BART AIR MODELING PROTOCOL

INDIVIDUAL SOURCE VISIBILITY 

IMPAIRMENT ANALYSIS

March, 2006

State of Wyoming

Department of Environmental Quality

Air Quality Division

Cheyenne, WY 82002
TABLE OF CONTENTS

Section Page

1.0 INTRODUCTION ...............................................................................................................1

2.0 OVERVIEW ........................................................................................................................2

3.0 POLLUTANTS AND EMISSIONS....................................................................................4

4.0 MODELING DOMAIN AND METEOROLOGICAL DATA .........................................10

5.0 CALMET INPUT ..............................................................................................................14

6.0 CALPUFF MODELING....................................................................................................16

7.0 POSTPROCESSING .........................................................................................................18

8.0 DETERMINATION OF SOURCES SUBJECT TO BART .............................................22

Tables

1 WYOMING BART – ELIGIBLE SOURCES.....................................................................3

2 MAXIMUM 24-HR EMISSION RATES FOR BART

 SOURCE ATTRIBUTION MODELING.......................................................................5

3 STACK PARAMETERS FOR BART SOURCE ATTRIBUTION MODELING .............8

4 SOURCE-SPECIFIC MODEL DOMAINS AND METEROLOGICAL 

 DATA YEARS .............................................................................................................12

5 CALMET INPUT PARAMETERS THAT ARE APPLICATION-

 SPECIFIC OR DIFFER FROM DEFAULT VALUES................................................15

6 CALPUFF INPUT PARAMETERS THAT ARE APPLICATION-

 SPECIFIC OR DIFFER FROM DEFAULT VALUES................................................17

7 MONTHLY F(RH) FACTORS FOR CLASS I AREAS ..................................................20

8 NATURAL BACKGROUND CONCENTRATIONS OF AEROSOL 

 COMPONENTS FOR 20% BEST DAYS FOR BART ANALYSIS (μg/m3).............21
1

1.0 INTRODUCTION

The U.S. EPA has issued final amendments to the Regional Haze Regulations, 

along with Guidelines for Best Available Retrofit Technology (BART) Determinations.(1) 

The Guidelines address methodology for determining which facilities must apply BART 

(Sources subject to BART) and the evaluation of control options.

The State of Wyoming will apply air quality modeling, using the CALPUFF 

modeling system, in accordance with the EPA Guidelines to determine the Wyoming 

sources which are subject to BART. This protocol defines the specific methodology to 

be used by the State. The Protocol may also be used for determining the improvement in 

visibility to be achieved by potential BART controls.

 

(1) 40 CFR Part 51: Regional Haze Regulations and Guidelines for Best Available Retrofit Technology 

(BART) Determinations; Final Rule. 70 Federal Register, 39103-39172, July 6, 2005.
2

2.0 OVERVIEW

The Wyoming Air Quality Division has determined that there are fourteen 

“BART-Eligible” sources in the State. The sources are listed in Table 1. Modeling and 

analyses described in this Protocol will be used to determine, for each of these sources, 

whether they are “subject to BART”. A source will be subject to BART if air quality 

modeling indicates that the source causes or contributes to visibility impairment in any

Class I area, within or outside of Wyoming.

Class I areas that could potentially experience visibility impairment due to 

emissions from Wyoming sources were defined as those within 300 km of any BARTeligible source

In Wyoming: N. Absaroka WA

Washakie WA

Yellowstone NP

Teton WA

Fitzpatrick WA

Bridger WA

Grand Teton NP

In Colorado: Rawah WA

Rocky Mountain NP

Mt. Zirkel NP

Flat Tops WA

Eagles Nest WA

Maroon Bells – Snowmass WA

West Elk WA

In Utah: Arches NP

In South Dakota: Badlands NP

Wind Cave NP

In Idaho: Craters of the Moon NP

Not all of these Class I areas will be included in modeling analyses. For each BARTeligible source, those Class I areas deemed most likely to experience visibility impacts on 

the basis of distance, meteorology, and terrain will be assessed in the analysis. Modeling 

domains, sources, and Class I areas are described in subsequent parts of this Protocol.

Wyoming will follow the BART guideline for evaluation of all BART-eligible 

sources. The threshold for classification of a source as subject to BART will be a 

visibility impact of 0.5 deciview (dv) compared to the 20% best days natural background, 

based on the 98th percentile of modeled 24-hour delta – deciview values.
3

Table 1. Wyoming BART – Eligible Sources

Basin Electric Laramie River Power Plant Boilers # 1, 2, 3

Black Hills P & L Neil Simpson # 1 Power Plant Boiler

Dyno Noble N.A. Cheyenne Plant Misc. Sources

FMC Corporation Granger Soda Ash Plant Boilers # 1, 2

FMC Corporation Green River Sodium Plant Three boilers

General Chemical Co. Green River Soda Ash Two boilers

OCI Wyoming Big Island Soda Ash Plant Boilers # 4, 5, 6

P4 Production Rock Springs Coking Plant Calciner

PacifiCorp Dave Johnson Power Plant Boilers # 3, 4

PacifiCorp Jim Bridger Power Plant Boilers # 1 – 4

PacifiCorp Naughton Power Plant Boilers # 1, 2, 3

PacifiCorp Wyodak Power Plant Boiler

Sinclair Oil Co. Casper Refinery Boiler # 7

Sinclair Oil Co. Sinclair Refinery Misc. Sources
4

3.0 POLLUTANTS AND EMISSIONS

The pollutants to be included in the CALPUFF modeling analyses will be sulfur 

dioxide (SO2), nitrogen oxides (NOx), and particulate matter. Where data are available, 

particulate matter emissions will be quantified separately as fine particles (PM2.5) and 

coarse particles (PM10). If no particle size information is available, all particles will be 

assumed to be PM2.5. Emissions of ammonia (NH3) and volatile organic compounds 

(VOC) will not be considered. It is believed that emissions of these pollutants have 

negligible impact on regional haze in Wyoming.

Table 2 presents a listing of emission rates to be modeled for all BART-eligible 

sources. The emission rates were determined by the Division based upon existing 

permits, allowable rates, and emissions reporting data. They represent the best available 

estimates of maximum 24-hour average emissions for each source and pollutant. Table 3 

lists stack parameters for each source. The data in Tables 2 and 3 will be used for all 

BART modeling to define the individual source’s impact on visibility.
5

Table 2. Maximum 24-hr Emission Rates for BART Source Attribution Modeling

Source Emission Unit NOx SO2 PM2.5 PM10

lb/hr g/s lb/hr g/s lb/hr g/s lb/hr g/s

Basin Electric Boiler # 1 2619.4 330.04 1284.0 161.78 158.3 19.95 387.7 48.85

Laramie River Boiler # 2 1502.3 189.29 1284.0 161.78 158.3 19.95 387.7 48.85

Boiler # 3 2719.2 342.61 1320.0 166.32 158.9 20.02 389.1 49.03

Black Hills Boiler 112.8 14.21 351.6 44.30 28.0 3.53 68.7 8.66

Neil Simpson # 1

Dyno Nobel IC Engine 170.6 21.50

IC Engine 170.6 21.50

Nitric Acid Plant 6.0 0.76

Nitric Acid Plant 6.0 0.76

Prill Tower 54.0 6.80

Wet Scrubber 19.7 2.48

Wet Scrubber 15.8 1.99

Prill Tower 43.1 5.43

Boiler 6.1 0.77

FMC Corporation Boiler # 1 251.0 31.63 71.7 9.03 35.8 4.51

Granger Soda Ash Boiler # 2 251.0 31.63 71.7 9.03 35.8 4.51

FMC Corporation NS-1A Boiler 523.3 65.93 1064.4 134.1 45.0 5.67

Sodium Products NS-1B Boiler 374.3 47.16 1064.4 134.1 45.0 5.67

PH-3 Boiler 43.4 5.47 8.4 1.06

General Chemical Boiler C 245.6 30.95 640.8 80.74 50.0 6.30

Green River Soda Ash Boiler D 501.6 63.20 1056.0 133.05 80.0 10.08
6

Table 2. Maximum 24-hr Emission Rates for BART Source Attribution Modeling (Continued)

Source Emission Unit NOx SO2 PM2.5 PM10

lb/hr g/s lb/hr g/s lb/hr g/s lb/hr g/s

OCI Boiler # 4 13.3 1.68 7.5 0.95

Big Island Boiler # 5 13.3 1.68 8.6 1.08

Boiler # 6 13.3 1.68

P4 Production Calciner 72.9 9.19 218.0 27.47 22.0 2.77

Pacificorp Boiler # 3 1194.2 150.47 2671.0 336.54 149.0 18.77 364.9 45.98

Dave Johnston Boiler # 4 1447.3 182.36 1737.0 218.86 372.5 46.93 357.9 45.09

Pacificorp Boiler # 1 2008.4 253.05 1683.0 212.05 162.7 20.50 398.3 50.19

Jim Bridger Boiler # 2 1716.7 216.30 1683.0 212.05 162.7 20.50 398.3 50.19

Boiler # 3 1941.1 244.57 1683.0 212.05 162.7 20.50 398.3 50.19

Boiler # 4 2030.8 255.88 1004.0 126.50 145.6 18.35 356.4 44.91

Pacificorp Boiler # 1 1079.8 136.05 2218.8 279.56 129.5 16.32 317.1 39.95

Naughton Plant Boiler # 2 1322.5 166.63 2844.0 358.34 159.0 20.03 389.3 49.05

Boiler # 3 2052.9 258.66 1839.5 231.77 228.6 28.80 559.7 70.52

Pacificorp Boiler 1189.0 149.81 2050.0 258.30 118.9 14.98 291.1 36.68

Wyodak

Sinclair Boiler 26.7 3.36 98.2 12.37 15.6 1.97

Casper Refinery
7

Table 2. Maximum 24-hr Emission Rates for BART Source Attribution Modeling (Continued)

Source Emission Unit NOx SO2 PM2.5 PM10

lb/hr g/s lb/hr g/s lb/hr g/s lb/hr g/s

Sinclair Heater 15.6 1.97

Refinery Heater 7.9 1.00

Heater 5.5 0.69

Heater 5.5 0.69

Heater 9.6 1.21 1.0 0.13

Heater 2.1 0.26 0.5 0.06

Heater 2.6 0.33 0.3 0.04

Heater 3.2 0.40

Heater 3.8 0.48

Heater 8.6 1.08

Heater 3.4 0.43

Heater 9.0 1.13

Heater 1.8 0.23

Heater 4.3 0.54

Boiler # 10 23.0 2.90 2.3 0.29

SRU # 1 5.1 0.64 340.0 42.84
8

Table 3. Stack Parameters for BART Source Attribution Modeling

Source Emission Unit Height Temp Vel Dia

(m) (K) (m/s) (m)

Basin Electric Boiler # 1 182.9 332.2 15.2 9.54

Laramie River Boiler # 2 182.9 332.2 15.2 9.54

Boiler # 3 182.9 358.2 22.9 8.66

Black Hills Boiler 76.2 442.4 21.9 1.83

Neil Simpson # 1

Dyno Nobel IC Engine 13.41 485.9 16.78 1.68

IC Engine 13.41 485.9 16.78 1.68

Nitric Acid Plant 21.34 438.7 11.88 0.61

Nitric Acid Plant 21.34 438.7 11.88 0.61

Prill Tower 39.93 310.4 24.23 1.52

Wet Scrubber 23.62 294.3 51.51 0.46

Wet Scrubber 51.21 369.3 6.89 0.46

Prill Tower 41.76 357.6 2.72 3.55

Boiler 21.34 505.4 5.15 0.33

FMC Corporation Boiler # 1 45.72 333.2 21.29 1.98

Granger Soda Ash Boiler # 2 45.72 333.2 21.2 1.98

FMC Corporation NS-1A Boiler 91.44 414.8 54.49 1.83

Sodium Products NS-1B Boiler 91.44 410.9 54.96 1.83

PH-3 Boiler 21.34 394.3 8.34 2.29

General Chemical Boiler C 47.55 452.6 15.98 3.05

Green River Soda Ash Boiler D 47.55 460.9 25.12 3.05

OIC Boiler # 4 16.76 434.1 18.39 1.98

Big Island Boiler # 5 24.38 440.1 21.53 1.83

Boiler # 6 24.38 434.1 21.53 1.83

P4 Production Calciner 39.62 1255.4 12.24 4.75

Pacificorp Boiler # 2 152.4 444.8 33.22 4.57

Dave Johnston Boiler # 3 76.2 325.9 8.53 7.01
9

Table 3. Stack Parameters for BART Source Attribution Modeling (Continued)

Source Emission Unit Height Temp Vel Dia

(m) (K) (m/s) (m)

Pacificorp Boiler # 1 152.4 333.2 22.3 7.32

Jim Bridger Boiler # 2 152.4 333.2 22.3 7.32

Boiler # 3 152.4 333.2 25.2 7.32

Boiler # 4 152.4 333.2 13 9.45

Pacificorp Boiler # 1 60.96 410.8 26.52 4.27

Naughton Plant Boiler # 2 68.58 410.8 21.34 4.88

Boiler # 3 137.16 323 19.81 8.08

Pacificorp Boiler 121.9 346.9 22.6 6.1

Wyodak

Sinclair Boiler 41.39 549.8 12.9 1.37

Casper Refinery

Sinclair Heater 33.53 672 10.3 0.76

Refinery Heater 36.6 477.6 4.39 1.22

Heater 27.43 1016.5 9.33 1.22

Heater 33.53 630.4 8.09 0.91

Heater 45.72 727.6 5.88 1.52

Heater 19.2 855.4 2.08 1.22

Heater 41.15 635.9 2.24 1.22

Heater 36.6 838.7 6.49 1.22

Heater 36.6 866.5 3.4 1.52

Heater 36.6 866.5 6.96 1.52

Heater 36.6 833.2 5.15 1.52

Heater 36.6 683.2 13.47 0.91

Heater 12.8 810.9 8.35 0.91

Heater 27.4 716.5 5.03 1.22

Boiler # 10 15.24 449.8 9.08 1.52

SRU # 1 49.1 810.9 1.4 1.52
10

4.0 MODELING DOMAIN AND METEOROLOGICAL DATA

The determination of those sources subject to BART will be carried out through 

application of the CALPUFF modeling system. Wind and meteorological fields for 

CALPUFF will be developed using the CALMET meteorological processor; input data 

will consist of MM5 prognostic model fields, augmented with surface and upper air data 

from observing stations within the model domain.

The Class I areas potentially affected by Wyoming sources are located over a 

large area within and surrounding the state. Wyoming BART – eligible sources are 

widely distributed within the state. Therefore, a very large modeling domain would be 

required to address all sources and Class I areas. It is also desired to utilize to the extent 

possible meteorological data fields previously used for long-range transport modeling 

that have been checked and quality assured. For these reasons, BART modeling will 

utilize three separate modeling domains and meteorological data sets.

The model domains will include a Southwest Wyoming domain, utilizing 

meteorological data for 1995, 1996, and 2001, and Northeast and Southeast Wyoming 

domains with 2001, 2002, and 2003 meteorological data. The southwest and northeast 

data sets have been developed for prior Wyoming analyses and can be used directly for 

BART modeling. The southeast domain meteorological fields will be developed 

specifically for the BART analyses. MM5 model output for the appropriate area and 

years have been acquired by the Division, and will be used in CALMET (with additional 

surface and upper air data) to generate required input meteorological fields for 

CALPUFF. The three modeling domains, along with the locations of Class I areas and 

BART – eligible sources, are shown in Figure 1.

Model domains and Class I areas to be used for each source’s BART evaluation 

are shown in Table 4. The assignments in Table 4 were developed on the basis of 

source/Class I area locations, distances to each Class I area, and professional judgment 

considering meteorological and terrain factors. All source-Class I area distances exceed 

50 km, and are less than 300 km, thus falling within the range of recommended 

application for CALPUFF.

Only those Class I areas that are most likely to be impacted by each source will be 

modeled. Areas greater than 300 km from a source have been excluded. Also, when 

several Class I areas are located in the same direction from a source, only the closest 

Class I areas will be evaluated. If impacts are less than the BART threshold for the Class 

I areas modeled, it can be reasonably assumed that areas at a greater distance and in 

directions of less frequent plume transport will not experience significant impacts.

 Possible exceptions exists in the case of the Laramie River and Dave Johnston 

power plants. These sources are generally upwind of the Wind Cave and Badlands Class 

I areas, and therefore will be modeled in the northeast Wyoming domain. However, all 

of these sources are approximately the same distance from northern Colorado Class I 

areas as they are from Wind Cave and Badlands. Because of prevailing wind directions, 
11

it is unlikely that impacts in Colorado exceed those in the South Dakota areas. However, 

if one or more of these sources are not subject to BART based on the Wind 

Cave/Badlands modeling, it will be appropriate to remodel such sources in the southeast 

Wyoming domain to confirm that they do not exceed the subject-to-Bart threshold at the 

nearest Colorado areas.
12

Table 4. Source-Specific Model Domains and Meteorological Data Years

Source Modeling

Domain

Meteorological

Data Years

Class I Areas

to be Evaluated

1. Basin Electric Northeast WY 2001, 2002, 2003 Wind Cave, Badlands

 Laramie River

2. Black Hills Northeast WY 2001, 2002, 2003 Wind Cave, Badlands

 Neil Simpson # 1

3. Dyno Nobel Southeast WY 2001, 2002, 2003 Rocky Mountain NP, Rawah, 

 Cheyenne Plant Mt. Zirkel

4. FMC Corporation Southwest WY 1995, 1996, 2001 Bridger, Fitzpatrick

 Granger Soda Ash

5. FMC Corporation Southwest WY 1995, 1996, 2001 Bridger, Fitzpatrick 

 Sodium Products

6. General Chemical Southwest WY 1995, 1996, 2001 Bridger, Fitzpatrick

 Green River Soda Ash

7. OCI Southwest WY 1995, 1996, 2001 Bridger, Fitzpatrick 

 Big Island Plant

8. P4 Production Southwest WY 1995, 1996, 2001 Bridger, Fitzpatrick 

 Rock Springs Coking

9. Pacificorp Northeast WY 2001, 2002, 2003 Wind Cave, Badlands

 Dave Johnston
13

Table 4. Source-Specific Model Domains and Meteorological Data Years (Continued)

Source Modeling

Domain

Meteorological

Data Years

Class I Areas

to be Evaluated

10. Pacificorp Southwest WY 1995, 1996, 2001 Bridger, Fitzpatrick 

 Jim Bridger

11. Pacificorp Southwest WY 1995, 1996, 2001 Bridger, Fitzpatrick 

 Naughton Plant

12. Pacificorp Northeast WY 2001, 2002, 2003 Wind Cave, Badlands 

 Wyodak

13. Sinclair Northeast WY 2001, 2002, 2003 Wind Cave, Badlands

 Casper Refinery

14. Sinclair Southeast WY 2001, 2002, 2003 Rocky Mountain NP, Rawah, 

 Sinclair Refinery Mt. Zirkel
14

5.0 CALMET INPUT

Input options for CALMET generally applied default values except where 

application-specific choices are required. Because the CALMET meteorological 

processing for the three modeling domains have been done at different times for different 

applications, there are some minor differences in several individual inputs. None of these 

are likely to cause significant differences in the final meteorological fields produced by 

CALMET.

Table 5 summarizes the CALMET inputs. Where there were differences between 

the CALMET runs, a range of values is shown.

All MM5 prognostic data had 36 km resolution, except for the 1995 (southwest 

Wyoming) data set, which has 20 km resolution. The southwest Wyoming CALMET 

processing used, in addition to the MM5 input, data from 63 surface weather stations and 

249 precipitation stations. The northeast Wyoming analyses included 13 surface stations 

and 62 precipitation stations. (The number of stations may vary slightly between years 

due to data availability.) The southeast Wyoming analysis will use surface and 

precipitation data from a number of stations comparable to what was used for the 

northeast domain. The number of upper air stations used as input to the CALMET 

processing varied from none (1996 only) to three, depending upon the ready availability 

of data from representative stations. For 1996, the MM5 data fields were used alone to 

define upper air conditions.
15

Table 5. CALMET Input Parameters that are Application-Specific or Differ from Default 

Values

CALMET Input Parameter Value(s)

Input Group 2

 Map projection (PMAP) Lambert Conformal

 Grid spacing (DGRIDKM) 4

 Number vertical layers (NZ) 8 – 10

 Top of lowest layer (m) 20 

 Top of highest layer (m) 2980 – 3500

Input Group 4

 Observation mode (NOOBS) 0 or 1

Input Group 5

 Prog. Wind data (IPROG) 4 or 14

 (RMAX1) 20 – 30

 (RMAX2) 40 – 50

 (RMAX3) N/A

 Terrain influence (TERRAD) 15

 (R1) 5

 (R2) 15 – 25

Input Group 6

 Max mixing ht (ZIMAX) 2980 - 3500

CALMET Model Version 5.53A, Level 040716
16

6.0 CALPUFF MODELING

CALPUFF modeling will be conducted for each BART-eligible source to 

calculate concentrations of each visibility-impairing pollutant in each Class I area. 

Source emissions, meteorological data, and model domains will be as prescribed in 

Sections 3.0 and 4.0 of this Protocol. Each source will be modeled separately for each of 

three years of meteorological data, utilizing the CALMET meteorological fields 

generated for the applicable years/model domains.

Concentration calculations will be made for all receptors within the designated 

Class I areas. Receptors will be defined for each area by the standard receptor points 

developed by the Federal Land Managers (FLMs) and available from the National Park 

Service web site database. 

CALPUFF model options will conform with standard default values with limited 

exceptions. Table 6 lists non-default and application-specific parameters that will be 

used in CALPUFF. Chemical transformation calculations will use MESOPUFF II 

equations, and dispersion coefficients will be PG coefficients (MDISP=3).

Since most of the sources to be evaluated have relatively tall stacks, and only 

impacts at distances greater than 50 km are of interest, building wake effects will not be 

considered for any sources. Puff splitting will not be used for CALPUFF modeling, in 

order to limit computation time to reasonable levels.

Background ozone concentrations will be considered in the model by use of 

hourly ozone files from representative monitoring sites in each model domain. A default 

ozone concentration (for missing data hours) will be specified as 44 ppb, based on 

average data from northeast Wyoming. The background ammonia concentration will be 

specified as 2.0 ppb for all model domains. The value of 2.0 is based on monitoring data 

from nearby states and IWAQM guidance. North Dakota has specified a background 

ammonia concentration of 2.0 ppb for the western part of the state, as indicated by 2000-

2001 monitoring data from Beulah. Colorado found background ammonia concentrations 

of 0.5 to 1.6 ppb for the Mt. Zirkel area. The IWAQM Phase II report suggests values of 

0.5 ppb for forested areas, 1.0 ppb for arid lauds, and 10.0 ppb for agricultural lauds 

(within a factor of two). Since a single background ammonia value must be specified 

within a model domain, a value of 2.0 ppb has been selected as reasonable given the 

available data. Experience has suggested that this value is conservative in that it is 

unlikely to significantly limit nitrate formation in the model.

Default particle size data will be used for all modeled particles except PM10. 

Since PM10 represents coarse particles with diameters between 2.5 and 10.0 μm, the 

mass-mean PM10 diameter has been estimated as 7.0 μm.
17

Table 6. CALPUFF Input Parameters that are Application-Specific or Differ From 

Default Values

CALPUFF Input Parameter Value(s)

Input Group 1

 Number of species modeled (NSPEC) 7

 Number of species emitted (NSE) 4

Input Group 2

 Dispersion coefficients (MDISP) 3

Input Group 3

 Species Modeled SO2, SO4, NOx, HNO3, NO3, PM10, PM25

 Species emitted SO2, NOx, PM10, PM25

Input Group 4

 Map projection (PMAP) Lambert Conformal

 Grid spacing (DGRIDKM) 4

 Cell face heights (ZFACE) same as used for applicable CALMET 

 

 Sampling grid F

 

Input Group 8 

 Particle size parameters mean 0.48, st. dev. 2.0

except 7.0, 2.0 for PM10

Input Group 11

 Ozone input option (MOZ) 1 (hourly ozone files)

 Monthly ozone (BCKO3) 44 ppb

 Monthly ammonia (BCKNH3) 2.0 ppb

Input Group 12

 Max mixing height (XMAXZ1) same as applicable CALMET

CALPUFF Model Version 5.711A, Level 040716
18

7.0 POSTPROCESSING

The CALPOST processor is used in the final processing step to calculate 24-hour 

average visibility results. Output will be specified in deciview (dv) units. The output 

consists of the highest deciview impact on each day from all receptors within each Class I 

area.

Calculations of light extinction are made for each pollutant modeled (sulfate, 

nitrate, coarse particles, and fine particles). The sum of all extinction values is then used 

to calculate the delta-dv change relative to natural background. Default extinction 

coefficients for each species, as given below, will be used.

Ammonium sulfate 3.0

Ammonium nitrate 3.0

PM Coarse (PM10) 0.6

PM Fine (PM25) 1.0

Monthly average relative humidity factors F(RH) will be used in the light 

extinction calculations to account for the hygroscopic characteristic of sulfate and nitrate 

particles. Monthly f(RH) values, from the EPA Guidance for Estimating Natural 

Visibility Conditions Under the Regional Haze Rule, are shown in Table 7 for each set of 

Class I areas. Values shown in Table 7 represent the average f(RH) for the Class I areas 

listed. In many cases the tabulated f(RH) values for a given month are the same for each 

Class I area within a group. Where differences occur, they are no greater than 0.3 units; 

the average values are within 0.1 to 0.2 units of the Guideline values for any individual 

area.

CALPOST visibility Method 6 (MVISBK=6) will be used for determination of 

background visibility. It should be noted that when Method 6 is used, the CALPOST 

calculation of f(RH) and the CALPOST designation of maximum relative humidity 

(RHMAX) are irrelevant and not used for calculations.

The natural background conditions as a reference for determination of the deltadv change due to a source should be representative of the 20% best natural visibility days. 

EPA BART guidance provides the 20% best days deciview values for each Class I area 

on an annual basis, but does not provide species concentration data for the 20% best 

background conditions. These concentrations are needed for input to CALPOST.

Annual species concentrations corresponding to the 20% best days have been 

calculated for each Class I area to be addressed, by scaling back the annual average 

concentrations given in Guidance for Estimating Natural Visibility Conditions Under the 

Regional Haze Rule (Table 2-1). A separate scaling factor was derived for each Class I 

area such that, when multiplied by the Guidance table annual concentrations, the 20% 

best days deciview value for that area would be calculated. The scaled aerosol 

concentrations were then averaged over the Class I areas to be addressed in each model 

domain to provide data (to be used for each month) for CALPOST input. The 20% best 
19

days aerosol concentrations to be used for Wyoming BART evaluations are listed in 

Table 8.

Concentrations were generally very similar at all of the Class I areas in a given 

model domain. Comparison of predicted deciview values for individual Class I areas 

(using annual average f(RH) and concentrations from Table 8) to the Guidance 20% best 

deciview tabulations show agreement within one percent for all Class I areas.
20

Table 7. Monthly f(RH) Factors for Class I Areas

Month Wind Cave

Badlands

Bridger, Fitzpatrick,

Grand Teton NP

Rocky Mt. NP,

Rawah, Mt. Zirkel,

Savage Run

 Jan 2.65 2.50 2.10

 Feb 2.65 2.30 2.13

 Mar 2.65 2.27 2.00

 Apr 2.55 2.10 2.10

 May 2.70 2.10 2.23

 Jun 2.60 1.83 1.87

 Jul 2.30 1.57 1.77

 Aug 2.30 1.53 1.83

 Sep 2.20 1.80 2.00

 Oct 2.25 2.03 1.87

 Nov 2.75 2.47 2.07

 Dec 2.65 2.43 2.03
21

Table 8. Natural Background Concentrations of Aerosol Components for 20% Best Days for BART Analyses (μg/m3).

Aerosol

Component

Wind Cave

Badland NP

Fitzpatrick WA

Bridger WA

Rawah WA

Rocky Mountain NP

Mt. Zirkel WA

 Ammonium Sulfate .047 .045 .045

 Ammonium Nitrate .040 .038 .037

 Organic Carbon .186 .178 .178

 Elemental Carbon .008 .008 .007

 Soil .198 .189 .189

 Coarse Mass 1.191 1.136 1.135
22

8.0 DETERMINATION OF SOURCES SUBJECT TO BART

Results of the CALPOST visibility processing for a BART-eligible source will be 

a listing of daily maximum ∆dv values for each Class I area and year of meteorological 

data. The visibility impact threshold to identify those sources subject to BART is a 98th

percentile change in visibility (∆dv) of 0.5 dv. Therefore, if the 8th highest ∆dv value is 

equal to or greater than 0.5 dv, the source will be considered to cause or contribute to 

visibility impairment in the subject Class I area.

The highest model-predicted daily ∆dv values for each Class I area and year will 

be ranked. If the 8th highest value for each of the three years at each Class I area in a 

given model domain is less than 0.5 dv, the source will have been demonstrated to have 

insignificant visibility impacts and will not be subject to BART. If the 8th high for any 

year or Class I area equals or exceeds 0.5 dv, the source will be subject to BART.