SUPPLEMENTARY INFORMATION:

Throughout this document “we,” “us,” and “our” refer to the EPA.

Table of Contents

I. Written Comments

II. Background

A. The 2010 SO₂ National Ambient Air Quality Standards

B. Designations, Classifications, and Attainment Dates for the 2010 SO₂ National Ambient Air Quality Standard

III. Proposed Determination

A. Applicable Statutory and Regulatory Provisions

B. Monitoring Network Considerations

C. Data Considerations and Proposed Determination

i. Emissions Information

ii. Monitoring Data

iii. Meteorology

iv. Modeling Information

v. Conclusion

IV. Proposed Action and Request for Public Comment

V. Environmental Justice Concerns

VI. Statutory and Executive Order Reviews

I. Written Comments

Submit your comments, identified by Docket ID No. EPA-R07-OAR-2021-0932, at https://www.regulations.gov. Once submitted, comments cannot be edited or removed from Regulations.gov. The 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. The 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.

II. Background

A. The 2010 SO₂ National Ambient Air Quality Standards

Under section 109 of the CAA, the 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. The primary NAAQS represent ambient air quality standards the attainment and maintenance of which the EPA has determined, including a margin of safety, are requisite to protect the public health. The secondary NAAQS represent ambient air quality standards the attainment and maintenance of which the EPA has determined are requisite to protect the public welfare from any known or anticipated adverse effects associated with the presence of such air pollutant in the ambient air.

Under the CAA, the EPA must establish a NAAQS for SO₂ . SO₂ is primarily released to the atmosphere through the burning of fossil fuels by power plants and other industrial facilities. SO₂ is also emitted from industrial processes including metal extraction from ore and heavy equipment that burn fuel with a high sulfur content. Short-term exposure to SO₂ can damage the human respiratory system and increase breathing difficulties. Small children and people with respiratory conditions, such as asthma, are more sensitive to the effects of SO₂. Sulfur oxides at high concentrations can also react with compounds to form small particulates that can penetrate deeply into the lungs and cause health problems.

The EPA first established primary SO₂ standards in 1971 at 0.14 parts per million (ppm) over a 24-hour averaging period and 0.3 ppm over an annual averaging period (36 FR 8186, April 30, 1971). In June 2010, the EPA revised the NAAQS for SO₂ to provide increased protection of public health, providing 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. The 2010 NAAQS is 75 parts per billion (ppb) (equivalent to 0.075 ppm) over a 1-hour averaging period (75 FR 35520, June 22, 2010). A violation of the 2010 1-hour SO₂ NAAQS occurs when the annual 99th percentile of ambient daily maximum 1-hour average SO₂ concentrations, averaged over a 3-year period, exceeds 75 ppb.

B. Designations, Classifications, and Attainment Dates for the 2010 SO₂ National Ambient Air Quality Standards

Following promulgation of any new or revised NAAQS, the EPA is required by CAA section 107(d) to designate areas throughout the nation as attaining or not attaining the NAAQS.

On August 5, 2013, the EPA finalized its first round of designations for the 2010 primary 1-hour SO₂ NAAQS (78 FR 47191). In the 2013 action, the EPA designated 29 areas in 16 states as nonattainment for the 2010 SO₂ NAAQS, including a portion of Muscatine County in Iowa. The designation was based on air quality monitoring data from 2009-2011 showing violations of the NAAQS. The EPA's initial round of designations for the 2010 SO₂ NAAQS including the Muscatine nonattainment area (NAA) became effective on October 4, 2013. Pursuant to CAA sections 172(a)(2) and 192(a), the maximum attainment date for the Muscatine NAA is October 4, 2018, five years after the effective date of the final action designating the area as nonattainment for the 2010 SO₂ NAAQS. ( printed page 3960)

III. Proposed Determination

A. Applicable Statutory and Regulatory Provisions

Section 179(c)(1) of the CAA requires the EPA to determine whether a nonattainment area attained an applicable standard by the applicable attainment date based on the area's air quality as of the attainment date.

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 (SLAMS) in a nonattainment area and entered into the EPA's Air Quality System (AQS) database. Data from ambient air monitors operated by state and local agencies in compliance with the 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.

Under EPA regulations in 40 CFR 50.17 and in accordance with 40 CFR part 50, appendix T, the 2010 1-hour annual SO₂ standard is met at an ambient air quality monitoring site when the design value is less than or equal to 75 ppb. Design values are calculated by computing the three-year average of the annual 99th percentile daily maximum 1-hour average concentrations. When calculating 1-hour primary standard design values, the calculated design values are rounded to the nearest whole number or 1 ppb by convention. An SO₂ 1-hour primary standard design value is valid if it encompasses three consecutive calendar years of complete data. A year is considered complete when all four quarters are complete, and a quarter is complete when at least 75 percent of the sampling days are complete. A sampling day is considered complete if 75 percent of the hourly concentration values are reported; this includes data affected by exceptional events that have been approved for exclusion by the Administrator.

B. Monitoring Network Considerations

Section 110(a)(2)(B)(i) of the CAA requires states to establish and operate air monitoring networks to compile data on ambient air quality for all criteria pollutants. The EPA's monitoring requirements are specified by regulation in 40 CFR part 58. These requirements are applicable to state, and where delegated, local air monitoring agencies that operate criteria pollutant monitors.

In section 4.4 of appendix D to 40 CFR part 58, the EPA specifies minimum monitoring requirements for SO₂ to operate at SLAMS. SLAMS produce data that are eligible for comparison with the NAAQS, and therefore, the monitor must be an approved Federal reference method (FRM) or Federal equivalent method (FEM) monitor.

The minimum number of required SO₂ SLAMS is described in sections 4.4.2 and 4.4.3 of appendix D to 40 CFR part 58. According to section 4.4.2, the minimum number of required SO₂ monitoring sites is determined by the population weighted emissions index for each state's core based statistical area. Section 4.4.3 describes additional monitors that may be required by an EPA regional administrator.

Under 40 CFR 58.10, states are required to submit annual monitoring network plans (AMNP) for ambient air monitoring networks for approval by the EPA. Within the Muscatine NAA, the State is responsible for assuring that each monitoring site meets air quality monitoring requirements. Iowa submits an AMNP to the EPA that describes the various monitoring sites operated by the State. Each AMNP discusses the status of the air monitoring network as required under 40 CFR 58.10 and addresses the operation and maintenance of the air monitoring network in the previous year. The EPA regularly reviews these AMNPs for compliance with the applicable reporting requirements in 40 CFR part 58. With the EPA's approval of Iowa's most recent AMNP, the State has met the applicable minimum monitoring requirements.^[1]

The EPA also conducts regular “technical systems audits” (TSAs) during which we review and inspect ambient air monitoring programs to assess compliance with applicable regulations concerning the collection, analysis, validation, and reporting of ambient air quality data.

During the 2015-2017 data period, Iowa operated three SO₂ SLAMS in the Muscatine SO₂ NAA: Greenwood Cemetery (AQS ID 19-139-0016); High School East Campus (AQS ID 19-139-0019); and Musser Park (AQS ID 19-139-0020).

C. Data Considerations and Proposed Determination

CAA section 179(c)(1) requires the Agency to “determine, based on the area's air quality as of the attainment date, whether the area attained the standard by that date.” The EPA first assessed what air quality information was available related to making a determination of attainment by the attainment date for the Muscatine area. The EPA chose to employ a weight-of-evidence approach for making this determination because the EPA does not have any analysis (including modeling) associated with the monitor siting to demonstrate that the monitors record maximum ambient SO₂ concentrations in the NAA, nor does EPA have modeling of actual emissions to support a determination based on modeled ambient concentrations whether the area attained the NAAQS by the attainment date. The available modeling of permitted allowable emissions in the area, as discussed later in this document, does not on its own provide a basis for determining whether the area attained by the attainment date. Thus, EPA relied upon SO₂ emissions data and trends, relevant air monitoring data and trends, SO₂ monitoring data incorporated with local meteorological data, as well as available modeling information in order to make its determination under CAA section 179(c)(1). The EPA believes our analysis of multiple types of air-quality related information to support our determination is consistent with section 179(c)(1)'s direction to determine the area's air quality as of the attainment date. Further detail on EPA's weight-of-evidence analysis is contained in the technical support document (TSD) included in the docket for this action.

i. Emissions Information

There are four facilities that emit or have historically emitted SO₂ located in or near the Muscatine NAA. Three are located within the nonattainment area—Grain Processing Corporation (GPC), Muscatine Power and Water (MPW), and Monsanto. Louisa Generating Station (LGS) is located south of the nonattainment area. Table 1 provides the annual emissions from 2011-2020 from each individual source along with the total combined emissions among the four facilities. In the 2011-2015 timeframe, GPC was the largest SO₂ source in the Muscatine area, with the majority of SO₂ emissions attributed to GPC's boilers using coal. A fuel switch at GPC's coal-fired boilers to natural gas occurred on July 14, 2015, and this change led to large reductions of SO₂ ( printed page 3961) emissions at GPC. Prior to 2018, Monsanto was fueled primarily by coal, with SO₂ emissions associated with its main boiler. As required by a construction permit, Monsanto converted its coal-fired boiler to use only natural gas in 2018 which eliminated nearly all SO₂ emissions from Monsanto.

The EPA first evaluated annual SO₂ emissions trends within the Muscatine nonattainment area. By 2017, total annual emissions in the Muscatine area had dropped approximately 72% from 2014 (24,181 tons per year (tpy) in 2014 to 6,781 tpy in 2017). Much of the reduction in emissions can be attributed to GPC's fuel conversion to natural gas in July of 2015, evident by the more than 50% reduction in annual SO₂ emissions at GPC from 2014 (13,075 tpy) to 2015 (6,191 tpy) and further reductions to below 200 tpy in 2016 and 2017. Overall, GPC's annual SO₂ emissions were reduced by 98.7% from 2014 to 2017.

In addition to emissions decreases within the nonattainment area, the EPA also looked at emissions at LGS, the nearby source located outside the nonattainment area. In the Louisa County Data Requirements Rule (DRR) modeling,^[2] Iowa modeled LGS using its permitted allowable rate of 4,270.89 lbs/hour,^[3] which would correspond to an annual total of 18,706 tpy. Actual annual emissions at LGS during the 2015-2017 timeframe ranged between 5,129 tpy and 6,098 tpy, significantly below the annual total of 18,706 tpy that corresponded with modeled attainment. In addition, the actual maximum hourly emission rate at LGS since 2011, as reported to EPA Clean Air Markets Division (CAMD) database ^[4] is 4,014.7 lb/hr, which is also below the 1-hour modeled emission rate.

EPA's evaluation of emissions at sources within and outside of the nonattainment area indicate significant reductions in emissions in the 2015-2017 timeframe compared to pre-2015 emissions.

ii. Monitoring Data

Under 40 CFR 58.15, monitoring agencies must certify, on an annual basis, data collected by FRM or FEMs at all SLAMS, including special purpose monitors, that meet EPA quality assurance requirements. In doing so, monitoring agencies must certify that the previous year of ambient concentration and quality assurance data are completely submitted to AQS and that the ambient concentration data are accurate to the best of their knowledge. Iowa annually certifies that the data it submits to AQS are quality assured, including data collected at monitoring sites in the Muscatine SO₂ NAA.

For the Muscatine SO₂ NAA the applicable attainment date is October 4, 2018. In accordance with appendix T to 40 CFR part 50, where determinations of SO₂ NAAQS compliance may be made based on well-sited air quality monitors, compliance with the NAAQS is based on three consecutive calendar years of data. The three calendar year period preceding the attainment date for the Muscatine SO₂ NAA is January 1, 2015-December 31, 2017.

The 3-year design values of 1-hour SO₂ from 2011 through 2020 for the three Muscatine area monitors are provided in Table 2 and the annual 99th percentile of 1-hour SO₂ concentrations are shown in Table 3. All monitor violations occur before the 2015-2017 timeframe, with all three monitors showing violations from 2011-2016. No monitor violation of the 3-year design value has occurred since 2016, with the largest of the three 2015-2017 1-hour SO₂ design values of 65 ppb at the Musser Park site. The trends indicated in the monitored design values are consistent with EPA's evaluation of the emissions trends discussed above. As emission reductions were implemented at the sources in the nonattainment area, SO₂ concentrations recorded at the area's air quality monitors decreased. Specifically, coal combustion at GPC ceased in mid-2015 and coal combustion at Monsanto ceased in late 2017. Significant decreases in 1-hour daily maximum SO₂ concentrations at the air quality monitors are consistent with that timeline. While the most recent complete and quality-assured design values (2018-2020) for the Greenwood Cemetery, High School East Campus, and Musser Park sites (15, 18, and 20 ppb, respectively) were recorded after the area's attainment date, they indicate the effectiveness of the area's control measures. These design values are no greater than 27% of the level of the 2010 1-hour SO₂ NAAQS.

iii. Meteorology

The EPA does not have conclusive evidence to support that the monitors are sited in the area of maximum ambient SO₂ concentrations. EPA would typically rely on the siting analysis performed to originally site the monitors or modeling of actual emissions to demonstrate the monitors are sited in the area of maximum concentrations. There is not a specific analysis associated with the siting of the monitors nor does EPA have access to modeling of actual emissions for sources in or near the nonattainment area to make such a determination. In the absence of that information, EPA has also evaluated local meteorology along with the monitored SO₂ values to evaluate the likelihood of maximum ambient concentrations occurring in locations that the monitors could not record. Hourly wind speeds and direction were collected from the Muscatine Airport, which is located approximately 8 kilometers southwest of GPC and the Musser Park and High School SO₂ monitors. The hourly winds were combined into a dataset with the coinciding one hour monitored SO₂ concentrations and plotted using SO₂ pollution roses. This analysis provides information to help determine from where (and potentially what source) the monitored impacts were coming. In summary, the monitors appear to be positioned in downwind areas of relatively high impacts as indicated by pollution roses. Full details of the local meteorology analysis and pollution roses are provided in the TSD.

iv. Modeling Information

The EPA considered relying on two separate modeling demonstrations for the Muscatine area. Modeling performed by the State of Iowa for purposes of the control strategy and attainment demonstration for the area was submitted to EPA in May 2016. EPA later approved the attainment plan and modeling in a final action in November 2020 (85 FR 73218). That final action has since been remanded without vacatur to EPA.^[5] The State of Iowa also submitted modeling pursuant to the SO₂ DRR for LGS in January 2017. This DRR modeling was the basis for EPA's Round 3 designation of Attainment/Unclassifiable for Louisa County (containing LGS) in December 2017 (83 FR 1098). Both sets of modeling rely on permitted allowable emissions rates ^{[6]  [7]} that were in place by the October 4, 2018, attainment date and were previously found by EPA to demonstrate attainment of the NAAQS as noted above. However, the EPA is not relying on Iowa's attainment demonstration modeling as a basis for our proposed determination, because that modeling may be revisited as part of the EPA's reconsideration action per the D.C. Circuit's remand of EPA's approval of Iowa's attainment plan. Rather, as discussed above, we are relying on the DRR modeling to provide a comparison between the much higher modeled emission rates at the sources and the actual recorded emissions to provide additional evidence that the entire area was attaining the NAAQs as of October 4, 2018.

v. Conclusion

In sum, and as discussed further in the TSD, we propose to find that the weight of the available evidence indicates that the Muscatine area attained the 2010 1-hour SO₂ NAAQS in the 2015-2017 timeframe by the October 4, 2018 attainment date. Specifically, the significant reductions in emissions during the relevant time period from sources within the nonattainment area and a nearby source outside the nonattainment area, coupled with corresponding decreased monitored SO₂ concentrations within the nonattainment area during that same time period lead us to our proposed determination that the area attained by its attainment date. Local meteorological data help confirm that the air quality monitors are unlikely to have missed high concentrations, and the available modeling information and emissions data of the nearby LGS source (which may not be reflected in the air quality monitoring data from within the nonattainment area) also supports the EPA's determination, as actual historical emissions from that source during the relevant time period were significantly below the emissions that were modeled to be consistent with attainment of the NAAQS.

IV. Proposed Action and Request for Public Comment

The EPA conducted a weight-of-evidence analysis, described in detail above and in the TSD, to determine if the Muscatine SO₂ nonattainment area attained the 2010 1-hour SO₂ NAAQS by the October 4, 2018 attainment date by evaluating all available technical information and data relevant to the SO₂ air quality ( e.g., emissions, monitoring, meteorological data, and modeling) in the Muscatine, Iowa, area. Based on the analysis and information presented in this document and the TSD contained in ( printed page 3963) the docket for this action, the EPA proposes to determine that the Muscatine SO₂ NAA attained the 2010 1-hour SO₂ standard by the applicable attainment date of October 4, 2018, consistent with CAA section 179(c)(1).

In addition, this action, if finalized, will address EPA's obligation under a consent decree in Center for Biological Diversity, et al. v. Regan, which establishes a deadline of March 31, 2022 for the EPA to determine under CAA section 179(c) whether the Muscatine County SO₂ nonattainment area attained the NAAQS by the October 4, 2018 attainment date.^[8]

This proposed action does not constitute a redesignation of the Muscatine SO₂ NAA to attainment for the 2010 1-hour SO₂ NAAQS under CAA section 107(d)(3) because we have not yet approved a maintenance plan for the area as meeting the requirements of section 175A of the CAA and have not determined that the area has met the other CAA section 107(d)(3)(E) requirements for redesignation. The classification and designation status in 40 CFR part 81 will remain nonattainment until the EPA has determined that Iowa has met the CAA requirements for redesignation to attainment for the Muscatine SO₂ NAA.

This is a proposed action and we are soliciting comments on this proposed action. Final rulemaking will occur after consideration of any comments.

V. Environmental Justice Concerns

When the EPA establishes a new or revised NAAQS, the CAA requires the EPA to designate all areas of the U.S. as either nonattainment, attainment, or unclassifiable. Area designations address environmental justice concerns by ensuring that the public is properly informed about the air quality in an area.

The EPA utilized the EJSCREEN tool to evaluate environmental and demographic indicators within the area. The tool outputs report is contained in the docket for this action. While the EPA's EJSCREEN tool demonstrates that demographic indicators are consistent or lower than national averages, there are vulnerable populations in the area including low-income populations and persons over 64 years of age.

This action addresses EPA's determination, as required by the CAA, of whether the Muscatine County, Iowa, area attained the 2010 1-hour SO₂ NAAQS by the relevant attainment date. This action proposes to determine an area has attained the NAAQS by the relevant attainment date, but it does not change the geographic status of the area nor does it impose additional or modify existing requirements on sources. Based on the information presented in this document and the associated technical support document, the EPA is proposing to determine that the air quality in the Muscatine County area is attaining the NAAQS. For these reasons, this proposed action does not result in disproportionately high and adverse human health or environmental effects on minority populations, low-income populations and/or indigenous peoples.

VI. Statutory and Executive Order Reviews

This action proposes to determine an area has attained the NAAQS by the relevant attainment date and does not impose additional or modify existing requirements. For that reason, this action:

• Is not a significant regulatory action subject to review by the Office of Management and Budget under Executive Orders 12866 (58 FR 51735, October 4, 1993) and 13563 (76 FR 3821, January 21, 2011);
• 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 an economically significant regulatory action based on health or safety risks subject toExecutive Order 13045 (62 FR 19885, April 23, 1997);
• Is not a significant regulatory action subject toExecutive Order 13211 (66 FR 28355, May 22, 2001);
• Is not subject to requirements of the National Technology Transfer and Advancement Act (NTTA) because this rulemaking does not involve technical standards; and
• This action does not have disproportionately high and adverse human health or environmental effects on minority populations, low-income populations and/or indigenous peoples, as specified inExecutive Order 12898 (59 FR 7629, February 16, 1994). The basis for this determination is contained in section V of this action, “Environmental Justice Concerns.”

For the reasons stated in the preamble, the EPA proposes to amend 40 CFR part 52 as set forth below:

1. The authority citation for part 52 continues to read as follows:

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

2. Revise § 52.834 to read as follows:

87 FR 3958