### WRAP BART CAL PUFF 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 th e implementation of Best Available Retrofit Technology (BART) on specific stationary emission sources that are reasonably anticipated to contribute to visibility impair ment 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 impa irment. The Western Regional Ai r Partnership (WRAP) Regional Modeling Center (RMC) has been performi ng subject-to-BART CALPUFF modeling of potential BART-eligible s ources 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 re visions 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 re garding how PM emissions were input into CALPUFF. Although not well documented, CALPU FF only reads 132 characters off its input file and since PM emissions were specified as the last species beyond 132 characters, they may2 have not been input correctly. Thus, the CALPUF F 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 we re made to the modeling:

• The maximum allowable mixing height wa s 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 a nd that a 4,500 m AGL is more appropriate for the west (CDPHE, 2005).

• The CALPUFF output was processed with th e POSTUTIL processor prior to obtaining the visibility impacts using the CALPOST processor to avoid the “double counting” of ammonia. Previously, the CALPOST proce ssor was applied directly to the CALPUFF output.

The post-April 2007 revised CALPU FF BART modeling results were mostly very similar to the pre-April 2007 CALPUFF BART modeling and all potential BA RT-eligible sources whose visibility impacts at a Class I area exceeded th e 0.5 change in deciview (dv) significance threshold 1 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 emis sions were expected to be small. However, some of the changes in visibility impacts in th e revised simulations were larger than expected.

The largest change in visibility impacts be tween 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 98 th percentile vi sibility impact of 8.2 dv was estimated. The post-April 2007 CALPUFF estimated visibility impact w ith the three changes (corrected PM emission inputs, 4,500 m AGL maximum mixing he ight and processing with POSTUTIL before CALPOST) obtaine d a maximum three-year average 98 th percentile visibility impacts of 5.4 dv (Table 2), whic h is substantially lower than be fore (8.2 dv). To determine the cause for the large changes in the pre- a nd post-April CALPUFF analysis the post-April CALPUFF results were reprocessed using CA LPOST without first pre-processing with POSTUTIL obtaining results given in Table 3. Ta ble 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 allo wable mixing height from 3,000 to 4,500 m AGL and estimates a maximum three-year average 98 th 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 du e to the use of the POSTUTIL processor on the CALPUFF output prior to cal culating 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 opin ion on whether a potential BART-eligible source is subject-to-BART or not, which is a State decision.3 Treatment of Backgro und Ammonia in CALPUFF

The CALPUFF modeling system has a very simplistic treatment of background ammonia concentrations. Despite the fact that atmospheric ammonia con centrations are known to have a high temporal and spatial variability, the CALPUFF modeling system assumes a single background ammonia concentration that is spat ially 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 th e other puffs to partition the tota l nitrate formed into particle NO3 and gaseous HNO3 using the user-specifi c background ammonia and estimated SO4 for that puff and an SO4/NO3NH4 equilibrium equati on. The total concentration impact at a given receptor (e.g., Class I area) is obtained by summi ng 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 recalcu lates 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 a mmonia 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 real ity, 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 M odeling (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 a nd forested lands, consequently a 1.0 ppb background ammonia value was select ed 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 measuremen ts in eastern Colorado and for their BART modeling recommended background ammonia valu es 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 base d on IWAQM (1998) recommendati ons, is highly uncertain. The pre-April/post-April POSTUTIL analysis sugg ests 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/ba rt/WRAP_RMC_BART_Protocol_Aug15_2006.pdf4 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 Minimum Distance 98th Percentile for Each Year 98th

Class I Area (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 1.625 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 Minimum Distance 98th Percentile for Each Year 98th

Class I Area (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 1.606 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 Minimum Distance 98th Percentile for Each Year 98th

Class I Area (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