 8-hour ozone NAAQS.^[1] Subsequently, on January 29, 2010, the Cabinet submitted a redesignation request and the first 10-year maintenance plan for the Northern Kentucky Area. In 2010, after the EPA's approval of the maintenance plan, the Northern Kentucky Area was redesignated to attainment for the 1997 8-hour ozone NAAQS.^[2]

The Northern Kentucky Area LMP for the 1997 8-hour ozone NAAQS, submitted by the Cabinet on July 23, 2024, is designed to maintain the 1997 8-hour ozone NAAQS within the Northern Kentucky Area through the end of the second 10-year portion of the maintenance period beyond redesignation (through August 5, 2030). The EPA is proposing to approve the plan because it meets all applicable requirements under CAA sections 110 and 175A.

II. Background

In 1979, under CAA section 109, the EPA established primary and secondary NAAQS for ozone at 0.12 parts per million (ppm), averaged over a 1-hour period.^[3] On July 18, 1997, the EPA revised the primary and secondary NAAQS for ozone to set the acceptable level of ozone in the ambient air at 0.08 ppm, averaged over an 8-hour period.^[4 5]

Following promulgation of a new or revised NAAQS, the EPA is required by the CAA to designate areas throughout the nation as attaining or not attaining the NAAQS. On April 30, 2004, the EPA designated the tri-state Cincinnati-Hamilton Area, which consists of the Kentucky counties of Boone, Campbell, and Kenton, the Ohio counties of Butler, Clermont, Clinton, Hamilton, and Warren, and a portion of Dearborn County of Indiana, as nonattainment for the 1997 8-hour ozone NAAQS. Those designations became effective on June 15, 2004.^[6]

The EPA has revised the ozone NAAQS twice since the 1997 standards were finalized. On July 20, 2012, the EPA designated areas as unclassifiable/attainment or nonattainment for the 2008 8-hour ozone NAAQS. At that time, the tri-state Cincinnati-Hamilton ( printed page 55301) Area was designated as nonattainment. The EPA redesignated the Kentucky portion of the tri-state Cincinnati-Hamilton Area to attainment for the 2008 8-hour ozone standard with an effective date of July 5, 2017.^[7] On November 16, 2017, the EPA designated areas for the 2015 8-hour ozone NAAQS. The tri-state Cincinnati-Hamilton Area was designated as nonattainment for the 2015 8-hour ozone NAAQS with an effective date of August 3, 2018.^[8] On June 9, 2022, the EPA designated the Ohio portion of the tri-state Area attainment for the 2015 8-hour ozone NAAQS with an effective date of June 9, 2022.^[9] On October 4, 2023, the EPA designated the Kentucky portion of the tri-state Cincinnati-Hamilton Area to attainment for the 2015 8-hour ozone standard with an effective date of November 3, 2023.^[10]

A State may submit a request that the EPA redesignate a nonattainment area that is attaining a NAAQS to attainment, and, if the area has met the criteria described in CAA section 107(d)(3)(E), the Agency may approve the redesignation request.^[11] One of the criteria for redesignation is for the area to have an approved maintenance plan under CAA section 175A. The maintenance plan must demonstrate that the area will continue to maintain the NAAQS for the period extending ten years after redesignation, and it must contain such additional measures as necessary to ensure maintenance and such contingency provisions as necessary to assure that violations of the NAAQS will be promptly corrected. Eight years after the effective date of redesignation, the State must also submit a second maintenance plan to ensure ongoing maintenance of the NAAQS for an additional ten years pursuant to CAA section 175A(b) ( i.e., ensuring maintenance for 20 years after redesignation).

The EPA has published long-standing guidance for states on developing maintenance plans. The Calcagni memo ^[12] provides that states may generally demonstrate maintenance by either performing air quality modeling to show that the future mix of sources and emission rates will not cause a violation of the NAAQS or by showing that projected future emissions of a pollutant and its precursors will not exceed the level of emissions during a year when the area was attaining the NAAQS ( i.e., attainment year inventory).^[13] The EPA clarified in three subsequent guidance memos that certain areas can meet the CAA section 175A requirement to provide for maintenance by showing that they are unlikely to violate the NAAQS in the future, using information such as the area design values ^[14] when the design values are well below the standard and have been historically stable.^[15] The EPA refers to a maintenance plan containing this streamlined demonstration as an LMP.

The EPA has interpreted CAA section 175A as permitting the LMP option because CAA section 175A does not define how areas may demonstrate maintenance, and in the Agency's experience implementing the various NAAQS, areas that qualify for an LMP and have approved LMPs have rarely, if ever, experienced subsequent violations of the NAAQS. As noted in the LMP guidance memoranda, states seeking an LMP must still submit the maintenance plan elements outlined in the Calcagni memo, including an attainment emissions inventory, provisions for the continued operation of the ambient air quality monitoring network, verification of continued attainment, and a contingency plan in the event of a future violation of the NAAQS. Moreover, a State seeking an LMP must still submit its section 175A maintenance plan as a revision to its SIP, with all attendant notice and comment procedures. While the LMP guidance memoranda were originally written with respect to certain NAAQS,^[16] the EPA has extended the LMP interpretation of section 175A to other NAAQS and pollutants not specifically covered by the previous guidance memos.^[17]

In this case, the EPA is proposing to approve the Northern Kentucky Area LMP because the Commonwealth has shown that ozone concentrations in the tri-state Cincinnati-Hamilton Area are well below the 1997 8-hour ozone NAAQS, have been historically stable, and that the Commonwealth has met the other maintenance plan requirements. The Cabinet submitted the LMP for the Northern Kentucky Area to fulfill the CAA's second maintenance plan requirement. The EPA's evaluation of the Northern Kentucky Area LMP is presented in section IV of this document, below.

On January 29, 2010, the Cabinet submitted a request to the EPA to redesignate the Northern Kentucky Area to attainment for the 1997 8-hour ozone NAAQS. This submittal included a plan, for inclusion in the Kentucky SIP, to provide for maintenance of the 1997 8-hour ozone NAAQS in the tri-state Cincinnati-Hamilton Area through 2030. The EPA approved the Northern Kentucky Maintenance Plan and the Commonwealth's requests to redesignate the Northern Kentucky Area to attainment for the 1997 8-hour ozone NAAQS, effective August 5, 2010.^[18] Kentucky's July 23, 2024, submittal contains the second 10-year maintenance plan for the 20-year maintenance period of the 1997 8-hour ozone NAAQS to ensure continued maintenance for the tri-state Cincinnati-Hamilton Area.

CAA section 175A(b) requires states to submit a revision to the first maintenance plan eight years after redesignation to provide for maintenance of the NAAQS for ten additional years following the end of the first 10-year period. However, the EPA's final implementation rule for the 2008 8-hour ozone NAAQS revoked the 1997 8-hour ozone NAAQS and stated that one consequence of revocation was that areas that had been redesignated to attainment ( i.e., maintenance areas) for the 1997 NAAQS no longer needed to submit a second 10-year maintenance plan under CAA section 175A(b).^[19]

In South Coast Air Quality Management District v. EPA, the United States Court of Appeals for the District ( printed page 55302) of Columbia Circuit vacated the EPA's interpretation that, because of the revocation of the 1997 8-hour ozone NAAQS, a second maintenance plan was not required for “orphan maintenance areas,” i.e., areas that had been redesignated to attainment for the 1997 8-hour ozone NAAQS maintenance areas and were designated attainment for the 2008 ozone NAAQS.^[20] Thus, states with these “orphan maintenance areas” under the 1997 8-hour ozone NAAQS must submit a maintenance plan for the second maintenance period. Accordingly, on July 23, 2024, Kentucky submitted a second maintenance plan for the Northern Kentucky Area which shows that the Area is expected to remain in attainment of the 1997 8-hour ozone NAAQS through 2030.

In recognition of the continuing record of relatively stable air quality monitoring data showing ambient 8-hour ozone concentrations in the tri-state Cincinnati-Hamilton Area well below the 1997 8-hour ozone NAAQS, the Cabinet chose the LMP option for the development of the second 1997 8-hour ozone NAAQS maintenance plan.

III. Kentucky's SIP Submittal

As mentioned previously, on July 23, 2024, the Cabinet submitted the Northern Kentucky Area LMP for the 1997 8-hour ozone NAAQS to the EPA as a revision to the Kentucky SIP. The submittal includes the LMP, air quality data, emissions inventory information, and appendices. Appendices to the plan include monitoring data along with documentation of public notice and statement of consideration.

IV. The EPA's Evaluation of Kentucky's SIP Submittal

The EPA has reviewed the Northern Kentucky Area LMP, which is designed to maintain the 1997 8-hour ozone NAAQS within the tri-state Cincinnati-Hamilton Area through the end of the 20-year period beyond redesignation, as required under CAA section 175A(b). The following is a summary of EPA's interpretation of the section 175A requirements ^[21] and the Agency's evaluation of how each requirement is met for the Northern Kentucky Area.

A. Attainment Emissions Inventory

The July 23, 2024, submittal includes an ozone attainment inventory that reflects typical summer day emissions in 2019. The submittal also includes a comparison of the 2019 attainment emissions inventory to the 2008 attainment inventory developed for the first 10-year maintenance plan.^[22] Comparing these inventories shows an overall decrease in emissions. Table 1 presents a summary of these inventories.

The Attainment Inventory portion of the July 23, 2024, submittal describes the methods, models, and assumptions used to develop the 2019 attainment inventory and indicates that this inventory was developed using the following combination of sources: (1) data from the 2017 National Emissions Inventory (NEI); (2) data required to be submitted by sources to the Division; ^[23] (3) mobile modeling conducted by the Ohio-Kentucky-Indiana (OKI) Regional Council of Governments; ^[24] and (4) EPA's 2016v2 modeling platform.^[25]

Based on our review of the methods, models, and assumptions used by Kentucky to develop the volatile organic compounds (VOC) and oxides of nitrogen (NO_X) estimates, the EPA proposes to find that the Northern Kentucky Area LMP includes a comprehensive, reasonably accurate inventory of actual ozone precursor emissions in attainment year 2019 and proposes to conclude that the plan's inventories are acceptable for the purposes of a subsequent maintenance plan under CAA section 175A(b).

B. Maintenance Demonstration

The maintenance demonstration requirement is satisfied in a LMP if the weight of evidence indicates that air quality in the area is well below the level of the NAAQS, that past air quality trends have been shown to be stable, and that the probability of the area ( printed page 55303) experiencing a violation over the second 10-year maintenance period is low.^[26] These criteria are evaluated below.

1. Evaluation of Ozone Concentrations

To attain the 1997 8-hour ozone NAAQS, the three-year average of the fourth-highest daily maximum 8-hour average ozone concentrations (design value or DV) at each monitor within an area must not exceed 0.08 ppm. Based on the rounding convention described in 40 CFR part 50, appendix I, the 1997 8-hour ozone NAAQS is attained if the design value is 0.084 ppm or below.

There are currently eleven ozone monitors in the tri-state Cincinnati-Hamilton Maintenance Area: one in Boone County, Kentucky; one in Campbell County, Kentucky; three in Butler County, Ohio; one in Clermont County, Ohio; one in Clinton County, Ohio; three in Hamilton County, Ohio; and one in Warren County, Ohio. Table 2 of this document presents the design values (in ppm) for each monitor in the tri-state Cincinnati-Hamilton Maintenance Area over the 2012-2025 period.

In the tri-state Cincinnati-Hamilton Area, all current design values are below 85 percent of the ozone NAAQS,^[27] with the exception of two monitors, one in Warren County, OH, (AQS ID: 39-165-0007) and one in Hamilton, OH (AQS ID: 39-061-0040). The Hamilton County monitor's 2025 design value is 0.074 ppm, which is 88 percent of the exceedance levels of the ozone NAAQS. All other monitors in the tri-state Cincinnati-Hamilton Area are below this 85 percent threshold. Based on the weight of evidence in support of Kentucky's submittal, the EPA is proposing to approve this LMP. No design values in the tri-state Cincinnati-Hamilton Area have exceeded the 1997 ozone NAAQS during the 2013-2025 time period depicted in Table 2. The design value data shown in Table 2 illustrates an overall downward trend in ozone levels at each monitor over this timeframe, and the 20-year maintenance period beyond redesignation ends in 2030, which leaves only a relatively short amount of remaining time based on the data in Table 2. Thus, the probability of the tri-state Cincinnati-Hamilton Area experiencing a violation during this second maintenance period is low. These aspects of the EPA's proposed approval are discussed in more detail below.

2. Stability of Ozone Levels

As discussed previously, the tri-state Cincinnati-Hamilton Area has maintained ozone concentrations below the 1997 8-hour ozone NAAQS over the past twelve 3-year design value periods. Additionally, the design value data shown in Table 2 illustrates an overall downward trend in ozone levels at each monitor over this timeframe. Over the past ten years, the maximum design value increase was 0.009 ppm, which occurred at the Boone, Kentucky ozone monitor (AQS ID: 21-015-0003) between the 2019-2021 DV and the 2023-2025 DV. After adding the maximum design value increase of 0.009 ppm to the maximum most recent design value (0.070 ppm), the resulting 0.079 ppm remains below the 0.084 ppm threshold to attain the 1997 8-hour ozone NAAQS. Additionally, the 20-year maintenance period beyond redesignation ends in 2030, which leaves a relatively short amount of remaining time during which significant increases in ozone levels are unlikely based on the data in Table 2. As a result, the EPA finds that there is a low probability of the tri-state Cincinnati-Hamilton Area violating the 1997 8-hour ozone NAAQS during this second maintenance period.

C. Monitoring Network and Verification of Continued Attainment

The EPA annually reviews the ozone monitoring network that the Cabinet operates and maintains in accordance with 40 CFR part 58. This network is described in the ambient air monitoring network plan that is developed by the Cabinet and submitted to the EPA annually, following a public notification and comment process. For further details on monitoring, the reader is referred to Kentucky's 2025 Ambient Air Monitoring Network Plan. The Cabinet submitted the 2025 Network Plan on July 1, 2025, and it received approval from the EPA on October 28, 2025.^[28]

As noted previously, the Cabinet's monitoring network in the Northern ( printed page 55304) Kentucky Area has been approved by the EPA in accordance with 40 CFR part 58, and the Cabinet has committed to continue to maintain a network in accordance with the EPA requirements. The EPA proposes to find that the Cabinet's monitoring network is adequate to verify continued attainment of the 1997 ozone NAAQS in the tri-state Cincinnati-Hamilton Area.

D. Contingency Plan

CAA section 175A(d) requires that a maintenance plan include contingency provisions. The purpose of such contingency provisions is to prevent future violations of the NAAQS or to promptly remedy any NAAQS violations that might occur during the maintenance period.

The Northern Kentucky Area LMP contingency plan includes tracking and triggering mechanisms to determine when control measures are needed, and a process for developing and adopting appropriate control measures. Specifically, any 8-hour ozone monitoring reading of 0.085 ppm or greater in the tri-state Cincinnati-Hamilton Area or periodic emissions inventory updates ^[29] that reveal excessive or unanticipated growth greater than 10 percent in either NO_X or VOC emissions over the attainment inventory for the tri-state Cincinnati-Hamilton Area will cause the Division to evaluate existing control measures to see if any further emission reduction measures should be implemented at that time. Implementation of necessary controls in response to an initial “indicator” will take place as expeditiously as possible, but no later than twelve months from the conclusion of the most recent ozone season (October 31) in which the annual fourth-highest monitored value of 0.085 ppm or greater occurred.

If a three-year average of annual fourth-highest monitored values of 0.085 ppm or greater ( i.e., a violation of the standard) occurs within the tri-state Cincinnati-Hamilton Area, an “action level” response will take effect. The Division, in conjunction with the metropolitan planning organization or regional council of governments, will determine additional control measures needed to ensure future attainment of the NAAQS for ozone. Appropriate contingency measures will be selected to be in place within twenty-four months of a monitored violation.

The contingency measures chosen will be selected based on their ability to bring the tri-state Cincinnati-Hamilton Area back into attainment. The LMP contains the following list of potential contingency measures:

• Implementation of a program to require additional emission reductions on stationary sources, including Reasonably Available Control Technology (RACT) for point sources of VOC and NO_X, and, specifically, the adoption of new and revised RACT rules based on Groups II, III, and IV Control Technique Guidelines;
• Implementation of a program to enhance inspection of stationary sources;
• Implementation of fuel programs, including incentives for alternative fuels;
• Restriction of certain roads or lanes to, or construction of such roads or lanes for use by, passenger buses or high-occupancy vehicles;
• Trip-reduction ordinances;
• Employer-based transportation management plans, including incentives;
• Programs for new construction and major reconstructions of paths or tracks for use by pedestrians or by non-motorized vehicles when economically feasible and in the public interest;
• Implementation of a modern vehicle inspection/maintenance program;
• Implementation of diesel retrofit programs, including incentives for performing retrofits for fleet vehicle operations;
• Additional engine idling reduction programs.

The Northern Kentucky LMP includes updates to the list of potential contingency measures in the first 10-year maintenance plan. New potential contingency measures include the implementation of a program to enhance inspection of stationary sources, a modern vehicle inspection/maintenance program, diesel retrofit programs with incentives for retrofitting fleet vehicles, and additional engine idling reduction programs. The second 10-year LMP no longer lists the programs from the first 10-year maintenance plan aimed at limiting or restricting vehicle use in downtown areas or other high-emission zones during peak periods as potential contingency measures.

The EPA proposes to find that the contingency provisions in Kentucky's second maintenance plan for Northern Kentucky for the 1997 8-hour Ozone NAAQS meet the requirements of the CAA section 175A(d).

V. Transportation Conformity

Transportation conformity is required by CAA section 176(c). Conformity to a SIP means that transportation activities will not produce new air quality violations, worsen existing violations or delay timely attainment of the NAAQS.^[30] The EPA's transportation conformity rule at 40 CFR part 93, subpart A, requires that transportation plans, programs, and projects conform to SIPs, and establishes the criteria and procedures for determining whether they conform.

Due to the decision in South Coast Air Quality Management v. EPA, the parts of the 1997 maintenance area that are also in maintenance for the 2008 and 2015 8-hour ozone NAAQS are not required to demonstrate conformity with the 1997 standard as they are currently demonstrating conformity with the more stringent 2008 and 2015 NAAQS with their respective approved motor vehicle emissions budgets.

The parts of the 1997 maintenance area that are not included in the maintenance area for the subsequent 2008 and 2015 NAAQS are considered a partial orphan area.^[31] Transportation conformity continues to apply for the 1997 NAAQS in the partial maintenance area. Within the partial orphan area, transportation conformity for transportation plans and transportation improvement programs for the 1997 ozone NAAQS can be demonstrated without a regional emissions analysis pursuant to 40 CFR 93.109(c) because the South Coast II court decision upheld EPA's revocation of the 1997 ozone NAAQS.

The Northern Kentucky 2008 8-hour ozone and 2015 8-hour ozone NAAQS Areas need to continue to meet all applicable requirements of the transportation conformity regulations, including the need for a regional emissions analysis and comparison of the results of the regional emissions analysis to the applicable budget for the respective 8-hour ozone NAAQS. A budget was approved for the 2008 8-hour ozone NAAQS,^[32] effective July 5, 2017.^[33] A budget was approved for the 2015 8-hour ozone NAAQS, effective November 3, 2023.^[34]

( printed page 55305)

VI. Proposed Action

Under CAA sections 110(k) and 175A and for the reasons set forth above, the EPA is proposing to approve the Northern Kentucky LMP for the 1997 8-hour ozone NAAQS, submitted by the Cabinet on July 23, 2024, as a revision to the Kentucky SIP. The EPA is proposing to approve the Northern Kentucky LMP because it includes an acceptable update of the various elements of the 1997 8-hour ozone NAAQS maintenance plan approved by the EPA for the first 10-year period (including emissions inventory, assurance of adequate monitoring and verification of continued attainment, and contingency provisions).

The EPA also finds that the Northern Kentucky Area qualifies for the LMP option and that, therefore, the Northern Kentucky Area LMP adequately demonstrates maintenance of the 1997 8-hour ozone NAAQS through documentation of monitoring data showing design values well below the NAAQS and continuation of existing control measures. The EPA believes that the Northern Kentucky Area LMP is sufficient to provide for maintenance of the 1997 8-hour ozone NAAQS in the tri-state Cincinnati-Hamilton Area over the remainder of the second 10-year maintenance period, through 2030, and thereby satisfies the requirements for such a plan under CAA section 175A(b).

VII. Statutory and Executive Order Reviews

Under the CAA, the Administrator is required to approve a SIP submission that complies with the provisions of the CAA and applicable Federal regulations.^[35] Thus, in reviewing SIP submissions, the EPA's role is to approve State choices, provided that they meet the criteria of the CAA. Accordingly, this proposed action merely proposes to approve State law as meeting Federal requirements and does not impose additional requirements beyond those imposed by State law. For that reason, this proposed rulemaking:

• Is not a significant regulatory action subject to review by the Office of Management and Budget underExecutive Order 12866 (58 FR 51735, October 4, 1993);
• Is not anExecutive Order 14192 (90 FR 9065, February 6, 2025) regulatory action because this action is not significant under Executive Order 12866;
• Does not impose an information collection burden under the provisions of the Paperwork Reduction Act (44 U.S.C. 3501 et seq.);
• Is certified as not having a significant economic impact on a substantial number of small entities under the Regulatory Flexibility Act (5 U.S.C. 601 et seq.);
• Does not contain any unfunded mandate or significantly or uniquely affect small governments, as described in the Unfunded Mandates Reform Act of 1995 (Pub. L. 104-4);
• Does not have federalism implications as specified inExecutive Order 13132 (64 FR 43255, August 10, 1999);
• Is not subject toExecutive Order 13045 (62 FR 19885, April 23, 1997) because it approves a State program;
• Is not a significant regulatory action subject toExecutive Order 13211 (66 FR 28355, May 22, 2001); and
• Is not subject to requirements of section 12(d) of the National Technology Transfer and Advancement Act of 1995 (15 U.S.C. 272 note) because application of those requirements would be inconsistent with the CAA.

In addition, the SIP is not approved to apply on any Indian reservation land or in any other area where the EPA or an Indian Tribe has demonstrated that a Tribe has jurisdiction. In those areas of Indian country, the rule does not have Tribal implications and will not impose substantial direct costs on Tribal governments or preempt Tribal law as specified by Executive Order 13175 (65 FR 67249, November 9, 2000).

Authority: 42 U.S.C. 7401 et seq.

91 FR 55299