1 

Draft Technical Support Document 

Colorado 

Area Designations for the 2010 SO2 Primary National Ambient Air Quality Standard 

Summary 

Pursuant to section 107(d) of the Clean Air Act (CAA), 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 dioxide (SO2) primary national ambient air quality 

standard (NAAQS). The CAA defines a nonattainment area as one that does not meet the 

NAAQS or that contributes to a violation in a nearby 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 available information as meeting or not meeting the 

NAAQS. 

Colorado submitted updated recommendations on September 18, 2015, ahead of a July 2, 2016, 

deadline for the EPA to designate certain areas 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 SO2 NAAQS. Table 1 below lists 

Colorado’s recommendations and identifies the counties or portions of counties in Colorado 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 combination of the above. 

Table 1. State’s recommended and EPA’s intended designations 

Area Colorado’s 

Recommended 

Area Definition 

Colorado’s 

Recommended 

Designation 

EPA’s Intended 

Area Definition 

EPA’s Intended 

Designation 

Colorado 

Springs, 

Colorado 

Unspecified (Area 

around source) 

Unclassifiable El Paso County, 

Colorado 

Unclassifiable 

Morgan 

County, 

Colorado 

Unspecified (Area 

around source) 

Unclassifiable Morgan County, 

Colorado 

Unclassifiable 

 
2 

Background 

On June 3, 2010, the EPA revised the primary (health based) SO2 NAAQS by establishing a new 

one-hour standard at a level of 75 parts per billion (ppb) which is attained when the three-year 

average of the 99th percentile of one-hour daily maximum concentrations does not exceed 75 

ppb. This NAAQS was published in the Federal Register on June 22, 2010 (75 FR 35520) and is 

codified at 40 CFR 50.17. The EPA determined this is the level necessary to protect public health 

with an adequate margin of safety, especially for children, the elderly and those with asthma. 

These groups are particularly susceptible to the health effects associated with breathing SO2. The 

two prior primary standards of 140 ppb evaluated over 24 hours, and 30 ppb evaluated over an 

entire year, codified at 40 CFR 50.4, remain applicable.1 However, the EPA is not currently 

designating areas on the basis of either of these two primary standards. Similarly, the secondary 

standard for SO2, 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 not later than one year after promulgation of a 

new or revised NAAQS, state governors must submit their 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 appropriate. If a state or tribe disagrees with the EPA’s 

intended designations, they are given an opportunity within the 120 day period to demonstrate 

why any proposed modification is inappropriate. 

On August 5, 2013, the EPA published a final rule establishing air quality designations for 29 

areas in the United States for the 2010 SO2 NAAQS, based on recorded air quality monitoring 

data from 2009 - 2011 showing violations of the NAAQS (78 FR 47191). In that rulemaking, the 

EPA committed to address, in separate future actions, the designations for all other areas for 

which the Agency was not yet prepared to issue designations. 

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 the 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 California. 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. 

 

1

 40 CFR 50.4(e) provides that the two prior primary NAAQS will no longer apply to an area one year after its 

designation under the 2010 NAAQS, except that for areas designated nonattainment under the prior NAAQS as of 

August 22, 2010, and areas not meeting the requirements 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. Neither El 

Paso County nor Morgan County were designated nonattainment under the prior NAAQS or were failing to meet 

SIP Call requirements under the prior NAAQS, so the exception does not apply. 
3 

According to the court-ordered schedule, the EPA must 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 that have newly monitored violations of the 

2010 SO2 NAAQS and (2) areas that contain any stationary sources that had not been announced 

as of March 2, 2015 for retirement and that according to the EPA’s Air Markets Database 

emitted in 2012 either (i) more than 16,000 tons of SO2 or (ii) more than 2,600 tons of SO2 with 

an annual average emission rate of at least 0.45 pounds of SO2 per one million British thermal 

units (lbs SO2/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 had announced through a company public 

announcement, public utilities commission filing, consent decree, public legal settlement, final 

state or federal permit filing, or other similar means of communication, by March 2, 2015, that it 

will cease burning coal at that unit. 

The last two deadlines for completing remaining designations are December 31, 2017, and 

December 31, 2020. The EPA has separately promulgated requirements for states and other air 

agencies to provide additional monitoring or modeling information on a timetable consistent with 

these designation deadlines. We expect this information to become available in time to help 

inform these subsequent designations. These requirements were promulgated on August 21, 2015 

(80 FR 51052), in a rule known as the SO2 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 memorandum supersedes earlier designation 

guidance for the 2010 SO2 NAAQS, issued on March 24, 2011, and it identifies factors that the 

EPA intends to evaluate in determining whether areas are in violation of the 2010 SO2 NAAQS. 

The guidance also contains the factors the EPA intends 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 via ambient monitoring or dispersion modeling 

results; 2) Emissions-related data; 3) Meteorology; 4) Geography and topography; and 5) 

Jurisdictional boundaries. This guidance was supplemented by two technical assistance 

documents intended to assist states and other interested parties in their efforts to characterize air 

quality through air dispersion modeling or ambient air quality monitoring for sources that emit 

SO2. Notably, the EPA released its most recent versions of documents titled, “SO2 NAAQS 

Designations Modeling Technical Assistance Document” (Modeling TAD) and “SO2 NAAQS 

Designations Source-Oriented Monitoring Technical Assistance Document” (Monitoring TAD) 

in December 2013. 

Based on ambient air quality data collected between 2012 and 2014, no violations of the 2010 

SO2 NAAQS have been recorded in any undesignated part of the State.2 However, there are 2 

 

2

 For designations based on ambient air quality monitoring data that violates the 2010 SO2 NAAQS, the consent 

decree directs the EPA to evaluate data collected between 2013 and 2015. Absent complete, quality assured 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 monitors that have recorded a violation of the 2010 SO2 NAAQS 
4 

sources in the State meeting the emissions 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 Colorado’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 data before us. 

The following are definitions of important terms used in this document: 

1) 2010 SO2 NAAQS – The primary NAAQS for SO2 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. See 40 CFR 50.17. 

2) Design Value - a statistic computed according to the data handling procedures of the 

NAAQS (in 40 CFR part 50 Appendix T) that, by comparison to the level of the NAAQS, 

indicates whether the area is violating the NAAQS. 

3) Designated nonattainment area – an area which the EPA has determined has violated the 

2010 SO2 NAAQS or contributed to a violation in a nearby area. A nonattainment 

designation reflects considerations of state recommendations and all of the information 

discussed in this document. The EPA’s decision is based on all available information 

including the most recent 3 years of air quality monitoring 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 2010 SO2 NAAQS. 

5) Designated unclassifiable/attainment area – an area which the EPA has determined to 

have sufficient evidence to find either is attaining or is likely to be attaining the NAAQS. 

The EPA’s decision is based on all available information including the most recent 3 

years of air quality monitoring data, available modeling analysis, and any other relevant 

information. 

6) Modeled violation – a violation based on air dispersion modeling. 

7) Recommended attainment area – an area a state or tribe 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 or tribe has recommended that the 

EPA designate as unclassifiable. 

10) Recommended unclassifiable/attainment area – an area 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. 

 

during these years have the option of submitting complete, quality assured and certified data for calendar year 2015 

by April 19, 2016to the EPA for evaluation. If after our review, the ambient air quality data for the area indicates 

that no violation of the NAAQS occurred between 2013 and 2015, the consent decree does not obligate the EPA to 

complete the designation. Instead, we may designate the area and all other previously undesignated areas in the State 

on a schedule consistent with the prescribed timing of the court order, i.e., by December 31, 2017, or December 31, 

2020. 
5 

Technical Analysis for the Colorado Springs, Colorado Area 

Introduction 

The Colorado Springs area contains a stationary source that according to the EPA’s Air Markets 

Database emitted in 2012 either more than 16,000 tons of SO2 or more than 2,600 tons of SO2

and had an annual average emission rate of at least 0.45 pounds of SO2 per one million British 

thermal units (lbs SO2/mmBTU). As of March 2, 2015, this stationary source had not met the 

specific requirements for being “announced for retirement.” Specifically, in 2012, the Martin 

Drake Power Plant emitted 4,792 tons of SO2, and had an emissions rate of 0.56 lbs 

SO2/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, Colorado recommended that the area surrounding the Martin Drake Power 

Plant be designated as unclassifiable 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 SO2 are expected. This assessment and 

characterization was based on consideration of the data available to the State, including attaining 

monitoring data with fewer than 3 total years of data that Colorado did not determine was located 

in the area of maximum concentration. After careful review of the State’s assessment, supporting 

documentation, and all available data, the EPA agrees with the State’s recommendation for the 

area, and intends to designate the areas as unclassifiable. However, because the State did not 

provide specific boundaries in its recommendation, the EPA’s intended boundary consists of El 

Paso County. 

The Martin Drake Power Plant is located in Colorado Springs, Colorado in the western portion of 

El Paso County. As seen in Figure 1 below, the facility is located in southwestern Colorado 

Springs. Also included in the figure are nearby emitters of SO2 (specifically the Nixon Power 

Plant, indicated in green). Figure 2, below, shows the intended unclassifiable area (El Paso 

County) in blue, with a red star which indicates the location of the Martin Drake facility. 

Figure 1. The EPA’s intended designation for El Paso County, Colorado 
6 

Figure 2. The EPA’s intended unclassifiable designation for El Paso County, Colorado 

 
7 

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 in accordance with the Modeling TAD, and 

the factors for evaluation contained in the EPA’s March 20, 2015 guidance, as appropriate. 

Detailed Assessment 

Air Quality Data 

This factor considers the SO2 air quality monitoring data in the area surrounding Martin Drake 

Power Plant. The facility is located in El Paso County, and the State included all available 

monitoring data from the one SO2 monitor currently operating in El Paso County. This monitor, 

which began collecting SO2 data in 2013, is located about .5 miles northwest of the Martin Drake 

Power Plant. The table below shows information provided by the State related to the monitor 

located in El Paso County. 

Table 2. Available Air Quality Data for the Area Closest to Martin Drake Power Plant 

County State 

Recommendation

Air Quality 

Systems 

(AQS) 

Monitor ID 

Monitor Location Distance to 

Martin Drake 

Power Plant 

(km) 

2013 – 2015 

SO2 Design 

Value in ppb 

El Paso Unclassified 08-041-0015 690 W. Highway 

24 

.8 km 56 ppb 
8 

Based on available ambient air quality collected at the Highway 24 monitor, the county 

surrounding Martin Drake Power Plant does not show a violation of the 2010 SO2 NAAQS at its 

monitor. Colorado did not attempt to use the Highway 24 monitoring data to justify an 

attainment designation, nor did the State attempt to demonstrate that the monitor is appropriately 

sited at a point of expected maximum SO2 concentration. EPA therefore does not consider the

Highway 24 data to be a sufficient technical justification to rule out that an exceedance of the 

2010 SO2 NAAQS may occur at another location near the facility. 

Emissions and Emissions-Related Data 

Evidence of SO2 emissions from the source meeting the emissions criteria of the March 2, 2015 

consent decree, i.e., Martin Drake Power Plant, is an important factor for determining whether 

the immediate area is experiencing elevated levels of SO2 concentrations. Other considerations 

for this factor include county level SO2 emissions data and data for sources located within 50 km. 

As part of its recommendation, Colorado included the annual emissions for all point sources in 

El Paso County emitting at or above 0.5 tons per year of SO2. Colorado obtained the data for 

these sources and their emissions from the yearly facility reports that are submitted to the State. 

These emissions data are summarized below. 

Table 3: SO2 Emissions from Martin Drake Power Plant and Other Local Sources 

County Company Distance to Martin 

Drake Power Plant 

2013 Actual SO2

Emissions 

(tons/year) 

El Paso Colorado Springs Utilities - 

Martin Drake Power Plant 

 4,580.3 

El Paso Colorado Springs Utilities – 

Nixon Power Plant 

24 km 3,955.1 

El Paso Fort Carson US ARMY 7.5 km 34.5 

El Paso Colorado Springs Utilities – 

Clear Spring Ranch 

23.5 km 23.6 

El Paso Schmidt Construction Company 

– Delta Drive Facility 

6.5 km 23.2 

El Paso Kiewit – Colorado Springs 

Asphalt Plant 

 16.0 

El Paso Martin Marietta Materials – 

Fillmore 

 8.2 

El Paso Schmidt Construction Company 

– Menzer Facility 

 3.7 

El Paso Penrose Community Hospital 3.7 

El Paso U.S. Air Force Academy 2.5 

El Paso Peterson Air Force Base 2.4 

El Paso Homeward Bound 2.3 
9 

El Paso Rocky Mountain Materials & 

Asphalt Inc. 

 2.2 

El Paso Fountain Landfill 1.6 

El Paso Federal Express – Colorado 

Tech 

 1.2 

El Paso Fountain Valley Power – Boca 

Raton 

 1.1 

El Paso Pikes Peak Pet Crematory 1.0 

El Paso Cheyenne Mountain Zoo 0.8 

El Paso Holt Family Funeral Homes 0.7 

El Paso UCH-MHS Central 0.6 

El Paso Hewlett-Packard Corporation 0.6 

El Paso Color Star Growers – Peyton 

Greenhouse 

 0.6 

* Distance was only measured for sources with emissions equal to or greater than 20 tons per 

year 

Emissions Controls 

The EPA recognizes that control strategies implemented after the release of the 2011 NEI may 

not be reflected, or may warrant further discussion. The State indicated that Colorado Springs 

Utilities (CSU) is currently in the process of installing SO2 controls on Units 5, 6 and 7 to 

comply with Regional Haze emission limits. At the time of this analysis, CSU plans to install dry 

sorbent injection (DSI) on Unit 5 and lime spray dryers or equivalent on Units 6 and 7 with 

target control operational dates by December 31, 2016 and compliance dates no later than 

December 31, 2017. 

Meteorology (Weather & Transport Patterns) 

In its designation recommendation, Colorado indicated that there are no available representative 

meteorological datasets for the transport and dispersion conditions at the Martin Drake Power 

Plant. Following its designation recommendation, Colorado submitted to EPA a document 

describing why it did not consider the nearest available meteorological data, at the Colorado 

Springs Airport over 11 kilometers east of the Martin Drake Power Plant, to be representative of 

EPA’s Guideline on Air Quality Models (40 CFR Part 51 Appendix W; November 2005). This 

document, “Meteorological Determination for the Martin Drake Power Plant,” was submitted to 

EPA on October 6, 2015. 

In its “Meteorological Determination” document, Colorado noted that winds at/near Martin 

Drake are expected to flow up the valley of Fountain Creek during most daytime hours and up 

the valley of Monument Creek at a much lower frequency due to differential heating and the 

proximity of the power plant to Pikes Peak (see Figure 3, below). During nighttime and early 

morning hours, drainage winds flowing from the valleys of Fountain Creek and Monument 

Creek towards the power plant are expected.
10 

Figure 3. Expected Wind Directions at the Martin Drake 

Power Plant and Colorado Springs Airport 

By contrast, the State determined that the upslope and downslope winds at the Colorado Springs 

Airport are driven by the higher terrain to the north of the airport, the Palmer Divide. There are 

no other significant terrain features that influence the winds at the airport. This makes the wind 

directions at the airport predominantly northerly (downslope) and southerly (upslope). 

The State also indicated that during inversion conditions with light surface winds during which 

the highest impacts from the power plant are expected to occur, the light drainage winds 

transporting the plumes from Martin Drake are especially likely to follow along Fountain Creek. 

Finally, Colorado stated that wind speeds at the airport are expected to be higher than at the 

Martin Drake Power Plant. 

All of these factors led Colorado to conclude that there are no available meteorological datasets 

that are representative of transport and dispersion conditions of the Martin Drake Power Plant 

plumes due to the differences in meteorological conditions between the Colorado Springs Airport 
11 

and the Martin Drake Power Plant. As a result, Colorado Springs Utilities erected a sound 

detection and ranging (SODAR) tower just south of Martin Drake in fall 2015 in order to gather 

representative meteorological data to more accurately inform future modeling. 

EPA agrees with Colorado’s determination that the meteorological data from the Colorado 

Springs Airport are not representative of meteorological conditions at the Martin Drake Power 

Plant. Therefore, EPA does not consider modeling which relies on meteorological data from the 

Colorado Springs Airport to be sufficient as the sole basis for a designation of the area impacted 

by emissions from the Martin Drake Power Plant. A discussion and analysis of other information 

the EPA received, including air dispersion modeling relying on these non-representative 

meteorological conditions, follows in the section titled, “Other Relevant Information.” 

Geography and Topography (Mountain Ranges or Other Air Basin Boundaries) 

The Martin Drake Power Plant is located near the confluence of two large creek drainages; 

Fountain Creek and Monument Creek. It is about two miles west of Rocky Mountain foothills, 

which rise dramatically in elevation. The wind flows impacting the plumes at Martin Drake 

Power Plant generally follow Fountain Creek. 

Jurisdictional Boundaries 

Once the geographic area associated with the immediate area surrounding the Martin Drake 

Power Plant, other nearby sources, and background concentrations are determined, existing 

jurisdictional boundaries are considered for the purpose of informing our intended unclassifiable 

area. Specific attention will be given to clearly defined legal boundaries. 

The EPA believes that our intended unclassifiable area, consisting of El Paso County, is 

comprised of clearly defined legal boundaries, and we find these boundaries to be a suitably clear 

basis for defining our intended unclassifiable area. 

Other Relevant Information 

The EPA received air dispersion modeling results from the Sierra Club (with modeling 

conducted by Wingra Engineering, S.C.), in which the submitter asserts that SO2 emissions from 

the Martin Drake Power Plant, when considered alone or in tandem with other local sources, are 

causing a violation of the NAAQS. 

In its modeling analysis, Sierra Club used meteorological data from the Colorado Springs 

Airport. As noted, the State has determined that the “meteorological data from the Colorado 

Springs Airport is not representative of transport and dispersion conditions of the Drake Power 

Plant plumes due to the differences in meteorological conditions at the Colorado Springs Airport 

and the Drake Power Plant. The use of the Colorado Springs Airport data to model the Drake 

Power Plant is inconsistent with the TAD and Appendix W.” EPA agrees with Colorado’s 

conclusion. 
12 

In addition to its review of meteorological data, the state of Colorado reviewed the methodology 

and model inputs in the Sierra Club’s modeling evaluation to determine if they were consistent 

with recommendations in the Modeling TAD, procedures for regulatory applications of 

AERMOD that are addressed in U.S. EPA’s Guideline on Air Quality Models ‐ Appendix W to 

40 CFR 51 (Appendix W) and the U.S. EPA’s AERMOD Implementation Guide (AIG), and 

relevant clarification memos issued by U.S. EPA.3 Based on its review, the state of Colorado 

determined that several inputs were inconsistent with EPA guidance. Specifically, the State 

concluded; 

1. A downwash analysis was not used in Sierra Club’s evaluation. Building parameters 

(including downwash) should be used in the modeling since there are structures at both 

plants that will cause plume downwash. 

2. The National Elevation Dataset (NED) in the Sierra Club evaluation was developed using 

base data from 1947. Since all of the modeled emission units were built after this date, 

plant survey data would provide more accurate base elevations. 

3. Sierra Club’s evaluation assumed that Martin Drake is located in a rural area based on 

examination of land use/cover. Colorado concluded that it would be more appropriate to 

model Drake as an urban source using the population density procedure and 2010 Census 

Tract data. 

4. Receptor elevations and hill heights were determined using the NED in the Sierra Club 

evaluation. Colorado noted use of higher resolution NED is more appropriate. 

5. A flagpole height of 1.5 meters was used for all receptors in the Sierra Club evaluation. 

Colorado concluded that ground-level receptors would be more appropriate per the TAD 

and Appendix W. 

 

Similar to the state of Colorado, EPA also has concerns with the assumptions used in the 

modeling conducted by the Sierra Club to support a nonattainment classification. EPA finds that 

certain modeling inputs do not conform to EPA guidance or are not sufficiently supported for use 

in AERMOD for the characterization of ambient air quality with significant SO2 emission 

sources. Specifically, the components of the Sierra Club modeling analysis that EPA finds 

inconsistent with EPA guidance or are not sufficiently supported include: 

1. The use of a version of AERMOD/AERMET dated 14134 and AERMINUTE dated 

14237. In a proposed rulemaking published in the July 29, 2015 Federal Register (80 FR 

45340), EPA released a revised version of AERMOD (15181), which replaces the 

previous version of AERMOD dated 14134. The latest version of AERMINUTE is also 

14337. The latest release of the AERMOD platform is recommended for use in regulatory 

applications. 

2. The use of receptors placed 1.5 meters above ground-level. EPA Modeling TAD and 

Appendix W do not recommend that receptors be placed at levels other than ground level 

for comparison to the NAAQS. 

 

3

 Colorado considered the memos “Applicability of Appendix W Modeling Guidance for the 1‐hour SO2 NAAQS”, 

August 23, 2010; “Additional Clarification Regarding Application of Appendix W Modeling Guidance for the 1‐

hour NO2 National Ambient Air Quality Standard” March 1, 2011.
13 

3. The use of actual stacks heights for modeling actuals and allowable emissions. For the 

purposes of modeling with actual emissions to characterize air quality for use in the SO2

designations process, EPA guidance recommends the use of actual stack heights. If 

allowable emissions are modeled, EPA guidance recommends that the GEP stack height 

policy be used in the model. 

4. Not considering building downwash. EPA guidance recommends careful consideration of 

the information for any nearby buildings, such as location and orientation relative to 

stacks and building size parameters. Given that there are structures at both the Drake and 

Nixon Power plant, building downwash should be considered to represent actual 

conditions. 

5. The use of rural dispersion coefficients. EPA’s guidance recommends two methods for 

determining land classification as either urban or rural. The methods include analyzing 

land use or population density with the most recent and representative information. It is 

not clear that the information used in the Sierra Club analysis is the most recent or 

representative of the area to support the use of rural dispersion coefficients. 

6. The assumed background concentration of 75.9 μg/m3 based on monitor information at a 

site about 100 km from the source. EPA’s guidance recommends that air quality data in 

the vicinity of the source should be used to determine the background concentrations. 

EPA is aware of monitoring data closer to the sources that should be considered for 

determining the background concentrations. Also, the assumed value may not be a 

representative background concentration because it may be impacted by other nearby 

sources. Based on the information provided by the Sierra Club, it does not appear that the 

calculation excluded periods when the dominant sources could have been influencing the 

monitored concentrations. EPA’s modeling TAD recommends approaches for correcting 

for source impacted background data which Sierra Club apparently did not take into 

account. 

For these reasons, and due to the use of unrepresentative meteorological data, EPA finds that the 

results of the Sierra Club modeling should not be used as the basis for a nonattainment 

designation for Colorado Springs, as Sierra Club has recommended in their evaluation of 

compliance with the 1-hour NAAQS for SO2. 

EPA has also received information from residents of Colorado Springs and nearby communities. 

Like Sierra Club, these individuals have advocated that the area around Martin Drake be 

designated as nonattainment for the 2010 SO2 NAAQS. EPA has not yet fully addressed the 

information provided by these individuals given time constraints and the expectation of 

additional technical information to support their specific claims. EPA will review and take in to 

account all of the information received by these and other parties as part of its final designation 

process for the Colorado Springs area. 

Conclusion 

After careful evaluation of the State’s recommendation and supporting information, as well as all 

available relevant information, the EPA intends to designate the area around Martin Drake Power 

Plant as unclassifiable for the 2010 SO2 NAAQS. Specifically, the boundaries are comprised of 

the entirety of El Paso County, Colorado. The EPA is basing this conclusion on the lack of 
14 

sufficient technical information on which to base a determination regarding whether the area is 

meeting or not meeting the NAAQS. 

At this time, our intended designations for the State only apply to this area. Consistent with the 

conditions in the March 2, 2015 court-ordered schedule, the EPA will evaluate and designate all 

remaining undesignated areas in Colorado by either December 31, 2017 or December 31, 2020. 

 
15 

Technical Analysis for the Morgan County, Colorado Area 

Introduction 

Morgan County, Colorado contains a stationary source that according to the EPA’s Air Markets 

Database emitted in 2012 either more than 16,000 tons of SO2 or more than 2,600 tons of SO2

and had an annual average emission rate of at least 0.45 pounds of SO2 per one million British 

thermal units (lbs SO2/mmBTU). As of March 2, 2015, this stationary source had not met the 

specific requirements for being “announced for retirement.” Specifically, in 2012, the Pawnee 

Power Plant emitted 13,510 tons of SO2, and had an emissions rate of 0.76 lbs SO2/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, Colorado recommended that the area surrounding the Pawnee Power Plant be 

designated as unclassifiable 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 SO2 are expected. This assessment and characterization was based 

on consideration of the data available to the State. After careful review of the State’s assessment, 

supporting documentation, and all available data, the EPA agrees with the State’s 

recommendation for the area, and intends to designate the areas as unclassifiable. However, 

because the State did not provide specific boundaries in its recommendation, the EPA’s intended 

boundary consists of Morgan County. 

The Pawnee Power Plant is located roughly 8.5 km southeast of Fort Morgan, Colorado in the 

central portion of Morgan County (see Figure 4, below). Also included in the figure are nearby 

emitters of SO2 (the green dot demarks Cargill Meat Solutions). Figure 5 shows the EPA’s 

intended unclassifiable designation for the area (i.e. Morgan County), in blue, with the Pawnee 

Power Plant indicated by the red star. 

Figure 4. Pawnee Power Plant and other sources in Morgan County, Colorado 
16 

Figure 5. EPA’s intended unclassifiable area, which consists of Morgan County, Colorado. 

 
17 

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 in accordance with the Modeling TAD, and 

the factors for evaluation contained in the EPA’s March 20, 2015 guidance, as appropriate. 

Detailed Assessment 

Air Quality Data 

 

This factor considers the SO2 air quality monitoring data in the area surrounding Pawnee Power 

Plant. The facility is located in Morgan County; however, there are no ambient air quality 

monitors located in this county. The State did not include the most recent 3 years of monitoring 

data, i.e., 2012 – 2014, in its recommendation for the closest neighboring county, i.e., Adams 

County. The table below shows information related to the monitor(s) located in Adams County, 

which were confirmed through the EPA’s 2014 design value report for SO2.

4

 

Table 4: Available Air Quality Data for the Area Closest to Pawnee Power Plant 

County State 

Recommendation

Air Quality 

Systems 

(AQS) 

Monitor ID 

Monitor Location Distance to 

Martin Drake 

Power Plant 

(km) 

2012 – 2014 

SO2 Design 

Value in ppb 

Adams None 08-001-3001 3174 E. 78th Ave. 

Welby, Colorado 

110 km 25 ppb 

 

 

4 The design value report for SO2, as well as each of the other NAAQS, can be found at this link: 

http://www3.epa.gov/airtrends/values.html 
18 

Due to the large distance from the nearest monitor (in Welby, Colorado) to the Pawnee Power 

Plant, the information from this monitor is not informative as to a designation of the area 

surrounding Pawnee. 

Emissions and Emissions-Related Data 

Evidence of SO2 emissions from the source meeting the emissions criteria of the March 2, 2015 

consent decree, i.e., Pawnee Power Plant, is an important factor for determining whether the 

immediate area is experiencing elevated levels of SO2 concentrations. Other considerations for 

this factor include county level SO2 emissions data and data for sources located within a 

significant concentration gradient.5 

As part of its recommendation, Colorado included the annual emissions for all point sources in 

Morgan County emitting SO2. Colorado obtained the data for these sources and their emissions 

from the yearly facility reports that are submitted to the State. These emissions data are 

summarized below. 

Table 5: SO2 Emissions from Martin Drake Power Plant and Other Local Sources 

County Company Distance to Pawnee 

Power Plant* 

2013 Actual SO2 

Emissions [tons/year] 

Morgan Public Service Co – Pawnee 

Power Plant 

 12,467.55 

Morgan Western Sugar Coop. 12 km 58.30 

Morgan Cargill Meat Solutions 8.6 km 48.84 

Morgan Brush Cogeneration 

Partners/Colorado Power 

 0.18 

Morgan Dairy Farmers of America, Inc. 0.16 

Morgan City of Brush 0.16 

Morgan Brushco Farms, Inc. 0.04 

Morgan Brush Meat Processors, Inc 0.02 

Morgan Public Service Co. – Roundup 

Station 

 0.01 

Morgan Heer Mortuary & Crematory 0.01 

Morgan Leprino Foods Co. <0.01 

 

5

 The State addressed the sources within a significant concentration gradient of Pawnee, citing EPA’s Modeling 

TAD. 
19 

* Distance was only measured for sources with SO2 emissions greater or equal to 20 tons per 

year. 

Emissions Controls 

The EPA recognizes that control strategies implemented after the release of the 2011 NEI may 

not be reflected, or may warrant further discussion. As part of its designation recommendation, 

Colorado submitted information on emissions reductions resulting from controls put into place 

after the date of the emissions inventory data provided in the table above. 

Pursuant to Colorado’s Regional Haze SIP (77 FR 76871, December 31, 2012), a semi-dry SO2

scrubber system was installed at the Pawnee Power Plant. The controls began operating in 

August 2014, and have resulted in an SO2 reduction of about 87%. Table 6, below, was included 

in Colorado’s recommendation and demonstrates the significant decrease in SO2 emissions 

following the installation of the new scrubber system. 

Table 6: Pawnee Power Plant Emissions 

Year Number of Months 

Reported 

SO2 Annual 

Emissions 

(tons/year) 

SO2 Annual 

Emission Rate 

(lb/MMBtu) 

2012 12 13,510 0.76 

2013 12 12,467 0.72 

2014 12 5,508* 0.34 

2015 (partial) 3 476 0.08 

2015 (projected) - ≈1,700 ≈0.10 

* SO2 lime spray dryer controls installed August 2014 

Meteorology (Weather & Transport Patterns) 

Evidence of source-receptor relationships between specific emissions sources and high SO2 

concentrations in the surrounding area is another important factor in determining the appropriate 

extent of the EPA’s intended unclassifiable area. 
20 

Geography and Topography (Mountain Ranges or Other Air Basin Boundaries) 

The Pawnee Power Plant is located on the plains of eastern Colorado about 7 km south of the 

South Platte River. There are no significant topographical features impacting the area near the 

Pawnee Power Plant. 

Jurisdictional Boundaries 

Once the geographic area associated with the immediate area surrounding the Pawnee Power 

Plant, other nearby sources, and background concentrations are determined, existing 

jurisdictional boundaries are considered for the purpose of informing our intended unclassifiable 

area. Specific attention will be given to clearly defined legal boundaries. 

The EPA believes that our intended unclassifiable area, consisting of Morgan County, is 

comprised of clearly defined legal boundaries, and we find these boundaries to be a suitably clear 

basis for defining our intended unclassifiable area. 

Conclusion 

After careful evaluation of the State’s recommendation and supporting information, as well as all 

available relevant information, the EPA intends to designate the area around Pawnee Power Plant 

as unclassifiable for the 2010 SO2 NAAQS. Specifically, the boundaries are comprised of the 

entirety of Morgan County, Colorado. 

EPA is basing this conclusion on the lack of sufficient technical information on which to base a 

determination regarding whether the area is meeting or not meeting the NAAQS. As indicated, 

no modeling has been conducted, and the appropriate modeling parameters are not understood. 

Further, ambient air quality data that properly represents the areas of predicted maximum 

concentrations in the vicinity of Pawnee Power Plant are not available. 

At this time, our intended designations for the State only apply to this area. Consistent with the 

conditions in the March 2, 2015 court-ordered schedule, the EPA will evaluate and designate all 

remaining undesignated areas in Colorado by either December 31, 2017 or December 31, 2020.