Air Plan Approval; Illinois; Alton Township 2010 Sulfur Dioxide Redesignation and Maintenance Plan

SUPPLEMENTARY INFORMATION:

Throughout this document whenever “we,” “us,” or “our” is used, we mean EPA.

I. Background and Redesignation Requirements

On June 22, 2010 (75 FR 35520), EPA revised the primary SO₂ NAAQS, establishing a new health-based 1-hour standard of 75 parts per billion (ppb). EPA promulgated designations for this standard in four rounds. On September 18, 2015, Illinois submitted its recommendations to EPA to designate certain areas of the State as part of the Round 2 designations. Illinois recommended a portion of southern Alton Township in Madison County be designated as nonattainment for the 2010 SO₂ NAAQS. EPA concurred with Illinois' analysis and published a final action designating the area as nonattainment for the 2010 SO₂ NAAQS effective September 12, 2016 (81 FR 45039).

Illinois submitted an attainment plan for the Alton Township nonattainment area on December 3, 2018. Under section 192(a) of the CAA, these plans are required to demonstrate that areas will attain the NAAQS as expeditiously as practicable, but no later than five years from the effective date of designation. The plan included modeling from emissions associated with the Alton Steel Inc. (Alton Steel) facility in Alton, Illinois, and emissions from the Ameren Missouri-Sioux Energy Center (Ameren-Sioux) power plant in Missouri, located about 13 kilometers west-northwest of the nonattainment area. The modeled emissions showed the nonattainment status of the area was mainly attributed to these two SO₂ sources. Unlike the Round 2 designations modeling, the Alton Township attainment demonstration did not include the Wood River Power Station among the sources modeled due to the retirement of the facility in June 2016 and its demolition in February 2021. On March 14, 2019, after Illinois' submission of the attainment plan, the Illinois Environmental Protection Agency issued Construction Permit #18020009 for the Alton Steel facility to operate a new ladle metallurgy facility (LMF) stack, removing the downfacing vents that were contributing to modeled nonattainment at the facility. EPA included additional dispersion modeling to supplement Illinois' attainment demonstration to demonstrate that the emission limits required by the Illinois SIP and submitted for EPA approval provide for modeled concentrations meeting the 2010 SO₂ NAAQS. EPA approved Illinois' attainment plan revision on February 21, 2023 (88 FR 10464). Additionally, under section 179(c)(1) of the CAA, EPA is required to determine whether a nonattainment area attained a standard by the applicable attainment date based on the area's air quality as of the attainment date. EPA is to issue this determination within six months of the attainment date. Thus, EPA had a mandatory duty under CAA section 179(c) to determine by March 12, 2022, whether the Alton Township area attained by September 12, 2021. As an additional action, EPA is proposing to determine the Alton Township area did attain the 2010 SO₂ NAAQS by the attainment date of September 12, 2021.

On October 2, 2023, Illinois submitted a redesignation request and maintenance plan for the Alton Township nonattainment area for the 2010 SO₂ NAAQS. Additionally, on March 31, 2022, Missouri adopted into its SIP a Consent Agreement between the Missouri Department of Natural Resources' Air Pollution Control Program and Ameren-Sioux, APCP-2021-018. EPA approved the revision to incorporate the Ameren-Sioux Consent Agreement on November 16, 2022 (87 FR 68634), establishing an enforceable SO₂ emissions limit for two coal-fired boilers at the facility. See Appendix B of the State's submittal included in the public docket of this action—Ameren-Sioux Consent Agreement. The submitted redesignation request and maintenance plan includes the Construction Permit issued to Alton Steel and the Consent Agreement issued to Ameren-Sioux approved into the Missouri SIP (87 FR 68634, November 16, 2022) with accepted SO₂ emissions limits. While the Ameren-Sioux facility is not in the Alton Township nonattainment area, Illinois included this facility in their modeling due to its close proximity to the nonattainment area and high SO₂ emissions, using information provided by the Missouri Department of Natural Resources. EPA's supplemental modeling with the updated limit of 7,342 pounds per hour (lbs/hr) averaged over a 24-hour block period for the Ameren-Sioux facility outlined in the Consent Agreement, provided for attainment of the 2010 SO₂ NAAQS, and thus is included in Illinois' submittal.

Under section 107(d)(3)(E) of the CAA, EPA may not promulgate a redesignation of a nonattainment area (or portion thereof) unless:

1. EPA has determined that the area has attained the NAAQS;

2. EPA has fully approved the applicable implementation plan for the area under section 110(k) of the CAA;

3. EPA has determined that improvement in air quality is due to permanent and enforceable reductions in emissions resulting from implementation of the applicable implementation plan and applicable Federal air pollution control regulations and other permanent and enforceable reductions;

4. EPA has fully approved a maintenance plan for the area under section 175A of the CAA; and

5. The State containing such area has met all requirements applicable to the area under section 110 of the CAA and part D.

II. Evaluation of Illinois' Redesignation Request and Maintenance Plan

On October 2, 2023, Illinois submitted a redesignation request for the Alton Township 2010 SO₂ nonattainment area to attainment and a SIP revision containing a maintenance plan for the area.

EPA's evaluation of Illinois' redesignation request and maintenance plan was based on consideration of the five redesignation criteria provided under CAA section 107(d)(3)(E).

Criteria (1)—The Alton Township SO ₂ Nonattainment Area Has Attained the 2010 SO ₂ NAAQS

In accordance with CAA section 107(d)(3)(E)(i), for redesignation of a nonattainment area to attainment, the CAA requires EPA to determine that the area has attained the applicable NAAQS. An area is attaining the 2010 SO₂ NAAQS at an ambient air quality monitoring site, in accordance with 40 CFR 50.17 and appendix T of part 50, when the three-year average of the annual (99th percentile) of the daily maximum 1-hour average concentrations is less than or equal to 75 ppb. As stated in EPA's April 23, 2014, “Guidance for 1-Hour SO₂ Nonattainment Area SIP Submissions” (“April 2014 SO₂ Guidance”), there are two components needed to support an attainment determination: (1) a review of representative air quality monitoring data located in the area of maximum concentration; and (2) a further analysis, where there are no monitors, using air quality dispersion modeling, which will generally be needed to estimate SO₂ concentrations throughout the nonattainment area to demonstrate that the entire area is attaining the applicable NAAQS, based on current actual emissions or the fully implemented control strategy. The April 2014 SO₂ Guidance further states that dispersion modeling should be conducted to estimate SO₂ concentrations throughout the nonattainment area using actual emissions and meteorological information for the most recent 3 calendar years. EPA considered Illinois' modeling analyses submitted in the State's attainment plan which incorporated reductions in allowable emissions without the LMF stack changes to Alton Steel, which showed Alton Steel was the principal contributor to the highest modeled violations. Subsequently, EPA considered the modeling analyses with changes to the LMF exhaust configuration as described in Illinois' Construction Permit issued to Alton Steel. This analysis determined that after changes to the LMF exhaust stack in Alton Steel, the analysis still modeled violations of the 2010 SO₂ NAAQS that were primarily due to the Ameren-Sioux facility. New SO₂ emission limits were established under the Ameren-Sioux Consent Agreement (87 FR 68634, November 16, 2022), and EPA conducted supplementary modeling that incorporated both of these new limits and the changes to the LMF exhaust stack at Alton Steel. The supplementary modeling showed design values that are below the 2010 SO₂ NAAQS for the Alton Township area.

The April 2014 SO₂ Guidance provides an extensive discussion of EPA's view that appropriately set comparably stringent limits based on averaging times as long as 30 days can be found to provide for attainment of the 2010 SO₂ NAAQS. The April 2014 SO₂ Guidance also offers specific recommendations for determining an appropriate longer-term average limit. Illinois' plan applies 1-hour average emission limits to Alton Steel. However, Illinois' attainment plan also considers the impact of an additional facility, Ameren-Sioux, that is subject to a 24-hour block average limit. In response, EPA provided additional discussion of its rationale for approving the use of longer-term average limits in plans designed to provide for attainment. More detail on EPA's analysis of the attainment plan for the Alton Township area can be found in the attainment plan approval (88 FR 10464, February 21, 2023).

Under 40 CFR 50.17(b), the 2010 SO₂ NAAQS is met at an ambient air quality monitoring site when the three-year average of the annual (99th percentile) daily maximum 1-hour average concentrations is less than or equal to 75 ppb, as determined in accordance with appendix T of 40 CFR part 50, at all relevant monitoring sites in the subject area. In a year with 365 days of valid monitoring data, the 99th percentile would be the fourth highest daily maximum 1-hour value. Attainment demonstrations for the 2010 SO₂ NAAQS should demonstrate future attainment and maintenance of the NAAQS in the entire area designated as nonattainment ( i.e., not just at the violating monitor) by using air quality dispersion modeling ( see appendix W to 40 CFR part 51) to show that the mix of sources and enforceable control measures and emission rates in an identified area will not lead to a violation of the 2010 SO₂ NAAQS. Illinois' redesignation request for the Alton Township nonattainment area relied upon the demonstration of attainment based on air dispersion modeling, which Illinois submitted to EPA as part of its December 2018 attainment SIP submittal. For a short-term ( i.e., 1-hour) standard, dispersion modeling, using allowable emissions and addressing stationary sources in the affected area (and in some cases those sources located outside the nonattainment area which may affect attainment in the area), is technically appropriate, efficient, and effective in demonstrating attainment in nonattainment areas because it takes into consideration combinations of meteorological and emission source operating conditions that may contribute to peak ground-level concentrations of SO_2.

Preferred air quality models for use in regulatory applications are described in appendix A of EPA's Guideline on Air Quality Models (40 CFR part 51, appendix W). In 2005, EPA promulgated AERMOD as the Agency's preferred near-field dispersion modeling for a wide range of regulatory applications addressing stationary sources (for example, in estimating SO₂ concentrations) in all types of terrain based on extensive developmental and performance evaluation. To support the SIP for bringing the Alton area into attainment of the 2010 SO₂ NAAQS, EPA assisted Illinois by conducting a modeling demonstration of the Alton area with updated emissions data relied upon in the model and by correcting the source characterization. Because Illinois' submittal predated the updated SO₂ emissions limit at the Ameren-Sioux facility necessary to model attainment in the area, EPA updated the emissions data at Ameren-Sioux in the supplemental model and corrected any other deficiencies in the emissions data. Additional updates and modifications in EPA's modeling analysis included: updated AERMOD version from version 18081 to 21112 (the most recent available version at the time), updated stack parameters to current operating conditions, and adjusted emission rates for the Ameren-Sioux facility to account for longer-term average limits. EPA used Illinois' receptor grid, meteorological surface and upper air stations, model settings, and some source parameters to develop the modeling demonstration. See Alton SO₂ Nonattainment Plan TSD, 87 FR 80509 (December 30, 2022). EPA used AERMOD version 21112, the most recent version of the AERMOD Modeling System available at the time, for the modeling analysis. In the time since the modeling was conducted by Illinois and later supplemented by EPA using AERMOD version 21112, EPA has released two AERMOD version updates. AERMOD 22112 was released in 2022 to replace the regulatory version 21112 and was a routine release that does not include any scientific updates to the regulatory formulation of AERMOD as described in appendix W. In 2023, AERMOD 23132 replaced the regulatory version 22112 which included small bug fixes and enhancements. Currently, AERMOD 23132 is the most recent available version. However, EPA does not require Illinois to use the current AERMOD version as Illinois used the AERMOD version that was most recent at the time it was developing the SIP and determining SO₂ emission limits.

Illinois' modeled demonstration of attainment for the Alton Township area was discussed in detail in EPA's proposed approval of the Alton Township SO₂ attainment plan. See87 FR 80509, (December 30, 2022). Illinois' original modeling submitted included Ameren-Sioux maximum allowable emissions and the reconfigured LMF stack emissions for Alton Steel, which yielded a maximum predicted 99th percentile 1-hour average concentration of 298.5 micrograms per cubic meter (µg/m³ ). EPA conducted a supplemental modeling run to evaluate the reconfigured stack requirements at Alton Steel in combination with Ameren-Sioux's updated 7,342 lbs SO₂ /hr 24-hour block limit found in the Missouri SIP (87 FR 68634, November 16, 2022). EPA's modeling demonstrated that the Alton area showed a design value of 74.9 ppb (196.2 µg/m³ ) with the revised SO₂ emission limits that will provide for attainment based on the new requirements for Alton Steel and Ameren-Sioux, which are both permanently and federally enforceable. As mentioned above, the changes at the Alton Steel facility were construction changes to the LMF stack parameters to be completed by July 31, 2019, modifying the dispersion of emissions, and they did not alter existing operating emission limit conditions that the reconfigured LMF stack was already subject to and approved for in the approved attainment plan. Stack test results of the LMF at the Alton Steel facility prior to construction of the modified LMF stack show compliance with the permitted values, as do updated stack test results of the LMF stack upon construction completion as shown in Table 1. EPA concluded that Illinois' modeling is a suitable demonstration that its requirements in the Construction Permit for Alton Steel and all other Illinois sources in the nonattainment area were properly addressed in the attainment plan. EPA's supplemental modeling demonstrated that the updated 24-hour block limit for Ameren-Sioux of 7,342 lbs/hr in combination with updated emissions at Alton Steel provide for attainment and is the primary basis to conclude that the area is attaining the 2010 SO₂ NAAQS.

EPA has also considered additional information which supports its proposed determination that the area has attained the NAAQS, including annual SO₂ emissions from the Alton Steel facility shown in Table 2. The annual tons per year emissions at the Alton facility show a decreasing trend overall. Annual SO₂ emissions from the Ameren-Sioux facility found in Table 2 also show a decreasing trend overall. EPA included data from the Ameren-Sioux facility showing that hourly emissions with a 24-hour average, are at or below the newly established limit of 7,342 lbs/hr, and have been since 2017. These data are included in the docket of this action. In this action, EPA is proposing to find that the Alton Township area has attained and will continue to attain the 2010 SO₂ NAAQS with respect to the redesignation criteria under CAA section 107(d)(3)(E)(i).