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# Technical Support Document

**Assessment of Sierra Club**  **’s 2015 1** 

**- Hour** 

**SO 2**  

# NAAQS Compliance Evaluation

**December 1 , 2015** 

**Colorado Department of Public Health and Environment** 

Air Pollution Control Division 4300 Cherry Creek Drive South Denver, Colorado 80246Page **2** of **5**



### Designation Recommendatio **n Addendum: Martin Drake Power Plant**

As requested by the United States Environmental Protection Agency (EPA), Region 8, on
November 12, 2015, t his 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 EP A’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 he ld 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 a ttainment 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 compl iance 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 imme diately adjacent to complex terrain that is substantially different from the flat terrain at the airport. M odeling with meteorological data that is not representative of site conditions would affect the magnitude

an d location of air quality impacts. The airport meteorological data is not representative in this case, as the Division explained on October 2, 2015

. 1 T herefore, it would be inappropriate to
determine an attainment or nonattainment designation based on modeling analyses that rel y 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
th e 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 e missions sources. The Division began operating an ambient SO2 monitor in Colorado Springs at Highway 24 and 8 th
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 r e q u i r e d S i t e M o d i f i c a t i o n f o r m was submitted to EPA in l a t e D e c e m b e r 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 s e v e r a l years. This monitor has noted four total exceedances of the 75 ppb standard since monitoring began in 2013 through November 2015. How ever, the 3

- year average 99 th percentile value, as of November 2015, remains below the 75 ppb 1

- hour NAAQS at approximately 6 1 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 99 th % 1

-

hour 08

- 041

- 0015 Highway

24 690 W.

Highway 24 2013 99 81 58

2014 83 72 5 7 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 accompan ied the Commission’s August 20, 2015 unclassif iable designation recommendation . Th is 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 18 th (dated August

5 th ) Sierra Club (SC) modeling ana l yses pertaining to source specific SO 2 NAAQS designations for areas surrounding the Martin Drake and Ray Nixon coal

- fired power plants

. Other s imilar third

party modeling analyses submitted to the Division, EPA, or the Colorado AQCC w ere not included in this review. The Division recognizes that these other submittals are important for

context purposes

. However, t he more recent Sierra Club modeling analysis is the most relevant to illustrate consistency with EPA modeling guidance an d guideline s

. The following EPA
documents were used in the technical review :

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

 Guidelines on Air Quality Models

– Appendix W to 40 CFR 51 (Appendix W)

 AERMOD Implementation Guide (A IG)

 Relevant clarification memos issued by EPA 3

2 Draft released in December 2013Page **4** of **5**

The Division reviewed key parameter inputs and identified several inputs that were in consistent
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 th at 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 m eteorological data

, the Division c onducted investigative modeling using the Sierra Club AERMOD Model input files but changed key modeling inputs
identified above to be consisten t with EPA modeling guidelines . The resulting AERMOD model output, shown in Table 2 below, yielded significant differences in the top ten modeled 4 th

- 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/m 3

. 4

**Table 2**  **: Differences in Maximum Concentrations for Drake Power Plant Impacts (μg/m 3**  **)**

Modeler (Range %
Differential ) Top 4 th

- high 1

- hr daily maximum

Concentration (μg/m 3 ) (without background)

( Modeled @ Actual Emission Rates) Top 4 th

- high 1

- hr daily maximum

Concentration (μg/m 3 ) (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/m 3 =(ppb)*12.187*(M)/(273.15+oC) where M is the molecular weight of the gaseous pollutant. An atmospheric pressure of 1 atm osphere and temperature of 25 degrees Celsius are assumed

.Page **5** of **5**

Springs area. It is important to note t he Division did not evaluate the appropriate ness of the
assumed SO2 emission rates as part of this review; therefore, SC’s allowable and actual emission rate inputs were not changed. Furthermore, t he 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 s ource specific designation should not rely on the SC modeling.