                       Technical Support Document (TSD)
                   Redesignation of Sumner County, Tennessee
                                       
                                    Summary
                                       
Pursuant to section 107(d) of the Clean Air Act (CAA or Act), the U.S. Environmental Protection Agency (EPA) designated Sumner County, Tennessee (hereinafter also referred to as the Sumner County Area or Area) as unclassifiable for the 2010 1-hour sulfur dioxide (SO2) primary national ambient air quality standard (NAAQS) in the second round of SO2 designations (81 FR 45039 published July 12, 2016).  Unclassifiable areas are defined as those that cannot be classified on the basis of available information as meeting or not meeting the NAAQS.  The unclassifiable designation for Sumner County, Tennessee was based on the information the State of Tennessee provided to EPA.  Specifically, the designation and associated boundaries were based on EPA's evaluation of the State's air dispersion modeling analysis for the area surrounding Tennessee Valley Authority (TVA) Gallatin Fossil Plant (hereinafter referred to as TVA-Gallatin) in Sumner County.  In summary, EPA's evaluation of the State's modeling as of EPA's July 2016 designations resulted in an unclassifiable designation because the State's modeling analysis relied upon a future allowable SO2 emissions limit, that was not yet federally-enforceable and that did not correspond to the State's preliminary permit limit's rate, to determine if the Area is meeting the 2010 1-hour SO2 NAAQS, and because the State did not appropriately account for background concentrations.

After careful evaluation of the State's recommendation, all timely comments and information received, EPA found that, based on available information, EPA could not determine whether the area around TVA-Gallatin was meeting or not meeting the NAAQS, and designated the Sumner County Area as unclassifiable for the 2010 1-hour SO2 NAAQS as part of EPA's SO2 Round 2 designations in July 2016.[,]  The boundaries for this designation consist of the entirety of Sumner County.
EPA has continued to work with Tennessee to revise and refine the modeling for the Sumner County Area to provide a reliable characterization of SO2 air quality for the Area.  Tennessee submitted a request on September 29, 2020, for EPA to change the designation of the Sumner County Area from unclassifiable to attainment/unclassifiable based on refined and updated modeling.
Section 107(d)(3)(A) provides that the Administrator may notify the Governor of any state that the designation of an area should be revised "on the basis of air quality data, planning and control considerations, or any other air quality-related considerations the Administrator deems appropriate."  The Act further provides in section 107(d)(3)(D) that even if the Administrator has not notified a state Governor that a designation should be revised, the Governor of any state may, on the Governor's own motion, submit a request to revise the designation of any area, and the Administrator must approve or deny the request.
When approving or denying a request to redesignate an area, EPA bases its decision on the air quality data for the area as well as the considerations provided under section 107(d)(3)(A).  EPA defines an attainment/unclassifiable area as: an area that, based on available information including (but not limited to) appropriate monitoring data and/or modeling analyses, EPA has determined meets the NAAQS and determined that the available information indicates that the area does not likely contribute to ambient air quality in a nearby area that does not meet the NAAQS.  EPA is proposing to find that the Sumner County Area would fall under the second definition.  EPA is proposing to approve Tennessee's request to redesignate all of Sumner County from unclassifiable to attainment/unclassifiable.


           Technical Analysis for the Sumner County, Tennessee Area
1.0 Introduction/Background

The Sumner County Area contains the TVA-Gallatin facility.  TVA-Gallatin is located in north-central Tennessee in the southern portion of Sumner County approximately 5 kilometers (km) southeast of the center of Gallatin and approximately 30 miles (48 km) northeast of Nashville.  In July 2016, EPA designated the entirety of Sumner County, as unclassifiable to meet the consent decree deadline of designating the area around TVA-Gallatin.  Figure 1 outlines EPA's July 2016 unclassifiable designation county boundary for the Area.
Figure 1:  EPA's July 2016 Area Designation for TVA-Gallatin (All of Sumner County, Tennessee).

There are no other emitters of SO2 greater than 100 tons per year (tpy) in the Sumner County Area.  Tennessee evaluated the following nearby sources for inclusion in the modeling:  Nashville International Airport (80.4 tpy; 33.8 km), Carlex Glass America, LLC (203.1 tpy; 44.7 km), Alternative Energy (6.6 tpy; 36.2 km), Ryman Hospitality Properties, Inc. (5.6 tpy; 29.6 km), and Hoeganaes Corp. (15.4 tpy; 6.5 km).  Based on the total emissions over distance (Q/d less than 20) methodology, Tennessee determined that none of the nearby sources needed to be included in the modeling analysis.
Table 1: Actual SO2 Emissions from sources emitting greater than 5 tpy in 2018 from Facilities in the Sumner County Area of Analysis
                                       
                                    County
                                 Facility Name
                           2018 SO2 Emissions (tpy)
                                    Sumner
                                 TVA-Gallatin
                                    1,740.5
                                    Sumner
                             Hoeganaes Corporation
                                     15.4
                                   Davidson
                        Nashville International Airport
                                     80.4
                                   Davidson
                           Carlex Glass America, LLC
                                     203.1
                                   Davidson
                              Alternative Energy
                                      6.6
                                   Davidson
                      Ryman Hospitality Properties, Inc.
                                      5.6

 Other Relevant Information

For the 2010 SO2 primary NAAQS, designations were based on the EPA's application of the nationwide analytical approach to, and technical assessment of, the weight of evidence for each area, including but not limited to available air quality monitoring data and air quality modeling results.  EPA issued updated SO2 designations guidance for the 2017 Round 3 designations through a July 22, 2016 , memorandum from Stephen D. Page, Director, U.S. EPA, Office of Air Quality Planning and Standards, to Regional Air Division Directors, U.S. EPA Regions 1-10.  This memorandum superseded earlier designation guidance for the 2010 1-hour SO2 NAAQS, issued on March 5, 2015 for the Round 2 designations.  The designation guidance referenced  two technical assistance documents intended to assist states and other interested parties in their efforts to characterize air quality through air dispersion modeling or ambient air quality monitoring for sources that emit SO2.  Notably, EPA released its modeling guidance titled, "SO2 NAAQS Designations Modeling Technical Assistance Document" (Modeling TAD) in December 2013.  EPA has since updated the Modeling TAD and the most recent version is dated August 2016.

The discussion and analysis that follows below will reference the State's use of the Modeling TAD, EPA's assessment of the State's modeling in accordance with the Modeling TAD, and the factors for evaluation contained in EPA's July 22, 2016, guidance, as appropriate.  EPA has included additional narrative and EPA's analysis of the modeling included in the September 29, 2020, redesignation package.

2.0 Detailed Assessment

2.1 Air Quality Data

There are no SO2 air quality monitors in the Sumner County Area.  Additionally, there are no SO2 air quality monitors in surrounding counties that are representative of the maximum or higher elevated levels of SO2 around the TVA-Gallatin facility.  The closest SO2 monitor to TVA-Gallatin is the East Health (Trinity Lane) monitor in Nashville (AQS ID: 47-037-0011), approximately 34 km southwest of TVA-Gallatin, which does not have a valid 2019 design value.  The 2019 design value for this monitor is invalid due to incomplete data in 2018 and 2019.  As discussed in Section 2.9 of this TSD, the 99[th] percentile 1-hour SO2 concentration in 2019 (8 ppb) measured at the East Health monitor was used in the modeling analysis to represent the ambient background concentration in the area.

2.2 Model Selection and Modeling Components

Model Selection and Modeling Components

EPA's Modeling TAD notes that for area designations under the 2010 1-hour SO2 NAAQS, the AERMOD modeling system should be used, unless use of an alternative model can be justified.  In some instances, the recommended model may be a model other than AERMOD, such as the BLP model for buoyant line sources.
The AERMOD modeling system contains the following components:
- AERMOD: the dispersion model
- AERMAP: the terrain processor for AERMOD
- AERMET: the meteorological data processor for AERMOD
- BPIPPRM: 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

The State used AERMOD version 19191, the current version of the model.  A discussion of the individual components will be referenced in the corresponding discussion that follows as appropriate.

2.3 Modeling Parameter: Rural or Urban Dispersion

EPA's recommended procedure for characterizing an area by prevalent land use is based on evaluating the dispersion environment within 3-km of the facility.  According to EPA's modeling guidelines, urban 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 urban. Otherwise, the source is considered a rural source.  When performing the modeling for the area of analysis, Tennessee determined that it was most appropriate to run the model in rural model.
The Auer method was used to determine the land use status of the area around TVA-Gallatin.  A 3-km radius was centered on the common location of the four boiler stacks, and the land use was categorized based on the Auer classifications (Auer, 1978).  The data source for the land cover was the 2011 National Land Cover Database (NLCD), with a data cell size (raster) of 30 meters (m) by 30 m.  The results of the Auer land use analysis for the TVA-Gallatin Area are presented in Table 2 and Figure 2 (Table 1 and Figure 2 in the TVA-Gallatin Redesignation Modeling Report).  The analysis indicates that the TVA-Gallatin Area is approximately 98.5 percent rural and 1.5 percent urban.  Additionally, Tennessee evaluated the latest U.S. Geological Survey (USGS) 2016 NLCD plot of the TVA-Gallatin Area and determined that the total urban categories areas comprise less than 50 percent of the total enclosed 3-km area around the TVA-Gallatin main stack(s) location, which can be seen in Figure 3 (Figure 3 in the TVA-Gallatin Redesignation Modeling Report).  Therefore, the State determined that is was most appropriate to run the model with rural dispersion coefficients based on the 2011 and 2016 USGS land cover data, and the preliminarily EPA agrees with this determination.
Table 2: Auer Land Use Percentages by Category  -  TVA-Gallatin Area  -  2011 Land Cover (Source: TVA-Gallatin Redesignation Modeling Report)
                                       

Figure 2: Auer Land Use Analysis  -  TVA-Gallatin Area (Source: TVA-Gallatin Redesignation Modeling Report)
                                       

Figure 3: Auer Land Use Analysis  -  TVA-Gallatin Area  -  2016 Land Cover (Source: TVA-Gallatin Redesignation Modeling Report)
                                       
                                       
2.4 Modeling Parameter: Area of Analysis (Receptor Grid)

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.

The TVA-Gallatin modeling was performed using a series of nested gridded receptor sets.  The receptor network contained 29,380 receptors.  Boundary receptors were placed along the perimeter of the fenced area of the property and spaced 50 m apart.  These boundary receptors corresponded to a permanent fence surrounding the property.  The nested receptor grids surrounded the facility site with the exception of those falling inside the fenced boundary area, which were removed.  Because concentration gradients are most pronounced near a source, the receptor spacing varied with distance from the site with those nearest the site more closely spaced than those further away.  The origin of each grid was located in the southwest corner.  The receptor spacing is provided in Table 3 (Table 2 of the TVA-Gallatin Redesignation Modeling Report).

Table 3: Receptor Grid Size and Spacing (Source: TVA-Gallatin Redesignation Modeling Report)
                             Receptor Spacing (m)
                                Grid Size (km)
                    Grid Origin (km south and west of site)
                                      100
                                    6 x 6
                                       3
                                      250
                                   40 x 40
                                      20

Figure 4 (Figure 7 of the TVA-Gallatin Redesignation Modeling Report) shows the State's receptor grid.  The 20-km modeling domain of TVA-Gallatin encompasses the following counties:  Sumner, Trousdale and Wilson. Davidson County (Nashville) is slightly touched on the north/northeast borderline as shown in Figure 5 (Figure 9 of the TVA-Gallatin Redesignation Modeling Report).


Figure 4: TVA-Gallatin Modeling Grid (Source: TVA-Gallatin Redesignation Modeling Report)
                                       

Figure 5: TVA-Gallatin 20-km Circular Domain (Source: TVA-Gallatin Redesignation Modeling Report)
                                       
Based on the information provided in Tennessee's recommendation, EPA preliminarily agrees with the area excluded from the modeling because these areas do not represent ambient air for the purposes of SO2 modeling.  EPA also preliminarily agrees that the grid selected by the State is adequate.  Therefore, EPA believes that Tennessee's receptor grid is appropriate for the characterization of the area, considering the impact of SO2 from this facility.

2.5 Modeling Parameter: Source Characterization

Section 6 of the Modeling TAD offers recommendations on source characterization including source types, use of accurate stack parameters, inclusion of building dimensions for building downwash (if warranted), and the use of actual stack heights with actual emissions or following Good Engineering Practices (GEP) policy with allowable emissions.

As discussed earlier in section 1.0, for the area around TVA-Gallatin, the State's modeling included no other emitters of SO2 within 50 km of the facility in any direction.  For the Sumner County Area, for the purpose of determining whether other sources should be included in the modeling analysis, Tennessee assessed SO2 emitting sources within 50 km of TVA-Gallatin that emitted 5 tpy or more based on 2018 emissions data.  Along with TVA-Gallatin in Sumner County, only one other source in the area of analysis emitted greater than 100 tpy during 2018:  Carlex Glass America, LLC in Davidson County.  Carlex Glass America, LLC is located approximately 45 km from TVA-Gallatin and 23 km from the Sumner County border and emitted 203 tons of SO2 in 2018.  This source was not included in the modeling analysis as it was represented by the urban background monitor Tennessee used, which is also in Davidson County.  Due to the distance from TVA-Gallatin, the relatively low SO2 emissions in 2018, and the urban background monitor Tennessee chose, EPA preliminarily supports the exclusion of this source from the modeling analysis.

The State characterized TVA-Gallatin within the area of analysis in accordance with the best practices outlined in the Modeling TAD.  Specifically, the State conservatively used GEP stack heights calculated using EPA's Building Profile Input Program for PRIME (BPIPPRM) preprocessor in conjunction with actual emissions for TVA-Gallatin.  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.  Additionally, the AERMOD component BPIPPRM was used to assist in addressing building downwash.

All SO2 emitting sources at TVA-Gallatin were modeled.  The TVA-Gallatin Redesignation 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).  Tennessee's modeling utilized the BPIPPRM preprocessor to evaluate the potential for building downwash and the modeling submittal provides a description of building layouts and locations which enable the use of BPIPPRM to adequately address building downwash.

EPA preliminarily agrees that Tennessee has appropriately characterized the area surrounding TVA-Gallatin.  Given the criteria for selecting nearby sources, EPA preliminarily agrees with the decision to only include TVA-Gallatin in the modeling analysis.  Also, the State appears to have appropriately used the actual emissions and conservatively used GEP stack heights for TVA-Gallatin and correctly accounted for the building downwash using BPIPPRM for AERMOD.

2.6 Modeling Parameter: Emissions

EPA's Modeling TAD notes that for the purposes of modeling to characterize air quality for use in designations, the recommended approach is to use the most recent 3 years of actual emissions data and concurrent meteorological data.  However, the TAD does provide for the flexibility of using allowable emissions in the form of the most recently permitted, (referred to as potential to emit (PTE) or allowable) emissions rate.

EPA believes that continuous emissions monitoring systems (CEMS) data provide acceptable historical emissions information when it is available, and that these data are available for many electric generating units.  In the absence of CEMS data, EPA's Modeling TAD highly encourages the use of AERMOD's hourly varying emissions keyword HOUREMIS, or using AERMOD's variable emissions factors keyword EMISFACT.  When choosing one of these methods, EPA believes that detailed throughput, operating schedules, and emissions information from the impacted 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 current 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 state implementation plan (SIP) demonstrations should contain the necessary emissions information for designations-related 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."

Figure 1 in Section 1.0 shows the SO2 emitters in the vicinity of TVA-Gallatin at the time of Round 2 designations.  EPA's review of current emission inventories for the Sumner County Area confirms that there are no other SO2 sources aside from TVA-Gallatin that emit over 100 tpy of SO2.  Tennessee used 2017-2019 actual emissions with meteorology data from 2017-2019 in the modeling demonstration.  Table 4 provides the annual emissions in tons/year from the sources modeled at the TVA-Gallatin facility.  Table 5 provides the basis of the short-term emission rates (lb/hr) used in the Tennessee's modeling analysis.


Table 4: SO2 Emissions at TVA Gallatin  -  (Tons/Year) (Source: TVA-Gallatin Redesignation Modeling Report)

                                       


Table 5: Source of SO2 Emissions at TVA-Gallatin (Source: TVA-Gallatin Redesignation)

                                       

Tennessee assessed Boilers 1-4 emissions and Gas Combustion Turbines 1-8 emissions using 2017-2019 CEMS data.  The other sources were modeled at their reported 2018 NEI (V1) actual emissions.  The Oil-Fired Auxiliary Boiler is utilized primarily for indoor heating for the facility at certain times of the year and not on a continuous basis.  This unit had zero emissions reported in 2014 and 2018 and was modeled at zero emissions.  The Emergency Diesel Generator was modeled at the annualized rate of its allowable hourly rate based on a maximum 500 hour per year of potential emissions operations.

EPA preliminarily agrees that emissions used in the modeling are appropriate for evaluating the SO2 concentrations in the Area.

2.7 Modeling Parameter: Meteorology and Surface Characteristics

The most recent 3 years of available 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, the Federal Aviation Administration, and military stations.

For the Sumner County Area, surface meteorology from the Nashville International Airport NWS station (BNA) in Nashville, Tennessee approximately 34 km (21 miles) to the southwest (located at Latitude 36.110535, Longitude -86.688137), 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.

The data were processed using the AERMET (version 19191) meteorological data preprocessor for AERMOD.  In addition, 1-minute meteorological data for the BNA NWS site was processed with AERMINUTE (version 15272) to generate hourly averaged wind speed and wind direction to supplement the standard hourly NWS observations.

The surface characteristics were obtained using, AERSURFACE (version 20060), which uses 2016 land cover data.  Two sets of processed meteorology were applied in the AERMOD modeling analysis by Tennessee to determine the overall highest impact from both runs.  The first set incorporates the surface characteristics at the NWS BNA airport location and the other set incorporates the surface characteristics at the TVA-Gallatin site.  EPA's "AERMOD Implementation Guide" (EPA-454/B-19-035 August 2019), recommends use of surface characteristics at the surface meteorology station for AERMOD modeling analyses.  Therefore, Tennessee's modeling results from the runs using the surface characteristics at the NWS BNA airport are being evaluated by EPA and are presented in Section 2.10 of this TSD.

Tennessee estimated values for 12 spatial sectors out to 1 km at a monthly temporal resolution for average moisture conditions in 2017 and wet moisture conditions in 2018 and 2019.  Tennessee also estimated values for albedo (the fraction of solar energy reflected from the earth back into space), the Bowen ratio (the method generally used to calculate heat lost or heat gained in a substance), and the surface roughness (sometimes referred to as "Zo" and is related to the height of obstacles to the wind flow, which is an important factor in determining the magnitude of mechanical turbulence in the boundary layer).  In Figure 6 (Figure 4 of the TVA-Gallatin Redesignation Modeling Report), the location of the Nashville, Tennessee, NWS station is shown relative to the TVA-Gallatin area of analysis.

Figure 6: TVA-Gallatin Facility location in relation to the Nashville International Airport NWS (Source: TVA-Gallatin Redesignation Modeling Report)
                                       
Meteorological data from the above surface and upper air station was used in generating AERMOD-ready files with the AERMET processor.  The output meteorological data created by the AERMET processor is suitable for being applied with AERMOD input files for AERMOD modeling runs.  Tennessee followed the methodology and settings presented in EPA's Modeling TAD in the processing of the raw meteorological data into an AERMOD-ready format, and used AERSURFACE to best represent surface characteristics.

Hourly surface meteorological data records are read by AERMET and include all the necessary elements for data processing.  However, wind data taken at hourly intervals may not always portray wind conditions for the entire hour, which can be variable in nature.  Hourly wind data may also be overly prone to indicate calm conditions, which are not modeled by AERMOD.  In order to better represent actual wind conditions at the meteorological tower, wind data of 1-minute duration was provided from the same instrument tower, but in a different formatted file to be processed by a separate preprocessor, AERMINUTE.  These data were subsequently integrated into the AERMET processing to produce final hourly wind records of AERMOD-ready meteorological data that better estimate actual hourly average conditions and that are less prone to over-report calm wind conditions.  This allows AERMOD to apply more hours of meteorology to modeled inputs, and therefore produce a more complete set of concentration estimates.  Tennessee used AERMINUTE to process 2017-2019 1-minute ASOS data from Nashville, Tennessee, to generate hourly average winds for input to AERMET.

As part of its recommendation, the State provided a 3-year surface wind rose for the BNA NWS station (Figure 5 of the TVA-Gallatin Redesignation Modeling Report) and the wind distributions frequencies (Figure 6 of the TVA-Gallatin Redesignation Modeling Report).  These figures are included in this TSD as Figure 7 and Figure 8.  In Figure 7, the frequency and magnitude of wind speed and direction are defined in terms of from where the wind is blowing.  The wind is predominantly blowing from the south with an average wind speed of 6.53 knots.  Figure 8 shows the wind frequency distribution.


Figure 7: Met Station (NWS/BNA) 2017-2019 Wind Rose (Source: TVA-Gallatin Redesignation Modeling Report)

                                       
Figure 8: Met Station (NWS/BNA) 2017-2019 Wind Frequency Distribution (Source: TVA-Gallatin Redesignation Modeling Report)
                                       
                                       
EPA preliminarily agrees with Tennessee's use of three years of meteorology (2017-2019) from the Nashville International Airport NWS station for the modeling analysis.  The meteorology data were processed appropriately for use in the modeling.

2.8 Modeling Parameter: Geography and Terrain

The terrain in the area of analysis is best described as hilly, with some significant elevated terrain features up to 300 feet above the TVA-Gallatin stack heights approximately 20 km to the northeast, within the modeling domain.  To account for these terrain changes, the AERMAP (version 18081) terrain program within AERMOD was used to specify terrain elevations for all the receptors.  The source of the elevation data incorporated into the model was the USGS National Elevation Database (NED).  EPA preliminarily agrees with the State's use of the USGS NED database and AERMAP terrain processor for AERMOD to account for the changes in elevation of the area to obtain a more accurate modeling result.

2.9 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, the State chose to use the East Health monitor (referred to as the Trinity Lane monitor in the TVA-Gallatin Redesignation Modeling Report) (AQS ID: 47-037-0011), an urban-based monitor in Nashville that is approximately 34 km southwest of TVA-Gallatin.  The 2019 design value for this monitor is invalid due to incomplete data in 2018 and 2019.  Therefore, Tennessee used the highest 99[th] percentile value from the 2017-2019 design value period, 8 parts per billion (ppb) in 2019, rather than the average over the three years, as the background concentration.

EPA preliminarily agrees that the use of the 99[th] percentile value for 2019 (8 ppb) from the East Health monitor is appropriate for the modeling analysis because it is the highest 99[th] percentile value over the 2017-2019 period and is also higher than the 2014-2016 design value (7 ppb) and the 2015-2017 design value (5 ppb).  Therefore, it conservatively represents the background concentration in the Nashville and the Sumner County Area.  The East Health monitor is also located in the urban Nashville area in the vicinity of SO2 sources that were not included in the modeling (Carlex Glass America, LLC., and the Nashville International Airport).  Therefore, this background monitor accounts for potential impacts from these unmodeled sources.

2.10 Summary of Modeling Inputs and Results

The AERMOD modeling parameters for the Sumner County Area of analysis are summarized below in Table 6.

Table 6: AERMOD Modeling Parameters for the Sumner County Area

                                Input Parameter
                                     Value
AERMOD Version 
19191 (regulatory options)
Dispersion Characteristics 
Rural 
Modeled Sources 
1 
Modeled Stacks 
17 
Modeled Structures 
48 
Modeled Fence lines 
1 
Total receptors 
29,380 
Emissions Type 
Hourly-varying actual emissions from CEMS for large emissions sources and constant actual/allowable emissions from small emissions sources
Emissions Years
2017-2019
Meteorology Years 
2017-2019 
Surface Meteorology Station 
Nashville, Tennessee 
Upper Air Meteorology Station 
Nashville, Tennessee 
NWS Station for Calculating Surface Characteristics
Nashville, Tennessee
Methodology for Calculating Background SO2 Concentration 
Tier 1 approach using highest 99[th] percentile value from 2017-2019 Design Value period for AQS site: 47-037-0011
Calculated Background SO2 Concentration 
8 ppb (2019)

The results presented below in Table 7 show the magnitude and geographic location of the highest predicted modeled concentration based on the input parameters.

Table 7. Maximum Predicted 99th Percentile Daily Maximum 1-Hour SO2 Concentrations in the Sumner County Area of Analysis Based on Actual Emissions (Source: TVA-Gallatin Redesignation Modeling Report)
                                   Scenario
                                   Surf Char
                                     Grid
                               Receptor Location
                                   Elev (m)
                                  1-hour SO2
                                       
                                       
                                       
                                     UTM E
                                     UTM N
                                       
                               Max Impact (ppb)*
                             Dist. to Impact (km)
                                     NAAQS
                                  Scenario 1
                                      NWS
                                     100m
                                    553080
                                    4021423
                                    136.84
                                     23.1
                                     2.3 N
                                      75
(*) Impacts reflect a 2-ppb background concentration increase from the modeled results/graphs having 6 ppb background concentration included.  This is due to the revised monitor background concentration (for East Health (Trinity Lane) monitoring site) from the modeled 6 ppb, 2019 incomplete design value to the revised 8 ppb (99[th] percentile, 2019), which is the highest value of the 3-year, 2017-2019 record.  The modeled background is applied on an annual basis.

The State's modeling indicates that the highest predicted 99th percentile daily maximum 1-hour concentration within the chosen modeling domain is 60.5 micrograms per cubic meter (μg/m[3]), equivalent to 23.1 ppb.  This modeled concentration included the background concentration of SO2 and is based on actual emissions from the facility.  Figure 9 below was included as part of the State's recommendation and indicates that the predicted value occurred north of TVA-Gallatin, approximately 2.3 km away.


Figure 9. Predicted 99th Percentile Daily Maximum 1-Hour SO2 Concentrations Averaged Over Three Years for the Area of Analysis for the Sumner County Area. (Source: TVA-Gallatin Redesignation Modeling Report)
                                       
The modeling submitted by the State does not indicate that the 2010 1-hour SO2 NAAQS is violated at the receptor with the highest modeled concentration.  The results from the modeling analysis indicate that SO2 concentrations in the area surrounding TVA-Gallatin are well below 50 percent of the 2010 1-hour SO2 NAAQS level of 75 ppb (196 μg/m[3]).
3.0 Jurisdictional Boundaries

Once the geographic area of analysis associated with TVA-Gallatin, other nearby sources of SO2, and background concentration were determined, existing jurisdictional boundaries were considered for the purpose of informing our redesignation of the unclassifiable area to attainment/unclassifiable, specifically with respect to clearly defined legal boundaries.

There is one large emitter of SO2 in Sumner County (large emitters are greater than 100 tpy), TVA-Gallatin.  In the July 2016 designation, EPA noted that according to the 2014 NEI, there were only two other sources in the area of analysis that emitted over 100 tpy, Vanderbilt University and Carlex Glass America, LLC in Davison County.  According to Tennessee and press releases, Vanderbilt University converted to natural gas operation and decommissioned its last coal-fired boiler in 2014.  Review of more recent 2018 NEI indicates that only Carlex Glass America, LLC is still emitting greater than 100 tpy.  This source is represented by the urban background monitor that Tennessee chose, which is also in Davidson County that was designated attainment/unclassifiable in the Round 3 SO2 Designations.  As a result, EPA does 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 Sumner County.  EPA believes that our previous designated unclassifiable area, consisting of the Sumner County Area, is comprised of clearly defined legal boundaries, and EPA found these boundaries to be a suitably clear basis for defining the initial designation.  In this action, EPA is proposing to approve Tennessee's request to redesignate this entire area (Sumner County) from unclassifiable to attainment/unclassifiable.
4.0 Other Relevant Information

No other relevant information is available for the Sumner County Area.
5.0 Conclusion

As discussed in detail in Section 2 of this TSD, and each of its subsections, EPA has evaluated Tennessee's modeling analysis and has preliminarily determined that it is technically sound and demonstrates attainment with the 1-hour SO2 NAAQS.  EPA is proposing to find that the modeling analysis meets the criteria in EPA's "Guideline on Air Quality Models" in 40 CFR Part 51, Appendix W, and EPA's SO2 Modeling TAD.  Therefore, after careful evaluation of the State's modeling analysis, the redesignation request, and supporting information, as well as all available relevant information, EPA proposes to redesignate the area around TVA-Gallatin from unclassifiable to attainment/unclassifiable for the 2010 1-hour SO2 NAAQS.  Specifically, EPA proposes to change the designation based on the updated modeling analysis and redesignate the Sumner County Area as attainment/unclassifiable.  The boundary is comprised of all area within the Sumner County, Tennessee border.
