Appendix L.4

BART Sensitivity Modeling Analysis 

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Appendix L.4 - 1

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1 

DRAFT 

BART in the VISTAS Region: 

Sensitivity to VOC, NH3 and Primary PM 

Emissions

29 April 2007 

VISTAS 

2090 U.S. 70 Highway 

Swannanoa, NC 28778 

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Appendix L.4 - 2

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Preface 

This report summarizes the results of efforts by several organizations and individuals. 

The VOC and NH3 sensitivity simulations with the CMAQ regional chemical transport 

model were carried out by the Georgia Institute of Technology. Calculations of the 

resulting impacts on regional haze (i.e., ∆dv, delta-deciviews) were performed by Dr. 

James Boylan of the State of Georgia Department of Natural Resources. 

The PM sensitivity simulations with the CMAQ regional chemical transport model were 

carried out by Alpine Geophysics. Calculations of the resulting impacts on regional haze 

were performed at ENVIRON Corporation. 

Dr. Ivar Tombach, VISTAS’ Technical Advisor, prepared this report from information 

provided by the above organizations and individuals. 

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1. Introduction 

The U.S. Environmental Protection Agency (US EPA) has established a program for 

reducing atmospheric haze in mandatory Class I Federal Areas through the application of 

best available retrofit technology (BART) emission controls to certain large sources.1

 

States are to ascertain the appropriate form of BART, if any, for each of these sources. 

The BART program addresses emissions of SO2, NOx, primary particulate matter (PM), 

and, at the option of the State, emissions of volatile organic compounds (VOCs) and 

ammonia (NH3). PM is inert, while SO2, NOx, NH3 and VOCs are precursors to 

secondary particulate matter in the forms of ammoniated sulfate (“sulfates”), ammonium 

nitrate (“nitrates”), and particulate organic matter (POM). The POM consists of organic 

carbon (OC) and of other species (such as hydrogen, oxygen, and halocarbons) attached 

to the OC. Sulfates, nitrates, POM, and PM in the atmosphere, as well as NO2 gas, all 

contribute to visibility impairment and haze.2

The US EPA has established that the controls on the emissions of SO2 and NOx that will 

be implemented under the Clear Air Interstate Rule (CAIR) will constitute BART for 

major electric generating units (EGUs),3 so that the State programs need only consider 

emissions of PM, ammonia, and VOCs from EGU sources. Furthermore, the US EPA 

guidance for implementing the BART program provides that States have the discretion of 

deciding whether or not ammonia and VOC emissions are to be considered in their 

BART programs, based on their evaluations of the contributions of the emissions of these 

species to haze at Class I areas in their areas of influence.4 The guidance also provides 

that one approach a State can use to ascertain whether applying BART will be needed is 

to evaluate the cumulative haze impacts of current emissions from all BART-eligible 

sources in the State. If their cumulative impact is less than the “contribution” threshold 

established by the State (0.5 deciviews for VISTAS states), then source-by-source 

analysis for BART will not be needed.5

In light of these drivers, VISTAS has undertaken sensitivity analyses with the CMAQ 

regional chemical transport model to ascertain the aggregate contributions of all 

VISTAS-region BART sources of VOCs, ammonia, and PM to haze in Class I areas. In 

this analysis, CMAQ simulations of the VISTAS region and surrounding states were 

carried out with all point sources included and then with selected emissions turned off. 

Comparisons of the haze impacts under the various emission scenarios indicated the 

contributions of the specific species emissions. 

 

1

 Federal Register, Vol. 70, No. 128, pp 39104-39172. 6 July 2005. 

2

 Although sulfates, nitrates, and POM are in the form of secondary particulate matter, in this 

report we will use PM to denote only primary particulate matter. 

3

 70FR39138 

4

 40CFR51, Appendix Y, Section II.A, Step 3 (or 70FR39160) 

5

 40CFR51, Appendix Y, Section III.A.3, Option 3 (or 70FR39163) 

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Appendix L.4 - 4

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The following emissions reduction scenarios were modeled with CMAQ and analyzed: 

• VOCs 

o 100% reduction in VOC emissions from all point sources (not just BARTeligible sources) in the VISTAS States 

• NH3

o 100 % reduction in ammonia emissions from all BART-eligible point 

sources in the VISTAS States 

• PM 

o 100% reduction in primary PM emissions from all EGU point sources in 

the VISTAS States 

o 100% reduction in primary PM emissions from all non-EGU point sources 

in the VISTAS States. 

In all cases, the CMAQ model, including secondary organic aerosol (SOA) chemistry 

modifications developed for VISTAS,6

 was applied to the VISTAS 12-km grid CMAQ 

modeling domain, which is shown in Figure 1-1. Note that the domain extends at least 

one state outside the VISTAS region, in order to include the effects of VISTAS emissions 

on Class I areas in other RPO regions. 

In order to keep the computational requirements manageable, simulations were not done 

for a full year. Rather, two periods were analyzed – a winter period and a summer period. 

The exact dates of these periods differed for the three analyses. 

Details of the analyses and their results appear in the following chapters, with VOCs 

addressed in Chapter 2, ammonia in Chapter 3, and PM in Chapter 4. The conclusions 

derived from this work are summarized in Chapter 5. 

 

6

 The SOA chemistry modifications are described in “Model sensitivity evaluation for organic 

carbon using two multi-pollutant air quality models that simulate regional haze in the 

southeastern United States” by Ralph E Morris, Bonyoung Koo, Alex Guenther, Greg Yarwood, 

Dennis McNally, T.W. Tesche, Gail Tonnesen, James Boylan, and Patricia Brewer. Atmospheric 

Environment, Volume 40, August 2006, pages 4960-4972. 

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Figure 1-1. The VISTAS 12-km CMAQ modeling domain (gridded area). 

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2. VOC Analyses 

The VOC sensitivity simulations were carried out using version 4.4 of the CMAQ model, 

including the VISTAS secondary organic aerosol (SOA) chemistry modifications. Two 

periods were modeled – a “summer” period of 1 June to 10 July 2002 and a “winter” 

period of 19 November to 19 December 2002. 

The emissions starting point for the simulations was the VISTAS 2009 OTW BaseD 

emissions inventory.7

 The contribution of point source VOC emissions at each Class I 

area in the VISTAS region and in adjacent States was then ascertained by turning off all

point source VOC emissions in the VISTAS states and comparing the CMAQ-modeled 

result with the initial simulation with all emissions. 

The results of this sensitivity analysis are shown in Figure 2-1, which displays the largest 

haze impact in deciviews, ∆dv, found over the two simulation periods at each Class I 

area. The ∆dv changes shown represent the contributions of all VOC emissions relative to 

assumed natural haze levels based on EPA-default annual average natural concentrations 

for the East and monthly-varying “climatologically representative” relative humidity at 

each Class I area. All of the impacts are below the BART contribution threshold of 0.5 

dv, although the contribution of all VOC point sources to Swanquarter (SWAN) haze is 

nearly 0.5 dv. 

Figure 2-1 overstates the impacts of VOC emissions from BART-eligible sources alone, 

however. Because BART-eligible sources typically account for only a small fraction of 

the total point source VOC emissions in each State, as shown in Table 2-1, the impacts of 

BART-eligible sources alone will be less that those shown in Figure 2-1. For example, 

averaged over the entire VISTAS region, only 9% of the point source VOC emissions are 

from BART-eligible sources. Consequently, to estimate the contributions to haze from 

BART eligible sources alone, the impacts on light extinction (bext) due to all VOC point 

sources in each VISTAS State were scaled by the factors listed in Table 2-1 and the 

results were again expressed as ∆dv. 

 

 

7

 OTW = On the way, i.e., including controls on the books in 2002, plus CAIR. (CAIR is now 

also on the books.) 

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Appendix L.4 - 7

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Maximum VOC Impact

0.0

0.1

0.2

0.3

0.4

0.5

0.6

BRET

BRIG

CACRCHAS

COHU

DOSO

EVER

GRSM

HEGL

JARI

LIGO

MACA

MING

OKEF

ROMA

SAMA

SHEN

SHRO

SIPS

SWAN

UPBU

WOLF

ΔdV

Figure 2-1. Maximum contributions of all VISTAS-region VOC point sources to 

haze in Class I areas during the CMAQ-modeled periods. 

Table 2-1. Fractions of point-source VOC emissions in VISTAS States that are due 

to BART-eligible sources. 

State Fraction of point-source 

VOC emissions due to 

BART-eligible sources

AL 0.13 

FL 0.11 

GA 0.14 

KY 0.08 

MS 0.14 

NC 0. 06 

SC 0.09 

TN 0.05 

VA 0.05 

WV 0.23 

VISTAS (total) 0.09 

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Two approaches, which bracket the range of possible conditions, were used for this 

scaling. In the first approach, the assumption was made that most of the POM at any 

Class I area originates from VOC emissions within the State within which the Class I 

area is located. In that case, the modeled impact on light extinction at that Class I area 

was multiplied by the factor in Table 2-1 for that State, and the result was converted to 

deciviews to produce an estimate of the impact of BART-eligible VOC point sources to 

haze in that Class I area. 

The result of such calculations is shown in Figure 2-2 (which has the same scale as 

Figure 2-1). The figure shows that none of the Class I areas is impacted by more than 

0.05 dv because of VOCs from BART-eligible sources, so all impacts are less than a 

tenth of US EPA’s recommended BART contribution threshold of 0.5 dv. 

 

Maximum VOC Impact

0.0

0.1

0.2

0.3

0.4

0.5

0.6

BRET

BRIG

CACRCHAS

COHU

DOSO

EVER

GRSM

HEGL

JARI

LIGO

MACA

MING

OKEF

ROMA

SAMA

SHEN

SHRO

SIPS

SWAN

UPBU

WOLF

ΔdV

Figure 2-2. Estimated maximum contributions of BART-eligible VISTAS-region 

VOC point sources to haze in Class I areas during the modeled periods, based on 

state-by-state scaling of CMAQ simulated impacts from all VOC point sources. 

For the second approach, the assumption was made that the POM at any Class I area 

originates from VOC emissions from many States, and therefore the ratio of BARTeligible VOC point source extinction impacts to all VOC point source extinction impacts 

is best represented by the VISTAS-average ratio of 0.09 in Table 2-1. In this case, 

therefore, the CMAQ-modeled impact on light extinction at every Class I area was 

multiplied by the 0.09 factor and the result was converted to deciviews. 

The result of such calculations is shown in Figure 2-3. Although there are some small 

differences between Figures 2-2 and 2-3, we see again that none of the Class I areas is 

impacted by more than 0.05 dv, a tenth of US EPA’s BART contribution threshold. 

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In reality, the contributions to haze at a Class I area are likely to lie between the results 

developed under the two approaches. Since both approaches yield estimates of 

cumulative impacts from BART-eligible VOC sources that are less than 0.05 dv, and 

therefore are substantially less than the 0.5 dv threshold, we conclude from this analysis 

that VOC emissions from individual BART-eligible sources will have minimal impact on 

visibility I Class I areas. VISTAS has therefore decided that effects of the VOC 

emissions from BART-eligible sources do not need to be modeled. 

Maximum VOC Impact

0.0

0.1

0.2

0.3

0.4

0.5

0.6

BRET

BRIG

CACRCHAS

COHU

DOSO

EVER

GRSM

HEGL

JARI

LIGO

MACA

MING

OKEF

ROMA

SAMA

SHEN

SHRO

SIPS

SWAN

UPBU

WOLF

ΔdV

Figure 2-3. Estimated maximum contributions of BART-eligible VISTAS-region 

VOC point sources to haze in Class I areas during the modeled periods, based on 

VISTAS region-average scaling of CMAQ simulated impacts from all VOC point 

sources. 

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3. Ammonia Analyses 

The NH3 sensitivity simulations were carried out using version 4.5 of the CMAQ model, 

including VISTAS secondary organic aerosol (SOA) chemistry modifications developed. 

Two periods were modeled – a “summer” period of 1 June to 10 July 2002 and a “winter” 

period of 19 November to 19 December 2002. 

The emissions starting point for the simulations was the VISTAS 2009 OTW BaseF4 

emissions inventory. The sensitivity of regional haze at each Class I area in the VISTAS 

region and in adjacent States to NH3 emissions was then ascertained by turning off NH3

emissions from all BART-eligible sources in the VISTAS states and comparing the 

CMAQ-modeled result with the initial simulation with all emissions. 

The results of this sensitivity analysis are shown in Figure 3-1, which displays the largest 

haze impact in deciviews, ∆dv, found over the two simulation periods at each Class I 

area. The ∆dv changes shown represent the haze contributions of all BART source NH3

emissions relative to assumed natural haze levels based on EPA-default annual average 

natural concentrations for the East and monthly-varying “climatologically representative” 

relative humidity at each Class I area. 

Maximum NH3 Impact

0.0

0.5

1.0

1.5

BRET

BRIG

CACRCHAS

COHU

DOSO

EVER

GRSM

HEGL

JARI

LIGO

MACA

MING

OKEF

ROMA

SAMA

SHEN

SHRO

SIPS

SWAN

UPBU

WOLF

ΔdV

Figure 3-1. Maximum contributions of all BART-eligible NH3 point sources in the 

VISTAS region to haze in Class I areas during the CMAQ-modeled periods. 

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Figure 3-1 shows that, during the two simulation periods, the NH3 emissions all BARTeligible sources contributed more than 0.5 dv to haziness at four Class I areas – 

Mammoth Cave National Park (MACA), Kentucky; Mingo Wilderness (MING), 

Missouri; Cape Romain Wilderness (ROMA), South Carolina, and Swanquarter 

Wilderness (SWAN), North Carolina. The largest impact under 0.5 dv was at Okefenokee 

Wilderness(OKEF), Georgia. All other modeled Class I areas had impacts well under 0.5 

dv. 

The majority of the NH3 emissions in the VISTAS region are from four large BARTeligible sources, as tabulated in Table 3-1. 

Table 3-1. Major BART NH3 Sources in the VISTAS Region 

Facility State NH3 Emissions 

(tpy) 

Distance to Nearest 

Class 1 Area (km) 

Meadwestvaco Corp. SC 174 30 

PCS Phosphate NC 206 31 

PCS Nitrogen TN 1252 180 

PCS Nitrogen GA 1765 220 

Analysis of the modeled spatial and temporal distributions of ammonia concentrations led 

to these four large sources as the probable principal contributors to the Class I area 

impacts described above. The principal NH3 contributor to Cape Romain haze appears to 

be the Meadwestvaco Corp. plant in South Carolina, located 30 km from that Class I area. 

The principal NH3 contributor to Swanquarter haze is the PCS Phosphate plant in North 

Carolina, which is located 31 km from that Class I area. 

At the other Class I areas the connection is not quite as clear because the major NH3

sources are more distant – roughly 200 km from the nearest Class I area -- but their 

emissions are some 6 to 9 times greater than those of the two plants mentioned above. It 

appears likely that the modeled haze impact at both Mammoth Cave and Mingo derives 

from NH3 emissions from the PCS Nitrogen plant in Tennessee, and the impact at 

Okefenokee is attributable to the NH3 emissions from the PCS Nitrogen plant in Georgia. 

The relevant States have the option of asking the few sources mentioned above to 

evaluate their NH3 impacts, in order to establish whether they should carry out a full 

BART analysis that covers conditions throughout the year. VISTAS has concluded that 

there is no need for any other sources to model their NH3 impacts. 

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4. PM Analyses 

The sensitivity simulations for primary PM were carried out by Alpine Geophysics using 

version 4.5 of the CMAQ model, again including the VISTAS secondary organic aerosol 

(SOA) chemistry modifications. All components of primary PM – primary sulfates, 

primary nitrates, primary OC, EC, soils, and coarse matter – were simulated. 

Two periods were modeled: the first quarter of 2002 (1 January – 31 March 2002), to 

represent winter conditions, and the 3rd quarter of 2002 (1 July-30 September 2002), to 

represent summer conditions. 

The emissions starting point for the PM simulations was the VISTAS 2002 BaseF4 

emissions inventory. The contributions of all EGU PM emissions and of all non-EGU 

emissions to haze at each Class I area in the VISTAS region and in adjacent States were 

then ascertained separately by turning off all EGU point sources and all non-EGU point 

sources, respectively, and comparing the CMAQ-modeled impact results with those of 

the initial simulation with all emissions. 

Results for the winter and summer modeling periods were evaluated separately. Figure 4-

1 shows the results for the winter period. Both the highest ∆dv and the 3rd highest ∆dv 

(approximating the 98th percentile) impacts are plotted for both EGU and non-EGU point 

sources. The reference against which impacts were calculated is the light extinction 

associated with the default annual-average natural background concentration. Figure 4-2 

is the corresponding plot for the summer period. 

Both figures show many impacts of PM emissions from the aggregates of all EGU 

sources and all non-EGU sources that exceed the 0.5-dv contribution threshold. The EGU 

impact is smaller than the non-EGU impact, and the impacts in winter tend to be greater 

than those in summer. 

Since not all of the sources modeled are BART-eligible, the impact of BART-eligible 

sources alone will be less that that displayed in Figures 4-1 and 4-2. However, most of 

the worst case impacts in those figures exceed the 0.5 dv contribution threshold by wide 

margins, and so it is likely that impacts of BART eligible sources will also often exceed 

that threshold. 

Consequently, VISTAS has decided that all BART-eligible sources should model PM 

emissions. 

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Appendix L.4 - 13

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13 

BART analysis with average natural conditions

First Quarter 2002ft4a

0.0

1.0

2.0

3.0

4.0

5.0

6.0

7.0

BRET1

BRIG1

CACR1

CHAS1

COHU1DOSO1

EVER1

GRSM1

HEGL1

JARI1

LIGO1

MACA1

MING1

OKEF1

ROMA1

SAMA1

SHEN1

SHRO1

SIPS1

SWAN1

UPBU1

delta dv

EGUs - max

EGUs - 3rd high

nonEGUs - max

nonEGUs - 3rd high

Figure 4-1. Maximum and 98th percentile contributions of all VISTAS-region EGU 

and non-EGU PM point sources to haze in Class I areas during the winter, based on 

CMAQ simulations. The solid line marks the 0.5 dv contribution threshold. 

BART analysis with average natural conditions

Third Quarter 2002ft4a

0.0

1.0

2.0

3.0

4.0

5.0

6.0

7.0

BRET1

BRIG1

CACR1

CHAS1

COHU1DOSO1

EVER1

GRSM1

HEGL1

JARI1

LIGO1

MACA1

MING1

OKEF1

ROMA1

SAMA1

SHEN1

SHRO1

SIPS1

SWAN1

UPBU1

delta dv

EGUs - max

EGUs - 3rd high

nonEGUs - max

nonEGUs - 3rd high

Figure 4-2. Maximum and 98th percentile contributions of all VISTAS-region EGU 

and non-EGU PM point sources to haze in Class I areas during the summer, based 

on CMAQ simulations. The solid line marks the 0.5 dv contribution threshold. 

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Appendix L.4 - 14

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14 

5. Conclusions 

The analyses described in the preceding chapters lead to different conclusions concerning 

the necessity of addressing VOC, ammonia, and PM in BART analyses. 

VOCs. The cumulative haze impacts from VOC emissions from all BART-eligible 

sources in the VISTAS region were found to be so small that there is no need to evaluate 

the VOC impacts of individual sources in BART analyses. 

Ammonia. Haze impacts from NH3 emissions from BART-eligible sources were greater 

than 0.5 dv at four Class I areas and approached 0.5 dv at another Class I area. Because 

there are relatively few large NH3 sources, these impacts likely derive from those 

sources. (The sources are listed in Table 3-1.) Consequently, modeling of the impacts of 

NH3 emissions from these sources throughout the year is appropriate, at the discretion of 

the State. Modeling of impacts by the remaining NH3 sources is not needed. 

PM. The cumulative haze impacts from primary PM emissions from all EGU sources and 

from all non-EGU sources were both found to often exceed the 0.5 dv contribution 

threshold by wide margins. Although the total of PM emissions from BART-eligible 

sources alone will be somewhat less, the evaluation of individual source PM impacts is 

warranted for BART analyses. 

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