On August 27, 2026, the U.S. Environmental Protection Agency (U.S. EPA) proposed the approval of Kentucky’s second 10-year Limited Maintenance Plan (LMP) for the 1997 8-hour ozone National Ambient Air Quality Standards (NAAQS). The proposal applies to Boone, Campbell, and Kenton counties, which make up the Kentucky portion of the Tri-State Cincinnati-Hamilton Maintenance area. Kentucky submitted the LMP to U.S. EPA as a revision to the Kentucky State Implementation Plan (SIP) on July 23, 2024. Comments on the proposed rule are due by September 17, 2026, under Docket ID No. EPA-R04-OAR-2024-0440-0008.
The SIP revision is specific to Northern Kentucky, although U.S. EPA’s evaluation considers air quality throughout the broader tri-state Cincinnati-Hamilton area. In addition to Boone, Campbell, and Kenton counties in Kentucky, the maintenance area includes Butler, Clermont, Clinton, Hamilton, and Warren counties in Ohio and a portion of Dearborn County in Indiana.
If finalized, U.S. EPA’s action would make the LMP’s maintenance-related commitments federally enforceable as part of the Kentucky SIP. Kentucky’s federally approved SIP is identified in 40 CFR 52 Subpart S, which already includes the first Northern Kentucky 8-hour ozone maintenance plan approved by U.S. EPA in 2010.
Why is Another Plan Needed for the 1997 Standard?
U.S. EPA established the 1997 ozone NAAQS at 0.08 parts per million (ppm) average over an 8-hour period. The Cincinnati-Hamilton area was designated nonattainment for that standard in 2004. Kentucky subsequently submitted a redesignation request and maintenance plan for Boone, Campbell, and Kenton counties, and U.S. EPA redesignated the Northern Kentucky area to attainment effective August 5, 2010.
Clean Air Act (CAA) Section 175A requires an initial maintenance plan covering the first 10 years after redesignation and a second plan providing for maintenance for an additional 10 years. Although U.S. EPA revoked the 1997 ozone standard when implementing the 2008 ozone NAAQS and initially concluded that certain maintenance areas would no longer need second maintenance plans, the Washington, D.C. Circuit Court of Appeals vacated that interpretation in South Coast Air Quality Management District v. EPA. As a result, states with these “orphan maintenance areas” must still submit second maintenance plans for the 1997 standard. Kentucky’s LMP is intended to demonstrate continued maintenance through August 5, 2030.
Why U.S. EPA Believes the Area Qualifies for the LMP Option
An LMP provides a streamlined way to demonstrate maintenance when the available evidence shows that monitored pollutant concentrations are well below the applicable NAAQS, historical air quality levels have been stable, and the likelihood of a future violation is low. An LMP must still include the principal maintenance-plan elements, including:
- An attainment-year emissions inventory;
- Continued operation of the ambient monitoring network;
- Procedures for verifying continued attainment; and
- Contingency provisions addressing future violations that may occur.
U.S. EPA’s proposed approval of the LMP indicates it agrees with Kentucky’s demonstration that ozone concentrations in the Cincinnati-Hamilton area have remained sufficiently below the 1997 standard, have been historically stable, and are unlikely to violate the standard during the remainder of the maintenance period.
Emissions Have Decreased Since the First Maintenance Plan
Kentucky’s LMP includes a 2019 attainment inventory representing typical summer-day emissions of nitrogen oxides (NOX) and volatile organic compounds (VOC), the two principal ozone precursors. The plan compares that inventory with the 2008 attainment inventory used for the first maintenance plan.
The data provided by U.S. EPA shows that total NOX emissions in Boone, Campbell, and Kenton counties decreased from 79.19 tons per summer day in 2008 to 25.44 tons per summer day in 2019 and that total VOC emissions decreased from 43.56 to 23.56 tons per summer day over the same period.
The 2019 inventory was developed using information from the 2017 National Emissions Inventory, data submitted by regulated sources to the Kentucky Division for Air Quality (KYDAQ), mobile-source modeling prepared by the Ohio-Kentucky-Indiana Regional Council of Governments, and U.S. EPA’s 2016v2 modeling platform. Based on its review, U.S. EPA is proposing to find that the inventory is comprehensive and reasonably accurate for purposes of the second maintenance plan.
Monitoring Data Continue to Show Attainment
For the 1997 ozone NAAQS, a monitor’s design value is the three-year average of its annual fourth-highest daily maximum 8-hour ozone concentration. Although the standard is expressed as 0.08 ppm, U.S. EPA’s applicable rounding convention means that a design value of 0.084 ppm or lower meets the standard.
The Tri-State Cincinnati-Hamilton maintenance area currently has 11 ozone monitors. The most recent design values for the 2023-2025 period range from 0.067 ppm to 0.074 ppm. U.S. EPA reports that no design value exceeded the 1997 ozone NAAQS during this period and that the monitoring data show an overall downward trend.
U.S. EPA also notes that the area has maintained concentrations below the standard over the past 12 three-year design-value periods. Based on those trends and the relatively limited time remaining before the maintenance periods in 2030, U.S. EPA is proposing to find that the probability of a violation during the second maintenance period is low.
Kentucky has committed to continue operating its ozone monitoring network in accordance with U.S. EPA requirements. U.S. EPA annually reviews that network, and U.S. EPA is proposing to find that it is adequate to verify continued attainment of the 1997 ozone NAAQS.
The Plan Includes Two Levels of Contingency Response
The LMP contains both monitoring and emissions-based mechanisms for determining when additional action may be needed.
An initial indicator would be triggered by either:
- An 8-hour ozone monitoring reading of 0.085 ppm or greater anywhere in the tri-state area; or
- An emissions inventory update showing unanticipated growth of more than 10 percent in either NOX or VOC emissions compared with the attainment inventory.
An indicator would require Kentucky to evaluate existing control measures and determine whether further reductions should be implemented. For an ozone-based indicator, necessary controls are to be implemented as expeditiously as possible and no later than 12 months after the conclusion of the relevant ozone season.
A three-year average of annual fourth-highest monitored values at or above 0.085 ppm would constitute a violation and trigger the plan’s action-level response. Kentucky would then work with the applicable metropolitan planning organization or regional council of governments to identify additional controls, with appropriate contingency measures to be in place within 24 months of the monitored violation.
Potential measures identified in the LMP include additional stationary-source emissions reductions, new or revised reasonably available control technology requirements, enhanced stationary-source inspections, alternative-fuel incentives, transportation-management measures, modern vehicle inspection and maintenance programs, diesel retrofit incentives, and additional engine-idling reduction programs. U.S. EPA is proposing to find that these provisions satisfy the contingency-plan requirements of CAA Section 175A(d).
Transportation Conformity Will Continue to Vary Across the Area
The proposed rule also addresses transportation conformity requirements associated with the 1997 ozone standard. Portions of the 1997 maintenance area that are also covered by the maintenance plans for the more stringent 2008 and 2015 ozone standards are not required to separately demonstrate conformity with the 1997 standard. Those areas must continue to conduct the applicable regional emissions analyses using the approved motor-vehicle emissions budgets for the 2008 and 2015 standards.
Transportation conformity for the 1997 standard continues to apply in the ”partial orphan area” — the southern portions of Boone, Campbell, and Kenton counties that were not included in the later 2008 and 2015 ozone areas. However, transportation plans and transportation improvement programs in that partial orphan area may demonstrate conformity with the 1997 standard without conducting a regional emissions analysis.
Importantly, this proposed rule applies only to Kentucky’s second 10-year LMP for the 1997 8-hour ozone standard. The references to the 2008 and 2015 ozone standards are included only to explain how transportation conformity requirements apply where the maintenance areas for the different ozone layers overlap; U.S. EPA is not proposing to approve or revise the maintenance plans for those standards as part of this action. In fact, U.S. EPA notes that Kentucky submitted a second 10-year maintenance plan for the 2008 8-hour ozone maintenance area on July 3, 2025, and that the plan “will be addressed in a separate future action.”
What Happens Next?
U.S. EPA is proposing to approve the Northern Kentucky LMP because the Agency believes it adequately updates the emissions inventory, monitoring, verification, and contingency elements of the first maintenance plan. U.S. EPA also proposes to find that the monitoring record and continued implementation of existing controls provide an adequate demonstration that the 1997 8-hour ozone NAAQS will be maintained through 2030.
U.S. EPA characterizes the action as approval of Kentucky’s existing state choices rather than the imposition of additional requirements beyond those established under state law. Nevertheless, final approval would place the LMP’s commitments within the federally enforceable SIP. Interested parties may submit comments under Docket ID No. EPA-R04-OAR-2024-0440-0008 through September 17, 2026.
ALL4 will continue to track policy updates, procedural regulatory actions, and rulemaking in the Commonwealth of Kentucky for all matters regulated under the CAA. If you have any questions on how this proposed rule could impact your organization, please contact Dylan Ernst at dernst@all4inc.com or Scott Kirkpatrick at skirkpatrick@all4inc.com.
