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Technical Support Document

1-Hour SO2 

National Ambient Air Quality Standard

Recommended Air Quality

Designations 

for

Specific Sources

September 18, 2015

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 Recommendations.......................................................................... 3

Overview ............................................................................................................ 3

History................................................................................................................ 4

EPA Guidance on SO2 Designations.................................................................... 4

Future EPA SO2 Designations Process ................................................................ 4

Analysis of Source Specific Designations.................................................5

Public Service Company of Colorado – Pawnee Power Plant.............................. 5

Pawnee Power Plant – SO2 Emissions Analysis...................................................................... 5

SO2 Emissions in Morgan County ........................................................................................... 5

Modeling of Significant SO2 Emission Sources....................................................................... 8

Colorado Springs Utilities – Martin Drake Power Plant....................................... 8

Martin Drake Power Plant – SO2 Emissions Analysis............................................................. 8

SO2 Emissions in El Paso County .......................................................................................... 10

Modeling of Significant SO2 Emission Sources..................................................................... 11

SO2 Ambient Monitoring in El Paso County ......................................................................... 12
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Introduction

Designation Recommendations

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

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

recommendations for the Public Service Company of Colorado – Pawnee Power Plant and 

Colorado Springs Utilities – Martin Drake Power Plant. The Division is requesting that the Air 

Quality Control Commission (Commission) recommend to the U.S. Environmental Protection 

Agency (EPA) that the nearby areas surrounding both power plants be designated as 

unclassifiable as insufficient data and analysis is available to ascertain compliance with the 1-hr 

SO2 NAAQS. The unclassifiable designation recommendations for both power plants will allow 

extra time to acquire more data to better analyze compliance with the SO2 standard.

Overview

In 2010, the EPA promulgated a new NAAQS for SO2 as a 1-hour average. The level was set at 

75 parts per billion (ppb) as the 3-year average of the 99th percentile 1-hour daily maximum 

values. In 2011, Colorado submitted final recommendations of “attainment” and 

“attainment/unclassifiable” as applicable for all air quality control regions in the state. In 

August 2013, EPA promulgated a final rule establishing nationwide SO2 non-attainment areas 

(none in Colorado) based on 2009-2011 monitoring data while acknowledging that EPA 

intended to address designations for other areas in separate future actions. EPA refers to the 

initial state designations based on 2009-2011 SO2 monitoring data as “Round 1” designations.

In August 2013, the Sierra Club and Natural Resources Defense Council filed a complaint in 

federal District Court alleging that EPA failed to promulgate SO2 designations for all areas of the 

country within the timeframe provided under the Clean Air Act (Act). On March 2, 2015, a 

Consent Decree was ordered that requires EPA to promulgate source specific designations on 

certain SO2 sources no later than July 2, 2016 – this set of designations is referred to as “Round 

2” designations. This section of the Consent Decree applies to any stationary source (1) 

emitting more than 16,000 tons/year (tpy) of SO2 in 2012, or (2) emitting more than 2,600 tpy 

of SO2 and having an annual average emission rate of 0.45 lbs SO2/MMBtu1

 1 MMBtu means one million British Thermal Units

or higher in 2012. 

Nationwide there are 68 coal-fired power plants that are specifically listed in the Consent 

Decree, including two in Colorado: Public Service Company of Colorado – Pawnee Power Plant 

and Colorado Springs Utilities – Martin Drake Power Plant. As a result of the Consent Decree, 

states with certain affected sources meeting the criteria above must submit source specific 

designation recommendations, to assist EPA in promulgating “Round 2” designations, by 

September 18, 2015. The timing of the Consent Decree designation process does not provide 

for consideration of post-2012 installed air pollution controls required pursuant to Regional 

Haze State Implementation Plan.
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History

Under the original annual and daily (24-hour) SO2 standards (1971) there was extensive 

nationwide monitoring that indicated few SO2 non-attainment problems; accordingly, the 

nationwide SO2 monitoring network decreased in size from approximately 1,496 sites in 1980 

to 488 sites operating in 2008. When the revised SO2 NAAQS was promulgated in 2010, only 

two SO2 monitors were operating in Colorado, both in the Denver Metro Area. Both monitors 

showed levels below the standard. Since there was very limited SO2 monitoring data on which 

to base designations, most of Colorado’s air quality control regions were recommended as 

“attainment/unclassifiable” during a public hearing of the Commission on March 17, 2011.

In response to the lack of SO2 monitoring data nationwide and associated concerns on whether 

large SO2 sources may emit levels exceeding the 1-hour standard, EPA proposed the SO2 Data 

Requirements Rule (DRR) in May 2014 that solicited public comment on future monitoring or 

modeling requirements for large SO2 sources based on three proposed SO2 emission threshold 

options. The final SO2 NAAQS DRR was signed on August 10, 2015.

EPA Guidance on SO2 Designations

In December of 2013, the EPA issued draft modeling guidance “SO2 NAAQS Designations 

Modeling Technical Assistance Document” (SO2 TAD Guidance) that provides direction to states 

on demonstrating compliance with the SO2 NAAQS through modeling of source impacts. Unlike 

ambient monitoring, which requires 3 years of data acquisition to ascertain compliance with 

the NAAQS, modeling provides for a relatively quick conservative assessment of ambient air 

quality from stationary source impacts. Accordingly, refined modeling is the preferred first 

approach to better characterize ambient air quality for both stationary source areas and the 

Division anticipates that refined modeling of the affected sources will be performed over an

extended period likely spanning into next year.

Future EPA SO2 Designations Process

The Data Requirements Rule establishes an SO2 emissions applicability threshold of 2,000 tons 

per year (tpy) that identifies priority sources subject to a source specific SO2 designations 

process, referred to as “Round 3” designations. This process includes both the Public Service 

Company of Colorado – Pawnee Power Plant and Colorado Springs Utilities – Martin Drake 

Power Plant. States have three options under the DRR to characterize current air quality in 

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

limits (under 2,000 tpy) by January 13, 2017; conduct air quality modeling by January 13, 2017;

or 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 

all other new monitored areas or remaining areas, EPA will promulgate “Round 4” SO2 

designations (by Court Order) no later than December 31, 2020. Consequently, the current 

Division recommendation of unclassifiable for both power plants results in only a temporary 

delay that will be resolved in future designation actions by EPA.
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Analysis of Source Specific Designations

Public Service Company of Colorado – Pawnee Power Plant

Pawnee Power Plant – SO2 Emissions Analysis

The March 2015 Consent Decree 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 coal2

Table 1

; consequently, the facility’s SO2 

emissions were over the Consent Decree 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 SIP (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 permit limitation of 0.12 lbs/MMBtu based on a 30-day 

rolling average emission rate. 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 87%.

Table 1: Pawnee Power Plant SO2 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 started in August 2014

SO2 Emissions in Morgan County

Table 2 includes the most current comprehensive emission inventory for Morgan County, which 

indicates that over 99% of the 2013 SO2 emissions in Morgan 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.

 2 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.
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Table 2: 2013 SO2 Emission Inventory for Morgan County

Source Category SO2 Emissions (tons/year)

Point Source 12,575.3

Agriculture Burning 41.5

Railroad 8.2

Motor Vehicles 2.2

Non-Road Vehicles 1.0

Wood Burning 1.0

Fuel Combustion 0.4

Oil & Gas Area 0.4

Oil & Gas Point 0.2

Aircraft 0.1

Total: 12,630.3

Table 3 includes the source specific emissions data from Air Pollutant Emissions Notice (APEN) 

reports filed with the Division in the spring of 2014 using actual SO2 emissions data from 2013 

or earlier. According to EPA Guidance3

Table 3: 2013 Point Source (Non-Oil & Gas) SO2 Emissions for Morgan County

on SO2 Designations Modeling, all sources expected to 

cause significant concentration gradients in the vicinity of the source of interest should be 

evaluated. Based on a review of actual SO2 emissions in Morgan County, there are potentially 

two significant SO2 sources (Western Sugar Cooperative and Cargill Meat Solutions) that need 

further evaluation for potential inclusion in a technical modeling analysis and demonstration

associated with the PSCo – Pawnee Power Plant.

Company 2013 Actual SO2 Emissions [tons/year]

Public Service Co – Pawnee Power Plant* 12,467.55

Western Sugar Coop. 58.30

Cargill Meat Solutions 48.84

Brush Cogeneration Partners/Colorado Power 0.18

Dairy Farmers of America, Inc. 0.16

City of Brush 0.16

Brushco Farms, Inc. 0.04

Brush Meat Processors, Inc 0.02

Public Service Co. – Roundup Station 0.01

Heer Mortuary & Crematory 0.01

Leprino Foods Co. <0.01

Total: 12,575.3

* Includes 0.95 tpy from other sources at the facility

 3 See Draft - SO2 NAAQS Designations Modeling Technical Assistance Document, US EPA OAR-OAQPS-AQAD 

December 2013
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The Western Sugar Cooperative – Fort Morgan Facility manufactures sugar and sugar byproducts from sugar beets in an annual 5-month period that starts in about mid-September. 

This period of operation, which can range from about 100 to 170 days and averages about 140 

days, is called a “campaign”. During the campaign the plant is in operation for 24 hours/day, 

seven days per week until all harvested beets have been processed. The facility has two 

boilers4 that fire low-sulfur coal. In 1975, an application was made to convert the boilers from 

firing natural gas to coal and install a venturi scrubber system for controlling emissions. Each 

boiler was equipped with a venturi scrubber system. The two boilers connect to common stack 

provided with a mist eliminator. The boilers do not have short-term SO2 emission limits, but 

are subject to Regulation Number 1 emission limit of 1.8 lbs/MMBtu5

Cargill Meat Solutions underwent a permit renewal in 2013 that specifies the SO2 emission 

limits. The facility has several boilers and a flare that fire either natural gas or bio-gas, which 

results in some SO2 emissions. However, the SO2 emissions are emitted from a number of 

emission points at relatively low concentrations. Thus, this source is not expected to have a 

significant impact on the 1-hr SO2 NAAQS in the local area.

. According to EPA SO2 

TAD Guidance, “air agencies may wish to conduct modeling using allowable or potential to emit 

(PTE) emissions.” In the situation of Western Sugar Cooperative, it is more appropriate to use 

actual emissions since the source only operates on a seasonal basis and the source PTE is much 

higher than actual emissions.

Figure 1 identifies the three SO2 sources to be evaluated. Cargill Meat Solutions is about 5.4 

miles and Western Sugar Company is about 7.3 miles from the Pawnee Power Plant 

respectively.

Figure 1: Significant SO2 Sources in Morgan County

 4 Boiler manufacturer is Babcock & Wilcox with Detroit Stoker Model Rotograte RG-4. Each boiler is rated at 196 

MMBtu/hour.

5 See Colorado Air Quality Control Commission (AQCC) Regulation Number 1, Section VI.A.3.a.(i)
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Modeling of Significant SO2 Emission Sources

In order to conduct the modeling necessary to predict the attainment status of the area 

surrounding the Pawnee Power Plant, the Division needs to develop appropriate short-term 

emission rates for the Pawnee Power Plant and the other two significant SO2 sources in 

consultation with the sources and EPA. Since the SO2 NAAQS is an hourly standard, these 

short-term emission rates are necessary model inputs to properly characterize SO2 emissions 

that might impact the ambient air quality in Morgan County. Since controls have only recently 

been installed, PSCo is still acquiring short-term emission rate data for Pawnee Power Plant. 

The Division anticipates that the refined modeling process could be completed in the next year, 

which will provide information on the attainment status for SO2 air quality for Morgan County

by the end of 2016.

Colorado Springs Utilities – Martin Drake Power Plant

Martin Drake Power Plant – SO2 Emissions Analysis

The March 2015 Consent Decree identified subject sources to include in “Round 2” based on 

2012 emission rates. In 2012, the Martin Drake Power Plant had no SO2 controls, aside from 

firing low-sulfur coal6

Drake Unit 5: 0.26 lb/MMBtu (30-day rolling average)

; consequently the facility SO2 emissions were over the Consent Decree

applicability threshold of 2,600 tpy along with an annual average emission rate exceeding 0.45 

lbs/MMBtu. 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. CSU submitted a compliance schedule to 

the Division on March 28, 2013, which was published for public comment and subsequently 

approved by the Division on November 4, 2013 (for SO2 compliance – the Division approved 

additional schedule changes for NOx in March 2015). The SO2 Best Available Retrofit 

Technology (BART) limits for the Martin Drake Power Plant are as follows:

Drake Unit 6: 0.13 lb/MMBtu (30-day rolling average)

Drake Unit 7: 0.13 lb/MMBtu (30-day rolling average)

Martin Drake Power Plant SO2 emissions by unit are indicated in Table 4 for the most recent 

three years of data that is reported to the EPA Air Markets Program Data system.

 6 The Martin Drake Power Plant generally fires a blend of sub-bituminous and bituminous coal that has an average 

sulfur content ranging from 0.21 to 0.53 percent from a variety of coal mines, including Power River Basin, 

ColoWyo, 20-Mile Foidel Creek, and West Elk, located in western Colorado and Wyoming, based on 2006-2015 

data.
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Table 4: Martin Drake Power Plant SO2 Emissions

Year* Unit

Number of 

Months 

Reported

SO2 Annual 

Emissions 

(tons/year)

SO2 30-day Rolling Emission 

Rate (lb/MMBtu)

2012

5

12 1,108 0.52

2013 12 982 0.49

Average 1,045 0.51

2012

6

12 1,680 0.55

2013 12 1,595 0.50

Average 1,637 0.52

2012

7

12 2,004 0.56

2013 12 2,004 0.51

Average 2,004 0.53

2012 All 12 4,707 0.56

2013 12 4,580 0.51

Average 4,686 0.52

**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 7 was down for 16 weeks. Therefore, 2014 is not included in recent 

emission evaluations.

In comparison, the BART limits were based on emissions averaged from the years 2006 – 2008, 

shown in Table 5.

Table 5: Martin Drake Power Plant BART baseline emissions (2006 - 2008)

Pollutant

Unit 5 Unit 6 Unit 7

Annual 

Emissions* 

(tpy)

Annual 

Emissions** 

(lb/MMBtu)

Annual 

Emissions* 

(tpy)

Annual 

Emissions** 

(lb/MMBtu)

Annual 

Emissions* 

(tpy)

Annual 

Emissions** 

(lb/MMBtu)

SO2 1,269 0.63 2,785 0.82 4,429 0.83

*Using daily CEMs data from 2006 – 2008 calendar years.

**The Division calculated average emission rate (lb/MMBtu) from the 2006 - 2008 calendar years based on 

average daily reported data for each unit.

Emission differences between these two time frames are significant. The percent in emission 

reductions and resultant tons per year reductions (average of three years for 2006 – 2008 

compared to 2012 – 2013,) are listed below:

Drake Unit 5: 18% SO2 emission reduction (224 tons/year)

Drake Unit 6: 41% SO2 emission reduction (1,148 tons/year)

Drake Unit 7: 55% SO2 emission reduction (2,425 tons/year)
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There are several reasons for lower emissions in recent years: an overall increase in natural gas 

consumption, additional outages due to installation of Regional Haze controls, and a shift in 

coal types to lower sulfur coal7

Drake Unit 5: 49% SO2 emission reduction

. The emission limits specified in the Regional Haze SIP will 

result in further SO2 emission reductions from 2012 – 2013 emissions. Using the most recent 

two full years of data, the following percent emission reductions (from 2012 – 2013 average) 

will occur by the end of 2017, based on the required 30-day rolling average current emissions 

and future limits:

Drake Unit 6: 75% SO2 emission reduction

Drake Unit 7: 76% SO2 emission reduction

SO2 Emissions in El Paso County

The 2013 emissions inventory is the most current inventory year that the Division has 

developed for El Paso County, except for motor vehicles, for which the most recent inventory is 

the year 2011. Table 6 below indicates that approximately 99% of emissions are from point 

sources. 

Table 6: 2013 SO2 Emission Inventory for El Paso County

Source Category SO2 Emissions (tons/year)

Point Source 8,668.5

Motor Vehicles 30.5

Wood Burning 21.8

Aircraft 15.3

Railroad 9.6

Fuel Combustion 7.2

Non-Road Vehicles 7.1

Agricultural Burning 0.3

Total: 8,760.4

Table 7 includes the source specific emissions data from Air Pollutant Emission Notice (APEN) 

reports filed with the Division in the spring of 2014 using actual SO2 emissions data from 2013 

or earlier. According to EPA Guidance8

 7 Sulfur content averaged 0.44% for 2006-2008 versus 0.25% for 2012 – June 2015 (plant-wide).

on SO2 Designations Modeling, all sources expected to 

cause significant concentration gradients in the vicinity of the source of interest should be 

evaluated. 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 below, 

air quality impacts from the Drake Power Plant have not been estimated due the lack of 

available representative meteorological data. When impacts from the Drake Power Plant are 

modeled, the Division will determine if contributions from Nixon and other off-site sources to 

cumulative impacts need to be addressed.

8 See Draft - SO2 NAAQS Designations Modeling Technical Assistance Document, US EPA OAR-OAQPS-AQAD 

December 2013
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Table 7: 2013 Point Source (Non-Oil & Gas) SO2 Emissions for El Paso County

Company 2013 Actual SO2 Emissions 

(tons/year)

Colorado Springs Utilities - Martin Drake Power Plant 4,580.3

Colorado Springs Utilities – Nixon Power Plant 3,955.1

Fort Carson US ARMY 34.5

Colorado Springs Utilities – Clear Spring Ranch 23.6

Schmidt Construction Company – Delta Drive Facility 23.2

Kiewet – Colorado Springs Asphalt Plant 16.0

Martin Marietta Materials – Fillmore 8.2

Schmidt Construction Company – Menzer Facility 3.7

Penrose Community Hospital 3.7

U.S. Air Force Academy 2.5

Peterson Air Force Base 2.4

Homeward Bound 2.3

Rocky Mountain Materials & Asphalt Inc. 2.2

Fountain Landfill 1.6

Federal Express – Colorado Tech 1.2

Fountain Valley Power – Boca Raton 1.1

Pikes Peak Pet Crematory 1.0

Cheyenne Mountain Zoo 0.8

Holt Family Funeral Homes 0.7

UCH-MHS Central 0.6

Hewlett-Packard Corporation 0.6

Color Star Growers – Peyton Greenhouse 0.6

Total: 8,668.5

*Sources reporting below 0.5 tpy were not included in this table due to extremely low emissions and 

contributions. This information is available upon request.

Modeling of Significant SO2 Emission Sources

In EPA’s draft guidance on state implementation of the 1-Hour SO2 NAAQS (issued September 

21, 2011), air quality modeling was identified as an alternative to monitoring for demonstrating 

attainment. In October 2011, to prepare for this potential modeling exercise, the Division 

completed an assessment to determine if there was any available meteorological dataset(s)

representative of transport and dispersion conditions at large stationary SO2 sources in 

Colorado. The Division assesses meteorological data representativeness on the factors

discussed in Section 8.3 of EPA’s Guideline on Air Quality Models (40 CFR Part 51 Appendix W; 

November 2005). The only available meteorological data for the area is measured at the 

Colorado Springs Airport. Martin Drake Power Plant is located at the confluence of two large 

creek drainages, within and adjacent to terrain features that cause different meteorological

conditions (e.g. wind directions/speeds, cloud cover) than at the Colorado Springs Airport.
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Therefore, the Division concluded that there was no available representative meteorological

dataset for the Drake Power Plant. 

The Division received air quality modeling from interested stakeholders regarding 1-hour SO2 

NAAQS compliance for the Drake Power Plant in October 2012 and September 2013. These 

modeling submittals use meteorological data from the Colorado Springs Airport. However, 

modeling with non-representative meteorological data affects the magnitude and location of 

impacts. Therefore, it would be inappropriate to base an attainment or nonattainment 

designation on the modeling analyses submitted to the Division, which rely upon 

meteorological data from the Colorado Springs Airport, a non-representative dataset.

The Division is working with CSU to gather representative meteorological data. One year of

meteorological data will be collected and modeling is anticipated to require about 6 months to 

develop and finalize. Consequently, the Division anticipates the final impact analysis to be 

complete in 2017.

SO2 Ambient Monitoring in El Paso County

The Division analyzed the potential to determine if the Drake Power Plant could be classified 

“attainment/nonattainment” based on current or historical ambient monitoring near the 

facility. Currently, there are four 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 may 

be relocated in the future when the Division approves a final modeling impact analysis based on 

maximum modeled impacts and siting relocation issues.

Monitoring data for SO2 has never approached the level of any SO2 standard until this monitor 

was added in January 2013. Table 8 below details the concentrations at the Highway 24 

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

standard since monitoring began in 2013 through July 2015. However, the 99th percentile 

value, as of July 2015, remains below the 75 ppb 1-hour NAAQS at approximately 60 ppb (2015 

data year is not complete).

Table 8: CDPHE Highway 24 Monitor Data (2013 - present)

AQS Site ID

Site 

“Name” Address Year

1st Max 1-

hour

2nd Max 1-

hr

99th% 1-

hour

08-041-

0015 Highway 24

690 W. 

Highway 24

2013 99 81 58

2014 83 57 56

2015* 87 68 68

*Data year is from January 1, 2015 – July 15, 2015 (all data has not been QA/QC’d)

Historically, there has been substantial SO2 monitoring in the Colorado Springs area, with up to 

ten monitors operating during different periods between 1988 and 2007, shown in Figure 2.
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Figure 2: Map of Historical and Current SO2 Monitoring Sites in Colorado Springs 

Metropolitan Area
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Monitors were dispersed across the metropolitan area, with multiple monitors located near the 

two major SO2 sources (Drake and Nixon Power Plants) and one located at higher elevation 

(Chipita Park). The monitoring sites, operator, and years of operation are shown in Table 9.

Table 9: Historical and Current Colorado Springs Area Monitors

AQS Site ID Site “Name” Address Operator Operating Years

08-041-0015 Highway 24 690 W. Highway 

24

CDPHE 2013 – present

08-041-6001 Nixon Base R.D. Nixon Power 

Plant (Exit 125 off 

Interstate I-25)

CSU 1988 – 2007

08-041-6002 Propane Air-Plant 3550 Marksheffel 

Road

CSU 1988 – 1990

08-041-6003 Nixon South R.D. Nixon Power 

Plan (Exit 123 off 

Interstate I-25)

CSU 1988 – 1991

08-41-6004 Woodmen 6000 Pulpit Rock 

Drive

CSU 1988 – 2007

08-41-6005 Pinello Ranch 4940 S. Highway 

85/87

CSU 1988 – 1996

08-41-6006 Chipita Park 9400 Chipita Park 

Road

CSU 1988 – 1996

08-41-6009 Nixon North R.D. Nixon Power 

Plan (Exit 123 off 

Interstate I-25)

CSU 1988 – 1996

08-41-6011 Colorado College 130 West Cache 

La Poudre

CSU 1989 – 2003

08-41-6013 Arvada Nevada 1699 S. Corona 

Avenue

CSU 1989 – 1996

08-41-6018 Sand Creek 4125 Center Park 

Drive

CSU 1997 – 2002

The monitors all showed widely variable concentration ranges from year to year. The two 

monitors closest to the Drake Power Plant (Colorado College and Arvada Nevada) ranged from 

39 ppb for the lowest annual 1st maximum value to 122 ppb for the highest. Monitors located 

further from the power plants, but still within the Colorado Springs urban area (Woodmen, 

Pinello Ranch, Sand Creek, and Propane-Air Plant) showed 1st maximum values well below the 

standard, ranging from 10-20 ppb in some years to maximum values between 50-60 ppb in 

other years. These values were all recorded in the late 1990’s. The higher elevation site also 

showed 1st maximum values well below the standard, with the lowest at 18 ppb and the highest 

at 30 ppb. These values were recorded in the late 1980’s and mid 1990’s. It is important to 

note that annual 99th percentile values from all historical monitors near the Drake Power Plant 

never came close to exceeding the current 75 ppb NAAQS, with values lower for 3-year 99th

percentiles. 
Page 15 of 15

Since the majority of SO2 emissions in El Paso County are from the Drake and Nixon Power 

Plants, the Division assessed how SO2 emissions have changed from these sources since the 

historical monitoring period. Figure 3 and Figure 4 illustrate there has been a downward trend 

from both plants in past 20 years. This trend will continue due to Regional Haze SIP 

requirements for both plants. Although monitors are no longer dispersed throughout the 

Colorado Springs area, this information demonstrates that prior monitored values were below 

the current 1-hour SO2 standard and that SO2 emission reductions have been both declining 

historically and will continue to decline in the future, resulting in lower ambient SO2 

concentrations.

Figure 3: Drake Power Plant Annual SO2 Emissions (tons/year) (1985-2014)

Figure 4: Nixon Power Plant Annual SO2 Emissions (tons/year) (1985-2014)

0

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