Technical Support Document

Arkansas

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) 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.

Arkansas submitted updated recommendations on September 11, 2015, ahead of a July 2, 2016, 

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

Arkansas’s recommendations and identifies the counties or portions of counties in Arkansas that 

the 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. Arkansas’ Recommended and the EPA’s Intended Designations

Area

Arkansas’

Recommended 

Area Definition

Arkansas’ 

Recommended 

Designation

EPA’s Intended 

Area Definition

EPA’s Intended 

Designation

Independence 

County, Arkansas

Independence 

County

Unclassifiable/

Attainment

Same as State’s 

Recommendation Unclassifiable

Jefferson County, 

Arkansas Jefferson County Unclassifiable/Attainment

Same as State’s 

Recommendation

Same as State’s 

Recommendation

Background

On June 3, 2010, the 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. The 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 
2

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

1 However, the 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 the EPA is also 

not currently designating areas on the basis of the secondary standard.

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 the 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, the EPA will 

promulgate the designations that it deems appropriate. If a state or tribe disagrees with the 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, the 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, the 

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 the 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 the 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 the EPA to complete the area designations 

according to the consent decree schedule.

According to the consent decree, the EPA must complete the remaining designations on a 

schedule that contains three specific deadlines. By no later than July 2, 2016 (16 months from the 

court’s order), the 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 the 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 

 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.
3

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. The 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 

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 the 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 the 

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

The guidance also contains the factors the 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, the EPA released its most recent versions of documents titled, “SO2 NAAQS 

Designations Modeling Technical Assistance Document” (Modeling TAD) and “SO2 NAAQS 

Designations Source-Oriented Monitoring Technical Assistance Document” (Monitoring TAD) 

in December 2013.

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

the 2010 SO2 NAAQS have been recorded in any undesignated part of the state of Arkansas.

2

However, there are 2 sources in the state meeting the emissions criteria of the consent decree for 

which the EPA must complete designations by July 2, 2016. In this technical support document, 

the EPA discusses its review and technical analysis of Arkansas’ updated recommendations for 

the areas that we must designate. The 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: 

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

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

certified data for 2015, the analyses of applicable areas for the 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 the 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 the EPA to 

complete the designation. Instead, we may designate the area and all other previously undesignated areas in the state 

on a schedule consistent with the prescribed timing of the consent decree, i.e., by December 31, 2017, or December 

31, 2020. 
4

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 the EPA has determined has violated the 

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

designation would reflect considerations of state recommendations and all of the 

information discussed in this document. The EPA’s decision would be 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 the 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 the EPA has determined to 

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

The EPA’s decision would be 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 the EPA 

designate as attainment. 

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

EPA designate as nonattainment. 

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

EPA designate as unclassifiable.

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

that the 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. 
5

Technical Analysis for Jefferson County, Arkansas Area

Introduction

The Jefferson County area contains a stationary source that according to the 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 White 

Bluff Steam Electric Station (White Bluff station) emitted 31,687 tons of SO2, and had an 

emissions rate of 0.59 lbs SO2/mmBTU. Pursuant to the March 2, 2015 consent decree, the EPA 

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

In its submission, Arkansas recommended that the area surrounding White Bluff station, 

specifically the entirety of Jefferson County, be designated as unclassifiable/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, the EPA agrees that the area is 

attaining the standard, and intends to designate Jefferson County as unclassifiable/attainment. 

The White Bluff station is located in central Arkansas in the northeastern portion of Jefferson

County. The facility is located approximately 38 km south of Little Rock, Arkansas. Included in 

Figure 1 is the EPA’s intended unclassifiable/attainment designation county boundary for the 

area, which is the same recommended area as the state’s unclassified/attainment designation.
6

Figure 1: The EPA’s Intended Area Designation for White Bluff Steam Electric Station

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

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

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

Detailed Assessment

Air Quality Data

There are no SO2 air quality monitors in Jefferson County. There are no SO2 air quality monitors 

in surrounding counties that are representative of the maximum or higher elevated levels of SO2

around the White Bluff station facility.

Model Selection and Modeling Components

The 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:

Evergreen Packaging 

(Background Source)

(20 miles SE of White Bluff)
7

- 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 the most recent AERMOD version 15181, and a discussion of the individual 

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

Modeling Parameter: Rural or Urban Dispersion

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

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

modeling guidelines, urban 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 urban. 

Otherwise, the source is considered a rural source. When performing the modeling for the area of 

analysis, the state ran the model using the rural mode. The submittal did not specifically discuss 

why the rural mode was chosen. However, based on our review of aerial photography of the area 

surrounding the facility, the determination to run the model in rural mode appears appropriate.

Modeling Parameter: Area of Analysis (Receptor Grid)

The EPA believes that a reasonable first step towards characterization of air quality in the area 

surrounding the White Bluff 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. 

The Cartesian receptor grid consisted of the following receptor spacing:

• 50-meter spacing along the facility fence line; 

• 100-meter spacing extending from the fence line to 5 kilometers; 

• 500-meter spacing extending from 5 to 10 kilometers; and 

• 1,000-meter spacing extending from 10 to 20 kilometers. 

The spacing is appropriate since it captures the gradient changes in the impact contours from the 

facility, which show approximately 30-40% decreases within in each spacing selection (see 

Figure 6). 

Figure 2 shows the state’s chosen area of analysis surrounding the White Bluff station, as well as 

receptor grid for the area of analysis.
8

Figure 2: White Bluff Steam Electric Station Receptor Grid for the Area of Analysis

The submitted modeling did not exclude any specific areas from receptor placement. Instead, 

receptors were placed throughout the modeled area, which is a conservative approach and 

consistent with EPA guidance. The impacts of the area’s geography and topography will follow 

in the appropriate section.

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 characterized the source’s building layout and 

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

diameter. Where appropriate, the AERMOD component BPIPPRIME was used to assist in 

addressing building downwash.
9

Modeling Parameter: Emissions

The 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 the most recently permitted, (referred to as 

PTE or allowable) emissions rate.

The 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, the 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, the EPA believes that detailed throughput, operating schedules, and 

emissions information from the impacted source(s) 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 the White Bluff station and no other emitters of SO2

within 20 km in the area of analysis. Evergreen Packaging is located 30 km southeast from 

White Bluff station, but the receptor placement is appropriate since it indicates no significant 

increases in the concentration gradients modeled beyond 20 km. This distance was selected 

because the state believes that this area of analysis adequately represents the area that could 

cause or contribute to a NAAQS violation in the vicinity of the affected source. No other sources 

beyond 20 km were determined by the state to have the potential to cause significant 

concentration gradient impacts within the area of analysis. The area of analysis and its associated 

annual actual SO2 emissions between 2012 and 2014 are summarized in Table 2 below. Modeled 

stack parameters for contributing sources in the area of analysis can be seen in Table 3.

Table 2: Actual SO2 Emissions 2012 – 2014 in the Jefferson County Area of Analysis

Company ID Facility Name SO2 Emissions tpy

2012 2013 2014

Entergy White Bluff Steam 

Electric Station 31,687 34,196 34,223
10

Table 3: Modeled Stack Parameters for Contributing Sources in Area of Analysis

For White Bluff station in the area of analysis, the state used actual emissions from the most 

recent 3-year data set, i.e., 2012 – 2014. CEMS data was used to generate hourly emissions files 

for the affected sources. 

There are two boilers in operation at the White Bluff station, Unit 1 and Unit 2. Units 1 and 2 are 

vented to a common, dual-flue stack. For these main units, three years (2012-2014) of actual 

hourly emissions, stack temperature, and exhaust flow rate data were input into the model. This 

emissions data was provided by Entergy from prior submittals to the EPA’s Clean Air Markets 

Database, while temperature and exhaust flow rates were provided by Entergy from the facility 

CEM system. As per the Modeling TAD, the actual 1000 ft. height of the main stack was 

represented in each case. The two Units at the facility were modeled as separate sources, each 

emitting from their own flue. 

The auxiliary boiler was also modeled using actual hourly emissions data. For this source, 

however, exhaust temperature and velocity were not available, so for all hours the exit 

temperature and velocity were set to the values located in the Arkansas Department of 

Environmental Quality (ADEQ) source registration tables for the auxiliary boiler. 

The Emergency Diesel Generator and Emergency Fire Pump Engine at the facility both have 

horizontal exhaust releases. This is represented in the modeling by setting the exit velocity of 

each source to 0.001 meters per second (m/s) to simulate the lack of vertical momentum out of 

the stack. Emissions data were only available on a monthly total emission basis for each engine. 

To convert that data into an emission rate for modeling, for each engine the total annual 

emissions for each year was determined and the highest annual total selected. That total was then 

divided by 52 to represent that the engines are tested once per week during the year. The 

resulting emission rate was then used as the lb/hr emission rate in the modeling. Based on 

information provided by Entergy employees, the emergency generator is typically tested weekly 

on Wednesdays, while the fire pump is typically tested on Friday evenings. To simulate this 

standard practice, the emergency generator was set in the modeling using the HRDOW7 

emission factor (i.e., variable by hour of day and 7 days per week) to emit during an 8 hour 

period on Wednesdays from 8 AM to 4 PM, and the fire pump was set to operate on Friday’s 

from 4 PM until Midnight. While this significantly overestimates the total emissions of the 

emergency engines, because the form of the 1-hour SO2 standard only considers the hour with 

the highest concentration each day, at least 7 of these hours are “dropped” and thus only one 

hour worth of emission is potentially included in the maximum daily impacts. We note that if 
11

these two emergency engines are operated less than an hour per week that it is likely 

conservative to add these sources in the modeling as it is unlikely they would impact the 

maximum modeled values if modeled with a temporally varying emission file.

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.

Three years (2012-2014) of surface observations from the NWS tower at Adams Field Airport in 

Little Rock, AR (WBAN No. 13963) and concurrent upper air data from North Little Rock 

Municipal Airport in North Little Rock, AR (WBAN No. 03952) were processed with the most 

recent version of AERMET (v.15181) the meteorological preprocessor for AERMOD, along 

with the two pre-processors to AERMET: AERSURFACE (v.13016) and AERMINUTE 

(v.14337). AERMET was applied to create the two meteorological data files required for input to 

AERMOD. 

AERMET requires specification of site characteristics including surface roughness (zo), albedo 

(r), and Bowen ratio (Bo). These parameters were developed according to the guidance provided 

by EPA in the AERMOD Implementation Guide (AIG) (EPA, 2008a) using AERSURFACE. 

The area within 1 km of the meteorological tower at Adams Field was broken into 12 sectors of 

30 degrees each to analyze the surface characteristics in each 30 degree arc around the tower. 

AERMET uses the surface characteristics in the sector from which the wind approaches the 

tower as part of the meteorological data processing for each hour. 

In AERSURFACE, the various land cover categories are linked to a set of seasonal surface 

characteristics. As such, AERSURFACE requires specification of the seasonal category for each 

month of the year. The following five seasonal categories are offered by AERSURFACE: 

1. Midsummer with lush vegetation; 

2. Autumn with unharvested cropland; 

3. Late autumn after frost and harvest, or winter with no snow; 

4. Winter with continuous snow on ground; and 

5. Transitional spring with partial green coverage or short annuals. 

The AERSURFACE run was performed using the seasonal temporal resolution option. The 

default seasonal distribution was used: December, January, and February were categorized as 

winter with no snow, March, April, and May as spring, June, July, and August as summer, and 
12

September, October, and November as fall. The precipitation was assumed to be average over 

the 3-year period. 

Additionally, 1-minute ASOS wind data, collected at the Adams Field meteorological tower, 

were processed using the AERMINUTE pre-processor for AERMET. Figure 3 shows the relative 

location of Adams Field and White Bluff station.

Figure 3: Meteorology Tower Location

The 3-year surface wind rose for Little Rock, Adam’s Field, is depicted in Figure 4. In this 

figure, the frequency and magnitude of wind speed and direction are defined in terms of where 

the wind is blowing from. The distance from White Bluff station is 21.7 miles and the average 

wind speed is 3.42 m/s.
13

Figure 4: Little Rock-Adams Field Cumulative Annual Wind Rose for Years 2012 – 2014

Modeling Parameter: Geography and Terrain

Terrain elevations from National Elevation Data (“NED”) from USGS were processed using the 

most recent version of AERMAP (v.11103) to develop the receptor terrain elevations required by 

AERMOD. NED data files contain profiles of terrain elevations, which in conjunction with 

receptor locations are used to generate receptor height scales. The height scale is the terrain 

elevation in the vicinity of a receptor that has the greatest influence on dispersion at that location 

and is used for model computations in complex terrain areas. 

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 
14

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

analysis, the state chose the second tier approach and calculated seasonal diurnal background 

concentrations at the Little Rock monitor (Monitor ID #05-119-0007). The background 

concentrations for this area of analysis are shown in Table 4, and these values were incorporated 

into the final AERMOD results. This approach is consistent with the EPA’s Modeling TAD and 

March 1, 2011 memorandum titled, “Additional Clarification Regarding Application of 

Appendix W Modeling Guidance for the 1-hour NO2 NAAQS.” While this memorandum 

nominally addresses NO2, the document and its recommended approaches also apply to the 1-

hour SO2 NAAQS.3

Table. 4. Seasonal Diurnal SO2 Concentrations at Little Rock Monitor (μg/m3)

 3

 See http://www3.epa.gov/scram001/guidance/clarification/Additional_Clarifications_AppendixW_Hourly-NO2-

NAAQS_FINAL_03-01-2011.pdf, p 19 - 20
15

Summary of Modeling Results

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

below in Table 5:

Table 5: AERMOD Modeling Parameters for the Jefferson County Area of Analysis

Jefferson County, OK Area of Analysis

AERMOD Version 15181

Dispersion Characteristics Rural

Modeled Sources 1

Modeled Stacks 5

Modeled Structures Yes

Modeled Fence Lines Yes (see Figure 5)

Total receptors -

Emissions Type Actual

Emissions Years 2012-2014 

Meteorology Years 2012-2014 

Surface Meteorology Station Adams Field in Little Rock

Upper Air Meteorology Station North Little Rock Airport

Methodology for Calculating 

Background SO2 Concentration 2nd tier monitoring data

Calculated Background SO2

Concentrations

See Table 4
16

Figure 5: White Bluff Steam Electric Station Modeled Fence Line

The results presented below in Table 6 show the magnitude of the highest predicted modeled 

concentration based on actual emissions.

Table 6: 2012 – 2014 Max Predicted 99th Percentile 1-Hour SO2 Concentration in the Jefferson 

County Area of Analysis based on Actual Emissions
17

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

within the chosen modeling domain is 162.4 μg/m3, or 62.04 ppb. This modeled concentration 

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

facility. Figure 6 shows the modeled impacts from White Bluff station, including background 

concentration, with the maximum impact location identified by the blue diamond.

Figure 6: White Bluff Station Modeled Impacts

Jurisdictional Boundaries:

Once the geographic area of analysis associated with the White Bluff station, other nearby 

sources, and background concentration is determined, existing jurisdictional boundaries are 

considered for the purpose of informing our intended unclassifiable/attainment area, specifically 

with respect to clearly defined legal boundaries. 

One facility in Jefferson County which was not addressed by the state is located approximately 

30 km southeast of the White Bluff station. According to the 2014 state emissions inventory, 

Evergreen Packaging in Pine Bluff emitted 1,077 tpy of SO2, which is a decrease from the 
18

reported 2011 NEI emissions of 1,755 tpy. Based on the distance from White Bluff station (20 

miles southeast) and available information, the EPA does not believe that emissions from 

Evergreen Packaging are likely to cause or contribute to a violation of the NAAQS within the 

Jefferson County area of analysis. For comparison, the modeled maximum concentration from 

White Bluff station which attains the NAAQS is less than 5 km from the facility. Based on 

available information and Evergreen Packaging’s decreasing emissions over time when 

compared to those from White Bluff station which have been modeled to attain the NAAQS, the 

EPA does not have reason to believe that emissions from Evergreen Packaging are causing or 

contributing to a violation of the NAAQS elsewhere in Jefferson County.

The EPA believes that our intended unclassifiable/attainment area, consisting of Jefferson

County, Arkansas, 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.

Other Relevant Information

Additional modeling for the White Bluff facility was received from the Sierra Club. The

submitter’s modeling results assert that there are impacts in excess of the 1-hour SO2 standard 

with a maximum-modeled concentration of 233.6 μg/m3. However, our review of Sierra Club’s 

modeling identified multiple errors in stack parameters, as well as less refined modeling 

approaches as compared with the state’s submittal (ex. variable stack velocity and temperature 

were not included). Stack parameters need to be accurate to calculate the emission rate, plume

dispersion effects, and ground level concentration that come from the stacks. The inaccuracy in 

Sierra Club’s modeling of stack parameters appears to result from incorrect units on the stack 

temperatures. The modeled stack temperature values for the two units at White Bluff station 

corresponds to stack temperatures in degrees Fahrenheit taken from the facility’s operating 

permit. Sierra Club did not convert these temperatures to Kelvin prior to running AERMOD 

resulting in a significantly lower modeled stack temperature and an underestimate in dispersion 

due to stack conditions (overestimating of off-site impacts). Sierra Club also did not include the 

additional refinement of variable stack parameters, as allowed by the modeling TAD, and was 

not as representative of daily operations when compared with the state’s analysis. The state’s 

modeling was determined to follow the modeling TAD more closely, and is more representative 

of actual operation conditions at White Bluff station. Therefore, our intended 

unclassifiable/attainment designation for Jefferson County is based on the state’s analysis. The 

Sierra Club’s modeling submittal, including modeling files and associated modeling report, are 

available for review as part of the docket for the SO2 designations action.

Conclusion

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

available relevant information, the EPA intends to designate the area around White Bluff station

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

unclassifiable/attainment area is comprised of the entirety of Jefferson County. 
19

When evaluating the modeling submitted by the state, no major issues were identified. The

modeling shows attainment, and the modeling closely follows EPA guidance, including the

TAD. Our decision to use Jefferson County as the boundary area for this designation is based 

upon the state’s recommendation and its submitted analysis. Additionally, the EPA has 

confirmed that there are no other sources in Jefferson County or near its borders that based on 

available information lead us to believe they are likely to cause or contribute to a violation of the 

NAAQS within Jefferson County.

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

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

2015 consent decree, the EPA will evaluate and designate all remaining undesignated areas in 

Arkansas by either December 31, 2017, or December 31, 2020. 
20

Technical Analysis for the Independence County, Arkansas Area

Introduction

The Independence County area contains a stationary source that according to the EPA’s Clean 

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 

Independence Steam Electric Station (Independence station) emitted 32,974 tons of SO2, and had 

an emissions rate of 0.59 lbs SO2/mmBTU. Pursuant to the March 2, 2015 consent decree, the 

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

In its submission, Arkansas recommended that the area surrounding the Independence station, 

specifically the entirety of Independence County, be designated as unclassifiable/attainment 

based on an assessment and characterization of impacts on 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, the EPA 

disagrees with the state’s recommendation for the area, and intends to designate the area, 

specifically the entirety of Independence County, as unclassifiable. 

The Independence station is located in northeastern Arkansas in the eastern portion of 

Independence County. The facility is located approximately 5 km southeast of Newark, 

Arkansas. Included in the figure is the EPA’s intended unclassifiable designation county 

boundary for the area, which is the same recommended area as the state’s unclassified/attainment

designation.
21

Figure 1: The EPA’s Intended Area Designation for Independence Steam Electric Station 

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

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

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

Detailed Assessment

Air Quality Data

There are no SO2 air quality monitors in Independence County. There are no SO2 air quality 

monitors in surrounding counties that are representative of the maximum or higher elevated 

levels of SO2 around the Independence station facility

Model Selection and Modeling Components

The 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
22

- 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 the most recent AERMOD version 15181, and a discussion of the individual 

components will be referenced in the corresponding discussion that follows as appropriate.

Modeling Parameter: Rural or Urban Dispersion

The 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 the EPA’s 

modeling guidelines, urban 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 urban. 

Otherwise, the source is considered a rural source. When performing the modeling for the area of 

analysis, as indicated in the modeling files, the state used the rural dispersion approach. Based on 

our review of aerial photography surrounding the facility, rural mode appears appropriate.

Modeling Parameter: Area of Analysis (Receptor Grid)

The EPA believes that a reasonable first step towards characterization of air quality in the area 

surrounding the Independence 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. 

The Cartesian receptor grid consisted of the following receptor spacing: 

• 50-meter spacing along the facility fence line; 

• 100-meter spacing extending from the fence line to 5 kilometers; 

• 500-meter spacing extending from 5 to 10 kilometers; and 

• 1,000-meter spacing extending from 10 to 20 kilometers. 

Figure 2 shows the state’s chosen area of analysis surrounding the Independence station, as well 

as receptor grid for the area of analysis.
23

Figure 2: Independence Steam Electric Station Receptor Grid for the Area of Analysis

Consistent with the Modeling TAD, receptors for the purposes of this designation effort were 

placed only in areas where it would also be feasible to place a monitor and record ambient 

impacts. The impacts of the area’s geography and topography will follow in the appropriate 

section.

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 for Independence station. The state also correctly 

characterized the source’s building layout and location, as well as the stack parameters, e.g., exit 

temperature, exit velocity, location, and diameter. Where appropriate, the AERMOD component 

BPIPPRIME was used to assist in addressing building downwash.
24

Modeling Parameter: Emissions 

The 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 the most recently permitted, (referred to as 

PTE or allowable) emissions rate.

The 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, the EPA’s Modeling TAD 

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

using AERMOD’s variable emissions factors keyword EMISFACT. When choosing one of these 

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

information from the impacted source(s) 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.” 

 

The state did not include any additional off-site emitters of SO2 in its modeling analysis. As 

shown in Figure 1, Future Fuels is located approximately 11.5 km northwest of Independence 

station, within the area of analysis included in the modeled receptor grid. Future Fuels emitted 

3,421 tpy SO2 in 2012. The area of analysis and its associated annual actual SO2 emissions 

between 2012 and 2014 are summarized below. 

Table 1: Actual SO2 Emissions 2012 – 2014 in the Independence County Area of Analysis

Company ID Facility Name SO2 Emissions tpy

2012 2013 2014

Entergy Independence Steam 

Electric Station 32,974 28,854 30,029
25

Table 2: Modeled Stack Parameters for Sources in Area of Analysis

For Independence station, the state used actual emissions from the most recent 3-year data set, 

i.e., 2012 – 2014. CEMS data was used to generate hourly emissions files for the affected 

sources. 

There are two boilers in operation at the Independence station, Unit 1 and Unit 2. Units 1 and 2 

are vented to a common, dual-flue stack. For these main units, three years (2012-2014) of actual 

hourly emissions, stack temperature, and exhaust flow rate data were input into the model. This 

emissions data was provided by Entergy from prior submittals to the EPA’s Clean Air Markets 

Database, while temperature and exhaust flow rates were provided by Entergy from the facility 

CEM system. As per the Modeling TAD, the actual 1000 ft. height of the main stack was 

represented in each case. The two Units at the facility were modeled as separate sources, each 

emitting from their own flue. This is a conservative representation because it neglects potentially 

enhanced buoyancy from a combined plume from both flues. 

The auxiliary boiler was also modeled using actual hourly emissions data. For this source, 

however, exhaust temperature and velocity were not available, so for all hours the exit 

temperature and velocity were set to the values located in the ADEQ source registration tables 

for the auxiliary boiler. 

The two emergency engines at the facility both have horizontal exhaust releases. This is 

represented in the modeling by setting the exit velocity of each source to 0.001 m/s to simulate 

the lack of vertical momentum out of the stack. Emissions data were only available on a month 

by month total emission basis for each engine. To convert that data into an emission rate for 

modeling, for each engine the total annual emissions for each year was determined and the 

highest annual total selected. That total was then divided by 52 to represent that the engines are 

tested once per week during the year. The resulting emission rate was then used as the lb/hr 

emission rate in the modeling. Based on information provided by facility staff, the emergency 

generator is tested weekly on Wednesdays, while the fire pump is tested on Friday evenings. To 

simulate this standard practice, the emergency generator was set in the modeling using the 

HRDOW7 emission factor (i.e., variable by hour of day and 7 days per week) to emit during an 8 

hour period on Wednesdays from 8 AM to 4 PM, and the fire pump was set to operate on 

Friday’s from 4 PM until Midnight. While this tends to overestimate the total emissions of the 

emergency engines, because the form of the 1-hour SO2 standard only considers the hour with 

the highest concentration each day, at least 7 of these hours are “dropped” and thus only one 
26

hour worth of emission is potentially included in the maximum daily impacts. We note that if 

these two emergency engines are operated less than an hour per week that it is likely 

conservative to add these sources in the modeling as it is unlikely they would impact the 

maximum modeled values if modeled with a temporally varying emission file.

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.

Guidance for regulatory air quality modeling recommends the use of one year of on-site 

meteorological data or five years of representative off-site meteorological data. The SO2

Modeling TAD however, specifies that 3 years of meteorological data concurrent to the actual 

emissions data being input into the model be used. Since on-site data are not available for the 

Independence station, meteorological data available from the National Weather Service (NWS) 

were used in this analysis. 

Three years (2012-2014) of surface observations from the NWS tower at Adams Field Airport in 

Little Rock, Arkansas (WBAN No. 13963) and concurrent upper air data from North Little Rock 

Municipal Airport in North Little Rock, Arkansas (WBAN No. 03952) were processed with the 

most recent version of AERMET (v.15181) the meteorological preprocessor for AERMOD, 

along with the two pre-processors to AERMET: AERSURFACE (v.13016) and AERMINUTE 

(v.14337). AERMET was applied to create the two meteorological data files required for input to 

AERMOD. 

AERMET requires specification of site characteristics including surface roughness (zo), albedo 

(r), and Bowen ratio (Bo). These parameters were developed according to the guidance provided 

by EPA in the AERMOD Implementation Guide (AIG) (EPA, 2008a) using AERSURFACE. 

The area within 1 km of the meteorological tower at Adams Field was broken into 12 sectors of 

30 degrees each to analyze the surface characteristics in each 30 degree arc around the tower. 

AERMET uses the surface characteristics in the sector from which the wind approaches the 

tower as part of the meteorological data processing for each hour. 

In AERSURFACE, the various land cover categories are linked to a set of seasonal surface 

characteristics. As such, AERSURFACE requires specification of the seasonal category for each 

month of the year. The following five seasonal categories are offered by AERSURFACE: 

1. Midsummer with lush vegetation; 

2. Autumn with unharvested cropland; 

3. Late autumn after frost and harvest, or winter with no snow; 
27

4. Winter with continuous snow on ground; and 

5. Transitional spring with partial green coverage (short annuals). 

The AERSURFACE run was performed using the seasonal temporal resolution option. The 

default seasonal distribution was used: December, January, and February were categorized as 

winter with no snow, March, April, and May as spring, June, July, and August as summer, and 

September, October, and November as fall. The precipitation was averaged over the 3-year 

period. 

Additionally, 1-minute ASOS wind data, collected at the Adams Field meteorological tower, 

were processed using the AERMINUTE pre-processor for AERMET. Figure 3 shows the 3-year 

wind rose for Adams Field; Figure 4 shows the relative location of Adams Field and 

Independence station; and the data characteristics of Adams Field are shown in Table 3.

The 3-year surface wind rose for Little Rock, Adam’s Field, is depicted in Figure 3. In this 

figure, the frequency and magnitude of wind speed and direction are defined in terms of where

the wind is blowing from. The distance from Independence station is 80.1 miles and the average 

wind speed is 3.42 m/s.
28

Figure 3: Little Rock-Adams Field Cumulative Annual Wind Rose for Years 2012 – 2014
29

Figure 4: Meteorology Tower Location

Table 3: Characteristics of Adams Field Met Station

Modeling Parameter: Geography and Terrain

Terrain elevations from National Elevation Data (“NED”) from USGS were processed using the 

most recent version of AERMAP (v.11103) to develop the receptor terrain elevations required by 
30

AERMOD. NED data files contain profiles of terrain elevations, which in conjunction with 

receptor locations are used to generate receptor height scales. The height scale is the terrain 

elevation in the vicinity of a receptor that has the greatest influence on dispersion at that location 

and is used for model computations in complex terrain areas.

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 Independence County 

area of analysis, the state chose background concentrations based on the most recent complete 

years of available monitoring data. A review of the data showed that the most representative 

monitor for use in the modeling is located in Little Rock (Monitor ID# 05-119-0007). EPA’s 

Modeling TAD and other guidance allows for a temporally varying approach, based on the 99th

percentile monitored concentrations by hour of day and season or month. The modeling was 

performed with a set of seasonal diurnal values developed using the methodology described in 

the EPA’s March 1, 2011 Clarification Memorandum titled, “Additional Clarification Regarding 

Application of Appendix W Modeling Guidance for the 1-hour NO2 National Ambient Air 

Quality Standard.” While this memorandum nominally addresses NO2, the document and its 

recommended approaches also apply to the 1-hour SO2 NAAQS.4

 4

 See http://www3.epa.gov/scram001/guidance/clarification/Additional_Clarifications_AppendixW_Hourly-NO2-

NAAQS_FINAL_03-01-2011.pdf, p 19 - 20
31

Table 4: Background Seasonal Diurnal SO2 values for the Area of Analysis

Summary of Modeling Results

The AERMOD modeling parameters for the Independence County area of analysis are 

summarized below in Table 5:

Table 5: AERMOD Modeling Parameters for the Independence County Area of Analysis

Independence County, Arkansas Area of Analysis

AERMOD Version 15181

Dispersion Characteristics Rural

Modeled Sources 1

Modeled Stacks 5

Modeled Structures Yes

Modeled Fence Lines Yes (see Figure 5)

Total receptors Large Grid (20 km)
32

13,812 receptors

Emissions Type Actual

Emissions Years 2012-2014 

Meteorology Years 2012-2014 

Surface Meteorology Station Adams Field in Little Rock

Upper Air Meteorology Station North Little Rock Airport

Methodology for Calculating 

Background SO2 Concentration

Seasonal and Diurnal 

background Values Little Rock 

monitor

Calculated Background SO2

Concentration

Seasonal Diurnal Values Used 

(See Table 5)

The results presented below in Table 6 show the magnitude of the highest predicted modeled 

concentration based on actual emissions.

Table 6: 2012 – 2014 Max Predicted 99th Percentile 1-Hour SO2 Concentration in the 

Independence County Area of Analysis based on Actual Emissions
33

Figure 5: Independence Steam Electric Station Modeled Fence Lines

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

within the chosen modeling domain is 131.3 μg/m3, or 50.16 ppb. The modeled concentration is 

based on actual emissions from the facility and included the background concentration of SO2.
34

Figure 6: Independence Modeled Impacts

Jurisdictional Boundaries:

Once the geographic area of analysis associated with the Independence station and background is

determined, existing jurisdictional boundaries are considered for the purpose of informing our 

intended unclassifiable area, specifically with respect to clearly defined legal boundaries. 
35

There is one other emitter of SO2, Future Fuels, located approximately 12 km to the northwest of 

Independence station. The state did not include this facility in its modeling analysis, and the EPA 

is not prepared at this time to fully assess the expected maximum impacts from Future Fuels 

when considered alone, or in tandem with Independence station. We are also not prepared to 

assume that the emissions are captured fully by the background estimate since a temporally 

varying approach was used. As discussed in additional modeling we have reviewed, Future Fuels 

has very large impacts in Independence County and it is necessary to include Future Fuels 

directly in the modeling because of the size of their impacts. According to the 2011 NEI, there 

are no other sources emitting at or above 100 tpy of SO2 in Independence County, or in any 

neighboring county. As a result, we do not have reason to believe that sources in any neighboring 

county have the potential to cause or contribute to a violation of the NAAQS within 

Independence County. The EPA believes that our intended unclassifiable area, consisting of 

Independence County, Arkansas, is comprised of clearly defined legal boundaries, and we find 

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

Other Relevant Information:

Additional modeling for the Independence station was received from the Sierra Club on 

September 30, 2015. It showed impacts in excess of the 1-hour SO2 NAAQS. The modeling 

included emissions from two main Independence station stacks and Future Fuels. The modeling 

analysis indicated that many receptors were above the standard, with a maximum SO2

concentration of 586 μg/m3 (224 ppb). In November 2015, ADEQ and Entergy (Independence

station) submitted modeling to the EPA in response to Sierra Club’s September 30, 2015 

submission. The cumulative modeled emission results for Future Fuels were similar to those 

provided by Sierra Club. Both sets of modeling confirm that Future Fuels has exceedances, but 

there is disagreement on whether those exceedances actually contribute to the impacts from 

Independence station. Sierra Club reported that those exceedances contribute, whereas ADEQ 

and Entergy (Independence station) reported that they do not. 

The EPA’s review of the Sierra Club’s modeling analysis identified areas that were either 

inconsistent with, or as not as refined as the Modeling TAD recommends. For example, Sierra

Club did not include variable stack velocity and temperature for the Independence station stacks, 

and they utilized actual annualized emissions from the 2012 State Emissions Inventory to 

calculate Future Fuel’s emission rate. The Independence station modeling report submitted in 

November 2015 followed the Modeling TAD more closely by including source apportionment. 

This analysis indicated that the contribution of emissions from Independence station compared to 

the modeled exceedances was not above the interim 1-hr SO2 SIL. However, the emission rates

used in the Independence station modeling were obtained from Sierra Club modeling, which

were inconsistent with the Modeling TAD, specifically with respect to the emissions from Future 

Fuels. As a result, the EPA believes that the extent and temporal pattern of the potential modeled

nonattainment due to Future Fuels has not been adequately characterized. Consequently, the EPA 

does not have sufficient information to accurately assess the potential contribution of 

Independence station from either set of modeling. The analysis and evaluation of the 

Independence facility impacts cannot be completed until the modeling inputs are refined to 

include Future Fuels’ emissions in a manner that is consistent with the Modeling TAD.
36

Conclusion

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

available relevant information, the EPA intends to designate the area around Independence 

station as unclassifiable for the 2010 SO2 NAAQS. Specifically, the boundaries are comprised of 

all area within the Independence County borders. Our decision is based on the state’s 

recommendation and modeling results provided by various parties.

The initial modeling provided by the state did not include emissions from sources near 

Independence station which may have an impact on air quality. Specifically, the state did not 

include emissions from Future Fuels. Sierra Club’s modeling for the area around Independence 

station asserting violations of the NAAQS, which included emissions from Future Fuels, was 

premised on several factors that are inconsistent with the Modeling TAD. Lastly, while the state 

provided a response to Sierra Club’s modeling, the EPA believes that without a comprehensive 

emissions profile for the area, including emissions from Future Fuels, air quality in the area has 

not been sufficiently characterized to inform an intended designation other than unclassifiable. 

The EPA notes that the state has submitted its list of sources that must be characterized under the 

SO2 Data Requirements Rule,5 and Future Fuels’ inclusion on the list requires the state to 

characterize its emissions in accordance with all applicable regulatory requirements. Based on all 

available information, including the reasons discussed above, the EPA is unable at this time to 

determine whether the area is meeting or not meeting the NAAQS.

At this time, our intended designation for the state only applies to this area and the other area

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

2015 consent decree, the EPA will evaluate and designate all remaining undesignated areas in 

Arkansas by either December 31, 2017, or December 31, 2020. 

 5 80 FR 51052.