WRAP BART CALPUFF Modeling 

Explanation of Changes in the April 2007 Revised Final Modeling Analysis 

WRAP Regional Modeling Center 

May 18, 2007 

INTRODUCTION 

The Regional Haze Rule (RHR) has provisions for the implementation of Best Available Retrofit 

Technology (BART) on specific stationary emission sources that are reasonably anticipated to 

contribute to visibility impairment at a Class I area. The CALPUFF modeling system is the 

EPA-recommended model for demonstrating that a potential BART-eligible source contributes 

significantly to visibility impairment. The Western Regional Air Partnership (WRAP) Regional 

Modeling Center (RMC) has been performing subject-to-BART CALPUFF modeling of 

potential BART-eligible sources in 7 WRAP states: 

• Alaska 

• Arizona 

• Montana (with EPA Region 8) 

• New Mexico 

• Nevada 

• South Dakota 

• Utah 

Information on the WRAP RMC CALPUFF BART modeling, including Modeling Protocol, 

model inputs and summary results can be found on the WRAP RMC BART webpage: 

• http://pah.cert.ucr.edu/aqm/308/bart.shtml 

WRAP RMC CALPUFF BART MODELING RESULTS 

The WRAP RMC has performed numerous revisions of the CALPUFF subject-to-BART 

modeling for potential BART-eligible sources in the 7 States. These revisions have mostly been 

due to updated estimate estimates for maximum 24-hour actual emissions from the states, but 

have also included updated source coordinates, additional of sources left out of initial analysis 

and corrections to CALMET/CALPUFF inputs. 

In March 2007, a potential error was uncovered regarding how PM emissions were input into 

CALPUFF. Although not well documented, CALPUFF only reads 132 characters off its input 

file and since PM emissions were specified as the last species beyond 132 characters, they may 
have not been input correctly. Thus, the CALPUFF input files were corrected to limit the input 

file to 132 characters. Given the need to rerun all of the WRAP RMC CALPUFF BART 

modeling, two other updates were made to the modeling: 

• The maximum allowable mixing height was increased from the 3,000 m AGL value 

recommended by EPA (Atkinson and Fox, 2006) to 4,500 m AGL. The Colorado 

Division of Public Health and Environment (CDPHE) has noted that mixing heights in 

the western U.S. are higher than the east and that a 4,500 m AGL is more appropriate for 

the west (CDPHE, 2005). 

• The CALPUFF output was processed with the POSTUTIL processor prior to obtaining 

the visibility impacts using the CALPOST processor to avoid the “double counting” of 

ammonia. Previously, the CALPOST processor was applied directly to the CALPUFF 

output. 

The post-April 2007 revised CALPUFF BART modeling results were mostly very similar to the 

pre-April 2007 CALPUFF BART modeling and all potential BART-eligible sources whose 

visibility impacts at a Class I area exceeded the 0.5 change in deciview (dv) significance 

threshold1

 in the pre-April preliminary analysis also exceeded it in the post-April final analysis 

and vice versa. Since primary PM emissions are typically a small part of the visibility impacts, 

the differences due to the changes in the PM emissions were expected to be small. However, 

some of the changes in visibility impacts in the revised simulations were larger than expected. 

The largest change in visibility impacts between the pre-April and post-April 2007 CALPUFF 

modeling analysis was for the New Mexico SRC02 PNM San Juan (PNMSJ) source. Table 1 

displays the CALPUFF-estimated visibility impacts due to the PNMSJ source for the pre-April 

2007 analysis where a three-year average 98th percentile visibility impact of 8.2 dv was 

estimated. The post-April 2007 CALPUFF estimated visibility impact with the three changes 

(corrected PM emission inputs, 4,500 m AGL maximum mixing height and processing with 

POSTUTIL before CALPOST) obtained a maximum three-year average 98th percentile visibility 

impacts of 5.4 dv (Table 2), which is substantially lower than before (8.2 dv). To determine the 

cause for the large changes in the pre- and post-April CALPUFF analysis the post-April 

CALPUFF results were reprocessed using CALPOST without first pre-processing with 

POSTUTIL obtaining results given in Table 3. Table 3 isolates the differences between the preApril and post-April 2007 CALPUFF analysis due solely to the correction of the PM emission 

inputs and the increase in the maximum allowable mixing height from 3,000 to 4,500 m AGL 

and estimates a maximum three-year average 98th percentile visibility impact of 8.6 dv, which is 

much closer to the pre-April 2007 value (8.2 dv). Thus, we conclude that the large change in the 

visibility impacts for the NM PNMSJ source is due to the use of the POSTUTIL processor on the 

CALPUFF output prior to calculating the visibility impacts using CALPOST. 

 

1

 EPA recommends a visibility significance threshold for contributing to visibility impairment at a Class I area of a 

change in deciview over natural conditions of greater than 0.5 (EPA, 2006). However, other factors may be 

considered and the WRAP RMC does not express any opinion on whether a potential BART-eligible source is 

subject-to-BART or not, which is a State decision. 

2
Treatment of Background Ammonia in CALPUFF 

The CALPUFF modeling system has a very simplistic treatment of background ammonia 

concentrations. Despite the fact that atmospheric ammonia concentrations are known to have a 

high temporal and spatial variability, the CALPUFF modeling system assumes a single 

background ammonia concentration that is spatially and temporally invariant. CALPUFF 

represents a continuous plume by a series of overlapping puffs. A CALPUFF simulation may 

include many sources and each puff from each source uses the total background ammonia 

concentrations independently of the other puffs to partition the total nitrate formed into particle 

NO3 and gaseous HNO3 using the user-specific background ammonia and estimated SO4 for 

that puff and an SO4/NO3NH4 equilibrium equation. The total concentration impact at a given 

receptor (e.g., Class I area) is obtained by summing up all the impacts from all of the overlapping 

puffs from all the sources. This approach in effect “double counts” the background ammonia 

concentration across overlapping puffs in the partitioning of total nitrate to particle NO3 and 

gaseous HNO3. The POSTUTIL postprocessor recalculates the partitioning of total nitrate to 

NO3 and HNO3 using the combined SO4 and total nitrate from all CALPUFF overlapping puffs 

at a receptor with the user defined background ammonia concentration, therefore eliminating the 

“double counting” of ammonia. 

The use of POSTUTIL to eliminate the “double counting” of ammonia in the CALPUFF 

simulation sounds like better science. In reality, background ammonia has so much spatial and 

temporal variability that the assumption of a single value can not always be valid. The 

Interagency Workgroup on Air Quality Modeling (IWAQM, 1998) recommends three 

background values for CALPUFF modeling: 

• 0.5 ppb for forested lands; 

• 1.0 ppb for arid lands; and 

• 10 ppb for grasslands. 

Most of the western U.S. Class I areas are characterized by arid and forested lands, consequently 

a 1.0 ppb background ammonia value was selected for the WRAP RMC BART CALPUFF 

modeling. Ammonia measurements are difficult to make and very few have been taken in the 

western U.S. The CDPHE collected measurements in eastern Colorado and for their BART 

modeling recommended background ammonia values ranging from 1 ppb to 44 ppb (CDPHE, 

2005). The selection of the 1 ppb background value used in the WRAP BART CALPUFF 

modeling, although justified based on IWAQM (1998) recommendations, is highly uncertain. 

The pre-April/post-April POSTUTIL analysis suggests that the whether the CALPUFF estimated 

visibility impacts at Class I areas is greater than 0.5 dv or not is not that sensitive to the userspecific background ammonia value, which is reassuring. 

REFERENCES 

All references cited can be found in the WRAP RMC CALPUFF BART Modeling Protocol: 

http://pah.cert.ucr.edu/aqm/308/bart/WRAP_RMC_BART_Protocol_Aug15_2006.pdf

3
Table 1. Original (pre-April 2007) visibility impacts using CALMET/CALPUFF without

running CALPOST. 

NM SRC02 PNM SJ #1-4: SO2 = 35,735; NOx = 38,763; PM = 3,884 TPY 

Annual Average Natural Conditions 

Class I Area with at least 1 receptor within 300 km of source

Class I Area 

Minimum 

Distance 98th Percentile for Each Year 98th 

(km) 2001 2002 2003 

3 year 

AVG 

Mesa Verde NP 40 7.527 9.458 7.67 8.218 

Weminuche Wilderness 98 2.449 3.248 2.589 2.762 

San Pedro Parks Wilderness 155 3.393 4.71 3.974 4.026 

La Garita Wilderness 169 1.556 1.876 1.866 1.766 

Canyonlands NP 170 8.227 4.795 5.125 6.049 

Black Canyon Gunnison NM 203 2.336 2.519 2.709 2.521 

Bandelier NM 210 2.259 3.64 3.313 3.071 

Petrified Forest NP 213 1.549 1.259 1.001 1.27 

West Elk Wilderness 216 1.999 2.208 2.228 2.145 

Arches NP 222 5.227 4.42 4.685 4.777 

Capitol Reef NP 232 4.333 1.975 2.515 2.941 

Pecos Wilderness 248 2.074 2.717 2.682 2.491 

Wheeler Peak Wilderness 258 1.959 1.719 1.812 1.83 

Great Sand Dunes NM 269 1.435 1.56 1.67 1.555 

Maroon Bells-Snowmass WA 271 1.145 1.257 1.024 1.142 

Grand Canyon NP 285 2.149 1.586 1.126 1.62 

4
Table 2. Revised (post-April 2007) visibility impacts using CALMET/CALPUFF with 

corrected PM emissions, 4.500 m AGL maximum mixing height and with running CALPOST. 

NM SRC02 PNM SJ #1-4: SO2 = 35,735; NOx = 38,763; PM = 3,884 TPY 

Annual Average Natural Conditions 

Class I Area with at least 1 receptor within 300 km of source

Class I Area 

Minimum 

Distance 98th Percentile for Each Year 98th 

(km) 2001 2002 2003 

3 year 

AVG 

Mesa Verde NP 40 5.54 5.34 5.30 5.40 

Weminuche Wilderness 98 2.24 2.99 2.41 2.55 

San Pedro Parks Wilderness 155 3.80 4.07 4.14 4.01 

La Garita Wilderness 169 1.63 1.82 1.77 1.74 

Canyonlands NP 170 6.21 4.33 4.44 4.99 

Black Canyon Gunnison NM 203 2.38 2.27 2.43 2.36 

Bandelier NM 210 2.47 2.90 3.08 2.82 

Petrified Forest NP 213 1.62 1.27 1.03 1.31 

West Elk Wilderness 216 2.14 1.90 2.20 2.08 

Arches NP 222 4.06 3.71 3.59 3.79 

Capitol Reef NP 232 4.00 2.02 2.35 2.79 

Pecos Wilderness 248 2.17 2.63 2.81 2.53 

Wheeler Peak Wilderness 258 1.94 1.73 1.97 1.88 

Great Sand Dunes NM 269 1.47 1.59 1.74 1.60 

Maroon Bells-Snowmass WA 271 1.19 1.27 1.15 1.21 

Grand Canyon NP 285 2.12 1.50 1.18 1.60 

5
6

Table 3. Revised (post-April 2007) visibility impacts using CALMET/CALPUFF with 

corrected PM emissions, 4.500 m AGL maximum mixing height and without running 

CALPOST. 

NM SRC02 PNM SJ #1-4: SO2 = 35,735; NOx = 38,763; PM = 3,884 TPY 

Annual Average Natural Conditions 

Class I Area with at least 1 receptor within 300 km of source

Class I Area 

Minimum 

Distance 98th Percentile for Each Year 98th 

(km) 2001 2002 2003 

3 year 

AVG 

Mesa Verde NP 40 8.05 9.68 7.98 8.57 

Weminuche Wilderness 98 2.84 3.42 2.64 2.96 

San Pedro Parks Wilderness 155 3.91 4.86 4.51 4.42 

La Garita Wilderness 169 1.82 1.96 1.95 1.91 

Canyonlands NP 170 8.27 4.87 5.50 6.21 

Black Canyon Gunnison NM 203 2.45 2.60 2.70 2.58 

Bandelier NM 210 2.57 3.76 3.63 3.32 

Petrified Forest NP 213 1.63 1.28 1.11 1.34 

West Elk Wilderness 216 2.15 2.25 2.31 2.24 

Arches NP 222 5.34 4.51 4.80 4.89 

Capitol Reef NP 232 4.44 2.09 2.56 3.03 

Pecos Wilderness 248 2.24 2.77 3.32 2.77 

Wheeler Peak Wilderness 258 2.23 1.74 2.00 1.99 

Great Sand Dunes NM 269 1.64 1.59 1.75 1.66 

Maroon Bells-Snowmass WA 271 1.19 1.28 1.17 1.21 

Grand Canyon NP 285 2.19 1.65 1.20 1.68