Document ID: EPA-R05-OAR-2018-0841-0001
Agency: epa
Document Type: Proposed Rule
Title: Proposed approval of the Illinois Alton Township 2010 SO2 Attainment Plan
Posted Date: 2022-12-30T05:00Z

[Federal Register Volume 87, Number 250 (Friday, December 30, 2022)]
[Proposed Rules]
[Pages 80509-80519]
From the Federal Register Online via the Government Publishing Office [www.gpo.gov]
[FR Doc No: 2022-28158]

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ENVIRONMENTAL PROTECTION AGENCY

40 CFR Part 52

[EPA-R05-OAR-2018-0841; FRL-10489-01-R5]

Air Plan Approval; Illinois; Alton Township 2010 SO2 Attainment 
Plan

AGENCY: Environmental Protection Agency (EPA).

ACTION: Proposed rule.

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SUMMARY: The Environmental Protection Agency (EPA) is proposing to 
approve the State Implementation Plan (SIP) revision which Illinois 
submitted to EPA on December 31, 2018, for attaining the 2010 sulfur 
dioxide (SO2) primary national ambient air quality standard 
(NAAQS) for the Alton Township nonattainment area in Madison County. 
This plan (herein called a ``nonattainment plan'') includes Illinois' 
attainment demonstration and other elements required under the Clean 
Air Act (CAA), including the requirement for meeting reasonable further 
progress (RFP) toward attainment of the NAAQS, reasonably available 
control measures and reasonably available control technology (RACM/
RACT), base-year and projection-year emission inventories, enforceable 
emission limitations and control measures, nonattainment new source 
review (NNSR), and contingency measures. EPA is proposing to approve 
Illinois' submission as a SIP revision for attaining the 2010 primary 
SO2 NAAQS in the Alton township nonattainment area, finding 
that Illinois has adequately demonstrated that the plan provisions 
provide for attainment of NAAQS in the nonattainment area and that the 
plan meets the other applicable requirements under the CAA.

DATES: Comments must be received on or before January 30, 2023.

ADDRESSES: Submit your comments, identified by Docket ID No. EPA-R05-
OAR-2018-0841 at https://www.regulations.gov, or via email to 
[email protected]. For comments submitted at Regulations.gov, follow 
the

[[Page 80510]]

online instructions for submitting comments. Once submitted, comments 
cannot be edited or removed from Regulations.gov. For either manner of 
submission, 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, please contact the person identified in 
the FOR FURTHER INFORMATION CONTACT section. For the full EPA public 
comment policy, information about CBI or multimedia submissions, and 
general guidance on making effective comments, please visit https://www2.epa.gov/dockets/commenting-epa-dockets.

FOR FURTHER INFORMATION CONTACT: Andrew Lee, Physical Scientist, 
Attainment Planning and Maintenance Section, Air Programs Branch (AR-
18J), Environmental Protection Agency, Region 5, 77 West Jackson 
Boulevard, Chicago, Illinois 60604, (312) 353-7645, 
[email protected]. The EPA Region 5 office is open from 8:30 a.m. to 
4:30 p.m., Monday through Friday, excluding Federal holidays and 
facility closures due to COVID-19.

SUPPLEMENTARY INFORMATION: Throughout this document whenever ``we,'' 
``us,'' or ``our'' is used, we mean EPA.

Table of Contents

I. Why was Illinois required to submit an SO2 plan for 
the Alton township area?
II. Requirements for SO2 Nonattainment Area Plans
III. Attainment Demonstration and Longer-Term Averaging
IV. Review of Modeled Attainment Plan
    A. Model Selection and General Model Inputs
    B. Meteorological Data
    C. Modeled Emissions Data
    D. Emission Limits
    E. Background Concentrations
    F. Summary of Results
V. Review of Other Plan Requirements
    A. Emissions Inventory
    B. RACM/RACT and Emissions Limitations and Control Measures
    C. New Source Review (NSR)
    D. RFP
    E. Contingency Measures
VI. EPA's Proposed Action
VII. Incorporation by Reference
VIII. Statutory and Executive Order Reviews

I. Why was Illinois required to submit an SO[bdi2] plan for the Alton 
township area?

    On June 22, 2010, EPA published a new 1-hour primary SO2 
NAAQS of 75 parts per billion (ppb), which is met at an ambient air 
quality monitoring site when the 3-year average of the annual 99th 
percentile of daily maximum 1-hour average concentrations does not 
exceed 75 ppb, as determined in accordance with appendix T of 40 CFR 
part 50. See 75 FR 35520, codified at 40 CFR 50.17(a)-(b). EPA has 
promulgated designations for this standard in four rounds. Alton 
Township, Illinois was designated nonattainment by EPA on June 30, 
2016, as part of the Agency's Round 2 designations.
    In the Round 2 designations, EPA designated areas including power 
plants exceeding certain emissions criteria, specifically including the 
Wood River power plant located in Wood River, Illinois. The modeling 
that Illinois submitted in support of its Round 2 designations 
recommendations included both the Wood River power plant and an 
additional source, the Alton Steel, Inc. steel mill in Alton, Illinois 
(Alton Steel). Alton Steel was included in the modeling analysis 
because its SO2 emissions showed the potential for creating 
significant SO2 concentration gradients within the modeling 
domain. The modeling was done using the AERMOD air dispersion modeling 
software utilizing data based on actual emissions from the Wood River 
Power Station and Alton Steel.
    The state found that the highest modeled NAAQS violations in the 
area were almost entirely due to Alton Steel emissions and especially 
occurred along or near Alton Steel's north fence line. The Alton Steel 
facility consists of a melt shop and a rolling mill in which steel 
scrap is melted (electric arc furnace), refined/alloyed (ladle 
metallurgical furnace), and then cast/formed into blooms and slabs. 
Illinois provided suitable evidence that Wood River should be judged 
not to contribute to the modeled violation as the facility was shut 
down in 2016. As such, Illinois recommended the designation of 
nonattainment for Alton Township to focus on the NAAQS violations 
caused by Alton Steel.
    The state's modeling in support of its designation recommendation 
indicated that the predicted 99th percentile 1-hour average 
concentration within the chosen modeling domain was 456.40 micrograms 
per cubic meter ([mu]g/m\3\), or 174.2 ppb. This modeled concentration 
included the background concentration of SO2 and was based 
on actual emissions from the facilities in the area. Illinois performed 
a culpability analysis which demonstrated that only a small group of 
receptors violated the 2010 SO2 NAAQS, and these receptors 
were primarily affected by emissions from Alton Steel, which were 
greatly influenced by downwash. High concentrations near Alton Steel 
were a consequence of building downwash combined with downward pointing 
vents, and primarily occurred when winds were blowing from the 
southwest, a direction that maximized the impact of the Alton Steel 
building in causing downwash and downwash-influenced concentrations in 
nearby ambient air locations.
    On September 18, 2015, Illinois submitted its recommendations for 
EPA to designate certain areas of the state as part of the Round 2 
designations. In its submission, Illinois recommended that a portion of 
Madison County be designated as nonattainment for the 2010 
SO2 NAAQS--specifically, a portion of southern Alton 
Township. EPA, agreeing with Illinois' analysis of the area, concurred 
with the state's proposed finding of nonattainment for Alton Township. 
EPA published a final action designating the area as nonattainment on 
July 12, 2016 (81 FR 45039), which became effective September 12, 2016. 
In response to EPA's designation of the Alton Township area, Illinois 
submitted an attainment plan on December 13, 2018, to EPA for approval. 
Under CAA section 192(a), these plans are required to demonstrate that 
their respective areas will attain the NAAQS as expeditiously as 
practicable, but no later than five years from the effective date of 
designation, which was September 12, 2021.
    Unlike in the Round 2 designations modeling, the Alton Township 
attainment demonstration does not include the Wood River Power Station 
among the sources modeled. Wood River was excluded from the 
nonattainment area because in November 2015, the facility owner 
(Dynegy, Inc.) publicly announced that the power plant would be 
closing, pending approval of the electrical transmission system 
operator (Midcontinent Independent System Operator). The facility was 
retired in June 2016 and ceased emitting SO2 at that point, 
and was demolished in February 2021.

[[Page 80511]]

II. Requirements for SO[bdi2] Nonattainment Area Plans

    Nonattainment area SO2 SIPs must meet the applicable 
requirements of the CAA, and specifically CAA sections 110, 172, 191 
and 192. EPA's regulations governing nonattainment area SIPs are set 
forth at 40 CFR part 51, with specific procedural requirements and 
control strategy requirements residing at subparts F and G, 
respectively. Soon after Congress enacted the 1990 amendments to the 
CAA, EPA issued comprehensive guidance on SIPs in a document entitled 
the ``General Preamble for the Implementation of Title I of the Clean 
Air Act Amendments of 1990,'' published at 57 FR 13498 (April 16, 1992) 
(General Preamble). Among other things, the General Preamble addressed 
SO2 SIPs and fundamental principles for SIP control 
strategies. Id. at 13545-49, 13567-68. On April 23, 2014, EPA issued 
guidance and recommendations for meeting the statutory requirements in 
SO2 SIPs addressing the 2010 primary NAAQS, in a document 
entitled, ``Guidance for 1-Hour SO2 Nonattainment Area SIP 
Submissions'' (April 2014 guidance), available at https://www.epa.gov/sites/production/files/2016-06/documents/20140423guidance_nonattainment_sip.pdf. In the April 2014 guidance, EPA 
described the statutory requirements for a complete nonattainment area 
SIP, which includes an accurate emissions inventory of current 
emissions for all sources of SO2 within the nonattainment 
area; an attainment demonstration; enforceable emissions limitations 
and control measures; demonstration of RFP; implementation of RACM 
(including RACT); NNSR; and adequate contingency measures for the 
affected area.
    In order for EPA to fully approve a SIP as meeting the requirements 
of CAA sections 110, 172 and 191-192 and EPA's regulations at 40 CFR 
part 51, the SIP for the affected area needs to demonstrate to EPA's 
satisfaction that each of the aforementioned requirements have been 
met. Under CAA sections 110(l) and 193, EPA may not approve a SIP that 
would interfere with any applicable requirement concerning NAAQS 
attainment and RFP, or any other applicable requirement, and no 
requirement in effect (or required to be adopted by an order, 
settlement, agreement, or plan in effect before November 15, 1990), in 
any area which is a nonattainment area for any air pollutant, may be 
modified in any manner unless it ensures equivalent or greater emission 
reductions of such air pollutant.

III. Attainment Demonstration and Longer-Term Averaging

    CAA section 172(c)(1) directs states with areas designated as 
nonattainment to demonstrate that the submitted plan provides for 
attainment of the NAAQS. 40 CFR part 51, subpart G further delineates 
the control strategy requirements that SIPs must meet, and EPA has long 
required that all SIPs and control strategies reflect the four 
fundamental principles of quantification, enforceability, 
replicability, and accountability. See General Preamble, at 13567-68. 
SO2 attainment plans must consist of two components: (1) 
emission limits and other control measures that assure implementation 
of permanent, enforceable and necessary emission controls, and (2) a 
modeling analysis which meets the requirements of 40 CFR part 51, 
appendix W which demonstrates that these emission limits and control 
measures provide for timely attainment of the primary SO2 
NAAQS as expeditiously as practicable, but by no later than the 
attainment date for the affected area. In all cases, the emission 
limits and control measures must be accompanied by appropriate methods 
and conditions to determine compliance with the respective emission 
limits and control measures, and must be quantifiable (i.e., a specific 
amount of emission reduction can be ascribed to the measures), fully 
enforceable (specifying clear, unambiguous and measurable requirements 
for which compliance can be practicably determined), replicable (the 
procedures for determining compliance are sufficiently specific and 
non-subjective so that two independent entities applying the procedures 
would obtain the same result), and accountable (source specific limits 
must be permanent and must reflect the assumptions used in the SIP 
demonstrations).
    EPA's April 2014 guidance recommends that the emission limits be 
expressed as short-term average limits (e.g., addressing emissions 
averaged over one or three hours), but also allows for emission limits 
with longer averaging times of up to 30 days so long as the state meets 
various suggested criteria. See April 2014 guidance, pp. 22 to 39. The 
guidance recommends that, should states and sources utilize a longer-
term average limit, the limit should be set at an adjusted level that 
reflects a stringency comparable to the 1-hour critical emission value 
shown to provide for attainment that the plan otherwise could have set 
as a 1-hour emission limit.
    Illinois' plan applies 1-hour average emission limits to Alton 
Steel. However, Illinois' plan also considers the impact of an 
additional facility that is about 12 kilometers from Alton Steel, 
namely Ameren's Portage des Sioux Power Center (``Sioux'' or ``Ameren-
Sioux'') in St. Charles County, Missouri, a facility that is subject to 
a 24-hour block average limit. Therefore, EPA is providing the 
following discussion of its rationale for approving the use of longer-
term average limits in plans designed to provide for attainment.
    The April 2014 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. In evaluating this option, 
EPA considered the nature of the standard, conducted detailed analyses 
of the impact of the use of 30-day average limits on the prospects for 
attaining the standard, and carefully reviewed how best to achieve an 
appropriate balance among the various factors that warrant 
consideration in judging whether a state's plan provides for 
attainment. See id.; see also id. at appendices B, C and D.
    As specified in 40 CFR 50.17(b), the 1-hour primary SO2 
NAAQS is met at an ambient air quality monitoring site when the 3-year 
average of the annual 99th percentile of daily maximum 1-hour average 
concentrations is less than or equal to 75 ppb. In a year with 365 days 
of valid monitoring data, the 99th percentile would be the fourth 
highest daily maximum 1-hour value. The 2010 SO2 NAAQS, 
including this form of determining compliance with the standard, was 
upheld by the U.S. Court of Appeals for the District of Columbia 
Circuit in Nat'l Envt'l Dev. Ass'n's Clean Air Project v. EPA, 686 F.3d 
803 (D.C. Cir. 2012). Because the standard has this form, a single 
exceedance of the level of the NAAQS does not create a violation of the 
standard. Instead, at issue is whether a source operating in compliance 
with a properly set limit reflecting a longer-term average could cause 
hourly exceedances of the NAAQS level, and if so the resulting 
frequency and magnitude of such hourly exceedances, and in particular 
whether EPA can have reasonable confidence that a properly set longer-
term average limit will provide that the 3-year average of the annual 
fourth highest daily maximum hourly value will be at or below 75 ppb. 
The following is a synopsis of EPA's review of how to judge whether 
such plans ``provide for

[[Page 80512]]

attainment,'' based on modeling of projected allowable emissions and in 
light of the NAAQS' form for determining attainment at monitoring 
sites.
    For plans for SO2 based on 1-hour emission limits, the 
standard approach is to conduct modeling using fixed emission rates. 
The maximum emission rate that would be modeled to result in attainment 
(i.e., in an ``average year'' \1\ shows three, not four days with 
maximum hourly levels exceeding 75 ppb, over three consecutive years) 
is labeled the ``critical emission value.'' The modeling process for 
identifying this critical emission value inherently considers the 
numerous variables that affect ambient concentrations of 
SO2, such as meteorological data, background concentrations, 
and topography. In the standard approach, the state would then provide 
for attainment by setting a continuously applicable 1-hour emission 
limit at this critical emission value. This is the approach Illinois 
took for setting limits at Alton Steel.
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    \1\ An ``average year'' is used to mean a year with average air 
quality. While 40 CFR 50 appendix T provides for averaging three 
years of 99th percentile daily maximum values (e.g., the fourth 
highest maximum daily concentration in a year with 365 days with 
valid data), this discussion and an example below uses a single 
``average year'' in order to simplify the illustration of relevant 
principles.
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    EPA recognizes that some sources have highly variable emissions, 
for example due to variations in fuel sulfur content and operating 
rate, that can make it extremely difficult, even with a well-designed 
control strategy, to ensure in practice that emissions for any given 
hour do not exceed the critical emissions value. EPA also acknowledges 
the concern that longer-term emission limits can allow short periods 
with emissions above the critical emissions value, which, if coincident 
with meteorological conditions conducive to high SO2 
concentrations, could in turn create the possibility of a NAAQS level 
exceedance occurring on a day when an exceedance would not have 
occurred if emissions were continuously controlled at the level 
corresponding to the critical emissions value. However, for several 
reasons, EPA believes that the approach recommended in its guidance 
document suitably addresses this concern. First, from a practical 
perspective, EPA expects the actual emission profile of a source 
subject to an appropriately set longer-term average limit to be like 
the emission profile of a source subject to an analogous 1-hour average 
limit. EPA expects this similarity because it has recommended that the 
longer-term average limit be set at a level that is comparably 
stringent to the otherwise applicable 1-hour limit (reflecting a 
downward adjustment from the critical emissions value) and that takes 
the source's emissions profile into account. As a result, EPA expects 
either form of emissions limit to yield comparable air quality.
    Second, from a more theoretical perspective, EPA has compared the 
likely air quality with a source having maximum allowable emissions 
under an appropriately set longer-term limit, as compared to the likely 
air quality with the source having maximum allowable emissions under 
the comparable 1-hour limit. In this comparison, in the 1-hour average 
limit scenario, the source is presumed at all times to emit at the 
critical emissions level, and in the longer-term average limit 
scenario, the source is presumed occasionally to emit more than the 
critical emissions value but on average, and presumably at most times, 
to emit well below the critical emissions value. In an ``average 
year,'' compliance with the 1-hour limit is expected to result in three 
exceedance days (i.e., three days with an hourly value above 75 ppb) 
and a fourth day with a maximum hourly value at 75 ppb. By comparison, 
with the source complying with a longer-term limit, it is possible that 
additional exceedances would occur that would not occur in the 1-hour 
limit scenario (if emissions exceed the critical emissions value at 
times when meteorology is conducive to poor air quality). However, this 
comparison must also factor in the likelihood that exceedances that 
would be expected in the 1-hour limit scenario would not occur in the 
longer-term limit scenario. This result arises because the longer-term 
limit requires lower emissions most of the time (because the limit is 
set well below the critical emissions value), so a source complying 
with an appropriately set longer term limit is likely to have lower 
emissions at critical times than would be the case if the source were 
emitting as allowed with a 1-hour limit.
    As a hypothetical example to illustrate these points, suppose a 
source that always emits 1,000 pounds of SO2 per hour, which 
results in air quality at the level of the NAAQS (i.e., results in a 
design value of 75 ppb). Suppose further that in an ``average year,'' 
these emissions cause the 5 highest maximum daily average 1-hour 
concentrations to be 100 ppb, 90 ppb, 80 ppb, 75 ppb, and 70 ppb. Then 
suppose that the source becomes subject to a 30-day average emission 
limit of 700 pounds per hour. It is theoretically possible for a source 
meeting this limit to have emissions that occasionally exceed 1,000 
pounds per hour, but with a typical emissions profile, emissions would 
much more commonly be between 600 and 800 pounds per hour. This 
simplified example assumes a zero-background concentration, which 
allows one to assume a linear relationship between emissions and air 
quality. (A nonzero background concentration would make the mathematics 
more difficult but would give similar results.) Air quality will depend 
on what emissions happen on what critical hours, but suppose that 
emissions at the relevant times on these 5 days are 800 pounds per 
hour, 1,100 pounds per hour, 500 pounds per hour, 900 pounds per hour, 
and 1,200 pounds per hour, respectively. (This is a conservative 
example because the average of these emissions, 900 pounds per hour, is 
well over the 30-day average emission limit.) These emissions would 
result in daily maximum 1-hour concentrations of 80 ppb, 99 ppb, 40 
ppb, 67.5 ppb, and 84 ppb. In this example, the fifth day would have an 
exceedance that would not otherwise have occurred, but the third day 
would not have an exceedance that otherwise would have occurred, and 
the fourth day would have been below, rather than at, 75 ppb. In this 
example, the fourth highest maximum daily concentration under the 30-
day average would be 67.5 ppb.
    This simplified example encapsulates the findings of a more 
complicated statistical analysis that EPA conducted using a range of 
scenarios using actual plant data. As described in appendix B of EPA's 
April 2014 guidance, EPA found that the requirement for lower average 
emissions is highly likely to yield better air quality than is required 
with a comparably stringent 1-hour limit. Based on analyses described 
in appendix B of its 2014 guidance, EPA expects that an emissions 
profile with maximum allowable emissions under an appropriately set, 
comparably stringent 30-day average limit is likely to have the net 
effect of having a lower number of hourly exceedances of the NAAQS 
level and better air quality than an emission profile with maximum 
allowable emissions under a 1-hour emission limit at the critical 
emissions value.\2\ This

[[Page 80513]]

result provides a compelling policy rationale for allowing the use of a 
longer averaging period, in appropriate circumstances where the facts 
indicate this result can be expected to occur.
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    \2\ See also further analyses described in rulemaking on the 
SO2 nonattainment plan for Southwest Indiana. In response 
to comments expressing concern that the emissions profiles analyzed 
for appendix B represented actual rather than allowable emissions, 
EPA conducted additional work formulating sample allowable emission 
profiles and analyzing the resulting air quality impact. These 
analyses provided further support for the conclusion that an 
appropriately set longer term average emission limit in appropriate 
circumstances can suitably provide for attainment. The rulemaking 
describing these further analyses was published on August 17, 2020, 
at 85 FR 49967, available at https://www.govinfo.gov/content/pkg/FR-2020-08-17/pdf/2020-16044.pdf. A more detailed description of these 
analyses is available in the docket for that action, specifically at 
https://www.regulations.gov/document?D=EPA-R05-OAR-2015-0700-0023.
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    The question then becomes whether this approach--which is likely to 
produce a lower number of overall hourly NAAQS level exceedances even 
though it may produce some unexpected exceedances above the critical 
emission value--meets the requirement in section 110(a)(1) and 
172(c)(1) for state implementation plans to ``provide for attainment'' 
of the NAAQS. For SO2, as for other pollutants, it is 
generally impossible to design a nonattainment plan in the present that 
will guarantee that attainment will occur in the future. A variety of 
factors can cause a well-designed attainment plan to fail and 
unexpectedly not result in attainment, for example if meteorology 
occurs that is more conducive to poor air quality than was anticipated 
in the plan. Therefore, in determining whether a plan meets the 
requirement to provide for attainment, EPA's task is commonly to judge 
not whether the plan provides absolute certainty that attainment will 
in fact occur, but rather whether the plan provides an adequate level 
of confidence of prospective NAAQS attainment. From this perspective, 
in evaluating use of a 30-day average limit, EPA must weigh the likely 
net effect on air quality. Such an evaluation must consider the risk 
that occasions with meteorology conducive to high concentrations will 
have elevated emissions leading to NAAQS level exceedances that would 
not otherwise have occurred and must also weigh the likelihood that the 
requirement for lower emissions on average will result in days not 
having hourly exceedances that would have been expected with emissions 
at the critical emissions value. Additional policy considerations, such 
as in this case the desirability of accommodating real world emissions 
variability without significant risk of NAAQS violations, are also 
appropriate factors for EPA to weigh in judging whether a plan provides 
a reasonable degree of confidence that the plan will lead to 
attainment. Based on these considerations, especially given the high 
likelihood that a continuously enforceable limit averaged over as long 
as 30 days, determined in accordance with EPA's guidance, will result 
in attainment, EPA believes as a general matter that such limits, if 
appropriately determined, can reasonably be considered to provide for 
attainment of the 2010 SO2 NAAQS.
    The April 2014 guidance offers specific recommendations for 
determining an appropriate longer-term average limit. The recommended 
method starts with determination of the 1-hour emission limit that 
would provide for attainment (i.e., the critical emissions value), and 
applies an adjustment factor to determine the (lower) level of the 
longer-term average emission limit that would be estimated to have a 
stringency comparable to the otherwise necessary 1-hour emission limit. 
This method uses a database of continuous emission data reflecting the 
type of control that the source will be using to comply with the SIP 
emission limits, which (if compliance requires new controls) may 
require use of an emission database from another source. The 
recommended method involves using these data to compute a complete set 
of emission averages, computed according to the averaging time and 
averaging procedures of the prospective emissions limit. In this 
recommended method, the ratio of the 99th percentile among these long-
term averages to the 99th percentile of the 1-hour values represents an 
adjustment factor that may be multiplied by the candidate 1-hour 
emission limit to determine a longer-term average emission limit that 
may be considered comparably stringent.\3\ The guidance also addresses 
a variety of related topics, such as the potential utility of setting 
supplemental emission limits, such as mass-based limits, to reduce the 
likelihood and/or magnitude of elevated emission levels that might 
occur under the longer-term emission rate limit.
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    \3\ For example, if the critical emission value is 1,000 pounds 
of SO2 per hour, and a suitable adjustment factor is 
determined to be 70 percent, the recommended longer term average 
limit would be 700 pounds per hour.
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    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. Supplemental 
guidance on modeling for purposes of demonstrating attainment of the 
SO2 standard is provided in appendix A to the April 2014 
guidance document referenced above. Appendix A provides extensive 
guidance on the modeling domain, the source inputs, assorted types of 
meteorological data, and background concentrations. Consistency with 
the recommendations in this guidance is generally necessary for the 
attainment demonstration to offer adequately reliable assurance that 
the plan provides for attainment.
    As stated previously, attainment demonstrations for the 2010 1-hour 
primary SO2 NAAQS must 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 SO2 
NAAQS. For a short-term (i.e., 1-hour) standard, EPA believes that 
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 SO2.
    The meteorological data used in the analysis should generally be 
processed with the most recent version of AERMET. Estimated 
concentrations should include ambient background concentrations, should 
follow the form of the standard, and should be calculated as described 
in section 2.6.1.2 of the August 23, 2010, clarification memo on 
``Applicability of Appendix W Modeling Guidance for the 1-hr 
SO2 National Ambient Air Quality Standard'' (U.S. EPA, 
2010).

IV. Review of Modeled Attainment Plan

    This section generally discusses EPA's evaluation of the modeled 
attainment demonstration for Illinois' plan. A more detailed discussion 
is also presented in a technical support document (TSD) contained in 
the public docket for this proposed approval of Illinois' SIP.

A. Model Selection and General Model Inputs

    As part of its SIP development process, Illinois used EPA's 
regulatory dispersion model, AERMOD, to help determine the 
SO2 emission limit

[[Page 80514]]

revisions that would be needed to bring the Alton Township 
nonattainment area into attainment of the 2010 SO2 NAAQS. 
For its 2018 Alton Township attainment plan, Illinois has relied upon 
AERMOD Version 18081 and the companion AERMOD User Guide documentation 
in developing this attainment demonstration. Regulatory default options 
were specified in developing the attainment demonstration that are 
consistent with established practices for use of AERMOD in determining 
NAAQS compliance for SIP revisions. Included among those default 
options are stack tip downwash, buoyancy induced dispersion, default 
wind profile coefficients, default vertical potential temperature 
gradients, and final plume rise. EPA finds these selections 
appropriate.
    This attainment demonstration uses a modeling domain that reflects 
the geographic extent of emission sources included in the Round 2 
modeling for the Wood River Power Plant. The most significant sources 
addressed in the modeling for the area are the Alton Steel facility and 
the Ameren-Sioux power center in Missouri about 13 kilometers west-
northwest of the nonattainment area. These two facilities are the 
principal causes of the modeled violations in the area. Illinois 
modeled several other, relatively minor sources within the area that 
did not contribute significantly to the violation. Illinois performed a 
culpability analysis to quantify the impacts of these various minor 
sources to determine their contribution to the modeled violations. At 
the highest concentrations the model estimated in the area, all other 
sources combined, aside from Ameren-Sioux and Alton Steel, contributed 
less than 2 [mu]g/m\3\ in total to the modeled violations. The way 
these sources are modeled are discussed in detail below.
    The receptor network encompasses the nonattainment area and 
consists of discrete fence line receptors spaced at approximately 50-
meter intervals and a gridded receptor array with 100-meter interval 
spacings. The receptor density is consistent with standard modeling 
guidance for adequately capturing and resolving SO2 
concentration maxima. See TSD pg. 3.
    Selection of terrain data corresponds to the geographic area 
represented by the Alton Township nonattainment area, as well as the 
locations of facilities nearby that influence concentrations in the 
area. U.S. Geological Survey (USGS) National Elevation Dataset (NED) 
data were obtained in an appropriate format for use in AERMAP and used 
for generating the necessary terrain inputs. Elevations from the NED 
data were determined for all sources and structures, and both 
elevations and representative hill heights were determined for 
receptors.
    A detailed site characterization of the Alton Steel facility, 
Ameren-Sioux power center, and pertinent other sources provided 
dimensional and locational data for structures and stacks necessary for 
addressing building-induced plume downwash. Stacks constructed to less 
than good engineering practice (GEP) height and within the ``zone of 
influence'' of a nearby structure have plumes that are potentially 
subject to excessive downwash. Illinois used EPA's Building Profile 
Input Program with PRIME algorithm (BPIPPRM, version 04274) to generate 
direction-specific building parameters for modeling building wake 
effects. The location and height of each stack and flare to be 
evaluated, and the locations and heights of nearby structures, were 
processed in BPIPPRM to produce the building parameters required by 
AERMOD.
    Most of the stacks modeled by Illinois are modeled at heights that 
BPIPPRM considers to be at or below GEP height. However, two sources in 
this analysis were modeled by Illinois with stacks above GEP height. 
The stack at the Ameren-Sioux facility is constructed above GEP height 
and was modeled by Illinois at actual height. Additionally, at WRB 
Refining, several stacks have been constructed with heights above GEP 
height and were modeled at the actual stack height and at full 
potential to emit. WRB Refining, despite being modeled above GEP 
height, is not considered a significant contributor to the violations 
in the area. Illinois performed a culpability analysis and concluded 
that WRB has a very low contribution, less than 1 [mu]g/m\3\ in all 
modeled scenarios, to the modeled violations. As such, Illinois 
modeling that facility at GEP height would change little about the 
principal sources of SO2 pollution in the area. Ameren-Sioux 
was modeled at above GEP height and was determined to be a significant 
contributor to the violations in the area. EPA has conducted 
supplemental modeling to correct any deficiencies in Illinois's 
modeling related to the characterization of emissions in the area. EPA 
used Illinois' receptor grid, meteorological surface and upper air 
stations, model settings, and some source parameters to develop the 
modeling demonstration. EPA is relying on our supplemental modeling to 
support the attainment plan and establish that the area is now modeling 
attainment. See TSD pg. 6. More discussion on this topic is included in 
the sections below.

B. Meteorological Data

    Procedures for selecting and developing meteorological data have 
been provided in the draft document ``Regional Meteorological Data 
Processing Protocol, EPA Region 5 and States.'' \4\ This document 
describes selection criteria for surface meteorological data that 
address the representativeness of the meteorological data collection 
site to the emission source/receptor impact area. There are two 
specific criteria to be considered: (1) the suitability of 
meteorological data for the study area, and (2) the similarity of 
surface conditions and surroundings at the emission source/receptor 
impact area compared to characteristics at the location of the 
meteorological instrumentation tower.
---------------------------------------------------------------------------

    \4\ Draft--Regional Meteorological Data Processing Protocol. EPA 
Region 5 and States (August 2014), available in the docket for this 
action.
---------------------------------------------------------------------------

    In its 2018 submission, Illinois used the then-most recent five 
years (2012-2016) of surface meteorological data from St. Louis, 
Missouri (WBAN No. 13994, 28 kilometers to the southwest) and 
coincident upper air data from Lincoln, Illinois (WBAN No. 4833, 157 km 
to the northeast). These data were determined to be representative of 
the NAA's airshed. These data, in combination with surface 
characteristics data, were processed using AERSURFACE (version 13016) 
to prepare the meteorological data for simulating the area's planetary 
boundary layer turbulence structure. Illinois utilized AERMET (version 
16216) to process the raw meteorological data. Illinois obtained 
Automated Surface Observing Systems (ASOS) one-minute wind speed and 
wind direction data for NWS surface stations and processed it using 
AERMINUTE (version 15272). EPA utilized the meteorological data 
processed by Illinois in its supplemental modeling. See TSD pg. 13.
    The frequency and magnitude of wind speed and direction are defined 
in terms of where the wind is blowing from, parsed out in sixteen 22.5-
degree wind sectors. The predominant wind direction during the five-
year period is from the south, occurring approximately 9.8% of the 
time. The highest percentage wind speed range, occurring 34.5% of the 
time, was in the 3.6-5.7 meters per second range.

C. Modeled Emissions Data

    In its 2018 submittal, Illinois provided an analysis modeling other

[[Page 80515]]

SO2 sources in the area, including GBC Metals, Olin 
Corporation, National Maintenance & Repair, Alton Water Treatment 
Facility, Conoco-Phillips Hartford Plant, Alton Memorial Hospital, St. 
Anthony's Hospital, St. Claire's Hospital, the Charles E. Mahoney 
Plant, WRB Refinery, and most notably including the Alton Steel 
facility and the Ameren-Sioux facility. Data for detailed site 
characterization (stack locations, fence line locations, building 
dimensions, etc.) of these sources were gathered and/or generated to 
support development of specific AERMOD inputs. Illinois used EPA's 
Building Profile Input Program with PRIME algorithm (BPIPPRM, version 
04274) to generate direction-specific building inputs for modeling 
building wake effects within AERMOD. Building-induced plume downwash 
was addressed for all stacks and flares. The flares, all of which are 
located at WRB Refining, were modeled with adjusted release parameters 
including fixed values for temperature, exit velocity, and modified 
values for release height and diameter. Illinois relied upon the 
AERSCREEN User's Guide \5\ to calculate the effective height and 
diameter for modeling the flares. Following the submittal from 
Illinois, EPA performed a supplemental modeling run to evaluate changes 
in allowable emissions that occurred after Illinois submitted the 
attainment plan and to correct any deficiencies in the emissions data 
or source characterization that could potentially cause reduced 
concentrations. See TSD p. 2.
---------------------------------------------------------------------------

    \5\ AERSCREEN User's Guide. EPA-454/B-16-004. December 2016. 
U.S. Environmental Protection Agency, Research Triangle Park, NC.
---------------------------------------------------------------------------

    The most significant sources affecting the nonattainment area were 
Alton Steel and the Ameren-Sioux facility in Missouri. While the 
Ameren-Sioux facility is not in the nonattainment area, Illinois 
modeled this facility due to its proximity to the nonattainment area 
and its high SO2 emissions, yielding an impact of up to 
283.4 [mu]g/m\3\ on the air quality in the area. Illinois modeled 
numerous minor point sources in the nonattainment area as well. 
Illinois did not explicitly model emissions from non-point sources, for 
example mobile emissions, incineration, agricultural field burning, 
etc., in AERMOD but instead represented the impact of these sources via 
monitored background data.
    Illinois' SIP submittal describes an exploratory run that Illinois 
conducted in order to define the air quality problem in the area and to 
determine the most appropriate remedy. Notably, the baghouse at Alton 
Steel was originally configured to emit out of downward pointing vents, 
which Illinois modeled using the POINTHOR option in AERMOD to consider 
the horizontally pointing vents. Based on the results of these runs in 
which Alton Steel was the principal contributor to the highest modeled 
violations, Illinois chose to mandate construction of a single vertical 
unobstructed stack for this emission unit. Thus, Illinois' attainment 
demonstration modeling represented this emission point (and all other 
emission points) as a vertical unobstructed stack release. Flares were 
modeled with adjusted release parameters, consistent with EPA's 
guidance for modeling flares presented in the AERSCREEN User's 
Guide.\6\ The adjusted parameters include fixed values for temperature 
(1,273 degrees Kelvin) and exit velocity (20 meters/second) and 
modified values for release height and diameter.
---------------------------------------------------------------------------

    \6\ See supra n.5.
---------------------------------------------------------------------------

    Ameren-Sioux operates two coal-fired boilers. Illinois modeled this 
source using information provided by the Missouri Department of Natural 
Resources. Illinois' modeling indicated that the limit on Ameren-Sioux 
in Missouri's SIP of 4.8 lbs/MMBtu did not ensure attainment inside the 
Alton nonattainment area. Illinois' modeling run evaluating the impact 
of maximum allowable emissions from Ameren-Sioux also reflecting the 
reconfigured ladle metallurgy facility (LMF) stack for Alton Steel 
yielded a maximum predicted 99th percentile 1-hour average 
concentration of 298.5 [mu]g/m\3\, and Illinois concluded that scaling 
this result down to reflect a temporally representative operating rate 
(either a 60th or a 70th percentile rate) for Ameren-Sioux would also 
show violations.
    EPA conducted a supplemental modeling run to correct deficiencies 
in the characterization of emissions in Illinois's modeling. EPA 
evaluated the estimated concentrations based on application of a new 
limit of 7,342 lbs/hour averaged over a 24-hour block period on the 
Ameren-Sioux facility published on November 16, 2022 (87 FR 68634). The 
adopted new limit is substantially lower than the previous SIP limit of 
4.8 lbs/MMBtu. Each of the facility's two boilers are rated to have a 
maximum heat input capacity of 4,920 MMBtu/hr and when applied to the 
former rate limit, add up to an effective rate of 47,232 lbs/hour on a 
facility-wide basis. The newly adopted limit marks a significantly 
reduced emission rate for the facility. EPA's supplemental modeling was 
based on the modeling runs submitted by Illinois, which modeled maximum 
uncontrolled emissions limits for all sources at the time but did not 
consider the revised limit at Ameren-Sioux. EPA's supplemental model 
run revised the modeled emissions for Ameren-Sioux to reflect the new 
24-hour block limit and modeled the facility at GEP height.
    The revised limit on Ameren-Sioux is on a 24-hour block average 
basis. Much of EPA's 2014 guidance addresses the situation in which 
modeling is used to determine the 1-hour critical emissions value used 
to calculate a limit necessary to provide for attainment, in which an 
adjustment factor is determined and applied to identify a reduced 
longer-term average limit to correspond to the modeled 1-hour value. 
The comparable stringency methodology provided in the guidance could 
also be utilized to estimate a 1-hour emission rate that may be used in 
a dispersion modeling run. Specifically, a preexisting longer-term 
average limit can be divided by the appropriate adjustment factor to 
determine an hourly modeled emission rate that is commensurate with the 
longer-term limit. Application of an adjustment factor means modeling 
this source using an hourly emission rate to which the 24-hour block 
limit established in Missouri's SIP is comparably stringent.
    In EPA's supplemental modeling run, the emissions from Boilers 1 
and 2 were treated as merged for a combined emissions rate from Ameren-
Sioux. EPA's stack height regulations restrict the circumstances under 
which plume merging is creditable. Under 40 CFR 51.100(hh), plume 
merging is defined to be a prohibited dispersion technique except, in 
the case of merging occurring after July 8, 1985, for cases in which 
such merging is part of a change in operation at the facility that 
includes the installation of pollution controls and is accompanied by a 
net reduction in the allowable emissions of a pollutant. (See 40 CFR 
51.100(hh)(2)(B)). The stack height regulations also note that this 
exclusion from the definition of dispersion techniques shall apply only 
to the emission limitation for the pollutant affected by such change in 
operation. To reduce its SO2 emissions, Ameren-Sioux began 
operation of flue gas desulfurization of the emissions from Boilers 1 
and 2 on November 15, 2010, and October 26, 2010, respectively. The 
construction of the new stack to vent the emissions from these units 
was part of the same project as installation of flue gas 
desulfurization equipment. Although Missouri did not adjust its SIP 
emission limit to reflect the reduction of allowable emissions

[[Page 80516]]

until several years after the installation of the pollution controls, 
the merging accompanied the installation of controls and may also be 
considered to accompany a net reduction in allowable emissions because 
the initial request for credit for merging was accompanied by a limit 
that required the net emission reduction that the Ameren-Sioux control 
project achieved. See TSD at 5.
    The final SO2 emission rate modeled for the merged 
Boilers 1 and 2 stack at Ameren-Sioux was 10,301.669 lbs/hr (1,297.988 
g/s). Based on guidance from the 2014 U.S. EPA's SO2 NAAQS 
Designations Modeling Technical Assistance Document, a ratio of 1-hour 
to 24-hour block average 99th percentile SO2 emission rates 
in lbs/hr were calculated using data collected from 2016-2020. This 
resulted in an adjustment factor of 2,007 lbs/hr/2,816 lbs/hr = 0.7127. 
When the adjustment factor of 0.7127 is applied to the 24-hour block 
limit of 7,342 lbs/hr, a 1-hour emission rate to which the longer-term 
limit would be comparably stringent to would be 10,301.669 lbs/hr. The 
merged stack was modeled using the GEP stack height of 145.41 meters.
    The other model inputs of EPA's supplemental run, i.e., receptor 
grid, background concentrations, meteorological data, and list of 
modeled sources, were consistent with the Illinois submitted modeling. 
Stack heights for the merged two vents at Ameren-Sioux and two stacks 
at WRB Refining were modified in the supplemental run to be consistent 
with GEP stack heights. The supplemental run used version 21112 of 
AERMOD. Results of these runs are described below.

D. Emission Limits

    A key element of Illinois' attainment plan is a change in Alton 
Steel's LMF exhaust configuration from the four downward-angled vents 
to a single 70-foot high, three-foot diameter stack with an 
unobstructed (no rain cap), vertically directed exhaust stream, which 
is represented in their final modeling. This change was mandated in 
Illinois' Construction Permit #18020009. As required by the 
construction permit, the SO2 emissions of this furnace shall 
not exceed 0.10 pound/ton of steel produced, 11.20 pounds per hour and 
37.50 tons per year. The first two of these limits apply on an hourly 
basis, such that Illinois' plan is designed to provide for attainment 
based on emission limits for the primary source in the area that apply 
every hour. Illinois is not relying on the limit on annual emissions to 
provide for attainment.
    An important prerequisite for approval of an attainment plan is 
that the emission limits that provide for attainment be quantifiable, 
fully enforceable, replicable, and accountable. See General Preamble at 
13567-68. The revised SO2 emission SIP limit at Ameren-Sioux 
is expressed as a 24-hour block average limit. Therefore, part of the 
review of Illinois' attainment plan must address the use of these 
limits, both with respect to the general suitability of using this 
limit for this purpose and with respect to whether the particular 
limits included in and/or credited by the plan have been suitably 
demonstrated to provide for attainment. The first subsection that 
follows addresses the enforceability of the limits in and/or credited 
by the plan, and the second subsection that follows addresses the 
credited 24-hour block limit.
1. Enforceability
    The change to Alton Steel's LMF exhaust configuration from the four 
downward-angled vents to a single 70-foot high, three-foot diameter 
stack with an unobstructed (no rain cap), vertically directed exhaust 
was mandated in Illinois Construction Permit #18020009, which is being 
incorporated into Illinois' SIP in the present action. This permitting 
action provides the federal enforceability supporting this portion of 
the attainment demonstration element of the revised SIP. As required by 
the construction permit, the SO2 emissions of this furnace 
shall not exceed 0.10 pound per ton of steel produced, 11.20 pounds per 
hour and 37.50 tons per year. EPA considers these emission limits and 
source configuration requirements, specified in Construction Permit 
Number #18020009, to be suitably enforceable. The facility must submit 
annual compliance certifications to ensure that the facility is 
meetings its SIP limits. Additionally, the facility must submit a semi-
annual Monitoring Report to the Illinois EPA, Air Compliance Section, 
summarizing required monitoring and identifying all instances of 
deviation from the permit. Stack testing must be done to verify the 
margin of compliance with the SO2 limit.
    For Ameren-Sioux, EPA has approved a more stringent 24-hour block 
limit submitted by Missouri that is aimed at reducing the facility's 
allowable emissions to levels that will allow the Alton nonattainment 
area to be modeled in attainment.\7\ Ameren-Sioux will be subject to 
the more restrictive limit of 7,342 lbs/hour of SO2 averaged 
over a 24-hour block period. Being a large coal fired EGU, the Ameren-
Sioux facility is required to monitor its release of SO2 via 
CEMS for other reasons such as the acid rain program and the Cross-
State Air Pollution Rule (CSAPR). This requirement also provides for a 
means to measure compliance at the source to ensure that the facility 
does not exceed its permanent and enforceable limit. To demonstrate 
compliance, Ameren must calculate the calendar day 24-hour block 
average emission for each unit subject to the facility wide emission 
limit. Unit level emission rates will then be summed together to 
determine a facility wide emission rate. Only valid operating hours 
will be included in the calculations for the daily emission rates. 
Valid operating hours include only hours that meet the primary 
equipment hourly operating requirements of 40 CFR 75.10(d). For 
example, if the source only meets 40 CFR 75.10(d) operational 
requirements for one hour in a particular 24-hour block period, the 
compliance with the emissions limit would be calculated by the total 
emissions divided by the one hour of operation that meets 40 CFR 
75.10(d). Therefore, any day with at least one hour that meets 
operational requirements will have a calculated block average that will 
be used to demonstrate compliance with the emissions limit. Hours when 
the units are experiencing startup, shutdown, or malfunction conditions 
will be used for the calculation if they meet the primary equipment 
hourly operating requirements of 40 CFR 75.10(d).
---------------------------------------------------------------------------

    \7\ See 87 FR 68634.
---------------------------------------------------------------------------

2. Longer-Term Average Limits
    As noted above, while Illinois considered only the 1-hour average 
limits it adopted for Alton Steel, EPA also considered the updated 24-
hour block limit approved into the Missouri SIP for the Ameren-Sioux 
facility. Therefore, the hypothetical critical emissions value to which 
Ameren-Sioux's 24-hour block average limit would be comparably 
stringent, and that is used in the attainment modeling for the area, 
would reflect an upward adjustment from the 7,342 lbs/hour averaged 
over a 24-hour block period. EPA conducted a site-specific analysis of 
variability at Ameren-Sioux using 2016-2020 CEMS data from EPA's Clean 
Air Markets Division's MySQL database, which was the most up to date 
information available at the time of analysis. EPA employed the method 
detailed in our 2014 guidance and used the historic 1-hour 99th 
percentile of SO2 emissions against the 99th percentile 24-
hour block average to derive an appropriate adjustment factor.

[[Page 80517]]

EPA determined that the adjustment factor for the Ameren-Sioux facility 
is 0.7127 and that it would be appropriate to apply this adjustment 
factor to Ameren-Sioux's long term averaging limit in order to estimate 
a 1-hour emission rate for modeling purposes. After applying the 
adjustment factor, EPA determined that a 1-hour emission rate used for 
modeling purposes would be 10,301.669 lbs/hour. EPA has determined 
through our supplemental modeling that an hourly emissions rate of 
10,301.669 lbs/hour is protective of the standard. As such, EPA 
determines that Ameren-Sioux's updated limit of 7,342 lbs/hour will 
provide for attainment in the nonattainment area.

E. Background Concentrations

    The Illinois demonstration of modeled attainment of the 2010 
SO2 NAAQS is based upon the combined impacts of facility-
specific emission rates together with monitored background 
concentrations integrated into the simulations. Regional sources not 
explicitly modeled in AERMOD, but which are contributors to ambient 
SO2 loadings within the nonattainment area, are represented 
via background monitoring data. In accordance with a ``Tier 2'' 
approach in EPA's guidance on background concentrations, Illinois 
identified separate background values for each hour of the day for each 
of the four seasons, for a total of 96 background values. Each of these 
values represents a three-year average (2014-2016) of the second 
highest hourly concentration for the applicable hour of the day for the 
applicable season. The seasonal, hourly-averaged 2014-2016 
SO2 background values for the attainment demonstration were 
developed from data collected at the East St. Louis monitor. See TSD at 
13. These values range from 6.81 to 27.4 ppb, with an average value of 
14.94 ppb.

F. Summary of Results

    Illinois evaluated many factors in their modeling runs to evaluate 
measures needed to ensure attainment in the area. In their modeling 
runs, Illinois indicated that the prior limit in Ameren-Sioux's 
Missouri's SIP did not ensure attainment. Illinois determined that the 
impact of maximum allowable emissions from Ameren-Sioux also reflecting 
the reconfigured LMF stack for Alton Steel yielded a maximum predicted 
99th percentile 1-hour average concentration of 298.5 [mu]g/m\3\, and 
Illinois concluded that scaling this result down to reflect a 
temporally representative operating rate (either a 60th or a 70th 
percentile rate) would also show violations.
    EPA concludes that Illinois' modeling is a suitable demonstration 
that its requirements in the new permit for Alton Steel and all other 
Illinois sources in the nonattainment area were properly addressed in 
the attainment plan. EPA's supplemental modeling has demonstrated that 
the updated 24-hour block limit for Ameren-Sioux of 7,342 lbs 
SO2/hr and the revised limits at Alton Steel provide for 
attainment. For reasons described above, EPA considers the limits 
relied upon in this plan to be permanent and enforceable. EPA's 
modeling suitably demonstrates that the Ameren-Sioux limit (in 
combination with requirements for Alton Steel) provides for attainment.
    As noted above, EPA conducted a supplementary modeling run to 
evaluate the Ameren-Sioux facility subject to the updated 7,342 lbs 
SO2/hr 24-hour block limit that is found in the Missouri 
SIP. Since this limit is evaluated on a 24-hour block basis, EPA 
applied a 71.27 percent adjustment factor, modeling a 1-hour emissions 
rate of 10,300.666 lbs SO2 per hour to which the 24-hour 
block limit is comparably stringent. The modeled design value from 
EPA's supplemental run was 196.2 [mu]g/m\3\, or 74.9 ppb. This run used 
GEP stack heights, which for two facilities were slightly lower than 
the heights Illinois modeled; a separate supplementary run without 
these corrections yielded essentially identical results. These results 
confirm Illinois' demonstration that with the applicability and 
creditability of revised limits for Alton Steel and Ameren-Sioux, 
Illinois' plan provides for attainment. EPA believes that this 24-hour 
block average emission limit, in combination with the requirements for 
Alton Steel, are suitable elements of a plan that appropriately 
provides for attainment.

V. Review of Other Plan Requirements

A. Emissions Inventory

    The Round 2 Wood River Study Area emission inventory was used as 
the starting point for creating the Alton Township NAA modeling 
inventory. A re-evaluation of sources was instituted, which reflected a 
shift in modeling focus from Dynegy's Wood River Power Station to the 
Alton Steel ``mini-mill.'' This re-evaluation was also driven by the 
need to address allowable emissions (for the SIP revision) rather than 
actual emissions (for an area designation recommendation).
    The emissions inventory and source emission rate data for an area 
serve as the foundation for air quality modeling and other analyses 
that enable states to: (1) estimate the degree to which different 
sources within a nonattainment area contribute to violations within the 
affected area; and (2) assess the prospects for attaining the standard 
based on alternative control measures. As noted above, the state must 
develop and submit to EPA a comprehensive, accurate, and current 
inventory of actual emissions from all sources of SO2 
emissions in each nonattainment area, as well as any sources located 
outside the nonattainment area which may affect attainment in the area. 
See CAA section 172(c)(3).
    Illinois provided a comprehensive, accurate, and current inventory 
of emissions of SO2 in and within 10 kilometers of the Alton 
township area. Illinois additionally examined whether any large sources 
beyond 10 kilometers of the nonattainment area might also have 
significant air quality impacts in the area, resulting in the addition 
of Ameren-Sioux to the inventory. By this means, Illinois has developed 
a thorough list of the sources with any potential to cause impacts that 
warrant including in the area's attainment modeling.
    Illinois included the sources of WRB Refining Inc. (formerly named 
ConocoPhillips), National Maintenance and Repair Inc., GBC Metals LLC 
(d/b/a Olin Brass), Olin Corporation, Alton Water Treatment Facility, 
ConocoPhillips Hartford Lubricant Plant, Alton Memorial Hospital, St. 
Anthony's Hospital, St. Clare's Hospital, and Charles E. Mahoney 
Company along with Alton Steel. The emission sources at Alton Steel, as 
well as those for many of the modeled nearby Illinois facilities, do 
not operate with variable loads but rather as ``on-off'' process 
operations, with the notable exception of Ameren-Sioux. The emissions 
inventory that Illinois submitted reflects actual emissions of these 
sources.

[[Page 80518]]

 Table 1--Alton Township NAA Modeling Inventory--Actual Alton Area 2017
                       SO2 Point Source Emissions
------------------------------------------------------------------------
                                                         Emission rate
                  Source description                    (tons per year)
------------------------------------------------------------------------
Alton Steel..........................................              45.39
National Maintenance & Repair........................               3.93
GBC Metals...........................................               0.64
Olin Corporation.....................................               0.12
Alton Water Treatment Facility.......................               2.40
Conoco Philips Hartford Lubricant Plant..............               0.00
Ameren-Sioux Power Center............................          2,722.267
Alton Memorial Hospital..............................               0.15
St. Anthony's Hospital...............................               1.67
St. Clare's Hospital.................................               0.02
Charles E. Mahoney...................................               4.70
WRB..................................................           1,494.59
Ardent Mills LLC.....................................              0.006
Bluff City Minerals ACQ LLC..........................               0.04
Precor Refining Group Inc............................              0.001
Linde LLC............................................              0.005
Apex Oil Co Inc......................................              0.014
Shell Oil Products US................................             0.0012
Koch Fertilizer LLC..................................             0.0042
------------------------------------------------------------------------

                      Table 2--Total SO2 Emissions
------------------------------------------------------------------------
                                                        Emissions (tons
                       Category                            per year)
------------------------------------------------------------------------
Non-EGU Point........................................           1,559.34
EGU Point............................................          2,722.267
Area.................................................            81.5196
On-Road Mobile.......................................            11.2065
Off-Road Mobile......................................            41.8851
                                                      ------------------
    Total............................................         4,415.9512
------------------------------------------------------------------------

B. RACM/RACT and Emissions Limitations and Control Measures

    Section 172(c)(1) of the CAA requires states to adopt and submit 
all RACM, including RACT, as needed to attain the standards as 
expeditiously as practicable. Section 172(c)(6) requires the SIP to 
contain enforceable emission limits and control measures necessary to 
provide for timely attainment of the standard. Illinois has required 
the principal contributor to the NAAQS violations, Alton Steel, to 
build a stack aimed at reducing the facility's contribution to the 
nonattainment area. Alton Steel built a stack to disperse emissions 
more appropriately from their facility; this change, along with 
establishment of suitable emission limits in their construction permit, 
along with the proposed limit on Ameren-Sioux to be found in the 
Missouri SIP, ensures that the area will attain the SO2 air 
quality standard. Consequently, consistent with EPA policy that 
reasonable measures do not extend beyond a set of measures that provide 
for attainment, Illinois asserts, and EPA concurs, that the state's 
plan satisfies requirements for RACM/RACT.

C. New Source Review (NSR)

    EPA approved Illinois' nonattainment new source review rules on 
December 17, 1992 (57 FR 59928); September 27, 1995 (60 FR 49780); and 
May 13, 2003 (68 FR 25504). These rules provide for appropriate new 
source review for SO2 sources undergoing construction or 
major modification in the Alton Township area without need for 
modification of the approved rules. Although these rules predated 
promulgation of the 2010 SO2 standards, these rules are 
written in a manner such that new sources within areas that become 
designated nonattainment for this new standard, such as the Alton 
Township area, become subject to these nonattainment new source review 
requirements. Therefore, this requirement has been met for this area.

D. RFP

    Section 172 of the CAA requires Illinois' Alton Township Attainment 
Plan SIP to provide for reasonable further progress toward attainment. 
For SO2 SIPs, which address a small number of affected 
sources, requiring expeditious compliance with attainment emission 
limits can address the RFP requirement. Alton Steel was required to 
complete its stack construction and meet its emission limits by 
December 31, 2018. For Ameren-Sioux, a new limit was approved into the 
Missouri SIP establishing a more stringent limit by establishing a 
limit of 7,342 lbs/hour averaged over a 24-hour block period. EPA 
approved Ameren-Sioux's new limit on November 16, 2022 (87 FR 68634) 
and is permanent and enforceable. EPA concludes that the timely 
requirements in the state's plan, including revised limits and 
construction of a 70-foot-tall stack for the Alton Steel facility and 
the SIP approved limit of Ameren-Sioux, represent implementation of 
control measures as expeditiously as practicable. This plan shows that 
Illinois can provide for attaining the standard. Accordingly, EPA 
proposes to find that Illinois' plan provides for RFP.

E. Contingency Measures

    Section 172 of the CAA requires that nonattainment plans include 
additional measures which will take effect if an area fails to meet RFP 
or fails to attain the standard by the attainment date. As noted above, 
EPA guidance describes special features of SO2 planning that

[[Page 80519]]

influence the suitability of alternative means of addressing the 
requirement in section 172(c)(9) for contingency measures for 
SO2. An appropriate means of satisfying this requirement is 
for the state to have a comprehensive enforcement program that 
identifies sources of violations of the SO2 NAAQS and for 
the state to undertake aggressive follow-up for compliance and 
enforcement. Illinois' plan provides for satisfying the contingency 
measure requirement in this manner for sources in the state. EPA 
concurs and proposes to approve Illinois' plan for meeting the 
contingency measure requirement in this manner.

VI. EPA's Proposed Action

    EPA is proposing to approve Illinois' submission as a SIP revision, 
which the state submitted to EPA on December 31, 2018, for attaining 
the 2010 SO2 NAAQS for the Alton Township nonattainment 
area. As part of this action, EPA is proposing to incorporate Illinois' 
Permit to Construct Number #18020009, applicable to Alton Steel, by 
reference into the SIP. The permit requires that Alton Steel operates a 
new LMF stack to replace the four downward facing vents on the 
individual compartments on the LMF stack. The SO2 emissions 
from the LMF stack must not exceed 0.10 pound per ton of steel 
produced, 11.20 pounds per hour, and 37.50 tons per year.
    This SO2 nonattainment plan includes Illinois' 
attainment demonstration for the Alton township SO2 
nonattainment area. Although Illinois did not explicitly model air 
quality based on Ameren-Sioux's updated limit, Illinois provided 
sufficient information and modeling to enable EPA to conduct 
additionally necessary supplemental modeling to demonstrate that the 
revised limit at the Alton Steel facility, that will drastically reduce 
any contributions from Illinois to the violations modeled in the NAA, 
and a lower limit imposed on Ameren-Sioux by Missouri would allow the 
area to meet the standard. Therefore, EPA concludes that the modeling 
in Illinois' plan, as supplemented by EPA, adequately demonstrates that 
the control requirements that apply to relevant sources in and near the 
area, including the revised 24-hour block SO2 limit for 
Ameren-Sioux, provide for attainment in the area. As previously 
explained, EPA conducted a confirmatory model run explicitly applying 
the more stringent limit at Ameren-Sioux, and factoring a historically 
representative adjustment factor, showing more directly that the 
measures in Illinois' plan as supplemented by this limit provide for 
attainment. This nonattainment plan also addresses requirements for 
emission inventories, RACT/RACM, RFP, and contingency measures. 
Illinois has previously addressed requirements regarding nonattainment 
area NSR. EPA has determined that Illinois' SO2 
nonattainment plan meets the applicable requirements of CAA sections 
172, 191, and 192. EPA is taking public comments for thirty days 
following the publication of this proposed action in the Federal 
Register. EPA will take these comments into consideration in our final 
action.

VII. Incorporation by Reference

    In this rule, EPA is proposing to include in a final EPA rule 
regulatory text that includes incorporation by reference. In accordance 
with requirements of 1 CFR 51.5, EPA is proposing to incorporate by 
reference the Illinois construction permit for Alton Steel, Inc., 
issued March 5, 2018, as described in section VI. of this preamble. EPA 
has made, and will continue to make, these documents generally 
available through www.regulations.gov and at the EPA Region 5 Office 
(please contact the person identified in the FOR FURTHER INFORMATION 
CONTACT section of this preamble for more information).

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. 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 
action merely approves state law as meeting Federal requirements and 
does not impose additional requirements beyond those imposed by state 
law. 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 in 
Executive Order 13132 (64 FR 43255, August 10, 1999);
     Is not an economically significant regulatory action based 
on health or safety risks subject to Executive Order 13045 (62 FR 
19885, April 23, 1997);
     Is not a significant regulatory action subject to 
Executive Order 13211 (66 FR 28355, May 22, 2001);
     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; and
     Does not provide EPA with the discretionary authority to 
address, as appropriate, disproportionate human health or environmental 
effects, using practicable and legally permissible methods, under 
Executive Order 12898 (59 FR 7629, February 16, 1994).
    In addition, the SIP is not approved to apply on any Indian 
reservation land or in any other area where EPA or an Indian tribe has 
demonstrated that a tribe has jurisdiction. In those areas of Indian 
country, the rule does not have tribal implications and will not impose 
substantial direct costs on tribal governments or preempt tribal law as 
specified by Executive Order 13175 (65 FR 67249, November 9, 2000).

List of Subjects in 40 CFR Part 52

    Environmental protection, Air pollution control, Incorporation by 
reference, Intergovernmental relations, Sulfur oxides.

    Dated: December 21, 2022.
Debra Shore,
Regional Administrator, Region 5.
[FR Doc. 2022-28158 Filed 12-29-22; 8:45 am]
BILLING CODE 6560-50-P