UNITED STATES ENVIRONMENTAL PROTECTION AGENCY RESEARCH TRIANGLE PARK, NC 27711



### JUL 1 9 2006 OFFICE OF AIR QUALITY PLANNING AND STANDARDS 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 thatprovide 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 BARTrequirements. As you know, BART applies to existing sources ofa certaintype, age, and size that "emit any air pollutant which mayreasonably be anticipated to cause or contribute to any impairmentof visibility in any [Class I] area." CAA §169A(b)(2)(A). One approach discussed in the Guidelines for determining that a source does notmeet the threshold test for BART is to use the air quality model CALPUFF.

We understand that many States and Regional Planning Organizations (RPOs) are currently considering the use ofCALPUFF 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 ofvisibility-impairing pollutants. We have also received a question regarding the process for estimating natural background conditions, one ofthe factors used to estimate a source's impact on visibility. This memo addresses these two questions. Pollutant-Specific CALPUFF Analyses Because ofthe complexity and nonlinear nature ofatmospheric 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 thresholdtest for BART. ln

Internet Address (URL)• http://www.epa.gov Recycled/Recyclable• Printed with Vegetable Oii Based Inks on Recycled Paper (Minimum 25% Postconsumer)2 certain situations, however, it may be appropriate to dojustthat. For example, ifa State chooses to adopt the Clean Air Interstate Rule (CAIR) programto address emissions ofS02 andNO, from electric generating units (EGUs), the CAIRmay satisfythe requirements for BART for these pollutants from these sources. However, the State must determine whether its BARTeligible EGUs are subjectto review under BART for direct emissions ofparticulate matter (PM). Because the task ofpredicting the impacts ofPM onvisibility is a relatively straight-forward exercise, unlike predicting the impacts ofS02 and NOx, we would recommend the use of CALPUFF on a pollutant specific basis to model only the impact ofPM emissions on visibility. Using the results ofsuch an analysis, States may then determine whether a source shouldbe subjectto review for PMcontrols, or alternatively, thatthe source is not subjectto BART for PM. EstimatingNatural Visibilitv Conditions The BART Guidelines explainthat States should estimate a source's impact onvisibility 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 ofestimating natural visibility conditions, 

### see "Guidance for EstimatingNatural Visibility Conditions Underthe Regional Haze Rule," EPA-454/B-3-005 (September 2003), butneither the BART Guidelines nor the guidance described above specify whether for purposes ofdetermining whether a source is subjectto BART, States should use annual values in calculating natural backgroundvisibility estimates or some other averaging period. The preambleto the BART Guidelines, however, states that the BART Guidelines suggestthat States use a natural visibilitybaseline for the 20% best days for determining a source's impact on visibility.

We are clarifying here thatthe EPA did not intend to limit States to the use ofthe 20% best visibility days for this comparisonthrough the statement inthe preamble describing the BART Guidelines. States mayuse the 20% best visibility days or an annual average. TheBART Guidelines allowfor this flexibility, and we believe that either value would allowfor States to determine appropriately whether a source is reasonably anticipated to cause or contribute to any impairment in visibility.

I amrequesting that inyour role as sublead Region for PM andRegional Haze, you transmit this
memo to the other Regions. I would like to thank you in advance for your assistance.
Ifyou have any questions about either ofthese issues, please contact either Kathy Kaufman or Todd Hawes in my office.