1 Draft Technical Support Document

North Dakota Area Designations for the 2010 SO 2 Primary National Ambient Air Quality Standard

Summary

Pursuant to section 107(d) of the Clean Air Act (CA A), the U.S. Environmental Protection Agency (EPA, or the Agency) must designate areas as either “unclassifiable,” “attainment,” or
“nonattainment” for the 2010 one-hour sulfur dioxid e (SO 2 ) primary national ambient air quality standard (NAAQS). The CAA defines a nonattainment a rea as one that does not meet the NAAQS or that contributes to a violation in a nearb y area. An attainment area is defined as any
area other than a nonattainment area that meets the NAAQS. Unclassifiable areas are defined as
those that cannot be classified on the basis of ava ilable information as meeting or not meeting the NAAQS.

North Dakota submitted updated recommendations on S eptember 16, 2015, ahead of a July 2, 2016, deadline for the EPA to designate certain are as established by the U.S. District Court for
the Northern District of California. This deadline is the first of three deadlines established by the court for the EPA to complete area designations for the 2010 SO 2 NAAQS. Table 1 below lists
North Dakota’s recommendations and identifies the c ounties or portions of counties in North Dakota that the EPA intends to designate by July 2, 2016 based on an assessment and characterization of air quality through ambient air quality data, air dispersion modeling, other evidence and supporting information, or a combinati on of the above.

Table 1: North Dakota’s Recommended and EPA’s inten ded designations
Area North Dakota’s

Recommended
Area Definition North Dakota’s

Recommended Designation EPA’s Intended

Area Definition EPA’s Intended

Designation

McLean County/Eastern Mercer County,
North Dakota Unspecified

(Area around source) Attainment McLean Co., North Dakota (full county),

Within Mercer
Co.:
Area east of CR37/ND 31,
east/north of ND 200 ALT, west of
the eastern border
of Mercer County/Missouri River, south of the
Knife River
National Historic
Site. Unclassifiable2

Central Mercer County, North
Dakota Unspecified

(Area around source) Attainment Within Mercer
Co.: Area west of
ND 49/61 st Ave SW, north of – Co.
Rd 15/17 th St. SW, east of Co. Rd 13,
south and east of the town Zap,
south of 8 th St.
SW/ND 200 Unclassifiable/

Attainment

Background

On June 3, 2010, the EPA revised the primary (healt h based) SO 2 NAAQS by establishing a new one-hour standard at a level of 75 parts per billio n (ppb) which is attained when the three-year average of the 99th percentile of one-hour daily ma ximum concentrations does not exceed 75 ppb. This NAAQS was published in the Federal Regist er on June 22, 2010 (75 FR 35520) and is
codified at 40 CFR 50.17. The EPA determined this i s the level necessary to protect public health with an adequate margin of safety, especially for c hildren, the elderly and those with asthma. These groups are particularly susceptible to the he alth effects associated with breathing SO 2 . The two prior primary standards of 140 ppb evaluated ov er 24 hours, and 30 ppb evaluated over an entire year, codified at 40 CFR 50.4, remain applic able. 1 However, the EPA is not currently designating areas on the basis of either of these t wo primary standards. Similarly, the secondary
standard for SO 2, set at 500 ppb evaluated over 3 hours has not been revised, and the EPA is also not currently designating areas on the basis of the secondary standard.

General Approach and Schedule

Section 107(d) of the Clean Air Act requires that n ot later than one year after promulgation of a new or revised NAAQS, state governors must submit t heir recommendations for designations and boundaries to EPA. Section 107(d) also requires the EPA to provide notification to states no less than 120 days prior to promulgating an initial area designation that is a modification of a
state’s recommendation. If a state does not submit designation recommendations, the EPA will promulgate the designations that it deems appropria te. If a state or tribe disagrees with the EPA’s intended designations, they are given an opportunit y within the 120 day period to demonstrate why any proposed modification is inappropriate.

1 40 CFR 50.4(e) provides that the two prior primary NAAQS will no longer apply to an area one year aft er its designation under the 2010 NAAQS, except that for a reas designated nonattainment under the prior NAAQS as of August 22, 2010, and areas not meeting the requirem ents of a SIP Call under the prior NAAQS, the prior NAAQS will apply until that area submits and EPA approves a SIP providing for attainment of the 2010 NAAQS. North
Dakota contains no such areas.3

On August 5, 2013, the EPA published a final rule e stablishing air quality designations for 29
areas in the United States for the 2010 SO 2 NAAQS, based on recorded air quality monitoring data from 2009 - 2011 showing violations of the NAA QS (78 FR 47191). In that rulemaking, the EPA committed to address, in separate future action s, the designations for all other areas for which the Agency was not yet prepared to issue desi gnations.

Following the initial August 5, 2013 designations, three lawsuits were filed against the EPA in different U.S. District Courts, alleging the Agency had failed to perform a nondiscretionary duty under the CAA by not designating all portions of th e country by the June 2013 deadline. In an effort intended to resolve the litigation in one of those cases, plaintiffs Sierra Club and the
Natural Resources Defense Council and the EPA filed a proposed consent decree with the U.S.
District Court for the Northern District of Califor nia. On March 2, 2015, the court entered the
consent decree and issued an enforceable order for the EPA to complete the area designations according to the court-ordered schedule.

According to the court-ordered schedule, the EPA mu st complete the remaining designations by three specific deadlines. By no later than July 2, 2016 (16 months from the court’s order), the EPA must designate two groups of areas: (1) areas t hat have newly monitored violations of the
2010 SO 2 NAAQS and (2) areas that contain any stationary sou rces that had not been announced as of March 2, 2015 for retirement and that accordi ng to the EPA’s Air Markets Database emitted in 2012 either (i) more than 16,000 tons of SO 2 or (ii) more than 2,600 tons of SO 2 with an annual average emission rate of at least 0.45 po unds of SO 2 per one million British thermal units (lbs SO 2 /mmBTU). Specifically, a stationary source with a coal-fired unit that as of January 1, 2010 had a capacity of over 5 megawatts and otherwise meets the emissions criteria, is excluded from the July 2, 2016 deadline if it ha d announced through a company public announcement, public utilities commission filing, c onsent decree, public legal settlement, final state or federal permit filing, or other similar me ans of communication, by March 2, 2015, that it will cease burning coal at that unit.

The last two deadlines for completing remaining des ignations are December 31, 2017, and December 31, 2020. The EPA has separately promulgat ed requirements for states and other air agencies to provide additional monitoring or modeli ng information on a timetable consistent with these designation deadlines. We expect this informa tion to become available in time to help inform these subsequent designations. These require ments were promulgated on August 21, 2015 (80 FR 51052), in a rule known as the SO 2 Data Requirements Rule (DRR).

Updated designations guidance was issued by the EPA through a March 20, 2015 memorandum from Stephen D. Page, Director, U.S. EPA, Office of Air Quality Planning and Standards, to Air Division Directors, U.S. EPA Regions I-X. This memo randum supersedes earlier designation guidance for the 2010 SO 2 NAAQS, issued on March 24, 2011, and it identifies factors that the EPA intends to evaluate in determining whether area s are in violation of the 2010 SO 2 NAAQS. The guidance also contains the factors the EPA inte nds to evaluate in determining the boundaries for all remaining areas in the country, consistent with the court’s order and schedule. These factors include: 1) Air quality characterization vi a ambient monitoring or dispersion modeling results; 2) Emissions-related data; 3) Meteorology; 4) Geography and topography; and 5) Jurisdictional boundaries. This guidance was supple mented by two technical assistance4

documents intended to assist states and other inter ested parties in their efforts to characterize air quality through air dispersion modeling or ambient air quality monitoring for sources that emit
SO 2 . Notably, the EPA released its most recent version s of documents titled, “SO 2 NAAQS Designations Modeling Technical Assistance Document ” (Modeling TAD) and “SO 2 NAAQS Designations Source-Oriented Monitoring Technical A ssistance Document” (Monitoring TAD)
in December 2013.

Based on ambient air quality data collected between 2012 and 2014, no violations of the 2010
SO 2 NAAQS have been recorded in the State. 2 It should be noted, however, that the EPA identified the Tioga area as one that was potential ly subject to the initial obligation to designate by July 2, 2016, based on data collected during thi s time period. However, this data have been found as not comparable to the NAAQS due to certifi cation and performance and benchmark issues. As a result, the consent decree no longer o bligates us to designate the area by July 2, 2016. Instead, we will designate the Tioga area by one of the remaining deadlines, i.e., December 31, 2017, or December 31, 2020, consistent with the conditions in the consent decree. There are 3 sources in the State meeting the emissi ons criteria of the consent decree for which the EPA must complete designations by July 2, 2016. In this draft technical support document, the EPA discusses its review and technical analysis of North Dakota’s updated recommendations for the areas that we must designate. The EPA also discusses any intended modifications from
the State’s recommendation based on all available d ata before us.

The following are definitions of important terms us ed in this document: 1) 2010 SO 2 NAAQS – The primary NAAQS for SO 2 promulgated in 2010. This NAAQS is 75 ppb, based on the three year average of the 99th percentile of the annual distribution of daily maximum one-hour average concentrations. S ee 40 CFR 50.17. 2) Design Value - a statistic computed according to th e data handling procedures of the NAAQS (in 40 CFR part 50 Appendix T) that, by compa rison to the level of the NAAQS, indicates whether the area is violating the NAAQS. 3) Designated nonattainment area – an area which the E PA has determined has violated the
2010 SO 2 NAAQS or contributed to a violation in a nearby ar ea. A nonattainment designation reflects considerations of state recomm endations and all of the information
discussed in this document. The EPA’s decision is b ased on all available information including the most recent 3 years of air quality mo nitoring data, available modeling analysis, and any other relevant information. 4) Designated unclassifiable area – an area which the EPA cannot determine based on all
available information whether or not it meets the 2 010 SO 2 NAAQS.

2 For designations based on ambient air quality monit oring data that violates the 2010 SO 2 NAAQS, the consent
decree directs the EPA to evaluate data collected b etween 2013 and 2015. Absent complete, quality ass ured and certified data for 2015, the analyses of applicable areas for the EPA’s intended designations will be informed by data
collected between 2012 and 2014. States with monit ors that have recorded a violation of the 2010 SO 2 NAAQS during these years have the option of submitting co mplete, quality assured and certified data for cale ndar year 2015 by April 19, 2016 to the EPA for evaluation. If af ter our review, the ambient air quality data for th e area indicates that no violation of the NAAQS occurred between 201 3 and 2015, the consent decree does not obligate th e EPA to complete the designation. Instead, we may designat e the area and all other previously undesignated ar eas in the State on a schedule consistent with the prescribed timing of the court order, i.e., by December 31, 20 17, or December 31, 2020.5

5) Designated unclassifiable/attainment area – an area which the EPA has determined to
have sufficient evidence to find either is attainin g or is likely to be attaining the NAAQS.
The EPA’s decision is based on all available inform ation including the most recent 3 years of air quality monitoring data, available mod eling analysis, and any other relevant
information. 6) Modeled violation – a violation based on air disper sion modeling. 7) Recommended attainment area – an area a state or tr ibe has recommended that the EPA designate as attainment. 8) Recommended nonattainment area – an area a state or tribe has recommended that the EPA designate as nonattainment. 9) Recommended unclassifiable area – an area a state o r tribe has recommended that the EPA designate as unclassifiable. 10) Recommended unclassifiable/attainment area – an are a a state or tribe has recommended that the EPA designate as unclassifiable/attainment

. 11) Violating monitor – an ambient air monitor meeting all methods, quality assurance and siting criteria and requirements whose valid design value exceeds 75 ppb, based on data analysis conducted in accordance with Appendix T of 40 CFR part 50.6

Technical Analysis for the McLean County/Eastern Me rcer County Area

Introduction

The southern portion of McLean County contains a st ationary source that according to the EPA’s
Air Markets Database emitted in 2012 either more th an 16,000 tons of SO 2 or more than 2,600
tons of SO 2 and had an annual average emission rate of at leas t 0.45 pounds of SO 2 per one million British thermal units (lbs SO 2 /mmBTU). As of March 2, 2015, this stationary sourc e had not met the specific requirements for being “announ ced for retirement.” Specifically, in 2012, the Coal Creek Station emitted 16,273 tons of SO 2 , and had an emissions rate of 0.34 lbs
SO 2 /mmBTU. Pursuant to the March 2, 2015 court-ordered schedule, the EPA must designate the area surrounding the facility by July 2, 2016.

On the eastern border of Mercer County, about 16.5 km southwest of the Coal Creek facility is a stationary source that according to the EPA’s Air M arkets Database emitted in 2012 more than 16,000 tons of SO 2 and more than 2,600 tons of SO 2 and had an annual average emission rate of at least 0.45 pounds of SO 2 per one million lbs SO 2 /mmBTU. As of March 2, 2015, this stationary source had not met the specific requirem ents for being “announced for retirement.” Specifically, in 2012, the Leland Olds Station emit ted 38,323 tons of SO 2 , and had an emissions
rate of 2.06 lbs SO 2 /mmBTU. Pursuant to the March 2, 2015 court-ordered schedule, the EPA must also designate the area surrounding the facili ty by July 2, 2016.

In its submission, North Dakota recommended that th e area surrounding both the Coal Creek and Leland Olds Stations be designated as attainment ba sed on an assessment and characterization of air quality from the facilities and other nearby so urces (specifically, the Stanton Station near Leland Olds) which may have a potential impact in t he area of analysis where maximum
concentrations of SO 2 are expected. This assessment and characterization was performed using air dispersion modeling software, i.e., AERMOD, ana lyzing both actual emissions (for Stanton Station) and allowable emissions (for Leland Olds S tation). After careful review of the State’s assessment, supporting documentation, and all avail able data, the EPA does not agree with the
State’s recommendation for the combined area becaus e the Leland Olds allowable emissions rate was not adequately adjusted to account for the 2010 SO 2 NAAQS, and therefore intends to designate the area as unclassifiable. Specifically, the boundaries consist of McLean County, North Dakota, and within Mercer County, the area ea st of CR-37/ND 31, east/north of ND 200 ALT, west of the eastern border of Mercer County/Mi ssouri River, and south of the Knife River
National Historic Site.

The Coal Creek Station is located in central North Dakota in the southern portion of McLean County. As seen in Figure 1 below, the facility is located in southern McLean County, North Dakota, 10 km north of the Missouri River which act s as the border of McLean and Mercer
counties.

The Leland Olds Station is located in central North Dakota near the eastern border of Mercer County. As seen in Figure 2 below, the facility is located just south of the Missouri River. Also7

included in Figure 2 are nearby emitters of SO 2 (specifically, the Stanton Station). Figure 3, below, shows the EPA’s intended unclassifiable desi gnation for the area in blue.

Figure 1. The EPA’s intended designation for McLean County (full county) and a segment of eastern Mercer County, North Dakota

Figure 2: Close up of the Mercer County portion of EPA’s intended McLean and Mercer County combined designation.8

Figure 3. The EPA’s intended unclassifiable area, w hich appears in blue.9

The discussion and analysis that follows below will reference the State’s use of the Modeling TAD, the EPA’s assessment of the State’s modeling i n accordance with the Modeling TAD, and
the factors for evaluation contained in the EPA’s M arch 20, 2015 guidance, as appropriate.

Detailed Assessment

Air Quality Data

This factor considers the SO 2 air quality monitoring data in the area of the Coa l Creek and
Leland Olds Stations. There are no SO 2 monitors located in McLean County. The SO 2 monitors in Mercer County are all located at least 30 kilome ters (km) west of the Leland Olds facility. The
nearest SO 2 monitor to both the Coal Creek and Leland Olds sta tions is located in Oliver County, roughly 13 km southwest of Leland Olds and 28 km so uthwest of Coal Creek. This and all other monitors in the State are too distant to reliably m easure impacts from either of these facilities.

Model Selection and Modeling Components

The EPA’s Modeling TAD notes that for area designat ions under the 2010 SO 2 NAAQS, the AERMOD modeling system should be used, unless use o f an alternative model can be justified. In some instances the recommended model may be a mo del other than AERMOD, such as the BLP model for buoyant line sources. The AERMOD mode ling system contains the following components:

- AERMOD: the dispersion model

- AERMAP: the terrain processor for AERMOD

- AERMET: the meteorological data processor for AERMO D

- BPIPPRIME: the building input processor

- AERMINUTE: a pre-processor to AERMET incorporating 1-minute automated surface observation system (ASOS) wind data

- AERSURFACE: the surface characteristics processor f or AERMET

- AERSCREEN: a screening version of AERMOD

The State used the most recent version of AERMOD, 1 5181, and a discussion of the individual components will be referenced in the corresponding discussion that follows, as appropriate.

Modeling Parameter: Rural or Urban Dispersion

US EPA-recommended procedure for characterizing an area by prevalent land use is based on evaluating the dispersion environment with 3 km of the facility. According to US EPA modeling guidelines, rural dispersion coefficients are to be used in the dispersion modeling analysis if
more than 50% of the area within a 3-km radius of t he facility is classified as rural. Conversely, if more than 50% of the area is urban, urban disper sion coefficients should be used in the modeling analysis. As shown in the Figure 4, the 3- km area surrounding each of the facilities is rural. Therefore, the State determined that is was most appropriate to run the model with rural dispersion coefficients for each of the facilities being modeled.

Modeling Parameter: Area of Analysis (Receptor Grid )10

The EPA believes that a reasonable first step towar ds characterization of air quality in the area surrounding the Coal Creek and Leland Olds Stations is to determine the extent of the area of analysis, i.e., receptor grid. Considerations prese nted in the Modeling TAD include but are not
limited to: the location of the SO 2 emission sources or facilities considered for mode ling; the extent of significant concentration gradients of ne arby sources; and sufficient receptor coverage and density to adequately capture and resolve the m odel predicted maximum SO 2 concentrations.
For the area near the Coal Creek and Leland Olds St ations, the State has included 1 other emitter
of SO 2 within the modeling domain. The additional facilit y included in the modeling was Stanton Station, which lies about 1 km west of Leland Olds Station. There are no other emitters that lie within 10 km of either facility in any direction. T he State determined that this was the appropriate distance in order to adequately charact erize air quality from the facility and other nearby sources which may have a potential impact in the area of analysis where maximum
concentrations of SO 2 are expected, citing EPA’s March 1, 2011 Memorandu m and the analysis presented at a 2011 EPA modeling workshop. The grid receptor spacing for the combined area of analysis chosen by the State is as follows:

- 0 km to 5 km with 100 meters spacing from Coal Cree k Station

- 5 km to 10 km with 250 meter spacing from Coal Cree k Station

- 0 km to 5 km with 100 meters spacing from Leland Ol ds Station

- 5 km to 10 km with 250 meter spacing from Leland Ol ds Station

The receptor network contained 30,163 receptors, an d the network covered portions of Mercer, Oliver, and McLean counties in North Dakota. Figure s 4 and 5, which was included in the State’s recommendation, show the area surrounding t he Coal Creek and Leland Olds Stations, as well as receptor grid, assumed for the modeling ana lysis.

Consistent with the Modeling TAD, receptors for the purposes of this designation effort were placed only in areas where it would also be feasibl e to place a monitor and record ambient air impacts. The impacts of the area’s geography and to pography will be discussed later within this
document.

Figure 4: Area of Analysis1112

Figure 5: Receptor Grid for the Area of Analysis

Modeling Parameter: Source Characterization

The State characterized the sources within the area of analysis in accordance with the best practices outlined in the Modeling TAD. Specificall y, the State used actual stack heights in conjunction with actual emissions for the Coal Cree k and Stanton stations. The State also followed EPA’s good engineering practices (GEP) pol icy in conjunction with allowable
emissions limits for the Leland Olds station. A GEP stack height analysis was performed for the sources with US EPA’s Building Profile Input Progra m (BPIP). BPIP was used to develop the building information to simulate building downwash in the modeling analysis. The building layout and location, as well as the stack parameter s (e.g., exit temperature, exit velocity, location, and diameter), were adequately characterized in the modeling analysis. Table 2 presents the assumed source characterizations for each facility provided by the State.

Table 2. Modeling Source Parameters
Parameter Leland Olds Station

(unit 1 and unit 2
modeled as a combined source) Stanton Station

(unit 1 and unit 10
modeled as a combined source) Coal Creek Station

(unit 1 and unit 2 modeled separately)

Stack Height 182.88 m 77.724 m 205.74 m13

Exit Temperature 335 K Actuals - hourly Actuals - hourly

Exit Velocity 21.0 m/sec Actuals - hourly Actuals - hourly
Diameter 9.97 m 4.6 m 7.8 m
Base Elevation 519 m 517 m 591 m

Modeling Parameter: Emissions

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

The EPA believes that continuous emissions monitori ng systems (CEMS) data provide acceptable historical emissions information, when i t is available, and that these data are available for many electric generating units. In the absence of CEMS data, the EPA’s Modeling TAD highly encourages the use of AERMOD’s hourly varyin g emissions keyword HOUREMIS, or through the use of AERMOD’s variable emissions fact ors keyword EMISFACT. When choosing one of these methods, the EPA believes that detaile d throughput, operating schedules, and emissions information from the impacted sources sho uld be used.

In certain instances, states and other interested p arties may find that it is more advantageous or simpler to use PTE rates as part of their modeling runs. Specifically, a facility may have recently adopted a new federally enforceable emissions limit , been subject to a federally enforceable consent decree, or implemented other federally enfo rceable mechanisms and control technologies to limit SO 2 emissions to a level that indicates compliance wit h the NAAQS. These new limits or conditions may be used in the applica tion of AERMOD. In these cases, the Modeling TAD notes that the existing SO 2 emissions inventories used for permitting or SIP planning demonstrations should contain the necessar y emissions information for designationsrelated modeling. In the event that these short-ter m emissions are not readily available, they may be calculated using the methodology in Table 8-1 of Appendix W to 40 CFR Part 51 titled, “Guideline on Air Quality Models.”

As previously noted, the State included the Coal Cr eek, Leland Olds, and Stanton stations in the area of analysis. North Dakota selected these facil ities because the State believes that this area of analysis adequately represents the area where maxim um concentrations of SO 2 are expected and adequately includes the sources which might contrib ute to those concentrations. The State determined that no other sources beyond 10 km would have the potential to cause significant concentration gradients within the area of analysis , citing EPA’s March 1, 2011 Memorandum and the analysis presented at a 2011 EPA modeling w orkshop. For this area of analysis, the State has opted to use a hybrid approach, where emissions from certain facilities are expressed as actual emissions, and those from other facilities a re expressed as allowable rates.

For the Coal Creek and Stanton stations, the State included annual actual SO 2 emissions based on
CEMs data between 2012 and 2014. Coal Creek Station features two units with a total generation14

capacity of more than 1100 megawatts. Each unit was modeled separately. Stanton Station has one turbine generator rated at 188 megawatts that i s supplied by two boilers. Emissions from the two boilers are exhausted through a single stack.

For Leland Olds Station, the State modeled the faci lity using the most recent federally
enforceable PTE limits for SO 2 . Leland Olds Station consists of two coal-fired un its; unit 1 is a 220-megawatts unit (2622 mmbtu/hr), and unit 2 is a 440-megawatt (5130 mmbtu/hr) unit. The two boilers’ emissions are exhausted into a single 600-foot dual flue stack. Basin Electric
installed wet scrubbers to control SO 2 emissions on both units and redirected the exhaust s from two separate stacks into a common stack. Therefore, using a 3-year modeling period would not be representative of the current and future SO 2 emissions at this station. The State determined
that a PTE rate of 0.15 lb/mmbtu of SO 2 (1162.8 lb/hr) would be appropriate for the analys is because there is not yet a full three-year record o f actual post-wet scrubber SO 2 emissions. To be conservative, the State also assumed a constant max imum post-scrubber SO 2 emission rate of 1162.8 lb/hr for the entire model simulation, which overstates the emissions after the scrubber installation. An equivalent diameter for the two fl ues and the use of the 95 th percentile flow rate and temperature were also assumed to represent the post-scrubber conditions for each hour of the
model simulation. The EPA notes that the PTE limit for Leland Olds Station was the result of wet scrubbers being installed at the facility, whos e limits have been approved into the SIP under the regional haze section (77 FR 20894, April 6, 20 12). The PTE limits became effective in June
2013 for unit 1 and October 2012 for unit 2. Table 3 summarizes the SO 2 emissions assumed for each source in the modeling analysis.

Table 3: SO 2 Emissions Between 2012 – 2014 from Coal Creek, Sta nton, and Leland Olds
stations.

Facility Name Units Type of
Emissions SO 2 Emissions (tons per year)

2012 2013 2014

Coal Creek Station Unit 1 Actuals 8030.85 8241.61 7713.68

Unit 2 Actuals 8240.38 7340.04 7900.64

Stanton Station Unit 1/Unit 10 Modeled as Single

Unit Actuals 2379.36 2061.10 2573.12

Leland Olds Station 1 Unit 1/Unit 2 Modeled as Single

Unit PTE 5093.075 5093.075 5093.075

1 BART Permit SO 2 Limit – 0.15 lb/mmbtu: Unit 1 rated at 2662 mmbtu/ hr and Unit 2 rated at 5130 mmbtu/hr.

The emissions rate used for Leland Olds (1162.8 lb/ hr) was based on continuous operation at the facility’s SIP-approved maximum allowable 30-day ro lling average rate of 0.15 lb/mmbtu. However, to properly account for short-term emissio ns spikes that can impact a one-hour rate but be smoothed out over a 30-day rate, the EPA recomme nds that an adjustment factor be applied to the modeled hourly emissions rate (See EPA’s April 23, 2014 SO 2 Nonattainment Area Guidance at 25-37, and Appendices B, C and D). AECO M did not apply such a factor when modeling Leland Olds. Therefore, EPA finds that the AECOM modeling analysis cannot be relied upon for the purposes of designating the are a of McLean County (full) and Mercer County (partial) as attainment, as the State recommended. Should the State submit an updated modeling analysis which meets EPA guidance and includes an a ppropriately adjusted emission rate for Leland Olds, EPA may base its final designation on that new information.15

Modeling Parameter: Meteorology and Surface Charact eristics

The most recent 3 years of meteorological data (con current with the most recent 3 years of emissions data) should be used in designations effo rts. As noted in the Modeling TAD, the selection of data should be based on spatial and cl imatological (temporal) representativeness. The representativeness of the data are based on: 1) the proximity of the meteorological monitoring site to the area under consideration, 2) the complexity of terrain, 3) the exposure of the meteorological site, and 4) the period of time during which data are collected. Sources of meteorological data include National Weather Servic e (NWS) stations, site-specific or onsite data, and other sources such as universities, Feder al Aviation Administration (FAA), and military stations.

The State determined that surface meteorological da ta from the state-operated Beulah 10-meter tower and upper air observations from a NWS site in Bismarck, North Dakota would best represent the meteorological conditions within the area of analysis. Missing surface data and upper air soundings were substituted with data from the Garrison airport and Glasgow station, respectively. The State selected meteorological dat a for the period 2012 to 2014, which is concurrent with the emissions period.

The State used AERSURFACE version 13016 using data primarily around the Beulah tower and a secondary set around the NWS Hazen airport to est imate the surface characteristics of the area of analysis. A 1-km radius circular area centered a t the Beulah primary meteorological station site was assumed and divided into twelve sectors, e ach with its own homogeneous land use that was distinctly different from the other sectors. As recommended in the AERSURFACE User's Guide, the State determine the surface moisture con dition for each season by comparing precipitation for the period of data to be processe d to the 30-years of Garrison airport (Hazen airport precipitation data has poor data capture an d, therefore, was not used) precipitation records, selecting "wet" conditions if precipitatio n is in the upper 30th percentile, "dry" conditions if precipitation is in the lower 30th pe rcentile, and "average" conditions if precipitation is in the middle 40th percentile. The monthly designations of surface moisture input
to AERSURFACE are summarized in Table 4.

Table 4. AERSURFACE Bowen Ratio Condition Designati ons for Beulah Site.16

The State used AERSURFACE to develop seasonal categ ories by month for each modeled year and they were applied for the primary (Beulah site) and secondary (Hazen airport) site, as shown
in Table 5. A month was selected as a "winter with continuous snow on the ground" if a month had at least half of the days with recorded snow on the ground. Daily snow cover records were obtained for the Garrison and Bismarck airports fro m the National Climatic Data Center (NCDC).

Table 5. Selected Seasonal Categories for AERSURFAC E.

As part of its recommendation, the State provided t he 3-year surface wind rose for Beulah, North Dakota. In Figure 6, the frequency and magnitude of wind speed and direction are defined in terms of from where the wind is blowing. Based on t he wind rose, the meteorological conditions
include:

• average wind speed: 5 meters per second;

• calm winds occur about six percent of the time; and

• predominant winds are from the northwest (about 8% of the time) and from the southeast (about 6% of the time).

Figure 6: Beulah, North Dakota Cumulative Annual Wi nd Rose for Years 2012 – 2014.17

Meteorological data from the above surface and uppe r air stations were used in generating AERMOD-ready files with the AERMET processor. The o utput meteorological data created by the AERMET processor is suitable for being applied with AERMOD input files for AERMOD modeling runs. The State followed the methodology a nd settings presented in US EPA’s SO 2 Modeling TAD, AERMOD/AERMET User Guides, and the AE RMOD Implementation Guide in the processing of the raw meteorological data into an AERMOD-ready format, and used AERSURFACE to best represent surface characteristic s.

Hourly surface meteorological data records are read by AERMET, and include all the necessary elements for data processing. However, wind data ta ken at hourly intervals may not always portray wind conditions for the entire hour, which can be variable in nature. Hourly wind data18

may also be overly prone to indicate calm condition s, which are not modeled by AERMOD. In order to better represent actual wind conditions at the meteorological tower, the State introduced Garrison airport data as 1-minute ASOS in Stage 2 o f AERMET. The 1-minute data was processed by a separate preprocessor, AERMINUTE (ve rsion 14337). These data were subsequently integrated into the AERMET processing to produce final hourly wind records of AERMOD-ready meteorological data that better estima te actual hourly average conditions and that are less prone to over-report calm wind condit ions. This allows AERMOD to apply more hours of meteorology to modeled inputs, and therefo re produce a more complete set of concentration estimates. As a guard against excessi vely high concentrations that could be produced by AERMOD in very light wind conditions, t he State set a minimum threshold of 0.5 meters per second in processing meteorological data for use in AERMOD. This approach is consistent with a March 2013 EPA memo titled, “Use of ASOS meteorological data in AERMOD dispersion Modeling.” In setting this thres hold, no wind speeds lower than this value would be used for determining concentrations. This threshold was specifically applied to the one
minute wind data.

Modeling Parameter: Geography and Terrain

The terrain in the area of analysis is best describ ed as flat. To account for these terrain changes, the AERMAP (version 11103) terrain program within A ERMOD was used to specify terrain elevations for all the receptors. The source of the elevation data incorporated into the model is

from the 10-meter USGS National Elevation Database.

Modeling Parameter: Background Concentrations of SO 2

The Modeling TAD offers two mechanisms for characte rizing background concentrations of SO 2 that are ultimately added to the modeled design val ues: 1) a “first tier” approach, based on monitored design values, or 2) a temporally varying approach, based on the 99 th percentile monitored concentrations by hour of day and season or month.

The State determined the background concentrations of SO 2 based on information provided in the EPA March 1, 2011 Memorandum and the analysis p resented at the 2011 EPA modeling workshop, specifically noting that the selection of regional background sources should be limited
to 10 kilometers from the source location. The near est large SO 2 source to Coal Creek and Leland Old stations is Stanton, which was modeled i n the analysis. The next large source is more than 20 km away (Milton R. Young Station), and this source was expected to produce a uniform background influence. Therefore, this and any more distant sources were not expected to interact with the modeled sources to cause a significant con centration gradient. For this 1-hour SO 2 NAAQS analysis, the State determined that the Stant on Station was the only background source to consider in this modeling. The total concentrati on for 1-hour SO 2 NAAQS compliance was computed by adding the Leland Olds, Coal Creek, and Stanton stations predicted concentration to the regional background concentrations from the state-approved Dunn Center monitor. The Dunn Center monitor is appropriate for characterizi ng background concentrations because it is a regional site that is located away from the area of interest but can be impacted by similar natural and distant man-made sources. The background concen tration was calculated as a 3-year (2012-19

2014) average of the 99 th percentile by season and hour-of-day and added int ernally in AERMOD to the AERMOD-predicted concentration for co mparison with the 1-hour SO 2 NAAQS of 196.5 μ g/m 3 (75 ppb 3 ). The Dunn Center seasonal SO 2 concentrations are displayed in Figure 7.

Figure 7. 2012-2014 Average 99 th Percentile Concentration at Dunn Center Monitor.

Summary of Modeling Results

The AERMOD modeling parameters for the Leland Olds, Coal Creek, and Stanton stations modeling analysis are summarized below in Table 6.

Table 6. AERMOD Modeling Parameters for the Leland Olds, Coal Creek, and Stanton stations modeling analysis.

AERMOD Version 15181 Dispersion Characteristics Rural

Modeled Sources 3

Modeled Stacks 4

Modeled Structures 4

Modeled Fencelines 0

3 The conversion factor for SO 2 (at the standard conditions applied in the ambient SO 2 reference method) is 1ppb = approximately 2.62 μ g/m 3

.20 Total receptors 30,163

Emissions Type Coal Creek: Actual
Stanton: Actual
Leland: PTE

Emissions Years Actuals: 2012-2014 (Coal Creek and Stanton) PTE: Unit 1 2013/ Unit 2 2012 (Leland

Olds) Meteorology Years 2012-2014 Surface Meteorology Station Beulah, North Dakota Upper Air Meteorology Station Bismarck, North Dakot a Methodology for Calculating Background SO 2 Concentration Temporal Varying Calculated Background SO 2 Concentration 0.80 ppb to 4.07 ppb

The State’s modeling indicates that the predicted 9 9 th percentile 1-hour average concentration within the selected modeling domain is 163.8 μ g/m 3 . This modeled concentration included the background concentration of SO 2 , and is based on actual emissions from the Coal Cr eek and Stanton stations, and PTE emissions from the Leland Olds Station. The results presented below in Table 7 show the magnitude and geographic locati on of the highest predicted modeled concentration from the modeling analysis.

Table 7. Maximum Predicted 99th Percentile 1-Hour S O 2 Concentration

Averaging Period Data Period Receptor Location SO 2 Concentration ( μ g/m 3 )

UTM/Latitude UTM/Longitude Modeled (including background) NAAQS
99th Percentile 1-Hour Average 2012-2014 320045 5238393 163.8 196.5 * *Equivalent to the 2010 SO 2 NAAQS set at 75 ppb.

Figure 8 below was included as part of the State’s recommendation, and indicates that the predicted value occurred northwest of the Coal Cree k station. The State also provided modeling analyses that were based on modeling that used non- default or beta configuration options for treating low-wind conditions. These non-default opt ions include adjustments to the computation of the friction velocity (ADJ_U*) in the AERMET met eorological pre-processor and lateral wind speed standard deviation computations incorporated into AERMOD ("LOWWIND3" option). While the State provided these additional analyses, these analyses were not evaluated or approved by EPA prior to the utilization of these o ptions in the modeling. At this time, EPA will only support the modeling analyses that used the cu rrent regulatory defaults to characterize SO 2 concentrations for the designations due July 2, 201 6.21

Figure 8: Maximum Predicted 99 th Percentile 1-Hour SO 2 Concentrations from the Modeling Analysis.

Jurisdictional Boundaries:

Once the geographic area of analysis associated wit h the Coal Creek, Leland Olds, Stanton stations, and background concentration is determine d, existing jurisdictional boundaries are considered for the purpose of informing our intende d unclassifiable/attainment area, specifically with respect to clearly defined legal boundaries.

The EPA notes that our intended unclassifiable area extends only into a portion of Mercer County. This is due to the fact that the consent de cree also obligates us to designate the area around another source in Mercer County, i.e., Coyot e Station. A discussion and analysis of our intended designation and associated boundaries for that area is elsewhere in this document. Except for Coal Creek Station, there are no other s ources within McLean County that according22

to the 2011 NEI, emit at or above 100 tpy of SO 2 . As a result, the EPA believes that a countywide designation of unclassifiable is reasonable, a s sources within McLean County are unlikely to cause or contribute to a violation of the NAAQS within the county or its neighboring areas.

The EPA believes that our intended unclassifiable, consisting of the entirety of McLean County, North Dakota, and a portion of Mercer County as des cribed in Table 1, is comprised of clearly defined legal boundaries, and we find these boundar ies to be a suitably clear basis for defining
our intended unclassifiable.

Other Relevant Information

EPA did not receive any additional information on t his designation aside from that submitted by
the State.

Conclusion

After careful evaluation of the State’s recommendat ion and supporting information, as well as all available relevant information, the EPA intends to designate the area around the Coal Creek,
Leland Olds and Stanton stations as unclassifiable for the 2010 SO 2 NAAQS. Specifically, the boundaries are comprised of McLean County, and with in Mercer County: the area east of CR37/ND 31, east/north of ND 200 ALT, west of the eas tern border of Mercer County/Missouri River, and south of the Knife River National Histor ic Site (see Figures 1-3).

As previously discussed, the emissions rate for Lel and Olds was based on continuous operation at the facility’s SIP-approved maximum allowable 30 -day rolling average rate of 0.15 lb/mmbtu. However, no adjustment was made to this rate, consi stent with the EPA’s guidance, to identify a
critical value of 1-hour emissions and to demonstra te that the 30-day emissions rate is of comparable stringency to that critical value. As a result, the EPA believes that the modeling performed by the State may not be a reliable indica tor of attainment status, especially with respect to impacts from the Leland Olds station. Th erefore, EPA is unable to determine at this time whether the area is attaining the NAAQS. It sh ould be noted that our intended unclassifiable area that includes a portion of Mercer County does not comprise the entire county, due to the fact that the court order also directs us to designate t he area around Coyote Station. Coyote Station is also located in Mercer County, and a discussion and analysis of our intended designation and
associated boundaries is elsewhere in this document . At this time, our intended designations for the State only apply to this area and the other are a presented in this technical support document. Consistent with the conditions in the March 2, 2015 court-ordered schedule, the EPA will evaluate and designate all remaining undesignated a reas in North Dakota by either December 31, 2017, or December 31, 2020.

Technical Analysis for Central Mercer County, North Dakota (Area Surrounding Coyote Station)

Introduction23

The central portion of Mercer County, North Dakota contains a stationary source that according
to the EPA’s Air Markets Database emitted in 2012 e ither more than 16,000 tons of SO 2 or more than 2,600 tons of SO 2 and had an annual average emission rate of at leas t 0.45 pounds of SO 2 per one million British thermal units (lbs SO 2 /mmBTU). As of March 2, 2015, this stationary source had not met the specific requirements for be ing “announced for retirement.” Specifically, in 2012, the Coyote Station emitted 10,639 tons of SO 2 , and had an emissions rate of 0.79 lbs
SO 2 /mmBTU. Pursuant to the March 2, 2015 court-ordered schedule, the EPA must designate the area surrounding the facility by July 2, 2016.

In its submission, North Dakota recommended that th e area surrounding the Coyote Station be designated as attainment based on an assessment and characterization of air quality from the facility and other nearby sources which may have a potential impact in the area of analysis where
maximum concentrations of SO 2 are expected. This assessment and characterization was performed using air dispersion modeling software, i .e., AERMOD, analyzing actual emissions. After careful review of the State’s assessment, sup porting documentation, and all available data, the EPA agrees that the area is attaining the NAAQS , and intends to designate it as unclassifiable/attainment. However, because the Sta te did not provide specific boundaries in its recommendation, the EPA’s intended boundaries consi st of the area west of ND 49/61 st Ave SW,
north of Co. Rd 15/17 th St. SW, east of Co. Rd 13, south and east of the t own Zap, and south of

8 th St. SW/ND 200.

The Coyote Station is located in central North Dako ta in the central portion of Mercer County. As seen in Figure 9 below, the facility (indicated by a red star) is located approximately 3 km southwest of Beulah, North Dakota. Also included in the figure are nearby emitters of SO 2 (none) and the EPA’s intended unclassifiable/attain ment designation for the area (shown in yellow).

Figure 9. The EPA’s intended designation for Centra l Mercer County, North Dakota24

The discussion and analysis that follows below will reference the state’s use of the Modeling TAD, the EPA’s assessment of the State’s modeling i n accordance with the Modeling TAD, and
the factors for evaluation contained in the EPA’s M arch 20, 2015 guidance, as appropriate.

Detailed Assessment

Air Quality Data

This factor considers the SO 2 air quality monitoring data in the area (within 20 km) of Coyote Station. The facility is located in Mercer County, and the state included the most recent 3 years of monitoring data from the five SO 2 monitors currently operating in Mercer County. The se monitors are all located north of Coyote Station. T he table below shows information provided by
the state related to these monitors.

Table 8: Available Air Quality Data for all SO 2 Monitors in Mercer County25 County State

Recommendation Monitor ID Monitor Location 2012-2014
SO 2 Design Value in ppb Mercer Attainment 38-057-0004 6024 Highway 200 23 Mercer Attainment 38-057-0102 Dgc #12 33 Mercer Attainment 38-057-0118 Dgc #14 26 Mercer Attainment 38-057-0123 Dgc # 16 21 Mercer Attainment 38-057-0124 Dgc # 17 19

Based on available ambient air quality collected at these monitors, the county surrounding Coyote Station does not show a violation of the 201 0 SO 2 NAAQS. North Dakota used this monitoring information as part of the showing, whic h combined with the modeling information below indicates that the area around Coyote Station should be designated as attainment for the

2010 SO 2 NAAQS. EPA does not consider these monitoring data to be conclusive as to the designation of the proposed unclassifiable/attainme nt area provided in Figure 9, because they are all located over 10 km from the Coyote Station.

Model Selection and Modeling Components

The EPA’s Modeling TAD notes that for area designat ions under the 2010 SO 2 NAAQS, the AERMOD modeling system should be used, unless use o f an alternative model can be justified. In some instances, the recommended model may be a m odel other than AERMOD, such as the BLP model for buoyant line sources. The AERMOD mode ling system contains the following components:

- AERMOD: the dispersion model

- AERMAP: the terrain processor for AERMOD

- AERMET: the meteorological data processor for AERMO D

- BPIPPRIME: the building input processor

- AERMINUTE: a pre-processor to AERMET incorporating 1-minute automated surface observation system (ASOS) wind data

- AERSURFACE: the surface characteristics processor f or AERMET

- AERSCREEN: a screening version of AERMOD

The State used the most recent version of AERMOD, 1 5181, and a discussion of the individual components will be referenced in the corresponding discussion that follows, as appropriate.

Modeling Parameter: Rural or Urban Dispersion

US EPA-recommended procedure for characterizing an area by prevalent land use is based on evaluating the dispersion environment with 3 km of the facility. According to US EPA modeling guidelines, rural dispersion coefficients are to be used in the dispersion modeling analysis if
more than 50% of the area within a 3-km radius of t he facility is classified as rural. Conversely, if more than 50% of the area is urban, urban disper sion coefficients should be used in the modeling analysis. As shown in Figure 10, the 3-km area surrounding each of the facilities is rural. Therefore, the State determined that is was most appropriate to run the model with rural dispersion coefficients for each of the facilities being modeled.26

Modeling Parameter: Area of Analysis (Receptor Grid )

The EPA believes that a reasonable first step towar ds characterization of air quality in the area surrounding the Coyote Station is to determine the extent of the area of analysis, i.e., receptor grid. Considerations presented in the Modeling TAD include but are not limited to: the location
of the SO 2 emission sources or facilities considered for mode ling; the extent of significant concentration gradients of nearby sources; and suff icient receptor coverage and density to adequately capture and resolve the model predicted maximum SO 2 concentrations. For the area near Coyote Station, the State has not included any other emitters of SO 2 within, as none lie within 10 km of the facility Station in any directi on. The State cited EPA’s March 1, 2011 Memorandum and the analysis presented at a 2011 EPA modeling workshop in support of the 10 km distance. The nearest large SO 2 sources are more than 15 km away, which would plac e them at a distance for which a uniform background influe nce would be expected. Therefore, these more distant sources would not be expected to inter act with Coyote to cause a significant concentration gradient near Coyote. In addition, as the State is aware, there are 5 monitors in the vicinity of the sources to the north (Antelope Vall ey Station and the Great Plains Synfuels Plant) that show NAAQS compliance by a wide margin. Theref ore, for this 1-hour SO 2 NAAQS analysis, no nearby background sources were conside red in the modeling. Additionally, the State determined that the 10 km area was the appropriate area in order to adequately characterize air quality from the facility and other nearby sources, which may have a potential impact in the area of analysis where maximum concentrations of SO 2 are expected. The grid receptor spacing for the area of analysis chosen by the State is as foll ows:

- Along the fenceline with 50-m spacing

- Fenceline to 5 km with 100 meters spacing

- 5 km to 10 km with 250 meters spacing

- 10 km to 20 km with 500 meters spacing

- 20 km to 50 km with 1,000 meter spacing

The receptor network contained 8,599 receptors, and an additional receptor to represent the source. The network mostly covers Mercer County, bu t portions of Dunn, Oliver, and McLean counties are also included in the analysis. Figures 10 and 11, included in the State’s recommendation, show the area of analysis surroundi ng the Coyote Station, as well as receptor grid for the area of analysis.

Consistent with the Modeling TAD, receptors for the purposes of this designation effort were placed only in areas where it would also be feasibl e to place a monitor and record ambient air impacts. The impacts of the area’s geography and to pography will be discussed later within this
document.

Figure 10. Area of Analysis27

Figure 11. Receptor Grid for the Area of Analysis28

Modeling Parameter: Source Characterization

The State characterized the source within the area of analysis in accordance with the best practices outlined in the Modeling TAD. Specificall y, for the Coyote station the State used actual stack heights in conjunction with actual emissions. The State also followed the EPA’s good engineering practices (GEP) policy in their GEP sta ck height analysis to provide input information for the building dimensions into AERMOD . A GEP stack height analysis was performed for the sources with US EPA’s Building Pr ofile Input Program (BPIP). BPIP was used to develop the building information to simulat e building downwash in the modeling analysis. A total of six buildings were included in to the GEP analysis: a multi-tiered boiler building structure (88 meters), the baghouse struct ure (24 meters), the scrubber structure (44 meters), recycle fly ash silo (32 meters), fly ash silo (43 meters), and lime silo (45 meters). The building layout and location, as well as the stack parameters (e.g., exit temperature, exit velocity, location, and diameter), were adequately characteri zed in the modeling analysis. Table 9 presents
the assumed source characterizations for each facil ity provided by the State.

Table 9. Modeling Source Parameters Parameter Stack Height Exit Temperature Exit Veloci ty Diameter Base Elevation Coyote Station 151.79 m 386.48 K 33.89 m/sec 6.40 m 590.52 m29

Modeling Parameter: Emissions

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

The EPA believes that continuous emissions monitori ng systems (CEMS) data provide acceptable historical emissions information, when i t is available, and that these data are available for many electric generating units. In the absence of CEMS data, the EPA’s Modeling TAD highly encourages the use of AERMOD’s hourly varyin g emissions keyword HOUREMIS, or through the use of AERMOD’s variable emissions fact ors keyword EMISFACT. When choosing one of these methods, the EPA believes that detaile d throughput, operating schedules, and emissions information from the impacted sources sho uld be used.

In certain instances, states and other interested p arties may find that it is more advantageous or simpler to use PTE rates as part of their modeling runs. Specifically, a facility may have recently adopted a new federally enforceable emissions limit , been subject to a federally enforceable consent decree, or implemented other federally enfo rceable mechanisms and control technologies to limit SO 2 emissions to a level that indicates compliance wit h the NAAQS. These new limits or conditions may be used in the applica tion of AERMOD. In these cases, the Modeling TAD notes that the existing SO 2 emissions inventories used for permitting or SIP planning demonstrations should contain the necessar y emissions information for designationsrelated modeling. In the event that these short-ter m emissions are not readily available, they may be calculated using the methodology in Table 8-1 of Appendix W to 40 CFR Part 51 titled, “Guideline on Air Quality Models.”

As previously noted, North Dakota included only the Coyote Station in the area of analysis. The State selected only this facility because there are no other SO 2 sources within 10 km of the facility, and the State believes that this area of analysis adequately represents the area where
maximum concentrations of SO 2 are expected and adequately includes the sources w hich might
contribute to those concentrations. No other source s beyond 10 km were determined by the State to have the potential to cause significant concentr ation gradients within the area of analysis.

For the Coyote Station, the State included annual a ctual SO 2 emissions based on CEMs data between 2012 and 2014. Coyote Station is a 427-mega watt (net) coal-fired power plant located in Beulah, North Dakota. The station operates a sin gle boiler exhausting through a 151-meter tall
stack.

Table 10: SO 2 Emissions Between 2012 – 2014 from Coyote Station.

Facility Name Units Type of Emissions SO 2 Emissions (tons per year)

2012 2013 2014 Coyote Station Unit 1 Actuals 10645.58 12584.67 12785.7630

Modeling Parameter: Meteorology and Surface Charact eristics

The most recent 3 years of meteorological data (con current with the most recent 3 years of emissions data) should be used in designations effo rts. As noted in the Modeling TAD, the selection of data should be based on spatial and cl imatological (temporal) representativeness. The representativeness of the data are based on: 1) the proximity of the meteorological monitoring site to the area under consideration, 2) the complexity of terrain, 3) the exposure of the meteorological site, and 4) the period of time during which data are collected. Sources of meteorological data include National Weather Servic e (NWS) stations, site-specific or onsite data, and other sources such as universities, the F ederal Aviation Administration (FAA), and military stations.

The State determined that surface meteorological da ta from the state-operated Beulah 10-meter tower and upper air observations from a NWS site in Bismarck, North Dakota would best represent the meteorological conditions within the area of analysis. Missing surface data and upper air soundings were substituted with data from the Garrison airport and Glasgow station, respectively. The State select meteorological data for the period 2012 to 2014, which is concurrent with the emissions period.

The State used AERSURFACE version 13016 using data primarily around the Beulah tower and a secondary set around the NWS Hazen airport to est imate the surface characteristics of the area of analysis. A 1-km radius circular area centered a t the Beulah primary meteorological station site was assumed and divided into twelve sectors, e ach with its own homogeneous land use that was distinctly different from the other sectors. As recommended in the AERSURFACE User's Guide, the State determine the surface moisture con dition for each season by comparing precipitation for the period of data to be processe d to the 30-years of Garrison airport (Hazen airport precipitation data has poor data capture an d, therefore, was not used) precipitation records, selecting "wet" conditions if precipitatio n is in the upper 30th percentile, "dry" conditions if precipitation is in the lower 30th pe rcentile, and "average" conditions if precipitation is in the middle 40th percentile. The monthly designations of surface moisture input
to AERSURFACE are summarized in Table 11.

Table 11. AERSURFACE Bowen Ratio Condition Designat ions for Beulah Site.31

The State used AERSURFACE to develop seasonal categ ories by month for each modeled year and they were applied for the primary (Beulah site) and secondary (Hazen airport) site, as shown
in Table 12. A month was selected as a "winter with continuous snow on the ground" if a month had at least half of the days with recorded snow on the ground. Daily snow cover records were obtained for the Garrison and Bismarck airports fro m the National Climatic Data Center (NCDC).

Table 12. Selected Seasonal Categories for AERSURFA CE.

As part of its recommendation, the State provided t he 3-year surface wind rose for Beulah, North Dakota. In Figure 12, the frequency and magnitude o f wind speed and direction are defined in terms of from where the wind is blowing. Based on t he wind rose, the meteorological conditions
include:

• average wind speed: 5 meters per second;

• calm winds occur about six percent of the time; and

• predominant winds are from the northwest (about 8% of the time) and from the southeast (about 6% of the time).

Figure 12. Beulah, North Dakota Cumulative Annual W ind Rose for Years 2012 – 2014.32

Meteorological data from the above surface and uppe r air stations were used in generating AERMOD-ready files with the AERMET processor. The o utput meteorological data created by the AERMET processor is suitable for being applied with AERMOD input files for AERMOD modeling runs. The State followed the methodology a nd settings presented in US EPA’s SO 2 Modeling TAD, AERMOD/AERMET User Guides, and the AE RMOD Implementation Guide in the processing of the raw meteorological data into an AERMOD-ready format, and used AERSURFACE to best represent surface characteristic s.

Hourly surface meteorological data records are read by AERMET, and include all the necessary elements for data processing. However, wind data ta ken at hourly intervals may not always33

portray wind conditions for the entire hour, which can be variable in nature. Hourly wind data may also be overly prone to indicate calm condition s, which are not modeled by AERMOD. In order to better represent actual wind conditions at the meteorological tower, the State introduced Garrison airport data as 1-minute ASOS in Stage 2 o f AERMET. The 1-minute data was processed by a separate preprocessor, AERMINUTE (ve rsion 14337). These data were subsequently integrated into the AERMET processing to produce final hourly wind records of AERMOD-ready meteorological data that better estima te actual hourly average conditions and that are less prone to over-report calm wind condit ions. This allows AERMOD to apply more hours of meteorology to modeled inputs, and therefo re produce a more complete set of concentration estimates. As a guard against excessi vely high concentrations that could be produced by AERMOD in very light wind conditions, t he State set a minimum threshold of 0.5 meters per second in processing meteorological data for use in AERMOD. This approach is consistent with a March 2013 EPA memo titled, “Use of ASOS meteorological data in AERMOD dispersion Modeling.” In setting this thresh old, no wind speeds lower than this value would be used for determining concentrations. This threshold was specifically applied to the one
minute wind data.

Modeling Parameter: Geography and Terrain

The terrain in the area of analysis is best describ ed as flat. To account for these terrain changes, the AERMAP (version 11103) terrain program within A ERMOD was used to specify terrain elevations for all the receptors. The source of the elevation data incorporated into the model is
from the 10-meter USGS National Elevation Database.

Modeling Parameter: Background Concentrations of SO 2

The Modeling TAD offers two mechanisms for characte rizing background concentrations of SO 2 that are ultimately added to the modeled design val ues: 1) a “first tier” approach, based on monitored design values, or 2) a temporally varying approach, based on the 99 th percentile monitored concentrations by hour of day and season or month. For the area near Coyote, the State chose the temporally varying approach. The ba ckground concentration for this area of analysis was determined by the State to be from 2.0 9 to 10.66 micrograms per cubic meter ( μ g/m 3 ), or 0.80 ppb to 4.07 ppb, 4 and that value was incorporated into the final AER MOD
results.

The State determined the background concentrations of SO 2 based on information provided in the EPA March 1, 2011 Memorandum and the analysis p resented at the 2011 EPA modeling workshop, specifically noting that the selection of regional background sources should be limited
to 10 kilometers from the source location. The near est large SO 2 sources are more than 15 km away, which would place them at a distance for whic h a uniform background influence would be expected. Therefore, these more distant sources wou ld not be expected to interact with Coyote to

4 The conversion factor for SO 2 (at the standard conditions applied in the ambient SO 2 reference method) is 1ppb = approximately 2.62 μ g/m 3

.34

cause a significant concentration gradient near Coy ote. In addition, there are 5 monitors in the vicinity of the sources to the north (Antelope Vall ey Station and the Great Plains Synfuels Plant) that show NAAQS compliance by a wide margin. The to tal concentration for 1-hour SO 2 NAAQS compliance was computed by adding the Coyote Station predicted concentration to the regional background concentrations from the state-a pproved Dunn Center monitor. The Dunn Center monitor is appropriate for characterizing ba ckground concentrations because it is a regional site that is located away from the area of interest but can be impacted by similar natural and distant man-made sources. The background concen tration was calculated as a 3-year (2012- 2014) average of the 99 th percentile by season and hour-of-day and added int ernally in AERMOD to the AERMOD-predicted concentration for co mparison with the 1-hour SO 2 NAAQS of 196.5 μ g/m 3 (75 ppb 5 ). The Dunn Center seasonal SO 2 concentrations are displayed in Figure 13.

Figure 13. 2012-2014 Average 99 th Percentile Concentration at Dunn Center Monitor.

Summary of Modeling Results

The AERMOD modeling parameters for the Coyote Stati on modeling analysis are summarized
below in Table 13.

Table 13. AERMOD Modeling Parameters for the Coyote station modeling analysis.

AERMOD Version 15181

5 Id.35

Dispersion Characteristics Rural

Modeled Sources 1

Modeled Stacks 1

Modeled Structures 1

Modeled Fencelines 1 Total receptors 8600 Emissions Type Actuals

Emissions Years 2012-2014 Meteorology Years 2012-2014 Surface Meteorology Station Beulah, North Dakota Upper Air Meteorology Station Bismarck, North Dakot a Methodology for Calculating Background SO 2 Concentration Temporal Varying Calculated Background SO 2 Concentration 0.80 ppb to 4.07 ppb

The State’s modeling indicates that the predicted 9 9 th percentile 1-hour average concentration within the selected modeling domain is 115.88 μ g/m 3 . This modeled concentration included the background concentration of SO 2 , and is based on actual emissions from the Coyote Station. The results presented below in Table 14 show the magnit ude and geographic location of the highest predicted modeled concentration from the modeling a nalysis.

Table 14: Maximum Predicted 99th Percentile 1-Hour SO 2 Concentration

Averaging Period Data Period Receptor Location SO 2 Concentration ( μ g/m 3 )

UTM/Latitude UTM/Longitude Modeled (including background) NAAQS
99th Percentile 1-Hour Average 2012-2014 289650 5235850 115.88 196. 5* *Equivalent to the 2010 SO 2 NAAQS set at 75 ppb.

Figure 14 below was included as part of the State’s recommendation, and indicates that the predicted value occurred northeast of the Coyote st ation. The State also provided modeling analyses that were based on modeling that used non- default or beta configuration options for treating low-wind conditions. These non-default opt ions include adjustments to the computation of the friction velocity (ADJ_U*) in the AERMET met eorological pre-processor and lateral wind speed standard deviation computations incorporated into AERMOD ("LOWWIND3" option). While the State provided these additional analyses, these analyses were not evaluated or approved by EPA prior to the utilization of these o ptions in the modeling. At this time, EPA will only support the modeling analyses that used the cu rrent regulatory defaults to characterize SO 2 concentrations for the designations required by Jul y 2, 2016.36

Figure 14. Maximum Predicted 99 th Percentile 1-Hour SO 2 Concentrations from the Modeling Analysis.

Jurisdictional Boundaries:

Once the geographic area of analysis associated wit h the Coyote Station and background concentration is determined, existing jurisdictiona l boundaries are considered for the purpose of informing our intended unclassifiable/attainment ar ea, specifically with respect to clearly defined legal boundaries.

The EPA has determined that there are no other sign ificant sources of SO 2 in the intended unclassifiable/attainment area emitting at or above 20 tpy but for Coyote Station. As previously mentioned, there are 2 facilities, i.e., Antelope V alley Station and Great Plains Synfuels, located approximately 8 km north of the EPA’s intended uncl assifiable/attainment area. The design values at ambient air quality monitors near these f acilities, and summarized in Table 8 above, indicate that at distances of approximately 2.5 km downwind of these facilities, the concentration
of SO 2 is approximately half of the NAAQS. As a result, t he EPA does not believe that37

emissions from either Antelope Valley Station or Gr eat Plains Synfuels are likely to cause or contribute to a violation of the NAAQS within the i ntended unclassifiable/attainment area.

The EPA believes that our intended unclassifiable/a ttainment area, consisting of County Road 13, and the outer border of Zap, ND as the western border, 8 th St. SW/ND 200 as the northern border, 61 st Ave SW/ND 49 as the eastern border, and 17 th St. SW/County Road 15 as the southern border, are comprised of clearly defined l egal boundaries, and we find these boundaries to be a suitably clear basis for defining our inten ded unclassifiable/attainment area.
Other Relevant Information

EPA did not receive any additional information on t his designation aside from that submitted by
the State.

Conclusion

After careful evaluation of the State’s recommendat ion and supporting information, as well as all available relevant information, the EPA intends to designate the area around the Coyote Station
as attainment/unclassifiable for the 2010 SO 2 NAAQS. Specifically, the boundaries are comprised of; County Road 13, and the outer border of Zap, ND as the western border, 8 th St. SW/ND 200 as the northern border, 61 st Ave SW/ND 49 as the eastern border, and 17 th St. SW/County Road 15 as the southern border (see Figur e 9).

At this time, our intended designations for the Sta te only apply to this area and the other area presented in this technical support document. Consi stent with the conditions in the March 2, 2015 court-ordered schedule, the EPA will evaluate and designate all remaining undesignated areas in North Dakota by either December 31, 2017, or December 31, 2020.