1

Draft Technical Support Document

Tennessee

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.

Tennessee submitted updated recommendations on September 16, 2015, and an updated

submission on October 30, 2015, ahead of a July 2, 2016, deadline for the EPA to designate 

certain areas. This deadline established by the U.S. District Court for the Northern District of 

California 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 Tennessee’s recommendations and 

identifies the counties or portions of counties in Tennessee 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. Tennessee’s Recommended and the EPA’s Intended Designations

Area

Tennessee’s 

Recommended Area 

Definition

Tennessee’s 

Recommended 

Designation

The EPA’s 

Intended Area 

Definition

The EPA’s 

Intended 

Designation

Sumner 

County, 

Tennessee

Area

Vicinity of the TVA 

Gallatin Fossil Plant 

in Sumner County 

within a 50 km 

radius of the 

facility, centered on 

the following 

coordinates: 

36.3165, -86.4033 

Attainment Sumner County Unclassifiable
2

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 

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 CAA 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 court-ordered schedule.

 

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 state implementation plan (SIP) Call under the prior 

NAAQS, the prior NAAQS will apply until that area submits and the EPA approves a SIP providing for attainment 

of the 2010 NAAQS. The Tennessee area is not designated nonattainment under the prior NAAQS nor is it an area 

not meeting the requirements of a SIP Call under the prior NAAQS.
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According to the court-ordered schedule, the EPA must complete the remaining designations by

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 

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.2 However, there 

 

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 
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is one source in the state meeting the emissions criteria of the consent decree for which the EPA 

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

discusses its review and technical analysis of Tennessee’s 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: 

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 reflects considerations of state recommendations and all of the information 

discussed in this document. The 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 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 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 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. 

 

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 court order, i.e., by December 31, 2017, or December 31, 

2020. 
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Technical Analysis for the TVA Gallatin – Sumner County Area

Introduction

Sumner County, Tennessee 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 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, TVA Gallatin Power Plant (TVA Gallatin) emitted 21,731

tons of SO2 and had an emissions rate of 0.62 lbs SO2/mmBTU. Pursuant to the March 2, 2015

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

In its submission, Tennessee recommended that the area surrounding TVA Gallatin, specifically 

the 50 kilometer (km) radius of the facility centered on coordinates 36.3165 and -86.4033, be 

designated as attainment based on an assessment and characterization of air quality from the 

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

where maximum concentrations of SO2 are expected. This boundary represents 12 whole and 

partial counties including all of Sumner and Trousdale, and portions of Wilson, Smith, Davidson 

and Macon, Robertson, Rutherford, DeKalb, Williamson, Cannon and Cheatham. This 

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

AERMOD, analyzing allowable emissions.

After careful review of the state’s assessment, supporting documentation, and all available data, 

the EPA does not agree with the state’s modeling analysis because it does not use the necessary 

technical information and approach to determine if the area is meeting the SO2 NAAQS. 

Specifically, the EPA reviewed two scenarios that Tennessee provided in its submission and does 

not believe that either of these scenarios were performed consistent with the Modeling TAD or 

the EPA guidance on modeling for the SO2 NAAQS. One scenario involved the consideration of 

TVA Gallatin’s 2012-2014 actual emissions whereas the other scenario involved the 

consideration of future allowable potential to emit (PTE) emission limits based on installation of 

SO2 controls (see consent decree discussion below). The latter scenario is the basis for 

Tennessee’s attainment designation recommendation. The EPA has identified input errors for 

Tennessee’s modeling analysis, and areas where further refinements of the modeling are 

necessary to be most consistent with the modeling approaches found in the Modeling TAD and 

the EPA guidance. Therefore, the EPA intends to designate Sumner County in its entirety as

unclassifiable. The remainder of the counties recommended by the state as attainment, in 

addition to any remaining undesignated area of Tennessee, will be addressed by either December 

31, 2017, or December 31, 2020, consistent with the deadlines in the final consent decree. 

The EPA notes that the future allowable emission rates modeled for the Sumner County Area 

(and which the state uses to support its attainment recommendation) have not yet been 
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established as federally-enforceable3by the state of Tennessee. It is for this reason as well, that 

the EPA intends to designate Sumner County as unclassifiable. In order for the EPA to consider 

the future allowable emission limit for TVA Gallatin for the designations to be finalized no later 

than July 2, 2016, the state will have to ensure that these limits are federally-enforceable (and the 

EPA has confirmed such) by the time this round of designations is complete. Regarding the TVA 

Gallatin future emission limit, if a limit with an averaging time longer than 1-hour is desired, 

supporting technical information will also need to include explanation, of whether the longer 

term average limit, that the EPA determines is comparatively stringent to a 1-hour limit at the 

critical emission value, ensures attainment of the SO2 NAAQS.

According to an April 14, 2011, TVA Federal Facilities Compliance Agreement (FFCA)4, all 

TVA units are limited to a system-wide annual tonnage limitations for SO2. This consent decree 

requires TVA Gallatin to install and commence continuous operations of SO2 controls (e.g., flue 

gas desulfurization (FGD), renewable biomass, repower, or retirement) for units 1-4 no later than 

December 31, 2017. In its September 16, 2015 designation recommendation, Tennessee 

indicated that TVA Gallatin was installing FGD on two of the four coal-fired units to comply 

with the TVA agreement and expected the control upgrades to be complete and operational by 

April 16, 2016. Tennessee specified that currently two FGD systems are already on-line and 

operating. Tennessee stated that enforceable emission limits associated with these control

upgrades are expected to be included in TVA Gallatin’s title V permit by April 8, 2016. On 

October 30, 2015, Tennessee supplemented their recommendation to include a technical analysis 

to support their September 16, 2015 recommendation of attainment, including a modeling 

demonstration indicating that FGD on unit 4 was operational and that control of the remaining 

three units would be completed by April 2016.

In January 2016, Tennessee provided additional information to support the state’s 

recommendation. The EPA has not yet had time to complete a full review of this information but 

will consider it prior to finalizing designations. If the state provides additional information prior 

to promulgation of final designations, the EPA will consider this information in our final 

designation for the Sumner County Area. 

TVA Gallatin is located in central Tennessee in the southern portion of Sumner County. As seen 

in Figure 1 below, the facility is located approximately 5 kilometers (km) southeast of the center 

of Gallatin. Also included in the figure are the state’s recommended area for the attainment

designation, and the EPA’s intended unclassifiable designation.

 

3 Consistent with past interpretations of legal requirements, control measures, emission limits and other curtailments 

need to be installed, operational and federally-enforceable to be considered when informing final designation 

decisions. The mechanisms for establishing federally enforceable emission limits, control measures, or curtailments 

for the purpose of informing SO2 designations include: a source-specific state implementation plan (SIP)-approved 

by the EPA, a minor new source review (NSR) permit, a title v permit or a consent decree established through 

Federal civil litigation.

4 The FFCA requires TVA to reduce SO2, nitrogen oxide and particulate matter emissions from coal-fired units. This 

includes addressing 92 percent of TVA's coal-fired system between 2011 and 2018 by either the installation of stateof-the-art pollution controls such as selective catalytic reduction (SCR), FGD, retirement, or repowering to 

renewable biomass. See http://www2.epa.gov/sites/production/files/documents/tva-ffca.pdf
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Figure 1. The EPA’s intended designation for Sumner County Area

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

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:
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 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 15131, 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, rural dispersion coefficients are to be used in the dispersion modeling 

analysis if more than 50 percent of the area within a 3 km radius of the facility is classified as 

rural. Conversely, if more than 50 percent of the area is urban, urban dispersion coefficients 

should be used in the modeling analysis. When performing the modeling for the area of analysis, 

the state determined that it was most appropriate to use the rural dispersion coefficients.

Tennessee did not provide information to support using the rural option for modeling the TVA 

Gallatin neither the future allowable PTE emissions scenario nor the 2012-2014 actual emissions

scenario. The information that has been provided to date does not enable the EPA to determine if 

rural dispersion coefficients are 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 TVA Gallatin 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 

Sumner County Area, the state assessed sources within the 12 county, 50 km area of analysis in 

all directions that emitted over 100 tpy based on 2014 emissions. The entire area of analysis 

covers over 100 SO2 sources including TVA Gallatin. Only two other sources within 

Tennessee’s analysis area (i.e., other than TVA Gallatin) emitted over 100 tpy of SO2 in 2014. 

These sources are Vanderbilt University and Carlex Glass America, LLC, both located in

Davidson County. Other emitting-sources in the area of analysis counties covers 100 sources that 

according to 2014 emissions inventory emitted a cumulative total of 67 of SO2. Tennessee 

determined that none of these SO2 emissions sources needed to be modeled because they would 

not likely cause concentration gradients in the area due to their 2014 SO2 emissions inventory, 

and because of their distance to TVA Gallatin and Sumner County. Additionally, the state 

indicated that any impacts from other sources would be accounted for by using an appropriate 

background concentration from the chosen ambient air monitor in Nashville (Davidson County), 
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Tennessee. The state did multiple modeling runs using multiple receptor grids. The receptor 

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

 Initial receptor grid: 50-km polar grid centered in the middle of the facility with 250 m 

spacing;

 Refined 10 x 10 km Cartesian grid system (with 10404 receptors) was implemented with 

a 100 meter spacing receptor grid to assess the location of maximum impacts ;

 A smaller Cartesian grid with 1708 discrete receptors at 100-m resolution was applied for 

the final modeling analysis. 

The final receptor network contained 10,404 receptors, and the network covered a small portion 

of Sumner and Wilson Counties in Tennessee. 

Figures 2 and 3, included in the state’s recommendation, show the state’s chosen area of analysis 

surrounding the TVA Gallatin as well as the 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 air 

impacts. The impacts of the area’s geography and topography will be discussed later within this 

document.

Figure 2: Sumner County Area of Analysis - Source: Tennessee’s Round 2 – Sulfur Dioxide 

Designations Recommendations prepared by TDEC October 30, 2015
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Figure 3: Final Receptor Grid for the Sumner County Area of Analysis - Source: Tennessee’s 

Round 2 – Sulfur Dioxide Designations Recommendations prepared by TDEC October 30, 2015
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Modeling Parameter: Source Characterization

The modeling report and supporting model input and output files provide information for 

characterizing the sources that were modeled including source locations and stack parameters 

(stack heights, exit temperatures, exit velocities, and diameters). Information provided by the 

state indicates that the BPIPPRIME preprocessor was used to evaluate the potential for building 

downwash for both the future allowable PTE emissions scenario and the 2012-2014 actual 

emissions scenario. However, information was not provided describing the building layouts and 

locations that were input into BPIPPRIME. Therefore, the EPA is unable to determine if the 

building downwash analysis using BPIPPRIME is acceptable. Also, for the future allowable PTE 

emissions modeling scenario, actual stack heights were used for modeling the four coal-fired 

boilers. This deviates from the Modeling TAD which recommends using good engineering 

practice (GEP) stack heights when modeling allowable PTE emissions. 

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 sources should be used. For the 2012-2014 actual 

emissions modeling scenario, the state indicated that 2012-2014 CEMS data were used for the 

four primary coal fired boilers to model hourly varying emissions using the HOUREMIS 

keyword. However, the hourly varying emissions files containing the CEMS data were not 

provided by the state and no details were provided by the state to explain how the data in the 

hourly varying files were generated. The information provided the state indicates that hourly 

varying SO2 emission rates were used in the modeling, but it appears that single, non-varying 

values of stack exit temperature and stack exit velocity were paired with these hourly emissions. 

If hourly varying temperatures and velocities are available, they should have been used in the 

modeling. 

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 
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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 discussed in the “Introduction” section above, TVA Gallatin and the state chose to use 

modeling based upon future allowable PTE emissions limits as the basis for their attainment 

designation recommendation. The future allowable PTE emissions are based upon additional 

FGD controls being installed on TVA Gallatin’s four coal fired boilers pursuant to a 2011 FFCA 

consent decree. The future allowable PTE controlled emissions used by Tennessee for the 

AERMOD modeling were calculated by reducing the average 2012-2014 actual emissions by 90 

percent, which is the estimated control efficiency of the controls on the four coal fired boilers. If 

the state is able to address all of the modeling concerns described throughout this Technical 

Support Document, the future allowable PTE emissions modeling may be determined to be 

acceptable, provided that appropriate federally enforceable limits are established. These units 

would need to restrict operation of the units to meet the conditions that have been included in 

revised modeling consistent with the EPA’s guidance. Tennessee has indicated that they expect 

the control upgrades for the four coal-fired boilers to be complete and operational by April 16, 

2016, noting that two FGD systems are already on-line and operating. Furthermore, the state has 

also noted that enforceable emission limits associated with these control upgrades are expected 

to be included in TVA Gallatin’s title V permit by April 8, 2016.

For the Sumer County Area, Tennessee assessed SO2 emitting sources within a 12 county, 50 km 

area of analysis that emitted 100 tpy or more based on 2014 emissions data. This area of analysis 

was chosen because Tennessee believes it represents the area where maximum concentrations of 

SO2 are expected. Along with TVA Gallatin in Sumner County, only 2 other sources in the area 

of analysis, Vanderbilt University and Carlex Glass America, LLC in Davidson County emitted 

over 100 tpy in 2014. Vanderbilt University, located approximately 41 km southwest of TVA 

Gallatin and 20 km, emitted 843.8 tons in 2014, and according to Tennessee shutdown its coalfired boiler in 2014. According to a press release5

, Vanderbilt converted to natural gas operation 

and decommissioned its last coal-fired boiler in 2014. The Carlex Glass America, LCC, located 

approximately 45 km from TVA Gallatin and 23 km from the Sumner County border, emitted 

133.3 tons of SO2 in 2014. Tennessee did not include either of these sources in the modeling 

analysis due to the shutdown of Vanderbilt University and the 2014 emissions relative to the 

distance for Carlex Glass America. However, the background monitor is located within the 

vicinity of both Vanderbilt University and Carlex Glass America, and therefore these sources’

impacts are likely captured by the background concentration. 

The remaining emissions inventory (excluding those sources over 100 tpy) within the 12 county 

area of analysis (Sumner, Davidson, Wilson, Smith, Davidson, Macon, Robertson, Rutherford, 

DeKalb, Williamson, Cannon and Cheatham) covers over 100 sources (some which reported no

2014 SO2 emissions) that according to 2014 emissions inventory emitted a cumulative total of 67

tons of SO2. Tennessee determined that none of these SO2 emissions sources within the area of 

 

5 See http://news.vanderbilt.edu/2015/05/vu-exhaust-stack-demolition-complete/ and 

http://news.vanderbilt.edu/2014/12/vanderbilt-power-plant-is-now-coal-free/
13

analysis needed to be modeled because they would not likely cause concentration gradients in the 

area due to their 2014 SO2 emissions inventory and their distance to TVA Gallatin and Sumner 

County. The EPA does not believe these sources would contribute to a violation of the SO2

NAAQS near the Sumner County area of analysis nor cause concentration impacts within their 

respective county borders. 

Table 2: Actual SO2 Emissions from sources emitting greater than 100 tpy in2014 from Facilities 

in the Sumner County Area of Analysis

County Facility Name

2014 SO2 Emissions 

(tons per year (tpy))

6

Sumner TVA: Gallatin 19437.58

Davidson

Vanderbilt University(unit 

shutdown in 2014) 843.8

Davidson

Carlex Glass America, 

LLC 133.3

As previously noted, the state’s modeling only included emissions from TVA Gallatin and did 

not include other emitter of SO2 within 50 km in the area of analysis. Tennessee determined that 

none of these SO2 emissions sources within the area of analysis needed to be modeled because 

they would not likely cause concentration gradients in the area due to their 2014 SO2 emissions 

inventory and their distance to TVA Gallatin and Sumner County. The EPA does not believe 

these sources would contribute to a violation of the SO2 NAAQS near the Sumner County area of 

analysis nor cause concentration impacts within their respective county borders. 

The state has chosen to model the facility using the future federally-enforceable PTE limits for 

SO2 established pursuant to the Federal Facilities Compliance Agreement discussed above as the 

basis for their designation recommendation. The future allowable PTE rates are summarized 

below. 

 

6 2014 annual emissions data were provided by the state and represent emissions reported to the Emissions Inventory 

System (EIS) gateway, in which states report emissions pursuant to 40 CFR Part 51, Subpart A. The EIS gateway 

can be accessed via: http://www3.epa.gov/ttnchie1/eis/gateway/.
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Table 3: SO2 Emissions based on future allowable PTE from Facilities in the Sumner County

Area of Analysis

Facility Name Unit ID

Modeled SO2 Emissions 

(pounds per hour, based on 

PTE)7

TVA Gallatin

TVA Gallatin CSGA12 227.44

TVA Gallatin CSGA34 216.69

TVA Gallatin CT1 0.00001

TVA Gallatin CT2 0.00556

TVA Gallatin CT3 0.00635

TVA Gallatin CT4 0.00556

TVA Gallatin AUXBLR 0.00000

TVA Gallatin CT5 0.02302

TVA Gallatin CT6 0.02302

TVA Gallatin CT7 0.04603

TVA Gallatin CT8 0.02302

TVA Gallatin NGH1 0.00002238

TVA Gallatin NGH2 0.00000706

TVA Gallatin NGH3 0.00000573

Total Emissions All Units 444.2626

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, and military 

stations.

For the Sumner County Area of analysis, surface meteorology from the Nashville International 

Airport NWS station (BNA) in Nashville, Tennessee approximately 20 miles to the southwest, 

and coincident upper air observations from the same NWS station (BNA) in Nashville, 

Tennessee were selected as best representative of meteorological conditions within the area of 

analysis. 

 

7 The total tons per year values based upon the new future allowable PTE emissions were not provided by the state, 

so this table provides the modeled pound per hour SO2 emissions. 
15

Figure 4: TVA Gallatin Facility location in relation to the Nashville International Airport NWS 

and the Trinity Lane background monitor

As part of its recommendation, the state provided the 5-year surface wind rose for Nashville, 

Tennessee. In Figure 5, the frequency and magnitude of wind speed and direction are defined in 

terms of from where the wind is blowing. The wind rose shows that the winds blow 

predominately from the South with mid-high wind speeds.
16

Figure 5: Nashville, Tennessee Cumulative Annual Wind Rose for Years 2010 – 2014 - Source: 

Tennessee’s Round 2 – Sulfur Dioxide Designations Recommendations prepared by TDEC 

October 30, 2015

Documentation provided by the state indicates that the meteorological data from the above 

surface and upper air stations were used in generating AERMOD-ready files with the AERMET 

and AERMINUTE processors. These meteorological files were used by the state both the future 

allowable PTE emissions modeling and the 2012-2014 actual emissions modeling. Specific 

details of the AERMET and AERMINUTE processing of the raw meteorological data, including

input and output files, were not provided by the state. Additionally, no information was provided 

by the state regarding the determination of surface characteristics (surface roughness, albedo and 

Bowen ratio) used in the AERMET processing. No indication was provided whether the EPA’s 

AERSURFACE tool was used for determining the appropriate surface characteristics or whether 

an alternate methodology was used. The detailed procedures for determining the surface 

characteristic parameters should be provided by the state.
17

Modeling Parameter: Geography and Terrain

The terrain in the area of analysis is best described as generally rolling to hilly. To account for 

these terrain changes, the modeling documentation submitted by the state indicates that the 

AERMAP terrain program within AERMOD was used to specify terrain elevations for all the 

receptors for both the future allowable PTE emissions scenario and the 2012-2014 actual 

emissions scenario. The source of the elevation data incorporated into the model is from the 

United States Geological Survey National Elevation Database (NED). However, the actual 

AERMAP input and output files and support NED files were not proved by the state to enable 

the EPA to review the procedures used in the AERMAP processing. These files should be 

provided by the state.

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 Sumner County Area of 

analysis, the state chose the “first tier” approach and used a single monitored design value from 

the Nashville/Davidson/Trinity Lane ambient monitor (AQS ID 47-037-0011). Tennessee 

described the Nashville monitor as a “conservative urban-based” background monitor. The 

background concentration for this area of analysis was determined by the state to be 28.8

micrograms per cubic meter (μg/m3), or 11 ppb,

8

and that value was incorporated into the final 

AERMOD results. The Trinity Lane monitor is located in the vicinity (approximately 10 km) of 

both the Vanderbilt University and Carlex Glass facilities, as shown in Figure 2. This 

background monitor likely covers the impacts from these two facilities. Additionally, the monitor 

is located between the two facilities and the TVA Gallatin facility so would account for any 

impacts from the facilities in the area of analysis around the TVA Gallatin facility.

Summary of Modeling Results

The AERMOD modeling parameters for the Sumner County Area of analysis are summarized 

below in Table 4.

 

8 The conversion factor for SO2 (at the standard conditions applied in the ambient SO2 reference method) is 1ppb = 

approximately 2.62μg/m3.
18

Table 4: AERMOD Modeling Parameters for the Sumner County Area of Analysis

Sumner County Area of Analysis

AERMOD Version 15181

Dispersion Characteristics Rural

Modeled Sources 1

Modeled Stacks 14

Modeled Structures not available

Modeled Fencelines 1

Total receptors 10,404

Emissions Type Future Allowable PTE

Emissions Years

New allowable limit effective 

in 2016/TBD

Meteorology Years 2010-2014 

Surface Meteorology Station Nashville, Tennessee

Upper Air Meteorology Station Nashville, Tennessee 

Methodology for Calculating 

Background SO2 Concentration 1

st tier single value

Calculated Background SO2

Concentration 11 ppb or 28.8 μg/m3

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

highest predicted modeled concentration based on future allowable PTE emissions as this is the 

modeling scenario that the state relied upon for its designation recommendation.

Table 5: Maximum Predicted 99th Percentile 1-Hour SO2

Concentration in the Sumner County Area of Analysis Based on or PTE Emissions

*Equivalent to the 2010 SO2 NAAQS set at 75 ppb

Averaging 

Period Data Period

Receptor Location SO2 Concentration (μg/m3)

UTM/Latitude UTM/Longitude

Modeled

(including 

background) NAAQS

99th Percentile 

1-Hour Average 2010-2014 552000 4017500 40.5 196.5*

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

within the chosen modeling domain is 40.5 μg/m3, or 15.5 ppb. This modeled concentration 

included the background concentration of SO2, and is based on future allowable PTE emissions 

from the facility. Figure 6 below was included as part of the state’s recommendation, and 

indicates that the predicted value occurred southwest of TVA: Gallatin. The state’s receptor grid 

is also shown in the figure. Consistent with past interpretations of legal requirements emission 

limits and other curtailments need to be installed, operational and federally-enforceable by April 
19

19, 2016 to be considered when informing final designation decisions. The EPA notes that the 

future allowable emission rates modeled for the Sumner County Area have not been established 

as federally-enforceable9by the state of Tennessee. If Tennessee: (1) addresses the technical 

modeling issues represented in this document; (2) demonstrates that the area meeting the SO2 

NAAQS; (3) and establishes TVA Gallatin’s future modeled allowable emission rate federallyenforceable (through a federally-enforceable mechanism)

10 and pursuant to the EPA’s 2014 SO2

Nonattainment Area SIP Guidance relating to a longer term average limit (if applicable) that the 

EPA can determine is comparatively stringent to a 1-hour limit at the critical emission value, the 

EPA may revise the designation to unclassifiable/attainment.

Tennessee has indicated that they expect the control upgrades for the four coal-fired boilers to be 

complete and operational by April 16, 2016 noting that two FGD systems are already on-line and 

operating. Furthermore, the state has also noted that enforceable emission limits associated with 

these control upgrades are expected to be included in TVA Gallatin’s title V permit by April 8, 

2016.

 

10 For example, a source-specific SIP approved by the EPA, a minor NSR permit, a title v permit or a consent decree 

established through Federal civil litigation
20

Figure 6: Maximum Predicted 99th Percentile 1-Hour SO2 Concentrations in the 

Sumner County Area of Analysis Based on future allowable PTE Emissions 

Jurisdictional Boundaries:

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

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

purpose of informing our intended unclassifiable area, specifically with respect to clearly defined 

legal boundaries. Tennessee’s boundary recommendation for TVA Gallatin is comprised of 

portions of 12 counties within a 50 km radius of the facility. However, based upon the EPA’s 

review of the state’s assessment, supporting documentation, and all available data, the modeling 

was not performed consistent with the Modeling TAD or the EPA’s guidance. Consequently, the 

EPA intends to designate Sumner County in its entirety as unclassifiable. 

Along with TVA Gallatin in Sumner County, only 2 other sources in the area of analysis, 

Vanderbilt University and Carlex Glass America, LLC in Davidson County emitted over 100 tpy 
21

in 2014. According to Tennessee and press releases, Vanderbilt University converted to natural 

gas operation and decommissioned its last coal-fired boiler in 2014. The Carlex Glass America, 

LCC, located approximately 45 km from TVA Gallatin and 23 km from the Sumner County 

border, emitted 133.3 tons of SO2 in 2014. Tennessee did not include either of these sources in 

the modeling analysis due to the shutdown of Vanderbilt University and the 2014 emissions 

relative to the distance for Carlex Glass America. However, their emissions are likely captured 

by the background concentration. All other SO2 emitting sources in the 12 county area of 

analysis (Sumner, Davidson, Wilson, Smith, Davidson, Macon, Robertson, Rutherford, DeKalb, 

Williamson, Cannon and Cheatham) covers over 100 sources that according to 2014 emissions 

inventory emitted a cumulative total of 67 tons SO2. Tennessee determined that none of these

SO2 emissions sources within the area of analysis needed to be modeled because they would not 

likely cause concentration gradients in the area due to their 2014 SO2 emissions inventory and 

their distance to TVA Gallatin and Sumner County. The EPA does not believe these sources are 

likely to contribute to a violation of the SO2 NAAQS near the Sumner County area of analysis 

nor cause concentration impacts within their respective county borders. As a result, our intended 

unclassifiable area consisting of Sumner County in its entirety, is a suitable, defined legal

boundary.

Other Relevant Information

The EPA has conducted a thorough review of the modeling performed by Tennessee for the 

TVA Gallatin facility to support their designation recommendation.11 The EPA’s review has 

identified a number of unresolved issues regarding the procedures and data used in the modeling 

which have led to the EPA intent to designate Sumner County as unclassifiable. The following is 

a brief summary of significant issues identified in the EPA’s review of the modeling:

 The future allowable (PTE) controlled emissions used by Tennessee for the 

AERMOD modeling were calculated by reducing the average 2012-2014 actual 

emissions by 90 percent, which is the estimated control efficiency of the controls that 

have been installed or are in the process of being installed on TVA Gallatin’s four 

units in order to meet the future enforceable terms of the FFCA consent decree. This 

may be acceptable if the TVA Gallatin permit is modified to incorporate these new 

allowable emission rates as 1-hour emissions limits and not as longer term averaging 

time limits (e.g., 30-day rolling average). If Tennessee and TVA Gallatin desire to 

use an averaging time longer than 1-hour, the recommendations provided in the 

EPA’s 2014 SO2 Nonattainment Area SIP Guidance should be followed. This 

guidance indicates that modeling to determine attainment with the 1-hour SO2 

NAAQS should use a higher 1-hour “critical value” to account for hourly emissions 

variability in longer-term (e.g., 30-day rolling average) allowable (PTE) limits. The 

details for the EPA’s recommended procedure for calculating the adjustment factor 

between the 1-hour critical value and the equivalent 30-day rolling average emissions 

limit are provided in Appendices B and C of this EPA’s 2014 SO2 Nonattainment 

 

11 These comments pertain to Tennessee’s October 30, 2015, updated recommendation, which includes a technical 

analysis and modeling to support their September 16, 2015 recommendation of attainment.
22

Area SIP Guidance.

4 The EPA has informed Tennessee that an appropriate 1-hour 

critical value should be used in the modeling of future allowable PTE emissions if an 

averaging time longer than 1-hour is desired for the new allowable permit limits. 

 Tennessee’s modeling report indicates that the stack exit temperatures and velocities 

used for modeling the future allowable (PTE) controlled emissions are the same as 

the average of the 2012-2014 actual values. It is likely that the stack exit temperatures 

will be significantly reduced once new scrubber controls are installed. This would 

likely increase the modeled concentration results because the plume heights may be 

significantly lower due to a lower thermal buoyancy. Tennessee should verify that the 

stack parameters used in modeling the future allowable emissions reflect the actual 

design values from TVA Gallatin.

 For the 2012-2014 actual emissions modeling scenario, the state indicated that 2012-

2014 CEMS data were used for the four primary coal fired boilers to model hourly 

varying emissions using the HOUREMIS keyword. However, the hourly varying 

emissions files containing the CEMS data were not provided by the state and no 

details were provided by the state to explain how the data in the hourly varying files 

was generated. The information provided by the state indicates that hourly varying 

SO2 emission rates were used in the modeling, but it appears that single, non-varying 

values of stack exit temperature and stack exit velocity were paired with these hourly 

emissions. If hourly varying temperatures and velocities are available, they should be

used in the modeling. 

 For both the future allowable PTE emissions scenario and the 2012-2014 actual 

emissions scenario, Tennessee’s modeling report and supporting modeling files do 

not provide information about how the surface characteristics (surface roughness, 

albedo and Bowen ratio) were calculated for the modeling. There is no indication 

whether the EPA AERSURFACE tool was used, and if so, what options and 

procedures were followed. Additionally, information has not been provided regarding 

how the AERMET and AERMINUTE processing was conducted, including how the 

surface characteristics were treated in AERMET. Tennessee should provide this 

information to the EPA for review.

 Tennessee did not provide information to support using the rural option for modeling 

the TVA Gallatin emissions or to enable the EPA to determine if rural dispersion 

coefficients are appropriate for both the future allowable PTE emissions scenario and 

the 2012-2014 actual emissions scenario. The EPA informed Tennessee of the need 

for this information. To date, the requested information has not been provided for 

review. 

 Information provided by the state indicates that the BPIPPRIME preprocessor was 

used to evaluate the potential for building downwash for both the future allowable 

PTE emissions scenario and the 2012-2014 actual emissions scenario. However, 

information was not provided describing the building layouts and locations that were 

input into BPIPPRIME. This information should be provided by the state. Also, for 
23

the future allowable PTE emissions modeling scenario, actual stack heights were used 

for modeling the four coal-fired boilers. This deviates from the Modeling TAD which 

recommends using GEP stack heights when modeling allowable PTE emissions. The 

future allowable PTE emissions modeling should be revised using the appropriate 

GEP stack heights. 

The EPA did not receive any additional information about the area in the immediate vicinity of 

the TVA Gallatin facility to inform our intended designations.

Conclusion

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

available relevant information, the EPA intends to designate Sumner County as unclassifiable for 

the 2010 SO2 NAAQS due to unresolved issues with the state’s modeling analysis. Based on all 

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

determine whether the area is meeting or not meeting the NAAQS. Specifically, our intended 

boundary consists of Sumner County in its entirety which differs from the state’s 

recommendation of 12 counties within a 50 km radius from TVA Gallatin. A summary of 

unresolved issues is provided in the discussion above. 

If the state provides additional information prior to promulgation of final designations that 

adequately addresses the issues identified above, the EPA may consider this information in our 

final designation for the Sumner County Area. Tennessee has indicated that they expect the FGD 

control upgrades (pursuant to the 2011 FFCA) for the four coal-fired boilers to be complete and 

operational by April 16, 2016, noting that two FGD systems are already on-line and operating. In 

its technical submission and in discussions, the state has also noted that enforceable emission 

limits associated with these control upgrades are expected to be included in TVA Gallatin’s title 

V permit by April 8, 2016.and, establishes TVA Gallatin’s future modeled allowable emission 

rate federally-enforceable (through a federally enforceable mechanism) and, as discussed above 

include a longer term average limit (if applicable) that the EPA determines is comparatively 

stringent to a 1-hour limit at the critical emission value. These issues would also have to be 

resolved to support an ultimate unclassifiable/attainment designation, if appropriate.

At this time, our intended designations for the state only apply to this area (Sumner County). 

Consistent with the conditions in the March 2, 2015 court-ordered schedule, the EPA will 

evaluate and designate all remaining undesignated areas in Tennessee by either December 31, 

2017, or December 31, 2020.