1

Review and Analysis of Sierra Club Submitted Zimmer Generating Station 

Evaluation of Compliance with the 1-hour NAAQS for SO2

Ohio EPA

Division of Air Pollution Control

Introduction

The United States Environmental Protection Agency (U.S. EPA) established a new 

National Ambient Air Quality Standard (NAAQS) for SO2 on June 22, 2010, of 75 ppb, 

as the 99th percentile of maximum daily values, averaged over three years. In addition, 

U.S. EPA revoked the primary annual and 24-hour standards. 

On August 5, 2013 (75 FR 47191), effective October 4, 2013, U.S. EPA promulgated 

the initial SO2 nonattainment areas for the newly established SO2 standard across the 

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

accepted as an enforceable order an agreement between the U.S. EPA and Sierra Club 

and the Natural Resources Defense Council to resolve litigation concerning the deadline 

for completing designations. The court’s order directs U.S. EPA to complete 

designations in three steps: the first by July 2, 2016; the second by December 31, 2017 

and the third by December 31, 2020. As part of the first round of designations, U.S. 

EPA has identified areas with newly monitored violations of the standard, or areas that 

contain stationary sources that emitted more than 16,000 tons of SO2 in 2012 or emitted 

more than 2,600 tons of SO2 and had an emission rate of at least 0.45 lbs SO2/MMBtu 

in 2012. The U.S. EPA has identified two facilities in Ohio as meeting one or more of 

these criteria: the General James M. Gavin Plant and the W.H. Zimmer Generating 

Station.

On September 16, 2015, Ohio EPA submitted recommendations of attainment for both 

the General James M. Gavin (including Kyger Creek) and W.H. Zimmer Generating 

Station source areas based on a five factor analysis, inclusive of refined dispersion 

modeling. On this same date, Sierra Club submitted dispersion modeling results and a 

recommendation of nonattainment for both areas. 

This document is Ohio EPA’s analysis and response to Sierra Club’s modeling and 

recommended designation for the W.H. Zimmer Generating Station source area. Ohio 

EPA has included for completeness Sierra Club’s modeling analysis report for both the 

W.H. Zimmer and General James M. Gavin power plants as Appendix A.

Modeling Approach

Per U.S. EPA’s Modeling Technical Assistance Document (TAD),

“Since the purpose here pertains to designations, this guidance supports 

analyses of existing air quality rather than analyses of emissions limits necessary 

to provide for attainment. Consequently, the guidance in this TAD differs in 
2

selected respects from the guidance published in Appendix W. These 

differences include:

 Placement of receptors only in areas where it is feasible to place a 

monitor vs. all ambient air locations (NSR, PSD, and SIP)

 Use of the most recent 3 years of actual emissions (designations) vs. 

maximum allowable emissions (NSR, PSD, and SIP)

 Use of 3 years of meteorological data (designations) vs. one to five 

years (NSR, PSD, and SIP)

 Use of actual stack height for designations using actual emissions vs. 

Good Engineering Practice (GEP) stack height for other regulatory 

applications (NSR, PSD, and SIP)”

The Sierra Club incorporated some, but not all, of the items listed above. Contrary to the 

specifications identified above, the Sierra Club also modeled allowable SO2 limits and 

proposed an emissions reduction for the W.H. Zimmer Generating Station, despite the 

fact their modeling does not show the W.H. Zimmer Generating Station causing or 

contributing to any exceedance of the standard over the period modeled. Further, Sierra 

Club placed receptors in areas that would be considered “inside the fenceline” for all 

sources included in the modeling domain. These are severe flaws in the analysis.

The averaging period for the 2010 SO2 NAAQS is the 99th percentile of maximum 

monitored daily values, averaged over three years. Per the Modeling TAD, three years 

of National Weather Service data is sufficient to allow the modeling to simulate a 

monitor. Thus, the modeled form of the standard is expressed as the 99th percentile of 

maximum daily values averaged over three years (herein referred to as “design value”)

for the purposes of designation.

The recommended dispersion model for modeling for SO2 designations is the American 

Meteorological Society/Environmental Protection Agency Regulatory Model (AERMOD) 

modeling system. There are two input data processors that are regulatory components 

of the AERMOD modeling system: AERMET, a meteorological data preprocessor that 

incorporates air dispersion based on planetary boundary layer turbulence structure and 

scaling concepts, and AERMAP, a terrain data preprocessor that incorporates complex 

terrain using United States Geological Survey (USGS) Digital Elevation Data. 

Additionally, Sierra Club utilized the AERMINUTE module to incorporate 1-minute 

ASOS meteorological data into the hourly surface input file. Sierra Club utilized the most 

up-to-date versions of AERMOD and the associated preprocessors available at the time 

of modeling analyses. Those versions used are as follows: AERMOD version 14134, 

AERMET version 14134, AERMINUTE version 14237, and AERMAP version 11103.

Meteorological Data

Meteorological data from 2012-2014 from the Cincinnati, OH surface station (Station # 

93814) located at the Cincinnati/Northern Kentucky International Airport and the 

Wilmington, OH upper air station (Station # 13841) located at the Wilmington Airborne 

Airpark were used in the Sierra Club analyses. These sites have previously been
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determined by Ohio EPA to be representative of Clermont County, OH and the 

surrounding region. AERSURFACE was run using twelve sectors and four seasons, 

centered on the location of the meteorological station, using average moisture 

conditions and continuous winter snow cover. The Sierra Club used a meteorological 

data set for their modeling analyses consistent with what Ohio EPA would use to 

perform modeling in this area.

Background

A background concentration of 23.5 g/m3(8.98 ppb), was included in all modeling 

analyses conducted by Sierra Club in the W.H. Zimmer source area. This background 

was based on the design value recorded at the Allen County air quality monitor, 39-003-

0009. Sierra Club chose this monitor because it recorded the lowest monitored design 

value in Ohio for the 2011-2013 period. This is not a background monitor. This monitor 

is located downwind of the city of Lima, Ohio and a large refining/petrochemical facility. 

Additionally, there is a major interstate highway close to this location. There are several 

other sources of information that would indicate a background SO2 concentration of 

28.776 g/m3for the specific source area, as used in the Ohio EPA modeling. Sierra 

Club utilized the 2011-2013 period, rather than 2012-2014, which would encompass 

those years actually modeled. Why this period was chosen is unclear. This source 

area has multiple SO2 monitors which were active during the 2012-2014 period. Ohio 

EPA further notes that the background used in Ohio’s recommended designation was 

11 ppb (28.776g/m3), and was based on a more local monitor with careful 

consideration of those sources not explicitly modeled in the domain. 

Emission Sources

The Sierra Club modeled emissions from the W.H. Zimmer Generating Station, DTE St. 

Bernard, and the Spurlock Station. Of the three SO2 sources at the W.H. Zimmer 

Generating Station, Sierra Club modeled only the main boiler. The single SO2 source at 

DTE St. Bernard was modeled as a single egress point, and the four SO2 sources at 

Spurlock Station were modeled as four separate egress points. The stack parameters 

for each source included in Sierra Club’s analyses are shown in Table 1.

Table 1: Release point parameters for all sources included in Sierra Club modeling, W.H. Zimmer 

source area.

Ohio EPA has serious reservations and concerns with respect to the quality of 

emissions data modeled and the choice of facilities to include in the modeling domain. 

These concerns are detailed here.

Emissions Data

Source ID Source Description Easting (X) Northing (Y) Base Elevation Stack Height Temperature Exit Velocity Stack Diameter SO2

(m) (m) (m) (m) (K) (m/s) (m) (g/s)

S01 Spurlock Unit 1 776858 4288455 161.92 245.364 424.26 32.65647 4.572 Variable

S02 Spurlock Unit 2 776823 4288378 162.99 245.364 424.26 32.65647 4.572 Variable

S03 Spurlock Unit 3 776742 4288261 164.17 198.12 333.15 18.20541 4.572 Variable

S04 Spurlock Unit 4 776742 4288261 164.17 219.456 333.15 16.00085 4.877 Variable

B01 DTE St. Bernard Boiler 1 715789 4339198 154.32 64.922 427.59 23.99779 2.134 47.91699398

Z01 W.H. Zimmer Unit 1 740441 4305880 154.82 174.65 337.04 15.928 12.192 Variable
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Sierra Club used hourly variable emissions inputs for the W. H. Zimmer Generating 

Station and Spurlock Station for years 2012-2014. These data were obtained from U.S. 

EPA’s Air Markets Program Data. Ohio EPA does not consider Air Markets Program 

Data ideally acceptable for use as input into a refined dispersion model for multiple 

reasons. Firstly, data reported to the Air Markets Program is done so for the accounting 

of emission allowance consumption, in accordance with Part 75 monitoring and 

reporting requirements. Accordingly, the data tends to overstate emissions when there 

are errors in the monitoring equipment or periods of missing data, as described in the 

Part 75 monitoring and reporting requirements. The data substitution methodologies 

employed via Part 75 requirements can lead to severe overestimations of emissions 

data. These data were never intended to serve as a repository for hourly emissions 

suitable for refined dispersion modeling. In addition to overstating emissions, the 

database does not contain hourly exit temperature values, which are a critical 

component necessary to accurately model hourly plume rise and exit velocities. The 

shortcomings of the Air Markets Program data are made more apparent when applied to 

stringent 1-hour standards. The hourly emissions database utilized by the Ohio EPA 

was the result of extensive analysis and consultation with relevant plant operators and 

technical staff to identify spurious emissions data and to compile a complete and 

accurate hourly database of emissions, exit temperatures, and exit velocities for each 

source included in the modeling domain. In addition to the use of inappropriate hourly 

emissions data, the Sierra Club modeled the DTE St. Bernard facility at a fixed emission 

rate based on 2014 actual SO2 emissions, averaged across 8,760 hours of operation. 

Fixed emission rates are not representative of operations at this facility, and all but 

guarantee that worst-case, unrealistic impacts are generated. This is particularly so 

when modeling for stringent 1-hour standards.

Sierra Club states that their use of fixed maximum exhaust flow rates and temperatures 

tends to under-predict impacts. Ohio EPA’s experience would indicate that this is not 

the case, and neglects the complexities of, for example, AERMOD’s treatment of the 

penetrated plume and the impact of temperature inversions on ground level impacts. 

Ohio EPA contends that an accurate treatment of actual emissions necessitates the 

inclusion of variable exhaust velocities and temperatures in addition to variable 

emissions. This is of particular importance when the intent of the designations modeling 

is to treat receptors as a surrogate for air quality monitors.

In the second portion of Sierra Club’s modeling analysis, allowable emissions were 

modeled for all sources listed in Table 1, above. Ohio EPA did not extensively analyze 

this portion of Sierra Club’s analysis, as the modeling of allowable emissions is 

inconsistent with intent of the both the consent decree and the final Data Requirements 

Rule. U.S. EPA’s March 20, 2015 memorandum entitled Updated Guidance for Area 

Designations for the 2010 Primary Sulfur Dioxide National Ambient Air Quality Standard

lists the following recommendations for designating source areas via modeling explicitly:

 ° using actual emissions as an input for assessing violations to provide results that 

reflect current actual air quality (i.e., modeling that simulates a monitor);
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 ° using 3 years of modeling results to calculate a simulated design value consistent 

with the 3-year monitoring period required to develop a design value for 

comparison to the NAAQS;

 ° placing receptors only in locations where a monitor could be placed; and

 ° using actual stack heights rather than following the Good Engineering Practice 

stack height policy when using actual emissions.

Thus, Sierra Club’s inclusion of allowable emissions is inconsistent with U.S. EPA policy

for the purposes of designations and the conclusion drawn by Sierra Club from this 

modeling should be dismissed. Further, there is no regulatory rationale for the Sierra 

Club’s suggested emission reductions at the W.H. Zimmer facility based on these

flawed results. Sierra Club’s own modeling indicates that the W.H. Zimmer facility does 

not cause or contribute to an exceedance of the standard over the 2012-2014 period. 

Explicitly Modeled Facilities

As stated previously, the Sierra Club modeled the DTE St. Bernard facility and Spurlock 

Station in addition to the William H. Zimmer Generating Station. While these facilities 

do fall within a 50 km radius of the source area, both facilities are located more than 40 

km distant, and are highly unlikely to cause a significant concentration gradient within 

the area impacted by the William H. Zimmer Generating Station. Indeed, Sierra Club’s 

modeling analysis demonstrates this clearly. Further, DTE St. Bernard is converting its 

single coal-fired boiler to natural gas by the beginning of 2016 and accepting new 

federally enforceable permit limits (Appendix B). Under the Data Requirements Rule,

any facility which will have a federally enforceable permit limit below 2,000 TPY of SO2

in place by January 2017 can be excluded from the modeling domain. Further, DTE St. 

Bernard falls below the 2,000 TPY emission limit established in the Data Requirements 

Rule with 2014 SO2 emissions of 1,666 tons. According to Appendix B, DTE St. Bernard

anticipates its post-conversion “potential” emissions to be less than 56 TPY. As such, it 

is inappropriate to include DTE St. Bernard in the modeling domain considering 2014 

emissions places the facility outside of the scope of the Data Requirements Rule, the 

facility has exceptionally low post-conversion emissions, and the facility is at a distance 

of greater than 40 kilometers from the source area. Ohio EPA’s modeling experience 

with the 1-hour SO2 standard suggests that, even for sources much larger than DTE St. 

Bernard, ground level impacts decrease dramatically within a few kilometers of the 

source. If it was Sierra Club’s intention to model all major sources operating in the W.H. 

Zimmer source area during years 2012-2014, it is unusual and arbitrary that DTE St. 

Bernard would be included, while the W.C. Beckjord facility, the largest source of SO2 in 

the source area during that time period, would be excluded. Sierra Club did not provide 

an explanation for why DTE St. Bernard was included.

In addition to the DTE St. Bernard facility, the Sierra Club explicitly modeled emissions 

from the Spurlock power plant. This facility is located approximately 40 kilometers to 

the southwest of the W.H. Zimmer facility. Given this distance and considering the 

primary southwest to northeast of the prevailing winds in the source area, Ohio EPA 

determined that this facility was unlikely to contribute to a concentration gradient beyond 
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what was accounted for in Ohio EPA’s conservative and carefully considered 

background of 10 ppb. To demonstrate this, Ohio EPA conducted an analysis of the 

impacts of Spurlock station at all receptors demonstrating a concentration greater than 

one-half of the standard (98.1 g/m3and greater) in Sierra Club’s modeling analysis. To 

avoid double counting of the background concentration, Ohio EPA did not include Sierra 

Club’s background in this analysis. These results are shown in Figure 1, below.

Figure 1: Sierra Club modeled impacts of Spurlock power plant on W.H. Zimmer source area, 2012-

2014. Concentrations in g/m3, without background.

Figure 1 demonstrates that the impacts of Spurlock station at critical receptors in the 

W.H. Zimmer source area are between 3 and 10 g/m3. Ohio EPA notes here that 

receptors to the north, near the DTE St. Bernard facility, were excluded from Figure 1 

for clarity. Impacts at those receptors are between 2 and 5 g/m3. The maximum 

impact of the Spurlock facility in the critical receptor grid is 10.16 g/m3, or 3.9 ppb. 

Ohio EPA notes here that these impacts are based on an inappropriate and overlyconservative emissions database. It is likely that, were a refined and representative 

emissions database utilized for the Spurlock facility, these impacts would lessen further. 

Ohio EPA also notes that, with a carefully considered and more representative 
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background such as that used by Ohio EPA, the impacts of the Spurlock station on the 

W.H. Zimmer source area would be adequately and conservatively represented. 

Receptors

The Sierra Club conducted modeling analyses on a three-tiered receptor grid consisting 

of 21,201 discrete receptors. The first grid is a grid of 100 meters spacing extending 

from the W.H. Zimmer Generating Station to 5 km. The second is a grid of 500 meters 

spacing extending to 10 kilometers. The final gird has a 1,000 meters spacing and 

extends to 50 kilometers. The elevations of these receptors were determined using 

AERMAP v. 11103 and 1 arc-second resolution National Elevation Dataset (NED) files. 

No attempt was made by Sierra Club to exclude receptors located in bodies of water, in 

locations where monitor placement would not be feasible, or in areas that would be 

considered “inside the fenceline” of the various sources. Figure 2 shows the receptor 

grid utilized by Sierra Club and the location of the explicitly modeled facilities. Note that 

Ohio EPA included an additional receptor at the location of AQS monitor 39-061-0040. 

Ohio EPA included this additional receptor as it is customary and appropriate modeling 

practice to include receptors at the location of ambient air quality monitors whenever 

possible. Inclusion of relevant monitors provides a convenient, transparent, and 

scientifically valid means to evaluate and assess model performance, and is done so by 

Ohio EPA in all situations where a representative monitor is located within a modeling 

domain. Furthermore, this receptor falls within a “hotspot” of modeled SO2 impacts and 

can be used to assess the accuracy of Sierra Club’s modeling results. Ohio EPA 

suggests that Sierra Club’s omission of this critical receptor represents an incomplete 

consideration of the particular aspects of the modeling domain, a lack of adherence and 

consideration of proper modeling practices, and the avoidance of a transparent and 

convenient evaluation of model performance in the source area. The location of this 

additional receptor is indicated in Figure 2.
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Figure 2: Sierra Club receptors and source locations, W.H. Zimmer source area.

Analysis

Using the model input supplied by Sierra Club, Ohio EPA replicated the results and 

analysis of actual emissions, years 2012-2014, as conducted by Sierra Club. It should 

be noted that at the time of evaluation, Ohio EPA had already updated to AERMOD v. 

15181. The difference in model versions did not impact final model results, and Ohio 

EPA was able to replicate those modeled values reported by Sierra Club. Note that the 

additional receptor at the location of monitor 39-061-0040 was the only modification 

introduced by Ohio EPA. Ohio EPA also included the erroneous background

determined by Sierra Club for this source area, 23.5 g/m3, as an additional source in 

AERMOD. This was done to simplify the analysis and graphical presentation of Sierra 

Club’s results.

Results

Ohio EPA’s replication of Sierra Club’s modeling results with variable emissions, years 

2012-2014, are in agreement with those results reported in Table 1 of Sierra Club’s 

submittal for the W.H. Zimmer source area. Sierra Club modeled exceedances of the 

standard at 23 receptors, located near DTE St. Bernard. These receptors and their 
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respective modeled design values, inclusive of a 23.5 g/m3background, are shown in 

Figure 3. 

Figure 3: Receptors with design values exceeding the 1-hour SO2 standard, 2012-2014, in the Sierra 

Club’s W.H. Zimmer source area modeling analysis. 

Sierra Club modeling indicates a maximum modeled design value of 519.2 g/m3,

approximately 3 kilometers to the northwest of DTE St. Bernard. At the additional 

receptor representing monitor 39-061-0040, a design value of 269.9 g/m3 was 

modeled. Table 2 shows the maximum modeled design value reported by Sierra Club 

for each facility individually, as well as the cumulative modeled design value. For 

completeness, Ohio EPA has included the design values modeled at the location of 

monitor 39-061-0040 in Table 2.
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Sierra Club 2012-2014 Modeled Design Values, W.H. Zimmer 

Source Area

Source Group

MAX 1-hour SO2 

Design Value 

(g/m3)

MAX 1-hour SO2 Design 

Value (g/m3) at 

Monitor Location

W.H. Zimmer 132.6235 50.03979

DTE St. 

Bernard 519.1875 269.91089

Spurlock 71.20649 32.75963

ALL 519.1882 269.9111

Table 2: Sierra Club maximum modeled design values and design values at location of monitor 39-061-

0040, 2012-2014.

Ohio EPA notes that the maximum impacts obtained by Sierra Club across the entirety 

of the receptor grid and at the location of monitor 39-061-0040 are almost exclusively 

attributable to DTE St. Bernard, as demonstrated by the results in Table 2 (comparing 

DTE St. Bernard results to those for source group ALL). Further, Sierra Club modeling 

indicates that W.H. Zimmer does not cause or contribute to any modeled exceedances 

of the standard. This is demonstrated in Figure 4, which shows the maximum modeled 

design values from DTE St. Bernard emissions alone, which can be compared to Figure 

3 for reference. It is clearly demonstrated that the emissions modeled by Sierra Club for 

DTE St. Bernard yield impacts nearly identical to those generated by the cumulative 

modeling at these monitors (Figure 3).
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Figure 4: Receptors with design values exceeding the 1-hour SO2 standard, 2012-2014, in the Sierra 

Club’s W.H. Zimmer source area modeling analysis, DTE St. Bernard contributions only.

Lastly, Ohio EPA notes that the design value modeled at the location of monitor 39-061-

0040, 269.9 g/m3, or 103.2 ppb, is 29 ppb greater than the actual monitored 2012-

2014 design value of 74 ppb. Ohio EPA interprets this as a clear demonstration that the 

Sierra Club’s modeling analysis severely over-predicts SO2 impacts. 

Conclusion

The flawed modeling submitted by Sierra Club for the W. H. Zimmer source area 

indicates a maximum design value of 519.2 g/m3. Sierra Club’s modeling also

indicates that the sole contributor to all modeled exceedances of the standard is the 

DTE St. Bernard facility, which does not fall under the criteria of Data Requirements 

Rule and is also exempted by a commitment to convert to natural gas which will occur 

by the end of 2015. (Appendix B) Further, this facility was modeled with a fixed 

emission rate, which is not representative of actual facility operations. Fixed emissions 

rates are likely to generate over-predictions for an hourly standard, and are 
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inappropriate for the analysis of the 1-hour SO2 standard in all but the rarest of 

instances. Even further, post-2014 potential emissions will be below 56 TPY for this 

facility. Sierra Club also relied on emissions data from the Air Markets Program, which 

contains erroneous and overestimated Part 75 emissions data and lacks temperature 

and exit velocity data. These data are therefore unsuitable for direct use in a refined 

modeling analysis. Lastly, the overly conservative nature of Sierra Club’s modeling 

submittal is clearly demonstrated when a receptor placed at the location of a nearby air 

quality monitor over-predicts the monitored design value by approximately 40%.

The modeling analysis and accompanying report submitted by Sierra Club for the W.H. 

Zimmer source area is a significant departure from what Ohio EPA would consider 

proper with respect to the area designations process, as described in U.S. EPA’s March 

20, 2015 Updated Guidance for Area Designations for the 2010 Primary Sulfur Dioxide 

National Ambient Air Quality Standard. This guidance suggests that if an area is found 

to be nonattainment of the standard, then a nonattainment area should be suggested 

based on jurisdictional boundaries or other easily definable features, as well as air 

quality data, emissions, meteorology, and geography. This five-factor approach was 

utilized by Ohio EPA when establishing the recommended attainment area designation 

for the W.H. Zimmer source area in accordance with U.S. EPA guidance. Sierra Club 

did not attempt to follow this process. Rather, emission reductions were suggested for 

the W.H. Zimmer plant based on allowable emissions, and no attempt to define a 

nonattainment area was made. Because Sierra Club provided U.S. EPA with a flawed 

and incomplete analysis for the purposes of designation, U.S. EPA cannot put any 

credence in Sierra Club’s submittal and should rely on the analyses submitted by Ohio 

EPA. 

In conclusion, there are multiple sources of error, over-prediction, and erroneous data, 

and Sierra Club’s recommendation of nonattainment must be disregarded. This 

conclusion is strengthened and supported by Sierra Club’s own modeling of “actual” 

emissions, which shows that the W. H. Zimmer facility did not cause or contribute to any 

exceedance of the 2010 1-hour SO2 standard, and that the only source causing or 

contributing to exceedances of the standard is a unit that does not meet the emissions 

criteria of the Data Requirements Rule and is converting to natural gas prior to January 

2017. Further, Sierra Club suggests that W.H. Zimmer be subjected to emission 

reductions based on the modeling of allowable emission rates, when that same facility 

was demonstrated by Sierra Club to not cause or contribute to any exceedances of the 

NAAQS when following U.S. EPA’s guidance with respect to modeling actual emissions.

Ohio EPA believes that the arbitrary inclusion of DTE St. Bernard is not justified, mostnotably in light of the fact that Sierra Club did not consider future operational changes, 

nor was any attempt made by Sierra Club to establish a realistic and representative 

emissions database for this source. The southwest Ohio area has a number of ambient 

sulfur dioxide monitors in the urban area around Cincinnati. None of these monitors 

measure values above the ambient air quality standard, which is another reflection of 

the over-predictive nature of the Sierra Club modeling. Ohio EPA also notes the 

inclusion of the Spurlock power plant in Sierra Club’s modeling, which has been 

demonstrated to impact critical receptors well below that which would be represented by 
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a carefully considered and representative background concentration. That Sierra Club 

also attempted to suggest emission reductions at the W.H. Zimmer facility without any 

rationale, except for the fact that the source is a coal-fired electric generating unit. As 

such, Ohio EPA contends that the modeling submitted by Ohio EPA Division of Air 

Pollution control is more technically sound and conforms more completely to U.S. EPA’s 

Data Requirements Rule. Further, Ohio EPA’s analysis presented a five factor analysis 

of the W.H. Zimmer source area in accordance with U.S. EPA guidance. Therefore, 

Ohio EPA reiterates here its original recommendation of attainment for the William H. 

Zimmer source area, as described in Ohio’s September 16, 2015 submittal.
1

Review and Analysis of Sierra Club Submitted Gavin Power Plant Evaluation of 

Compliance with the 1-hour NAAQS for SO2

Ohio EPA

Division of Air Pollution Control

Introduction

The United States Environmental Protection Agency (U.S. EPA) established a new 

National Ambient Air Quality Standard (NAAQS) for SO2 on June 22, 2010, of 75 ppb, 

as the 99th percentile of maximum daily values, averaged over three years. In addition, 

U.S. EPA revoked the primary annual and 24-hour standards. 

On August 5, 2013 (75 FR 47191), effective October 4, 2013, U.S. EPA promulgated 

the initial SO2 nonattainment areas for the newly established SO2 standard across the 

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

accepted as an enforceable order an agreement between the U.S. EPA and Sierra Club 

and the Natural Resources Defense Council to resolve litigation concerning the deadline 

for completing designations. The court’s order directs U.S. EPA to complete 

designations in three steps: the first by July 2, 2016; the second by December 31, 2017 

and the third by December 31, 2020. As part of the first round of designations, U.S. 

EPA has identified areas with newly monitored violations of the standard, or areas that 

contain stationary sources that emitted more than 16,000 tons of SO2 in 2012 or emitted 

more than 2,600 tons of SO2 and had an emission rate of at least 0.45 lbs SO2/MMBtu 

in 2012. The U.S. EPA has identified two facilities in Ohio as meeting one or more of 

these criteria: the General James M. Gavin Plant and the W.H. Zimmer Generating 

Station.

On September 16, 2015, Ohio EPA submitted recommendations of attainment for both 

the General James M. Gavin (including Kyger Creek) and W.H. Zimmer Generating 

Station source areas based on a five factor analysis, inclusive of refined dispersion 

modeling. On this same date, Sierra Club submitted dispersion modeling results and a 

recommendation of nonattainment for both areas. 

This document is Ohio EPA’s analysis of, and response to, Sierra Club’s modeling and 

recommended designation for the General James M. Gavin source area. Ohio EPA has 

included, for completeness, Sierra Club’s modeling analysis report for both the W.H. 

Zimmer and General James M. Gavin power plants as Appendix A of this submittal.

Modeling Approach

Per U.S. EPA’s Modeling Technical Assistance Document (TAD),

“Since the purpose here pertains to designations, this guidance supports 

analyses of existing air quality rather than analyses of emissions limits necessary 

to provide for attainment. Consequently, the guidance in this TAD differs in 
2

selected respects from the guidance published in Appendix W. These 

differences include:

 Placement of receptors only in areas where it is feasible to place a 

monitor vs. all ambient air locations (NSR, PSD, and SIP)

 Use of the most recent 3 years of actual emissions (designations) vs. 

maximum allowable emissions (NSR, PSD, and SIP)

 Use of 3 years of meteorological data (designations) vs. one to five 

years (NSR, PSD, and SIP)

 Use of actual stack height for designations using actual emissions vs. 

Good Engineering Practice (GEP) stack height for other regulatory 

applications (NSR, PSD, and SIP)”

The Sierra Club incorporated some, but not all, of the items listed above. Contrary to the 

specifications listed above, the Sierra Club also modeled allowable SO2 limits and 

proposed an emissions reduction for the General James M. Gavin plant. Further, Sierra 

Club placed receptors in areas that would be considered “inside the fenceline” for all 

sources included in the modeling domain. These are serious flaws in the analysis.

The averaging period for the 2010 SO2 NAAQS is the 99th percentile of maximum 

monitored daily values, averaged over three years. Per the Modeling TAD, three years 

of National Weather Service data is sufficient to allow the modeling to simulate a 

monitor. Thus, the modeled form of the standard is expressed as the 99th percentile of 

maximum daily values averaged over three years (herein referred to as “design value”)

for the purposes of designation.

The recommended dispersion model for modeling for SO2 designations is the American 

Meteorological Society/Environmental Protection Agency Regulatory Model (AERMOD) 

modeling system. There are two input data processors that are regulatory components 

of the AERMOD modeling system: AERMET, a meteorological data preprocessor that 

incorporates air dispersion based on planetary boundary layer turbulence structure and 

scaling concepts, and AERMAP, a terrain data preprocessor that incorporates complex 

terrain using United States Geological Survey (USGS) Digital Elevation Data. 

Additionally, Sierra Club utilized the AERMINUTE module to incorporate 1-minute 

ASOS meteorological data into the hourly surface input file. Sierra Club utilized the most 

up-to-date versions of AERMOD and the associated preprocessors available at the time 

of the modeling analyses. Those versions used are as follows: AERMOD version 

14134, AERMET version 14134, AERMINUTE version 14237, and AERMAP version 

11103.

Meteorological Data

Meteorological data from 2012-2014 from the Huntington, WV surface station (Station # 

3860) located at the Huntington Tri-State Airport and the Pittsburgh, PA upper air 

station (Station # 94823) located at the Greater Pittsburgh International Airport were 

used in the Sierra Club analyses. These sites have previously been determined by Ohio 
3

EPA to be representative of Gallia County, OH and the surrounding region. 

AERSURFACE was run using twelve sectors and four seasons, centered on the 

location of the surface meteorological station, using average moisture conditions and 

continuous winter snow cover. The Sierra Club used a meteorological data set for their 

modeling analyses consistent with what Ohio EPA would use to perform modeling in 

this area.

Background

A background concentration of 28.8 g/m3(11.01 ppb) was included in all modeling 

analyses conducted by Sierra Club in the Gavin source area. This background was 

based on the design value recorded at the Allen County air quality monitor, 39-003-

0009. Sierra Club chose this monitor because it recorded the lowest monitored design 

value in Ohio for the 2011-2013 period. This is not a background monitor. This monitor 

is located downwind of the city of Lima, Ohio and a large refining/petrochemical facility. 

Additionally, there is a major interstate highway close to this location. There are several 

other sources of information that would indicate a background SO2 concentration of 

26.16 g/m3for this specific source area, as was used in the Ohio EPA modeling. 

Sierra Club utilized the 2011-2013 period, rather than 2012-2014, which would 

encompass those years actually modeled. Why this period was chosen is unclear. 

This source area has multiple SO2 monitors which were active during the 2012-2014 

period. Ohio EPA further notes that the background used in Ohio’s recommended 

designation was 10 ppb (26.16 g/m3), and was based on a more local monitor with 

careful consideration of those sources not explicitly modeled in the domain.

Emission Sources

The Sierra Club explicitly modeled emissions from the General James M. Gavin plant, 

Kyger Creek Station, and the Mountaineer Power Plant. The two SO2 sources at the 

General James M. Gavin plant were modeled by Sierra Club as two separate egress

points, as the two boilers vent emissions to individual stacks. The five emission sources 

at Kyger Creek were incorrectly modeled by Sierra Club as venting to a single common 

stack, with an incorrect diameter of 11.887 meters. A correct representation of the 

egress points at Kyger Creek would represent the emissions from Units 1 and 2 as 

being emitted from a stack with a 7.53 meter diameter, and emissions from Units 3, 4, 

and 5 as being emitted from a stack with a diameter of 9.2 meters. Ohio EPA notes 

here that the modeling submitted to U.S. EPA by Ohio EPA as part of a five factor 

analysis on September 16, 2015 correctly represented the egress points at Kyger Creek 

Station. Sierra Club modeled the emissions from Mountaineer Plant from a single 

egress point. The stack parameters for each source included in Sierra Club’s analyses 

are shown in Table 1.
4

Table 1: Release point parameters for all sources included in Sierra Club modeling, Gen. James M. 

Gavin source area.

Ohio EPA has serious reservations and concerns with respect to the quality of 

emissions data modeled and the choice of facilities to include in the modeling domain. 

These concerns are detailed here.

Emissions Data

Sierra Club used hourly variable emissions inputs for the General James M. Gavin 

Plant, Kyger Creek Station, and Mountaineer Power Plant, years 2012-2014. These 

data were obtained from U.S. EPA’s Air Markets Program Data. Ohio EPA does not 

consider Air Markets Program Data ideally acceptable for use as input into a refined 

dispersion model for multiple reasons. Firstly, data reported to the Air Markets Program 

is done so for the accounting of emission allowance consumption, in accordance with 

Part 75 monitoring and reporting requirements. Accordingly, the data tends to overstate 

emissions when there are errors in the monitoring equipment or during periods of 

missing data, as described in the Part 75 monitoring and reporting requirements. The 

data substitution methodologies employed via Part 75 requirements for the purposes of 

allowance accounting can lead to severe overestimations of emissions data, and 

consequently ground level impacts determined from refined dispersion modeling. 

These data were never intended to serve as a repository for hourly emissions suitable 

for refined dispersion modeling. In addition to overstating emissions, the database does 

not contain hourly exit temperature values, which are a critical component necessary to 

accurately model hourly plume rise and exit velocities. The shortcomings of the Air 

Markets Program data are made more apparent when applied to stringent 1-hour 

standards. 

It should be noted here that the hourly emissions database utilized by the Ohio EPA for 

this round of designation modeling was the result of intensive analysis and consultation 

with relevant plant operators and technical staff to identify spurious emissions data and 

to compile a complete and accurate hourly database of emissions, exit temperatures, 

and exit velocities for each source included in the modeling domain. A description of 

the problematic data and subsequent refinement of the emissions database used by 

Ohio EPA for the General James M. Gavin source area is provided as Appendix C of 

this submittal. This document was originally submitted by Ohio EPA as an Appendix to 

Ohio’s September 16, 2015 recommendation submittal.

In addition to using an inappropriate emissions database, Sierra Club states that their 

use of fixed maximum exhaust flow rates and temperatures tends to under-predict 

impacts. Ohio EPA’s experience would indicate that this is not the case, and neglects 

the complexities of, for example, AERMOD’s treatment of the penetrated plume and the 

impact of temperature inversions on ground level impacts. Ohio EPA contends that an 

Source ID Source Description Easting (X) Northing (Y) Base Elevation Stack Height Temperature Exit Velocity Stack Diameter SO2

(m) (m) (m) (m) (K) (m/s) (m) (g/s)

G01 Gavin Unit 1 403338 4310252 174.29 252.98 323.15 8.058 12.802 Variable

G02 Gavin Unit 2 403295 4310124 174.3 252.98 323.15 8.058 12.802 Variable

K01 Kyger Units 1-5 402253 4308114 172.36 255.42 327.594 15.512 11.887 Variable

M01 Mountaineer Unit 1 419097 4314859 178.8 304.8 327.039 15.119 12.954 Variable
5

accurate treatment of actual emissions necessitates the inclusion of variable exhaust 

velocities and temperatures in addition to variable emissions. This is of particular 

importance when the stated intent of the designations modeling is to treat receptors as 

a surrogate for air quality monitors.

In the second portion of Sierra Club’s modeling analysis, allowable emissions were 

modeled for all sources listed in Table 1, above. Ohio EPA did not extensively analyze 

this portion of Sierra Club’s analysis, as the modeling of allowable emissions is well 

beyond the scope and intent of the both the consent decree and the final Data 

Requirements Rule. U.S. EPA’s March 20, 2015 memorandum entitled Updated 

Guidance for Area Designations for the 2010 Primary Sulfur Dioxide National Ambient 

Air Quality Standard lists the following recommendations for designating source areas 

via modeling explicitly:

 ° using actual emissions as an input for assessing violations to provide results that 

reflect current actual air quality (i.e., modeling that simulates a monitor);

 ° using 3 years of modeling results to calculate a simulated design value consistent 

with the 3-year monitoring period required to develop a design value for 

comparison to the NAAQS;

 ° placing receptors only in locations where a monitor could be placed; and

 ° using actual stack heights rather than following the Good Engineering Practice 

stack height policy when using actual emissions.

Thus, Sierra Club’s inclusion of allowable emissions is inconsistent with U.S. EPA policy

for the purposes of designation and the conclusion drawn by Sierra Club from this 

modeling should be dismissed. Further, there is no regulatory rationale for the Sierra 

Club’s suggested emission reductions at the General James M. Gavin facility based on 

these flawed results. 

Explicitly Modeled Facilities

As stated previously, the Sierra Club modeled Kyger Creek Station and the Mountaineer 

Power Plant in addition to the General James M. Gavin facility. While these facilities do 

fall within a 50 km radius of the source area, the Mountaineer Power Plant was 

determined by Ohio EPA to be too distant and having emissions not significant enough 

to impact the source area above background. Ohio EPA contends that this conclusion 

is supported by Sierra Club’s modeling, and is discussed further in the Results section 

of this document. 

Receptors

The Sierra Club conducted modeling analyses on a three-tiered receptor grid consisting 

of 21,201 discrete receptors. The first grid is a grid of 100 meters spacing extending 

from the General James M. Gavin Plant to 5 kilometers. The second is a grid of 500 

meters spacing extending to 10 kilometers. The final gird has a 1,000 meters spacing 

and extends to 50 kilometers. The elevations of these receptors were determined using 
6

AERMAP v. 11103 and 1 arc-second resolution National Elevation Dataset (NED) files. 

No attempt was made by Sierra Club to exclude receptors located in bodies of water, in 

locations where monitor placement would not be feasible, or in areas that would be 

considered “inside the fenceline” of the various explicitly modeled sources. Figure 1 

shows the receptor grid utilized by Sierra Club and the location of the explicitly modeled 

facilities. Note that Ohio EPA included an additional receptor at the location of AQS 

monitor 39-105-0003 when conducting the analysis of Sierra Club’s modeling. This 

monitor is located approximately 13 kilometers to the northeast of the General James 

M. Gavin plant, and is situated such that prevailing winds from the southwest carry 

emissions from both the General James M. Gavin plant and Kyger Creek Station to the 

monitor location. Ohio EPA included this additional receptor as it is customary and 

appropriate modeling practice to include receptors at the location of ambient air quality 

monitors whenever possible. Inclusion of relevant monitors provides a convenient, 

transparent, and scientifically valid means to evaluate and assess model performance, 

and is done so by Ohio EPA in all situations where a representative monitor is located 

within a modeling domain. Furthermore, this receptor falls within a “hotspot” of modeled 

SO2 impacts and can be used to assess the accuracy of Sierra Club’s modeling results. 

Ohio EPA suggests that Sierra Club’s omission of this critical receptor represents an 

incomplete consideration of the particular aspects of the modeling domain, a lack of 

adherence and consideration of proper modeling practices, and the avoidance of model 

performance evaluation in the area around the source. The location of this additional 

receptor is indicated in Figure 1.
7

Figure 1: Sierra Club receptors and source locations, Gen. James M. Gavin source area.

Analysis

Using the model input supplied by Sierra Club, Ohio EPA replicated the results and 

analysis of actual emissions, years 2012-2014, as conducted by Sierra Club. It should 

be noted that at the time of evaluation, Ohio EPA had already updated to AERMOD v. 

15181. The difference in model versions did not impact final model results, and Ohio 

EPA was able to replicate those modeled values reported by Sierra Club. Note that the 

additional receptor at the location of monitor 39-105-0003 was the only modification 

introduced by Ohio EPA. Ohio EPA also included the erroneous background

determined by Sierra Club for this source area, 28.8 g/m3, as an additional source in 

AERMOD. This was done to simplify the analysis and graphical presentation of Sierra 

Club’s results.

Results

Ohio EPA’s replication of Sierra Club’s modeling results with variable emissions, years 

2012-2014, are in agreement with those results reported in Table 1 of Sierra Club’s 

submittal for the General James M. Gavin source area. Sierra Club modeled 

exceedances of the standard at 8,608 receptors, including the additional receptor 
8

placed by Ohio EPA at the location of monitor 39-105-0003. Exceedances were 

modeled within 14.5 kilometers of the General James M. Gavin Plant. These receptors 

and their respective modeled design values, inclusive of a 28.8 g/m3background, are 

shown in Figure 2. 

Figure 2: Receptors with design values exceeding the 1-hour SO2 standard, 2012-2014, in the Sierra 

Club’s General James M. Gavin source area modeling analysis. 

Sierra Club modeling indicates a maximum modeled design value of 318.5 g/m3

approximately 2.8 kilometers to the northwest of the General James M. Gavin plant. At 

the additional receptor representing monitor 39-105-0003, a design value of 197.4

g/m3 was modeled. Table 2 shows the maximum modeled design value reported by 

Sierra Club for each facility individually, as well as the cumulative modeled design 

value. For completeness, Ohio EPA also included the design values modeled at the 

location of monitor 39-105-0003 in Table 2.
9

Sierra Club 2012-2014 Modeled Design Values, General James 

M. Gavin Source Area

Source Group

MAX 1-hour SO2 

Design Value 

(g/m3)

MAX 1-hour SO2 Design 

Value (g/m3) at 

Monitor Location

Gen J.M. Gavin 291.5320 170.83095

Kyger Creek 101.1416 65.79408

Mountaineer 53.09317 38.95341

ALL 318.5353 197.38876

Table 2: Sierra Club maximum modeled design values and design values at location of monitor 39-105-

0003, 2012-2014.

The flawed Sierra Club modeling indicates that Kyger Creek Station and Mountaineer 

Power Plant do not cause any modeled exceedances of the standard, and that the 

General James M. Gavin Plant is the largest contributor to modeled exceedances of the 

standard. Further, modeled results at the location of monitor 39-105-0003 demonstrate 

a modeled exceedance of the standard. Ohio EPA sited that monitor in the vicinity of 

the General James M. Gavin plant to specifically measure SO2 impacts from that plant. 

This source oriented monitor can be used as an indicator of model performance and 

ambient air quality in the area around the source.

Ohio EPA notes that monitor 39-105-0003 is in attainment of the 1-hour SO2 standard, 

with a 2012-2014 monitored design value of 30 ppb, while Sierra Club modeling 

indicates a modeled design value of 75.5 ppb for the same period. This large 

discrepancy prompted Ohio EPA to conduct an analysis of modeled concentrations at 

the monitor location against monitored values. Ohio EPA first compared the first 

through fifth highest modeled and monitored maximum daily values for each year 2012-

2014. The results of this analysis are shown in Table 3.
10

Year Rank Modeled 

Concentration (ppb)

Monitored 

Concentration (ppb)

39-105-0003

2012

1 133 34

2 100 34

3 77 32

4 70 26

5 66 24

2013

1 86 63

2 75 55

3 73 50

4 71 33

5 70 33

2014

1 143 85

2 93 47

3 87 33

4 85 32

5 80 30

Table 3: 1st through 5th highest modeled and monitored values at monitor location 39-105-0003, 2012-

2014.

Table 3 demonstrates that Sierra Club’s modeling significantly over-predicts the 

maximum daily values with respect to those recorded at monitor 39-105-0003 and is not 

representative of actual air quality. During the 2012-2014 period, only a single 

maximum daily concentration greater than 75 ppb was recorded at the monitor location. 

Over the same period, Sierra Club modeled nine maximum daily values greater than 75 

ppb at the monitor location. To further assess the performance of Sierra Club’s 

modeling against the monitor data, Ohio EPA evaluated the modeled and monitor 

maximum daily values at the 95th, 90th, 75th, 50th, and 25th percentiles for each year of 

the 2012-2014 period. These results are shown Table 4.
11

Year Percentile Modeled 

Concentration (ppb)

Monitored 

Concentration (ppb)

39-105-0003

2012

95th 51 17

90th 44 9

75th 31 5

50th 18 2

25th 12 1

2013

95th 60 20

90th 45 12

75th 31 6

50th 16 3

25th 11 1

2014

95th 65 23

90th 55 17

75th 33 9

50th 16 4

25th 12 1

Table 4: Modeled and monitored maximum daily values at various percentiles at monitor location 39-

105-0003, years 2012-2014.

The results shown in Table 4 indicate that Sierra Club modeling systematically overpredicts, in some cases severely, across a wide range of percentiles, for all years 

modeled. Ohio EPA interprets these data as indicative of a systemic conservative bias 

in the Sierra Club modeling. Based on Ohio EPA’s analysis of the modeling submittal, 

this consistent over-prediction originates from Sierra Club’s use of an inappropriate and 

overly conservative emissions database.

In addition to the significant over-prediction demonstrated by Sierra Club’s modeling, 

Ohio EPA does not believe that the inclusion of the Mountaineer Power Plant was 

appropriate for this source area. Ohio EPA determined that the relatively low emissions 

from the Mountaineer Plant, considering local meteorology and its distance from the 

General James M. Gavin Plant (4,411 TPY, 16.7 km), would not necessitate explicit 

modeling as it is unlikely to impact ambient SO2 concentrations in the source area 

beyond what is accounted for in the background concentration. This conclusion is 

supported by the fact that emissions from the General James M. Gavin Plant and Kyger 

Creek Station, which were approximately 11 times higher than Mountaineer Power Plant 

in 2014, did not demonstrate modeled exceedances of the standard, and had maximum 

impacts relatively close to the facilities. Ohio EPA analyzed the impact of the 

Mountaineer Power Plant at the 8,608 receptors for which Sierra Club modeling 

indicating an exceedance of the standard, without further alteration to the modeling 

inputs. The results of this modeling for only the Mountaineer Power Plant are shown in 
12

Figure 3. Note that for clarity and to avoid double counting of the background, the 

concentrations shown in Figure 3 do not include a background.

Figure 3: Mountaineer Power Plant impacts, 2012-2014, without background.

The results shown in Figure 3 demonstrate that the impacts of the Mountaineer Plant on 

those receptors demonstrated by Sierra Club to exceed the standard are relatively low. 

Within the fine grid around the General James M. Gavin and Kyger Creek facilities 

where the largest impacts were modeled, the impacts of Mountaineer are between 5 

and 8 g/m3. The greatest modeled impact of Mountaineer in this receptor grid was 
13

12.38 g/m3, or 4.7 ppb. Ohio EPA contends that had Sierra Club utilized a carefully 

considered and more representative monitor to derive a background for this analysis, 

then the impacts of the Mountaineer Power plant would be conservatively represented, 

eliminating the need to explicitly model this source. It should be noted that Ohio EPA’s 

background for this area, 10 ppb, adequately and conservatively represents the impacts 

of the Mountaineer Power Plant in the General James M. Gavin source area. Sierra 

Club’s inclusion of this source, in addition to a non-representative background, provides

an additional unnecessary and inappropriate level of conservatism in the modeling 

analysis.

Conclusion

Modeling submitted by Sierra Club for the General James M. Gavin source area 

indicates a maximum design value of 318.5 g/m3. Sierra Club relied on emissions data 

from the Air Markets Program, which contains erroneous and overly conservative Part 

75 substitution data, and lacks variable temperature and exit velocity data. These data 

are therefore unsuitable for direct use in a refined modeling analysis. The overly 

conservative nature of Sierra Club’s modeling submittal is clearly demonstrated when 

modeled results are compared to monitor data recorded at monitor 39-105-0003, 

located 13 kilometers away and sited to monitor emissions from both the General 

James M. Gavin and Kyger Creek power plants. Ohio EPA’s analysis indicates that 

Sierra Club’s modeling severely over-predicts maximum daily values for all years, 

across a broad range of percentiles. The modeled design value at the monitor location, 

75.5 ppb, is approximately two-and-half times greater than the monitored design value 

at the same location, years 2012-2014. Ohio EPA notes that if the maximum modeled 

design value obtained by Sierra Club, 318.5 g/m3is scaled by a factor of 2.5, a design 

value of 127 g/m3is obtained, inclusive of background. Although Ohio EPA 

acknowledges that such scaling is inappropriate for deriving valid design values, it is a 

striking indication of the overly conservative nature of Sierra Club’s modeling in this

source area. 

In conclusion, there are multiple sources of error, over-prediction, and erroneous data, 

and Sierra Club’s recommendation of nonattainment must be disregarded. As such, 

Ohio EPA contends that the modeling submitted by Ohio EPA Division of Air Pollution 

control is more technically sound and conforms more completely to U.S. EPA’s Data 

Requirements Rule. Therefore, Ohio EPA reiterates here its original recommendation 

of attainment for the General James M. Gavin source area, as described in Ohio’s 

September 16, 2015 submittal.