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Technical Support 

Document

1-Hour SO2

National Ambient Air Quality Standard

Recommended Air Quality

Designation Boundaries

for

“Round 2” Sources

April 19, 2016

Colorado Department of Public Health and Environment

Air Pollution Control Division

4300 Cherry Creek Drive South

Denver, Colorado 80246
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Table of Contents

Introduction ..........................................................................................3

Designation Boundary Recommendations..................................................... 3

EPA Guidance on SO2 Designations ................................................................ 3

Future EPA SO2 Designations Process ............................................................ 3

Analysis of Source Specific Designations ..........................................4

Public Service Company of Colorado – Pawnee Power Plant..................... 4

Air Quality Characterization: Ambient Monitoring or Dispersion Modeling Results . 4

Emissions-Related Data....................................................................................................... 5

Meteorology (Weather & Transport Patterns) ................................................................ 6

Geography and Topography (Mountain Ranges or Other Air Basin Boundaries)........ 7

Jurisdictional Boundaries.................................................................................................... 7

Other Relevant Information ............................................................................................... 8

Conclusion ............................................................................................................................. 8

Colorado Springs Utilities – Martin Drake Power Plant............................. 10

Air Quality Characterization: Ambient Monitoring or Dispersion Modeling Results 10

SO2 Ambient Monitoring in El Paso County ................................................................... 10

Dispersion Modeling ........................................................................................................... 10

Emissions-Related Data..................................................................................................... 10

Meteorology (Weather & Transport Patterns) .............................................................. 11

Geography and Topography (Mountain Ranges or Other Air Basin Boundaries)...... 12

Jurisdictional Boundaries.................................................................................................. 12

Other Relevant Information ............................................................................................. 12

Conclusion ........................................................................................................................... 13
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Introduction

Designation Boundary Recommendations

This Technical Support Document (TSD) provides the basis for the source specific 

sulfur dioxide (SO2) National Ambient Air Quality Standard (NAAQS) air quality 

designation boundary recommendations for the Public Service Company of Colorado –

Pawnee Power Plant and Colorado Springs Utilities – Martin Drake Power Plant. These 

sources must be designated pursuant to “Round 2” of EPA’s Data Requirements Rule

(DRR) (80 FR 51056). The Colorado Department of Public Health and Environment 

(CDPHE) Air Pollution Control Division (Division) recommended, and the U.S. 

Environmental Protection Agency (EPA) proposed, to designate specific areas 

surrounding both power plants as unclassifiable regarding compliance with the 1-hr 

SO2 NAAQS. These areas will be subject to future additional designation requirements

under the Data Requirements Rule (DRR). 

EPA Guidance on SO2 Designations

Pursuant to the letter EPA sent to Governor Hickenlooper on February 16, 2016, the 

Division is providing additional information that EPA should consider prior to finalizing 

these designations. In the attached Technical Support Document of EPA’s letter, it is 

noted that updated designations guidance issued by EPA through a March 20, 2015 

memorandum supersedes earlier designation guidance and identifies factors that the 

EPA intends to evaluate in determining area boundaries under the 2010 SO2 NAAQS. 

These five factors are: 

1) Air quality characterization via ambient monitoring or dispersion modeling results; 

2) Emissions-related data;

3) Meteorology;

4) Geography and topography;

5) Jurisdictional boundaries.

In this TSD, CDPHE considers each of these five factors along with other relevant 

information regarding each of the detailed area boundary recommendations for 

Pawnee and Drake Power Plants. 

Future EPA SO2 Designations Process

The DRR establishes an SO2 emissions applicability threshold of 2,000 tons per year 

(tpy) that identifies priority sources subject to a source specific SO2 designation 

process. States have three options under the DRR to characterize current air quality 

in areas with large SO2 sources (2,000 tpy or greater): (1) establish federally 

enforceable emission limits (under 2,000 tpy) by January 13, 2017; (2) conduct air 

quality modeling by January 13, 2017; or (3) begin operating an appropriate 

monitoring network by January 1, 2017. The EPA will promulgate Round 3 SO2

designations (by Court Order) no later than December 31, 2017 for areas with sources 

that are modeled. For sources that are monitored and any remaining undesignated 

areas, EPA will promulgate Round 4 SO2 designations (by Court Order) no later than 

December 31, 2020. The Pawnee Power Plant and Martin Drake Power Plant will be 

subject to future DRR promulgations.
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Analysis of Source Specific Designations

This section of the TSD analyzes each of the five factors set forth in EPA’s March 20, 

2015 memorandum for the Pawnee Power Plant and the Martin Drake Power Plant. 

Boundary recommendations are presented at the conclusion of each five factor 

analysis.

Public Service Company of Colorado – Pawnee Power Plant

Air Quality Characterization: Ambient Monitoring or Dispersion Modeling Results

As EPA noted previously, the nearest ambient SO2 monitor in Adams County is over 60 

miles away to the southwest. Due to the fact that there has never been ambient SO2

monitoring in Morgan County, there is not any current or historical ambient 

monitoring for the Pawnee Power Plant available. Therefore, refined dispersion 

modeling is currently being conducted for this facility in consultation with the source

and EPA. 

For DRR modeling, EPA recommends that the model domain be based on the 

determination of significant concentration gradients and distance from source. The 

SO2 Modeling Technical Assistance Document (TAD) states the following:

“Concentration gradients associated with a particular source will be generally largest 

between the source and the distance to the maximum ground level concentrations 

from the source. Beyond that distance, gradients tend to be smaller and more 

spatially uniform...a general guideline that the distance between a source and its 

maximum ground level concentration is generally 10 times the stack height1in flat 

terrain. However, the potential influence of terrain can impact the location and 

magnitudes of significant concentration gradients. The use of significant 

concentration gradients can help inform the decision on the size of the modeling 

domain and sources to consider for modeling.” The Division notes that 10 times the 

stack height of Pawnee Power Plant is: 550 feet x 10 = 5,500 feet (roughly 1 mile or 

1.7 kilometers (km)). This radius would be considered too narrow considering the 

meteorological data, including potential terrain influences, and other nearby sources 

in the area.

EPA’s March 1, 2011 Memo, which is referenced in the TAD, also states the following:

“Even accounting for some terrain influences on the location and gradients of 

maximum 1-hour concentrations, these considerations suggest that the emphasis on 

determining which nearby sources to include in the modeling analysis should focus on 

the area within about 10 kilometers of the project location2in most cases. The 

routine inclusion of all sources within 50 kilometers of the project location, the 

nominal distance for which AERMOD is applicable, is likely to produce an overly 

 

1

EPA SO2 NAAQS Designations Modeling Technical Assistance Document, page 8.

2

Additional Clarification Regarding Application of Appendix W Modeling Guidance for the 1-hour NO2 

National Ambient Air Quality Standard, March 1, 2011, page 16.
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conservative result in most cases.” Based on the TAD and this Memo, the Division 

believes the modeling analysis should focus on the area within a 10 km radius of the 

Pawnee Power Plant. 

Figure 1 identifies SO2 sources in the vicinity of the Pawnee Power Plant. Cargill Meat 

Solutions is about 5.4 miles (8.7 kilometers) and Western Sugar Company is about 7.3 

miles (11.8 kilometers) from the Pawnee Power Plant respectively. 

Figure 1: Significant SO2 Sources in Morgan County 

When CDPHE initially submitted Round 2 designation recommendations on September 

18, 2015, the appropriate area for air quality characterization had not yet been 

determined. Upon review of the EPA TAD and Memo, CDPHE proposes that a radius of 

10 km captures potential terrain influences and the affected populations in the town 

of Brush and a portion of the city of Fort Morgan. The Division anticipates that the 

refined modeling process to be completed in 2016, which will then provide additional 

information for the attainment status of the Pawnee Power Plant. 

Emissions-Related Data 

Pawnee Power Plant – SO2 Emissions Analysis

The March 2015 Consent Decree (CD) identified subject sources for inclusion in Round 

2 designations based on 2012 continuous emissions monitoring data. In 2012, the 

Pawnee Power Plant had no SO2 controls aside from firing low-sulfur coal3

;

consequently, the facility’s SO2 emissions were over the CD’s applicability threshold 

of 2,600 tpy along with an annual average emission rate exceeding 0.45 lbs/MMBtu. 

In 2014, pursuant to Colorado’s Regional Haze State Implementation Plan (SIP) 

3

The Pawnee Power Plant generally fires sub-bituminous coal that has an average sulfur content 

ranging from 0.24 to 0.42 percent from the Powder River Basin area in Wyoming.

Copyrighted map removed
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(approved by EPA in January 2013), the Public Service Company of Colorado (PSCo) 

installed and commenced operation of a semi-dry SO2 scrubber system (lime spray 

dryer) at the Pawnee Power Plant. Pursuant to the Regional Haze SIP, the scrubber 

system is subject to a federally enforceable (allowable) permit limitation of 0.12 

lbs/MMBtu based on a 30-day rolling average emission rate. Table 1 provides SO2

emissions data for the Pawnee Power Plant that is reported to the EPA Air Markets 

Program Data system. As indicated in the below table, the recently installed semi-dry 

SO2 scrubber has resulted in a significant SO2 emission reduction of about 86%.

Table 1: Pawnee Power Plant SO2 Emissions (updated with 2015 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 12 1,810 0.08

*SO2 lime spray dryer controls started in August 2014

SO2 Emissions in Morgan County

The most current comprehensive emission inventory available from Division records 

for Morgan County indicates that over 99% of the 2013 SO2 emissions in the county are 

from point sources. As indicated above, the Pawnee Power Plant began operating a 

semi-dry scrubber system in August of 2014 that will significantly lower SO2 emissions 

in the future. The Pawnee Power Plant comprised 98.7% of the SO2 emissions in 

Morgan County (as of 2013).

Meteorology (Weather & Transport Patterns)

The local winds are a combination of the mountain/valley winds in the South Platte 

and Beaver Creek Valleys and large scale weather features (synoptic winds). During 

the night and early morning hours the down valley winds in the South Platte River or 

Beaver Creek Valleys dominate. The winds in Beaver Creek Valley are most likely 

enhanced by synoptic conditions. The annual 60-meter windrose, which is similar to 

winds at plume height, is depicted in Figure 2 below. Generally, over a year, a 

variety of synoptic type forcings cause winds to flow from either the west through 

northeast or from the south through southeast most of the time.
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Figure 2: Annual 60-meter Windrose (Pawnee Power Plant meteorological data)

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

The Pawnee Power Plant is located on the southern slope of the South Platte River 

Valley and on the west side of Beaver Creek. Near this location the South Platte River 

turns from the southeast to the northeast and continues in this direction into 

Nebraska. Beaver Creek flows from the south to the north into the South Platte River. 

Outside of the South Platte River Valley the terrain gently slopes up, north to the 

Cheyenne Ridge and south to Monument Ridge.

Jurisdictional Boundaries

The Pawnee Power Plant is not located within any defined town or city boundaries, 

but rather within the rural areas of Morgan County. The closest town is Brush

(population 5,501 as of 2013), which is located approximately three miles (4.8 km) to 

the northeast. The closest city is Fort Morgan (population 11,407 as of 2013), located 

approximately five miles (8 km) to the northwest. As of 2013, the population of 

Morgan County was 28,404 people4. The populations of these two nearby communities 

comprise about 60% of Morgan County’s population.

 

4

All population statistics are from the United States Census Bureau.
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Other Relevant Information

None.

Conclusion

Upon consideration of the five factors, CDPHE recommends applying a radius of 10 km 

(6.2 miles) around the Pawnee Power Plant as the boundary for the unclassifiable area

designation, shown in Figure 3. This is based on the following information:

 EPA’s Modeling TAD and March 1, 2011 Memo distance guidelines indicate that 

a 10 km radius normally establishes an appropriate modeling domain. SO2

modeling for the area around the Pawnee Power Plant will not be completed 

before the court ordered deadline for making Round 2 designations. No SO2

ambient monitoring data is available for this area. Without completed 

modeling results or ambient monitoring data, it is appropriate to base the SO2

area boundary on the 10 km radius outlined in EPA’s March 1, 2011 Memo;

 SO2 emissions from the Pawnee Power Plant have been significantly reduced

since the installation of the semi-dry lime scrubber in 2014. Emissions from 

Cargill Meat Solutions are captured by the 10 km radius. The Western Sugar 

Cooperative facility is located further than 10 km from the Pawnee Power 

Plant, and has significantly lower emissions than the power plant;

 There are no significant meteorological, geographical or topographical features 

that make the 10 km radius an inappropriate area designation boundary;

 The nearby town of Brush and the affected areas of the city of Fort Morgan 

closest to the Pawnee Power Plant are included in this proposed radius5.

 

5

Town and city boundaries applicable as of April 2016.
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Figure 3: Pawnee Power Plant Boundary Recommendation
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Colorado Springs Utilities – Martin Drake Power Plant

Air Quality Characterization: Ambient Monitoring or Dispersion Modeling Results

SO2 Ambient Monitoring in El Paso County

Currently, there are four SO2 monitoring locations in operation in Colorado, of which 

one is located in Colorado Springs at Highway 24 and 8th Street (AQS-ID: 08-041-

0015). This site might be relocated in the future, depending on the results of any

final modeling impact analysis and site availability.

Historical monitoring data for SO2 in Colorado Springs (and the rest of the state) never 

approached the level of any SO2 standard prior to the installation of the Highway 24

monitor in January 2013. There has been substantial SO2 monitoring in the Colorado 

Springs area, with up to ten monitors operating during different periods between 1988 

and 2007. Table 2 below details the concentrations at the Highway 24 monitor over 

the past three years. This monitor has noted four total exceedances of the 75 ppb 

standard since monitoring began in 2013 through the end of 2015. However, these 

exceedances do not indicate nonattainment because the 99th percentile value through 

the end of 2015 remains below the 75 ppb 1-hour NAAQS, with the 2013-2015 design

value at 56 ppb. The last exceedance was in March 2015.

Table 2: CDPHE Highway 24 Monitor Data (2013 - present)

AQS Site 

ID

Site 

“Name” Address Year

1st Max 1-

hour (ppb)

2nd Max 1-

hour (ppb)

99th% 1-hour

(ppb)

08-041-

0015

Highway 

24

690 W. 

Highway 24

2013 99 81 58

2014 83 57 57

2015 87 70 53

2013-

2015

n/a n/a 56

Dispersion Modeling

The Division concluded that there was no available representative meteorological 

dataset for the Drake Power Plant in September 2015 (and provided EPA with

additional information in December 2015). Colorado Springs Utilities (CSU) began 

gathering representative meteorological data in October 2015. At least one year of 

meteorological data will be collected. Subsequently, after the quality assurance 

process is completed for the entire year of acquired meteorological data, modeling is 

anticipated to span about six months. Consequently, the Division anticipates the final 

impact analysis to be complete in 2017.

Emissions-Related Data

Martin Drake Power Plant – SO2 Emissions Analysis

CSU is currently in the process of installing SO2 controls on Units 6 and 7 to comply 

with Regional Haze SIP emission limits effective December 31, 2017. The scrubber for 

Drake Unit 7 is installed and in testing phases while the scrubber for Unit 6 will begin 

operational testing at the end of 2016. Emissions from the Drake Power Plant are 
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shown in Table 3 below (from the EPA Air Markets Program Data system). 

Additionally, the CSU Board voted January 20, 2016 to decommission Drake Unit 5 by 

December 31, 2017. CSU plans to accept a federally enforceable plant-wide SO2

emission limit of less than 2,000 tons per year per the DRR, effective on or before 

January 13, 2017. This limit will reduce actual SO2 emissions by approximately 2,800 

tons in the year 2017 (based on initial pre-application meetings with the Division).

Table 3: Martin Drake Power Plant SO2 Emissions

Year* Unit

Number of 

Months 

Reported

SO2 Annual 

Emissions 

(tons/year)

SO230-day Rolling 

Emission Rate (lb/MMBtu)

2012

5

12 1,108 0.52

2013 12 982 0.49

2015 12 580 0.42

2012

6

12 1,680 0.55

2013 12 1,595 0.50

2015 12 1,448 0.47

2012

7

12 2,004 0.56

2013 12 2,004 0.51

2015 12 2,004 0.53

2012

All

12 4,707 0.56

2013 12 4,580 0.51

2015 12 3,960 0.45

2015** 6 & 7 12 3,452 0.47

*Drake Power Plant experienced a fire event that resulted in the units not operating for various periods 

of time. Unit 6 was down for nine weeks. Unit 5 was down for 16 weeks. Therefore, 2014 is not 

included in recent emission evaluations.

**This row is included since Unit 5 operated at 70% of average operating hours (years 2006-2013) in the 

year 2015 to give perspective regarding the operation of Units 6 and 7.

SO2 Emissions in El Paso County

The most current comprehensive emission inventory available from Division records 

for El Paso County indicates that approximately 99% of the 2013 SO2 emissions in the 

county are from point sources. The Nixon Power Plant, approximately 15 miles away 

from Drake, is the only other significant SO2 source in El Paso County, and contributed 

about 46% of point source SO2 emissions in 2013 while Drake contributed 53%. As 

discussed in the modeling section, air quality impacts from the Drake Power Plant 

have not been estimated due to the lack of available representative meteorological 

data. However, preliminary analysis indicates that SO2 emission concentration 

gradients and potential hotspots from the Drake and Nixon Power Plants do not 

overlap because of geographical distance and terrain features that result in each 

plant existing in separate airsheds with regard to SO2.

Meteorology (Weather & Transport Patterns)

As noted, the Division determined there is not currently available representative 

meteorological datasets for the transport and dispersion conditions at the Drake 

Power Plant. EPA agreed with this determination in its February 16, 2016 letter and 
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associated draft technical support document. The Division anticipates representative 

meteorological data will be collected by the end of 2016 that will then be quality 

assured to be used for air dispersion modeling purposes along with allowing for the 

development of a representative annual wind rose for the Drake Power Plant.

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

Fountain Creek and Monument Creek converge just to the west of the Drake Power 

Plant, which directly impact the plume and wind flows. There are two prominent

terrain features that affect wind conditions at the Drake Power Plant: Pikes Peak 

approximately two miles to the west and the Palmer Divide approximately five miles 

to the north.

Jurisdictional Boundaries

Clearly defined legal boundaries can be used to determine an appropriate geographic 

area regarding potential NAAQS impacts from the Martin Drake Power Plant. The city 

of Colorado Springs is the largest city in Colorado by land area, encompassing 194 

square miles, and the second largest in population (445,830 as of 2014

6

). Many 

scattered unincorporated county designated areas are enclosed within city limits. 

The Drake Power Plant is located at the western edge of downtown Colorado Springs. 

Therefore, it is appropriate to examine roadway intersections as major defining lines 

for the designation boundary since city limits extend much farther east than the areas 

that monitoring, meteorological and topographical analyses suggest are being 

impacted. The Division also considers the city of Manitou Springs to be a clearly 

defined legal boundary that should be included in the designation area due to 

Fountain Creek airflows and potential impacts in the areas of the foothills.

Other Relevant Information

The Division used CDPHE’s public-facing Community Health Equity Map (2010-2014 

data)7to examine census-tract level geographic disparities for selected social 

determinants of health, including income/poverty levels and race/ethnicity 

population percentages, and key health conditions and outcomes, including asthmarelated hospitalization rates, heart disease mortality rates, and preventable 

conditions. An example of these maps (income/poverty rates) is shown below in 

Figure 4. Several areas in the southern area of the city clearly indicate above average 

poverty rates, increased minority populations as well as higher than state averages for 

asthma-related hospitalization rates, heart disease mortality rates, and preventable 

condition hospitalization rates. The Division also reviewed population density using 

this Map to incorporate appropriate potentially affected populations and assess

population density in the mountains west of Manitou Springs and Colorado Springs. 

This information is important in considering boundary area recommendations for a 

source located in an urbanized area with varied population factors.

 

6

All population statistics are from the United States Census Bureau.

7

http://www.cohealthmaps.dphe.state.co.us/cdphe_community_health_equity_map/ -
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Figure 4: CDPHE Community Health Equity Map Example (Income/Poverty Rate)

Conclusion

Upon consideration of the five factors and other relevant information, CDPHE 

recommends a boundary around a portion of the city of Colorado Springs, including 

enclosed unincorporated county areas, bounded to the north by East Woodmen Road, 

North Academy Boulevard, and city limits, to the east by North/South Powers 

Boulevard, and to the south and west by city limits, with the addition of the ‘census 

designated place’ termed “Stratmoor” bounded by South Academy Boulevard for the 

designation boundary, shown in Figure 5

8

. This conclusion is based on the following 

information:

 Preliminary monitoring and emissions-related data shows that this boundary 

incorporates the primary source (Martin Drake Power Plant). Emissions from 

the only other notable SO2 source in the vicinity are in a separate airshed;

 Meteorological and topographical information indicate that potential impacts 

would be contained within this boundary;

 The mountains that generally begin just to the west of the cities of Colorado 

Springs and Manitou Springs constitute a geographical boundary limit;

 Basing the area boundary on the guideline 10 km radius for a modeling domain 

is not appropriate because of the complex terrain and urban demographics; 

 Affected populations, including sensitive subpopulations, within the city of 

Colorado Springs, including those living in unincorporated enclosed county 

areas, and the city of Manitou Springs are located within this boundary. EPA 

has previously approved area boundaries for other Round 2 SO2 designation 

 

8

City boundaries applicable as of April 2016.
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boundaries that align with roadways, rather than the outermost jurisdictional 

boundaries of a unit of local government9.

 

9 North Dakota Mercer County (portion) SO2 Designation: road boundaries specified in Technical Support 

Document (February 16, 2016)
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Figure 5: Martin Drake Power Plant Boundary Recommendation