Page 1 of 5

Technical Support Document

Assessment of Sierra Club’s 2015 1-Hour 

SO2 NAAQS Compliance Evaluation

December 1, 2015

Colorado Department of Public Health and Environment

Air Pollution Control Division

4300 Cherry Creek Drive South

Denver, Colorado 80246
Page 2 of 5

Designation Recommendation Addendum: Martin Drake Power Plant

As requested by the United States Environmental Protection Agency (EPA), Region 8, on 

November 12, 2015, this Technical Support Document (TSD) evaluates third-party modeling 

that was submitted to the Air Pollution Control Division (Division) and the EPA regarding the 

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

designation recommendations for the Colorado Springs Utilities (CSU) – Martin Drake Power 

Plant. This document details both qualitative and quantitative comparisons to determine 

consistency with EPA’s current modeling guidelines and guidance. 

Background

The classification recommendation (“unclassifiable”) for the area surrounding Drake Power 

Plant was adopted by the Colorado Air Quality Control Commission (Commission) following a 

public hearing held on August 20, 2015. An additional informational briefing, including an 

update by CSU on the meteorological tower and data gathering, was held on October 15, 2015. 

The Commission considered third-party modeling that projects nonattainment of the NAAQS 

and monitoring data that is indicative of attainment as part of this specific classification 

recommendation. The Commission voted to recommend designating the area as unclassifiable, 

which will allow extra time to acquire more data to better analyze compliance with the SO2 

standard.

The Division received a number of unsolicited third-party external modeling analyses related to 

compliance with the 1-hour SO2 NAAQS for the Martin Drake Power Plant on the following 

dates:

 Sierra Club/Wingra Engineering (dated October 4, 2012)

 Air Expertise Colorado (dated September 23, 2013)

 Sierra Club/Wingra Engineering (dated August 5, 2015)

 Air Expertise Colorado (given to the Commission on October 15, 2015)

The Division received the 2015 Sierra Club modeling on September 18, 2015, on the same date 

as EPA. The above listed modeling submittals utilized meteorological data from the Colorado 

Springs Airport. However, the Martin Drake Power Plant is located immediately adjacent to 

complex terrain that is substantially different from the flat terrain at the airport. Modeling with 

meteorological data that is not representative of site conditions would affect the magnitude 

and location of air quality impacts. The airport meteorological data is not representative in this 

case, as the Division explained on October 2, 2015.

1

 Therefore, it would be inappropriate to 

determine an attainment or nonattainment designation based on modeling analyses that rely

upon meteorological data from the Colorado Springs Airport. 

Monitoring is warranted because of the fact that there are two facilities with significant SO2 

emissions in a complex terrain region. Additionally, the ambient monitoring requirements for 

the 2010 1-hour SO2 NAAQS included a new SO2 monitor in the Colorado Springs metropolitan 

 

1

CDPHE, Technical Services Program, Air Pollution Control Division, “Review of Martin Drake Power 

Plant/Colorado Springs, Colorado/Evaluation of Compliance with the 1-hour NAAQS for SO2 dated 

August 5, 2015” (Oct. 2, 2015)
Page 3 of 5

area. In order to site the monitor in the most appropriate location, the Division conducted site 

reviews along with examinations of area topography, meteorology, and emissions sources. The 

Division began operating an ambient SO2 monitor in Colorado Springs at Highway 24 and 8th

Street (AQS-ID: 08-041-0015) on January 10, 2013. This was an existing carbon monoxide site 

that was deemed to be appropriate to characterize SO2 in the area to meet regulatory 

requirements. A required Site Modification form was submitted to EPA in late December 2012 

and was also included in Colorado’s approved 2013 Network Monitoring Plan.

Table 1 below details the concentrations at the Highway 24 monitor over the past several

years. This monitor has noted four total exceedances of the 75 ppb standard since monitoring 

began in 2013 through November 2015. However, the 3-year average 99th percentile value, as 

of November 2015, remains below the 75 ppb 1-hour NAAQS at approximately 61 ppb (2015 

data year is not complete).

Table 1: 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 72 57

2015* 87 70 69

*Data year is from January 1, 2015 – November 17, 2015 (all data has not been QA/QC’d)

Monitoring data from a number of other SO2 monitors that operated during various years in 

the Colorado Springs metropolitan area are summarized in the September 18, 2015 Technical 

Support Document that accompanied the Commission’s August 20, 2015 unclassifiable 

designation recommendation. This historical data is indicative of attainment of the 75 ppb 1-

hour NAAQS. 

Assessment of Compliance Evaluation

On September 22, 2015, the Division completed a review of the September 18th (dated August 

5

th) Sierra Club (SC) modeling analyses pertaining to source specific SO2 NAAQS designations for

areas surrounding the Martin Drake and Ray Nixon coal-fired power plants. Other similar thirdparty modeling analyses submitted to the Division, EPA, or the Colorado AQCC were not 

included in this review. The Division recognizes that these other submittals are important for 

context purposes. However, the more recent Sierra Club modeling analysis is the most relevant 

to illustrate consistency with EPA modeling guidance and guidelines. The following EPA 

documents were used in the technical review:

 Draft SO2 NAAQS Designations Modeling Technical Assistance Document (TAD)2

 Guidelines on Air Quality Models – Appendix W to 40 CFR 51 (Appendix W)

 AERMOD Implementation Guide (AIG)

 Relevant clarification memos issued by EPA3

 

2 Draft released in December 2013
Page 4 of 5

The Division reviewed key parameter inputs and identified several inputs that were inconsistent 

with current EPA guidance/guidelines, as noted below:

1. A downwash analysis was not used in SC’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 SC 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. SC’s evaluation assumed that Martin Drake is located in a rural area based on 

examination of land use/cover. The Division 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 SC 

evaluation. The Division noted use of higher resolution NED is more appropriate.

5. A flagpole height of 1.5 meters was used all receptors in the SC evaluation. The Division 

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

Appendix W.

Despite the lack of representative meteorological data, the Division conducted investigative

modeling using the Sierra Club AERMOD Model input files but changed key modeling inputs

identified above to be consistent with EPA modeling guidelines. The resulting AERMOD model 

output, shown in Table 2 below, yielded significant differences in the top ten modeled 4th-high 

1-hr daily maximum concentrations for the Drake Power Plant impacts. For clarification 

purposes, the 1-hour SO2 NAAQS of 75 ppb equals 196.5 μg/m3.

4

Table 2: Differences in Maximum Concentrations for Drake Power Plant Impacts (μg/m3)

Modeler (Range % 

Differential)

Top 4th-high 1-hr daily maximum 

Concentration (μg/m3) (without 

background)

(Modeled @ Actual Emission Rates)

Top 4th-high 1-hr daily maximum 

Concentration (μg/m3) (without 

background)

(Modeled @ Allowable Emission 

Rates)

Sierra Club (0%) 1,585.3 567.5

Division (-88.5% to 

-83.5%)

182.3 – 263.2 65.3 – 94.2

The Division’s investigative modeling was intended only to assess SC’s modeling analysis. The 

Division is not endorsing its results as an estimate of SO2 concentrations in the Colorado 

 

3

“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

4

The general equation is μg/m3=(ppb)*12.187*(M)/(273.15+oC) where M is the molecular weight of the 

gaseous pollutant. An atmospheric pressure of 1 atmosphere and temperature of 25 degrees Celsius

are assumed.
Page 5 of 5

Springs area. It is important to note the Division did not evaluate the appropriateness of the 

assumed SO2 emission rates as part of this review; therefore, SC’s allowable and actual 

emission rate inputs were not changed. Furthermore, the assumed representative background 

SO2 concentration was not assessed by the Division as part of this review. Because the SC 

modeling did not use representative meteorological data, and because correcting several inputs

to the SC modeling yielded significantly different results, the Division determined that a source 

specific designation should not rely on the SC modeling.