2010 Primary Sulfur Dioxide (SO2)

National Ambient Air Quality Standard (NAAQS)

Technical Analyses and Information Supporting the

Designation Recommendation for Oklahoma

Introduction and Summary

Air Quality Division

Oklahoma Department of Environmental Quality

September 18, 2015
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Introduction, Summary, and State Recommendation

In 2010, the U.S. Environmental Protection Agency (EPA) promulgated a new National Ambient 

Air Quality Standard (NAAQS) for sulfur dioxide (SO2) of 75 parts per billion (ppb) on a 1-hour 

average. In response, Governor Mary Fallin submitted a letter dated May 27, 2011 providing the 

State of Oklahoma’s recommendations for designations under the new NAAQS. The State 

recommended a designation of unclassifiable for Muskogee and Tulsa counties, and attainment 

for all the other 75 counties within the state. EPA identified areas of the U.S. with monitored 

violations of the 2010 SO2 NAAQS, designated those areas as nonattainment in July 2013, and 

deferred designations for remaining areas. No areas in Oklahoma were included in the July 2013 

designations. 

On March 2, 2015, the U.S. District Court for the Northern District of California entered a 

federal consent decree between EPA and Sierra Club (SC) and Natural Resources Defense 

Council (NRDC), which resolved litigation over the deadline for EPA’s completion of 

designations. The court order established that EPA must complete designations in three 

additional rounds. The first round of designations must be completed by July 2, 2016, and must 

be made for 1) areas that have newly monitored violations of the 2010 SO2 standard and 2) areas 

that contain any stationary source that according to the EPA’s Air Markets Database either 

emitted more than 16,000 tons of SO2 in 2012 or emitted more than 2,600 tons of SO2 and had an 

emission rate of at least 0.45 lbs SO2/MMBTU in 2012 and that has not been announced for 

retirement (as of March 2, 2015). 

In a letter dated March 20, 2015 to Scott Thompson, Executive Director, Oklahoma Department 

of Environmental Quality (DEQ), EPA Acting Assistant Administrator Janet McCabe identified 

three affected electric power plant sources in Oklahoma that meet the criteria for designation by 

July 2, 2016: Western Farmers Electric Cooperative (WFEC) Hugo Generating Station in 

Choctaw County, Oklahoma Gas and Electric (OG&E) Sooner Generating Station in Noble 

County, and OG&E Muskogee Generating Station in Muskogee County. EPA stated that the 

most recent information available would be used in making designations and boundary decisions 

for these areas in Oklahoma, and provided an opportunity for Oklahoma to submit an updated 

recommendation and supporting information to inform the designations for the affected areas. 

Any updated recommendation and supporting information was to be submitted to EPA by 

September 18, 2015, and was to conform to the “Updated Guidance for Area Designation for the 

2010 Primary Sulfur Dioxide National Ambient Air Quality Standard, March 20, 2015.”

On August 17, 2015 Governor Fallin sent a letter to EPA recommending designations of 

unclassifiable for Muskogee County and attainment for Noble and Choctaw Counties for the July 

2, 2016 designations. That recommendation is based on supporting information contained within 

this package.
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General Overview of Impact Analysis Approach and Modeling Methodology

DEQ evaluated pertinent source-oriented monitoring data and conducted air dispersion modeling 

for the three affected facilities to determine impacts, and to inform the State’s recommendations 

for the 2010 1-hour SO2 NAAQS designations. 

Ambient impacts of SO2 are largely source-oriented, meaning the highest concentrations will be 

exhibited in close proximity to the emitting sources. The 2010 1-hour SO2 NAAQS replaced the 

existing annual and 24-hour standards (140 ppb and 30 ppb, respectively) with a one-hour 

standard of 75 ppb. This substantial change to the form of the standard indicated more focus on 

short-term exposure to high levels of SO2. These facts, as well as limited state and federal 

resources, have led to EPA’s “hybrid” approach to attainment designations – relying on 

modeling to overcome limitations of the SO2 monitoring network.

40 CFR Part 51, Appendix W, Guideline on Air Quality Models, recommends the use of 

AMS/EPA Regulatory Model (AERMOD), citing the dispersion model's capability to address 

multiple sources, both rural and urban areas, flat and complex terrain, and surface and elevated 

emission release points. The enclosed “Modeling Compliance with the 1-Hour SO2 NAAQS” 

(“Modeling Report”) describes the significant effort DEQ has expended to determine, obtain and 

refine the data required to complete the modeling analysis. Due to the unique nature of utilizing 

model predictions as a surrogate to monitored air quality data, model inputs were chosen in a 

manner that best reflects actual conditions. 

The 2010 1-hour SO2 NAAQS is attained when the three-year average of the 99th percentile of 

the daily maximum one-hour average does not exceed 75 ppb (or 196.4 μg/m3)

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. The “design 

value” used to compare to the 75 ppb standard is calculated as the three-year average of the 

highest fourth highest (H4H) SO2 value for a particular monitor or modeling receptor. 

Attainment of the standard would be demonstrated if the modeled cumulative impact of the 

affected sources and other contributing nearby sources added to the background concentration is 

less than 75 ppb. The 2012-2014 monitoring design value (9.6 μg/m3) from the monitor (ID 40-

109-1037) in Oklahoma County was used as a background concentration for the modeling 

analyses, since it is the only SO2 monitor located in Oklahoma that is not directly impacted by a 

large SO2 emission source. 

For each affected facility/area, the Modeling Report describes the determination of the 

appropriate modeling domain, receptor grid, meteorological and terrain data, ambient monitored 

background concentrations, and emissions from other contributing nearby sources. Actual source 

emissions data were gathered or calculated for the years 2012-2014. Hourly emissions data from 

the affected sources were taken from continuous emissions monitoring systems (CEMS) when 

available. If CEMS data were unavailable, the emissions were calculated using throughputs and 

 

1

For all ambient measurements/projections, EPA reference conditions are used in converting between ppb and 

μg/m3(see 40 CFR §50.3).
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operational data. Federally enforceable requirements will result in significant SO2 emissions 

reductions from the OG&E Sooner and Muskogee Stations within the next five years. As 

described in more detail below, modeling was also performed using alternate operating scenarios 

to project the impact of these changes on ambient air quality. Oklahoma DEQ primarily utilized 

meteorological data from Oklahoma Mesonet sites in combination with data from the National 

Climatic Data Center's (NCDC) Integrated Surface Hourly Database (ISHD) to generate more 

representative surface meteorological conditions. For the Muskogee area, only data from the 

ISHD was utilized. The OG&E Muskogee facility section below, along with the additional 

documents, discusses questions regarding meteorological data representativeness, effects of 

complex terrain, and other issues for the Muskogee area. The following additional reports have 

been prepared and included to support modeling report’s conclusions: 

1. Comparison of Modeled and Monitored SO2 Concentrations in Muskogee, Oklahoma

2. Effects of Terrain on Wind Flow in Muskogee, Oklahoma

3. Source-Oriented SO2 Monitoring Placement for the Area of Muskogee, Oklahoma –

Modeling Analysis Report

A description of the three affected electric generating stations and modeling analysis and results 

for each are described.

Choctaw County WFEC Hugo Generating Station

The WFEC – Hugo Generating Station in Choctaw County is located three miles west of Ft. 

Towson on US-70, 12 miles east of Hugo, Oklahoma. 

The coal-fired complex at the Hugo power plant consists of one main boiler with a maximum 

heat input of 184 million British thermal units per hour (MMBTU/hr) and an auxiliary boiler 

unit. The main boiler has a capacity of 446 megawatts (MW) electrical output and primarily 

combusts sub-bituminous coal to produce steam used to generate electric power (SIC 4911). The 

flue gas is exhausted through a single 500 foot stack. An auxiliary unit is used as backup during 

turnaround periods, and is fueled exclusively by No. 2 fuel oil. The Hugo power plant was 

constructed in 1978 as authorized by Permit Number PSD-OK-053 and has no SO2 controls 

installed. The Hugo plant is operating under permit number 2008-337-TVR (M-1), which limits 

SO2 emissions to 1.2 lbs/MMBTU, 4,591.8 lbs/hr, and 16,404.1 tons per year. 

There is no ambient monitoring site in close proximity to the WFEC Hugo Generating Station. 

The facility’s SO2 emissions would not be expected to have a significant impact on the 

Muskogee monitoring site (40-101-0167), which is the Oklahoma monitoring site located closest 

to the Hugo plant.

DEQ performed modeling to confirm the State’s recommended designation of attainment with 

the new SO2 standard for the areas affected by the Hugo plant emissions. The modeling utilized

actual emissions data for years 2012 through 2014 obtained from EPA’s Clean Air Markets 

Database (CAMD). The modeling demonstrated compliance with the 2010 1-hour SO2 NAAQS 

of 75 ppb for the areas surrounding the WFEC Hugo Generating Station, with a three-year 
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average of the H4H daily maximum impacts of 45 ppb (118.2 μg/m3).(See Modeling Report,

Section 7.1)

Noble County – OG&E Sooner Generating Station

The OG&E Sooner Generating Station in Noble County is located 19 miles North of Stillwater

on US-177, six miles east of Red Rock, Oklahoma. The Sooner Generating Station is a coalfired steam boiler electric generation facility consisting of two units that use sub-bituminous 

low-sulfur Wyoming coal as the primary fuel and No. 2 fuel oil as a start-up fuel. Neither boiler 

is designed to operate on oil continuously. 

Both Units 1 and 2 can potentially combust approximately 300 tons per hour of coal to produce 

3.8 million pounds per hour of steam each. Both boilers were designed with a maximum heat 

input of 5,116 MMBTU/hr and a nominal 550 MW electrical output. Flue gas is exhausted 

through two 500-foot stacks. The Sooner plant currently operates under AQ Permit Number 

2010-338-TVR2 (M-2), which limits SO2 emissions to 1.2 lbs/MMBTU for Units 1 and 2. 

However, under the Regional Haze Program, the Sooner plant is required to meet limits that 

represent Best Available Retrofit Technology (BART) to control SO2 emissions from Units 1 

and 2. OG&E Sooner Generating Station is subject to a Federal Implementation Plan (FIP) (40 

CFR §52.1923), which requires Units 1 and 2 to meet a SO2 emission limit of 0.06 lb/MMBTU 

by January 4, 2019. OG&E has committed2to meet the federal BART limit by using flue gas 

desulfurization to control SO2 emissions from Units 1 and 2. 

There is no ambient monitoring site in close proximity to the OG&E Sooner Generating Station. 

No violation of the 2010 SO2 NAAQS has been detected at the Ponca City monitoring site (40-

071-0604) in Kay County, which is the site located closest to the Sooner Plant, for the last three 

years of available certified data. The design value for the Ponca City site for the years 2012 

through 2014 is 37 ppb.

DEQ performed modeling to confirm the State’s recommended designation of attainment with 

the new SO2 standard for the areas affected by the Sooner plant emissions. Since the BART 

emission reductions will occur after the court-ordered deadline for the designation decision, the 

air quality analyses for this facility involved three operating scenarios.

For Scenario 1:

Modeled using actual emissions data for OG&E Unit 1 & Unit 2 from EPA’s Clean Air Markets 

Database for years 2012 through 2014.

For Scenario 2:

Modeled potential emissions from OG&E Unit 1 using the BART emission limit (0.06 

lb/MMBTU) and projected actual emissions from OG&E Unit 2 using the 2012 through 2014 

three year average of the 99th percentile emission rate and heat input.

 

2

See “OG&E’s Public Statements on Commitments to Meet EPA’s Regional Haze FIP Requirements.”
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For Scenario 3:

Modeled potential emissions from OG&E Unit 1 and Unit 2 using the BART emission 

limit (0.06 lb/MMBTU).

The modeled three-year average of the H4H daily maximum impacts for each operating scenario 

are:

Scenario 1 - 52 ppb (136.6 μg/m3)

Scenario 2 - 62 ppb (161.7 μg/m3)

Scenario 3 - 17 ppb (43.5 μg/m3)

The modeling review demonstrated compliance with the 2010 1-hour SO2 NAAQS of 75 ppb, 

for the areas surrounding the OG&E Sooner Generating Station, with a three-year average of the 

H4H daily maximum impact of 62 ppb (161.7 μg/m3).(See Modeling Report, Section 7.3)

Muskogee County – OG&E Muskogee Generating Station

The OG&E Muskogee Generating Station in Muskogee County is located near Muskogee on 

US-62 on the East bank of the Arkansas River, in Fort Gibson, Oklahoma.

The Muskogee Generating Station is a coal-fired steam boiler electric generation facility 

consisting of three units (Units 4, 5 and 6) that use sub-bituminous low-sulfur Wyoming coal as 

the primary fuel and natural gas as a start-up fuel. Units 1, 2, and 3, the oldest units, have been 

retired.

Units 4, 5, and 6 can each potentially combust approximately 300 tons per hour of coal to 

produce 3.8 million pounds per hour of steam each. These units each have a nominal capacity of 

550 MW electrical output. Units 4 and 5 each have a 350 foot stack, and Unit 6 has a 500 foot 

stack. The Muskogee plant is operating under AQ Permit Number 2005-271-TVR (M-7), which 

contains SO2 emission limits of 1.2 lbs/MMBTU for units 4 and 5, and 1.2 lbs/MMBTU and 

6,180.0 lbs/hr for unit 6. However, under the Regional Haze Program, the Muskogee plant is 

required to meet limits that represent Best Available Retrofit Technology (BART) to control SO2

emissions from Units 4 and 5. OG&E Muskogee Generating Station is subject to a Federal 

Implementation Plan (FIP) (40 CFR §52.1923), which requires Units 4 and 5 to meet a SO2

emission limit of 0.06 lb/MMBTU by January 4, 2019. OG&E has committed3to meet the 

federal BART limit by conversion of Unit 4 and Unit 5 to natural gas.

A source-oriented SO2 monitoring site (40-101-0167) has been operated in close proximity to the 

OG&E Muskogee Generating Station since 19814. No violation of the 2010 SO2 NAAQS has 

been detected at the Muskogee monitoring site, located 4 km NNW of the facility, for the last 

 

3

Ibid.

4

See “Source-Oriented SO2 Monitoring Placement for the Area of Muskogee, Oklahoma – Modeling Analysis 

Report”
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three years of available certified data. The design value for the Muskogee site for 2012 through 

2014 is 49 ppb.

DEQ performed modeling to confirm the State’s recommended designation of unclassifiable for 

the new SO2 standard for the areas affected by the Muskogee plant emissions. Since the BART 

emission reductions will occur after the court-ordered deadline for the designation decision, the 

air quality analyses for this facility involved three operating scenarios. The future operating 

scenarios were modeled using continuous operation of all the affected emission units. 

For Scenario 1:

Modeled using actual emissions data for OG&E Unit 4, Unit 5, & Unit 6 from EPA’s Clean Air 

Markets Database for years 2012 through 2014.

For Scenario 2:

Modeled potential emissions from OG&E Unit 4 using the natural gas fired emission rate 

of 0.0006 lb/MMBTU (which is lower than the BART emission limit of 0.06 lb/MMBTU),

and projected actual emissions from OG&E Unit 5 & Unit 6 using the 2013 and 2014 twoyear average of the 99th percentile emission rate and heat input.

For Scenario 3:

Modeled potential emissions from OG&E Unit 4 and Unit 5, using the natural gas fired 

emission rate of 0.0006 lb/MMBTU (which is lower than the BART emission limit of 0.06 

lb/MMBTU) and projected actual emissions from OG&E Unit 6, using the 2013 and 2014 

two-year average of the 99th percentile emission rate and heat input. 

The modeled three-year average of the H4H daily maximum impacts for each operating scenario 

are:

Scenario 1 - 85 ppb (223.6 μg/m3)

Scenario 2 - 72 ppb (189.5 μg/m3)

Scenario 3 - 50 ppb (129.7 μg/m3)

The modeling review demonstrated compliance with the 2010 1-hour SO2 NAAQS of 75 ppb

(196.4 μg/m3) for the areas surrounding the OG&E Muskogee Generating Station for Scenarios 2 

and 3. However, the modeling review was not able to demonstrate compliance for Scenario 1. 

Under a certain combination of meteorological and operating conditions, a potential violation 

was modeled for a small area (approximately one quarter-section). (See Modeling Report, 

Section 7.2) 

Uncertainty for the Muskogee Area Modeling and Monitoring

DEQ’s assessment for the Muskogee area reinforces the premise that, although both sourceoriented monitoring sites and dispersion modeling are valuable evaluation tools, neither is 
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definitive in characterizing ambient air quality conditions for NAAQS designation purposes. 

Despite the considerable efforts of DEQ and others, much uncertainty remains as to whether 

there is a potential for a NAAQS violation within a small area during the relatively short window 

of time before BART is fully implemented. The modeling analysis for domain ID 36 

(Muskogee) raised significant questions regarding representativeness of available meteorological 

data, effects of complex terrain, and other issues. These questions must be taken into account if 

modeling is used as a decision-making tool in the attainment status designation process for the 

areas surrounding the OG&E Muskogee Generating Station. This is particularly important since 

a designation of “nonattainment” is unlikely to have a practical effect on air quality before the

implementation of “on-the-books” requirements that will soon remove the concern.

While under Scenario 1, modeling indicated there is a small area near the OG&E Muskogee 

Generating Station with potential impacts greater than the 2010 1-hour SO2 NAAQS, under 

Scenarios 2 and 3 the modeling demonstrates compliance with the 2010 1-hour SO2 NAAQS. 

The federally enforceable BART reductions reflected in Scenarios 2 and 3 are to be obtained on 

or before January 4, 2019. The last three years of certified monitoring data indicate compliance 

with the standard. Based on the Monitor vs. Model Report5, DEQ determined that terrain effects 

on wind direction and speed in the area surrounding the modeling domain were not adequately 

characterized by the available meteorological data. The modeling results did not correlate well 

with monitored values; therefore, there is uncertainty regarding the modeling results. 

The modeling of the OG&E Muskogee area was first performed using the Porter Mesonet site for

meteorological data. Those modeling results provided insight as to how sensitive the model was 

to topographic features near the plant, which explain in part why the SO2 concentrations 

predicted by the model do not reflect the lower concentrations recorded at the monitor for the 

same time period. See the separate DEQ report6on effects of terrain on wind speeds for more 

information. For the final modeling analysis, the nearest National Weather Service 

meteorological site available (KMKO) was used. KMKO is located 13 kilometers SSW of the 

OG&E Muskogee Generating Station. The results of the final modeling analysis show that the 

hourly modeling outputs do not correlate well with the actual hourly impacts recorded at the 

monitor (40-101-0167). The lack of correlation is a result of how the model attempts to replicate 

atmospheric stability (See Model vs. Monitor Report7).

Modeling for the area surrounding the OG&E Muskogee Generating Station, using the described 

meteorological data, can demonstrate attainment by assuming BART emission reductions that 

are federally enforceable, but are not required to be fully implemented until after EPA’s courtordered deadline for final designation action.

 

5

“Comparison of Modeled and Monitored SO2 Concentrations In Muskogee, Oklahoma”

6

“Effects of Terrain on Wind Speeds in and near Muskogee, Oklahoma”

7 Comparison, op cit.
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Conclusion

Choctaw County, including the area surrounding WFEC Hugo Generating Station, should be 

designated as “attainment” for 2010 1-hour SO2 NAAQS, based on the modeling results and no 

available monitoring data indicating a contrary conclusion. Noble County, including the area 

surrounding OG&E Sooner Generating Station, should be designated as “attainment” for 2010 1-

hour SO2 NAAQS, based on monitoring data and modeling results. Muskogee County, including 

the area surrounding OG&E Muskogee Generating Station, should be designated as 

“unclassifiable” or “unclassifiable/attainment” based on the current monitoring data, which

indicates compliance with the standard, and other important considerations. OG&E Muskogee 

Generating Station is already under legal obligation to make SO2 reductions that would 

essentially remove all concerns over a potential violation of the 2010 SO2 NAAQS. Also, 

additional questions regarding meteorological data representativeness and the ability of the 

model to accurately determine the atmospheric stability cast doubt over whether a significant 

potential for such a violation even exists under current operations and conditions.