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2.0 BART MODELING REQUIREMENTS 

The final version of EPA’s Regional Haze Regulations was published in the Federal Register on 

6 July 2005 (70 FR 39104). One of the provisions of the program is the requirement that certain existing 

stationary sources emitting visibility-impairing air pollutants install and operate the Best Available 

Retrofit Technology (BART). The regulations require case-by-case BART determination to define 

specific emissions limitations representing BART and schedules for compliance for each source ‘subject 

to BART’. These requirements would be part of the SIP revisions that each State must submit to EPA by 

17 December 2007. This chapter summarizes the requirements of EPA’s BART Rule, particularly with 

respect to the modeling analyses that are required to determine (a) if a source is ‘subject to BART’, and 

(b) what levels of emissions controls might be necessary for sources shown to cause or contribute to 

visibility impairment at federally-protected Class I areas. 

2.1 Regional Haze Rule and BART Guidelines 

 Section 169A of the Clean Air Act (CCA) sets forth a national goal for visibility which is the 

“prevention of any future, and the remedying of any existing, impairment of visibility in Class I areas 

which impairment results from manmade air pollution.” In 1999, EPA published a final rule to address 

a type of visibility impairment known as regional haze (64 FR 35714). The Regional Haze Rule (RHR) 

requires States to submit implementation plans (SIPs) to address regional haze visibility impairment in 

156 federally-protected parks and wilderness areas (i.e., the Class I scenic areas identified in the Clean 

Air Act). The 1999 rule was issued to fulfill a long-standing EPA commitment to address regional haze 

under the authority and requirements of sections 169A and 169B of the CAA. As required by the CAA, 

the final RHR included a requirement for Best Available Retrofit Technology (BART) for certain large 

stationary sources that were put in place between 1962 and 1977. The regional haze rule addresses 

visibility impairment resulting from emissions from a multitude of sources located across a wide 

geographic area. Because the problem of regional haze is believed to be caused in large measure by 

long-range transport of emissions from multiple sources, EPA adopted an approach that requires states 

to look at the contribution of all BART sources to the problem of regional haze in determining both 

applicability and the appropriate level of control. If a source potentially subject to BART is located in 

an area from which pollutants may be transported to a Class I area, that source ‘‘may reasonably be 

anticipated to cause or contribute’’ to visibility impairment in the Class I area. 

The BART guidelines were written primarily for the benefit of state, local and Tribal agencies, 

and describe a process for making the BART determinations and establishing the emission limitations 

that must be included in their SIPs or Tribal implementation plans (TIPs). Because the individual states 

have the authority to require source operators to assume part of the analytical burden in the BART 

analysis, there may be some differences in how the analyses are carried out across the CENRAP 

region. The BART guidelines also recognize that data collection, analysis, and rule development may 

be performed by Regional Planning Organizations, for adoption within each SIP or TIP. 

The BART guidelines provide a process for making BART determinations that states can use in 

implementing the regional haze requirements on a source-by-source basis. States must follow the 

guidelines in making BART determinations on a source-by-source basis for > 750 megawatt (MW) 

power plants but are not required to use the process in the guidelines when making BART 

determinations for other types of sources, i.e., states retain the discretion to adopt approaches that 

differ from the guidelines. 
 

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Sources are BART-eligible if they meet three criteria: (a) emissions of at least 250 tons per 

year of a visibility-impairing pollutant, (b) in operation between 7 August 1962 and 7 August 1977, 

and (c) one of 26 listed source categories in the guidance. BART controls are required for any BARTeligible source which can be reasonably expected to cause or contribute to impairment of visibility in 

any of the 156 federal parks and wilderness (Class I) areas protected under the regional haze rule. Air 

quality modeling is an important tool available to the States in determining whether a source can be 

reasonably expected to contribute to visibility impairment in a Class I area. 

In EPA’s recent 1 August 2005 proposed rulemaking (70 FR 44154) entitled “Revisions to 

Provisions Governing Alternative to Source-Specific Best Available Retrofit Technology 

Determinations” the agency addressed BART for electric generating units (EGUs). Theses newlyproposed guidelines establish certain control levels or emission rates as presumptive standards for 

facilities of greater than 200 MW capacity at plants with total generating capacity in excess of 750 

MW. These presumptive levels, developed through a formal rulemaking process, are to be applied on a 

source-specific basis. EPA believes that it appropriate to apply them in a trading context where the 

burden to meet BART-equivalent reductions may be shared among non-BART eligible sources as well. 

Thus, when states estimate emissions reductions achievable from source-by-source BART, they must 

assume that all EGUs which would otherwise be subject to BART will control at the presumptive 

level, unless they demonstrate such presumptions are not appropriate at particular units. 

2.2 Role of Air Quality Models 

The EPA guidelines present several options for assessing whether or not sources are subject to 

BART. The options, relying on different modeling and/or emissions analysis approaches, are provided 

as guidance and the states are entitled to use other reasonable approaches for analyzing the visibility 

impacts of an individual source or group of sources. The options are: 

Option 1: Individual Source Attribution. States can use dispersion modeling to determine 

that an individual source cannot reasonably be anticipated to cause or contribute to visibility 

impairment in a Class I area and thus is not subject to BART. Under this option, states can 

analyze an individual source’s impact on visibility as a result of its emissions of SO2, NOx and 

direct PM emissions. Because EPA’s recommended CALPUFF dispersion model cannot 

currently be used to reliably estimate the predicted impacts on visibility from an individual 

source’s emissions of VOC or ammonia, states may elect to use a more qualitative assessment 

to determine on a case-by-case basis which sources of VOC or ammonia emissions may be 

likely to impair visibility and should therefore be subject to BART review. EPA approved 

models should be used to predict the visibility impacts from a single source at a Class I area. 

Option 2: Use of ‘Model’ Plants. Under this option, analysis of model (or prototypical) 

plants could be used to exempt certain BART-eligible sources that share specific 

characteristics. EPA (2005) provides a model plant analysis assuming potential emissions as 

an example of how states might use this approach. The plume and stack characteristics of the 

model plants were developed considering a broad range of sources, and the analysis was based 

on impacts at two hypothetical Class I areas. States may develop their own model plant 

analyses to account for the meteorology and terrain in their region. Alternatively, a state could 

determine that the assumptions in EPA’s model plant analyses accurately reflect the 

characteristics of the sources and region at issue and no further analysis would be necessary. 

Because PM was not evaluated in EPA’s model plant analysis, states must determine whether 

PM emissions are significant enough to warrant further analysis. 
 

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It may be most useful to use this type of analysis to identify the types of small sources that do 

not cause or contribute to visibility impairment for purposes of BART, and thus should not be 

subject to a BART review. Different Class I areas may have different characteristics, however, 

so care must be taken to ensure that the criteria developed are appropriate for the applicable 

cases. States could use modeling analyses of representative plants to reflect groupings of 

specific sources with important common characteristics. Based on these analyses, states may 

find that certain types of sources are clearly anticipated to cause or contribute to visibility 

impairment. States can then choose to categorically require those types of sources to undergo a 

BART determination. 

Option 3: Cumulative Modeling. States may also submit to EPA a demonstration, based on 

an analysis of overall visibility impacts, that emissions from BART-eligible sources in a state, 

considered together, are not reasonably anticipated to cause or contribute to any visibility 

impairment in a Class I area and thus no source should be subject to BART. States may do this 

on a pollutant-by-pollutant basis or for all visibility-impairing pollutants to determine if 

emissions from these sources contribute to visibility impairment. 

While EPA identifies several options for determining whether a source is “subject to BART”, the most 

credible method is the use of dispersion modeling. Air quality modeling allows a state or source 

operator to analyze an individual source’s impact on visibility as a result of its gaseous SO2, NOx and 

direct PM emissions. 

EPA assumes in the BART guidance that dispersion modeling cannot currently be used to 

estimate the predicted impacts on visibility from an individual source’s emissions of VOC or ammonia 

owing to uncertainties in the ammonia inventories1. Presumably, EPA believes that the uncertainty in 

current biogenic and anthropogenic ammonia inventories are sufficiently large that dispersion 

modeling of such sources would not provide meaningful results. However, this view does not comport 

with EPA’s CAIR modeling or ongoing RPO regional haze visibility modeling in which existing 

ammonia inventories are indeed used to calculate daily and annual PM2.5 and visibility impact 

assessments. EPA suggests a more qualitative assessment to determine on a case-by-case basis which 

sources of VOC or ammonia emissions may be likely to impair visibility and should therefore be 

subject to BART review. CENRAP is considering the use of advanced one-atmosphere models 

(CAMx) to investigate whether VOC and ammonia emissions from BART eligible sources in the 

region constitute a problem. Findings from these CENRAP modeling studies are expected to provide 

information helpful to the States or source operators in deciding whether and to what extent ammonia 

and VOCs need to be considered in the BART modeling. 

2.3 EPA Guidance on Air Quality Models 

The BART determination under the Regional Haze Rule seeks to quantify the impact of source 

emissions of SO2, NOx, and direct PM (PM2.5 and/or PM10) on 24-hr visibility impairment at receptors 

located within downwind Class I areas. Since visibility is defined in the context of light extinction, 

which itself is determined by atmospheric concentrations of specific fine particulate species -- 

 

1

 We believe this viewpoint is outdated, give recent improvements in modeling data bases and one-atmosphere models by 

EPA ORD/OAQPS and others (Seigneur et al., 2000; Arnold et al., 2003; Morris et al., 2005a,b; Tesche et al, 2005). 
 

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ammonium, sulfate, nitrate, organic carbonaceous matter, elemental carbon, and coarse mass – logic 

dictates that the modeling method(s) used must be capable of simulating these components reliably. 

EPA’s position on recommended models for fine particulate and visibility estimation from 

single point sources is clearly set out in the Final BART Rule and in the BART Modeling guidance 

document. The Final BART Rule (pg 101) states “Because the air quality model CALPUFF is 

currently the best application available to predict the impact of a single source on visibility in a Class I 

area, we proposed that CALPUFF assessment be used as the preferred approach first, for determining 

whether an individual source is subject to BART, and second, in the BART determination process. 

CALPUFF can be used to estimate not only the effects of directly emitted PM2.5 emissions from a 

source, but also to predict the visibility impacts from the transport and chemical transformation of fine 

particle precursors.” The Rule goes on to state (pg 110) that “regional scale modeling typically 

involves use of a photochemical grid model that is capable of simulating aerosol chemistry, transport, 

and deposition of airborne pollutants, including particulate matter and ozone. Regional scale air quality 

models are generally applied for geographic scales ranging from a multi-state to the continental scale. 

Because of the design and intended applications of grid models, they may not be appropriate for BART 

assessments, so States should consult with the appropriate EPA Regional Office prior to carrying out 

any such modeling”. 

In contrast, EPA’s “Guidance for Demonstrating Attainment of the Air Quality Goals for PM2.5

and Regional Haze” (EPA, 2001) sets forth the types of models that should be used for simulating 

secondary fine particulate and visibility for SIPs. EPA states (pg 169): “States should use a regional 

scale photochemical grid model to estimate the effects if a control strategy on secondary components 

of PM. Changes in primary components may be estimated using a numerical grid model (with no 

chemistry), a Lagrangian model, or in some cases a receptor model”. Thus, in its Regional Haze and 

PM2.5 SIP modeling guidance, EPA indicates that CALPUFF (a Lagrangian non-steady-state Gaussian 

puff model) should not be used for secondary PM and visibility impacts at Class I areas, but rather is 

relegated to the category of estimating primary species. 

So, on the one hand, EPA maintains that CALPUFF is the “best regulatory modeling 

application currently available for predicting a single source’s contribution to visibility impairment” 

and notes that “it is the only EPA-approved model for use in estimating single source pollutant 

concentrations resulting from the long range transport of primary pollutants”. On the other hand, only 

regional grid models with appropriate chemistry are to be used in developing PM2.5 and Regional Haze 

SIPs. EPA attempted to reconcile these two positions in the Final BART Rule by asserting that (a) 

regional models were not developed to treat individual point sources and (b) CALPUFF’s secondary 

aerosol chemistry is adequate for estimating relative benefits of controls on BART sources. The first 

point ignores the significant amount of advanced plume-in-grid model development carried out at EPA 

and elsewhere over the past 20 years, while the second point is the focus of continuing debate in the 

scientific modeling community. Regardless, the BART modeling guidance stands and CALPUFF is 

the recommended model for BART determinations. EPA does allow for the use of alternative models 

on a case-by-case basis. Thus, this guidance addresses both approaches with CALPUFF being the 

primary tool for most BART visibility assessments. 

2.4 Steps in the BART Modeling Process 

The BART guidelines identify four steps required to determine emission limitations for 

affected sources. 
 

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Identify BART-Eligible Sources. The first step is to identify whether a source is “BARTeligible” based on its source category, when it was put in service, and the magnitude of its 

emissions of one or more “visibility-impairing” air pollutants. The BART guidelines list 26 

source categories of stationary sources that are BART-eligible. Sources must have been put in 

service between 7 August 1962 and 7 August 177 in order to be BART-eligible. Potential 

emissions of 250 tons per year or more of a visibility-impairing air pollutant are required to 

make a source eligible for BART. Qualifying pollutants include primary particulate matter 

(PM10) and gaseous precursors to secondary fine particulate matter such as SO2 and NOx. 

Whether ammonia and VOCs should be included as visibility-impairing pollutants for BART 

eligibility is left for the states to determine on a case-by-case basis. The guidance states that 

high molecular weight VOCs with 25 or more carbon atoms and low vapor pressure should be 

considered as primary PM2.5 emissions and not VOCs for BART purposes. 

Determine Sources Subject to BART. To determine whether a source or facility is ‘subject to 

BART’, EPA recommends modeling all units and all pollutants together. This entails using 

plant-wide emissions and considering the effect of all pollutants combined for comparison to 

the visibility threshold. EGUs participating in the CAIR program may be BART-eligible for 

PM if plant wide potential emissions of PM are greater than de minimus. If the EGU is BARTeligible for PM and if the source emissions result in impairment above the state-determined 

threshold, the source may be deemed subject to BART. 

To determine whether a BART-eligible source can be excluded from BART controls, a 

modeling demonstration is required. EPA’s preferred approach is an assessment with 

CALPUFF followed by comparison of the estimated 24-hr visibility impacts against a threshold 

above estimated natural conditions to be determined by the states. The threshold to determine 

whether a single source “causes” visibility impairment is set at 1.0 deciview change from 

natural conditions over a 24-hour averaging period. Any exceedance of this threshold would 

trigger a BART review. The guidance also states that the proposed threshold at which a source 

may “contribute” to visibility impairment should not be higher than 0.5 deciviews, although 

depending on factors affecting a specific Class I area, it may be set lower than 0.5 deciviews. 

EPA’s guidance builds upon the 1990 National Acid Precipitation Assessment Program 

(NAPAP) that found that a 5% change in light extinction will evoke a just noticeable change in 

most landscapes. Converting the 5% change in light extinction to a change in deciviews yields 

a change of approximately 0.5 deciviews. EPA believes that this is a natural breakpoint at 

which to set the BART exemption levels. Since visibility degradation may begin to be 

recognized by a human observer at this extinction level, the guidance uses a 0.5 deciview 

change on a 24-hour average basis for determining that an individual source is causing 

visibility impairment at a Class I area. This level would be calculated by comparing the air 

quality model’s results for an individual source against’ natural visibility’ conditions. A 

source’s impact is assessed by comparing the 98th percentile modeled value (8th highest day 

annually at a receptor or 22nd highest over 3-years) with the 0.5 deciview threshold. An impact 

is declared if the source contributes to visibility impairment. If so, it is ‘subject to BART’. 

If individual BART-eligible units at a facility have impacts that are less than 0.5 dv but in 

aggregate with other BART eligible sources at the facility are greater than 0.5 dv, the source is 

‘subject to BART’. In other words, if the combined impact of all units is greater than the 

threshold for contribution determined by the state (such as 0.5 dv), the source is declared 
 

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‘subject to BART’. This does not imply that controls are necessary for each unit; the control 

technology analysis, which follows the ‘subject to BART’ test, can be a unit-by-unit evaluation 

and a visibility analysis can be conducted for single units and individual pollutants. 

Determine Appropriate Types and Levels of Control. The third step is to determine BART 

for the source by considering various control options and selecting the “best” alternative, taking 

into consideration: (a) any pollution control equipment in use at the source which affects the 

availability of options and their impacts, (b) the costs of compliance with control options, (c) 

the remaining useful life of the facility, (d) the energy and non air-quality environmental 

impacts of compliance, and (e) the degree of improvement in visibility that may reasonably be 

anticipated to result from the use of such technology. 

Incorporate BART-Determinations into the State’s Regional Haze SIP. Results of the 

BART determinations must be incorporated into the state’s regional haze SIP which is due 17 

December 2007. Instead of applying BART on a source-by-source basis, a state (or a group of 

states) has the option of implementing an emissions trading program designed to achieve 

regional haze improvements that are greater than the visibility improvements that could be 

expected from BART2

 alone. If the geographic distributions of emissions under the two 

approaches are similar, determining whether trading is “better than BART” may be possible by 

simply comparing emissions expected under the trading program against the emissions that 

could be expected if BART was applied to eligible sources. If the geographic distributions of 

emissions are likely to be different, however, regional air quality modeling comparing the 

expected improvements in visibility from the trading program and from BART would be 

required. 

According to the BART guidance, a modeling protocol should be submitted for all modeling 

demonstrations regardless of the distance from the BART-eligible source to the Class I area. EPA’s 

role in the development of the protocol is only advisory as the “States better understand the BARTeligible source configurations” and factors affecting their particular Class I areas. 

2.5 CALPUFF Modeling Recommendations 

To evaluate the visibility impacts of a BART-eligible source at Class I areas beyond 50 km 

from the source, the EPA guidance recommends the use of the CALPUFF model. For modeling 

sources closer than 50 km to a Class I area, the guidance recommends that expert modeling judgment 

be used “giving consideration to both CALPUFF and other methods.” While the PLUVUE-II model is 

mentioned as a possible tool to consider in addition to CALPUFF within 50 km of a source, it seems 

unlikely this technique would be any more credible than CALPUFF. The EPA guidance notes that 

regional scale photochemical grid models may have merit, but such models are resource intensive 

relative to CALPUFF. Photochemical grid models are clearly more appropriate than CALPUFF for 

cumulative modeling options such as in the determination of the aggregate contribution of all-BARTeligible sources to visibility impairment, but such use should involve consultation with the appropriate 

EPA Regional Office. 

 

2

 EPA’s BART guidance suggests that CAIR would satisfy the BART requirements for affected EGUs, which 

could affect the modeling efforts a State would need to undertake. 
 

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For both BART exemption and BART determination modeling with CALPUFF, emissions 

reflecting periods of start-up, shutdown and malfunction are not to be considered in determining the 

appropriate emission rates. The EPA recommends that the states use the highest 24-hour average actual 

emission rate for the most recent five-year period (excluding periods with start-up, shutdown and 

malfunctions). Visibility improvements may be evaluated on a pollutant-specific basis. States are 

“encouraged to account for the magnitude, frequency, and duration of the contributions to visibility 

impairment caused by the source based on the natural variability of meteorology. 

EPA does not recommend CALPUFF for addressing visibility impacts from VOCs because its 

capability to simulate secondary organic aerosol formation from VOC emissions is deficient, 

especially for anthropogenic emissions. Currently, VISTAS is performing a weight-of-evidence 

analysis using EPA’s CMAQ regional air quality model to assess whether VOC emissions from all 

BART-eligible and non-BART point sources in each VISTAS State contribute to visibility impairment. 

If the impact of eliminating all VOC emissions from all point sources in a VISTAS state is shown to be 

less than 0.5 dv, then, logically, the impact of any one BART-eligible source would be less than 0.5 dv. 

If the VISTAS regional modeling analysis confirms this expectation, the CENRAP States might argue 

similarly that VOC emissions should not be subject to BART. 

EPA has given states the option to address ammonia (NH3) emissions from BART-eligible 

sources. VISTAS weight-of-evidence modeling with CMAQ is also addressing ammonia in the same 

way VOCs are being examined. If the CMAQ sensitivity evaluations shows that the collective impact 

of all point NH3 emissions in a state is less than 0.5 dv, then the impact of a single BART eligible 

source would be less than 0.5 dv. Should the VISTAS modeling confirm the expected insignificant 

role of direct NH3 emissions on Class 1 area visibility, the CENRAP States might argue similarly that 

VOC emissions should not be subject to BART. CENRAP is presently considering similar modeling 

for both ammonia and VOCs. If the VISTAS modeling demonstrates the importance of VOCs and/or 

NH3 emissions on Class I area visibility, then CENRAP will need to consider the use of advanced 

regional models to assist the States in establishing BART controls for these pollutants. 

2.6 Alternative Models for BART Analyses 

All air quality models share a common foundation: the species-conservation (or atmospheric 

diffusion) equation. Source-oriented air quality models including CALPUFF derive from this equation 

(Tesche, 1983) which applies equally to one source or a million or more sources. The distinction lies 

in how the various terms are treated (CAMx, CMAQ) or neglected (CALPUFF) in the governing 

equations and the choice of coordinate system (Lagrangian or Eulerian). As shown in the next 

Chapter, much of the simplicity of the CALPUFF model derives from the fact that many chemical and 

physical processes known to influence visibility are simply ignored. More sophisticated plume models 

such as SCIPUFF and SCICHEM (Karamchandani et al., 2000) rely on a second order closure 

treatment of turbulent mixing and detailed photochemical reaction schemes and to provide more 

realistic simulations than simpler approaches such as CALPUFF. Full-science regional models such as 

CAMx and CMAQ treat the various physical and chemical processes in detail, albeit at the expense of 

greater computer resources. Furthermore, detailed plume-in-grid treatment is available in several 

regional scale models, at least in CAMx, it is now possible to perform particulate and gas-phase source 

apportionment within the plume-in-grid formulation. Because EPA allows the use of alternative 

models on a case-by-case basis, these CENRAP guidelines include descriptions of these models and 

how they can be used. 
 

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EPA essentially dismisses the use of regional scale models because they are “generally applied 

for geographic scales ranging from a multi-state to the continental scale and that by design they may 

not be appropriate for simulating individual point sources”. This posture ignores a very substantial 

body of research and model development carried out at the agency and elsewhere in the U.S. over the 

past 20 years. Modern one-atmospheric regional photochemical grid models, employing multi-scale 

nested grids (Kumar and Russell, 1996) and plume-in-grid techniques (Karamanchandani et al., 2002), 

are fully applicable to the analysis of point source plumes, most especially when reactive atmospheric 

chemistry occurs. If they were not, then they would not be reliable in simulating the combined effects 

of the wide array of anthropogenic and biogenic emissions that cause gas phase, particulate, secondary 

aerosol, and visibility air pollution problems—precisely the problem currently being addressed by all 

five RPOs. Furthermore, the convergence of fast commodity-based Linux computer clusters and the 

recently-developed RPO regional modeling emissions, meteorological, and air quality data bases make 

application of these comprehensive models no longer a research or academic exercise. While regional 

scale modeling clearly requires expertise to perform properly, the actual program costs to conduct a 

CMAQ or CAMx regional modeling study are today quite comparable with and occasionally less that a 

traditional PSD modeling study using ISC, CALPUFF, or AERMOD. Given grid nesting and plumein-grid technology, modern regional models are applicable to a very broad range of scales from 10-20 

km to continental scale. Regional photochemical grid models are clearly more appropriate than 

CALPUFF for cumulative modeling requirements such as in the determination of the aggregate 

contribution of all-BART-eligible sources to visibility impairment. 

States may elect to perform cumulative modeling in order to examine the feasibility of 

exempting BART-eligible sources, but this is certainly not an EPA or BART Rule requirement. The 

regional haze rule requires States to consider the degree of visibility improvement resulting from a 

source's installation and operation of retrofit technology, along with the other statutory factors when 

making a BART determination. However, there is no longer a cumulative modeling requirement in 

determining the degree of visibility improvement. 

For the vast majority of potential BART-eligible sources in the CENRAP domain, application 

of the CALPUFF modeling system will in all likelihood be sufficient to address the needs of the source 

operator, the state, EPA and the FLM. For those special cases where more accurate modeling results 

are required, appropriate full-science modeling systems and supporting RPO data bases are available. 

Their broad use by the five RPOs in the past three years amply demonstrate that these comprehensive 

models are no longer arcane academic/research institution projects but applications-ready tools 

suitable for regulatory decision-making.