Review of

Martin Drake Power Plant / Colorado Springs, Colorado /

Evaluation of Compliance with the 1‐hour NAAQS for SO2 dated

August 5, 2015

Modeling, Meteorology, and Emission Inventory Unit (Modeling Unit)

Air Pollution Control Division (APCD) / Technical Services Program (TSP)

Colorado Department of Public Health & Environment

October 2, 2015

Per a request from the TSP program manager on September 22, 2015, a review was completed

for the analysis titled Martin Drake Power Plant / Colorado Springs, Colorado / Evaluation of

Compliance with the 1‐hour NAAQS for SO2 / August 5, 2015 that was submitted by the Sierra

Club to U.S. EPA Region 8 on September 18, 2015 regarding SO2 NAAQS designation of areas

surrounding the Martin Drake (Drake Power Plant) and Ray D. Nixon (Nixon Power Plant) coal‐

fired power plants.  

A previous, similar evaluation, titled Martin Drake Power Plant / Colorado Springs, Colorado /

Sierra Club Evaluation of Compliance with the 1‐hour SO2NAAQS / October 4, 2012, was

submitted by the Sierra Club to U.S. EPA on March 18, 2013 in response to EPA’s Responses to

State and Tribal 2010 Sulfur Dioxide Designation Recommendations (the “120‐day letters”). A

review of the Sierra Club’s 2012 evaluation was not requested from the Modeling Unit; thus,

the 2012 evaluation was not reviewed.

U.S. EPA’s most recent applicable guidance document was released in December 2013 in draft

form: SO2 NAAQS Designations Modeling Technical Assistance Document (TAD). Per the TAD,

“States should submit a modeling and analysis protocol that details the methodology and

model inputs before commencement of the modeling exercise”, which would provide

information to support the states’ modeling decisions and recommended designations, and

provide a basis for U.S. EPA’s evaluation of the decisions and recommendations. A modeling

and analysis protocol for the Drake Power Plant has not been submitted to U.S. EPA Region 8

due to the delay in obtaining the necessary data, representative meteorological data, needed

to complete a modeling analysis based on recommendations in the TAD.  

In the absence of a designations modeling and analysis protocol for the Drake Power Plant that

has been evaluated by U. S. EPA, the methodology and model inputs in the Sierra Club’s 2015

evaluation (SC’s evaluation) have been reviewed to determine if they are consistent with

recommendations in the 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

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W) and the U.S. EPA’s AERMOD Implementation Guide (AIG), and relevant clarification memos

issued by U.S. EPA1

.

Model Selection

The model versions used in SC’s evaluation are presented in the table below along with the

versions that are regulatory (or most recent, if not a regulatory model) when this review was

conducted.

Program Version Used in SC’s Evaluation Regulatory/Most Recent Version

AERMOD 14134 15181

AERMAP 11103 11103

AERMET 14134 15181

AERMINUTE 14237 14337

AERSURFACE 13016 13016

BPIPPRM Not Used 04274

The use of AERMOD 15181 in SC’s evaluation resulted in no differences in the top 10 modeled

4th‐high 1‐hr daily max concentrations.

Meteorological Data

Three years of meteorological data collected at the Colorado Springs Airport NWS station were

used in SC’s evaluation. Per the TAD and Appendix W, representative meteorological data

should be used. SC’s evaluation does not assess whether the Colorado Springs Airport NWS

data is representative of transport and dispersion conditions at the Drake Power Plant.

Per the Division’s assessment, 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 Division’s meteorological data assessment is presented in Attachment A. The

use of the Colorado Springs Airport data to model the Drake Power Plant is inconsistent with

the TAD and Appendix W.

SC’s evaluation also includes the Nixon Power Plant, which is located approximately 24 km

southeast of the Drake Power Plant and 20 km south of the Colorado Springs Airport NWS

station, but does not assess whether the Colorado Springs Airport NWS data is representative

of transport and dispersion conditions at the Nixon Power Plant.  Meteorological data

                                                      

1 “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

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previously obtained at the Nixon Power Plant indicate wind conditions that are different than

those at the Colorado Springs Airport NWS, which questions the representativeness of the NWS

data. The use of the Colorado Springs Airport data to model the Nixon Power Plant is

inconsistent with the TAD and Appendix W.

It is unknown whether the high modeled concentrations reported in SC’s evaluation will

increase or decrease with the use of representative meteorological data.

Modeling Domain

The modeling domain (50 km x 50 km) is centered on the Drake Power Plant in SC’s evaluation.

The extent of the modeling domain is expected to be adequate to account for all areas of

impact from the Drake Power Plant and the Nixon Power Plant.

Source Characterization

The emission releases at the Drake Power Plant and the Nixon Power Plant were modeled as

un‐capped, vertical point sources. The modeled stack parameters (release height, exit

temperature, exit velocity, inside stack diameter) for the three boilers at the Drake Power Plant

match the modeled stack parameters used in Colorado’s Regional Haze Plan analyses. Modeled

stack parameters for the boiler at the Nixon Power Plant either match or are similar to the

modeling stack parameters used in Colorado’s Regional Haze Plan analyses.

Actual stack heights were used in SC’s evaluation for modeling actual and allowable emissions.

As explained in the TAD, modeling with actual emissions and actual stack heights is an attempt

to mimic a monitor. The TAD also clarifies that if allowable emissions are modeled, the GEP

stack height policy should be used. SC’s evaluation did not include a downwash analysis to

justify GEP heights greater than 65 meters; therefore, the maximum stack height for modeling

with allowable emissions is 65 meters. SC’s evaluation of impacts from allowable emissions

modeled several stack heights greater than 65 meters: Unit 7 at the Drake Power Plant (76.2 m)

and the Nixon Power Plant stack (140.21 m). To represent actual conditions as closely as

possible, building parameters should be used in AERMOD since there are structures at both the

Drake Power Plant and the Nixon Power Plant to cause downwash of the plumes from these

plants.  

Elevations for all stacks were determined with AERMAP using the USGS National Elevation

Dataset (NED) at 1 arc‐second resolution for SC’s evaluation. For the 7.5 minute quadrangle in

which the Drake Power Plant is located in, the base data source used in developing the NED is

from 1947. All of the modeled emission units were built after 1947; thus, as discussed in the

AIG, the use of plant survey data would provide more accurate base elevations.

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The TAD and Appendix W describes two (2) methods for determining if an area should be

considered urban: land use procedure and population density procedure. SC’s evaluation

assumes the facility is located in a rural location based on the examination of land use/cover

within 3 km of the Drake Power Plant using AERSURFACE and USGS 1992 National Land Cover

Dataset (NLCD). The 1992 NLCD is over 20 years old and its representativeness of the current

conditions in Colorado is highly questionable. The Drake Power Plant is located in the city of

Colorado Springs, the 2nd largest city in Colorado with an estimated population of 416,000 to

445,000 for years 2010 to 2014. The most recent USGS NLCD is for the year 2011. The execution

of the land use procedure is hindered by the 2011 NLCD land cover classifications that do not

correlate directly to the land use types specified in the TAD and Appendix W (based on a 1978

journal publication).

Using the population density procedure and U.S. 2010 Census Tract data, the city of Colorado

Springs has numerous tracts with population density greater than 750 people/km2

. While the

Drake Power Plant is located in a tract with a population density less than 750 people/km2, it

would experience the urban heat island effect since it is surrounded by tracts with population

densities ranging from 750 people/km2 to greater than 2,000 people/km2 (Attachment B).

Based on the results of the population density procedure, it would be more appropriate to

model Drake as an urban source than as rural source. When SO2 is modeled in an urban area, a

half‐life of 4 hours is applied by AERMOD.

Receptors

In SC’s evaluation, receptors were placed using three nested grids:

 100‐meter spacing within a 10 km x 10 km square centered on the Drake Power Plant;

 500‐meter spacing extending 5 km in all cardinal directions from the edge of the 100‐

meter receptor grid;

 1‐km spacing extending 40 km in all cardinal directions from the edge of the 500‐meter

receptor grid.

The highest impacts from the Drake Power Plant are expected to occur within the 100‐meter

receptor grid. Due to the coarse receptor spacing around the Nixon Power Plant, the highest

impacts may not be captured with any of the nested receptor grids.

Receptor elevations and hill heights were determined with AERMAP using the USGS National

Elevation Dataset (NED) at 1 arc‐second resolution for SC’s evaluation. Due to the non‐simple

terrain features in the modeling domain and per the AIG, use of higher resolution NED (1/3 arc‐

second) is more appropriate.

A flagpole height of 1.5 meters was used at all receptors in SC’s evaluation. The TAD states that

the use of flagpole receptors is not necessary and Appendix W refers to maximum ground‐level

concentrations.  

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Background Concentration

SC’s evaluation used a 1‐hr SO2 background concentration of 75.9 g/m3 based on measured

concentrations in Adams County, which is located over 100 km away in the Denver

Metropolitan Area. Per the TAD and Appendix W, air quality data in the vicinity of the source is

used to determine the background concentration.  Per the Division’s analysis (Attachment C), a

1‐hr SO2 background concentration of 0.058 ppm, or 152 g/m3

, based on measured

concentrations in Colorado Springs at the Division’s Highway 24 monitoring station is

recommended. To avoid double counting, an alternative is to exclude the concentrations

measured at the Highway 24 monitoring station that are impacted by the Drake Power Plant;

this technique is discussed in the TAD and Appendix W.

Emissions Inputs

Two sets of emission rates were modeled in SC’s evaluation: allowable and actual. The modeled

allowable emission rates match those summarized in Table 4 of SC’s evaluation report. The

actual hourly emission rates were downloaded from U.S. EPA’s Air Markets Program Data

webpage. The determination of whether these emission rates are consistent with the

TAD/Appendix W/EPA guidance is not within the purview of this review; this determination is

deferred to technical staff in the Stationary Sources Program or the Planning and Policy

Program.

Results

From some exploratory modeling, the use of ground level receptors, the urban option for the

Drake Power Plant plumes, estimated building dimensions to account for downwash effects on

the Drake Power Plant plumes, and receptor elevations/hill heights derived from NED at 1/3

arc‐second resolution in SC’s evaluation yields differences in the top 10 modeled 4th‐high 1‐hr

daily max concentrations for Drake Power Plant impacts ranging from ‐88.5% to ‐83.4% and for

Nixon Power Plant impacts ranging from ‐1.7% to 1.3%. It is unknown whether the high

modeled concentrations reported in SC’s evaluation will increase or decrease with the use of

representative meteorological data.

   

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Attachment A

Meteorological Determination for the Martin Drake Power Plant

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Meteorological Determination for the 

Martin Drake Power Plant 

Modeling, Meteorology, and Emission Inventory Unit

Air Pollution Control Division/ Technical Services Program

Colorado Department of Public Health & Environment

September 3, 2015

The process completed by the Modeling, Meteorology, and Emission Inventory Unit (MMEIU) to 

determine if any of the available meteorological datasets are representative of transport and dispersion 

of the Martin Drake Power Plant plumes for 1-hr SO2 NAAQS designations are formally documented in 

this report. 

On September 21, 2011, U.S. EPA issued draft guidance on state implementation of the 1-Hour SO2

NAAQS. This guidance identifies air quality modeling as an alternative to monitoring for demonstrating 

attainment with the 1-hr SO2 NAAQS. In October 2011, to prepare for this potential modeling exercise, 

MMEIU completed an assessment to determine if there are any available meteorological datasets that 

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

including the Martin Drake Power Plant. The 2011 assessment was documented in brief and informal 

notes/figures that would need to be edited to fully convey the elements of the assessment if formal 

documentation is needed. As a result of comments from the Air Quality Control Commission regarding 

meteorological data representativeness for the Drake Power Plant during the August 20, 2015 hearing, 

formal documentation of the meteorological determination completed for this power plant is provided 

below. 

Meteorological data requirements for regulatory applications of AERMOD are specified in U.S. EPA’s 

Guideline on Air Quality Models (40 CFR Part 51 Appendix W; November 2005), which is referenced in all 

of U.S. EPA guidance documents for SO2 designations modeling. The Guideline describes factors that 

need to be considered when determining representativeness of meteorological data: 

“The meteorological data used as input to a dispersion model should be selected on the basis of spatial and 

climatological (temporal) representativeness as well as the ability of the individual parameters selected to 

characterize the transport and dispersion conditions in the area of concern. The representativeness of the 

data is dependent on: (1) The proximity of the meteorological monitoring site to the area under 

consideration; (2) the complexity of the terrain; (3) the exposure of the meteorological monitoring site; and 

(4) the period of time during which data are collected.” 

Regarding regulatory applications of AERMOD, the Guideline explains the following: 

“Of paramount importance is the requirement that all meteorological data used as input to AERMOD must 

be both laterally and vertically representative of the transport and dispersion within the analysis domain. 

Where surface conditions vary significantly over the analysis domain, the emphasis in assessing 

representativeness should be given to adequate characterization of transport and dispersion between the 

source(s) of concern and areas where maximum design concentrations are anticipated to occur.”

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To determine if available meteorological data is representative of transport and dispersion of plumes 

from the Martin Drake Power Plant, it is necessary to obtain an understanding of the driving forces 

behind wind conditions at the facility. With the exception of mesoscale or synoptic weather systems 

moving through the area, local wind flows are driven by differential heating of the surrounding terrain 

by the sun. 

The Martin Drake Power Plant is located adjacent to the foothills of Colorado Springs directly east of 

Pikes Peak, near the confluence of Monument Creek and Fountain Creek. Fountain Creek flows 

southeast down Ute Pass through Manitou Springs along U.S. Route 24 into Colorado Springs, merging 

with Monument Creek near the Drake Power Plant, and into the Arkansas River in Pueblo. Monument 

Creek flows from the foothills near Palmer Lake (western end of Palmer Divide) to near Monument, then 

along Interstate 25 to its confluence with Fountain Creek. (Figure 1) 

The terrain features that would affect wind conditions at the power plant are Pikes Peak to the west and 

Palmer Divide to the north. The sides of Pikes Peak through which Fountain Creek flows are east facing 

and the uplifted land of the Palmer Divide is south facing. The east facing sides of Pikes Peak will receive 

more direct sunshine in the morning than the Palmer Divide, which will cause the air to rise sooner over 

the mountain than the air over the Palmer Divide. Due to differential heating and the proximity of the 

power plant to the mountain, winds at/near the power plant will be upslope winds that 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. During nighttime and early morning hours, drainage winds flowing from the 

valleys of Fountain Creek and Monument Creek towards the power plant are expected. (Figure 2) 

The highest impacts from the power plant are expected to occur when the plumes are trapped under an

inversion with light surface winds. For the majority of the time when this condition exists, there would 

be light drainage winds transporting the power plant plumes down along Fountain Creek to the 

southeast. There will also be a limited number of inversion events when there will be upslope winds that 

transport the power plant plumes up the valley of Fountain Creek or Monument Creek due to a synoptic 

weather system. High impacts are also expected when the plume impacts the higher terrain southwest 

through northwest of the power plant. This will occur under stable to neutral conditions along with 

upslope winds. These upslope winds could be driven by local winds or synoptic weather systems. 

The only available meteorological data collected in the Colorado Springs area is measured at the airport, 

which is just over 11 km east of the power plant. At the airport, the upslope and downslope winds 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). 

Since the wind directions at the power plant are also expected to have a northerly component for some 

hours due to the Palmer Divide/Monument Creek and some synoptic weather systems would cause 

wind conditions to be comparable within the region, there will be a portion of the winds at the airport 

and the power plant that are similar; however, the airport data will not be representative of winds for all 

other periods when Pikes Pike/Fountain Creek has a greater influence than the Palmer 

Divide/Monument Creek on plume transport, including the periods when the inversions are expected to 

cause the highest power plant impacts. It is also important to consider that the power plant will have a 

lower frequency of sunshine due to a higher occurrence of convective-type clouds and the upslope 

cloud cover that will last longer in the winter at the power plant than at the airport, both of these 

weather conditions will affect plume dispersion and are conducive for higher impacts. 

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The wind speeds at the airport are expected to be greater than the wind speeds at the power plant due 

to the low roughness length of the land cover around the airport anemometer. Also, wind speed and 

direction vary with height above terrain; thus, for elevated plumes such as those at the power plant 

(200+ feet), it is necessary to obtain wind conditions above 10 meters (anemometer height at the 

Colorado Springs Airport) to construct realistic wind profiles to model plume transport. Modeling with 

non-representative meteorological data affects the location and magnitude of plume impacts. 

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 and the Martin Drake Power Plant and no other meteorological 

dataset is available for the area. The collection of meteorological data at multiple levels up to plume 

height, or to a minimum height of 60 meters needs to be conducted to obtain representative 

meteorological data for modeling Martin Drake Power Plant plumes with AERMOD. 

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PALMER DIVIDE

PIKES

PEAK

Fountain Creek

Monument Creek

Fountain Creek

CO Springs

Airport

Martin Drake

Power Plant

I-25

I-25

Hwy 24

Hwy 24

¢ 0 1.25 2.5 5 Miles

Figure 1. Locations of Martin Drake Power Plant

and Colorado Springs Airport

Legend

EcoBase.DBO.HIGHWAYS_CDOT

National Elevation Dataset

Elevation (meters)

High : 4397

Low : 50

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PALMER DIVIDE

PIKES

PEAK

Fountain Creek

Monument Creek

Fountain Creek

CO Springs

Airport

Martin Drake

Power Plant

upslope

downslope

downslope

upslope

¢ 0 1.25 2.5 5 Miles

Figure 2. Expected Wind Directions at the

Martin Drake Power Plant and 

Colorado Springs Airport

Legend

National Elevation Dataset

Elevation (meters)

High : 4397

Low : 50

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Attachment B

Urban Determination for Colorado Springs

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Legend

U.S. 2010 Census Tract

people/sq mi

0 - 1295 (0-500 people/sq km)

1296 - 1943 (500-750 people/sq km)

1944 - 2590 (750-1000 people/sq km)

2591 - 3885 (1000-1500 people/sq km)

3886 - 5180 (1500-2000 people/sq km)

5181 - 31336 (> 2000 people/sq km)

Colorado Springs

Urban Determination - Population Density Procedure

Drake Power Plant (purple marker)

CDPHE/APCD/TSP/MMEIU 9/30/2015

¢ 0 3 6 12 km

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Attachment C

1‐hr SO2 Background Concentration

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4300 Cherry Creek Drive S., Denver, CO 80246-1530 P 303-692-2000 www.colorado.gov/cdphe

John W. Hickenlooper, Governor | Larry Wolk, MD, MSPH, Executive Director and Chief Medical Officer

Doris Jung 

By email: doris.jung@state.co.us 

 September 29, 2015 

Dear Doris, 

You have requested an estimate of air pollution background levels at the Martin Drake power 

plant located in Colorado Springs. Unfortunately, we do not have recent data that we can 

unequivocally state represents a background level for SO2 (i.e. a level that does not include plant 

impacts) currently existing at this location. The only current monitor for SO2 in Colorado Springs is 

our site at Highway 24. Therefore, this is the best estimate of “background” we have. Keep in 

mind that this measurement includes current plant impacts, so using it as the background level for 

modeling will overestimate SO2 levels. In the past, Colorado Springs Utilities has collected SO2 data 

at a number of locations. However, these data are from the year 2007, and many years prior to 

that. In 2007, new diesel fuel rules went in to effect, and SO2 levels have dropped since then. 

 The “Evaluation of Compliance with the 1-hour NAAQS for SO2” analysis that you have cites 

numbers from Adams County (our Welby site). Although the analysis refers to the Welby data as 

“the lowest measured background concentration in the state”, that is not accurate. I have 

measured data from a number of private facilities in the state that are lower than the Welby 

concentrations. However, many of these are rural areas that would not reflect the situation in the 

city of Colorado Springs. Although Welby could be used, in a permitting analysis we would likely 

use the Highway 24 value, because it is a Colorado Springs value, rather than a Denver one. The 

Highway 24 value for an SO2 1-hour 99th percentile, averaging data from 2013 and 2014, is 0.058 

ppm, or 152 ug/m3. 

Sincerely, 

 Nancy D. Chick 

 Environmental Protection Specialist 

 Air Pollution Control Division 

 

C:\BackgroundConc\Excel XML Form Requests\ABC Requests El Paso County\ElPasocountyMartinDrakeABC.docx 

\Cc: D. Jung, 

 E. Malone 

 Pierce, Gordon 

 Wike, Cindy

Dedicated to protecting and improving the health and environment of the people of Colorado 

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