UNITED STATES ENVIRONMENTAL PROTECTION AGENCY 

RESEARCH TRIANGLE PARK, NC 27711 

JUL 1 9 2006 

OFFICE OF 

AIR QUALITY PLANNING 

AND ST AND ARDS 

MEMORANDUM 

SUBJECT: Regional Haze Regulations and Guidelines for Best Available Retrofit 

Technology (BART) Determinations 

FROM: Joseph W. Paisie, Group Lea~ w~ Geographic Strategies Group CMC~) ~ 

TO: Kay Prince, Branch Chief 

EPA, Region 4 

In July 2005, EPA issued BART Guidelines that provide guidance to the States in making BART 

determinations for large power plants and other BART sources. In the BART Guidelines, we 

described several approaches that States could use to determine whether a source should be 

subject to review for BART, or whether it should be exempt from the BART requirements. As 

you know, BART applies to existing sources of a certain type, age, and size that "emit any air 

pollutant which may reasonably be anticipated to cause or contribute to any impairment of 

visibility in any [Class I] area." CAA §169A(b)(2)(A). One approach discussed in the 

Guidelines for determining that a source does not meet the threshold test for BART is to use the 

air quality model CALPUFF. 

We understand that many States and Regional Planning Organizations (RP Os) are currently 

considering the use of CALPUFF for making BART determinations. We have received a 

question asking whether States can, or should, allow sources to use CALPUFF to estimate 

visibility impacts on a pollutant specific basis, or whether EPA intended CALPUFF to be used to 

model a source's visibility impacts based on its total emissions of visibility-impairing pollutants. 

We have also received a question regarding the process for estimating natural background 

conditions, one of the factors used to estimate a source's impact on visibility. This memo 

addresses these two questions. 

Pollutant-Specific CALPUFF Analyses 

Because of the complexity and nonlinear nature of atmospheric chemistry and chemical 

transformation among pollutants, EPA does not generally recommend that CALPUFF be used on 

a pollutant specific basis to determine whether a source meets the threshold test for BART. ln 

Internet Address (URL)• http://www.epa.gov 

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certain situations, however, it may be appropriate to do just that. For example, if a State chooses 

to adopt the Clean Air Interstate Rule (CAIR) program to address emissions of S02 and NO, 

from electric generating units (EGU s ), the CAIR may satisfy the requirements for BART for 

these pollutants from these sources. However, the State must determine whether its BARTeligible EGUs are subject to review under BART for direct emissions of particulate matter (PM). 

Because the task of predicting the impacts of PM on visibility is a relatively straight-forward 

exercise, unlike predicting the impacts of S02 and NOx, we would recommend the use of 

CALPUFF on a pollutant specific basis to model only the impact of PM emissions on visibility. 

Using the results of such an analysis, States may then determine whether a source should be 

subject to review for PM controls, or alternatively, that the source is not subject to BART for 

PM. 

Estimating Natural Visibilitv Conditions 

The BART Guidelines explain that States should estimate a source's impact on visibility by 

"calculat[ing] daily visibility values for each receptor as the change in deciviews compared 

against natural visibility conditions." 70 Fed. Reg. 39104, 39162 (July 6, 2005). EPA has 

provided guidance to the States specifically for the complex task of estimating natural visibility 

conditions, see "Guidance for Estimating Natural Visibility Conditions Under the Regional Haze 

Rule," EPA-454/B-3-005 (September 2003), but neither the BART Guidelines nor the guidance 

described above specify whether for purposes of determining whether a source is subject to 

BART, States should use annual values in calculating natural background visibility estimates or 

some other averaging period. The preamble to the BART Guidelines, however, states that the 

BART Guidelines suggest that States use a natural visibility baseline for the 20% best days for 

determining a source's impact on visibility. 

We are clarifying here that the EPA did not intend to limit States to the use of the 20% best 

visibility days for this comparison through the statement in the preamble describing the BART 

Guidelines. States may use the 20% best visibility days or an annual average. The BART 

Guidelines allow for this flexibility, and we believe that either value would allow for States to 

determine appropriately whether a source is reasonably anticipated to cause or contribute to any 

impairment in visibility. 

I am requesting that in your role as sublead Region for PM and Regional Haze, you transmit this 

memo to the other Regions. I would like to thank you in advance for your assistance. 

If you have any questions about either of these issues, please contact either Kathy Kaufman or 

Todd Hawes in my office.