## Sibley Generating Station Sibley, Missouri Evaluation of Compliance with the 1-hour NAAQS for S02 

## September 3, 2015

Conducted by: 

### Steven Klajka, P.E., BCEE

Wingra Engineering , S.C. Madison, WisconsinEvaluation of Compliance with the 1

-hour NAAQS for S02 Se

pt

em ber 3, 2015 Page2 1. 

### Introduction Wingra Engineering, S.C. was hired by Sierra Club to conduct an air modeling impact analysis to help the U.S. Environmental Protection Agency (USEPA) and state and local air agencies identify fac ilitie s that are likely causing exceedanc es of the 1-hour sulfur dioxide (S02) national ambient air quality standard (NAAQS). This document describe s the results and procedures for an evaluation conducted for the Sibley Generating Station located in Sibley, Mis souri. To ensure the modeling analysis reflected the cumulative concentration of S02 emissions , it included emissions from the following additional sources of so2 emissions located within 50 kilometers of the Sib ley Generating Station:

• Hawthorne Generating Station

- Kansas City, Missouri • Veolia Energy- Kansas City- Kansas City, Missouri • Neannan Creek Power Station- Kansas City, Kansas The dispersion modeling analy s is predicted ambient air concentrations for comparison with t h e 1- hour S02 NAAQS. The modeling was performed u sing the most recent version of AERMOD, AERMET, and AERMINUTE, with data provided to Sierra Club by regulatory air agencies or obtained through other publicly-available sources as documented below
. The analysis was conducted in adherence to all available USEPA guidance for evaluating source impacts on attainment ofthe 1-hour S0 2 NAAQS via aerial dispersion modeling, including the AERMOD Implementation Guide; USEPA's Applicab ility of Appendix W Modeling Guidance for the 1-hour S02 National Ambient Air Quality Standard, August 2 3 , 2010; modeling guidance promulgated by USEP A in Appendix W to 40 CFR Part 51; USEPA's March 2011 Modeling Guidance for 

### SOz NAAQS Designations; 1 and USEPA's December 2013 S02 NAAQS De signations Technical Assistance Document? 

### 2. Compliance with the 1-hour S02 NAAQS 

### 2.1 1-hour S02 NAAQS The 1-hour S02 NAAQS takes the form of a three-year average of the 99 1h-percenti le of the annual distribution of daily maximum 1-hour concentrations, which cannot exceed 75 parts per billion (ppb). 3 Comp liance with this standard was verifie d using USEPA's AERMOD air dispersion model, which produces air concentrations in units of**~g/m .**  The 1-hour S02 NAAQS of75 ppb equals

1 http: // www .epa.gov / scramOO 1 /s o2
_ modeling_guidance.htm 2 http: //www.epa.gov / oaqp s OO 1 / sulfurdioxide/pdfs/ S02ModelingT AD.pdf 3 USEPA , Applicability of Appendix W Modeling Guidance for the 1-hour S02 National Ambient Air Quality Standard, Augu st 23 , 2010.Evaluation of Compliance with the 1-ltour NAAQS for S02 Se

ptember 3, 2015 Page3 196.2 J.lg/m3 , and this is the value used for determining whether modeled impacts exceed the NAAQS . 4 The 99th-percentile of the annual distribution of daily maximum 1-hour concentrations corresponds to the fourth-highest value at each receptor for a given year. 2.2 Mo deling Results Model results for all sources included in the S02 analysis are summarized in Table 1 . Results are provided for each source alone, and for all sources combined. Modeling results for S ibley Generating Station and the other three power plants are summarized in Table 1. It was determined that based on either current allowable emissions or measured actual emissions, the Sibley Generating Station is estimated to create downwind S02 concentrations which exceed the 1-hour NAAQS. More specifically, the modeling results presented in Table 1, show exceedances of the NAAQS by the plant 's allowable and actua l emissions. "Allowable" is the peak emission rate from each unit as approved by the current air quality operation permit for the facility. " Actual" are the measured emissions for each hour between January 1, 2012 and December 31 , 2014 as taken from USEPA 

### Air Markets Program Data. 5 In addition, the emissions from the Hawthorne, Nearman Creek and Veolia plants significantly contribute to the ambient S02 concentration in the area impacted by Sibley Generating Station. Air quality impacts in Missouri are based on a background concentration of23.5 J.lg/m3

. This is the 2011 -13 design valu e for Monroe County , Missouri

- the lowest measured background concentration in the state . This is the most recently available design value. See Section 5 for further discussion of the background concentrations used for this analysis.

4 The ppb to**~g/m 3**  conversion is found in the source code to AERMOD v. 14134, subroutine Modules. The co nversion calculation is 75 /0.3823 = 196.2**~glm •**  5 http: //ampd.epa.gov/ampd/Evaluation of Compliance with the /-hour NAAQSfor S0 2 September 3, 2015 Page4 Table 1 - S02 Modeling Results for Sibley Generating Station ModelingAnalysis Averaging 99th Percentile 1-hour Daily Maximum (f.1g/m3 ) Emission Rates Period Impact Background Total NAAQS Al lowable Sibley 1,197. 1 23.5 1 ,220.6 196.2

Actual Boiler 70 47 . 8 23.5 71.3 196.2 Allowable Sibley 1,411.9 23.5 1,435.4 196.2 Boilers Actual 50,60 & 70 50 .8 23.5 74.3 196 .2 Allowable 379.7 23.5 403.2 196 .2 Hawthorne Actual 23 . 5 23.5 47.0 196.2 Allowab le Nearman 126.3 23 .5 149.8 196.2

Actual Creek 81.4 23.5 104 .9 1 96.2 Allowable 1,614.5 23.5 1,638.0 196.2 Veolia Actual 407.6 23.5 431.1 196.2 Allowable All With 1,706.5 23.5 1,730 .0 196.2 Sibley Actual Boiler 70 41 1. 7 23 . 5 435.2 196.2 Allowable All With 1,739 .0 23.5 1,762.5 196.2 Sib ley Actual Boi lers 411.7 23.5 435.2 196.2 50,60 & 7 0 Complies with NAAQS? No Yes No Yes No Yes Yes Yes No No No No No NoEv aluation of Complianc e with th e 1-hour NAAQS for SOz Se

pt

emb e r 3, 2015 Page 5 The emissions u sed for the modeling analysis are s ummarized in Table 2. 

### Table 2- Modeled S02 

### Emissions 6 Stack Unit

ID ID Boiler 50 SOl Boiler 60 Boiler 70 Sibley Subtotal

HOI Unit 5A H awthorne Subtotal N01 Unit 1 Nearman Creek Subtotal V02 Boiler 6 Boiler 8 Veolia Subtotal Facility Total Allowable Emissions (lbslhr) 5,486.4 5,645.7 36,576.9 47,709.0 10,792 .0 10,792.0 2,919.7 2,919.7 3,434.3 3,604.0 7,038.2 68,459.0 Based on the modeling results, Table 3 provides the emission reductions from current allowable rates necessary to achieve compliance with the 1-hour NAAQS. This assumes a one-hour averaging period for the emission rate and that the emission rate is binding at all times. However, given the conservative aspects of this modeling protocol, it is extreme ly likely that this limit is too high to protect the NAAQS. For example , startup or shutdown periods were not evaluated. During these periods , decreased gas velocities and temperatures may lead to greater ambient impacts at ground level. Further, the hypothetical emission limitation in Table 3 would allow Sibley Generating Station to consume the ent ire NAAQS, leaving little to no room for any other source of S0 2 in the area. No margin of safety has been included in the hypothetical emission limitation.

6 S ibley a llowable emiss ions were taken from Missouri DNR, Part 70 Permit to Operate, No. OP2006-063, Augu st 31 , 2 006 . Hawthorne allowable emis sions were taken from Missouri DNR, Part 70 Permit to Operate, No. OP2013-0024 , July 12, 2013 . Nearman Creek allowab le emissions were taken from Kansas, Air Emission Source Class I Operating Permit, ID 2 090008 , March 24 , 2004. Veolia allowab le emis sions were taken from Missouri DNR, Part 70 Permit to Operate , No. OP2012-050, No. OP2012-050 , March 4 , 2013.Evaluation of Compliance with the 1-hour NAAQSfor S01 September 3, 2015 Page6 

### Table 3 -Required Emission Reductions from Sibley Generating Stationfor Compliance with the 1-hourNAAQS for S02 Acceptable Impact Required Required Required (NAAQS

- Background) Total Facility Total Facility Total Facility 99th Percentile Reduction Based on Emission Rate 1-hour Average 1-hour Daily Max Allowable Emissions (lbs/br) Emission Rate**(~g/m )**  (%) (lbs/mmbtu) 172 .7 90% 4 ,738 .0 0.89 Predicted exceedances of the 1-hour NAAQS for S02 bas ed on allowable emissions extend throughout the region to a maximum distance of 50 kilometers. Figure 1 shows the extent ofNAAQS violations based on allowable emissions from all sources . Figure 2 shows the extent ofNAAQS violations based on actual hourly emiss ions from all sources. 

### 2.3 Conservative Modeling Assumptions A di spersion modeling analysis requires the selection of numerous parameters which affect the predicted concentrations. For the enclosed analysis, several parameters were selected which underpredict facility impacts. Assumptions used in this modeling analysis which likely under-estimate concentrations include the following:
• Allowable emissions are based on a limitation with an averaging period which is greater than the 1-hour average u sed for the S02 air quality standard
. Emissions and impacts during any 1-hour period may be higher than assumed for the modeling analysis. • 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 di s persion and increasing predicted air quality impacts. • No consideration ofbuilding or structure downwash. These downwash effects typically increase predicted concentrations near the facility. • Except for the Hawthorne, Nearman Creek and Veolia plants, no consideration of off-site sources. Any other off-site sources of S02 will increase the predicted impacts. 

### 2.4 Comparison with Existing Ambient Monitoring An ambient monitor for S02 is operated approximately 1.2 krn or 0. 7 miles southeast of Veolia Energy. The USEPA database for ambient monitoring measurements identifies MonitorEvaluation of Compliance with the 1-hour NAAQS for SOz September 3, 2015 Page 7 #7 290950034 as the TROOST: 724 Troost (Rear), Kansas City, MO 64106 monitor. 7 The 2012-14 design value measured at this monitor was 150 ppb or 392.4 J.lg/m3

. Based on actual hourly emissions from the Sib ley plant and other regional sources, the modeling analysis predicted a concentration near the monitor location of 382.5 J.lg/m3

. Minus the assumed background of23.5 J.!g/m3 the predicted impact due to regional S0 2 sources is 359 J..Lg/m3

. Since the predicted and actual measured design values are in close agreement, this comparison supports the accuracy of the modeling results presented in this report.

7 http: //www.epa .gov/airtrends/pdfs/S02

_DesignValues_ 20 12 20 14_FINAL
_ 8

_3

_ 15.xJsxEvaluation of Compliance with the 1-hour NAA QSfor S02 September 3, 2015 Pa ge8

## ~ .,

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. • I '**~~!J:lOO** y,oooo**~SOOO** 370000**)7~00** )8t000 );!.000 J(Nt)()O**3c-~o ~0(10**  JOSOOO J IIXOO J 1 !000 4,0000**.t~..OOO** -llOOCO**J~.QO** .uXICO UTMEdsl(m) 1-hour average S02 concentrations (ug per cubic meter) - All colored areas exceed the NAAQS.

I 196 400 600 800 1000 Figure 1 -Regional View of Impa cts Based on Allowable Emissions from All SourcesEvaluation of Compliance with th e 1

- hour NAAQSfor SOz September 3, 2015 Page9

o · Sibley Generating Station

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### ..

...............**37~0 ~'C«>>**  ;;,ocoo un.1 East {m] ·+



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1-hour average S02 concentrations ( ug per cubic meter) -All colored areas exceed the NAAQS
. 196 300 400 Figure 2 -Regional View of Impacts Based on ActualEmissions from All Sources <'



## ..Evaluation of Compliance with the 1-hour NAAQS for S02 September 3, 2015 Page 10 

### 3. Modeling Methodology 3.1 Air Dispersion Model The modeling analysis used USEPA's AERMOD program, v. 14134. AERMOD, as available from the Support Center for Regulatory Atmospheric Modeling (SCRAM) website, was us ed in conjunction with a third-party modeling s oftware program, AERMOD View, sold by Lakes Environmental Software. 

### 3.2 Control Options The AERMOD model was run with the following control options: • 1-hour average air concentrations • Regulatory defaults • Flagpole receptors To reflect a representative inhalation 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 determine if the modeled facility 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. 8 For urban sources, the URBANOPT option is used in conjunction with the urban population from an appropriate nearby city and a de fault surface roughness of 1.0 meter. Methods described in Section 4.1 were u sed to determine whether rural or urban dispersion coefficients were appropriate for the modeling analysis. 

### 3.3 Output Options The AERMOD analysis was based on three years ofrecent meteorological data. The modeling analyses used one run with three years of sequential meteorological data from 2012-2014. Consistent with USEPA's Modeling Guidance for S02 NAAQS Designations, AERMOD provided a table of fourth -hi gh 1-hour S02 impacts concentrations con s istent with the form of the 1-hour S02NAAQS. 9 Please refer to Table 1 for the modeling results.

8 USEPA, Rev is ion to the Guideline on Air Quality Mode ls: Adoption of a Preferred General Purpo se (F lat and Com plex Terrain) Di spers ion Model and Other Revisions, Appendix W to 40 CFR Part 51, November 9, 2005. 9 USEPA, Area Designations for the 20 10 Revi sed Primary Sulfur Dioxide National Ambient Air Quality Standards , Attachment 3, March 24,2011 , pp. 24

- 26.Evaluation of Complian ce with th e 1

- hour NAA QSfor SOz Se

ptemb er 3 , 2015 Page 11 

### 4. Model lnputs 4.1 Geographical Inputs The "ground floor" of all air dispersion modeling analyses is estab li shing a coordinate system for identifying the geographical location of emission sources and receptors. These geographical locations are used to determine loc al characteristics (such as land u se and e levation), and also to ascertain source to receptor distances and relationships. The Universal Transverse Mercator (UTM) NAD83 coordinate system was used for identifying the easting (x) and northing (y) coordinates ofthe modeled sources and receptors. Stack locations were obtained from facility permits and prior modeling files provided by the state regulatory agency. The stack locations were then verified using aerial photographs. The facility was evaluated to determine if it should be modeled using the rural or urban dispersion coefficient option in AERMOD. A Geographic Information System (GIS) was used to determine whether rural or urban dispersion coefficients apply to a site. Land use within a three

-kilometer radius circle surrounding the facility was considered. USEPA guidance states that urban dispersion coefficients are u sed if more than 50% of the area within 3 kilometers has urban land uses. Otherwise, rural dispersion coefficients are appropriate. 10 USEPA's AERSURFACE v. 13016 was used to develop the meteorological data for the modeling analysis. Thi s model was also used to evaluate surrounding land use within 3 kilometers. Based on the output from the AERSURFACE, approximately l.5% of surrounding land use around the modeled facility was of urban land use types including Type 21 -Low Intensity Residential, Type 22 -High Intensity Residential and Type 23

- Commercial 

### I Industrial 

### I Transportation. This is less than the 50% value cons idered appropriate for the use of urban dispersion coefficients. Based on the AERSURF ACE analysis , it was concluded that the rural option would be used for the modeling summarized in this report. Please refer to Section 4.5.3 for a discussion of the AERSURF ACE analysis.

10 USEPA, Revi s ion to the Guide line on Air Quality Models: Adoption of a Pref e rred General Purpose (Flat and C omplex T errain) Di spersion Model and Other Re vi sions , Appendix W to 40 CFR Part 51 , November 9, 2005 , Section 7. 2 . 3 .Evaluation of Compliance with the 1-hour NMQS for S02 Se

ptember 3, 2015 Page 12 

### 4.2 Emission Rates and Source Parameters The modeling analysis considered S02 emi ssions from the Sibley, Hawthorne, Nearman Creek and Veolia power plants. Other off

- site sources were not considered. Concentrations were predicted for the scenarios shown in Tables 1 and 2: 1) allowable emissions based on the current permit issued by the regulatory agency, and 2) actual hourly emissions for Sibley, Hawthorne and Nearman Creek were measured each hour between January 1, 2012 and December 31, 2014 as taken from USEPA Air Markets Program Data. 11 Actual emissions for Veolia were the annual average based on the emissions inventory report for 20 14 . Stack parameters and emissions used for the modeling analys is are summarized in Table 4. 

### Table 4- Facility Stack Parameters and Emissions 12 Facility Sibley Hawthorn Nearman Veolia

Stack SOl S03 HOl NO! VOl Description 50,60 and 70 Boiler 70 Unit SA Unit 1 6 and 8 X Coord. [m] 397718 397718 372264 353388 363375 Y Coord. [m] 4337279 4337279 4332322 4337135 4330433 Base Elevation [m] 221.13 221.13 226.06 229.41 234 . 12 Release Height [m] 213.36 213.36 182.88 121.92 85.95 Gas Exit Temperature [° K] 422.039 422.039 338.706 410.928 395 . 92 8

Gas Exit Velocity [rnls] 37.087 28 .43 27.344 13.373 1.79 8

Inside Diameter [m] 4.115 4 . 115 6.462 6.096 5 .1 05 Allowab le Emi s sion Rate [g/s] 6,011 4 , 609 1 ,3 60 367 . 9 886 .8

Actual Emission Rate [g/s) 

## - - - - 223 .9 The above stack parameters and emissions were obtained from regulatory agency documents and databases identified in Section 2.2. The analysis was conducted based on 100% operating load using maximum exhaust flow rates and temperatures. Operation at less than full capacity loads was not considered. This assumption under-predicts impacts since stack parameters such as exit flow rate and temperature are lower at less than fu ll load , reducing pollutant dispersion and increasing predicted air quality impacts. Stack location, height and diameter were verified using aerial photographs, and flue gas flow rate and temperature were verified using combustion calculations.

11 http :// ampd.epa.gov/ampd/ 12 Sibley, Hawthorne and Nearman Creek stack parameters obtained from U.S. Energy Information Administration, Form ElA-923 Detailed Data, Schedule 6, Stack & Flue Data, http://www.eia.g o v /electricity/data/eia923 / . Veolia stack parameters obtained from Missouri DNR Emissions Inventory Questionnaire for 2014 .Evaluation of Compliance with the 1-hour NAAQSfor S02 September 3, 2015 Page 13 

### 4.3 Building Dimensions No building dimensions or prior downwash evaluations were available. Therefore this modeling analysis di d not address the effects of down was h and this may under-predict impacts. 

### 4.4 Receptors For S ibley Generating Station, three receptor grids were emp loyed: 1. A 1 00-meter Cartesian receptor grid centered on S ibley Generating Station and extending out 5 kilometers. 2. A 500

-meter Cartesian receptor grid centered on Sibley Generating Stat ion and extending out 10 kilometers. 3. A 1,000-meter Cartesian receptor grid centered on Sib ley Generating Station and extend ing out 50 kilometers. 50 kilometers is the maximum distance accepted by USEPA for the use of the AERMOD di spersion model. 13 A flagpole height of 1.5 meters was used for all these receptors. Elevations from stacks and receptors were ob tained from National Elevation D ataset (NED) GeoTiff data. GeoTiff is a binary file that includes data descriptors and geo-referencing information necessary for extracting terrain elevations. These elevations were extracted from 1 arc-second (30 meter) resolution NED files. The USEPA software program AERMAP v. 11103 i s used for these tasks. 

### 4.5 Meteorological Data To improve the accuracy of the modeling analysis, recent meteorological data for the 2012-2014 period were prepared using the USEPA's program AERMET which creates the model-ready surface and profi le data file s required by AERMO D . Required data inputs to AERMET included surface meteorological measurements , twice -daily soundings of upper air measurements, and the micrometeorological parameters surface roughness , albedo , and Bowen ratio. One-minute ASOS data were available s o USEPA methods were used to reduce calm and missing hours. 14 The USEPA software program AERMINUTE v. 14237 is used for these tasks. This section discusses how the meteoro lo g ic al data was prepared for u se in the 1-hour S02 NAAQS

1 3 USEPA, Revision to the Guideline on Air Quality Models: Adoption of a Preferred General Purpose (Flat and Complex Terrain) Dispersion Model and Other Revi sions, Appendix W to 40 CFR Part 5 I, Section A.l.(l), November 9, 2005. 1 4 USEPA , Area Designations for the 2010 Revised Primary Sulfur Dioxide National Ambient Air Quality Standards, Attachment 3, March 24, 20 11, p . 19 .Evaluation of Compliance with the !-hour NAAQS for S02 September 3, 2015 Page 14 modeling analyses. The USEPA software program AERMET v. 14134 is used for these tasks. 4.5.1 Surface Meteorology Surface meteorology was obtained for Kansas City Downtown Airport located near the Sibley Generating S tation. Integrated Surface Hourly (ISH) data for the 2012-2014 period were obtained from the National Climatic Data Center (NCDC). The ISH surface data was processed through AERMET Stage 1, which performs data extraction and quality control checks. 4.5.2 Upper Air Data Upper-air data are collected by a "weather balloon" that is released twice per day at selected locations. As the balloon is released, it rises through the atmosphere, and radios the data back to the surface. The measuring and transmitting device is known as either a radiosonde, or rawindsonde. Data collected and radioed back include: 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 Sibley Generating Station, the concurrent 2012-2014 upper air data from twice-daily radiosonde measurements obtained at the most representative location were used. This location was the Topeka, Kansas measurement station. These data are in Forecast Systems Laboratory (FSL) format and were downloaded in ASCII text format from NOAA's FSL website. 15 All reporting levels were downloaded and processed with AERMET. 4 .5.3 AERSURFACE AERSURF ACE is a program that extracts surface roughne ss, albedo, and daytime Bowen ratio for an area surrounding a given location. AERSURF ACE uses land u se and land cover (LULC) data in the U.S. Geological Survey's 1992 National Land Cover Dataset to extract the necessary micrometeorological data. LULC data was used for processing meteorological data sets used as input to AERMOD. AERSURFACE v. 13016 was used to develop surface roughness, albedo, and daytime Bowen ratio value s in a region surrounding the meteorological data collection site. AERSURF ACE was used to develop surface roughness in a one kilometer radius surrounding the data collection s ite. Bowen ratio and albedo was developed for a 1 0 kilometer by 10 kilometer area centered on the meteorological data collection s ite. These micrometeorological data were processed for seasonal periods u sing 30-degree sectors. Seasonal moisture conditions were considered average with winter

15 Available at: http: //esrl.noaa.gov /raobs/Evaluation of Compliance with the 1-hour NAAQS for S02 Se

ptember 3, 2015 Page 15 months having continuous snow cover. 

### 4.5.4 Data Review Missing meteorological data were not filled as the data file met USEPA's 90% data comp leteness requirement. 16 The AERMOD output file shows there were 0.31% missing data. To confirm the representativeness of the airport meteorological data, the surface characteristics of the airport data collection s ite and the modeled source location were compared. Since the Kansas City Downtown Airport is located close to Sibley Generating Station, this meteorological data set was considered appropriate for this modeling analysis. 17 Additionally, this weather station provided high quality surface measurements for the most recent 3

-year time, and had similar land use, surface characteristics, terrain features and climate. 

### 5. Background SOz Concentrations Background concentrations were determined consistent with USEPA's Modeling Guidance for S02 NAAQS Designations. 18 ' 19 To preserve the form ofthe 1-hour S02 standard , based on the 99th percentile of the annual distribution of daily maximum 1-hour concentrations averaged across the number ofyears modeled, the background fourth -highest daily maximum 1 -hour S0 2 concentration was added to the modeled fourth-highest daily maximum 1-hour S0 2 concentration. 20 Background concentrations were based on the 2011

- 13 design value measured by the ambient monitors located in Missouri. 21 

### 6. Reporting All files from the programs used for this modeling analysis are availab le to regulatory agencies
. These include analyses prepared with AERSURFACE, AERMET, AERMAP, and AERMOD.

16 USEPA, Meteorological Monitoring Guidance for Regulatory Modeling Applications, EPA-454/R

- 99

- 05 , February 2000 , Section 5.3.2 , pp. 5-4 to 5-5. 1 7 USEPA, AERMOD Implementation Guide, March 19,2009, pp. 3-4. 1 8 USEPA, Area Designations for the 2010 Revised Primary Sulfur Dioxide National Ambient Air Quality Standards, Attachment 3 , March 24,2011, pp. 20-23. 19 USEPA, S0 2 NAAQS De signations Modeling Technical Assistance Docwnent, Dec. 2013, section 8.1, pp 27-28. 20 USEPA , Applicability of Appendix W Modeling Guidance for the !-hour S02 National Ambient Air Quality Standard, August 23,2010 , p . 3. 2 1 http :// www.epa .gov/ airtrends/values.html