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TABLE OF CONTENTS 

SECTION PAGE 

1.0 EXECUTIVE SUMMARY ............................................................................................. 1-1

2.0 INTRODUCTION/BACKGROUND .............................................................................. 2-1

3.0 PLANT DESCRIPTION .................................................................................................. 3-1

4.0 EMISSIONS DATA ........................................................................................................ 4-1

5.0 CALPUFF MODELING SYSTEM OVERVIEW........................................................... 5-1

6.0 GEOPHYSICAL AND METEOROLOGICAL DATA .................................................. 6-1

6.1 Modeling Domain ......................................................................................................... 6-1

6.2 Terrain ........................................................................................................................... 6-1

6.3 Land Use ....................................................................................................................... 6-2

6.4 Meteorological Data Base ............................................................................................. 6-2

7.0 AIR QUALITY MODELING METHODOLOGY ......................................................... 7-1

7.1 Model Selection ............................................................................................................. 7-1

7.2 Meteorological Modeling Options ................................................................................ 7-1

7.3 CALPUFF Computational Domain and Receptors ....................................................... 7-2

7.4 Dispersion Modeling Options ....................................................................................... 7-2

7.5 Air Quality Monitoring Data ......................................................................................... 7-3

7.6 Regional Haze ............................................................................................................... 7-4

7.7 Regional Haze Impact Calculation Methodology ......................................................... 7-4

7.8 Background Concentrations and Relative Humidity Adjustment Factors .................... 7-5

8.0 BART PROTOCOL MODELING RESULTS ................................................................ 8-1

9.0 SUMMARY ..................................................................................................................... 9-1
TABLE OF CONTENTS 

(Continued) 

TABLES 

Table ES-1: Modeled Visibility 20% Best Days*.................................................................................... 1-4

Table 3-1 Colstrip Plant Stack Parameters .......................................................................................... 3-4

Table 4-1: Colstrip Clean Air Markets Summary – Maximum Daily Data .......................................... 4-3

Table 4-2: Colstrip Particulate Matter Emission Factors from Stack Testing ...................................... 4-3

Table 4-3: Summary of Colstrip Unit 3 September 1999 Coal Analysis .............................................. 4-4

Table 4-4: Colstrip Units 1 and 2 Annual Average Emissions Data ..................................................... 4-4

Table 4-5: Colstrip Unit Cases, English Units ...................................................................................... 4-5

Table 4-6: Colstrip Unit Cases, Metric Units........................................................................................ 4-8

Table 4-7: Colstrip - Modeling Scenarios ........................................................................................... 4-11

Table 6-3. Meteorological Data Sources and Parameters Available ................................................... 6-7

Table 6-4. NWS Upper Air Stations ................................................................................................... 6-7

Table 6-5. NWS Hourly Surface Stations ........................................................................................... 6-8

Table 6-6. NWS Precipitation Stations ............................................................................................... 6-9

Table 7-5. Ozone Stations ................................................................................................................... 7-7

Table 7-6. Summary of Background Natural Visibility Conditions .................................................... 7-9

Table 7-7. Monthly f(RH) values based on Representative IMPROVE Site Locations...................... 7-9

Table 8-1 Class I Impacts (Deciviews) Method 8 (8th Highest Annual Average)* ............................ 8-1

Table 8-2 Class I Impacts (Deciviews) Method 8 20% Best Days Condition .................................... 8-2

FIGURES 

ES-1 Colstrip Location and Class I Areas Evaluated ............................................................................ 1-3

3-1 Colstrip Power Plant Location ...................................................................................................... 3-2

3-2 Plot Plan of the Colstrip Facility Showing the Relative Locations of Units 1-4. ......................... 3-3

6-1 Terrain Contours ........................................................................................................................... 6-4

6-2 Dominant Land Use Categories .................................................................................................... 6-5

6-3 Surface Stations ............................................................................................................................ 6-6

7-1 Locations of Receptors ................................................................................................................. 7-6

7-4 Locations of Ozone Monitoring Stations used in the CALPUFF Modeling ................................. 7-7
1-1 

1.0 EXECUTIVE SUMMARY 

Atmospheric dispersion and chemistry modeling was used to predict the possible 

visibility impacts of emissions from the Colstrip generating plant in Colstrip, MT at Prevention 

of Significant Deterioration (PSD) Class I areas. The Class I areas within 300 kilometers (km) 

of Colstrip are the UL Bend National Wildlife Refuge (200 km to the northwest), the North 

Absaroka Wilderness Area (254 km to the west), the Theodore Roosevelt National Park (260 km 

to the northeast), the Washakie Wilderness Area (278 km to the southwest) and Yellowstone 

National Park (281 km to the west). These Class I areas are shown in relation to the Colstrip 

plant in Figure ES-1. 

The CALPUFF model was used to evaluate the visibility impacts of both historical and 

present emissions from Colstrip as well as a variety of emission control alternatives. The model 

setup and input meteorological data were obtained from Environmental Protection Agency 

(EPA)/Western Regional Air Partnership (WRAP) files. Colstrip Units 1&2 are rated at 

333gross megawatts (MW) each and Units 3&4 are rated at 805 gross MW each. Emissions data 

were from various sources representing historical worst-case actual emissions (2001-2003, the 

baseline Best Available Retrofit Technology (“BART”) modeling period), current worst-case 

actual emissions (reference modeling period) and several potential control strategies to identify 

the “best system of continuous emission reduction” applicable to the plant for filterable 

particulate, sulfur dioxide (SO2) and nitrogen oxides (NOx) control. These units already have 

relatively low emissions due to the use of wet scrubbers that remove both particulate and sulfur 

dioxide, low sulfur Powder River Basin coal and good combustion control using low NOx

burners (LNB). A range of demonstrated control alternatives for sub-bituminous coal-fired 

boilers was modeled: 

 Baseline case controls – those controls in-place in the baseline period (2001-2003). 

These are wet scrubbers and LNB. 

 Reference case controls – those controls in-place as of November 2011. 

 For Units 1 and 2, the reference case controls are wet scrubbers, upgraded 

LNB, and digital boiler controls. 

 For Units 3 and 4, the reference case controls are wet scrubbers, upgraded 

LNB, Separated Over-Fire Air (SOFA), and digital boiler controls. 

 Potential add-on controls 

 SOFA for Units 1 and 2 (including further upgrades to the LNB). 

 Selective Non-Catalytic Reduction (SNCR) for all units. 
1-2 

 Selective Catalytic Reduction (SCR) for all units. 

 Enhanced SO2 controls with lime injection for Units 1 & 2. 

 Enhanced particulate matter controls for Units 1 & 2. 

The modeling was conducted by TRC and a summary of the visibility modeling results 

for the 20 percent best natural background visibility days is provided in ES-1. The results are 

presented as the 98th percentile predicted delta-deciview impacts from natural background 

conditions where 1.0 delta-deciview is generally considered the average limit of human 

perception. Incremental changes of less than 1.0 between cases will not be generally perceptible. 

Use of the 98th percentile predicted visibility impact is conservative because predicted visibility 

impacts of this magnitude or greater only occur eight days per year, but these predicted impacts 

are compared against all natural visibility conditions. Moreover, as elaborated herein, several 

additional conservative assumptions are built into the modeling protocol, so the actual visibility 

impacts should be regarded as less than the predicted values presented in Table ES-1.
1-3 

Figure ES-1: Colstrip Location and Class I Areas Evaluated 
1-4 

Table ES-1: Modeled Visibility 20% Best Days* 

*Scenario visibility predictions include application of indicated controls on all Units listed in the sub-headings (i.e. U

3&4). 
2-1 

2.0 INTRODUCTION/BACKGROUND 

The owners of the Colstrip power plant, PPL Montana, Puget Sound Energy Inc., 

Portland General Electric Company, Avista Corporation, PacifiCorp and NorthWestern Energy, 

are working with the U.S. Environmental Protection Agency (EPA) Region 8 to assess the 

efficacy of emission control options vis-à-vis the facility’s predicted visibility impacts at 

Prevention of Significant Deterioration (PSD) Class I areas. Regional haze is caused by 

naturally occurring atmospheric particles and humidity as well as by anthropogenic emissions of 

primary particulate matter and of secondary emissions resulting from atmospheric gas to particle 

conversions of sulfur dioxide (SO2) and nitrogen oxides (NOx). The Regional Haze Regulations 

are intended to improve visibility in PSD Class I Areas. 

The Colstrip plant is located in Colstrip, Montana. Units 1 and 2 began operating in 1975 

and 1976, and Units 3 and 4 began operation in 1984 and 1986. Colstrip Units 1&2 each have a 

gross generating capacity of 333 megawatts (MW), and have low emissions due to the use of low 

sulfur coal and venturi scrubbers. Colstrip Units 3&4 each have a gross generating capacity of 

805 MW and likewise use low sulfur coal and scrubbers as well as SOFA. 

Previous reports, Best Available Retrofit Technology (BART) Assessment of Colstrip 

Generating Station, August 2007 and, responding to EPA’s comments, Addendum to PPL 

Montana’s Colstrip BART Report, June 2008, have been prepared and submitted relative to 

control strategies and predicted visibility impacts for Units 1&2 under the BART rules. The 

more recent, NOx Control Update to PPL Montana’s Colstrip Generating Station BART Report, 

September 2011, updated cost estimates, but did not contain any new visibility modeling. This February 

2012 report addresses modeled visibility impacts of various control strategies for Units 1 through 

4. This report describes the facility emissions for a “baseline case” as existed in 2001-2003 

(used for the BART modeling), the emissions for a “reference case” as exists for current 

operations, and for various potential emission control options. The inputs to the CALPUFF 

model, used to predict the visibility impacts, are described and the predictions are presented. 
3-1 

3.0 PLANT DESCRIPTION 

The Colstrip power plant has four coal-fired generating units. Units 1 and 2 began 

operation in 1975 and 1976 respectively, and Units 3 and 4 began operations in 1984 and 1986, 

respectively. Units 1 and 2 are rated at 333 megawatts of gross generating capacity each and 

Units 3 and 4 are rated at 805 megawatts of gross generating capacity each. The facility burns 

low sulfur Powder River Basin sub-bituminous coal. A map of the plant location is shown in 

Figure 3-1. 

The air pollution control devices for Units 1 and 2, include wet scrubbers installed as 

original equipment, upgraded low NOx burners installed on both units by 2006, and digital boiler 

controls installed on both units by 2007. Units 3 and 4 have wet scrubbers installed as original 

equipment, as well as upgraded low NOx burners, Separated Over-Fire Air (SOFA) and digital 

boiler controls installed on Unit 3 in 2007, and on Unit 4 in 2009. Figure 3-2 presents a plot plan 

of the facility showing the west to east arrangements of Units 1 through 4. 

Each of the four units is serviced by its own, single flue stack. Table 3-1 presents the 

stack parameter data used for modeling input for all modeling scenarios. 
3-2 

Figure 3-1 Colstrip Power Plant Location 
3-3 

Figure 3-2 Plot Plan of the Colstrip Facility Showing the Relative Locations of Units 1-
3-4 

Table 3-1 Colstrip Plant Stack Parameters 

Stack ID # Lambert Conformal 

Coordinates1 

Stack Height Base 

Elevation 

Diameter Gas Exit 

Velocity 

 LCC East LCC North 

 km km M m m m/sec 

Unit 1 823.887 185.596 152.4 988 5.03 29.7 

Unit 2 823.930 185.598 152.4 988 5.03 28.44 

Unit 3 824.044 185.514 210.9 988 7.32 33.5 

Unit 4 824.142 185.516 210.9 988 7.32 33.5 

 

1

 The LCC projection is based on standard parallels at 45.0 and 49.0 North, and origin at 44.25 North 

and 109.5 West, false easting of 600 km and NAD83. 
4-1 

4.0 EMISSIONS DATA 

The maximum actual one-day (midnight to midnight) pollutant emission rates were 

compiled for modeling input. SO2, NOx and Particulate Matter (PM) emission rates were 

assembled. The maximum actual one-day emissions for each pollutant were not restricted to be 

concurrent, and thus the modeled emissions do not represent a real emission day, but assume the 

worst-case emissions for each pollutant occur on all days of the modeling period. This is an 

unrealistic, conservative assumption based upon EPA’s BART modeling protocol since the 

maximum emission rates of each pollutant do not, in fact, occur simultaneously and the actual 

daily emissions over the modeling period are lower than the maximum actual daily emissions. 

The U.S. Environmental Protection Agency (EPA) Clean Air Markets (CAM) web site1

contains the raw Continuous Emissions Monitoring Systems (CEMS) data from the Electronic 

Data Reports (EDRs) submitted by power plants pursuant to Federal Acid Rain program and 

other market-based regulatory programs. Pre-packaged data sets2

 for January 2001 through 

December 2003 (the “baseline” period used for BART analysis) and January 2008 through June 

2011 (the “reference” period for current emissions) were downloaded from the CAM web site. 

The maximum one-day SO2 emission rate (pound per hour, lb/hr), NOx emission rate (lb/hr), and 

heat input rate (million British Thermal Units per hour, MMBtu/hr) in each period are 

summarized in Table 4-1. Note that “substitute data” are not included in Table 4-1, and were not 

used for the visibility evaluations, and that the maximum one-day emissions for the pollutants 

are not concurrent. The use of CAM data, even absent substituted data, adds another layer of 

conservativeness to the model results. The monitoring protocols are designed to ensure that if a 

bias is present in the data, the emissions bias will always be positive (i.e., the bias will result 

only in overestimated emissions and impacts). Furthermore experience in measuring heat input 

at Colstrip indicates that the data reported in CAM contains positive bias. 

PM consists of Filterable PM (FPM) and Condensable PM (CPM). FPM and CPM 

emission factors were calculated from testing of Units 1 and 3 in April 2011 and June 2010, 

respectively. These test results are summarized in 4-2. CPM and FPM emissions employed for 

modeling are simply the emission factor multiplied by the maximum one-day heat input rate for 

the period of interest. 

PM speciation was performed using the method in the workbook provided by the 

National Park Service (NPS) for dry-bottom pulverized-coal boilers with wet scrubbing3

. The 

 

1http://camddataandmaps.epa.gov/gdm/index.cfm?fuseaction=emissions.wizard, accessed October 11, 2011.

2 Each prepackaged data set covers one calendar quarter. 

3http://www.nature.nps.gov/air/permits/ect/ectCoalFiredBoiler.cfm, accessed October 12, 2011. 
4-2 

NPS spreadsheet speciates CPM into organic and inorganic fractions, and FPM into coarse soil, 

fine soil, and elemental carbon fractions. The NPS speciation is based on PM with an 

aerodynamic diameter of 10 microns or less (PM10). All CPM is PM10. The stack tests, 

however, measure total FPM. Therefore, the total FPM emission factors from stack tests are 

multiplied by 0.71 (obtained from AP-424

) to obtain FPM with an aerodynamic diameter of 10 

microns or less (FPM10). The coal properties (summarized in 4-3) input to the NPS workbook 

were obtained from a 1999 stack test report5 for Unit 3. 

The control technologies considered include the following: 

 Baseline case controls – those controls in-place as of 2003. These are wet scrubbers 

and LNB. 

 Reference case controls – those controls in-place as of November 2011. 

 For Units 1 and 2, the reference case controls are wet scrubbers, upgraded LNB, 

and digital boiler controls. 

 For Units 3 and 4, the reference case controls are wet scrubbers, upgraded LNB, 

SOFA and digital controls. For each of these units, the maximum NOx

emission rate after the installation date is used as the SOFA emission rate. 

Substitute CAM data are excluded. 

 Potential add-on controls 

 SOFA for Units 1 and 2 (including upgraded LNB). The assumed NOx emissions 

rate of 0.266 lb/MMBtu (24 hour emission rate) is based on the average NOx 

emissions rate from Units 1 and 2 during the period from 2006 through 2010 

(refer to 4-4), and the expected emission rate improvement. 

 Selective Non-Catalytic Reduction (SNCR) for all units. A 10 percent reduction 

in emissions relative to the cases with LNB plus SOFA. 

 Selective Catalytic Reduction (SCR) for all units. The assumed NOx emissions 

rate of 0.084 lb/MMBtu (24-hour average). 

 Enhanced SO2 controls with lime injection for Units 1&2. Cases with 0.38 

lb/MMBtu emissions (24 hour average) - approximately a 50% further 

reduction in emission (equivalent to an 88 percent reduction from the 

uncontrolled emission on a 30 day average). 

 Enhanced particulate matter controls, with PM10 emission rate of 0.015 lb/MMBtu 

for Units 1 & 2 

Table 4-5 presents the emission control cases for each of the four Colstrip Units in 

English units. Table 4-6 presents these cases in metric units, which are used by the air dispersion 

model. Table 4-7 presents the modeling scenarios, which are combinations of the units’ cases in 

Table 4-6. 

 

4AP-42, Fifth Edition, Compilation of Air Pollutant Emission FactorsVolume I, Stationary Point and Area Sources, 

Table 1.1-6.

5Source Emissions Survey of Montana Power Company Colstrip Project Division Unit Number 3-6 Scrubber Inlet Duct and Unit 

Number 3 Stack Colstrip, Montana, September 1999, 36404 File Number 99-166 

http://www.epa.gov/ttnatw01/combust/utiltox/mercury/colstrpr.pdf, accessed October 12, 2011. 
4-3 

Table 4-2: Colstrip Particulate Matter Emission Factors from Stack Testing 

Colstrip 

Units 

Filterable 

Particulate 

Matter 

(FPM) 

(lb/MMBtu) 

Condensable 

Particulate 

Matter 

(CPM) 

(lb/MMBtu) 

Comments 

Units 1&2 0.0155 0.0214 From April 2011 test of Unit 1. Average of 3 runs. 

Units 3&4 0.0226 0.0049 From June 2010 test of Unit 3. Average of 3 runs. 

Table 4-1: Colstrip Clean Air Markets Summary – Maximum Daily Data 

Year Unit Max Heat 

Input2 

(MMBtu) 

Date On 

Which 

 Max Heat 

Input 

Occurred 

Max 

SO2Emissio

ns2 (lb/hr) 

Date On 

Which 

Max SO2

Emissions 

Occurred 

Max 

NOxEmissio

ns3 

(lb/MMBtu) 

Max 

NOxEmission

s2 (lb/hr) 

Date On 

Which Max 

NOx

Emissions 

Occurred 

20111 1 3,692 01/04/2011 2,367 01/04/2011 0.38 1,388 05/04/2011 

20111 2 3,507 01/01/2011 1,649 01/08/2011 0.37 1,297 02/24/2011 

20111 3 7,990 03/29/2011 790 03/18/2011 0.17 1,397 03/30/2011 

20111 4 8,120 01/11/2011 814 01/03/2011 0.17 1,398 02/01/2011 

2010 1 3,732 04/14/2010 2,645 12/22/2010 0.41 1,529 04/28/2010 

2010 2 3,708 03/02/2010 2,250 03/30/2010 0.46 1,710 02/24/2010 

2010 3 8,616 02/14/2010 785 04/06/2010 0.17 1,479 02/05/2010 

2010 4 8,980 03/02/2010 830 07/09/2010 0.16 1,443 01/04/2010 

2009 1 3,758 11/15/2009 2,652 11/15/2009 0.40 1,503 12/15/2009 

2009 2 3,647 12/08/2009 2,453 10/21/2009 0.41 1,508 02/09/2009 

2009 3 8,686 12/15/2009 820 01/29/2009 0.17 1,491 12/15/2009 

2009 4 8,656 12/15/2009 782 12/20/2009 0.27 2,324 11/30/2009 

2008 1 3,844 03/22/2008 2,342 01/21/2008 0.43 1,662 02/05/2008 

2008 2 3,991 01/30/2008 2,368 01/21/2008 0.37 1,487 04/22/2008 

2008 3 8,336 08/05/2008 799 09/19/2008 0.18 1,501 08/15/2008 

2008 4 8,795 01/04/2008 1,054 09/19/2008 0.40 3,559 02/14/2008 

2001-

2003 

1 4,012 10/01/2003 2,812 01/03/2002 0.49 1,971 10/25/2003 

2001-

2003 

2 3,903 08/01/2002 2,909 04/06/2002 0.54 2,125 02/20/2001 

2001-

2003 

3 8,777 01/03/2002 969 10/29/2003 0.46 4,029 03/15/2002 

2001-

2003 

4 9,462 05/12/2001 914 10/05/2003 0.43 4,082 02/17/2001 

1. January through June 

2. Data are the 24 hour average of the maximum day (midnight to midnight), substitute data excluded, as 

reported by EPA Clean Air Markets website 

3. Approximate 
4-4 

Table 4-3: Summary of Colstrip Unit 3 September 1999 Coal Analysis 

Coal Analysis Parameter Test 1 Test 2 Test 3 Average 

Sulfur (% dry) 0.72 0.74 0.76 0.74 

Ash (% dry) 16.8 18 17.3 17.4 

HHV (Btu/lb, as fired) 9,550 9,300 9,470 9,440 

Table 4-4: Colstrip Units 1 and 2 Annual Average Emissions Data 

From EPA Clean Air Markets 

Unit ID Year Operating 

Time 

Avg. NOx

Rate 

(lb/MMBtu) 

NOx Tons Heat Input 

(MMBtu) 

1 1997 7,406 0.390 4,702 24,119,389 

1 1998 7,828 0.393 5,273 26,858,555 

1 1999 7,278 0.407 5,305 26,069,267 

1 2000 8,270 0.342 4,390 25,642,024 

1 2001 7,620 0.399 5,370 26,939,103 

1 2002 8,346 0.401 5,811 28,955,991 

1 2003 7,460 0.378 4,983 26,328,944 

1 2004 8,474 0.346 5,321 30,770,151 

1 2005 8,428 0.348 5,145 29,574,218 

1 2006 7,259 0.310 3,726 24,003,758 

1 2007 8,353 0.306 4,428 28,950,693 

1 2008 8,198 0.319 4,317 27,066,347 

1 2009 7,519 0.304 3,854 25,314,407 

1 2010 8,160 0.303 4,138 27,353,513 

2 1997 7,517 0.417 5,933 28,484,395 

2 1998 8,204 0.435 5,452 25,057,443 

2 1999 8,085 0.381 5,517 28,933,076 

2 2000 7,667 0.424 5,359 25,302,330 

2 2001 8,563 0.430 6,339 29,450,519 

2 2002 7,124 0.406 4,967 24,447,383 

2 2003 8,228 0.402 5,773 28,747,739 

2 2004 8,335 0.405 5,848 28,891,409 

2 2005 7,418 0.366 4,678 25,570,756 

2 2006 8,335 0.281 4,020 28,630,015 

2 2007 8,154 0.302 4,363 28,936,718 

2 2008 7,109 0.311 3,744 24,082,853 

2 2009 8,590 0.297 4,117 27,731,018 

2 2010 7,964 0.320 4,224 26,425,208 
4-5 

Table 4-5: Colstrip Unit Cases, English Units 

Case 

ID 

Case 

Description 

SO2

(lb/hr) 

NOx 

(lb/ 

MMBtu) 

NOx

(lb/hr) 

Particulate Matter (lb/hr) Stack Pa

Filterable Condensable Height 

(ft) 

Inner 

Diameter

(ft) 

Coarse

Soil 

Fine 

Soil 

Elemental

Carbon 

Inorganic 

(SOx) 

Organic 

1X Unit 1 

Baseline 

Case 

2,812@ 0.491@ 1,971@ 12.6∆ 30.4∆ 1.2∆ 68.7∆ 17.2∆ 500 16.5 

1R Unit 1 

Reference 

Case 

(Digital 

Controls) 

2,652* 0.432* 1,662* 12.1! 29.1! 1.1! 65.8! 16.5! 500 16.5 

1A Unit 1 

SOFA 

2,652* 0.266^ 1,022^ 12.1! 29.1! 1.1! 65.8! 16.5! 500 16.5 

1B Unit 1 88% 

SO2 Control 

1,461‡ 0.432* 1,662* 12.1! 29.1! 1.1! 65.8! 16.5! 500 16.5 

1C Unit 1 

SOFA w/ 

88% SO2

Control 

1,461‡ 0.266^ 1,022^ 12.1! 29.1! 1.1! 65.8! 16.5! 500 16.5 

1D Unit 1 

SOFA + 

SCR 

2,652* 0.084^ 323^ 12.1! 29.1! 1.1! 65.8! 16.5! 500 16.5 

1E Unit 1 PM10

= 0.015 

lb/MMBtu 

2,652* 0.432* 1,662* 5.6† 13.5† 0.5† 30.5† 7.6† 500 16.5 

2X Unit 2 

Baseline 

Case 

2,909@ 0.544@ 2,125@ 12.3∆ 29.5∆ 1.1∆ 66.8∆ 16.7∆ 500 16.5 

2R Unit 2 

Reference 

Case 

(Digital 

Controls) 

2,453* 0.461* 1,710* 12.5! 30.2! 1.2! 68.3! 17.1! 500 16.5 

2A Unit 2 

SOFA 

2,453* 0.266^ 1,062^ 12.5! 30.2! 1.2! 68.3! 17.1! 500 16.5 

2B Unit 2 88% 

SO2 Control 

1,517‡ 0.461* 1,710* 12.5! 30.2! 1.2! 68.3! 17.1! 500 16.5 
4-6 

Table 4-5: Colstrip Unit Cases, English Units 

Case 

ID 

Case 

Description 

SO2

(lb/hr) 

NOx 

(lb/ 

MMBtu) 

NOx

(lb/hr) 

Particulate Matter (lb/hr) Stack Pa

Filterable Condensable Height 

(ft) 

Inner 

Diameter

(ft) 

Coarse

Soil 

Fine 

Soil 

Elemental

Carbon 

Inorganic 

(SOx) 

Organic 

2C Unit 2 

SOFA w/ 

88% SO2

Control 

1,517‡ 0.266^ 1,062^ 12.5! 30.2! 1.2! 68.3! 17.1! 500 16.5 

2D Unit 2 

SOFA + 

SCR 

2,453* 0.084^ 335^ 12.5! 30.2! 1.2! 68.3! 17.1! 500 16.5 

2E Unit 2 PM10

= 0.015 

lb/MMBtu 

2,453* 0.461* 1,710* 5.8† 14.0† 0.5† 31.6† 7.9† 500 16.5 

3X Unit 3 

Baseline 

Case 

969@ 0.459@ 4,029@ 40.2$ 96.9$ 3.7$ 34.4$ 8.6$ 692 24.0 

3R Unit 3 

Reference 

Case 

(SOFA) 

820© 0.180© 1,501© 39.8# 95.9# 3.7# 34.0# 8.5# 692 24.0 

3A Unit 3 

SOFA + 

SNCR 

820© 0.162& 1,351& 39.8# 95.9# 3.7# 34.0# 8.5# 692 24.0 

3B Unit 3 

SOFA + 

SCR 

820© 0.084¶ 730¶ 39.8# 95.9# 3.7# 34.0# 8.5# 692 24.0 

4X Unit 4 

Baseline 

Case 

914@ 0.431@ 4082@ 43.4$ 104.4$ 4.0$ 37.1$ 9.3$ 692 24.0 

4R Unit 4 

Reference 

Case 

(SOFA) 

830© 0.172© 1,443© 41.2# 99.1# 3.8# 35.2# 8.8# 692 24.0 

4A Unit 4 

SOFA + 

SNCR 

830© 0.155& 1,299& 41.2# 99.1# 3.8# 35.2# 8.8# 692 24.0 
4-7 

Table 4-5: Colstrip Unit Cases, English Units 

Case 

ID 

Case 

Description 

SO2

(lb/hr) 

NOx 

(lb/ 

MMBtu) 

NOx

(lb/hr) 

Particulate Matter (lb/hr) Stack Pa

Filterable Condensable Height 

(ft) 

Inner 

Diameter

(ft) 

Coarse

Soil 

Fine 

Soil 

Elemental

Carbon 

Inorganic 

(SOx) 

Organic 

4B Unit 4 

SOFA + 

SCR 

830© 0.084¶ 754¶ 41.2# 99.1# 3.8# 35.2# 8.8# 692 24.0 

§ From August 2007 BART Report, Tables 2-1 & 2-2 @ Data are based on maximum 24-hour average data from U.S. EPA Clean Air Markets, 2001 - 2003. Substitute data are not include∆ Data are based on maximum 24-hour average heat input data from U.S. EPA Clean Air Markets, 2001 - 2003 and emissions factors 

of Unit 1. Speciation per NPS guidance and coal data from PPL. 

! Data are based on maximum 24-hour average heat input data from U.S. EPA Clean Air Markets, 2008 - 2011 and emissions factors 

Unit 1. Speciation per NPS guidance and coal data from PPL. 

* Data are based on maximum 24-hour average data from U.S. EPA Clean Air Markets, 2008 - 2011. Substitute data are not included

^ Data are based on maximum 24-hour average heat input data from U.S. EPA Clean Air Markets, 2008 - 2011, and an emissions fact

reduction from the average of NOx emission reported for Units 1 and 2 from 2006 through 2010 in EPA Clean Air Markets. 

& Data are based on 10% reduction relative to the SOFA case. ‡ Percent SO2 control is relative to uncontrolled operation. 

† Data are based on maximum 24-hour average heat input data from U.S. EPA Clean Air Markets, 2008 - 2011 and emissions factors 

of Unit 1. Speciation per NPS guidance and coal data from PPL. 

© Data are based on maximum 24-hour average data from U.S. EPA Clean Air Markets. Only CAM data after 1/1/08 and 1/19/10 for 

respectively were used. 

# Based on maximum 24-hour average heat input data from U.S. EPA Clean Air Markets and emissions factors from the June 2010 te

installation dates are 1/1/08 and 1/19/10 for Units 3 and 4, respectively. Only CAM data after these dates were used. Speciation per 

$ Data are based on maximum 24-hour average heat input data from U.S. EPA Clean Air Markets, 2001 - 2003 and emissions factors 

of Unit 3. Speciation per NPS guidance and coal data from PPL. 

¶ Data are based on maximum 24-hour average heat input data from U.S. EPA Clean Air Markets, 2008 - 2011, and an emissions fact

reduction from the average of NOx emission reported for Units 3 and 4 from 2006 through 2010 in EPA Clean Air Markets. 
4-8 

Table 4-6: Colstrip Unit Cases, Metric Units

Case 

ID 

Case 

Description 

SO2

(g/s) 

NOx

(g/s) 

Particulate Matter (g/s) Stack Param

Filterable Condensable Height 

(m) 

Inner 

Diameter

(m) 

T

Coarse

Soil 

Fine 

Soil 

Elemental

Carbon 

Inorganic

(SOx) 

Organic 

1X Unit 1 

Baseline 

Case 

354@ 248@ 1.59∆ 3.83∆ 0.15∆ 8.66∆ 2.16∆ 152 5.03 

1R Unit 1 

Reference 

Case 

(Digital 

Controls) 

334* 209* 1.52! 3.67! 0.14! 8.29! 2.07! 152 5.03 

1A Unit 1 

SOFA 

334* 129^ 1.52! 3.67! 0.14! 8.29! 2.07! 152 5.03 

1B Unit 1 88% 

SO2

Control 

184‡ 209* 1.52! 3.67! 0.14! 8.29! 2.07! 152 5.03 

1C Unit 1 

SOFA w/ 

88% SO2

Control 

184‡ 129^ 1.52! 3.67! 0.14! 8.29! 2.07! 152 5.03 

1D Unit 1 

SOFA + 

SCR 

334* 41^ 1.52! 3.67! 0.14! 8.29! 2.07! 152 5.03 

1E Unit 1 

PM10 = 

0.015 

lb/MMBtu 

334* 209* 0.705† 1.697† 0.065† 3.838† 0.96† 152 5.03 

2X Unit 2 

Baseline 

Case 

367@ 268@ 1.55∆ 3.72∆ 0.14∆ 8.42∆ 2.10∆ 152 5.03 

2R Unit 2 

Reference 

Case 

(Digital 

Controls) 

309* 215* 1.58! 3.81! 0.15! 8.61! 2.15! 152 5.03 

2A Unit 2 

SOFA 

309* 134^ 1.58! 3.81! 0.15! 8.61! 2.15! 152 5.03 
4-9 

Table 4-6: Colstrip Unit Cases, Metric Units

Case 

ID 

Case 

Description 

SO2

(g/s) 

NOx

(g/s) 

Particulate Matter (g/s) Stack Param

Filterable Condensable Height 

(m) 

Inner 

Diameter

(m) 

T

Coarse

Soil 

Fine 

Soil 

Elemental

Carbon 

Inorganic

(SOx) 

Organic 

2B Unit 2 88% 

SO2 Control 

191‡ 215* 1.58! 3.81! 0.15! 8.61! 2.15! 152 5.03 

2C Unit 2 

SOFA w/ 

88% SO2

Control 

191‡ 134^ 1.58! 3.81! 0.15! 8.61! 2.15! 152 5.03 

2D Unit 2 

SOFA + 

SCR 

309* 42^ 1.58! 3.81! 0.15! 8.61! 2.15! 152 5.03 

2E Unit 2 PM10

= 0.015 

lb/MMBtu 

309* 215* 0.732† 1.762† 0.068† 3.985† 1.00† 152 5.03 

3X Unit 3 

Baseline 

Case 

122@ 508@ 5.07$ 12.21$ 0.469$ 4.33$ 1.084$ 211 7.32 

3R Unit 3 

Reference 

Case 

(SOFA) 

103© 189© 5.02# 12.08# 0.464# 4.29# 1.072# 211 7.32 

3A Unit 3 

SOFA + 

SNCR 

103© 170& 5.02# 12.08# 0.464# 4.29# 1.072# 211 7.32 

3B Unit 3 

SOFA + 

SCR 

103© 92¶ 5.02# 12.08# 0.464# 4.29# 1.072# 211 7.32 

4X Unit 4 

Baseline 

Case 

115@ 514@ 5.47$ 13.16$ 0.506$ 4.67$ 1.168$ 211 7.32 

4R Unit 4 

Reference 

Case 

(SOFA) 

105© 182© 5.19# 12.49# 0.480# 4.44# 1.109# 211 7.32 

4A Unit 4 

SOFA + 

SNCR 

105© 164& 5.19# 12.49# 0.480# 4.44# 1.109# 211 7.32 
4-10 

Table 4-6: Colstrip Unit Cases, Metric Units

Case 

ID 

Case 

Description 

SO2

(g/s) 

NOx

(g/s) 

Particulate Matter (g/s) Stack Param

Filterable Condensable Height 

(m) 

Inner 

Diameter

(m) 

T

Coarse

Soil 

Fine 

Soil 

Elemental

Carbon 

Inorganic

(SOx) 

Organic 

4B Unit 4 

SOFA + 

SCR 

105© 95¶ 5.19# 12.49# 0.480# 4.44# 1.109# 211 7.32 

§ From August 2007 BART Report, Tables 2-1 & 2-2 @ Data are based on maximum 24-hour average data from U.S. EPA Clean Air Markets, 2001 - 2003. Substitute data are not include∆ Data are based on maximum 24-hour average heat input data from U.S. EPA Clean Air Markets, 2001 - 2003 and emissions factors 

of Unit 1. Speciation per NPS guidance and coal data from PPL. 

! Data are based on maximum 24-hour average heat input data from U.S. EPA Clean Air Markets, 2008 - 2011 and emissions factors 

Unit 1. Speciation per NPS guidance and coal data from PPL. 

* Data are based on maximum 24-hour average data from U.S. EPA Clean Air Markets, 2008 - 2011. Substitute data are not included

^ Data are based on maximum 24-hour average heat input data from U.S. EPA Clean Air Markets, 2008 - 2011, and an emissions fact

reduction from the average of NOx emission reported for Units 1 and 2 from 2006 through 2010 in EPA Clean Air Markets. 

& Data are based on 10% reduction relative to the SOFA case. ‡ Percent SO2 control is relative to uncontrolled operation. 

† Data are based on maximum 24-hour average heat input data from U.S. EPA Clean Air Markets, 2008 - 2011 and emissions factors 

of Unit 1. Speciation per NPS guidance and coal data from PPL. 

© Data are based on maximum 24-hour average data from U.S. EPA Clean Air Markets. Only CAM data after 1/1/08 and 1/19/10 for 

respectively were used. 

# Based on maximum 24-hour average heat input data from U.S. EPA Clean Air Markets and emissions factors from the June 2010 te

installation dates are 1/1/08 and 1/19/10 for Units 3 and 4, respectively. Only CAM data after these dates were used. Speciation per 

$ Data are based on maximum 24-hour average heat input data from U.S. EPA Clean Air Markets, 2001 - 2003 and emissions factors 

of Unit 3. Speciation per NPS guidance and coal data from PPL. 

¶ Data are based on maximum 24-hour average heat input data from U.S. EPA Clean Air Markets, 2008 - 2011, and an emissions fact

reduction from the average of NOx emission reported for Units 3 and 4 from 2006 through 2010 in EPA Clean Air Markets. 
4-11 

Table 4-7: Colstrip - Modeling Scenarios 

Scenario 

ID 

Scenario Description Unit Case IDs Meteorology 

1.0 Units 1 & 2 combined with a baseline of 2001 ( 2001 

emissions and met data 2001-2003) 

1X 2X 2001 - 2003 

1.1 Units 1 & 2 combined reference case 2008 (using 2008-

2011 emissions and met data 2001-2003) 

1R 2R 2001 - 2003 

1.2 SOFA 1 & 2 with the 2008-2011 reference case emissions 

held constant 

1A 2A 2001 - 2003 

1.3 Lime addition 88% control with reference case emissions 

held constant 

1B 2B 2001 - 2003 

1.4 Units 1 & 2 Lime addition 88% and SOFA with reference 

case emissions held constant 

1C 2C 2001 - 2003 

1.5 SCR on top of SOFA with reference case emissions held 

constant 

1D 2D 2001 - 2003 

1.6 New PM controls to 0.015 lb/mmBtu control level and 

reference case emissions 

1E 2E 2001 - 2003 

2.0 Units 3 & 4 combined baseline of 2001 (2001 emissions 

and met data 2001-2003) 

 3X 4X 2001 - 2003 

2.1 Units 3 & 4 combined reference case 2008 (using 2008-

2011 emissions data and met 2001-2003) Units 3&4 data 

reflect reduction in NOx and PM from operation of SOFA 

 3R 4R 2001 - 2003 

2.2 Units 3 & 4 with SCNR and reference case emissions 3A 4A 2001 - 2003 

2.3 Units 3 & 4 with SCR and reference case emissions 3B 4B 2001 - 2003 
5-1 

5.0 CALPUFF MODELING SYSTEM OVERVIEW 

To determine the long range transport of pollutants emitted from a source, the U.S. EPA 

Guideline on Air Quality Models (Revised) (U.S. EPA, 2005)6

, the IWAQM Phase 2 Report (U.S. EPA, 

1998)7 and the FLAG Phase I Report – Revised (2010) (FLAG, 2010)8 recommend the CALPUFF 

dispersion modeling system. 

CALPUFF is a multi-layer, multi-species, non-steady-state, puff dispersion model. It can 

simulate the effects of time- and space-varying meteorological conditions on pollutant transport, 

transformation, and removal. It also contains algorithms for near-source effects, such as building 

downwash, transitional plume rise, partial plume penetration, sub-grid scale terrain interactions, and 

longer range effects, such as pollutant removal (wet scavenging and dry deposition), chemical 

transformation, vertical wind shear, and overwater transport. 

Running CALPUFF requires extensive preparation of data for input to the preprocessing 

programs of the modeling system. The following steps are considered necessary to determine the 

potential impacts in the Class I areas due to the proposed project: 

 Determine an appropriate modeling domain, which encompasses the Project site and 

the Class I areas with sufficient buffer for recirculation effects; 

 Obtain three years of the appropriate Mesoscale Model (MM5) datasets, hourly 

surface meteorological data, hourly precipitation data, and twice-daily sounding data; 

 Process the meteorological data for input to the CALPUFF preprocessing programs; 

 Develop an appropriate land use classification for each grid cell of the modeling 

domain; 

 Develop the terrain profile for the modeling domain; 

 Locate receptors within the Class I area; 

 Execute all the preprocessing programs of the CALPUFF dispersion modeling 

system, including CALMET (as discussed further below); and 

 Execute the post-processing programs of the CALPUFF dispersion modeling system, 

namely CALPOST, for each of the desired pollutants, analyses (i.e., concentration, 

visibility, and deposition) and Class I areas as appropriate. 

The steps presented above for BART visibility modeling are based on the guidance provided by 

the FLAG Phase I Report – Revised (2010) (FLAG, 2010) and IWAQM Phase 2 Report (U.S. EPA, 

1998) for a refined Class I analysis. 

The CALPUFF dispersion modeling system consists of three main programs: CALMET, 

CALPUFF, and CALPOST. CALMET is used to process the land use, terrain, and meteorological data 

for use in the CALPUFF program. CALPUFF uses the output from CALMET, along with the source data 

(i.e., exhaust characteristics and emission rates) to calculate hourly concentration and deposition values 

 

6

http://www.epa.gov/scram001/guidance/guide/appw_05.pdf 7

http://www.epa.gov/scram001/7thconf/calpuff/phase2.pdf 8

http://www.nature.nps.gov/air/pubs/pdf/flag/FLAG_2010.pdf
5-2 

for each receptor. CALPOST uses the hourly output from CALPUFF and calculates the pollutant-specific 

and averaging period-specific concentration, change in light extinction, and deposition rates. In addition 

to these main programs, the CALPUFF dispersion modeling system requires the use of many other 

preprocessing programs. The following sections present the model inputs and how each was developed 

for input to the programs of the CALPUFF dispersion modeling system. 

It is noted that the CALPOST program was updated to include the latest methods recommended 

by the Interagency Monitoring of Protected Visual Environments (IMPROVE) Steering Committee 

(IMPROVE 2005)9

 on calculating light extinction. It differs from the previous algorithm used in the 

previous guidance for regional haze analyses in six areas: 

1. Extinction efficiencies of sulfates, nitrates, and organics vary with concentration, as 

there are distinct efficiencies for "small" and "large" particles and the ratio of smallto-large particle fraction decreases as the concentration increases. Furthermore, the 

extinction efficiency for sulfates no longer equals that for nitrates. 

2. A separate hygroscopic humidity enhancement factor curve is prescribed for "small" 

and "large" particles, fS(RH) and fL(RH). These curves apply to sulfates and 

nitrates. 

3. Light extinction due to scattering by sea salt (with its own hygroscopic humidity 

enhancement factor fSS(RH)) is included (not expected to be a significant particulate 

component in Montana). 

4. Light absorption by nitrogen dioxide (NO2) gas is included in the extinction. 

5. Background particulate organic matter concentration is taken to be 1.8 times 

measured organic carbon concentrations, instead of 1.4. 

6. Rayleigh scattering extinction varies with site elevation and mean temperature. 

The new IMPROVE algorithm was developed in an effort to mitigate some of the technical 

criticism of the original equation. The FLAG, 2010 guidance recognizes the many enhancements of the 

IMPROVE group and recommends the use of Method 8, sub-mode 5 as the default method to calculate 

regional haze impacts. The new guidance has been coded in CALPOST (Version 6.221) and is available 

as a Beta version of the model that has been reviewed and accepted by the FLMs. This method was used 

for all visibility modeling analyses. 

The model was run in its regulatory default mode, in accordance with the approved modeling 

protocol and the “EPA Model Clearinghouse Memorandum Clarification On EPA-FLM Recommended 

Settings For CALMET”, of August 31ST, 2009 and “Federal Land Managers’ Air Quality Related Values 

Work Group (FLAG) Phase I Report—Revised (2010)” of October, 2010. 

These documents are available at: 

(http://www.epa.gov/ttn/scram/CALMET%20CLARIFICATION.pdf) and 

(http://www.nature.nps.gov/air/pubs/pdf/flag/FLAG_2010.pdf) respectively.

 

9

http://vista.cira.colostate.edu/IMPROVE/Publications/GrayLit/019_RevisedIMPROVEeq/RevisedIMPROVEAlgorith

m3.doc
6-1 

6.0 GEOPHYSICAL AND METEOROLOGICAL DATA 

6.1 Modeling Domain 

The CALMET computational domain was designed to cover an area of 780 km by 570 

km (Figure 6-1). It includes Colstrip, all Class I areas within 300 km from it. and extends beyond 

the Class I areas by at least 50 km as recommended by the IWAQM Phase 2 Report (U.S. EPA, 

1998). These Class I areas are: North Absaroka WA, Theodore Roosevelt NP, UL Bend WA, 

Washakie WA and Yellowstone NP. 

Consistent with the “EPA Model Clearinghouse Memorandum Clarification On EPAFLM Recommended Settings For CALMET”, of August 31, 2009, a horizontal grid resolution of 

4 km was used to represent the variations of the terrain elevations and land use within the 

domain. Also consistent with that guidance, the modeling domain has 10 vertical layers, with the 

top of the layers defined as (in meters): 0, 20, 40, 80, 160, 320, 640, 1200, 2000, 3000, and 

4000. 

The resulting domain is captured by the 196 x 143 grid cells, a workable number of cells. 

The CALPUFF computational domain is the same as the CALMET domain. The domain extends 

beyond the Class I areas in order to provide an adequate buffer zone at the boundaries, and to 

allow the effects of flow curvature and possible small-scale recirculation to be evaluated. 

Lambert conformal coordinates (based on a Lambert Conformal Conic Projection) were 

used to define the modeling domain. The projection origin of the modeling domain is 44.25 

degrees North Latitude and 109.5 degrees West Longitude with two standard parallels set to 45.0 

degrees and 49.0 degrees North Latitude. A False Easting of 600 kilometers at the projection 

origin was used. The datum is set to WGS-84. 

6.2 Terrain 

Gridded terrain elevations for the modeling domain are derived from 3-sec Shuttle Radar 

Topo Mission files, or SRTM3 digital elevation files, produced by the United States Geological 

Survey (USGS). Data are provided in files covering 3 arc seconds by 3 arc seconds of latitude 

and longitude. 

SRTM3 DEM data elevations are in meters relative to mean sea level, and the spacing of 

the elevations along each profile is 3 arc-seconds, which corresponds to a spacing of 

approximately 90 meters. 

Figure 6-1 shows contours of the terrain averaged to 2 km. The USGS elevation records 

located within each grid cell in the computational domain were averaged to produce a mean 
6-2 

elevation at each grid point. This figure also shows locations of Colstrip and a circle with 300-

km radius around the facility. North Absaroka, Theodore Roosevelt, UL Bend, Washakie and 

Yellowstone Class I areas are within the 300 km from the Colstrip. 

6.3 Land Use 

A standard USGS dataset was used to assign a land use classification for each grid cell: 

the 30-m resolution U.S. Geological Survey (USGS) 1992 National Land Cover Data (NLCD). 

This dataset was downloaded from the USGS website in GeoTIFF format. A total of 6 files, 

covering the states of Idaho, Montana, Nebraska, North Dakota, South Dakota, and Wyoming 

were used. 

These files were then processed using the CTGPROC preprocessing program that is part 

of the CALPUFF version 5.8 dispersion modeling system. 

The land use classifications and associated geophysical parameters such as albedo, 

surface roughness, etc. are assigned to each grid cell using the MAKEGEO program, which is 

also a part of the CALPUFF version 5.8 dispersion modeling system. The final output from 

MAKEGEO is the GEO.DAT file 

Figure 6-2 shows the dominant land use categories used in the modeling. 

6.4 Meteorological Data Base 

The CALMET model requires meteorological information from the surface and upper air 

as well as geophysical information about the land use and terrain heights. Specifically, CALMET 

requires surface observations of wind speed, wind direction, temperature, cloud cover, ceiling 

height, surface pressure, relative humidity, and precipitation type (e.g., snow, rain, etc.). These 

variables are routinely measured at the National Weather Service (NWS) surface stations. The 

upper air data include twice-daily observations of vertical profiles of wind speed, wind direction, 

temperature, and pressure. 

Three-dimensional gridded data from the prognostic numerical model MM5 provided by 

WRAP were also included with a resolution of 36-km for 2001 through 2003. Hourly 

precipitation data, which are required for wet deposition calculations in CALPUFF, were derived 

from the MM5 data. 

The NWS collects twice-daily upper air data vertical profiles of the atmosphere taken at 

00 Greenwich Mean Time (GMT) and 12 GMT at each station. Seven stations are available 

within the modeling domain, or in close proximity. These stations are Aberdeen, SD; Bismark, 
6-3 

ND; Boise, ID; Glasgow, MT; Riverton, WY; Great Falls, MT; and Rapid City, SD (6-4). The 

upper air data were acquired from the National Oceanic and Atmospheric Administration 

(”NOAA”) (http://raob.fsl.noaa.gov/) and processed using the READ62 program. 

A total of 60 surface meteorological stations for the year 2001, and 57 stations for the 

years 2002 and 2003 were used in the refined runs (Figure 6-3 and 6-5). The hourly surface 

meteorological data was obtained from the National Climatic Data Center (NCDC) in Integrated 

Surface Hourly (ISH) format, and were processed by the SMERGE program for the creation of 

the SURF.DAT file 

A total of 94 precipitation stations were used for all three years of the analyses. Hourly 

precipitation data for the precipitation stations included in the modeling analyses, were obtained 

in TD-3240 format from the NCDC. This data was processed using two programs from the 

CALPUFF dispersion modeling system: PXTRACT and PMERGE. 
6-4 

Figure 6-1. Terrain Contours 

Figure 6-1. Terrain contours (m MSL) at 2 km resolution for the 780 km by 570 km 

CALMET and CALPUFF computational domains. The Class I areas and the Colstrip facility are 

also shown. 

350 400 450 500 550 600 650 700 750 800 850 900 950 1000 1050 1100

LCCx (km), WGS-84

-100

-50

0

50

100

150

200

250

300

350

400

450

LCCy (km), 

WGS-84

Yellowstone

Washakie

UL Bend

Theodore Roosevelt - N

Theodore Roosevelt - C

Theodore Roosevelt - S

Teton

Red Rock Lakes North Absaroka

Gates of the Mountains

Colstrip

Map projection: LCC, WGS-84

Feast = 600 km, Fnorth = 0 km

LATo = 44.25N, LANo = 109.5W

LAT1 = 45 N, LAT2 = 49 N

ID

MT

ND

SD

WY

PPL, Montana

Terrain

(meters)

-1

100

300

500

700

900

1100

1300

1500

1700

1900

2100

2300

2500

2700

2900

3100

3300

3500

3700

3900

4500

300 km circle around facility
6-5 

Figure 6-2. Dominant Land Use Categories 

Figure 6-2. Dominant land use categories at 2 km resolution on the CALMET and CALPUFF 

modeling domains. Class I areas and the location of Colstrip PPL are also shown. 

350 400 450 500 550 600 650 700 750 800 850 900 950 1000 1050 1100

LCCx (km), WGS-84

-100

-50

0

50

100

150

200

250

300

350

400

450

LCCy (km), WGS-84

Yellowstone

Washakie

UL Bend

Theodore Roosevelt - N

Theodore Roosevelt - C

Theodore Roosevelt - S

Teton

Red Rock Lakes North Absaroka

Gates of the Mountains

Colstrip

Map projection: LCC, WGS-84

Feast = 600 km, Fnorth = 0 km

LATo = 44.25N, LANo = 109.5W

LAT1 = 45 N, LAT2 = 49 N

ID

MT

ND

SD

WY

PPL, Montana

300 km circle around facility

10

15

20

25

30

35

40

45

50

55

60

65

70

75

80

85

90

95

100

Snow/Ice

Tundra

Barren

Wetland

Water

Forest

Range

Agriculture

Urban/Built-Up
6-6 

Figure 6-3. Surface Stations 

Figure 6-3. Surface stations. The stations included in the SURF.DAT files are plotted in 

purple 

150 200 250 300 350 400 450 500 550 600 650 700 750 800 850 900 950 1000 1050 1100 1150 1200 1250

LCCx (km), WGS-84

-200

-150

-100

-50

0

50

100

150

200

250

300

350

400

450

500

550

600

LCCy (km), WGS-84

Yellowstone

Washakie

UL Bend

Theodore Roosevelt - N

Theodore Roosevelt - C

Theodore Roosevelt - S

Teton

Red Rock Lakes North Absaroka

Gates of the Mountains

KPIH

KIDA KSUN

KBYI

KSMN

KGDV

KBIL

KLWT

KBHK

KBTM

KDLN

KBZN

KLVM

KGGW

KJDN

KOLF

KSDY

KHLN

KMSO

KGTF

KHVR

KFCA

KCTB

KMLS

KHEI

KDIK

KISN

KMIB

KMOT

KN60

CYEN

KCUT

KPHP

KIEN

KD07

KRAP

KRCA

KDGW KCPR

KLND

KRIW

KJAC

KBYG KGCC

KSHR

P60

KWRL

KCOD KGEY

TH28

SP26

PE25

SA32

GL27

MO39 GA38 BA37 K2WX

KBPI

KRXE

MIN2

LAN3

SHE4

BIL5

LEW6

MIL7

CAS8

RAP9

GR11

HE12

KA13

MI14

PO15

GL17 WI18

BO19

BU20

CU21

DI22

HA23

LI24

PE25

SP26

GL27

TH28

YE29

RE31

SA32

DI33

RI34

ES35

BA37 GA38 MO39

Colstrip

Map projection: LCC, WGS-84

Feast = 600 km, Fnorth = 0 km

LATo = 44.25N, LANo = 109.5W

LAT1 = 45 N, LAT2 = 49 N

MM5 grid points 

Surface met stations

Facility

BC

AL SA

ID

MT

ND

SD

WY

PPL, Montana

MODELING DOMAIN

300 km circles around facility
6-7 

Table 6-3. Meteorological Data Sources and Parameters Available 

Type of Dataset Frequency Source Parameters 

Surface observations Hourly NWS Wind speed, wind 

direction, temperature, 

ceiling height, cloud 

cover, relative humidity, 

surface pressure, 

precipitation type 

Precipitation Observations Hourly NWS Precipitation

 

Modeled Profiles 

MM5 for 2001, 2002 

and 2003 

Hourly WRAP Hourly, gridded fields of 

winds, temperature, 

pressure, and humidity and 

liquid water content on 

36 km grid for 2003, 2004 

and 2005. 

Table 6-4. NWS Upper Air Stations 

City Code ID X Cord. (km) Y Cord. 

(km) 

Time Zone 

Aberdeen, SD ABR 14929 1463.662 194.558 6 

Bismark, ND BIS 24011 1266.552 317.294 6 

Boise, ID BOI 24131 57.175 -52.317 6 

Glasgow, MT GGW 94008 813.956 442.858 7 

Riverton, WY RIW 24061 684.046 -131.806 7 

Great Falls, MT TFX 4102 458.315 357.276 7 

Rapid City, SD UNR 94043 1103.706 0.223 7 

 
6-8 

Table 6-5. NWS Hourly Surface Stations 

WMO WBAN Station Name State Latitude 

(deg.) 

Longitude 

(deg.) 

CALL ID LCCX1

(km) 

LCCY1

(km) 

 Colstrip region MT 46.90 -103.40 TH28 824.377 178.618 

 Border of WY & MT close to KSHR MT 45.10 -106.90 SP26 805.708 99.912 

 Colstrip region MT 45.80 -106.60 PE25 269.113 482.966 

 Border of ID & MT close to KSMN ID 45.10 -113.50 SA32 1064.318 312.121 

 Glacier WA area MT 48.50 -114.00 GL27 283.839 99.808 

 Colstrip region 2

 MT 45.70 -106.50 MO39 828.340 159.976 

 Colstrip region 2

 MT 45.60 -106.50 GA38 836.341 154.721 

 Colstrip region 2

 MT 45.70 -106.60 BA37 831.370 162.314 

718620 99999 ESTEVAN AIRPORT SA 49.22 -102.97 CYEN 1075.221 572.157 

725686 99999 DOUGLAS WY 42.80 -105.38 KDGW 937.563 -152.417 

725690 24089 CASPER NATRONA CO INTL A WY 42.90 -106.47 KCPR 847.878 -145.371 

725760 24021 LANDER HUNT FIELD WY 42.82 -108.73 KLND 663.094 -158.761 

725765 99999 RIVERTON MUNICIPL AP WY 43.05 -108.45 KRIW 685.694 -132.898 

725776 99999 JACKSON HOLE (AWOS) WY 43.60 -110.73 KJAC 500.578 -71.501 

725780 24156 POCATELLO REGIONAL AP ID 42.92 -112.57 KPIH 348.939 -143.020 

725785 24145 IDAHO FALLS FANNING FIEL ID 43.52 -112.07 KIDA 392.002 -77.767 

725865 99999 HAILEY/FRIEDMAN MEM ID 43.50 -114.30 KSUN 211.558 -71.499 

725867 24133 BURLEY MUNICIPAL ARPT ID 42.53 -113.77 KBYI 248.522 -181.773 

726514 99999 CUSTER (RAMOS) SD 43.73 -103.62 KCUT 1073.773 -40.031 

726516 99999 PHILIP ARPT SD 44.05 -101.60 KPHP 1232.468 9.686 

726517 99999 PINE RIDGE SD 43.03 -102.52 KIEN 1169.119 -110.323 

726539 99999 FAITH SD 45.03 -102.02 KD07 1188.561 114.823 

726620 24090 RAPID CITY REGIONAL ARPT SD 44.05 -103.05 KRAP 1116.674 -0.951 

726625 99999 ELLSWORTH AFB SD 44.15 -103.10 KRCA 1111.770 9.802 

726627 99999 BUFFALO (AMOS) SD 45.57 -103.52 K2WX 1066.215 164.509 

726650 99999 GILLETTE (AMOS) WY 44.35 -105.53 KGCC 916.548 19.141 

726654 99999 JOHNSON CO ARPT WY 44.38 -106.72 KBYG 821.593 18.385 

726660 24029 SHERIDAN COUNTY ARPT WY 44.77 -106.97 KSHR 800.273 61.038 

726664 94173 YWLLOESTONE LAKE RAMOS WY 44.55 -110.42 P60 526.875 33.779 

726665 24062 WORLAND MUNICIPAL WY 43.97 -107.95 KWRL 724.471 -29.901 

726667 99999 SOUTH BIG HORN CO WY 44.52 -108.08 KGEY 712.923 31.039 

726676 99999 GLENDIVE(AWOS) MT 47.13 -104.80 KGDV 956.165 330.710 

726700 99999 CODY MUNI (AWOS) WY 44.52 -109.02 KCOD 638.174 30.132 

726710 99999 BIG PINEY (AMOS) WY 42.57 -110.10 KBPI 550.622 -186.713 

726770 24033 BILLINGS LOGAN INT'L ARP MT 45.80 -108.55 KBIL 673.824 172.705 

726776 24036 LEWISTOWN MUNICIPAL ARPT MT 47.05 -109.45 KLWT 603.797 311.134 

726777 99999 BAKER MUNI ARPT MT 46.33 -104.25 KBHK 1003.738 244.677 

726785 24135 BUTTE BERT MOONEY ARPT MT 45.95 -112.50 KBTM 367.559 193.375 

726796 99999 DILLON AIRPORT MT 45.25 -112.55 KDLN 360.659 115.807 

726797 24132 BOZEMAN GALLATIN FIELD MT 45.80 -111.15 KBZN 471.785 173.608 

726798 24150 LIVINGSTON MISSION FIELD MT 45.70 -110.45 KLVM 526.041 161.596 

726818 99999 REXBURG ID 43.83 -111.80 KRXE 414.859 -43.982 

726865 99999 SALMON/LEMHI (AWOS) ID 45.12 -113.88 KSMN 255.576 106.332 

727584 99999 HETTINGER ND 46.02 -102.65 KHEI 1129.513 219.869 

727645 24012 DICKINSON MUNICIPAL AP ND 46.80 -102.80 KDIK 1110.543 305.206 

727670 94014 WILLISTON SLOULIN INTL A ND 48.20 -103.65 KISN 1034.294 455.151 

727675 99999 MINOT AFB ND 48.42 -101.35 KMIB 1201.973 494.726 

727676 24013 MINOT FAA AP ND 48.27 -101.28 KMOT 1208.871 478.684 

727677 99999 GARRISON (AMOS) ND 47.65 -101.43 KN60 1204.887 408.989 

727680 94008 GLASGOW INTL ARPT MT 48.22 -106.62 KGGW 813.875 445.081 

727684 99999 JORDAN (RAMOS) MT 47.33 -106.93 KJDN 794.107 345.428 

727686 99999 WOLF POINT INTL MT 48.10 -105.58 KOLF 891.719 435.111 

727687 99999 SIDNEY-RICHLAND MT 47.70 -104.20 KSDY 997.286 396.803 

727720 24144 HELENA REGIONAL AIRPORT MT 46.60 -111.97 KHLN 410.886 264.119 

727730 24153 MISSOULA INTERNATIONAL A MT 46.92 -114.10 KMSO 250.034 306.969 

727750 24143 GREAT FALLS INTL ARPT MT 47.47 -111.38 KGTF 458.369 359.499 

727770 94012 HAVRE CITY-COUNTY AP MT 48.55 -109.77 KHVR 580.071 477.866 

727790 24146 KALISPELL GLACIER PKINT MT 48.32 -114.25 KFCA 248.065 462.938 

727796 24137 CUT BANK MUNI AP MT 48.60 -112.37 KCTB 388.414 487.267 

742300 24037 MILES CITY MUNICIPAL ARP MT 46.43 -105.88 KMLS 877.984 248.676 

1 The Lambert Conformal Conic (LCC) projection is based on standard parallels of 45.0 N and 49.0 N, an origin latitude 

and longitude of 44.25 N and 109.5 W, false easting is 600 km, and datum WGS-84. 

2 Available for year 2001 only 
6-9 

Table 6-6. NWS Precipitation Stations 

Station 

# 

Code LCCX1 (km) LCCY1 (km) 

1 102707 384.179 3.054

2 104456 414.496 ‐97.83

3 104598 450.949 20.568

4 109065 457.016 ‐42.187

5 240019 608.743 144.598

6 240165 1000.85 98.169

7 240330 851.652 154.443

8 240364 381.95 364.385

9 240807 674.436 173.49

10 241008 398.496 223.504

11 241047 471.102 159.731

12 241102 646.324 119.765

13 241127 919.923 140.95

14 241309 363.69 187.631

15 241318 367.545 195.043

16 241500 409.693 182.013

17 241737 398.556 400.264

18 241875 818.853 315.938

19 241984 745.578 415.841

20 241995 562.873 85.142

21 242414 355.318 100.002

22 242584 444.483 401.422

23 242689 984.011 194.513

24 243176 830.225 422.568

25 243489 355.513 377.545

26 243581 962.534 328.424

27 243751 458.225 359.835

28 243753 457.965 358.396

29 244038 454.666 70.289

30 244143 610.515 333.696

31 244241 409.064 307.639

32 244442 960.363 260.842

33 244820 416.337 41.508

34 244904 642.972 227.183

35 244983 604.404 311.356

36 245030 354.764 48.082

37 245040 359.981 305.525

38 245086 526.738 161.364

39 245106 767.83 120.773

40 245122 448.108 184.266

41 245387 535.71 250.371

42 245690 877.768 248.331

43 245706 458.006 309.604

44 245791 641.199 170.619

45 246008 503.089 310.922

46 246946 596.886 161.814

47 247258 483.489 424.972

48 247610 383.28 164.443

49 248101 349.712 440.303

50 248169 903.889 321.125

51 248329 462.783 235.703

52 248511 798.078 245.511

53 248866 473.254 47.536

54 248927 492.377 255.769

55 249044 686.942 306.878

56 249052 699.148 299.624

57 249240 722.38 119.254

58 249900 672.688 408.034

59 320995 1069.91 233.549

60 322183 1110.78 305.115

61 322188 1108.84 315.208

62 322365 1116.27 366.482

63 324180 1129.9 218.118

64 328812 1022.96 352.255
6-10 

Station 

# 

Code LCCX1 (km) LCCY1 (km) 

65 391114 1064.05 167.24

66 391294 1031.04 159.581

67 392557 1059.01 ‐88.635

68 392565 1050.72 ‐53.541

69 396304 1104.96 ‐74.095

70 396427 1082.03 ‐2.375

71 396937 1116.41 ‐1.753

72 397882 1047.24 45.423

73 399347 1087.2 ‐57.859

74 399537 1111.61 112.063

75 481000 708.358 ‐93.048

76 481165 821.651 14.493

77 482715 589.641 ‐79.28

78 482725 967.911 ‐82.53

79 484910 498.026 ‐84.364

80 485345 528.722 35.425

81 485371 994.597 ‐22.508

82 486395 965.28 6.992

83 486440 512.452 43.09

84 486603 1027.28 ‐81.928

85 486660 1025.02 ‐29.239

86 486935 1007.7 ‐17.014

87 487388 658.66 58.747

88 487545 898.839 61.78

89 488626 806.921 39.792

90 488852 769.79 ‐18.384

91 488858 771.919 ‐46.47

92 488875 704.667 ‐66.076

93 488888 664.779 ‐59.38

94 489770 722.855 ‐25.372
7-1 

7.0 AIR QUALITY MODELING METHODOLOGY 

7.1 Model Selection 

The CALPUFF modeling system (Scire et al., 2000a,b) was used to predict visibility 

impacts at UL Bend, North Absaroka, Theodore Roosevelt, Washakie and Yellowstone Class I 

areas. The EPA-approved version of CALPUFF modeling system, Version 5.8 was used in the 

modeling. 

7.2 Meteorological Modeling Options 

The CALMET model was run in its regulatory default mode, in accordance with the “EPA 

Model Clearinghouse Memorandum Clarification On EPA-FLM Recommended Settings For 

CALMET”, of August 31ST, 200910 ; and “Federal Land Managers’ Air Quality Related Values 

Work Group (FLAG) Phase I Report—Revised (2010)” of October, 201011. 

Initial Guess Field 

The MM5 gridded meteorological data were used to define the initial guess field for the 

CALMET simulations. MM5 data were available for years 2001-2003 with a horizontal grid 

spacing of 36 km and 34 vertical levels. 

Step 1 Field: Terrain Effects 

In developing the Step 1 wind field, CALMET adjusts the initial guess field to reflect 

kinematic effects of the terrain, slope flows and blocking effects. Slope flows are a function of the 

local slope and altitude of the nearest crest. The crest is defined as the highest peak within a radius 

TERRAD around each grid point. For the domain used to perform the modeling, a value of 

TERRAD of 15 km was used. This value has been recommended by MDEQ in the Montana 

Protocol and is considered appropriate for this modeling domain. The Step 1 field produces a flow 

field consistent with the fine-scale CALMET terrain resolution (2 km). 

Step 2 Field: Objective Analysis 

In Step 2, observations are incorporated into the Step 1 wind field to produce a final wind 

field. Each observation site influences the final wind field within a radius of influence (parameters 

RMAX1 at the surface and RMAX2 aloft). Observations and Step 1 field are weighted by means of 

parameters R1 at the surface and R2 aloft: at a distance R1 from an observation site, the Step 1 wind 

 

10 (http://www.epa.gov/ttn/scram/CALMET%20CLARIFICATION.pdf 11http://www.nature.nps.gov/air/pubs/pdf/flag/FLAG_2010.pdf
7-2 

field and the surface observations are weighted equally. RMAX1 and RMAX2 were set to 100 km 

and 200 km, while R1 and R2 were set to 50 km and 100 km, respectively. 

7.3 CALPUFF Computational Domain and Receptors 

The CALPUFF computational domain is the same as the CALMET computational domain. 

Receptor locations and terrain elevations within the Class I areas were obtained from the NPS 

website (http://www2.nature.nps.gov/air/Maps/Receptors/index.htm) for the this Class I Area 

analysis. Receptor locations and elevations were converted to the coordinate system of the 

modeling domain and input to the CALPUFF model to determine the maximum change in light 

extinction in each of the Class I areas as appropriate. The distribution of the receptors within UL 

Bend, Theodore Roosevelt, Washakie, North Absaroka, and Yellowstone are shown in Figure 7-1. 

7.4 Dispersion Modeling Options 

The CALMET, CALPUFF, and CALPOST programs of the CALPUFF dispersion modeling 

system require numerous inputs and the selection of different modeling options to develop the 

meteorological fields, calculate hourly concentration and deposition values, and calculate averaging 

period and pollutant-specific concentrations and deposition values, along with the change in light 

extinction due to a project. 

These model analyses for the BART protocol modeling were run in the regulatory default 

mode, in accordance with the “EPA Model Clearinghouse Memorandum Clarification On EPAFLM Recommended Settings For CALMET”, of August 31ST, 2009 and “Federal Land Managers’ 

Air Quality Related Values Work Group (FLAG) Phase I Report—Revised (2010)” of October, 

2010. If not addressed in the previously discussed documents, default values provided by A User’s 

Guide for the CALMET Meteorological Model (Version 5) (Earth Tech, 2000a) and A User’s 

Guide for the CALPUFF Dispersion Model (Version 5) (Earth Tech, 2000b) were used. 

The CALPUFF simulations were conducted using the following model options: 

 - Gaussian near-field distribution 

 - Transitional plume rise 

 - No building downwash 

 - Pasquill-Gifford (PG) dispersion coefficients (rural areas) and McElroy-Pooler 

coefficients (urban areas) 

 - Transition of σy to time-dependent (Heffter) growth rates 

 - Partial plume path adjustment for terrain 

 - Wet deposition, dry deposition, and chemical transformation using the 

MESOPUFF II scheme. 
7-3 

 - Regulatory option used 

Two important computational parameters in CALPUFF are XMXLEN (maximum length of 

an emitted puff, in grid units) and XSAMLEN (maximum travel distance of a puff, in grid units, 

during one time step). Both of these variables were set to 1.0 in the CALPUFF simulations in order 

to allow the strong wind channeling effects to be accounted for in the puff trajectory calculations. 

The first parameter ensures that the length of an emitted puff does not become so large so 

that it cannot respond to changes in the wind field on the scale of the meteorological grid. The 

model automatically increases the frequency of puff releases to ensure the length of a single puff is 

not larger than the grid size. The second parameter decreases the internal time step to ensure the 

travel distance during one time step does not exceed the grid size. 

The partial plume path adjustment option was used in CALPUFF for this analysis 

(MCTADJ=3). The CALMET wind field incorporates the effect of the terrain on the plume 

trajectories. The plume path coefficient is used to characterize the local effect on ground-level 

concentrations. The default plume path coefficients (PPC) used for this analysis are listed below: 

Stability Class A B C D E F 

PPC 0.5 0.5 0.5 0.5 0.35 0.35 

Nine species were modeled with CALPUFF for this analysis: SO2, SO4, NOx, NO3, HNO3, 

and four subcategories of PM10, (PMF, PMC, EC, and SOA). Of these nine species, seven are 

emitted by the Colstrip sources: SO2, SO4, NOx and four subcategories of PM10. 

The chemical mechanism computes transformation rates of SO2 to SO4 and NOx to 

NO3/HNO3. Hourly measured ozone concentrations are provided in an external OZONE.DAT file 

for use with the chemical transformation module. The ozone concentrations are used as a surrogate 

for OH radical concentrations during the day when the gas phase free radical chemistry is active. 

To allow for flexibility, each of the four units were modeled individually and their source 

contributions were summed in postprocessing steps. 

7.5 Air Quality Monitoring Data 

Ambient ozone monitoring data from the EPA AIRS and CASTNET networks were used to 

develop the hourly ozone monitoring data file (OZONE.DAT). The ozone stations used in the 

modeling are plotted in Figure7-4. These stations and their locations are listed in 7-5. 

Monthly background ammonia concentrations were calculated from hourly ammonia data 

collected from 2001 to 2002 at a site near Beulah, ND. The MDEQ believes these data are 
7-4 

representative of Montana background ammonia concentrations and the EPA concurs. These 

ammonia background concentrations, shown in 4.3.2 of the Montana Protocol were used in the 

CALPUFF modeling. 

7.6 Regional Haze 

Haze is a result of the suspension of small aerosols in the atmosphere, which individually 

are invisible, but collectively give the sky an opalescent appearance. Sources of aerosols are natural 

(e.g., sea spray, forest fires, dust storms) and anthropogenic (e.g., automobile and stack exhaust). 

Haze usually is associated with sub-saturated air, whereas fog or mist indicates saturated conditions. 

To measure the amount of haze, light extinction, i.e., the attenuation of light (defined as the 

reduction in luminance of a parallel beam of light as the light passes through a medium wherein 

absorption and scattering occur), is used. However, light extinction can also be caused by fog or 

mist and other naturally occurring atmospheric phenomena (e.g., precipitation). Therefore, the 

change in light extinction should be based upon the change from the existing or background light 

extinction to the potential light extinction caused by a proposed source. 

However, under the regulations promulgated for visibility protection (40 Code of Federal 

Regulations (CFR) Part 51.301(x)), visibility impairment is defined as, “...any humanly perceptible 

change in visibility (visual range, contrast, coloration) from that which would have existed under 

natural conditions.” Thus, the change in light extinction due to the plant is compared to the light 

extinction under natural conditions (i.e., no man-made pollution). Calculations of the impact of the 

simulated plume particulate matter component concentrations on light extinction were carried out in 

the CALPOST postprocessor. 

7.7 Regional Haze Impact Calculation Methodology 

Two regional haze analyses specified by FLAG 2010 were conducted at the Class I areas. 

Using the background values discussed in the next section, the visibility impacts were first assessed 

relative to the annual average background extinctions and the 98th percentile impact, and second 

relative to the 20% best days (B20%) extinctions and the 98th percentile impacts. 

CALPOST version 6.221 level 080724 was used for this assessment. CALPOST version 

6.221 was run in conformance with FLAG 2010 default settings (MVISCHECK=1). This is Method 

8, sub-mode 5, which conforms to using monthly RHFSML, RHFLRG, and RHFSEA with daily 

modeled and monthly background concentrations. 
7-5 

7.8 Background Concentrations and Relative Humidity Adjustment Factors 

To determine the change in visibility or light extinction in the Class I areas of interest, the 

natural background estimates for visibility reference levels, or background concentrations, must be 

quantified. These consist of ammonium sulfate, ammonium nitrate, organic matter, elemental 

carbon, soil, coarse matter, and sea salt. These concentrations are input into CALPOST variables 

BKSO4, BKNO3, BKOC, BKEC, BKSOIL, BKPMC and BKSALT, respectively. 

Tables 5 and 6 of FLAG 2010 provide the natural background concentrations for the Class I 

areas pertinent to this part of the analysis. Background concentrations used for these analyses are 

presented in 7-6. 

These estimates of background visibility assume no man-made pollution exists (i.e., relate to 

a time before humans) and were based on the natural background aerosol concentrations provided 

by the National Acid Precipitation Assessment Program (NAPAP) and adjusted to account for the 

effect of relative humidity on particle formation (namely sulfates and nitrates). 

The extinction due to Rayleigh scattering was set as listed in Tables 5 and 6 of FLAG 2010. 

The average monthly relative humidity adjustment factors from Tables 7 and 8 of FLAG 2010 (see 

7-7), along with the preceding extinction components, were used to determine the extinction change 

from the natural background visibility levels. 
7-6 

Figure 7-1. Locations of Receptors 

480 500 520 540 560 580 600 620 640

LCCx (km), WGS-84

-60

-40

-20

0

20

40

60

80

100

LCCy (km), WGS-84

Yellowstone

Washakie

Teton

North Absaroka

Map projection: LCC, WGS-84

Feast = 600 km, Fnorth = 0 km

LATo = 44.25N, LANo = 109.5W

LAT1 = 45 N, LAT2 = 49 N

1030 1040 1050 1060 1070 1080 1090

LCCx (km), WGS-84

300

310

320

330

340

350

360

370

380

390

400

LCCy (km), 

WGS-84

Theodore Roosevelt - N

Theodore Roosevelt - C

Theodore Roosevelt - S

705 710 715 720 725 730

LCCx (km), WGS-84

350

355

360

365

370

375

380

385

390

LCCy (km), WGS-84

UL Bend
7-7 

Figure 7-1. Locations of receptors at UL Bend, North Absaroka, Theodore Roosevelt, Washakie 

and Yellowstone Class I areas 

Table 7-5. Ozone Stations 

Station ID Latitude (deg.) Longitude (deg.) Land use Setting Site name 

160230101 43.46 113.56 Blighted Areas Rural 

300298001 48.51 114.00 Agricultural Rural Glacier National Park 

380070002 46.89 103.37 Agricultural Rural Painted Canyon 

380130004 48.64 102.40 Agricultural Rural LostwoodNwr 

380250003 47.32 102.53 Agricultural Rural Dunn Center 

380530002 47.60 103.26 Agricultural Rural Trnp-Nu 

380570004 47.30 101.77 Agricultural Rural Beulah North 

380650002 47.19 101.43 Agricultural Rural Hannover 

460711001 43.75 101.94 Agricultural Rural 

461030016 44.06 103.21 Residential Urban Near Robinsdale School 

560050123 44.67 105.29 Desert Rural 

560391011 44.57 110.40 Forest Rural Near the Water Tower of Hwy98 

560358001 42.93 109.79 Forest Rural Freemont Lake South, WV 

Figure 7-4. Locations of Ozone Monitoring Stations used in the CALPUFF Modeling 
7-8 

200 250 300 350 400 450 500 550 600 650 700 750 800 850 900 950 1000 1050 1100 1150 1200 1250

LCCx (km), WGS-84

-200

-150

-100

-50

0

50

100

150

200

250

300

350

400

450

500

550

LCCy (km), WGS-84

160230101

300298001

380070002

380130004

380250003

380530002

380570004

380650002

460711001

461030016

560050123

560391011

GLR468

THR422

WNC429

PND165

YEL408

Colstrip

Map projection: LCC, WGS-84

Feast = 600 km, Fnorth = 0 km

LATo = 44.25N, LANo = 109.5W

LAT1 = 45 N, LAT2 = 49 N

CASTNET

AIRS

ID

MT

ND

SD

WY

PPL, Montana

MODELING DOMAIN
7-9 

Table 7-6. Summary of Background Natural Visibility Conditions 

Class I area 

Average Natural 

Concentration West (μg/m3

) 

Dry Extinction Efficiency 

Coefficient (m3

/g)

Ammonium sulfate 0.12 3 

Ammonium nitrate 0.10 3 

Organic carbon mass 0.47 4 

Elemental carbon 0.02 10 

Soil 0.50 1 

Coarse Mass 3.00 0.6 

Source: Guidance for Estimating Natural Visibility Conditions Under the Regional Haze Rule (2003), 2-1

Table 7-7. Monthly f(RH) values based on Representative IMPROVE Site Locations 

Class I area Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec 

UL Bend WA 2.6 2.4 2.4 2.3 2.2 2.1 1.9 1.8 1.9 2.2 2.6 2.6 

Theodore Roosevelt NP 2.9 2.8 2.8 2.4 2.4 2.5 2.4 2.2 2.2 2.3 3.0 3.0 

Washakie WA 2.4 2.2 2.2 2.1 2.1 1.9 1.6 1.5 1.8 2.0 2.3 2.4 

North Absaroka WA 2.4 2.2 2.2 2.1 2.1 1.9 1.6 1.5 1.8 2.0 2.3 2.4 

Yellowstone NP 2.5 2.3 2.2 2.1 2.1 1.9 1.7 1.6 1.8 2.1 2.4 2.5 

Source: Guidance for Estimating Natural Visibility Conditions Under the Regional Haze Rule (2003), Appendix A 
8-1 

8.0 BART PROTOCOL MODELING RESULTS 

The changes in light extinction relative to the natural background due to SO2, NOx and 

PM10 from Colstrip Units 1&2, 3&4, and 1-4, were computed using the BART modeling 

protocol approach based on the base case emissions level during the years (2001-2003), the 

reference case emission levels during the years (2009-2010), and the control options analyzed. 

The emissions levels for the base case, the reference case, and the control alternatives are 

presented in Tables 4-5 and 4-6, English and metric units, respectively. 

The CALPUFF modeling predicted the effects of emissions from the units on visibility at 

the five Class I areas within 300 km of the facility. The FLAG 2010 methodology with 

CALPOST Method 8 and 98th percentile day results were used to predict visibility impacts. The 

light extinction impacts are presented for each modeled year in units of change in deciview (dv) 

for each of the five Class I areas, consistent with the procedures outlined in the Montana 

Protocol. As previously noted, the use of non-coincident actual daily maximum emissions from 

EPA’s CAM database and applying the 98th percentile in order to predict visibility impacts to all 

background visibility conditions result in conservative predictions that overestimate the facility’s 

visibility impact. 

Tables 8-1 and 8-2 summarize the delta in deciviews (i.e., the change in visibility) for the 

base case, the reference case, and each of the control alternatives from natural annual average 

conditions and conditions on the 20 percent best visibility days.The coefficients used in the 

IMPROVE equation for calculating light extinction as presented in the Federal Land Managers’ 

Air Quality Related Values Work Group (FLAG) Phase I Report – Revised (2010)12 are 

presented to either one or two significant digits. Using the rules of mathematical significance, 

the modeling results should only be presented to one significant digit precision; however, 

following EPA convention, the results presented herein are reported to two decimal places. It 

must be noted that the inclusion of a second significant digit in these results implies a level of 

model precision not warranted by the underlying equation. For each control alternative, the 

emissions were altered for only the specific pollutant for which the control alternative applied, 

with the other pollutants modeled at their base case emissions. Recall that changes of less than 

1.0 deciview are considered imperceptible. 

 

12 http://www.nature.nps.gov/air/pubs/pdf/flag/FLAG_2010.pdf
8-1 

Table 8-1 Class I Impacts (Deciviews) Method 8 (8th Highest Annual Average)* 

*Scenario visibility predictions include application of indicated controls on all Units listed in the sub-headings (i.e. Un
8-2 

Table 8-2 Class I Impacts (Deciviews) Method 8 20% Best Days Condition 
9-1 

9.0 SUMMARY 

Conservative BART Protocol CALPUFF visibility modeling was conducted for 

emissions from the Colstrip generating station. The approach conservatively uses non-coincident 

maximum daily emissions from EPA’s CAM database for NOx, SO2 and PM for all modeling 

days and assess the 98th percentile visibility impact. 

Modeling shows that the predicted visibility impacts of the Colstrip facility have 

decreased since the BART baseline years (2001-2003) due to implementation of additional 

controls and reduction of emissions. Colstrip burns low sulfur Powder River Basin Coal as its 

fuel and employs SO2 scrubbers and LNB technology, as well as SOFA on Units 3 & 4, resulting 

in low emission rates. 

A review of the model predicted visibility impacts of the various control techniques 

considered (Tables 8-1 and 8-2) shows that implementing these controls will not, in most cases, 

lead to perceptible visibility improvements (i.e., changes greater than 1.0 delta-deciview). Due 

to the conservative nature of the BART protocol predictions (unrealistic emission scenario, use 

of the 98th percentile prediction as the metric of performance), it is likely that the predicted 

impacts are being over-predicted and that actual visibility impacts and modeled improvements 

will be less than indicated on these tables.