# Air Plan Approval; South Carolina; Charlotte-Gastonia-Rock Hill Area Maintenance Plan for the 2008 8-Hour Ozone NAAQS
**AGENCY:**
Environmental Protection Agency (EPA).
**ACTION:**
Proposed rule.
**SUMMARY:**
On September 26, 2023, the State of South Carolina, through the South Carolina Department of Environmental Services (SCDES, formerly the “South Carolina Department of Health and Environmental Control”), submitted a request for the Environmental Protection Agency (EPA) to approve a State Implementation Plan (SIP) revision containing the State's plan for maintaining the 2008 ozone National Ambient Air Quality Standard (NAAQS or standard) through 2036 for the South Carolina portion of the bi-state Charlotte-Rock Hill, North Carolina-South Carolina 2008 8-hour ozone nonattainment area (the entire area is hereinafter referred to as the “bi-State Charlotte Area” and the South Carolina portion is hereinafter referred to as the “York County Area”). EPA is proposing to approve and incorporate this maintenance plan, including the 2018 and 2036 motor vehicle emission budgets (budgets) for nitrogen oxides (NOx) and volatile organic compounds (VOC) for the York County Area, into the SIP. EPA is also notifying the public of the status of EPA's adequacy determination for the budgets for the York County Area.
**DATES:**
Comments must be received on or before October 6, 2025.
**ADDRESSES:**
Submit your comments, identified by Docket ID No. EPA-R04-OAR-2024-0558 at *regulations.gov* . Follow the online instructions for submitting comments. Once submitted, comments cannot be edited or removed from *Regulations.gov* . EPA may publish any comment received to its public docket. Do not submit electronically any information you consider to be Confidential Business Information (CBI) 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. EPA will generally not consider comments or comment contents located outside of the primary submission ( *i.e.,* on the web, cloud, or other file sharing system). For additional submission methods, the full EPA public comment policy, information about CBI or multimedia submissions, and general guidance on making effective comments, please visit *https://www.epa.gov/dockets/commenting-epa-dockets.*
**FOR FURTHER INFORMATION CONTACT:**
Nelsha Athauda, Multi Air Pollutant Coordination Section, Air Planning and Implementation Branch, Air and Radiation Division, U.S. Environmental Protection Agency, Region 4, 61 Forsyth Street SW, Atlanta, Georgia 30303-8960. The telephone number is (404)-562-9360. Ms. Athauda can also be reached via electronic mail at *[email protected].*
**SUPPLEMENTARY INFORMATION:**
**Table of Contents**
I. Summary of EPA's Proposed Action
II. Background
III. EPA's Evaluation of South Carolina's SIP Submittal
A. Attainment Emissions Inventory
B. Maintenance Demonstration
C. Monitoring Network
D. Verification of Continued Attainment
E. Contingency Plan
IV. EPA's Analysis of South Carolina's Proposed NOx and VOC Budgets
V. EPA's Adequacy Determination for the Proposed NOx and VOC Budgets
VI. Effect of EPA's Proposed Action
VII. Proposed Action
VIII. Statutory and Executive Order Reviews
**I. Summary of EPA's Proposed Action**
In accordance with the Clean Air Act, 42 U.S.C. 7401, *et seq.* (CAA or Act), EPA is proposing to approve the York County Area's maintenance plan for the 2008 8-hour ozone NAAQS, adopted by SCDES [^1] on September 26, 2023, and submitted by SCDES as a revision to the South Carolina SIP on September 26, 2023. [^2]
[^1] On July 1, 2024, SCDES was restructured into a health agency, the Department of Public Health, and an environmental agency, the Department of Environmental Services (DES). In a letter dated June 20, 2024, South Carolina represented to EPA that all the functions, powers, and duties of the environmental divisions, offices, and programs of the South Carolina Department of Health and Environmental Control (SCDHEC), including the authority to administer and enforce state implementation plans, are retained and continued in full force and effect under DES. This letter is in the docket for this proposed rulemaking. Throughout this proposal, the terms, “Department”, “South Carolina Department of Health and Environmental Services”, “SCDHEC”, “South Carolina Department of Environmental Services”, and “SCDES” are interchangeable.
[^2] The September 26, 2023, SIP submission, with exception of the supporting modeling files, is included in the docket for this action. Due to size and compatibility limitations of the Federal Docket Management System, the supporting modeling files are instead available at the EPA Region 4 office. To request these files, please contact the person listed in this Notice of Proposed Rulemaking (NPRM) under the section titled *FOR FURTHER INFORMATION CONTACT* .
The York County Area's maintenance plan for the 2008 8-hour ozone NAAQS, submitted by SCDES on September 26, 2023, is designed to maintain the 2008 8-hour ozone NAAQS within the York County Area through the end of the second 10-year portion of the maintenance period beyond redesignation (through 2036). EPA is proposing to approve the plan because it meets all applicable requirements under CAA sections 110 and 175A. EPA is also proposing to approve the 2018 and 2036 NOx and VOC budgets in the York County Area second maintenance plan because they meet the applicable transportation conformity requirements under 40 CFR 93.118(e).
**II. Background**
On March 12, 2008, EPA promulgated a revised 8-hour ozone NAAQS of 0.075 parts per million (ppm). *See* 73 FR 16436 (March 27, 2008). Under EPA's regulations at 40 CFR part 50, the 2008 8-hour ozone NAAQS is attained when the 3-year average of the annual fourth highest daily maximum 8-hour average ambient air quality ozone concentrations is less than or equal to 0.075 ppm. *See* 40 CFR 50.15. Ambient air quality monitoring data for the 3-year period must meet a data completeness requirement. The ambient air quality monitoring data completeness requirement is met when the average percent of days with valid ambient monitoring data is greater than 90 percent, and no single year has less than 75 percent data completeness as determined in Appendix I of part 50.
Upon promulgation of a new or revised NAAQS, the CAA requires EPA to designate as nonattainment any area that is violating the NAAQS, based on the three most recent years of complete, quality assured, and certified ambient air quality data at the conclusion of the designation process. The bi-state Charlotte Area was designated marginal nonattainment for the 2008 8-hour ozone NAAQS on May 21, 2012, (effective July 20, 2012) using 2009-2011 ambient air quality data. *See* 77 FR 30088 (May 21, 2012). The Area attained the standard, and on April 17, 2015, SCDES submitted a redesignation request and the first 10-year maintenance plan for the York County Area. In the final implementation rule for the 2008 8-hour ozone NAAQS (SIP Implementation Rule), EPA established ozone nonattainment area attainment dates based on Table 1 of section 181(a) of the CAA. *See* 80 FR 12264 (March 6, 2015). This rule established an attainment date three years after the July 20, 2012, effective date for areas classified as marginal areas for the 2008 8-hour ozone nonattainment designations. Therefore, the bi-state Charlotte Area's attainment date was July 20, 2015. In 2015, the York County Area was redesignated to attainment for the 2008 8-hour ozone NAAQS, the 10-year maintenance plan was approved, and the public was notified that EPA found the NOx and VOC budgets adequate. *See* 80 FR 76865 (December 11, 2015).
The primary guidance on maintenance plans and redesignation requests is the September 4, 1992, memorandum from John Calcagni, titled “Procedures for Processing Requests to Redesignate Areas to Attainment” (“Calcagni Memorandum”). [^3] The Calcagni Memorandum outlines the key elements of a maintenance plan, which include the following: attainment emissions inventory, maintenance demonstration, monitoring network requirements, verification of continued attainment, and contingency plan elements.
[^3] “Procedures for Processing Requests to Redesignate Areas to Attainment,” Memorandum from John Calcagni, Director, Air Quality Management Division, September 4, 1992 (hereinafter referred to as the “Calcagni Memorandum”), available at *https://www.epa.gov/ground-level-ozone-pollution/procedures-processing-requests-redesignate-areas-attainment.*
On April 17, 2015, SCDES requested that EPA redesignate the South Carolina portion of the bi-state Charlotte Area to attainment for the 2008 8-hour ozone NAAQS and submitted a SIP revision containing the State's plan for maintaining attainment of the 2008 8-hour ozone standard in the Area, including the 2014 and 2026 budgets for NOx and VOC for the York County Area. In a NPRM published on October 14, 2015, EPA proposed to determine that the bi-state Charlotte Area is continuing to attain the 2008 8-hour ozone NAAQS; to approve and incorporate into the South Carolina SIP the State's plan for maintaining attainment of the 2008 8-hour ozone standard in the Area, including the 2014 and 2026 budgets for NOx and VOC for the South Carolina portion of the bi-state Charlotte Area; and to redesignate the South Carolina portion of the bi-state Charlotte Area to attainment for the 2008 8-hour ozone NAAQS. *See* 80 FR 61775 (October 14, 2015). EPA approved the York County maintenance plan and the State's requests to redesignate the York County Area to attainment for the 2008 8-hour ozone NAAQS, effective January 11, 2016. *See* 80 FR 76865 (December 11, 2015).
Section 175A(b) of the CAA requires states to submit a revision to the SIP eight years after redesignation to provide for maintenance of the NAAQS for ten additional years following the end of the first 10-year period. Accordingly, on September 26, 2023, South Carolina submitted the second maintenance plan for the York County Area showing that the Area is expected to remain in attainment of the 2008 8-hour ozone NAAQS through 2036.
EPA has revised the ozone NAAQS once since the 2008 standards were finalized. On October 1, 2015, the Agency revised both the primary and secondary NAAQS for ozone to a level of 0.070 ppm (annual fourth-highest daily maximum 8-hour average concentration, averaged over 3 years). *See* 80 FR 65296 (October 26, 2015). On November 16, 2017, EPA designated areas for the 2015 8-hour ozone NAAQS. The bi-state Charlotte Area was designated attainment for that standard with an effective date of August 3, 2018. *See* 83 FR 25776 (June 4, 2018).
**III. EPA's Evaluation of South Carolina's SIP Submittal**
As mentioned above, on September 26, 2023, SCDES submitted the York County Area's maintenance plan to EPA as a revision to the South Carolina SIP. The submittal includes the maintenance plan, air quality data, emissions inventory information, motor vehicle emissions budgets, and appendices.
EPA has reviewed the York County Area's maintenance plan, which is designed to maintain the 2008 8-hour ozone NAAQS within the bi-state Charlotte 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 [^4] and EPA's evaluation of how each requirement is met.
[^4]*See* Calcagni Memorandum.
**A. Attainment Emissions Inventory**
For maintenance plans, a state should develop a comprehensive, accurate inventory of actual emissions for an attainment year to identify the level of emissions which is sufficient to maintain the NAAQS. A state should develop this inventory consistent with EPA's most recent guidance on emissions inventory development. For ozone, the inventory should be based on typical summer day emissions of VOC and NOx, as these pollutants are precursors to ozone formation.
As discussed above, EPA determined that the bi-state Charlotte Area had attained the 2008 8-hour ozone NAAQS at the time that it redesignated the North Carolina portion of the Area to attainment. *See* 80 FR 44873. The bi-state Charlotte Area continues to attain the 2008 8-hour ozone NAAQS. South Carolina selected 2018 as the base year ( *i.e.,* attainment emissions inventory year) for developing a comprehensive emissions inventory for NOx and VOC, for which projected emissions could be developed for 2026 and 2036. The attainment inventory identifies a level of emissions in the Area that is sufficient to attain the 2008 8-hour ozone NAAQS. South Carolina began development of the attainment inventory by first generating a baseline emissions inventory for the State's portion of the bi-state Charlotte Area. South Carolina estimated projected summer day emission inventories using projected rates of growth in population, traffic, economic activity, and other parameters. In addition to comparing the final year of the plan (2036) to the base year (2018), South Carolina compared an interim year (2026) to the baseline to demonstrate that the years in between are also expected to show continued maintenance of the 2008 8-hour ozone standard.
The emissions inventory is composed of four major types of sources: point, nonpoint, onroad mobile, and nonroad mobile. South Carolina also included event sources ( *i.e.,* wildfires and prescribed fires) in the inventory. The complete descriptions of how the inventories were developed are discussed in Appendices A, B, C, D, and E of the September 26, 2023, submittal, which can be found in the docket for this proposed action.
The point source emissions were tabulated from data collected by direct on-site measurements of emissions or mass balance calculations utilizing approved emission factors. There are usually several emission sources for each facility. Emissions data is collected for each point source at a facility and the data is entered into an in-house database system. For the projected year's inventory, point sources are adjusted by growth factors. Growth rates for the industrial point sources were calculated via the EPA 2016v2 modeling platform data.
For nonpoint sources, emissions were estimated by multiplying an emission factor by some known indicator of collective activity such as production, number of employees, or population. The emission factors used were obtained from the Emission Inventory Improvement Program (EIIP) Tech Reports; [^5] the Procedures document [^6] or EPA's Compilation of Air Pollutant Emission Factors, Fifth Edition (AP-42); [^7] and the Nonpoint Method Advisory Committee (NOMAD) collaboration. [^8] These types of emissions were estimated on the county level. Various sources of data, such as population growth, energy consumption by sector, and county business patterns from the Census, were used to determine the growth projections.
[^5]*See* Air Emissions Inventory Improvement Program (EIIP) Technical Report Series, available at: *https://www.epa.gov/air-emissions-inventories/air-emissions-inventory-improvement-program-eiip.*
[^6]*See* Procedures for the Development of Emissions Factors from Stationary Sources, available at: *https://www.epa.gov/air-emissions-factors-and-quantification/procedures-development-emissions-factors-stationary.*
[^7]*See* AP-42, Compilation of Air Pollutant Emission Factors, 5th edition, available at: *https://www.epa.gov/air-emissions-factors-and-quantification/ap-42-compilation-air-emissions-factors-stationary-sources.*
[^8] EPA default data for nonpoint sources was developed by EPA with the help of the Nonpoint Method Advisory (NOMAD) committee. NOMAD is a group of inventory developers from a variety of State and local agencies that collaborate on the development of methodologies to aid EPA in the development of default data for the National Emissions Inventory (NEI).
For onroad mobile sources, South Carolina used the EPA mobile model MOVES3.1 [^9] to generate emissions. On January 7, 2021, EPA announced the availability of MOVES3 for official purposes outside of California. MOVES3 was the latest state of-the art upgrade to EPA's modeling tools for estimating emissions from cars, trucks, buses, and motorcycles based on the latest data and regulations and was available for use in SIPs and transportation conformity analyses outside of California. The notice of availability started a two-year grace period that ended on January 7, 2023, after which MOVES3 was required to be used in new regional-emissions and hot-spot analyses for transportation conformity determinations outside of California.
[^9]*See* 86 FR 1106.
EPA announced availability of a new versions of the MOVES model (MOVES4 and MOVES5) on September 12, 2023 [^10] and December 11, 2024, [^11] respectively. The CAA does not require states that have already submitted SIP revisions or will submit SIP revisions shortly after the release of a new model to revise these SIP revisions simply because a new motor vehicle emissions model is now available. Because South Carolina submitted the SIP on September 26, 2023, it used MOVES3.1 to estimate exhaust and evaporative emissions as well as brake and tire wear emissions from all types of on-road vehicles. The estimation of emissions involves multiplying an activity level by an emission factor and is done within the model. The activity level used by MOVES3.1 is vehicle miles traveled (VMT). For the future years' inventories, the MOVES3.1 mobile model takes into consideration expected federal tailpipe standards, fleet turnover, and new fuels.
[^10]*See* 88 FR 62567.
[^11]*See* 89 FR 99862.
EPA's MOVES3.1 mobile model was also used to calculate emissions for all nonroad sources except for railroad locomotive line haul emissions which are not included in the nonroad portion of the MOVES3.1 model and were calculated differently.
Events sources emissions estimates were calculated by EPA using the SMARTFIRE2 (SF2/B2) system, along with state provided inputs, including a list of all York County wildland fires (WLFs) and prescribed fires in 2017. [^12]
[^12]*See* Section 7 of the 2017 National Emissions Inventory (NEI) Technical Support Document for more details. Available at *https://www.epa.gov/air-emissions-inventories/2017-national-emissions-inventory-nei-technical-support-document-tsd.*
The 2018 NOx and VOC emissions for the South Carolina portion of the Area, as well as the emissions for other years, were developed consistent with EPA guidance and are summarized in Tables 1 through 3 of the following subsection discussing the maintenance demonstration. *See* Appendices A-E of the September 26, 2023, submission for more detailed information on the emissions inventory.
**B. Maintenance Demonstration**
The maintenance plan includes a maintenance demonstration that:
(i) Shows compliance with and maintenance of the 2008 8-hour ozone NAAQS by providing information to support the demonstration that current and future emissions of NOx and VOC remain at or below 2018 emissions levels.
(ii) Uses 2018 as the attainment year and includes future emissions inventory projections for 2018, 2026, and 2036.
(iii) Per 40 CFR part 93, NO <sub>X</sub> and VOC Budgets were established for the last year (2036) of the maintenance plan (see section VII below). Additionally, SCDES opted to establish a budget for an interim year (2026).
(iv) Provides actual (2018) and projected emissions inventories, in tons per ozone season day (tons/OSD), for the South Carolina portion of the bi-state Charlotte Area, as shown in Tables 1 through 3, below.
| Source category | 2018 | 2026 | 2036 |
| --- | --- | --- | --- |
| Nonpoint | 9.54 | 10.15 | 10.76 |
| Nonroad | 1.35 | 1.21 | 1.26 |
| Onroad | 2.82 | 1.72 | 1.38 |
| Point | 3.38 | 3.38 | 3.39 |
| Event | 0.18 | 0.18 | 0.18 |
| Total | 17.27 | 16.64 | 16.97 |
| Source category | 2018 | 2026 | 2036 |
| --- | --- | --- | --- |
| Nonpoint | 1.03 | 1.05 | 1.06 |
| Nonroad | 1.49 | 0.94 | 0.85 |
| Onroad | 6.86 | 3.47 | 2.51 |
| Point | 4.13 | 4.22 | 4.37 |
| Event | 0.02 | 0.02 | 0.02 |
| Total | 13.53 | 9.7 | 8.81 |
| | VOC | NO |
| --- | --- | --- |
| 2018 | 17.27 | 13.53 |
| 2036 | 16.97 | 8.81 |
| Difference from 2018 to 2036 | −0.30 | −4.72 |
Tables 1 through 3 summarize the 2018 and future projected emissions of NO <sub>X</sub> and VOC from the York County Area. In situations where local emissions are the primary contributor to nonattainment, the NAAQS should not be violated in the future if emissions from within the area remain at or below the baseline with which attainment was achieved. South Carolina has projected emissions as described previously and determined that emissions in the South Carolina portion of the bi-state Charlotte Area will remain below those in the attainment year inventory for the duration of the maintenance plan.
A “safety margin” is the amount by which the total projected emissions from all sources of a given pollutant are less than the total emissions that would satisfy the applicable requirement for reasonable further progress (RFP), attainment, or maintenance. *See* 40 CFR 93.101. The safety margin is calculated as the difference between emissions in an attainment year and projected emissions in the maintenance year. South Carolina selected 2018 as the attainment emissions inventory year for the York County Area. South Carolina calculated safety margins in its submittal for year 2018, 2026, and 2036. Because the initial budget year of 2018 is also the base year for the maintenance plan inventory, there is no safety margin, therefore, no adjustments were made to the budget for 2018. The State has allocated 100% of the 2036 safety margin to the 2036 budgets for the York County Area. Table 4 displays the established safety margins for the York County Area in tons per ozone season day (OSD).
| | VOC | NO |
| --- | --- | --- |
| 2018 | N/A | N/A |
| 2026 | 0.63 | 3.83 |
| 2036 | 0.30 | 4.72 |
The State decided to allocate one hundred percent of the 2036 safety margins to 2036 budgets to allow for unanticipated growth in VMT, changes and uncertainty in vehicle mix assumptions, etc., that will influence the emission estimations. After allocation of one hundred percent of the available 2036 safety margin, there is no remaining 2036 safety margin for NO <sub>X</sub> and VOC emissions. This allocation is discussed further in section IV of this proposed rulemaking along with the budgets to be used for transportation conformity proposes.
**C. Monitoring Network**
There are currently five Air Quality System (AQS) ozone monitors in the bi-State Charlotte Area: one in Lincoln County, North Carolina; two in Mecklenburg County, North Carolina; one in Rowan County, North Carolina; and one in Union County, North Carolina. No monitors are located within the South Carolina portion of the bi-State Charlotte Area. The plan presents the design values (DV) [^13] (in ppm) for the currently active monitors in the bi-state Charlotte Area from 2012 to 2024.
[^13] Design values are calculated as the 3-year average of the annual fourth-highest daily maximum 8-hour average ozone concentration.
| County | AQS Site ID | 2010- | 2011- | 2012- | 2013- | 2014- | 2015- | 2016- | 2017- | 2018- | 2019- | 2020- | 2021- | 2022- |
| --- | --- | --- | --- | --- | --- | --- | --- | --- | --- | --- | --- | --- | --- | --- |
| Crouse: Lincoln, NC | 37-109-0004 | 0.075 | 0.072 | 0.068 | 0.065 | 0.067 | 0.067 | 0.065 | 0.064 | 0.060 | 0.061 | 0.061 | 0.065 | 0.064 |
| Garinger: Mecklenburg, NC | 37-119-0041 | 0.083 | 0.078 | 0.070 | 0.068 | 0.069 | 0.069 | 0.068 | 0.070 | 0.067 | 0.066 | 0.064 | 0.069 | 0.069 |
| University Meadows: Mecklenburg, NC | 37-119-0046 | 0.083 | 0.078 | 0.073 | 0.067 | 0.070 | 0.070 | 0.070 | 0.069 | 0.067 | 0.066 | 0.064 | 0.068 | 0.069 |
| Rockwell: Rowan, NC | 37-159-0021 | 0.078 | 0.073 | 0.068 | 0.064 | 0.065 | 0.064 | 0.062 | 0.062 | 0.061 | 0.062 | 0.061 | 0.065 | 0.065 |
| Monroe: Union, NC | 37-179-0003 | 0.073 | 0.070 | 0.068 | 0.065 | 0.068 | 0.067 | 0.068 | 0.068 | 0.063 | 0.062 | 0.061 | 0.067 | 0.066 |
As shown in Table 5, the design values derived from the monitors in the bi-State Charlotte Area have been below the level of the 2008 8-hour ozone NAAQS since redesignation. Furthermore, the overall ozone concentrations for the Area decreased by 14 ppb between the 2010-2012 and 2022-2024 design values at one of the Mecklenburg, NC, monitors (AQS ID 37-119-0046). As the ozone levels have dropped and remain relatively stable, it is reasonable to conclude that the bi-State Charlotte Area will not exceed the 2008 8-hour ozone NAAQS during the second 10-year maintenance period.
As noted above, all the monitors in the bi-state Charlotte Area are in the State of North Carolina. North Carolina, through the North Carolina Department of Air Quality (NCDAQ), has committed to continue operation of all those monitors in compliance with 40 CFR part 58, [^14] addressing the requirement for monitoring. For further details on monitoring, see the 2024-2025 North Carolina Annual Monitoring Network Plan [^15] as well as EPA's approval letter for the 2024-2025 Annual Network Plan, which can be found in the docket for this proposed action.
[^14]*See* 80 FR 29250 (May 21, 2015); 80 FR 44873 (July 28, 2015).
[^15] See docket for Volume 1 (Network Descriptions (All Regions)) of the 2024-2025 Annual Network Plan and EPA's approval letter for the 2024-2025 Annual Network Plan. Volume 2 (Site Descriptions by Region) of the 2024-2025 Annual Network Plan is available online at: *https://www.deq.nc.gov/about/divisions/air-quality/air-quality-monitoring/annual-network-plan/2024-2025-annual-monitoring-network-plan-north-carolina-air-quality.*
**D. Verification of Continued Attainment**
The State of South Carolina, through SCDES, has the legal authority to enforce and implement the requirements of the South Carolina portion of the Area 2008 8-hour ozone maintenance plan. This includes the authority to adopt, implement, and enforce any subsequent emissions control contingency measures determined to be necessary to correct future ozone attainment problems.
Verification of continued attainment is accomplished through operation of the ambient ozone monitoring network and the periodic update of the York County Area's emission inventories. SCDES has been and will continue proactive efforts including reviewing monitoring data and evaluating trends in an effort to identify possible violations as early as possible. In addition, to track future levels of emissions, SCDES will continue to develop and submit to EPA updated emission inventories for all source categories at least once every three years consistent with the requirements of 40 CFR part 51, subpart A, and 40 CFR 51.122.
**E. Contingency Plan**
Section 175A of the CAA requires that a maintenance plan include such contingency measures as EPA deems necessary to assure that the state will promptly correct a violation of the NAAQS that occurs after redesignation. *See* CAA section 175A(d). The maintenance plan should identify the contingency measures to be adopted, a schedule and procedure for adoption and implementation, and a time limit for action by the state. A state should also identify specific indicators to be used to determine when the contingency measures need to be implemented. The maintenance plan must include a requirement that a state will implement all measures with respect to control of the pollutant that were contained in the SIP before redesignation of the area to attainment in accordance with section 175A(d).
In the September 25, 2023, maintenance plan, South Carolina affirms that all programs instituted by the State will remain enforceable and that sources are prohibited from reducing emissions controls unless such a change is first approved by EPA as a revision to the South Carolina SIP that is consistent with Section 110(l) of the CAA. The plan also states that SCDES will implement all control measures with respect to NOx, VOCs, and ozone that were contained in the SIP for the maintenance area before it was redesignated as attainment. The contingency plan included in the submittal includes a triggering mechanism to determine when contingency measures are needed and a process of developing and implementing appropriate control measures. The primary trigger of the contingency plan will be a certified design value that exceeds the 2008 8-hour ozone NAAQS ( *i.e.,* when the three-year average of the 4th highest values is equal to or greater than 0.076 ppm at any monitor in the bi-state Charlotte Area). If certified data indicates a violating design value, the triggering event will be the date of the design value violation, not the final QA/QC date. If initial monitoring data indicates a possible design value violation but later certification indicates that a NAAQS violation did not occur, a triggering event will not have occurred, and contingency measures will not need to be implemented.
If the primary trigger is activated, SCDES will begin analyses to determine the emission control measures that will be necessary for attaining or maintaining the 2008 8-hour ozone NAAQS and implement contingency measures within 24 months of a violation trigger to bring the area back into attainment. [^16] In the September 25, 2023, maintenance plan, the State identified the following contingency measures that may be considered for adoption upon a trigger of the contingency plan:
[^16] If SCDES determines that a longer schedule is required to implement specific contingency measures, then, upon selection of the appropriate measures, it will seek concurrence with EPA of the proposed schedule and provide sufficient information to demonstrate that the proposed measures are a prompt correction of the triggering event. Any extension would be subject to EPA's approval of the SIP revision containing the required contingency measure.
• Reasonably Available Control Technology (RACT) for NOx on existing stationary sources not subject to existing requirements;
• Implementation of diesel retrofit programs, including incentives for performing retrofits for fleet vehicle operations;
• Alternative fuel programs for fleet vehicle operations;
• Gas can and lawnmower replacement programs;
• Voluntary engine idling reduction programs;
• Other measures deemed appropriate at the time as a result of advances in control technologies.
Finally, the Department will monitor periodic emissions inventory updates during the triennial National Emissions Inventory and compare them to projected emissions. If actual emissions exceed by more than 10 percent the projected emissions in this maintenance plan, the Department will investigate the differences and develop an appropriate strategy for addressing these differences.
Generally, the maintenance plan for the York County Area relies on similar contingency measures as those in the SCDES plans for the first 10-year period. The only change is that the second 10-year maintenance plan no longer lists the “Take a Break from Exhaust program,” as a potential contingency measure. EPA proposes to find that the contingency provisions in South Carolina's second maintenance plan for the York County Area for the 2008 8-hour Ozone NAAQS meet the requirements of the CAA section 175A(d).
**IV. EPA's Analysis of South Carolina's Proposed NO
X
and VOC Budgets**
Under section 176(c) of the CAA, new transportation plans, programs, and projects, such as the construction of new highways, must “conform” to ( *i.e.,* be consistent with) the part of the state's air quality plan that addresses pollution from cars and trucks. Conformity to the SIP means that transportation activities will not cause new air quality violations, worsen existing violations, or delay timely attainment of the NAAQS or any interim milestones. Regulations at 40 CFR part 93 set forth EPA policy, criteria, and procedures for demonstrating and assuring conformity of such transportation activities to a SIP. The regional emissions analysis is one, but not the only, requirement for implementing transportation conformity. Transportation conformity is a requirement for nonattainment and maintenance areas. Maintenance areas are areas that were previously nonattainment for a particular NAAQS but have since been redesignated to attainment with an approved maintenance plan for that NAAQS.
Under the CAA, states are required to submit, at various times, control strategy SIPs and maintenance plans for nonattainment areas. These control strategy SIPs (including RFP and attainment demonstration requirements) and maintenance plans create budgets for criteria pollutants and/or their precursors to address pollution from cars and trucks. Per 40 CFR part 93, a budget must be established for the last year of the maintenance plan. A state may adopt budgets for other years as well. The budget is the portion of the total allowable emissions in the maintenance demonstration that is allocated to highway and transit vehicle use and emissions. *See* 40 CFR 93.101. The budget serves as a ceiling on emissions from an area's planned transportation system. The budget concept is further explained in the preamble to the November 24, 1993, Transportation Conformity Rule (58 FR 62188). The preamble also describes how to establish the budget in the SIP and how to revise the budgets.
In the first maintenance plan for the York County Area, SCDES used 2014 for the attainment year inventory because 2014 was one of the years in the 2012-2015 three-year design value period when the bi-state Charlotte Area first attained the 2008 ozone NAAQS. For the second maintenance plan, SCDES selected 2018 as the base emissions inventory. The base year of 2018 was selected because it is one of the more recent years for which the York County Area has an attaining design value for the 2008 8-hour ozone NAAQS while avoiding the potential underrepresentation of emissions caused by the response to the COVID-19 pandemic beginning in early 2020. According to the transportation conformity rule, a maintenance plan must establish budgets for the last year of the maintenance plan (in this case, 2036). *See* 40 CFR 93.118. The state may set a budget for an interim year (in this case 2026), but it is not a requirement. In the September 26, 2023, submittal, South Carolina requested that EPA replace the previous 2014 and 2026 NO <sub>X</sub> and VOC budgets from the first 10-year maintenance plan with the 2018 and 2036 budgets. If approved as proposed, the previous 2014 and 2026 budgets will no longer apply for transportation conformity purposes. Table 6, below, provides the NO <sub>X</sub> and VOC budgets in kilograms per day (kg/day), for 2018 and 2036.
| | 2018 | NO | VOC | 2036 | NO | VOC |
| --- | --- | --- | --- | --- | --- | --- |
| Base Emissions | 6224.03 | 2555.02 | 2273.63 | 1256.15 | | |
| Safety Margin Allocated to Budget | N/A | N/A | 4281.96 | 272.16 | | |
| Total Budget | 6224.03 | 2555.02 | 6555.59 | 1528.31 | | |
As mentioned above, South Carolina has chosen to allocate the available safety margin to the NO <sub>X</sub> and VOC budgets for 2036 for the York County Area.
Through this rulemaking, EPA is proposing to approve the budgets for NO <sub>X</sub> and VOC for 2018 and 2036 for the York County Area because EPA believes that the Area maintains the 2008 8-hour ozone NAAQS with the emissions at the levels of the budgets. EPA is also proposing to replace the previous 2014 and 2026 NO <sub>X</sub> and VOC budgets from the first 10-year maintenance plan. If EPA finalizes that replacement, the 2014 and 2026 budgets will no longer apply for transportation conformity purposes. EPA intends to make its determination on the adequacy of the 2018 and 2036 budgets for the York County Area for transportation conformity purposes in the near future by completing the adequacy process that was started on September 26, 2023. After EPA finds the 2018 and 2036 budgets adequate or approves them (whichever is completed first), they must be used for future conformity determinations. EPA is proposing to approve the budgets because they are consistent with maintenance of the 2008 8-hour ozone NAAQS through 2036.
**V. EPA's Adequacy Determination for the Proposed NO
X
and VOC Budgets**
When reviewing submitted “control strategy” SIPs or maintenance plans containing budgets, EPA may affirmatively find those budgets adequate for use in determining transportation conformity. Once EPA affirmatively finds the submitted budget is adequate for transportation conformity purposes, that budget must be used by state and Federal agencies in determining whether proposed transportation projects conform to the SIP as required by section 176(c) of the CAA.
EPA's substantive criteria for determining adequacy of budgets are set out in 40 CFR 93.118(e)(4). The process for determining adequacy consists of three basic steps: Public notification of a SIP submission, a public comment period, and EPA's adequacy determination. This process for determining the adequacy of submitted budgets for transportation conformity purposes was initially outlined in EPA's May 14, 1999, guidance, “Conformity Guidance on Implementation of March 2, 1999, Conformity Court Decision.” EPA adopted regulations to codify the adequacy process in the Transportation Conformity Rule Amendments for the “New 8-Hour Ozone and PM <sub>2.5</sub> National Ambient Air Quality Standards and Miscellaneous Revisions for Existing Areas; Transportation Conformity Rule Amendments—Response to Court Decision and Additional Rule Change,” on July 1, 2004 (69 FR 40004). Additional information on the adequacy process for transportation conformity purposes is available in the proposed rule entitled, “Transportation Conformity Rule Amendments: Response to Court Decision and Additional Rule Changes,” 68 FR 38974, 38984 (June 30, 2003).
As discussed earlier, South Carolina's September 26, 2023, maintenance plan includes NO <sub>X</sub> and VOC budgets for the York County Area for 2018, an interim year of the maintenance plan, 2026, and the last year of the maintenance plan, 2036. EPA is reviewing the NO <sub>X</sub> and VOC budgets through the adequacy process. The York County Area NO <sub>X</sub> and VOC budgets, opened for public comment on EPA's adequacy website on October 19, 2023, found at: *https://www.epa.gov/state-and-local-transportation/adequacy-review-state-implementation-plan-sip-submissions-conformity.* The EPA public comment period on adequacy for the budgets for 2018 and 2036 for the York County Area closed on November 20, 2023. No comments, adverse or otherwise, were received during EPA's adequacy process for the budgets associated with South Carolina's maintenance plan.
EPA intends to make its determination on the adequacy of the 2018 and 2036 budgets for the York County Area for transportation conformity purposes in the near future by completing the adequacy process that was started on September 26, 2023. If EPA finds the 2018 and 2036 budgets adequate or approves them, the new budgets for NO <sub>X</sub> and VOC must be used for future transportation conformity determinations. For required regional emissions analysis years that involve 2018 through 2036, the applicable 2018 budgets will be used and for 2036 and beyond, the applicable budgets will be the new 2036 budgets established in the maintenance plan, as defined in Section IV of this proposed rulemaking.
**VI. Effect of EPA's Proposed Action**
EPA's proposed action establishes the basis upon which EPA may take final action on the issues being proposed for approval. Approval of South Carolina's SIP revisions would incorporate a plan for maintaining the 2008 8-hour ozone NAAQS in the Area through 2036 into the SIP. This maintenance plan includes contingency measures to remedy any future violations of the 2008 8-hour ozone NAAQS and procedures for evaluation of potential violations. The maintenance plan also establishes NOx and VOC budgets for 2018 and 2036 for the York County Area. The budgets are listed in Table 4 in Section IV. Additionally, EPA is notifying the public of the status of EPA's adequacy determination for the newly established NOx and VOC budgets for 2018 and 2036 for the York County Area.
**VII. Proposed Action**
EPA is proposing to approve the second maintenance plan for the 2008 8-hour ozone NAAQS for the South Carolina portion of the bi-state Charlotte Area, including the NOx and VOC budgets for 2018 and 2036 (to replace the previous NOx and VOC budgets for 2014 and 2026 from the first 10-year maintenance plan), into the South Carolina SIP under CAA section 175A. The maintenance plan meets all applicable requirements for maintenance plans and related contingency provisions in CAA section 175A, including a demonstration that the bi-state Charlotte Area will continue to maintain the 2008 8-hour ozone NAAQS until January 11, 2036, the end of the 20-year maintenance period. Further, as part of this proposed action, EPA is describing the status of its adequacy determination for the NOx and VOC budgets for 2018 and 2036 in accordance with 40 CFR 93.118(f)(1). Within 24 months from the publication date of EPA's final rule for this action (if EPA approves this maintenance plan and the underlying budgets), or the effective date of EPA's adequacy determination for the budgets, whichever is earlier, the transportation partners will need to demonstrate conformity to the new NOx and VOC budgets pursuant to 40 CFR 93.104(e)(3).
**VIII. 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. *See* 42 U.S.C. 7410(k); 40 CFR 52.02(a). Thus, in reviewing SIP submissions, 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 action:
• Is not a significant regulatory action subject to review by the Office of Management and Budget under Executive Order 12866 (58 FR 51735, October 4, 1993);
• Is not subject to Executive Order 14192 (90 FR 9065, February 6, 2025) because SIP actions are exempt from review 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 in Executive Order 13132 (64 FR 43255, August 10, 1999);
• Is not subject to Executive Order 13045 (62 FR 19885, April 23, 1997) because it approves a state program;
• Is not a significant regulatory action subject to Executive 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.
Because 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, this proposed action for the State of South Carolina does not have Tribal implications as specified by Executive Order 13175 (65 FR 67249, November 9, 2000). Therefore, this proposed action will not impose substantial direct costs on Tribal governments or preempt Tribal law. The Catawba Indian Nation (CIN) Reservation is located within the boundary of York County, South Carolina. Pursuant to the Catawba Indian Claims Settlement Act, S.C. Code Ann. 27-16-120 (Settlement Act), “all state and local environmental laws and regulations apply to the [Catawba Indian Nation] and Reservation and are fully enforceable by all relevant state and local agencies and authorities.” The CIN also retains authority to impose regulations applying higher environmental standards to the Reservation than those imposed by state law or local governing bodies, in accordance with the Settlement Act.
List of Subjects in 40 CFR part 52 Environmental protection, Air pollution control, Incorporation by reference, Intergovernmental relations, Ozone, Reporting and recordkeeping requirements, Volatile organic compounds.
**Authority:**
42 U.S.C. 7401 *et seq.*
Dated: August 27, 2025.
Kevin McOmber,
Regional Administrator, Region 4.