Determination of Attainment by the Attainment Date; Michigan; St. Clair 2010 Sulfur Dioxide Nonattainment Area

SUPPLEMENTARY INFORMATION:

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

I. Background

A. The 2010 1-Hour Primary SO ₂ NAAQS

Under section 109 of the CAA, EPA has established primary and secondary NAAQS for certain pervasive air pollutants (referred to as “criteria pollutants”) and conducts periodic reviews of the NAAQS to determine whether they should be revised or whether new NAAQS should be established.

On June 22, 2010 (75 FR 35520), EPA published in the Federal Register a strengthened, primary 1-hour SO₂ NAAQS, establishing a new standard at a level of 75 ppb, based on the 3-year average of the annual 99th percentile of daily maximum 1-hour average concentrations of SO_2. This revised SO₂ NAAQS provided increased protection of public health and provided for revocation of the 1971 primary annual and 24-hour SO₂ standards for most areas of the country following area designations under the new NAAQS.

B. Designations and Attainment Dates for the 2010 SO ₂ NAAQS

Following promulgation of a new or revised NAAQS, EPA is required to designate all areas of the country as either “attainment,” “nonattainment,” or “unclassifiable,” pursuant to CAA section 107(d)(1). On July 12, 2016 (81 FR 45039), EPA finalized its second round of initial designations under the 2010 SO₂ NAAQS. During the second round of designations, the St. Clair area of Michigan was designated as nonattainment for the 2010 SO₂ NAAQS (40 CFR 81.323) based on modeling of actual emissions for the designated area.

CAA section 191(a) directs states containing an area designated nonattainment for the 2010 SO₂ NAAQS to develop and submit a nonattainment area State Implementation Plan (SIP) to EPA within 18 months of the effective date of an area's designation as nonattainment. The Michigan Department of Environment, Great Lakes, and Energy (EGLE) was required to submit a SIP by March 12, 2018, to bring the St. Clair area into attainment by the attainment date of September 12, 2021.

EGLE submitted a request for a Clean Data Determination (CDD) on July 24, 2020. When a nonattainment area is attaining the 2010 SO₂ NAAQS based on the most recent available data, EPA may issue a CDD suspending planning requirements. EPA issued a CDD for the St. Clair area based on monitoring and modeling data for the 2017-2019 period via a final rule published on December 7, 2021 (86 FR 69173).

C. Requirement To Determine Attainment by the Attainment Date

Section 179(c)(1) of the CAA requires EPA 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 area attained by September 12, 2021. This action proposes to determine the St. Clair area did attain the 2010 SO₂ NAAQS by the attainment date of September 12, 2021.

A determination of whether an area's air quality meets applicable standards is generally based upon the most recent three years of complete, quality-assured data gathered at established State and local air monitoring stations in a nonattainment area and entered into EPA's Air Quality System (AQS) database. Data from ambient air monitors operated by State and local agencies in compliance with EPA monitoring requirements must be submitted to AQS. Monitoring agencies annually certify that these data are accurate to the best of their knowledge. All data are reviewed to determine the area's air quality status in accordance with 40 CFR part 50, appendix T (for SO₂). In general, for SO₂, EPA does not rely exclusively on monitoring data to determine whether the NAAQS is met unless it has been demonstrated that the monitors were appropriately sited to record expected maximum ambient concentrations of SO₂ in an area. As such, monitoring data can be supplemented with other relevant information, including dispersion modeling and emissions inventories, for determining attainment.

II. Proposed Determination of Attainment by the Attainment Date

A. Area Characterization

The St. Clair area is located within the lower southeastern corner of Michigan northeast of Detroit and shares a border with Ontario, Canada along the St. Clair River. The area is defined by the St. Clair River for the eastern boundary, an extension from the St. Clair River straight west to the intersection of State Highway M-29 and St. Clair River Drive, continuing west on State Highway M-29 to Church Road to Arnold Road to County Line Road for the southern boundary, County Line Road and the Macomb/St. Clair County boundary to Stoddard Road to Wales Ridge Road for the western boundary, and Alpine Road to Fitz Road to Smith Creek Road to Range Road to Huron Avenue, extending straight east from the intersection of Huron Road and River Road to the St. Clair River for the northern boundary.

The St. Clair area contains two SO₂ -emitting facilities that are both coal-fired power plants. Additionally, the area contains two SO₂ monitors which reside near the facilities. The two monitors have been operating since 2016 and have had no recorded violations of the NAAQS. As these monitors were sited to operate under guidance per the “SO₂ NAAQS Designations Source-Oriented Monitoring Technical Assistance Document” (SO₂ Monitoring TAD), EPA believes that these monitors' locations adequately represent the locations of potential maximum SO₂ impacts from the two power plants.

B. St. Clair Nonattainment Area's Attainment of the 2010 SO ₂ NAAQS

We propose to determine that the St. Clair nonattainment area attained the 2010 SO₂ NAAQS by the attainment date of September 12, 2021. EPA previously determined that the St. Clair SO₂ nonattainment area was attaining the 2010 SO₂ NAAQS in its December 7, 2021 (86 FR 69173), CDD. EPA issued the CDD based on SO₂ monitoring and modeling data from EGLE. For this determination of attainment by the attainment date, EPA is in part relying on the approved CDD of the St. Clair area as well as additional supporting information. The data cited by the CDD demonstrated attainment for the 2017-2019 time period, with averaged SO₂ monitoring values of 54 ppb for the Belle River-Mills Monitor and 45 ppb for the St. Clair-Remer Monitor. The CDD modeled 2017-2019 emission sources for an overall maximum 99th percentile impact output of 64.4 ppb, which falls below the 2010 SO₂ NAAQS of 75 ppb.

As noted, determinations of whether areas attained the NAAQS by the attainment date are generally based on the area's design value as of the attainment date, i.e., the three most recent calendar years of data, in this case 2018-2020. Therefore, in this proposal EPA is closely examining monitoring and emissions data from 2020 to supplement the analysis already concluded in the CDD, which looked at air quality information from 2017-2019. In 2020, primary source SO₂ emissions and monitored SO₂ ambient air concentrations in the area continued to decline. The SO₂ emissions from the Belle River and St. Clair power plants decreased by an additional total of 8,996 tons per year from 2019 to 2020 (Table 1). As seen in Table 2, the 2018-2020 design values at the two air quality monitors in the area continued to show SO₂ levels below the 75 ppb level of the NAAQS and a decline in SO₂ concentration from 2017-2019. Therefore, the additional information EPA has examined for 2020, coupled with the existing CDD based on 2017-2019 monitoring and emissions data, leads the agency to conclude that the St. Clair nonattainment area attained by its attainment date.