# Exhibit 4

## Sooner Generating Station

Red Rock, Oklahoma

Evaluation of Compliance wi th the 1-hour NAAQS for SO 2



## June 3, 2014

Conducted by:

Steven Klafka, P.E., BCEE

Wingra Engineering, S.C.

Madison, WisconsinEvaluation of Compliance with the 1-hour NAAQS for SO 2 June 3, 2014 Page 2

1. Introduction

Wingra Engineering, S.C. was hi red by the Sierra Club to conduc t an air modeling impact analysis to help USEPA, state and local air a gencies identify facilities th at are likely causing violations of the 1- hour sulfur dioxide (SO 2 ) national ambient air qualit y standard (NAAQS). This document describes the results and procedures for a n evaluation conducted for the Sooner Generating Station located in Red Rock, Oklahoma.

The dispersion modeling analysis predicted ambient air concentr ations for comparison with the one
hour SO 2 NAAQS. The modeling was performe d using the most recent versi on of AERMOD, AERMET, and AERMINUTE, with data provided to the Sierra Club by regulatory air agencies and through other publicly-available so urces as documented below. The analysis was conducted in
adherence to all available USEP A guidance for evaluating source impacts on attainment of the 1-
hour SO 2 NAAQS via aerial dispersion mode ling, including the AERMOD Imp lementation Guide; USEPA's Applicability of Appendi x W Modeling Guidance for the 1 -hour SO 2 National Ambient Air Quality Standard, August 23, 2010; modeling guidance promul gated by USEPA in Appendix W to 40 CFR Part 51; and, USEPA’ s March 2011 Modeling Guidance fo r SO 2 NAAQS Designations. 1

2. Compliance with the 1-hour SO 2 NAAQS

2.1 1-hour SO 2 NAAQS

The 1-hour SO 2 NAAQS takes the form of a t hree-year average of the 99 th -percentile of the annual distribution of daily maximum 1- hour concentrations, which cann ot exceed 75 ppb. 2 Compliance
with this standard w as verified using USEPA’s AERMOD air disper sion model, which produces air concentrations in units of μg/m 3 . The 1-hour SO 2 NAAQS of 75 ppb equals 196.2 μg/m 3 , and this is the value used for determining whether modeled impacts exceed t he NAAQS. 3 The 99 th -percentile
of the annual distri bution of daily maximum 1-hour concentratio ns corresponds to the fourth-highest value at each receptor for a given year.

2.2 Modeling Results

Modeling results for Sooner Gene rating Station are summarized i n Table 1. It was determined that based on either currently permitted or measured actual emission s, the Sooner Gene rating Station is
estimated to create downwind SO 2 concentrations which exceed the 1-hour NAAQS.

1 http://www.epa.gov/scram001/ so2_modeling_guidance.htm

2 USEPA, Applicability of Appendix W Modeling Guidance for the 1- hour SO 2 National Ambient Air Quality Standard, August 23, 2010.

3 The ppb to μg/m 3 conversion is found in the source code to AERMOD v. 13350, sub routine Modules. The conversion calculation is 75/0.3823 = 196.2 μg/m 3

.Evaluation of Compliance with the 1-hour NAAQS for SO 2 June 3, 2014 Page 3

Table 1 presents the modeling results based on allowable and ma ximum emissions. “Allowable” is the peak emission rate from eac h unit as approved by the curren t air quality operation permit for the facility. “Maximum” is the highe st combined emission rate from all units during any single hour as measured during 2012 as taken from USEPA Air Markets Program Data

. 4

Air quality impacts in Oklahom a are based on a background conce ntration of 13.1 μg/m 3 . This is the 2010-12 design value for Oklahoma County, Oklahoma - the lowest measured background
concentration in the state. This is the most recently availabl e design value.

Table 1 - SO 2 Modeling Results for Sooner Gene rating Station Modeling Analysis

Emission Rates Averaging Period 99 th Percentile 1-hour Daily Maximum (μg/m 3 ) Complies with NAAQS? Impact Background Total NAAQS

Allowable 1-hour 315.2 13.1 328.3 196.2 No

Maximum 1-hour 212.8 13.1 225.9 196.2 No

The currently permitted emissions and measured maximum emission s used for the modeling analysis
are summarized in Table 2.

Table 2 - Modeled SO 2 Emissions from Sooner Generating Station 5 , 6

Stack

ID Unit
ID Allowable Emissions 3-hour Average (lbs/hr) Maximum Emissions 1-hour Average (lbs/hr) S01 Unit 1 6,139.2 4,228.0 S02 Unit 2 6,139.2 4,062.9 Total All Units 12,278.4 8,290.9

Based on the modeling results, emission reductions from current allowable rates considered necessary to achieve compliance with the 1-hour NAAQS were calc ulated and presen ted in Table 3.

4 http://ampd.epa.gov/ampd/

5 Oklahoma Department of Environmental Quality, Part 70 Permit N o. 2010-338-TVR2, November 21, 2011. Allowable emissions are based on a limitation of 1.2 lbs/mmbtu applicable to both Units 1 and 2.

6 Maximum emissions are based on th e measured hourly rates repor ted for 2012 in USEPA Air Markets Program Data

.Evaluation of Compliance with the 1-hour NAAQS for SO 2 June 3, 2014 Page 4

Table 3 - Required Emission Reduct ions for Compliance with the 1-hour NAAQS for SO 2

Acceptable Impact (NAAQS - Background) 99th Percentile 1-hour Daily Max (μg/m 3 ) Required Total Facility Reduction Based on Allowable Emissions (%) Required Total Facility Emission Rate (lbs/hr) Required Total Facility 1-hour Average Emission Rate

(lbs/mmbtu)
183.1 41.9% 7,132.5 0.70

Predicted exceedences of the 1-hour NAAQS for SO 2 based on allowable emissions extend throughout the region to a maxim um distance of 11 kilometers.

Figure 1 provides a regional vi ew of NAAQS violations based on allowable emissions.

Figure 2 provides a local view of NAAQS violations based on all owable emissions.

2.3 Conservative Modeling Assumptions

A dispersion modeling analysis requires the selection of numero us parameters which affect the predicted concentrations. For the enclosed analysis, several pa rameters were selected which underpredict facility impacts.

Assumptions used in this modeli ng analysis which likely under-e stimate concentrations include the following:

 Allowable emissions are base d on a limitation with an averaging period which is greater than the 1-hour average used for the SO 2 air quality standard. Emissions and impacts during any 1-hour period may be higher than assumed for the modeling analy sis.

 No consideration of facility operation at less than 100% load. Stack parameters such as exit flow rate and temperature are typically lower at less than full load, reducing pollutant dispersion and increasing predic ted air quality impacts.

 No consideration of off-site sources. These other sources of SO 2 will increase the predicted impacts.Evaluation of Compliance with the 1-hour NAAQS for SO 2 June 3, 2014 Page 5

Figure 1 - Regional View of SO 2 Concentrations Due to Sooner Generating Plant Allowable EmissionsEvaluation of Compliance with the 1-hour NAAQS for SO 2 June 3, 2014 Page 6

Figure 2 - Local View of SO 2 Concentrations Due to Sooner Ge nerating Plant Allowable EmissionsEvaluation of Compliance with the 1-hour NAAQS for SO 2 June 3, 2014 Page 7

3. Modeling Methodology

3.1 Air Dispersion Model

The modeling analysis used U SEPA’s AERMOD program, v. 13350. A ERMOD, as available from the Support Center for Regulator y Atmospheric Modeling (SCRAM) website, was used in conjunction with a third-par ty modeling software program, AERMOD View , sold by Lakes
Environmental Software.

3.2 Control Options

The AERMOD model was run with t he following control options:

 1-hour average air concentrations

 Regulatory defaults

 Flagpole receptors

To reflect a representative i nhalation level, a flagpole height of 1.5 meters was used for all modeled receptors. This parameter was added to the receptor file when running AERMAP, as described in
Section 4.4.

An evaluation was conducted to det ermine if the modeled facilit y was located in a rural or urban setting using USEPA’s methodology outlined in Section 7.2.3 of the Guideline on Air Quality
Models. 7 For urban sources, the URBANOPT option is used in conjunction with the urban population from an appropriate ne arby city and a default surfac e roughness of 1.0 meter. Methods
described in Section 4.1 were used t o determine whether rural o r urban dispersion coefficients were appropriate for the modeling analysis.

3.3 Output Options

The AERMOD analysis was based on three years of recent meteorol ogical data. The modeling analyses used one run with five year s of sequential meteorologi cal data from 2009 to 2013.
Consistent with USEPA’s Modeling Guidance for SO 2 NAAQS Designations, AERMOD provided a table of fourth-high 1-hour SO 2 impacts concentrations consiste nt with the form of the 1-hour SO 2 NAAQS. 8

Please refer to Table 1 for the modeling results.

7 USEPA, Revision to the Guidelin e on Air Quality Models: Adopti on of a Preferred General Purpose (Flat and Complex Terrain) Dispersion Model and Other Revisions, Appendix W to 40 CFR Part 51, November 9, 2005.

8 USEPA, Area Designations for the 2010 Revised Primary Sulfur Di oxide National Ambient Air Quality Standards, Attachment 3, March 24, 2011, pp. 24-26.Evaluation of Compliance with the 1-hour NAAQS for SO 2 June 3, 2014 Page 8

4. Model Inputs

4.1 Geographical Inputs

The “ground floor” of all air dis persion modeling analyses is e stablishing a coordinate system for identifying the geographical loc ation of emission sources and r eceptors. These geographical
locations are used to determine local characteristics (such as land use and elevation), and also to ascertain source to receptor di stances and relationships.

The Universal Transverse Merca tor (UTM) NAD83 coordinate system was used for identifying the easting (x) and northing (y) coordi nates of the modeled sources and receptors. Stac k locations were obtained from facility permits and prior modeling files provide d by the state regulatory agency. The
stack locations were then veri fied using aerial photographs.

The facility was evaluated to de termine if it should be modeled using the rural o r urban dispersion coefficient option in AERMOD. A GIS was used to determine whet her rural or urban dispersion coefficients apply to a site. L and use within a three-kilomete r radius circle surr ounding the facility was considered. USEPA guidance st ates that urban dispersion coe fficients are used if more than 50%
of the area within 3 kilometers has urban land uses. Otherwise, rural dispersion coefficients are appropriate. 9

USEPA’s AERSURFACE v. 13016 was use d to develop the meteorologi cal data for the modeling analysis. This model was also us ed to evaluate surrounding land use within 3 kilometers. Based on the output from the AERSURFAC E, approximately 2.6% of surroundi ng land use around the modeled facility was of urban l and use types including Type 21 – Low Intensity Residential, Type 22 – High Intensity Residential and Type 23 – Commercial / Indu strial / Transportation.

This is less than the 50% value c onsidered appropriate for the use of urban dispersion coefficients. Based on the AERSURFACE analysis, i t was concluded that the rur al option would be used for the modeling summarized in this re port. Please refer to Section 4. 5.3 for a discussion of the AERSURFACE analysis.

9 USEPA, Revision to the Guidelin e on Air Quality Models: Adopti on of a Preferred General Purpose (Flat and Complex Terrain) Dispersion Model and Other Revisions, Appendix W to 40 CFR Part 51, November 9, 2005, Section 7.2.3.Evaluation of Compliance with the 1-hour NAAQS for SO 2 June 3, 2014 Page 9

4.2 Emission Rates and Source Parameters

The modeling analyses only considered SO 2 emissions from the facilit y. Off-site sources were not considered. Table 1 presents conc entrations which were predicte d for the following scenarios:

1) approved or allowable emissions based on permits issued by t he regulatory agency, and

2) maximum emissions based on m easured actual hourly SO 2 emissions as taken from
USEPA Air Markets Program Data

. 10 To assure realistic emission rates were used, emissions
from all units at the facilit y were combined and the hour with the maximum total facility
emissions was used to determine the maximum emissions.

Stack parameters and emissions use d for the modeling analysis a re summarized in Table 4.

Table 4 – Facility Stack Parameters and Emissions 11

Stack S01 S02 Description Unit 1 Unit 2 X Coord. [m] 674574.02 674499.82 Y Coord. [m] 4036106.47 4036136.67 Base Elevation [m] 284.4 284.57 Release Height [m] 152.4 152.4 Gas Exit Temperature [°K] 402.04 402.04

Gas Exit Velocity [m/s] 31.01 31.01

Inside Diameter [m] 6.1 6.1 Allowable Emission Rate [g/s] 773.5 773.5 Maximum Emission Rate [g/s] 532.7 511.9

The above stack parameters and emissions were obtained from reg ulatory agency documents and
databases identified in Secti on 2.3. The analysis was conducted based on 100% operating load using
maximum exhaust flow rates and t emperatures. Operation at less than full capacity loads was not considered. This assumption te nds to under-predict impacts sinc e stack parameters such as exit flow rate and temperature are typically lower at less than full load , reducing pollutant dispersion and increasing predicted air qualit y impacts. Stack location, heigh t and diameter were verified using aerial photographs, and flue gas fl ow rate and temperature were verified using combustion
calculations.

10 http://ampd.epa.gov/ampd/

11 Email, E. Mulligan – OKDEQ to L. Williams – Sierra Club, Model ing Data, May 29, 2014. Stack parameters are taken from an AERMOD modeling file provided by OKDEQ - OGE_Soon er_5yrs_SO2.DTA.Evaluation of Compliance with the 1-hour NAAQS for SO 2 June 3, 2014 Page 10

4.3 Building Dimensions and GEP

Building dimensions from a prior downwash evaluation were avail able so this modeling analysis did
address the effects of downwash. 12 The BPIP model was used to re-evaluate downwash.

4.4 Receptors

For Sooner Generating Station, three receptor grids were employ ed:

1. A 100-meter Cartesian receptor grid centered on Sooner Generati ng Station and extending
out 5 kilometers.
2. A 500-meter Cartesian receptor grid centered on Sooner Generati ng Station and extending
out 10 kilometers.
3. A 1,000-meter Cartesian receptor grid centered on Sooner Genera ting Station and extending out 50 kilometers. 50 kilometers is t he maximum distance accept ed by USEPA for the use of the AERMOD dispersion model. 13

A flagpole height of 1.5 meters w as used for all these receptor s.
Elevations from stacks and recep tors were obtained from Nationa l Elevation Dataset (NED) GeoTiff data. GeoTiff is a binary file tha t includes data descriptors a nd geo-referencing information necessary for extract ing terrain elevations. These elevations w ere extracted from 1 arc-second (30 meter) resolution NED files. The U SEPA software program AERMAP v. 11103 is used for these
tasks.

4.5 Meteorological Data

To improve the accuracy of the modeling analysis, recent meteor ological data for the 2009-2013 period were prepared using the U SEPA’s program AERMET which cre ates the model-ready surface and profile data files requir ed by AERMOD. Required data inpu ts to AERMET included surface meteorological measurements, t wice-daily soundings of upper air measurements, and the micrometeorological parameters surface roughness, albedo, and B owen ratio. One-minute ASOS
data were available so USEPA met hods were used to reduce calm a nd missing hours. 14 The USEPA software program AERMINUTE v. 11325 is used for these tasks.

12 Email, E. Mulligan – OKDEQ to L. Williams – Sierra Club, Model ing Data, May 29, 2014. Building dimensions are taken from a BPIP modeling file provided by OKDEQ, PRIME.PIP.

13 USEPA, Revision to the Guideline on Air Quality Models: Adopti on of a Preferred General Purpose (Flat and Complex Terrain) Dispersion Model and Other Revisions, Appendix W to 40 CFR Part 51, Section A.1.(1), November 9, 2005.

14 USEPA, Area Designations for the 2010 Revised Primary Sulfur D ioxide National Ambient Air Quality Standards, Attachment 3, March 24, 2011, p. 19.Evaluation of Compliance with the 1-hour NAAQS for SO 2 June 3, 2014 Page 11

This section discusses how the meteorological data was prepared for use in the 1-hour SO 2 NAAQS modeling analyses. The USEPA software program AERMET v. 13350 is used for these tasks.

4.5.1 Surface Meteorology

Surface meteorology was obtained for Stillwater Regional Airpor t located near the Sooner Generating Station. Integrated S urface Hourly (ISH) data for th e 2009-2013 period were obtained
from the National Climatic Data Center (NCDC). The ISH surfac e data was processed through AERMET Stage 1, which performs data extraction and quality cont rol checks.

4.5.2 Upper Air Data

Upper-air data are collected by a “ weather balloon” that is rel eased twice per day at selected
locations. As the balloon is rel eased, it rises through the at mosphere, and radios the data back to the surface. The measuring and transmitting device is known as eit her a radiosonde, or rawindsonde.
Data collected and radioed back i nclude: air pressure, height, temperature, dew point, wind speed, and wind direction. The upper air data were processed through AERMET Stage 1, which performs data extraction and quality control checks.

For Sooner Generating Stati on, the concurrent 2009-2013 upper a ir data from twice-daily radiosonde
measurements obtained at the most representative location were used. This location was the Norman, Oklahoma measurement station. These data are in Forecas t Systems Laboratory (FSL)
format and were downloaded in ASC II text format from NOAA’s FSL website. 15 All reporting
levels were downloaded a nd processed with AERMET.

4.5.3 AERSURFACE

AERSURFACE is a program that ex tracts surface roughness, albedo , and daytime Bowen ratio for an area surrounding a given locatio n. AERSURFACE uses land use and land cover (LULC) data in the U.S. Geological Survey’s 1992 National Land Cover Dataset t o extract the necessary micrometeorological data. LULC data was used for processing me teorological data sets used as input to AERMOD.

AERSURFACE v. 13016 was used to deve lop surface roughness, albe do, and daytime Bowen ratio values in a region surrounding the meteorological data collecti on site. AERSURFACE was used to develop surface roughness in a one kilometer radius surrounding the data collection site. Bowen ratio and albedo was developed for a 10 kilometer by 10 kilomet er area centered on the meteorological data collection si te. These micrometeorological data were processed for seasonal

15 Available at: http://esrl.noaa.gov/raobs/Evaluation of Compliance with the 1-hour NAAQS for SO 2 June 3, 2014 Page 12

periods using 30-degree sectors . Seasonal moisture conditions w ere considered average with no
months with continuous snow cover.

4.5.4 Data Review

Missing meteorological data were not filled as the data file me t USEPA’s 90% data completeness requirement. 16 The AERMOD output file shows th ere were 0.73% missing data.

To confirm the representativene ss of the airport meteorological data, the surface characteristics of the airport data collection s ite and the modeled source locatio n were compared. Sin ce the Stillwater Regional Airport is located c lose to Sooner Generating Station, this meteorological data set was considered appropriate for this modeling analysis. 17 Additionally, this weathe r station provided high quality surface measurements for the most recent 5-year time, a nd had similar land use, surface characteristics, terra in features and climate.

5. Background SO 2 Concentrations

Background concentrations were determined consistent with USEPA ’s Modeling Guidance for SO 2 NAAQS Designations. 18 To preserve the form of the 1-hour SO 2 standard, based on the 99 th percentile of the annual distri bution of daily maximum 1-hour c oncentrations averaged across the number of years modeled, the b ackground fourth-highest daily ma ximum 1-hour SO 2 concentration
was added to the modeled fourth-h ighest daily maximum 1-hour SO 2 concentration. 19

Background concentrations were based on the 2010-12 design valu e measured by the ambient
monitors located in Oklahoma. 20

6. Reporting

All files from the programs used for this modeling analysis are available to regulatory agencies. These include analyses prepared w ith AERSURFACE, AERMET, AERMAP , and AERMOD.

16 USEPA, Meteorological Monitoring Guidance for Regulatory Modeli ng Applications, EPA-454/R-99-05, February 2000, Section 5.3.2, pp. 5-4 to 5-5.

17 USEPA, AERMOD Implementation Guide, March 19, 2009, pp. 3-4.

18 USEPA, Area Designations for the 2010 Revised Primary Sulfur D ioxide National Ambient Air Quality Standards, Attachment 3, March 24, 2011, pp. 20-23.

19 USEPA, Applicability of Appendix W Modeling Guidance for the 1- hour SO 2 National Ambient Air Quality Standard, August 23, 2010, p. 3.

20 http://www.epa.gov/airtrends/values.html