1

Technical Support Document

Illinois

Area Designations for the 2010 SO2 Primary National Ambient Air Quality Standard

Summary

Pursuant to section 107(d) of the Clean Air Act (CAA), the U.S. Environmental Protection 

Agency (EPA, or the Agency) must designate areas as either “unclassifiable,” “attainment,” or 

“nonattainment” for the 2010 one-hour sulfur dioxide (SO2) primary national ambient air quality 

standard (NAAQS). The CAA defines a nonattainment area as one that does not meet the 

NAAQS or that contributes to a violation in a nearby area. An attainment area is defined as any 

area other than a nonattainment area that meets the NAAQS. Unclassifiable areas are defined as 

those that cannot be classified on the basis of available information as meeting or not meeting the 

NAAQS.

Illinois submitted updated recommendations on September 18, 2015, ahead of a July 2, 2016, 

deadline for EPA to designate certain areas established by the U.S. District Court for the 

Northern District of California. This deadline is the first of three deadlines established by the 

court for EPA to complete area designations for the 2010 SO2 NAAQS. Table 1 below lists 

Illinois’ recommendations and identifies the counties or portions of counties in Illinois that EPA

intends to designate by July 2, 2016 based on an assessment and characterization of air quality 

through ambient air quality data, air dispersion modeling, other evidence and supporting 

information, or a combination of the above.

Table 1. Illinois’ Recommended and EPA’s Intended Designations

Area

Illinois’ 

Recommended 

Area Definition

Illinois’ 

Recommended 

Designation

EPA’s Intended 

Area Definition

EPA’s Intended 

Designation

Alton 

Township 

Area, Illinois

Within Alton 

Twp. of Madison 

Co.,

Area east of 

Corporal 

Belchik 

Memorial 

Expressway, 

south of East 

Broadway, south 

of Route 3, and 

north of

Route 143 

Nonattainment Same as State’s 

Recommendation Nonattainment
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Wood River 

Township 

Area, Illinois

Within Madison 

Co.,

All of Wood 

River Township, 

and the area 

north of Cahokia 

Diversion 

Channel in 

Chouteau 

Township.

Attainment Same as State’s 

Recommendation

Unclassifiable/ 

Attainment

Rest of 

Madison 

County, 

Illinois

Within Madison 

Co.:

Remainder of 

Madison 

County.

Unclassifiable Same as State’s 

Recommendation No designation 

Massac County

Area, Illinois Massac County Attainment Same as State’s Recommendation

Unclassifiable/ 

Attainment

Jasper County 

Area, Illinois

Jasper County Attainment Same as State’s 

Recommendation

Unclassifiable/ 

Attainment

Putnam/Bureau 

County Area, 

Illinois

Putnam County

and Bureau 

County

Attainment Same as State’s 

Recommendation

Unclassifiable/ 

Attainment

Williamson 

County Area, 

Illinois

Williamson 

County Attainment Same as State’s Recommendation Nonattainment

Background

On June 3, 2010, EPA revised the primary (health based) SO2 NAAQS by establishing a new 

one-hour standard at a level of 75 parts per billion (ppb) which is attained when the three-year 

average of the 99th percentile of one-hour daily maximum concentrations does not exceed 75 

ppb. This NAAQS was published in the Federal Register on June 22, 2010 (75 FR 35520) and is 

codified at 40 CFR 50.17. EPA determined this is the level necessary to protect public health 

with an adequate margin of safety, especially for children, the elderly, and those with asthma.

These groups are particularly susceptible to the health effects associated with breathing SO2. 

The two prior primary standards of 140 ppb evaluated over 24 hours, and 30 ppb evaluated over 

an entire year, codified at 40 CFR 50.4, remain applicable.

1 However, EPA is not currently 

designating areas on the basis of either of these two primary standards. Similarly, the secondary 

standard for SO2, set at 500 ppb evaluated over 3 hours has not been revised, and EPA is also not 

currently designating areas on the basis of the secondary standard.

 

1 40 CFR 50.4(e) provides that the two prior primary NAAQS will no longer apply to an area one year after its 

designation under the 2010 NAAQS, except that for areas designated nonattainment under the prior NAAQS as of 

August 22, 2010, and areas not meeting the requirements of a SIP Call under the prior NAAQS, the prior NAAQS 

will apply until that area submits and EPA approves a SIP providing for attainment of the 2010 NAAQS. The 

Illinois areas above are not subject to these exceptions.
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General Approach and Schedule

Section 107(d) of the Clean Air Act requires that not later than one year after promulgation of a 

new or revised NAAQS, state governors must submit their recommendations for designations 

and boundaries to EPA. Section 107(d) also requires EPA to provide notification to states no 

less than 120 days prior to promulgating an initial area designation that is a modification of a 

state’s recommendation. If a state does not submit designation recommendations, EPA will 

promulgate the designations that it deems appropriate. If a state or tribe disagrees with EPA’s 

intended designations, they are given an opportunity within the 120 day period to demonstrate 

why any proposed modification is inappropriate.

On August 5, 2013, EPA published a final rule establishing air quality designations for 29 areas 

in the United States for the 2010 SO2 NAAQS, based on recorded air quality monitoring data 

from 2009 - 2011 showing violations of the NAAQS (78 FR 47191). In that rulemaking, EPA

committed to address, in separate future actions, the designations for all other areas for which the 

Agency was not yet prepared to issue designations.

Following the initial August 5, 2013 designations, three lawsuits were filed against EPA in 

different U.S. District Courts, alleging the Agency had failed to perform a nondiscretionary duty 

under the CAA by not designating all portions of the country by the June 2013 deadline. In an 

effort intended to resolve the litigation in one of those cases, plaintiffs Sierra Club and the 

Natural Resources Defense Council and EPA filed a proposed consent decree with the U.S. 

District Court for the Northern District of California. On March 2, 2015, the court entered the 

consent decree and issued an enforceable order for EPA to complete the area designations 

according to the court-ordered schedule.

According to the court-ordered schedule, EPA must complete the remaining designations by 

three specific deadlines. By no later than July 2, 2016 (16 months from the court’s order), EPA

must designate two groups of areas: (1) areas that have newly monitored violations of the 2010 

SO2 NAAQS and (2) areas that contain any stationary sources that had not been announced as of 

March 2, 2015 for retirement and that according to EPA’s Air Markets Database emitted in 2012 

either (i) more than 16,000 tons of SO2 or (ii) more than 2,600 tons of SO2 with an annual 

average emission rate of at least 0.45 pounds of SO2 per one million British thermal units (lbs 

SO2/MMBTU). Specifically, a stationary source with a coal-fired unit that as of January 1, 2010 

had a capacity of over 5 megawatts and otherwise meets the emissions criteria, is excluded from 

the July 2, 2016 deadline if it had announced through a company public announcement, public 

utilities commission filing, consent decree, public legal settlement, final state or federal permit 

filing, or other similar means of communication, by March 2, 2015, that it will cease burning 

coal at that unit.

The last two deadlines for completing remaining designations are December 31, 2017, and 

December 31, 2020. EPA has separately promulgated requirements for states and other air 

agencies to provide additional monitoring or modeling information on a timetable consistent with 

these designation deadlines. We expect this information to become available in time to help 
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inform these subsequent designations. These requirements were promulgated on August 21, 

2015 (80 FR 51052), in a rule known as the SO2 Data Requirements Rule (DRR). 

 

Updated designations guidance was issued by EPA through a March 20, 2015 memorandum 

from Stephen D. Page, Director, U.S. EPA, Office of Air Quality Planning and Standards, to Air 

Division Directors, U.S. EPA Regions I-X. This memorandum supersedes earlier designation 

guidance for the 2010 SO2 NAAQS, issued on March 24, 2011, and it identifies factors that EPA

intends to evaluate in determining whether areas are in violation of the 2010 SO2 NAAQS. The 

guidance also contains the factors EPA intends to evaluate in determining the boundaries for all 

remaining areas in the country, consistent with the court’s order and schedule. These factors 

include: 1) Air quality characterization via ambient monitoring or dispersion modeling results; 2) 

Emissions-related data; 3) Meteorology; 4) Geography and topography; and 5) Jurisdictional 

boundaries. This guidance was supplemented by two technical assistance documents intended to

assist states and other interested parties in their efforts to characterize air quality through air 

dispersion modeling or ambient air quality monitoring for sources that emit SO2. Notably, EPA

released its most recent versions of documents titled, “SO2 NAAQS Designations Modeling 

Technical Assistance Document” (Modeling TAD) and “SO2 NAAQS Designations SourceOriented Monitoring Technical Assistance Document” (Monitoring TAD) in December 2013.

Based on ambient air quality data collected between 2012 and 2014, no violations of the 2010 

SO2 NAAQS have been recorded in any undesignated part of Illinois.

2 However, there are five 

sources in the state meeting the emissions criteria of the consent decree for which EPA must 

complete designations by July 2, 2016. In this draft technical support document, EPA discusses 

its review and technical analysis of Illinois’ updated recommendations for the areas that we must 

designate. EPA also discusses any intended modifications from the state’s recommendation 

based on all available data before us.

The following are definitions of important terms used in this document: 

1) 2010 SO2 NAAQS – The primary NAAQS for SO2 promulgated in 2010. This NAAQS 

is 75 ppb, based on the three year average of the 99th percentile of the annual distribution 

of daily maximum one-hour average concentrations. See 40 CFR 50.17. 

2) Design Value - a statistic computed according to the data handling procedures of the 

NAAQS (in 40 CFR part 50 Appendix T) that, by comparison to the level of the NAAQS, 

indicates whether the area is violating the NAAQS.

3) Designated nonattainment area – an area which EPA has determined has violated the 

2010 SO2 NAAQS or contributed to a violation in a nearby area. A nonattainment 

 

2 For designations based on ambient air quality monitoring data that violates the 2010 SO2 NAAQS, the consent 

decree directs EPA to evaluate data collected between 2013 and 2015. Absent complete, quality assured and 

certified data for 2015, the analyses of applicable areas for EPA’s intended designations will be informed by data 

collected between 2012 and 2014. States with monitors that have recorded a violation of the 2010 SO2 NAAQS 

during these years have the option of submitting complete, quality assured and certified data for calendar year 2015 

by April 19, 2016 to EPA for evaluation. If, after our review, the ambient air quality data for the area indicates that 

no violation of the NAAQS occurred between 2013 and 2015, the consent decree does not obligate EPA to complete 

the designation. Instead, we intend to designate the area and all other previously undesignated areas in the state on a 

schedule consistent with the prescribed timing of the court order, i.e., by December 31, 2017, or December 31, 2020. 
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designation reflects considerations of state recommendations and all of the information 

discussed in this document. EPA’s decision is based on all available information 

including the most recent 3 years of air quality monitoring data, available modeling 

analysis, and any other relevant information. 

4) Designated unclassifiable area – an area which EPA cannot determine based on all 

available information whether or not it meets the 2010 SO2 NAAQS. 

5) Designated unclassifiable/attainment area – an area which EPA has determined to have 

sufficient evidence to find either is attaining or is likely to be attaining the NAAQS.

EPA’s decision is based on all available information including the most recent 3 years of 

air quality monitoring data, available modeling analysis, and any other relevant 

information. 

6) Modeled violation – a violation based on air dispersion modeling.

7) Recommended attainment area – an area a state or tribe has recommended that EPA

designate as attainment. 

8) Recommended nonattainment area – an area a state or tribe has recommended that EPA

designate as nonattainment. 

9) Recommended unclassifiable area – an area a state or tribe has recommended that EPA

designate as unclassifiable.

10)Recommended unclassifiable/attainment area – an area a state or tribe has recommended 

that EPA designate as unclassifiable/attainment.

11) Violating monitor – an ambient air monitor meeting all methods, quality assurance and 

siting criteria and requirements whose valid design value exceeds 75 ppb, based on data 

analysis conducted in accordance with Appendix T of 40 CFR part 50. 
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Technical Analysis for the Madison County, Illinois Area

Introduction

Madison County, Illinois contains a stationary source that according to EPA’s Air Markets 

Database emitted in 2012 either more than 16,000 tons of SO2 or more than 2,600 tons of SO2

and had an annual average emission rate of at least 0.45 pounds of SO2 per one million British 

thermal units (lbs SO2/MMBTU). As of March 2, 2015, this stationary source had not met the 

specific requirements for being “announced for retirement.” Specifically, in 2012, the Wood 

River Power Station (“Wood River”) emitted 6,756 tons of SO2, and had an emissions rate of 

0.476 lbs SO2/MMBTU. Pursuant to the March 2, 2015 court-ordered schedule, EPA must 

designate the area surrounding the facility by July 2, 2016.

In its submission, Illinois recommended that a portion of Madison County be designated as 

nonattainment for the 2010 1-hour SO2 NAAQS – specifically, that portion of southern Alton 

Township that is east of the Corporal Belchik Memorial Expressway, south of East Broadway 

Street and Illinois Route 3, and north of Illinois Route 143. Illinois also recommended that all of 

Wood River Township and that portion of Chouteau Township north of the Cahokia Diversion 

Channel be designated as attainment. Lastly, Illinois recommended that the remainder of 

Madison County be designated as unclassifiable. These recommendations were based on an 

assessment and characterization of air quality from the facility and other nearby sources which 

may have a potential impact in the area of analysis where maximum concentrations of SO2 are 

expected. This assessment and characterization was performed using air dispersion modeling 

software, i.e., AERMOD, analyzing actual emissions. After careful review of the state’s 

assessment, supporting documentation, and all available data, EPA agrees that the area 

surrounding Wood River, specifically all of Wood River Township and the portion of Chouteau 

Township north of the Cahokia Diversion Channel is attaining standard, and intends to designate 

it as unclassifiable/attainment. Additionally, EPA agrees that portions of Madison County 

should be designated nonattainment, consistent with the state’s recommendation, i.e., the area 

east of Corporal Belchik Memorial Expressway, south of East Broadway, south of Route 3, and 

north of Route 143 within Alton Township in Madison County. Lastly, while the state has 

recommended that the remainder of Madison County be designated unclassifiable, EPA does not 

intend to promulgate any designation at this time with respect to this area. Instead, EPA intends 

to evaluate and designate the remainder of Madison County by either December 31, 2017, or 

December 31, 2020, consistent with the March 2, 2015 court-ordered schedule.

Wood River is located in south-central Illinois near St. Louis, Missouri in western Madison 

County. As seen in Figure 1 below, the facility is located near the junction of the Mississippi 

River and the river named Wood River. Also included in the figure are nearby emitters of SO2, 

and the state’s recommended area for the nonattainment designation. Figure 2 shows EPA’s 

intended nonattainment designation for the Alton Township area and unclassifiable/attainment 

designation for the Wood River Township area.
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Figure 1. Illinois’ recommended nonattainment area within Madison County, Illinois 

Figure 2. EPA’s intended designations in Madison County, Illinois
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The discussion and analysis that follows below will reference the state’s use of the Modeling 

TAD, EPA’s assessment of the state’s modeling in accordance with the Modeling TAD, and the 

factors for evaluation contained in EPA’s March 20, 2015 guidance, as appropriate.

Detailed Assessment

Model Selection and Modeling Components

EPA’s Modeling TAD notes that for area designations under the 2010 SO2 NAAQS, the 

AERMOD modeling system should be used, unless use of an alternative model can be justified. 

In some instances the recommended model may be a model other than AERMOD, such as the 

BLP model for buoyant line sources. The AERMOD modeling system contains the following 

components:

- AERMOD: the dispersion model

- AERMAP: the terrain processor for AERMOD

- AERMET: the meteorological data processor for AERMOD

- BPIPPRIME: the building input processor 

- AERMINUTE: a pre-processor to AERMET incorporating 1-minute automated surface 

observation system (ASOS) wind data 

- AERSURFACE: the surface characteristics processor for AERMET
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- AERSCREEN: a screening version of AERMOD

The state used AERMOD version 15181, the most recent version, and a discussion of the 

individual components will be referenced in the corresponding discussion that follows, as 

appropriate.

Modeling Parameter: Rural or Urban Dispersion

EPA’s recommended procedure for characterizing an area by prevalent land use is based on 

evaluating the dispersion environment with 3 km of the facility. According to EPA’s modeling 

guidelines, rural dispersion coefficients are to be used in the dispersion modeling analysis if 

more than 50% of the area within a 3 km radius of the facility is classified as rural. Conversely, 

if more than 50% of the area is urban, urban dispersion coefficients should be used in the 

modeling analysis. The state performed an Auer’s analysis for the area near Wood River in 

order to determine which mode was appropriate for the modeling. The analysis indicated that the 

study area is approximately 80.1% rural and 19.9% urban, showing that the rural option was 

appropriate to apply to all emission sources in the modeling domain. Figure 3 and Table 2 below 

show the results of the state’s Auer’s analysis.

Figure 3. Auer’s Analysis – Area Near Wood River 

Table 2. Auer’s Analysis Land Use Percentages by Category – Area Near Wood River 
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Modeling Parameter: Area of Analysis (Receptor Grid)

A reasonable first step towards characterization of air quality in the area surrounding Wood 

River is to determine the extent of the area of analysis, i.e., receptor grid. Considerations 

presented in the Modeling TAD include but are not limited to: the location of the SO2 emission 

sources or facilities considered for modeling; the extent of significant concentration gradients of 

nearby sources; and sufficient receptor coverage and density to adequately capture and resolve 

the model predicted maximum SO2 concentrations. For the Madison County area, the state has 

included four other emitters of SO2 within 10 kilometers (km) of Wood River in any direction. 

The state determined that, aside from exceptionally large sources, sources farther than 10 km 

from Wood River would not cause significant concentration gradients in the area near Wood 

River and therefore need not be modeled. The four emitters of SO2 within the 10 km radius 

included in the area of analysis other than Wood River are: WRB Refining Inc. (formerly named 

ConocoPhillips), Alton Steel, Inc., Christ Brothers Products LLC, and National Maintenance and 

Repair facilities. The Ameren UE Sioux Power Plant, located just west of Portage Des Sioux, 

Missouri, was also included in the modeling due to its tall stacks and high annual emissions. 

Ameren UE Sioux Power Plant is located approximately 18 kilometers west-northwest of the 

center of the study area.

The grid receptor spacing for the area of analysis chosen by the state is as follows:

- 50 meters along the fenceline (six facilities) 

- 100 meters from the fenceline out to 1.0 to 2.0 kilometers 

- 500 meters from 1.0 to 2.0 kilometers out to 9.0 kilometers 

The receptor network contained 11,746 receptors, and the network covered west-central Madison 

County in Illinois, and eastern edges of St. Louis and St. Charles Counties in Missouri.
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For the purposes of this designation effort, the Modeling TAD states that the receptor grid need 

not include receptors in areas where it would not be feasible to place a monitor and record 

ambient air impacts, such as bodies of water. With the exception of receptor locations within 

plant fencelines, Illinois conservatively included the full grid of receptors, including some over 

the Mississippi River. Figure 4 shows the receptor grid for the area of analysis.

Figure 4. Receptor Grid for the Madison County Area of Analysis

Modeling Parameter: Source Characterization

The state characterized the sources within the area of analysis in accordance with the best 

practices outlined in the Modeling TAD. Specifically, the state used actual stack heights in 

conjunction with actual emissions. The state also adequately characterized the modeled sources’

building layout and location, as well as the stack parameters, e.g., exit temperature, exit velocity, 

location, and diameter. The AERMOD component BPIPPRIME was used to assist in addressing 

building downwash.

Modeling Parameter: Emissions 

EPA’s Modeling TAD notes that for the purposes of modeling to characterize air quality for use 

in designations, the recommended approach is to use the most recent 3 years of actual emissions 

data and concurrent meteorological data. However, the TAD does provide for the flexibility of 

using allowable emissions in the form of a federally enforceable limit on the emissions rate 

(referred to as PTE or allowable emissions rate).
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EPA believes that continuous emissions monitoring systems (CEMS) data provide acceptable 

historical emissions information, when it is available, and that these data are available for many 

electric generating units. In the absence of CEMS data, EPA’s Modeling TAD highly 

encourages the use of AERMOD’s hourly varying emissions keyword HOUREMIS, or through 

the use of AERMOD’s variable emissions factors keyword EMISFACT. When choosing one of 

these methods, EPA believes that detailed throughput, operating schedules, and emissions 

information from the impacted sources should be used. 

In certain instances, states and other interested parties may find that it is more advantageous or 

simpler to use PTE rates as part of their modeling runs. Specifically, a facility may have recently 

adopted a new federally enforceable emissions limit, been subject to a federally enforceable 

consent decree, or implemented other federally enforceable mechanisms and control 

technologies to limit SO2 emissions to a level that indicates compliance with the NAAQS. These 

new limits or conditions may be used in the application of AERMOD. In these cases, the 

Modeling TAD notes that the existing SO2 emissions inventories used for permitting or SIP 

planning demonstrations should contain the necessary emissions information for designationsrelated modeling. In the event that these short-term emissions are not readily available, they may 

be calculated using the methodology in Table 8-1 of Appendix W to 40 CFR Part 51 titled, 

“Guideline on Air Quality Models.” 

 

As previously noted, the state included Wood River and four other emitters of SO2 within 10 km

of Wood River as well as one large source approximately 15 km from Wood River. The 

modeled facilities and their associated annual actual SO2 emissions between 2012 and 2014 are 

summarized below.

Table 3. Actual SO2 Emissions for 2012 – 2014 from Facilities in the Madison County, Illinois 

Area of Analysis

Facility Name

SO2 Emissions (tons per year) Approximate 

Distance to 

Wood River 

2012 2013 2014 (km)

Wood River 6,719.49 7,662.27 7,034.66 N/A

WRB Refining LLC 1,966.48 1,203.08 1,103.42 6.46

Alton Steel, Inc. 42.75 38.00 39.35 2.45

Christ Brothers Products 7.20 7.20 7.20 8.83

National Maintenance and Repair 3.93 3.93 3.93 4.28

Ameren Missouri Sioux Power 

Station 2,658.45 2,799.27 1,483.75 14.94

Total Emissions From All Facilities 

in the State’s Area of Analysis 11,398.30 11,713.75 9,672.31

For Wood River, the state used actual emissions from the most recent 3-year data set, i.e., 2012 –

2014. The state used CEMS SO2 emissions data provided by Wood River for its boiler stacks, 

along with temporally varying exit temperature and exit velocity.
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For WRB Refining LLC, the state used company-provided hourly varying emissions, 

temperature, and exit velocity. 

For Alton Steel, Inc., the state constructed a three-year emission profile for the Electric Arc 

Furnace (EAF) and Ladle Metallurgy Furnace (LMF) using company-provided operating 

schedule and yearly emissions. The state used a conservative worst-case emissions year for all 

three years for the two other sources at this facility.

For Christ Brothers Products, and National Maintenance and Repair, the state used the worstcase emission year for the entire simulation.

For Ameren Missouri Sioux Power Station, the state used hourly CEMS SO2 emissions data.

In instances where seasonal throughput was available, emissions were allocated appropriately via 

the EMISFACT keyword in AERMOD and applied to the three-year period. For sources lacking 

hourly varying temperature or exit velocity, replacement values were obtained either from the 

Illinois EPA database or from company-provided emission reports.

Modeling Parameter: Meteorology and Surface Characteristics

The most recent 3 years of meteorological data (concurrent with the most recent 3 years of 

emissions data) should be used in designations efforts. As noted in the Modeling TAD, the 

selection of data should be based on spatial and climatological (temporal) representativeness. 

The representativeness of the data are based on: 1) the proximity of the meteorological 

monitoring site to the area under consideration, 2) the complexity of terrain, 3) the exposure of 

the meteorological site, and 4) the period of time during which data are collected. Sources of 

meteorological data include National Weather Service (NWS) stations, site-specific or onsite 

data, and other sources such as universities, Federal Aviation Administration (FAA), and 

military stations.

For the Madison County area of analysis, surface meteorology from the NWS station in St. 

Louis, Missouri, 27 km to the southwest, and coincident upper air observations from the NWS 

station in Lincoln, Illinois, 158 km to the northeast were selected as best representative of 

meteorological conditions within the area of analysis.

The state used AERSURFACE version 13016 using data from the NWS station in St. Louis, 

Missouri located at (38.75, -90.37) to estimate the surface characteristics of the area of analysis. 

These surface characteristics are the albedo (the fraction of solar energy reflected from the earth 

back into space), the Bowen ratio (representing the ratio of sensible heat flux to latent heat flux 

at the ground level), and the surface roughness (representing the influence of ground features 

such as buildings and vegetation on surface wind flow). In Figure 5 below, generated by EPA, 

the location of the St. Louis, Missouri NWS station is shown relative to the Madison County area 

of analysis.
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Figure 5. Madison County Area of Analysis and the St. Louis, Missouri NWS

As part of its recommendation, the state provided the 3-year surface wind rose for St. Louis, 

Missouri. In Figure 6, the frequency and magnitude of wind speed and direction are defined in 

terms of from where the wind is blowing. The most predominant wind direction during the 

three-year time period used in the modeling is from the southeast to southwest, occurring 

approximately 9.6% of the time. The highest percentage wind speed range, occurring 33.8% of 

the time, was in the 3.6 – 5.7 m/s range.

Figure 6. St. Louis, Missouri Cumulative Annual Wind Rose for Years 2012 – 2014
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Meteorological data from the above surface and upper air stations were used in generating 

AERMOD-ready files with the AERMET processor. The output meteorological data created by 

the AERMET processor is suitable for being applied with AERMOD input files for AERMOD 

modeling runs. The state followed the methodology and settings presented in the U.S. EPA’s 

User’s Guide for the AERMOD Meteorological Preprocessor (November 2004) in the 

processing of the raw meteorological data into an AERMOD-ready format, and used 

AERSURFACE to best represent surface characteristics. 

Hourly surface meteorological data records are read by AERMET, and include all the necessary 

elements for data processing. However, wind data taken at hourly intervals may not always 

portray wind conditions for the entire hour, which can be variable in nature. Hourly wind data 

may also be overly prone to indicate calm conditions, which are not modeled by AERMOD. In 

order to better represent actual wind conditions at the meteorological tower, wind data of one 

minute duration was provided from the same instrument tower, but in a different formatted file to 

be processed by a separate preprocessor, AERMINUTE. These data were subsequently 

integrated into the AERMET processing to produce final hourly wind records of AERMODready meteorological data that better estimate actual hourly average conditions and that are less 
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prone to over-report calm wind conditions. This allows AERMOD to apply more hours of 

meteorology to modeled inputs, and therefore produce a more complete set of concentration 

estimates. As a guard against excessively high concentrations that could be produced by 

AERMOD in very light wind conditions, the state set a minimum threshold of 0.5 meters per 

second in processing meteorological data for use in AERMOD. This approach is consistent with 

a March 2013 EPA memo titled, “Use of ASOS meteorological data in AERMOD dispersion 

Modeling.” In setting this threshold, no wind speeds lower than this value would be used for 

determining concentrations. This threshold was specifically applied to the one minute wind data. 

Modeling Parameter: Geography and Terrain

The terrain in the area of analysis is best described as flat to gently rolling. To account for these 

terrain changes, the AERMAP terrain program within AERMOD was used to specify terrain 

elevations for all the receptors. The source of the elevation data incorporated into the model is 

from the USGS National Elevation Database (NED).

Modeling Parameter: Background Concentrations of SO2

The Modeling TAD offers two mechanisms for characterizing background concentrations of SO2

that are ultimately added to the modeled design values: 1) a “first tier” approach, based on 

monitored design values, or 2) a temporally varying approach, based on the 99th percentile 

monitored concentrations by hour of day and season. For the Madison County area of analysis, 

the state chose to use the latter. The background concentrations for this area of analysis ranged

from 7 to 39 micrograms per cubic meter (μg/m3), or approximately 3 to 15 ppb.3 AERMOD 

incorporated these hourly/seasonal values into the final results. 

Summary of Modeling Results

The AERMOD modeling parameters for the Madison County area of analysis are summarized 

below in Table 4.

Table 4. AERMOD Modeling Parameters for the Madison County, Illinois Area of Analysis

Madison County, Illinois Area of Analysis

AERMOD Version 15181

Dispersion Characteristics Rural

Modeled Sources 6

Modeled Stacks 82

Modeled Structures 527

Modeled Fencelines 10

Total receptors 11,746

Emissions Type Actual

Emissions Years 2012-2014

Meteorology Years 2012-2014

 

3 The conversion factor for SO2 (at the standard conditions applied in the ambient SO2 reference 

method) is 1ppb = approximately 2.62μg/m3.
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Surface Meteorology Station St. Louis, Missouri

Upper Air Meteorology Station Lincoln, Illinois

Methodology for Calculating 

Background SO2 Concentration Temporally Varying 

Calculated Background SO2

Concentration 3 to 15 ppb

The results presented below in Table 5 show the magnitude and geographic location of the 

highest predicted modeled concentration based on actual emissions.

Table 5. Maximum Predicted 99th Percentile 1-Hour SO2 Concentration in the Madison County

Area of Analysis Based on Actual Emissions

Averaging Period Data Period

Receptor Location SO2 Concentration (μg/m3

)

UTM/Latitude UTM/Longitude

Modeled 

(including 

background)

NAAQS

99th Percentile 

1-Hour Average 2012-2014 748051 4307978 456.40 196.4*

*Equivalent to the 2010 SO2 NAAQS set at 75 ppb

The state’s modeling indicates that the predicted 99th percentile 1-hour average concentration 

within the chosen modeling domain is 456.40 μg/m3, or 174.2 ppb. This modeled concentration 

included the background concentration of SO2, and is based on actual emissions from the 

facilities. Illinois performed a culpability analysis which demonstrated that only a small group 

of receptors exceeded the 2010 SO2 NAAQS, and these receptors were primarily affected by 

emissions from Alton Steel, which are greatly influenced by downwash. High concentrations 

near Alton Steel (over twice the standard) are a consequence of building downwash combined 

with stacks that point downward, and primarily occur when winds are blowing from the 

southwest, a direction that maximizes the impact of the Alton Steel building in causing 

downwash and downwash-influenced concentrations in nearby ambient air locations. Since 

Wood River is to the south-southeast of Alton Steel, the primary impacts of the power plant on 

the critical days are likely to be well to the east of Alton Steel, and the critical day impacts of the 

power plant at the location of modeled violations at the fenceline of Alton Steel are likely to be 

minimal. Although impacts of Wood River at the Alton Steel location are somewhat higher 

when the wind is blowing from the south-southeast, carrying emissions directly from the power 

plant to Alton Steel, these are not occurring on modeled violation days, and so are not indicative 

of the power plant’s emissions contributing to violations near Alton Steel. We have reviewed the 

pertinent evidence and the State’s analysis, and we agree with Illinois’ recommendation to 

conclude that Wood River does not contribute to the modeled violation near Alton Steel. Rather, 

the violation appears to be predominantly the result of Alton Steel’s building downwashinfluenced emissions, with minimal impact from Wood River or other sources.

Figure 7 below was included as part of the state’s recommendation, and indicates that the 

maximum estimated concentration occurred within the dense 100-meter grid near Alton Steel, 

approximately 2.5 kilometers northwest of the Wood River main stacks. The state’s receptor 

grid is also shown in the figure.
18

Figure 7. Maximum Predicted 99th Percentile 1-Hour SO2 Concentrations in the Madison County

Area of Analysis Based on Actual Emissions

As noted above, portions of Illinois’ modeling domain extend into Missouri and other areas 

beyond the area Illinois recommended be designated either as attainment or nonattainment. 

Illinois identified no violations in those portions of the modeling domain. However, Illinois has 

focused its analysis on the area relatively close to Wood River, the analysis is most reliable in 

this area, and so EPA intends to designate only the area that Illinois recommended either to be 

nonattainment or attainment. Missouri made no recommendations regarding this area, and EPA 

intends not to include any portion of Missouri in the designations for this area.

Jurisdictional Boundaries:

Once the geographic area of analysis associated with Wood River, other nearby sources, and 

background concentration is determined, existing jurisdictional boundaries are considered for the 

purpose of informing our intended designations, specifically with respect to clearly defined legal 

boundaries. EPA believes that Illinois’ recommended nonattainment area, consisting of a portion 

of southern Alton Township that is east of the Corporal Belchik Memorial Expressway, south of 

East Broadway Street and Illinois Route 3, and north of Illinois Route 143, is specified using

clearly defined boundaries, and we find these boundaries to be a suitably clear basis for defining 

our intended nonattainment area. The intended unclassifiable/attainment area is based on 

township boundaries and a well-known water body, which are also suitably clear for defining 

designated areas.

We find that Illinois’ recommended boundaries for this area are justified by the modeling data 

and the meteorology in the area. Illinois modeled attainment, and recommended a designation of 

attainment, for the area near Wood River (Wood River Township and part of Chouteau 

Township), but they modeled a violation, and recommended a designation of nonattainment, in 
19

the immediate vicinity of Alton Steel (southern part of Alton Township). The state found the 

violation to be predominantly associated with emissions from Alton Steel. Illinois provided 

suitable evidence that Wood River should be judged not to contribute to the modeled violation.

Illinois’ modeling included the nearby sources which are most likely to have a significant 

concentration gradient near Wood River. The recommended nonattainment area contains the 

region of modeled nonattainment and the sources contributing to the high concentrations. Wood 

River and the other modeled sources outside the recommended nonattainment area, as well as the 

more distant SO2 sources in Madison County and neighboring counties which were not included 

in Illinois’ Wood River modeling analysis, are unlikely to cause or contribute to violations in any 

portion of Madison County that EPA will be designating as nonattainment. 

Other Relevant Information

There was no additional relevant information submitted regarding Wood River or Madison 

County. 

Conclusion

After careful evaluation of the state’s recommendation and supporting information, as well as all 

available relevant information, EPA intends to designate the area around Wood River as 

unclassifiable/attainment for the 2010 SO2 NAAQS. Specifically, the unclassifiable/attainment 

area is comprised of Wood River Township and the portion of Chouteau Township north of the 

Cahokia Diversion Channel. EPA believes that Illinois’ modeling analysis supports this 

designation. EPA also intends to designate the area around Alton Steel as nonattainment. 

Specifically, the nonattainment boundaries are comprised of Corporal Belchik Memorial 

Expressway on the west, East Broadway Street and Illinois Route 3 on the north, and Alton 

Township borders on the east and south, extending to Illinois Route 143 as the remainder of the 

southern boundary. This area contains the region of modeled nonattainment and the sources 

contributing to the high concentrations. EPA reiterates that while Illinois recommended that the 

remainder of Madison County be designated as unclassifiable, we do not intend to designate the 

remainder of Madison County at this time. EPA views this deferral as consistent with Illinois’

recommendation that EPA defer any substantive designation of the remainder of Madison 

County (i.e., promulgating a designation of attainment, unclassifiable/attainment, or 

nonattainment), pending analysis of “additional large SO2 emitting facilities in other parts of the 

county that will likely need to be addressed later by the Illinois EPA in accordance with the Data 

Requirements Rule.” 

At this time, our intended designations for the state only apply to the specified nonattainment and 

unclassifiable/attainment areas described above and the other areas discussed elsewhere in this 

technical support document. Consistent with the conditions in the March 2, 2015 court-ordered 

schedule, EPA will evaluate and designate the remainder of Madison County and all other 

remaining undesignated areas in Illinois by either December 31, 2017, or December 31, 2020. 
20

Technical Analysis for the Massac County, Illinois Area

Introduction

Massac County, Illinois contains a stationary source that according to EPA’s Air Markets 

Database emitted in 2012 either more than 16,000 tons of SO2 or more than 2,600 tons of SO2

and had an annual average emission rate of at least 0.45 pounds of SO2 per one million British 

thermal units (lbs SO2/MMBTU). As of March 2, 2015, this stationary source had not met the 

specific requirements for being “announced for retirement.” Specifically, in 2012, the Joppa 

Power Station (“Joppa”) emitted 16,991 tons of SO2, and had an emissions rate of 0.475 lbs 

SO2/MMBTU. Pursuant to the March 2, 2015 court-ordered schedule, EPA must designate the 

area surrounding the facility by July 2, 2016.

In its submission, Illinois recommended that the area surrounding Joppa, specifically the entirety 

of Massac County, be designated as attainment based on an assessment and characterization of 

air quality from the facility and other nearby sources which may have a potential impact in the 

area of analysis where maximum concentrations of SO2 are expected. This assessment and 

characterization was performed using air dispersion modeling software, i.e., AERMOD, 

analyzing actual emissions. After careful review of the state’s assessment, supporting 

documentation, and all available data, EPA agrees that the area is attaining the standard, and 

intends to designate Massac County as unclassifiable/attainment. 

Joppa is located in southern Illinois in western Massac County. As seen in Figure 8 below, the 

facility is located approximately 1 km northwest of the center of the town of Joppa. Also 

included in the figure are nearby emitters of SO2, the state’s recommended area for the 

attainment designation, and EPA’s intended unclassifiable/attainment designation for the area.

Figure 8. EPA’s intended designation for Massac County, Illinois
21

The discussion and analysis that follows below will reference the state’s use of the Modeling 

TAD, EPA’s assessment of the state’s modeling in accordance with the Modeling TAD, and the 

factors for evaluation contained in EPA’s March 20, 2015 guidance, as appropriate.

Detailed Assessment

Model Selection and Modeling Components

EPA’s Modeling TAD notes that for area designations under the 2010 SO2 NAAQS, the 

AERMOD modeling system should be used, unless use of an alternative model can be justified. 

In some instances the recommended model may be a model other than AERMOD, such as the 

BLP model for buoyant line sources. The AERMOD modeling system contains the following 

components:

- AERMOD: the dispersion model

- AERMAP: the terrain processor for AERMOD

- AERMET: the meteorological data processor for AERMOD

- BPIPPRIME: the building input processor 

- AERMINUTE: a pre-processor to AERMET incorporating 1-minute automated surface 

observation system (ASOS) wind data
22

- AERSURFACE: the surface characteristics processor for AERMET

- AERSCREEN: a screening version of AERMOD

The state used AERMOD version 15181, the most recent version, and a discussion of the 

individual components will be referenced in the corresponding discussion that follows, as 

appropriate.

Modeling Parameter: Rural or Urban Dispersion

EPA’s recommended procedure for characterizing an area by prevalent land use is based on 

evaluating the dispersion environment with 3 km of the facility. According to EPA’s modeling 

guidelines, rural dispersion coefficients are to be used in the dispersion modeling analysis if 

more than 50% of the area within a 3 km radius of the facility is classified as rural. Conversely, 

if more than 50% of the area is urban, urban dispersion coefficients should be used in the 

modeling analysis. The state performed an Auer’s analysis for the Joppa study area in order to 

determine which mode was appropriate for the modeling. The analysis indicated that the study 

area is approximately 96% rural and 4% urban, showing that the rural option applied to all 

emission sources in the modeling domain. Figure 9 and Table 6 below show the results of the 

state’s Auer’s analysis.

Figure 9. Auer’s Analysis –Area Near Joppa
23

Table 6. Auer’s Analysis Land Use Percentages by Category – Area Near Joppa 

Modeling Parameter: Area of Analysis (Receptor Grid)

A reasonable first step towards characterization of air quality in the area surrounding Joppa is to 

determine the extent of the area of analysis, i.e., receptor grid. Considerations presented in the 

Modeling TAD include but are not limited to: the location of the SO2 emission sources or 

facilities considered for modeling; the extent of significant concentration gradients of nearby 

sources; and sufficient receptor coverage and density to adequately capture and resolve the 

model predicted maximum SO2 concentrations. For the Joppa area, the state has included four

other emitters of SO2 within approximately 10 km of Joppa in any direction. The state 

determined that, aside from exceptionally large sources, sources farther than 10 km from Joppa 

would not cause significant concentration gradients in the Joppa area and therefore need not be 

modeled. However, Illinois also included TVA-Shawnee Power Plant, which is approximately 

11 km from Joppa, based on a determination that this source had sufficient emissions to warrant 

its inclusion. In addition to Joppa and TVA-Shawnee Power Plant, the other emitters of SO2

included in Illinois’ modeling analysis, all within approximately 10 km of Joppa, are: Lafarge 

Midwest Inc., Midwest Electric Power Inc., Trunkline Gas Company, Honeywell International 

Inc. 

The grid receptor spacing for the area of analysis chosen by the state is as follows:

- 50 meters spacing along the fenceline (six facilities) 

- 100 meters out to approximately 3.0 kilometers 

- 500 meters from 3.0 kilometers out to approximately 8.0 kilometers 

- 1,000 meters from 8.0 kilometers out to approximately 12 kilometers 

The receptor network contained 25,649 receptors, and the network covered the northwestern 

portion of Massac County in Illinois, the northeastern portion of Ballard County in Kentucky, 

and the northwestern portion of McCracken County in Kentucky. 
24

Figure 10 below, included in the state’s recommendation, shows the receptor grid for the area of 

analysis.

For the purposes of this designation effort, the Modeling TAD states that the receptor grid need 

not include receptors in areas where it would not be feasible to place a monitor and record 

ambient air impacts, such as bodies of water. With the exception of receptor locations within 

plant fencelines, Illinois conservatively included the full grid of receptors, including some over 

the Ohio River. 

Figure 10. Receptor Grid for the Massac County, Illinois Area of Analysis

Modeling Parameter: Source Characterization

The state characterized the sources within the area of analysis in accordance with the best 

practices outlined in the Modeling TAD. Specifically, the state used actual stack heights in 

conjunction with actual emissions. The state also adequately characterized the modeled sources’ 

building layout and location, as well as the stack parameters, e.g., exit temperature, exit velocity, 

location, and diameter. The AERMOD component BPIPPRIME was used to assist in addressing

building downwash.

Modeling Parameter: Emissions

EPA’s Modeling TAD notes that for the purposes of modeling to characterize air quality for use 

in designations, the recommended approach is to use the most recent 3 years of actual emissions 

data and concurrent meteorological data. However, the TAD does provide for the flexibility of

using allowable emissions in the form of a federally enforceable limit on the emissions rate 

(referred to as PTE or allowable emissions rate).
25

EPA believes that continuous emissions monitoring systems (CEMS) data provide acceptable 

historical emissions information, when it is available, and that these data are available for many 

electric generating units. In the absence of CEMS data, EPA’s Modeling TAD highly 

encourages the use of AERMOD’s hourly varying emissions keyword HOUREMIS, or through 

the use of AERMOD’s variable emissions factors keyword EMISFACT. When choosing one of 

these methods, EPA believes that detailed throughput, operating schedules, and emissions 

information from the impacted sources should be used. 

In certain instances, states and other interested parties may find that it is more advantageous or 

simpler to use PTE rates as part of their modeling runs. Specifically, a facility may have recently 

adopted a new federally enforceable emissions limit, been subject to a federally enforceable 

consent decree, or implemented other federally enforceable mechanisms and control 

technologies to limit SO2 emissions to a level that indicates compliance with the NAAQS. These 

new limits or conditions may be used in the application of AERMOD. In these cases, the 

Modeling TAD notes that the existing SO2 emissions inventories used for permitting or SIP 

planning demonstrations should contain the necessary emissions information for designationsrelated modeling. In the event that these short-term emissions are not readily available, they may 

be calculated using the methodology in Table 8-1 of Appendix W to 40 CFR Part 51 titled, 

“Guideline on Air Quality Models.” 

As previously noted, the state included Joppa, one relatively large source 11 km from Joppa, and 

four other emitters of SO2 within approximately 10 km from Joppa. The facilities in the area of 

analysis, their associated annual actual SO2 emissions between 2012 and 2014 are summarized 

below, along with their distance from Joppa.

Table 7. Actual SO2 Emissions between 2012 – 2014 from Facilities in the Massac County, 

Illinois Area 

Facility Name

SO2 Emissions (tons per year) Approximate 

Distance to 

2012 2013 2014 Joppa (km)

Joppa PS (Electric Energy Inc.) 17,007.07 16,557.74 18,229.24 N/A

Lafarge Midwest Inc. 552.60 553.28 491.65 1.97

Midwest Electric Power Inc. (MEPI) 5.68 0.00 0.00 1.40

Trunkline Gas Company 0.866 0.866 0.866 1.42

Honeywell International Inc. 162.51 58.73 143.15 10.14

TVA – Shawnee Power Plant 27,114.87 27,210.73 29,734.54 11.41

Total Emissions From All Facilities 

in the State’s Area of Analysis 44,843.60 44,381.46 48,599.45

For Joppa, the state used actual emissions from the most recent 3-year data set, i.e., 2012 – 2014. 

The state used CEMS SO2 emissions data provided by the Joppa for its boiler stacks, along with 

hourly specific exit temperatures and exit velocities. 
26

For Lafarge Midwest Inc., the state used company-provided hourly varying emissions, 

temperature, and exit velocity. 

For MEPI, the state used company-provided hourly varying emissions.

For Trunkline Gas Company, the state used emissions found in the company’s Annual Emissions 

Reports, which included yearly emissions data and seasonal throughput. These seasonal 

emissions were averaged over three years (2012-2014), multiplied by a scalar (via EMISFACT 

keyword in AERMOD), and then applied to the three-year modeling period.

For Honeywell International Inc., the state constructed a three-year hourly profile based on 

seasonal throughput.

For TVA-Shawnee Boiler Stacks, the state used CEMS data obtained via U.S. EPA’s Clean Air 

Markets Division (CAMD) database to construct hourly emission rates.

For all the sources that lacked CEMS data, the state used constant values for exit temperature 

and exit velocity, which were obtained either from the Illinois EPA database or from companyprovided emission reports.

Modeling Parameter: Meteorology and Surface Characteristics

The most recent 3 years of meteorological data (concurrent with the most recent 3 years of 

emissions data) should be used in designations efforts. As noted in the Modeling TAD, the 

selection of data should be based on spatial and climatological (temporal) representativeness. 

The representativeness of the data are based on: 1) the proximity of the meteorological 

monitoring site to the area under consideration, 2) the complexity of terrain, 3) the exposure of 

the meteorological site, and 4) the period of time during which data are collected. Sources of 

meteorological data include National Weather Service (NWS) stations, site-specific or onsite 

data, and other sources such as universities, Federal Aviation Administration (FAA), and 

military stations.

For the Massac County area of analysis, surface meteorology from the NWS station in Paducah, 

Kentucky, 18 km to the southeast, and coincident upper air observations from the NWS station in 

Nashville, Tennessee, 228 km to the southeast were selected as best representative of 

meteorological conditions within the area of analysis. 

The state used AERSURFACE version 13016 using data from the NWS station in Paducah, 

Kentucky located at (37.05822 °N, 88.57251 °W) to estimate the surface characteristics of the 

area. These surface characteristics are the albedo (the fraction of solar energy reflected from the 

earth back into space), the Bowen ratio (representing the ratio of sensible heat flux to latent heat 

flux at the ground level), and the surface roughness (representing the influence of ground 

features such as buildings and vegetation on surface wind flow). In Figure 11 below, generated 

by EPA, the location of the Paducah, Kentucky NWS station is shown relative to the Joppa 

Power Station area of analysis.

Figure 11. Massac County Area of Analysis and the Paducah, Kentucky NWS
27

As part of its recommendation, the state provided the 3-year surface wind rose for Paducah, 

Kentucky. In Figure 12, the frequency and magnitude of wind speed and direction are defined in 

terms of from where the wind is blowing. The predominant wind direction during the three-year 

time period used for the modeling is from the southwest, occurring approximately 11.4% of the 

time. The highest percentage wind speed range, occurring 32.9% of the time, was in the 2.1 –

3.6 m/s range.

Figure 12. Paducah, Kentucky Cumulative Annual Wind Rose for Years 2012 – 2014
28

Meteorological data from the above surface and upper air stations were used in generating 

AERMOD-ready files with the AERMET processor. The output meteorological data created by 

the AERMET processor is suitable for being applied with AERMOD input files for AERMOD 

modeling runs. The state followed the methodology and settings presented in the U.S. EPA’s 

User’s Guide for the AERMOD Meteorological Preprocessor (November 2004) in the 

processing of the raw meteorological data into an AERMOD-ready format, and used 

AERSURFACE to best represent surface characteristics.

Hourly surface meteorological data records are read by AERMET, and include all the necessary 

elements for data processing. However, wind data taken at hourly intervals may not always 

portray wind conditions for the entire hour, which can be variable in nature. Hourly wind data 

may also be overly prone to indicate calm conditions, which are not modeled by AERMOD. In 

order to better represent actual wind conditions at the meteorological tower, wind data of one 

minute duration was provided from the same instrument tower, but in a different formatted file to 

be processed by a separate preprocessor, AERMINUTE. These data were subsequently 

integrated into the AERMET processing to produce final hourly wind records of AERMODready meteorological data that better estimate actual hourly average conditions and that are less 
29

prone to over-report calm wind conditions. This allows AERMOD to apply more hours of 

meteorology to modeled inputs, and therefore produce a more complete set of concentration 

estimates. As a guard against excessively high concentrations that could be produced by 

AERMOD in very light wind conditions, the state set a minimum threshold of 0.5 meters per 

second in processing meteorological data for use in AERMOD. This approach is consistent with 

a March 2013 EPA memo titled, “Use of ASOS meteorological data in AERMOD dispersion 

Modeling.” In setting this threshold, no wind speeds lower than this value would be used for 

determining concentrations. This threshold was specifically applied to the one minute wind data. 

Modeling Parameter: Geography and Terrain

The terrain in the area of analysis is best described as flat to gently rolling. To account for these 

terrain changes, the AERMAP terrain program within AERMOD was used to specify terrain 

elevations for all the receptors. The source of the elevation data incorporated into the model is 

from the USGS National Elevation Database (NED).

Modeling Parameter: Background Concentrations of SO2

The Modeling TAD offers two mechanisms for characterizing background concentrations of SO2

that are ultimately added to the modeled design values: 1) a “first tier” approach, based on 

monitored design values, or 2) a temporally varying approach, based on the 99th percentile 

monitored concentrations by hour of day and season or month. For the Joppa area of analysis, 

the state chose to use the latter. The background concentrations for this area of analysis ranged 

from 2.6 to 34 μg/m3, or 1 to 13 ppb.4 AERMOD incorporated these hourly/seasonal values into 

the final results. 

Summary of Modeling Results

The AERMOD modeling parameters for the Joppa area of analysis are summarized below in 

Table 8.

Table 8. AERMOD Modeling Parameters for the Massac County Area of Analysis

Massac County, Illinois Area of Analysis

AERMOD Version 15181

Dispersion Characteristics Rural

Modeled Sources 6

Modeled Stacks 18 

Modeled Structures 127

Modeled Fencelines 6

Total receptors 25,649

Emissions Type Actual

Emissions Years 2012-2014

 

4 The conversion factor for SO2 (at the standard conditions applied in the ambient SO2 reference 

method) is 1ppb = approximately 2.62μg/m3.
30

Meteorology Years 2012-2014

Surface Meteorology Station Paducah, Kentucky

Upper Air Meteorology Station Nashville, Tennessee

Methodology for Calculating 

Background SO2 Concentration Temporally Varying

Calculated Background SO2

Concentration 1 to 13 ppb

The results presented below in Table 9 show the magnitude and geographic location of the 

highest predicted modeled concentration based on actual emissions.

Table 9. Maximum Predicted 99th Percentile 1-Hour SO2 Concentration in the Massac County, 

Illinois Area of Analysis Based on Actual Emissions

Averaging Period Data Period

Receptor Location SO2 Concentration (μg/m3

)

UTM/Latitude UTM/Longitude

Modeled

(including 

background)

NAAQS

99th Percentile 

1-Hour Average 2012-2014 332800 4121600 168.29 196.4*

*Equivalent to the 2010 SO2 NAAQS set at 75 ppb

The state’s modeling indicates that the predicted 99th percentile 1-hour average concentration 

within the chosen modeling domain is 168.29 μg/m3, or 64.2 ppb. This modeled concentration 

included the background concentration of SO2, and is based on actual emissions from the 

facilities. Figure 13 below was included as part of the state’s recommendation, and indicates that 

the predicted value occurred within the dense 100-meter grid approximately 2.9 km northwest of 

the Joppa Power Station main stacks and 0.4 km northwest of the Lafarge northern fenceline. 

The state’s receptor grid is also shown in the figure.

Figure 13. Maximum Predicted 99th Percentile 1-Hour SO2 Concentrations in the 

Massac County, Illinois Area of Analysis Based on Actual Emissions
31

As noted above, portions of Illinois’ modeling domain extend into Kentucky and other areas 

beyond the area Illinois recommended be designated as attainment. Illinois identified no 

violations in those portions of the modeling domain. However, Illinois has focused its analysis 

on the area relatively close to Joppa, the analysis is most reliable in this area, and so EPA intends 

to designate only the area that Illinois recommended designating attainment. Kentucky made no 

recommendations regarding this area, and EPA intends not to include any portion of Kentucky in 

the designations for this area.

Jurisdictional Boundaries:

Once the geographic area of analysis associated with Joppa, other nearby sources, and 

background concentrations are determined, existing jurisdictional boundaries are considered for 

the purpose of informing our intended unclassifiable/attainment area, specifically with respect to 

clearly defined legal boundaries. EPA believes that our intended unclassifiable/attainment area, 

consisting of Massac County, is comprised of clearly defined legal boundaries, and we find these 

boundaries to be a suitably clear basis for defining our intended unclassifiable/attainment area. 

As previously discussed, emissions from 6 facilities were included in the modeling analysis for 

the area around Joppa, including Joppa and another large emitter of SO2, i.e., TVA’s Shawnee 

Power Station, which is located in neighboring McCracken County, Kentucky. The area of 

analysis was found to show compliance with the standard, and EPA has determined that there are 
32

no additional sources of SO2 within Massac County, but for those included in the state’s 

modeling analysis, emitting at or above 100 tpy, according to the 2011 NEI. As a result, EPA

does not believe that sources or emissions from Massac County cause or contribute to a violation 

of the NAAQS in any area. EPA anticipates that further analysis of air quality in McCracken 

County, Kentucky, notably to assess the impact of the TVA’s Shawnee Generating Station in that 

county, will be conducted pursuant to the data requirements rule, supporting designation action 

for that county at a future time.

There are two facilities in Calvert City, Marshall County, Kentucky, which are 13 km from the 

Massac County border. Given their emissions (1,976 tpy for Isp Chemicals and 860 tpy for CC 

Metals and Alloys in the 2011 NEI) and the distance, they are not likely to cause or contribute to 

violations in Massac County. Additionally, their emissions are likely to be accounted for in the 

background values measured at the Paducah monitor (AQS ID 21-145-1024), which is located 

between the Calvert City facilities and the Joppa study area. The 2012 – 2014 design value 

collected at this monitor was 21 ppb.

Other Relevant Information

The Sierra Club submitted a modeling analysis to EPA for the Joppa Power Station, asserting 

that Joppa was contributing to modeled violations. Sierra Club provided this information to 

Illinois, and Illinois has reviewed this information and identified several deviations from 

recommendations in the Modeling TAD. First, the Sierra Club’s analysis used a lower-thanactual stack height. The Sierra Club used a higher, fixed background value, taken from Oglesby, 

in north central Illinois, whereas Illinois’s background data for Joppa came from Paducah, 

Kentucky, much closer to the Joppa modeling domain. Sierra Club used fixed stack gas 

temperatures and flow rates, whereas Illinois used hourly varying data for these parameters for 

Joppa. These differences likely explain why the Sierra Club results were greater than Illinois’ 

modeling results. The Sierra Club analysis did not provide information refuting the 

appropriateness of Illinois’ analysis of the Joppa area for this round of SO2 designations, and 

EPA does not find that the Sierra Club analysis has provided compelling information for EPA’s 

designation decision for Massac County to differ from Illinois’ recommendation.

Conclusion

After careful evaluation of the state’s recommendation and supporting information, as well as all 

available relevant information, EPA intends to designate the area around Joppa as 

unclassifiable/attainment for the 2010 SO2 NAAQS. Specifically, EPA intends to designate an 

unclassifiable/attainment area consisting of the entirety of Massac County. 

At this time, our intended designations for the state only apply to this area and the other areas 

presented in this technical support document. Consistent with the conditions in the March 2, 

2015 court-ordered schedule, EPA will evaluate and designate all remaining undesignated areas 

in Illinois by either December 31, 2017, or December 31, 2020. 
33

Technical Analysis for the Jasper County, Illinois Area

Introduction

Jasper County, Illinois contains a stationary source that according to EPA’s Air Markets 

Database emitted in 2012 either more than 16,000 tons of SO2 or more than 2,600 tons of SO2

and had an annual average emission rate of at least 0.45 pounds of SO2 per one million British 

thermal units (lbs SO2/MMBTU). As of March 2, 2015, this stationary source had not met the 

specific requirements for being “announced for retirement.” Specifically, in 2012, Illinois Power 

Generating Company’s Newton Power Station (“Newton”) emitted 16,519 tons of SO2, and had 

an emissions rate of 0.590 lbs SO2/MMBTU. Pursuant to the March 2, 2015 court-ordered 

schedule, EPA must designate the area surrounding the facility by July 2, 2016.

In its submission, Illinois recommended that the area surrounding Newton, specifically the 

entirety of Jasper County, be designated as attainment based on an assessment and 

characterization of air quality from the facility and other nearby sources which may have a 

potential impact in the area of analysis where maximum concentrations of SO2 are expected. 

This assessment and characterization was performed using air dispersion modeling software, i.e., 

AERMOD, analyzing actual emissions. After careful review of the state’s assessment, 

supporting documentation, and all available data, EPA agrees that the area is attaining the 

standard, and intends to designate Jasper County as unclassifiable/attainment

Newton is located in southeastern Illinois in southwestern Jasper County. The facility is located 

approximately seven miles southwest of the City of Newton in a rural area bounded on the east 

and south by Newton Lake. Figure 14 shows EPA’s intended designation for this area.

Figure 14. EPA’s intended designation for the Jasper County Area
34

The discussion and analysis that follows below will reference the state’s use of the Modeling 

TAD, EPA’s assessment of the state’s modeling in accordance with the Modeling TAD, and the 

factors for evaluation contained in EPA’s March 20, 2015 guidance, as appropriate.

Detailed Assessment

Model Selection and Modeling Components

EPA’s Modeling TAD notes that for area designations under the 2010 SO2 NAAQS, the 

AERMOD modeling system should be used, unless use of an alternative model can be justified. 

In some instances the recommended model may be a model other than AERMOD, such as the 

BLP model for buoyant line sources. The AERMOD modeling system contains the following 

components:

- AERMOD: the dispersion model

- AERMAP: the terrain processor for AERMOD

- AERMET: the meteorological data processor for AERMOD

- BPIPPRIME: the building input processor 
35

- AERMINUTE: a pre-processor to AERMET incorporating 1-minute automated surface 

observation system (ASOS) wind data 

- AERSURFACE: the surface characteristics processor for AERMET

- AERSCREEN: a screening version of AERMOD

The state used AERMOD version 15181, the most recent version, and a discussion of the 

individual components will be referenced in the corresponding discussion that follows, as 

appropriate.

Modeling Parameter: Rural or Urban Dispersion

EPA’s recommended procedure for characterizing an area by prevalent land use is based on 

evaluating the dispersion environment with 3 km of the facility. According to EPA’s modeling 

guidelines, rural dispersion coefficients are to be used in the dispersion modeling analysis if 

more than 50% of the area within a 3 km radius of the facility is classified as rural. Conversely, 

if more than 50% of the area is urban, urban dispersion coefficients should be used in the 

modeling analysis. The state performed an Auer’s analysis for the Newton study area in order to 

determine which mode was appropriate for the modeling. The analysis indicated that the study 

area is approximately 98.2% rural and 1.8% urban, showing that the rural option applied to all 

emission sources in the modeling domain. Figure 15 and Table 10 below show the results of the 

state’s Auer’s analysis.

Figure 15. Auer’s Analysis – Area Near Newton 
36

Table 10. Auer’s Analysis Land Use Percentages by Category – Newton Study Area

Modeling Parameter: Area of Analysis (Receptor Grid)
37

A reasonable first step towards characterization of air quality in the area surrounding the Newton 

Power Station is to determine the extent of the area of analysis, i.e., receptor grid. 

Considerations presented in the Modeling TAD include but are not limited to: the location of the 

SO2 emission sources or facilities considered for modeling; the extent of significant 

concentration gradients of nearby sources; and sufficient receptor coverage and density to 

adequately capture and resolve the model predicted maximum SO2 concentrations. For the 

Jasper County area, the state identified no other significant emitters of SO2 in the area within or 

beyond 10 km of Newton in any direction. In the absence of exceptionally large sources within 

30 km of Newton, the state determined that 10 km was the appropriate distance within which to 

include moderate sized sources in order to adequately characterize air quality from the facility 

and other nearby sources which may have a potential impact in the area of analysis where 

maximum concentrations of SO2 are expected. Again, Newton is the only significant SO2 source 

within the 10-kilometer study area, and there are no sources beyond ten kilometers that Illinois 

EPA considers to have the potential to cause significant gradient impacts within the study area. 

The grid receptor spacing for the area of analysis chosen by the state is as follows:

- 50 meters along the fenceline

- 100 meters from the fenceline out to 5.0 kilometers 

- 500 meters from 5.0 kilometers out to 8.0 kilometers

The receptor network contained 12,165 receptors, and the network covered the southwestern 

section of Jasper County and small portions of Effingham and Clay counties.

Figures 16, included in the state’s recommendation, shows the state’s receptor grid for the area 

of analysis.

For the purposes of this designation effort, the Modeling TAD states that the receptor grid need 

not include receptors in areas where it would not be feasible to place a monitor and record 

ambient air impacts, such as bodies of water. With the exception of receptor locations within 

plant fencelines, Illinois conservatively included the full grid of receptors, including some over 

Newton Lake. 

Figure 16. Receptor Grid for the Jasper County, Illinois Area of Analysis
38

Modeling Parameter: Source Characterization

The state characterized the source within the area of analysis in accordance with the best 

practices outlined in the Modeling TAD. Specifically, the state used actual stack heights in 

conjunction with actual emissions. The state also adequately characterized Newton’s building 

layout and location, as well as the stack parameters, e.g., exit temperature, exit velocity, location, 

and diameter. The AERMOD component BPIPPRIME was used to assist in addressing building 

downwash.

Modeling Parameter: Emissions 

EPA’s Modeling TAD notes that for the purposes of modeling to characterize air quality for use 

in designations, the recommended approach is to use the most recent 3 years of actual emissions 

data and concurrent meteorological data. However, the TAD does provide for the flexibility of 

using allowable emissions in the form of a federally enforceable limit on the emission rate

(referred to as PTE or allowable emissions rate).

EPA believes that continuous emissions monitoring systems (CEMS) data provide acceptable 

historical emissions information, when it is available, and that these data are available for many 

electric generating units. In the absence of CEMS data, EPA’s Modeling TAD highly 

encourages the use of AERMOD’s hourly varying emissions keyword HOUREMIS, or through 

the use of AERMOD’s variable emissions factors keyword EMISFACT. When choosing one of 
39

these methods, EPA believes that detailed throughput, operating schedules, and emissions 

information from the impacted source should be used. 

In certain instances, states and other interested parties may find that it is more advantageous or 

simpler to use PTE rates as part of their modeling runs. Specifically, a facility may have recently 

adopted a new federally enforceable emissions limit, been subject to a federally enforceable 

consent decree, or implemented other federally enforceable mechanisms and control 

technologies to limit SO2 emissions to a level that indicates compliance with the NAAQS. These 

new limits or conditions may be used in the application of AERMOD. In these cases, the 

Modeling TAD notes that the existing SO2 emissions inventories used for permitting or SIP 

planning demonstrations should contain the necessary emissions information for designationsrelated modeling. In the event that these short-term emissions are not readily available, they may 

be calculated using the methodology in Table 8-1 of Appendix W to 40 CFR Part 51 titled, 

“Guideline on Air Quality Models.” 

As previously noted, the state included only Newton within its modeling analysis. The facility in 

the area of analysis and its associated annual actual SO2 emissions between 2012 and 2014 are 

summarized below. 

Table 11. Actual SO2 Emissions for 2012 – 2014 from Facilities in the Jasper County, Illinois 

Area of Analysis

Facility Name

SO2 Emissions (tons per year)

2012 2013 2014

Newton 16,533.83 16,144.5 16,372.76

Total Emissions From All Facilities 

in the State’s Area of Analysis 16,533.83 16,144.5 16,372.76

For Newton, the state used hourly varying 2012-2014 CEMS data, coupled with companyprovided hour-specific exit temperatures and exit velocities.

Modeling Parameter: Meteorology and Surface Characteristics

The most recent 3 years of meteorological data (concurrent with the most recent 3 years of 

emissions data) should be used in designations efforts. As noted in the Modeling TAD, the 

selection of data should be based on spatial and climatological (temporal) representativeness. 

The representativeness of the data are based on: 1) the proximity of the meteorological 

monitoring site to the area under consideration, 2) the complexity of terrain, 3) the exposure of 

the meteorological site, and 4) the period of time during which data are collected. Sources of 

meteorological data include National Weather Service (NWS) stations, site-specific or onsite 

data, and other sources such as universities, Federal Aviation Administration (FAA), and 

military stations.

For the Jasper County area of analysis, surface meteorology from the NWS station in Evansville, 

Indiana, 123 km to the southeast, and coincident upper air observations from the NWS station in 

Lincoln, Illinois, 164 km to the northwest were selected as best representative of meteorological 

conditions within the area of analysis. 
40

The state used AERSURFACE version 13016 using data from the NWS station in Evansville, 

Indiana located at (38.05 °N, 87.53 °W) to estimate the surface characteristics of the area of 

analysis. These surface characteristics are the albedo (the fraction of solar energy reflected from 

the earth back into space), the Bowen ratio (representing the ratio of sensible heat flux to latent 

heat flux at the ground level), and the surface roughness (representing the influence of ground 

features such as buildings and vegetation on surface wind flow). In Figure 17 below, generated 

by EPA, the location of the Evansville, Indiana NWS station is shown relative to the Jasper 

County area.

Figure 17. Jasper County Area and the Evansville, Indiana NWS

As part of its recommendation, the state provided the 3-year surface wind rose for Evansville, 

Indiana. In Figure 18, the frequency and magnitude of wind speed and direction are defined in 

terms of from where the wind is blowing. The predominant wind direction during the three-year 

time period represented in the modeling is from the southwest, occurring approximately 11.3% 

of the time. The highest percentage wind speed range, occurring 25.3% of the time, was in the 

3.6 – 5.7 m/s range.

Figure 18. Evansville, Indiana Cumulative Annual Wind Rose for Years 2012 – 2014
41

Meteorological data from the above surface and upper air stations were used in generating 

AERMOD-ready files with the AERMET processor. The output meteorological data created by 

the AERMET processor is suitable for being applied with AERMOD input files for AERMOD 

modeling runs. The state followed the methodology and settings presented in the U.S. EPA’s 

User’s Guide for the AERMOD Meteorological Preprocessor (November 2004) in the 

processing of the raw meteorological data into an AERMOD-ready format, and used 

AERSURFACE to best represent surface characteristics.

Hourly surface meteorological data records are read by AERMET, and include all the necessary 

elements for data processing. However, wind data taken at hourly intervals may not always 

portray wind conditions for the entire hour, which can be variable in nature. Hourly wind data 

may also be overly prone to indicate calm conditions, which are not modeled by AERMOD. In 

order to better represent actual wind conditions at the meteorological tower, wind data of one 

minute duration was provided from the same instrument tower, but in a different formatted file to 

be processed by a separate preprocessor, AERMINUTE. These data were subsequently 

integrated into the AERMET processing to produce final hourly wind records of AERMODready meteorological data that better estimate actual hourly average conditions and that are less 

prone to over-report calm wind conditions. This allows AERMOD to apply more hours of 

meteorology to modeled inputs, and therefore produce a more complete set of concentration 
42

estimates. As a guard against excessively high concentrations that could be produced by 

AERMOD in very light wind conditions, the state set a minimum threshold of 0.5 meters per 

second in processing meteorological data for use in AERMOD. This approach is consistent with 

a March 2013 EPA memo titled, “Use of ASOS meteorological data in AERMOD dispersion 

Modeling.” In setting this threshold, no wind speeds lower than this value would be used for 

determining concentrations. This threshold was specifically applied to the one minute wind data. 

Modeling Parameter: Geography and Terrain

The terrain in the area of analysis is best described as flat to gently rolling. To account for these 

terrain changes, the AERMAP terrain program within AERMOD was used to specify terrain

elevations for all the receptors. The source of the elevation data incorporated into the model is 

from the USGS NED. 

Modeling Parameter: Background Concentrations of SO2

The Modeling TAD offers two mechanisms for characterizing background concentrations of SO2

that are ultimately added to the modeled design values: 1) a “first tier” approach, based on 

monitored design values, or 2) a temporally varying approach, based on the 99th percentile 

monitored concentrations by hour of day and season or month. For the Jasper County area of 

analysis, the state chose to use the latter. The background concentrations for this area of analysis 

ranged from 3.3 to 15.5 μg/m3, or 1.3 to 6 ppb.5 AERMOD incorporated these hourly/seasonal 

values into the final results. 

Summary of Modeling Results

The AERMOD modeling parameters for the Jasper County area of analysis are summarized 

below in Table 12.

Table 12. AERMOD Modeling Parameters for the Jasper County, Illinois Area of Analysis

Jasper County, Illinois Area of Analysis

AERMOD Version 15181 

Dispersion Characteristics Rural 

Modeled Sources 1

Modeled Stacks 2

Modeled Structures 9

Modeled Fencelines 1

Total receptors 12,165

Emissions Type Actual

Emissions Years 2012-2014 

Meteorology Years 2012-2014

 

5 The conversion factor for SO2 (at the standard conditions applied in the ambient SO2 reference 

method) is 1ppb = approximately 2.62μg/m3.
43

Surface Meteorology Station Evansville, Indiana

Upper Air Meteorology Station Lincoln, Illinois

Methodology for Calculating 

Background SO2 Concentration Temporally Varying 

Calculated Background SO2

Concentration 1.3 to 6 ppb

The results presented below in Table 13 show the magnitude and geographic location of the 

highest predicted modeled concentration based on actual emissions.

Table 13. Maximum Predicted 99th Percentile 1-Hour SO2

Concentration in the Jasper County, Illinois Area of Analysis Based on Actual Emissions

Averaging Period Data Period

Receptor Location SO2 Concentration (μg/m3

)

UTM/Latitude UTM/Longitude

Modeled 

(including 

background) NAAQS

99th Percentile 

1-Hour Average 2012-2014 390400 4314200 138.89 196.4*

*Equivalent to the 2010 SO2 NAAQS set at 75 ppb

The state’s modeling indicates that the predicted 99th percentile 1-hour average concentration 

within the chosen modeling domain is 138.89 μg/m3, or 53.0 ppb. This modeled concentration 

included the background concentration of SO2, and is based on actual emissions from the 

facilities. Figure 19 below was included as part of the state’s recommendation, and indicates that 

the predicted value occurred approximately 3.85 km northeast of the Newton main stack. The 

state’s receptor grid is also shown in the figure.
44

Figure 19. Maximum Predicted 99th Percentile 1-Hour SO2 Concentrations in the 

Jasper County, Illinois Area of Analysis Based on Actual Emissions

Jurisdictional Boundaries

Once the air quality in the area of analysis associated with Newton is determined, existing 

jurisdictional boundaries are considered for the purpose of informing the boundaries of the 

intended area to be designated, specifically with respect to clearly defined legal boundaries. 

EPA believes that our intended unclassifiable/attainment area, consisting of Jasper County, has 

clearly defined legal boundaries, and we find these boundaries to be a suitable basis for defining 

our intended unclassifiable/attainment area.

There are no SO2 sources over 100 tpy in most of the neighboring counties surrounding Jasper 

County. According to the 2011 NEI, there are two sources emitting over 100 tpy of SO2 in

Crawford County, to the east. While the 2011 emissions would make both of these sources

candidates for future characterization under the DRR (with 2011 emissions for CII Carbon of 

5,521 tons/year and for Ameren Hutsonville of 3,167 tons/year), these facilities are 

approximately 50 km from Newton and 25-30 km from the Jasper County border. Based on the 

distance from these facilities to the county border, EPA does not believe that their emissions are 

likely to cause or contribute to a violation of the NAAQS in Jasper County. Therefore, EPA 
45

believes that it is reasonable to designate Jasper County as unclassifiable/attainment based on 

Illinois’ modeling analysis which shows that Newton is not causing violations of the 2010 SO2

NAAQS in conjunction with the fact that no other sources are expected to cause or contribute to 

violations in Jasper County.

Other Relevant Information

The Sierra Club submitted a modeling analysis to EPA for Newton asserting that the plant was 

contributing to modeled violations. The Sierra Club’s analysis used actual Air Markets Database 

emissions for Newton with fixed temperature and velocity values. Illinois used hourly 

temperature and velocity values. The Sierra Club used a much higher, fixed background value, 

taken from Oglesby, in north central Illinois, whereas Illinois’s background data for Newton, 

reflecting seasonal and hour-of-day variations in background concentrations, came from 

Nilwood, Macoupin County, Illinois, which is closer to the modeling domain and more likely to 

characterize background concentrations at Newton accurately. These differences appear to 

account for the differences between the Sierra Club results and Illinois’ results. Since Illinois’ 

modeling inputs reflect a more accurate characterization of relevant conditions, the Illinois 

results must be considered more reliable. The Sierra Club analysis did not provide information 

refuting the appropriateness of Illinois’ inputs to its analysis of the Jasper County area for this 

round of SO2 designations, and since Illinois’ inputs appear more appropriate, EPA does not find 

that the Sierra Club analysis has provided compelling information to revise EPA’s designation 

decision for Jasper County.

Conclusion

After careful evaluation of the state’s recommendation and supporting information, as well as all 

available relevant information, EPA intends to designate the area around Newton as 

unclassifiable/attainment for the 2010 SO2 NAAQS. Specifically, the area consists of the 

entirety of Jasper County.

At this time, our intended designations for the state only apply to this area and the other areas 

presented in this technical support document. Consistent with the conditions in the March 2, 

2015 court-ordered schedule, EPA will evaluate and designate all remaining undesignated areas 

in Illinois by either December 31, 2017, or December 31, 2020. 
46

Technical Analysis for the Putnam/Bureau Counties, Illinois Area

Introduction

Putnam County contains a stationary source that according to EPA’s Air Markets Database 

emitted in 2012 either more than 16,000 tons of SO2 or more than 2,600 tons of SO2 and had an 

annual average emission rate of at least 0.45 pounds of SO2 per one million British thermal units 

(lbs SO2/MMBTU). As of March 2, 2015, this stationary source had not met the specific 

requirements for being “announced for retirement.” Specifically, in 2012, the Hennepin Power 

Station (“Hennepin”) emitted 5,906 tons of SO2, and had an emissions rate of 0.501 lbs

SO2/MMBTU. Pursuant to the March 2, 2015 court-ordered schedule, EPA must designate the 

area surrounding the facility by July 2, 2016.

In its submission, Illinois recommended that the area surrounding Hennepin, specifically Putnam 

County and neighboring Bureau County, be designated as attainment based on an assessment and

characterization of air quality from the facility and other nearby sources which may have a 

potential impact in the area of analysis where maximum concentrations of SO2 are expected. 

This assessment and characterization was performed using air dispersion modeling software, i.e., 

AERMOD, analyzing actual emissions. After careful review of the state’s assessment, 

supporting documentation, and all available data, EPA agrees that the area is attaining the 

standard, and intends to designate Putnam and Bureau Counties as unclassifiable/attainment.

Hennepin is located in north-central Illinois at the northwestern edge of Putnam County on its 

border with Bureau County. As seen in Figure 20 below, the facility is located approximately 

5.6 km northeast of the town of Hennepin in a rural area bounded on the north by the Illinois 

River. Also included in the figure are nearby emitters of SO2, the state’s recommended area for 

the attainment designation, and EPA’s intended unclassifiable/attainment designation for the 

area.

Figure 20. EPA’s intended designation for Putnam and Bureau Counties, Illinois
47

The discussion and analysis that follows below will reference the state’s use of the Modeling 

TAD, EPA’s assessment of the state’s modeling in accordance with the Modeling TAD, and the 

factors for evaluation contained in EPA’s March 20, 2015 guidance, as appropriate.

Detailed Assessment

Model Selection and Modeling Components

EPA’s Modeling TAD notes that for area designations under the 2010 SO2 NAAQS, the 

AERMOD modeling system should be used, unless use of an alternative model can be justified. 

In some instances the recommended model may be a model other than AERMOD, such as the 

BLP model for buoyant line sources. The AERMOD modeling system contains the following 

components:

- AERMOD: the dispersion model

- AERMAP: the terrain processor for AERMOD

- AERMET: the meteorological data processor for AERMOD

- BPIPPRIME: the building input processor 

- AERMINUTE: a pre-processor to AERMET incorporating 1-minute automated surface 

observation system (ASOS) wind data 
48

- AERSURFACE: the surface characteristics processor for AERMET

- AERSCREEN: a screening version of AERMOD

The state used AERMOD version 15181, the most recent version, and a discussion of the 

individual components will be referenced in the corresponding discussion that follows, as 

appropriate.

Modeling Parameter: Rural or Urban Dispersion

EPA’s recommended procedure for characterizing an area by prevalent land use is based on 

evaluating the dispersion environment with 3 km of the facility. According to EPA’s modeling 

guidelines, rural dispersion coefficients are to be used in the dispersion modeling analysis if 

more than 50% of the area within a 3 km radius of the facility is classified as rural. Conversely, 

if more than 50% of the area is urban, urban dispersion coefficients should be used in the 

modeling analysis. The state performed an Auer’s analysis for the Putnam/Bureau Counties

study area in order to determine which mode was appropriate for the modeling. The analysis 

indicated that the study area is approximately 98.1% rural and 1.9% urban, showing that the rural 

option applied to all emission sources in the modeling domain. Figure 21 and Table 14 below 

show the results of the state’s Auer’s analysis.

Figure 21. Auer’s Analysis – Area Near Hennepin
49

Table 14. Auer’s Analysis Land Use Percentages by Category – Putnam/Bureau Counties Study 

Area

Modeling Parameter: Area of Analysis (Receptor Grid)

A reasonable first step towards characterization of air quality in the area surrounding Hennepin is 

to determine the extent of the area of analysis, i.e., receptor grid. Considerations presented in the 

Modeling TAD include but are not limited to: the location of the SO2 emission sources or 

facilities considered for modeling; the extent of significant concentration gradients of nearby 

sources; and sufficient receptor coverage and density to adequately capture and resolve the 

model predicted maximum SO2 concentrations. For the Putnam/Bureau Counties area, the state 

has included three other emitters of SO2 within 10 km of Hennepin in any direction. The state 

determined that, aside from exceptionally large sources, sources farther than 10 km from 

Hennepin would not cause significant concentration gradients in the Putnam/Bureau Counties

area and therefore need not be modeled. In addition to Hennepin, the other emitters of SO2

included in the area of analysis are: Washington Mills, Advanced Asphalt, and Marquis Energy. 

The state identified no sources beyond 10 km from Hennepin with sufficient emissions to 

warrant including in this analysis.

The grid receptor spacing for the area of analysis chosen by the state is as follows:

- 50 meters along the fenceline (four facilities)

- 100 meters from the fenceline out to a distance of approximately 4.0 kilometers

- 500 meters from 4.0 kilometers out to a distance of approximately 8.0 kilometers

- 1,000 meters from 8.0 kilometers out to a distance of approximately 11 kilometers

The receptor network contained 13,430 receptors, and the network covered the northern twothirds of Putnam County and the southeast portion of Bureau County.

Figure 22, included in the state’s recommendation, shows the state’s receptor grid for the area.
50

For the purposes of this designation effort, the Modeling TAD states that the receptor grid need 

not include receptors in areas where it would not be feasible to place a monitor and record 

ambient air impacts, such as bodies of water. With the exception of receptor locations within 

plant fencelines, Illinois conservatively included the full grid of receptors, including some over 

the Illinois River. 

Figure 22. Receptor Grid for the Putnam/Bureau Counties, Illinois Area of Analysis

Modeling Parameter: Source Characterization

The state characterized the sources within the area of analysis in accordance with the best 

practices outlined in the Modeling TAD. Specifically, the state used actual stack heights in 

conjunction with actual emissions. The state also adequately characterized the modeled sources’

building layout and location, as well as the stack parameters, e.g., exit temperature, exit velocity, 

location, and diameter. The AERMOD component BPIPPRIME was used to assist in addressing 

building downwash.

Modeling Parameter: Emissions 

EPA’s Modeling TAD notes that for the purposes of modeling to characterize air quality for use 

in designations, the recommended approach is to use the most recent 3 years of actual emissions 

data and concurrent meteorological data. However, the TAD does provide for the flexibility of 
51

using allowable emissions in the form of a federally enforceable limit on the emissions rate 

(referred to as PTE or allowable emissions rate).

EPA believes that continuous emissions monitoring systems (CEMS) data provide acceptable 

historical emissions information, when it is available, and that these data are available for many 

electric generating units. In the absence of CEMS data, EPA’s Modeling TAD highly 

encourages the use of AERMOD’s hourly varying emissions keyword HOUREMIS, or through 

the use of AERMOD’s variable emissions factors keyword EMISFACT. When choosing one of 

these methods, EPA believes that detailed throughput, operating schedules, and emissions 

information from the impacted sources should be used. 

In certain instances, states and other interested parties may find that it is more advantageous or 

simpler to use PTE rates as part of their modeling runs. Specifically, a facility may have recently 

adopted a new federally enforceable emissions limit, been subject to a federally enforceable 

consent decree, or implemented other federally enforceable mechanisms and control 

technologies to limit SO2 emissions to a level that indicates compliance with the NAAQS. These 

new limits or conditions may be used in the application of AERMOD. In these cases, the 

Modeling TAD notes that the existing SO2 emissions inventories used for permitting or SIP 

planning demonstrations should contain the necessary emissions information for designationsrelated modeling. In the event that these short-term emissions are not readily available, they may 

be calculated using the methodology in Table 8-1 of Appendix W to 40 CFR Part 51 titled, 

“Guideline on Air Quality Models.” 

 

As previously noted, the state included Hennepin and three other emitters of SO2 within 10 km in 

the area of analysis, and the state concluded that no significant sources existed beyond 10 km 

from Hennepin that warranted inclusion. The facilities in the area of analysis and their 

associated annual actual SO2 emissions between 2012 and 2014 are summarized below.

Table 15. Actual SO2 Emissions in 2012 – 2014 from Facilities in the Putnam/Bureau Counties, 

Illinois Area of Analysis

Facility Name

SO2 Emissions (tons per year) Approximate 

Distance to 

Hennepin

2012 2013 2014 (km)

Hennepin 5,911.25 4,274.35 3,965.36 N/A

Washington Mills 890.20 929.43 1,035.01 1.01

Advanced Asphalt 13.63 13.62 5.46 1.59

Marquis Energy 8.56 9.30 4.05 2.82

Total Emissions From All Facilities 

in the State’s Area of Analysis 6823.64 5226.70 5009.88

For Hennepin, the state used actual emissions from the most recent 3-year data set, i.e., 2012 –

2014. The state used CEMS SO2 emissions data provided by Hennepin, along with hourlyspecific exit temperature and exit velocity.
52

For Washington Mills, the state used company-provided hourly varying emissions, temperature, 

and exit velocities for the largest emitting furnace stack (99.9% of the facility emissions). For 

the two smaller emitting units, Washington Mills provided operating information that allowed 

the state to construct an hourly varying emissions rate coupled with constant temperature and 

exit velocity values.

For Advanced Asphalt, the state constructed an hourly profile based on company-provided 

seasonal throughput.

For Marquis Energy, the state used a combination of seasonal emissions factors (EMISFACT) 

and an hourly emissions profile.

Modeling Parameter: Meteorology and Surface Characteristics

The most recent 3 years of meteorological data (concurrent with the most recent 3 years of 

emissions data) should be used in designations efforts. As noted in the Modeling TAD, the 

selection of data should be based on spatial and climatological (temporal) representativeness. 

The representativeness of the data are based on: 1) the proximity of the meteorological 

monitoring site to the area under consideration, 2) the complexity of terrain, 3) the exposure of 

the meteorological site, and 4) the period of time during which data are collected. Sources of 

meteorological data include National Weather Service (NWS) stations, site-specific or onsite 

data, and other sources such as universities, Federal Aviation Administration (FAA), and 

military stations.

For the Putnam/Bureau Counties area of analysis, surface meteorology from the NWS station in 

Rockford, Illinois, 111 km to the north, and coincident upper air observations from the NWS 

station in Davenport, Iowa, 110 km to the northwest, were selected as best representative of 

meteorological conditions within the area of analysis. 

The state used AERSURFACE version 13016 using data from the NWS station in Rockford, 

Illinois located at (42.20 °N, 89.10 °W) to estimate the surface characteristics of the area of 

analysis. These surface characteristics are the albedo (the fraction of solar energy reflected from 

the earth back into space), the Bowen ratio (representing the ratio of sensible heat flux to latent

heat flux at the ground level), and the surface roughness (representing the influence of ground 

features such as buildings and vegetation on surface wind flow). In the figure below, generated 

by EPA, the location of the Rockford, Illinois NWS station is shown relative to the 

Putnam/Bureau Counties area.

Figure 23. Putnam/Bureau Counties Area and the Rockford, Illinois NWS
53

As part of its recommendation, the state provided the 3-year surface wind rose for Rockford, 

Illinois. In Figure 24, the frequency and magnitude of wind speed and direction are defined in 

terms of from where the wind is blowing. The predominant wind direction during the three-year 

time period used in the modeling is from the south, occurring approximately 12.8% of the time. 

The highest percentage wind speed range, occurring 31.4% of the time, was in the 3.6 – 5.7 m/s 

range.

Figure 24. Rockford, Illinois Cumulative Annual Wind Rose for Years 2012 – 2014
54

Meteorological data from the above surface and upper air stations were used in generating 

AERMOD-ready files with the AERMET processor. The output meteorological data created by 

the AERMET processor is suitable for being applied with AERMOD input files for AERMOD 

modeling runs. The state followed the methodology and settings presented in the U.S. EPA’s 

User’s Guide for the AERMOD Meteorological Preprocessor (November 2004) in the 

processing of the raw meteorological data into an AERMOD-ready format, and used 

AERSURFACE to best represent surface characteristics.

Hourly surface meteorological data records are read by AERMET, and include all the necessary 

elements for data processing. However, wind data taken at hourly intervals may not always 

portray wind conditions for the entire hour, which can be variable in nature. Hourly wind data 

may also be overly prone to indicate calm conditions, which are not modeled by AERMOD. In 

order to better represent actual wind conditions at the meteorological tower, wind data of one 

minute duration was provided from the same instrument tower, but in a different formatted file to 

be processed by a separate preprocessor, AERMINUTE. These data were subsequently 

integrated into the AERMET processing to produce final hourly wind records of AERMODready meteorological data that better estimate actual hourly average conditions and that are less 

prone to over-report calm wind conditions. This allows AERMOD to apply more hours of 
55

meteorology to modeled inputs, and therefore produce a more complete set of concentration 

estimates. As a guard against excessively high concentrations that could be produced by 

AERMOD in very light wind conditions, the state set a minimum threshold of 0.5 meters per 

second in processing meteorological data for use in AERMOD. This approach is consistent with 

a March 2013 EPA memo titled, “Use of ASOS meteorological data in AERMOD dispersion 

Modeling.” In setting this threshold, no wind speeds lower than this value would be used for 

determining concentrations. This threshold was specifically applied to the one minute wind data. 

Modeling Parameter: Geography and Terrain

The terrain in the area of analysis is best described as flat to gently rolling. To account for these 

terrain changes, the AERMAP terrain program within AERMOD was used to specify terrain 

elevations for all the receptors. The source of the elevation data incorporated into the model is 

from the USGS National Elevation Database (NED). 

Modeling Parameter: Background Concentrations of SO2

The Modeling TAD offers two mechanisms for characterizing background concentrations of SO2

that are ultimately added to the modeled design values: 1) a “first tier” approach, based on 

monitored design values, or 2) a temporally varying approach, based on the 99th percentile 

monitored concentrations by hour of day and season or month. For the Putnam/Bureau Counties

area of analysis, the state chose to use the latter. The background concentrations for this area of 

analysis ranged from 2 to 15 μg/m3, or approximately 0.8 to 5.7 ppb.6 AERMOD incorporated 

these hourly/seasonal values into the final results. 

Summary of Modeling Results

The AERMOD modeling parameters for the Putnam/Bureau Counties area of analysis are 

summarized below in Table 16.

Table 16. AERMOD Modeling Parameters for the Putnam/Bureau Counties, Illinois Area of 

Analysis

Putnam/Bureau Counties, Illinois Area of Analysis

AERMOD Version 15181 

Dispersion Characteristics Rural

Modeled Sources 4

Modeled Stacks 7

Modeled Structures 90

Modeled Fencelines 4

Total receptors 13,429

Emissions Type Actual 

Emissions Years 2012-2014 

 

6 The conversion factor for SO2 (at the standard conditions applied in the ambient SO2 reference 

method) is 1ppb = approximately 2.62μg/m3.
56

Meteorology Years 2012-2014 

Surface Meteorology Station Rockford, Illinois

Upper Air Meteorology Station Davenport, Iowa

Methodology for Calculating 

Background SO2 Concentration Temporally Varying

Calculated Background SO2

Concentration 0.8 to 5.7 ppb

The results presented below in Table 17 show the magnitude and geographic location of the 

highest predicted modeled concentration based on actual emissions.

Table 17. Maximum Predicted 99th Percentile 1-Hour SO2 Concentration in the Putnam/Bureau 

Counties Area Based on Actual Emissions

Averaging Period Data Period

Receptor Location SO2 Concentration (μg/m3

)

UTM/Latitude UTM/Longitude

Modeled 

(including 

background) NAAQS

99th Percentile 

1-Hour Average 2012-2014 311600 4570200 94.56 196.4*

*Equivalent to the 2010 SO2 NAAQS set at 75 ppb

The state’s modeling indicates that the predicted 99th percentile 1-hour average concentration 

within the chosen modeling domain is 94.56 μg/m3, or 36.1 ppb. This modeled concentration 

included the background concentration of SO2, and is based on actual emissions from the 

facilities. Figure 25 below was included as part of the state’s recommendation, and indicates that 

the predicted value occurred within the dense 100-meter grid at an elevated location 7.2 km 

southeast of Hennepin. The state’s receptor grid is also shown in the figure.
57

Figure 25. Maximum Predicted 99th Percentile 1-Hour SO2 Concentrations in the Putnam/Bureau 

Counties, Illinois Area of Analysis Based on Actual Emissions

Jurisdictional Boundaries:

Once the geographic area of analysis associated with Hennepin, other nearby sources, and 

background concentrations are determined, existing jurisdictional boundaries are considered for 

the purpose of informing our intended unclassifiable/attainment area, specifically with respect to 

clearly defined legal boundaries. EPA believes that our intended unclassifiable/attainment area, 

consisting of Putnam and Bureau Counties, are comprised of clearly defined legal boundaries, 

and we find these boundaries to be a suitably clear basis for defining our intended 

unclassifiable/attainment area. Illinois’ modeling demonstrated that Hennepin and the nearby 

SO2 sources did not cause or contribute to violations in the modeled domain, which included the 

portions of Putnam and Bureau Counties likely to experience the highest concentrations in the 

area. EPA reviewed the SO2 sources located in neighboring counties which were not included in 

Illinois’ modeling analysis of the Hennepin, and determined that none are expected to contribute 

to violations of the 2010 SO2 NAAQS in Putnam or Bureau Counties. 

EPA has confirmed that there are no additional sources in Putnam or Bureau Counties or any 

adjacent county that would be likely candidates for future characterization under the DRR. Two 

facilities located in neighboring LaSalle County, Pilkington and Owens-Brockway, emit over 

100 tpy, but are located over 20 km from the Putnam County line. An ambient air quality 
58

monitor (AQS ID 170-99-0007) located between these facilities and the county line recorded a 

2012 – 2014 design value of 8 ppb, which suggests that the impact of these facilities in Putnam 

County is even smaller. Therefore, EPA does not believe that these sources cause or contribute 

to a violation of the NAAQS in Putnam County. 

Sterling Steel, located approximately 20 km from the Bureau County border in neighboring 

Whiteside County, emitted 208 tpy of SO2 according to the 2011 NEI. Based on these levels of 

emissions in conjunction with the distance from the county border, EPA does not believe that 

emissions from Sterling Steel cause or contribute to a violation of the NAAQS in Bureau 

County. 

Additional sources of SO2 emitting above 100 tpy are all over 50 km from the Putnam and 

Bureau County borders. As a result, EPA does not believe that they cause or contribute to a 

violation of the NAAQS in either county. The consent decree directs EPA to designate the area 

around Hennepin, which may be confined to the county where the facility is located, i.e., Putnam 

County. In this case, however, the source is on the border between two counties, and the 

information provided by Illinois and confirmed by EPA equally addresses air quality in Putnam 

and Bureau Counties. Therefore, EPA intends to follow Illinois’ recommendation to include 

both counties in the unclassifiable/attainment area.

Other Relevant Information

There was no additional relevant information submitted for Hennepin or Putnam or Bureau 

Counties. 

Conclusion

After careful evaluation of the state’s recommendation and supporting information, as well as all 

available relevant information, EPA intends to designate the area around Hennepin as 

unclassifiable/attainment for the 2010 SO2 NAAQS. Specifically, the boundaries are comprised 

of the Putnam and Bureau County lines. 

At this time, our intended designations for the state only apply to this area and the other areas 

presented in this technical support document. Consistent with the conditions in the March 2, 

2015 court-ordered schedule, EPA will evaluate and designate all remaining undesignated areas 

in Illinois by either December 31, 2017, or December 31, 2020. 
59

Technical Analysis for the Williamson County, Illinois Area

Introduction

Williamson County contains a stationary source that according to EPA’s Air Markets Database 

emitted in 2012 either more than 16,000 tons of SO2 or more than 2,600 tons of SO2 and had an 

annual average emission rate of at least 0.45 pounds of SO2 per one million British thermal units 

(lbs SO2/MMBTU). As of March 2, 2015, this stationary source had not met the specific 

requirements for being “announced for retirement.” Specifically, in 2012, the Marion Power 

Station (“Marion”) emitted 5,850 tons of SO2, and had an emissions rate of 0.489 lbs

SO2/MMBTU. Pursuant to the March 2, 2015 court-ordered schedule, EPA must designate the 

area surrounding the facility by July 2, 2016.

In its submission, Illinois recommended that the area surrounding Marion, specifically the 

entirety of Williamson County, be designated as attainment based on an assessment and 

characterization of air quality from the facility and other nearby sources which may have a 

potential impact in the area of analysis where maximum concentrations of SO2 are expected. 

This assessment and characterization was performed using air dispersion modeling software, i.e., 

AERMOD, analyzing an emissions rate described as the “maximum actual emissions expected” 

from this plant. After careful review of the state’s assessment, supporting documentation, and all 

available data, EPA disagrees with the state’s recommendation. As discussed below, Illinois 

modeled Marion as emitting 5,512 tons per year, but this emission rate cannot be characterized 

as reflecting either actual emissions or allowable emissions. In this respect, Illinois’ modeling is 

contrary to the recommendations of the Modeling TAD. Illinois provided no justification for 

using this emission rate, and EPA is finding that the state’s modeling does not support an 

attainment or an unclassifiable/attainment designation for this area.

In contrast, Sierra Club provided modeling that relied on actual emissions data for 2012 to 2014, 

consistent with an approach recommended in the Modeling TAD. While this modeling addresses 

some modeling inputs in a less reliable fashion than Illinois, EPA finds overall, especially 

considering the superior treatment of emissions, that Sierra Club’s modeling provides a more 

reliable assessment of whether the area near Marion is meeting the SO2 NAAQS. Sierra Club 

estimated a maximum concentration in Williamson County of 110.3 ppb.

For 2012 to 2014, emissions of SO2 from Marion averaged 7,620 tons per year, approximately 38 

percent higher than the emission rate Illinois modeled. As a result, the results that Illinois 

obtained using an SO2 emission rate of 5,512 tons per year, finding a maximum estimated 

concentration of 74.3 ppb, suggests that modeling using actual emissions would also have shown 

a violation of the SO2 NAAQS. Consequently, based on Sierra Club’s modeling, with 

supplemental evidence from Illinois’ modeling, EPA concludes that the Williamson County area 

is violating the SO2 NAAQS.

Although Marion is only 1.5 km from the border of Williamson County with Johnson County, 

Sierra Club’s modeling using actual emissions indicates that violations are occurring only in 

Williamson County. Given the uncertainties in the available evidence, EPA is not attempting to 
60

determine whether portions of Williamson County might warrant being designated 

unclassifiable/attainment. Therefore, EPA’s current intent is to designate the entirety of 

Williamson County as nonattainment.

Marion is located in southern Illinois in southern Williamson County, in a rural area adjacent to

the Lake of Egypt on the east. As seen in Figure 26 below, the facility is located approximately 

11 km south-southwest of the center of Marion. Also included in the figure are nearby emitters 

of SO2, the state’s recommended attainment area, and EPA’s intended nonattainment area.

Figure 26. EPA’s intended designation for Williamson County, Illinois

The discussion and analysis that follows below will reference the state’s use of the Modeling 

TAD, EPA’s assessment of the state’s modeling in accordance with the Modeling TAD, and the 

factors for evaluation contained in EPA’s March 20, 2015 guidance, as appropriate.

Detailed Assessment

Model Selection and Modeling Components
61

EPA’s Modeling TAD notes that for area designations under the 2010 SO2 NAAQS, the 

AERMOD modeling system should be used, unless use of an alternative model can be justified. 

In some instances the recommended model may be a model other than AERMOD, such as the 

BLP model for buoyant line sources. The AERMOD modeling system contains the following 

components:

- AERMOD: the dispersion model

- AERMAP: the terrain processor for AERMOD

- AERMET: the meteorological data processor for AERMOD

- BPIPPRIME: the building input processor 

- AERMINUTE: a pre-processor to AERMET incorporating 1-minute automated surface 

observation system (ASOS) wind data 

- AERSURFACE: the surface characteristics processor for AERMET

- AERSCREEN: a screening version of AERMOD

The state used AERMOD version 15181, the most recent version, and a discussion of the 

individual components will be referenced in the corresponding discussion that follows, as 

appropriate.

Modeling Parameter: Rural or Urban Dispersion

EPA’s recommended procedure for characterizing an area by prevalent land use is based on 

evaluating the dispersion environment with 3 km of the facility. According to EPA’s modeling 

guidelines, rural dispersion coefficients are to be used in the dispersion modeling analysis if 

more than 50% of the area within a 3 km radius of the facility is classified as rural. Conversely, 

if more than 50% of the area is urban, urban dispersion coefficients should be used in the 

modeling analysis. The state performed an Auer’s analysis for the Marion study area in order to 

determine which mode was appropriate for the modeling. The analysis indicated that the study 

area is approximately 98.5% rural and 1.5% urban, showing that the rural option applied to all 

emission sources in the modeling domain. Figure 27 and Table 18 below show the results of the 

state’s Auer’s analysis.

Figure 27. Auer’s Analysis – Area Near Marion
62

Table 18. Auer’s Analysis Land Use Percentages by Category – Williamson County Study Area
63

Modeling Parameter: Area of Analysis (Receptor Grid)

A reasonable first step towards characterization of air quality in the area surrounding Marion is 

to determine the extent of the area of analysis, i.e., receptor grid. Considerations presented in the 

Modeling TAD include but are not limited to: the location of the SO2 emission sources or 

facilities considered for modeling; the extent of significant concentration gradients of nearby 

sources; and sufficient receptor coverage and density to adequately capture and resolve the 

model predicted maximum SO2 concentrations. The state determined that, aside from 

exceptionally large sources, sources farther than 10 km from Marion would not cause significant 

concentration gradients in the Williamson County area and therefore need not be modeled. For 

the Williamson County area, the state identified one other significant emitter of SO2 within 10 

km of Marion in any direction, and the state identified no significant emitter of SO2 beyond 10 

km that warranted inclusion in the analysis. In addition to Marion, the other emitter of SO2

included in the area of analysis is the United States Penitentiary.

The grid receptor spacing for the area of analysis chosen by the state is as follows:

- 50 meters along the fenceline (two facilities)

- 100 meters from the fenceline out to approximately 5.0 kilometers

- 500 meters from 5.0 kilometers out to approximately 8.0 kilometers

- 1,000 meters from 8.0 kilometers out to approximately 10 kilometers

The receptor network contained 25,118 receptors, and the network covered central and southcentral Williamson County, the northeast corner of Union County, and the northwest and northcentral part of Johnson County.

Figure 28, included in the state’s recommendation, shows the state’s chosen receptor grid for the 

area.

For the purposes of this designation effort, the Modeling TAD states that the receptor grid need 

not include receptors in areas where it would not be feasible to place a monitor and record 

ambient air impacts, such as bodies of water. With the exception of receptor locations within 

plant fencelines, Illinois conservatively included the full grid of receptors, including some over 

the Lake of Egypt and other nearby bodies of water. 

Figure 28. Receptor Grid for the Williamson County, Illinois Area of Analysis
64

Modeling Parameter: Source Characterization

The state characterized the physical plant of Marion in accordance with the best practices 

outlined in the Modeling TAD. Specifically, the state used actual stack heights. The state also 

adequately characterized the building layout and location for the modeled sources, as well as the 

stack parameters, e.g., exit temperature, exit velocity, location, and diameter. The AERMOD 

component BPIPPRIME was used to assist in addressing building downwash. 

Modeling Parameter: Emissions 

EPA’s Modeling TAD notes that for the purposes of modeling to characterize air quality for use 

in designations, the recommended approach is to use the most recent 3 years of actual emissions 

data and concurrent meteorological data. However, the TAD does provide for the flexibility of 

using allowable emissions in the form of a federally enforceable limit on the emissions rate 

(referred to as PTE or allowable emissions rate).

EPA believes that continuous emissions monitoring systems (CEMS) data provide acceptable 

historical emissions information, when it is available, and that these data are available for many 

electric generating units. In the absence of CEMS data, EPA’s Modeling TAD highly 

encourages the use of AERMOD’s hourly varying emissions keyword HOUREMIS, or through 

the use of AERMOD’s variable emissions factors keyword EMISFACT. When choosing one of 

these methods, EPA believes that detailed throughput, operating schedules, and emissions 

information from the impacted sources should be used. 
65

In certain instances, states and other interested parties may find that it is more advantageous or 

simpler to use PTE rates as part of their modeling runs. Specifically, a facility may have recently 

adopted a new federally enforceable emissions limit, been subject to a federally enforceable 

consent decree, or implemented other federally enforceable mechanisms and control 

technologies to limit SO2 emissions to a level that indicates compliance with the NAAQS. These 

new limits or conditions may be used in the application of AERMOD. In these cases, the 

Modeling TAD notes that the existing SO2 emissions inventories used for permitting or SIP 

planning demonstrations should contain the necessary emissions information for designationsrelated modeling. In the event that these short-term emissions are not readily available, they may 

be calculated using the methodology in Table 8-1 of Appendix W to 40 CFR Part 51 titled, 

“Guideline on Air Quality Models.” 

 

As previously noted, the state included Marion and one other emitter of SO2 within 10 km in the

area of analysis, namely the U.S. Penitentiary. No other sources within or beyond 10 km were 

determined by the state to have the potential to cause significant concentration gradient impacts 

within the area of analysis. 

Illinois modeled fixed emission rates for the two facilities in this area. Illinois’ submittal states, 

“The four Marion Power Station stacks were modeled at maximum actuals expected from the 

source.” Specifically, Illinois modeled this facility as having a fixed emission rate of 5,512 tons 

per year in each of 2012, 2013, and 2014.

However, the actual emission rates that the facility reported to the CAMD database are 

substantially higher than those that Illinois modeled. The reported emission rates are shown in 

Table 19 below. Based on these data, for 2012 to 2014, average actual emissions were 7,620 

tons per year, or approximately 38 percent higher than the emissions modeled by Illinois.

Illinois provides no discussion of this discrepancy. Illinois provides no discussion of the 

derivation of the particular emission rate that it modeled, and Illinois provides no rationale for 

modeling an emission rate lower than the recent emission rates at the facility. That is, while 

Illinois states that it modeled Marion at the maximum actual emission rate expected for the 

facility, Illinois identified no reason for EPA to believe that this facility can be expected to emit 

so much less than it emitted in 2012-2014. Normally, “maximum actual emissions” would mean 

the maximum emission rate that has actually occurred in the recent past. “Allowable emissions” 

would mean the maximum emission rate that would be allowed for the facility, which to be 

creditable for consideration in this designations process would need to meet relevant criteria, in 

particular being federally enforceable and requiring compliance adequately in advance of EPA’s 

promulgation of the designation to quality as representing current air quality as of the time of the 

designation. Illinois has provided no information to indicate that its modeled emission rate for 

Marion corresponds to either of these means of determining an emission rate to model. In short, 

Illinois has provided no justification for determining the designation for the Williamson County

area on the basis of an emission rate that is substantially below recent actual emission rates.

For the other source in the Williamson County area that Illinois modeled, namely the U.S. 

Penitentiary, Illinois states that in absence of data to indicate the temporal variability of this 

facility’s emissions, “the conservative worst-case year emissions were applied to each year and 
66

spread uniformly throughout every hour.” The emission rate for this facility in the 2011 NEI is 

0.02 tons per year, well below the 0.18 tons per year that Illinois modeled, so this emission rate 

indeed appears to provide a suitable, conservative assessment of this facility’s impact.

Table 19 shows actual emission rates for 2012 to 2014 along with the emission rates modeled by 

Illinois. For Marion, the actual emission rates reflect data reported to CAMD. Although annual 

emissions rates for the U.S. Penitentiary were not reported by Illinois and are not otherwise 

available, the emission rate modeled by Illinois appears conservative as compared to the 

emission rate included in the 2011 NEI.

Table 19. Actual SO2 Emissions between 2012 – 2014 from Facilities in the Williamson County, 

Illinois Area, Compared to the Emissions Illinois Modeled

Facility Name

Actual SO2 Emissions (tons per year) Emission Rate 

Illinois 

Modeled

Approximate 

Distance to 

Marion (km) 2012 2013 2014

Marion (SIPCO) 5850 8357 8652 5512.11 N/A

United States Penitentiary N/A N/A N/A 0.18 5.57

Total Emissions From All Facilities 

in the State’s Area of Analysis 5850 8357 8652 5512.29

Modeling Parameter: Meteorology and Surface Characteristics

The most recent 3 years of meteorological data (concurrent with the most recent 3 years of 

emissions data) should be used in designations efforts. As noted in the Modeling TAD, the 

selection of data should be based on spatial and climatological (temporal) representativeness. 

The representativeness of the data are based on: 1) the proximity of the meteorological 

monitoring site to the area under consideration, 2) the complexity of terrain, 3) the exposure of 

the meteorological site, and 4) the period of time during which data are collected. Sources of 

meteorological data include National Weather Service (NWS) stations, site-specific or onsite 

data, and other sources such as universities, Federal Aviation Administration (FAA), and 

military stations.

For the Williamson County area, surface meteorology from the NWS station in Paducah, 

Kentucky, 64km to the southeast, and coincident upper air observations from the NWS station in 

Nashville, Tennessee, 260 km to the southeast were selected as best representative of 

meteorological conditions within the area of analysis. 

The state used AERSURFACE version 13016 using data from the NWS station in Paducah, 

Kentucky located at (37.05822 °N, 88.77251 °W) to estimate the surface characteristics of the 

area of analysis. These surface characteristics are the albedo (the fraction of solar energy 

reflected from the earth back into space), the Bowen ratio (representing the ratio of sensible heat 

flux to latent heat flux at the ground level), and the surface roughness (representing the influence 

of ground features such as buildings and vegetation on surface wind flow). 
67

As part of its recommendation, the state provided the 3-year surface wind rose for Paducah, 

Kentucky. In Figure 29, the frequency and magnitude of wind speed and direction are defined in 

terms of from where the wind is blowing. The predominant wind direction during the three-year 

time period used in the modeling is from the southwest, occurring approximately 11.4% of the 

time. The highest percentage wind speed range, occurring 32.9% of the time, was in the 2.1 –

3.6 m/s range.

Figure 29. Paducah, Kentucky Cumulative Annual Wind Rose for Years 2012 – 2014

Meteorological data from the above surface and upper air stations were used in generating 

AERMOD-ready files with the AERMET processor. The output meteorological data created by 

the AERMET processor is suitable for being applied with AERMOD input files for AERMOD 

modeling runs. The state followed the methodology and settings presented in the U.S. EPA’s 

User’s Guide for the AERMOD Meteorological Preprocessor (November 2004) in the 

processing of the raw meteorological data into an AERMOD-ready format, and used 

AERSURFACE to best represent surface characteristics.

Hourly surface meteorological data records are read by AERMET, and include all the necessary 

elements for data processing. However, wind data taken at hourly intervals may not always 

portray wind conditions for the entire hour, which can be variable in nature. Hourly wind data 
68

may also be overly prone to indicate calm conditions, which are not modeled by AERMOD. In 

order to better represent actual wind conditions at the meteorological tower, wind data of one 

minute duration was provided from the same instrument tower, but in a different formatted file to 

be processed by a separate preprocessor, AERMINUTE. These data were subsequently 

integrated into the AERMET processing to produce final hourly wind records of AERMODready meteorological data that better estimate actual hourly average conditions and that are less 

prone to over-report calm wind conditions. This allows AERMOD to apply more hours of 

meteorology to modeled inputs, and therefore produce a more complete set of concentration 

estimates. As a guard against excessively high concentrations that could be produced by 

AERMOD in very light wind conditions, the state set a minimum threshold of 0.5 meters per 

second in processing meteorological data for use in AERMOD. This approach is consistent with 

a March 2013 EPA memo titled, “Use of ASOS meteorological data in AERMOD dispersion 

Modeling.” In setting this threshold, no wind speeds lower than this value would be used for 

determining concentrations. This threshold was specifically applied to the one minute wind data. 

Modeling Parameter: Geography and Terrain

The terrain in the area of analysis is best described as complex to gently rolling (south to north). 

To account for these terrain changes, the AERMAP terrain program within AERMOD was used 

to specify terrain elevations for all the receptors. The source of the elevation data incorporated 

into the model is from the USGS National Elevation Database (NED). 

Modeling Parameter: Background Concentrations of SO2

The Modeling TAD offers two mechanisms for characterizing background concentrations of SO2

that are ultimately added to the modeled design values: 1) a “first tier” approach, based on 

monitored design values, or 2) a temporally varying approach, based on the 99th percentile 

monitored concentrations by hour of day and season or month. For the Williamson County area 

of analysis, the state chose to use the latter. The background concentrations for this area of 

analysis ranged from 3.6 to 32.8μg/m3, or 1.4 to 12.5 ppb.7 AERMOD incorporated these 

hourly/seasonal values into the final results. 

Summary of Modeling Results

Illinois’ AERMOD modeling parameters for the Williamson County area of analysis are 

summarized below in Table 20.

Table 20. AERMOD Modeling Parameters for the Williamson County, Illinois Area of Analysis

Williamson County, Illinois Area of Analysis

AERMOD Version 15181

Dispersion Characteristics Rural

Modeled Sources 2

 

7 The conversion factor for SO2 (at the standard conditions applied in the ambient SO2 reference 

method) is 1ppb = approximately 2.62μg/m3.
69

Modeled Stacks 8

Modeled Structures 37

Modeled Fencelines 2

Total receptors 25,118

Emissions Type Not explained

Emissions Years Unspecified

Meteorology Years 2012-2014

Surface Meteorology Station Paducah, Kentucky 

Upper Air Meteorology Station Nashville, Tennessee 

Methodology for Calculating 

Background SO2 Concentration Temporally Varying

Calculated Background SO2

Concentration 1.4 to 12.5 ppb

The results presented below in Table 21 show the magnitude and geographic location of the 

highest predicted modeled concentration based on PTE emissions.

Table 21. Maximum Predicted 99th Percentile 1-Hour SO2 Concentration in the Williamson 

County, Illinois Area Based on Illinois’ Modeled Emissions

Averaging Period Data Period

Receptor Location SO2 Concentration (μg/m3

)

UTM/Latitude UTM/Longitude

Modeled 

(including 

background) NAAQS

99th Percentile 

1-Hour Average 2012-2014 327200 4166200 194.48 196.4*

*Equivalent to the 2010 SO2 NAAQS set at 75 ppb

The state’s modeling indicates that the predicted 99th percentile 1-hour average concentration 

within the chosen modeling domain is 194.48 μg/m3, or 74.3 ppb. This modeled concentration 

included the background concentration of SO2, and is based on emission values that, particularly 

for Marion, have not been justified to be an appropriate basis for designation of this area. Figure 

30 below was included as part of the state’s recommendation, and indicates that Illinois’

predicted value occurred within the dense 100-meter grid 1.2 km north-northwest of Marion. 

The state’s receptor grid is also shown in the figure.

As discussed above, a more appropriate basis for determining the designation for this area would 

be a modeling analysis that reflects actual emissions from Marion, which as discussed above 

averaged approximately 38 percent higher than the emission rate that Illinois modeled. A full 

modeling analysis would be necessary to determine precisely the air quality that would be 

estimated based on actual 2012 to 2014 emissions from this facility. Nevertheless, since Marion 

is the dominant source in the area, and insofar as impacts of this source may be assumed to be 
70

proportionate to emission levels8, Illinois’ modeling provides strong evidence that modeling

using actual emissions that average approximately 38 percent higher than the emission rate 

Illinois modeled would estimate concentrations well over the SO2 NAAQS. 

There are no other sources within Williamson County that emit over 100 tpy of SO2, but there 

are two sources with emissions above 100 tpy in neighboring counties. In Jackson County, 

Southern Illinois University is listed in the 2011 NEI as emitting 746 tons per year and is 

approximately 5.5 km to the west of the Williamson County border. This facility has not been 

listed for future characterization under the DRR. When considered with the distance from the 

county border, EPA finds it unlikely that this facility is causing or contributing to a violation of 

the NAAQS in Williamson County. (For comparison purposes, Illinois estimated the maximum 

modeled impact from Marion to be 1.2 km from the facility.) Similarly, in Union County, Clyde 

Choate Mental Health Center is listed in the 2011 NEI as emitting 740 tons per year and is 

approximately 16 km to the southwest of the Williamson County border. This facility also has 

not been listed for future characterization under the DRR, and is also sufficiently distant from the 

Williamson County border that EPA believes that it also is unlikely to cause or contribute to a 

violation of the NAAQS within Williamson County. 

As discussed elsewhere in this document, the Joppa Power Plant is 43 km south of the 

Williamson County border, and the area surrounding the facility has been found to be attaining 

the standard. Therefore, EPA does not believe that any of these sources cause or contribute to a 

violation of the 2010 SO2 NAAQS in Williamson County. Figure 30 shows Illinois’ modeling 

results, reflecting an emission rate for Marion that is inconsistent with recommendations in the 

Modeling TAD.

 

8 The difference in concentrations estimated by modeling actual emissions versus concentrations estimated by 

modeling a fixed 5,512 tons per year rate would depend on how emissions on the critical days compare to a 5,512 

tons per year emission rate. Nevertheless, the difference between the emission rate that Illinois modeled (5,512 

tons per year) and the average 2012-2014 emission rate (7,620 tons per year, on average 38 percent higher) 

provides an approximation of the likely difference in estimated concentrations that use of actual emissions would 

be expected to yield. 
71

Figure 30. Maximum Predicted 99th Percentile 1-Hour SO2 Concentrations in the Marion, Illinois

Area of Analysis Based on Inappropriate Emission Rates

Other Relevant Information

The Sierra Club submitted a modeling analysis to EPA for the Marion concluding that it was 

contributing to modeled violations. The Sierra Club’s analysis used actual Air Markets Database 

emissions for Marion. This modeling analysis used AERMOD. Sierra Club modeled both actual 

and allowable emissions for Marion. Because the analysis using actual emissions more 

accurately represents current air quality, EPA considers that analysis to be a more appropriate 

basis for determining the designation of this area. 

Sierra Club modeling used surface meteorological data from the NWS station at the Paducah 

Barkley Regional Airport and upper air data from the Nashville, Tennessee upper air station, 

which are the same data that Illinois used. Sierra Club used AERSURFACE output files 

provided by Illinois EPA. Sierra Club used the following Cartesian grid of receptors:

- Receptors every 100 meters from Marion out to 5 km from Marion,

- Receptors every 500 meters from 5 to 10 km, and 

- Receptors every 1,000 meters from 10 km to 50 km

“A flagpole height of 1.5 meters was used for all these receptors.” No building downwash was 

considered. Background concentrations were based on the design value from a monitor in 
72

LaSalle, Illinois, which Sierra Club identifies as the lowest design value in Illinois for 2011 to 

2013. Sierra Club modeled emissions from both Marion and Joppa (43 km south-southeast of 

Marion).

From this modeling, Sierra Club estimated a maximum concentration (based on actual 

emissions) in the Williamson County area of 288.9 μg/m3, or 110.3 ppb. A map of these results 

is shown in Figure 31. Significantly, although Marion is only 1.5 km from the border of 

Williamson County with Johnson County, these Sierra Club modeling results indicate that 

violations are occurring only in Williamson County. 

Figure 31. Sierra Club modeling results near Marion using actual emissions

Illinois has identified concerns with Sierra Club’s modeling. Illinois considers its background 

value, derived from a monitor in Randolph County (with exclusions for hours with potentially 

significant impacts from nearby sources), and reflecting hour-of-day and seasonal variability, to 

be more representative than a fixed value obtained from a monitor much further from Marion. 
73

Illinois expressed concern that Sierra Club used receptors 1.5 meters above ground, contrary to 

the recommendation of the Modeling TAD. Illinois noted that Sierra Club’s modeling used fixed 

stack parameters (flow rates and exit temperatures), whereas Illinois used hourly actual values.

Although a complete evaluation of the significance of these differences would require a full 

modeling analysis, which was not provided to EPA, EPA considered the likely significance of 

these differences. Compared to Sierra Club’s background concentration of 8.0 ppb, Illinois used 

background concentrations ranging from 1.4 to 12.5 ppb, with an average value of 4.5 ppb. Use 

of “flagpole height” receptors is not prone to affect concentrations substantially. Use of hourly 

varying stack parameters can yield a significant impact on estimated concentrations.

Nevertheless, the most critical difference between Illinois’ modeling and Sierra Club’s modeling 

is the fact that the Sierra Club modeled actual emissions, consistent with the recommendations in 

the Modeling TAD, whereas Illinois used a substantially lower emission rate with no evident 

relationship to either actual or allowable emission rates and no justification for its use. If one 

approximates the results that Illinois might have obtained by scaling the state’s results according 

to the degree by which Illinois understated recent average emissions, these results would be 

expected to be similar to the results obtained by the Sierra Club. Thus, while in other respects 

EPA finds Illinois’ modeling to be more reliable, the most important difference between the two 

analyses appears to be the emission rates used, in particular that the Sierra Club used emission 

rates consistent with the recommendations of the Modeling TAD and Illinois did not.

Sierra Club modeling shows that the Joppa plant, in Massac County, located 43 km to the south 

of Marion, has minimal impact on violations in Williamson County. Therefore, this analysis 

supports a conclusion that Joppa should be considered not to be a nearby contributor to the 

violations in Williamson County and should therefore not be included in the nonattainment area.

Jurisdictional Boundaries:

Once the nature of the designation of the Williamson County area is determined, and once EPA 

has reviewed other considerations as to the spatial extent of over which air quality is known and 

over which other nearby sources are located, existing jurisdictional boundaries are considered for 

the purpose of informing our intended area to be designated, specifically to assure that the area is 

defined using clearly defined legal boundaries. EPA believes that our intended nonattainment 

area, consisting of Williamson County, is comprised of clearly defined legal boundaries, and we 

find these boundaries to be a suitably clear basis for defining our intended nonattainment area. 

This is the same area that Illinois recommended be designated attainment. Although the 

available evidence suggests that portions of the county are attaining the SO2 NAAQS, EPA did 

not attempt to define a subcounty area to differentiate what portion of Williamson County is and 

is not violating the SO2 NAAQS. 

Conclusion

After careful evaluation of the state’s recommendation and supporting information, as well as all 

available relevant information, EPA intends to designate the area around Marion as 

nonattainment for the 2010 SO2 NAAQS. Specifically, the boundaries are comprised of the 

Williamson County boundary lines.
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At this time, our intended designations for the state only apply to this area and the other areas 

presented in this technical support document. Consistent with the conditions in the March 2, 

2015 court-ordered schedule, EPA will evaluate and designate all remaining undesignated areas 

in Illinois by either December 31, 2017, or December 31, 2020.