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6.32
UNITED STATES ENVIRONMENTAL PROTECTION AGENCY
Office of Air Quality Planning and Standards
Research Triangle Park, North Carolina 27711
MEMORANDUM
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DATE: March 31, 1989
SUBJECT: Applicability of Building Downwash in Prevention of
Significant Deterioration (PSD) Permit Analyses
FROM: John Calcagni, Director
Air Quality Management Division (MD-15)
TO: William B. Hathaway, Director
Air, Pesticides, and Toxics Division (6T)
Region VI
Thank you for your memorandum of March 8, 1989 in which you urge
consideration of changes to EPA's current policy of applying building
downwash to background sources in PSD modeling. Your memorandum describes
problems associated with the collection of building dimension data
necessary for downwash modeling, and you suggest that EPA might issue rules
and provide funding to collect this building data. Alternatively, you
believe that downwash modeling should not be required for any background
sources.
Members of my staff are currently analyzing several approaches for
handling background sources. This will be the subject of a future
conference call with the Regional Offices. In the interim, some of our
concerns regarding this issue and your specific suggestions are discussed
below.
The Guideline on Air Quality Models notes that background
concentrations are an essential part of the total air quality concentration
to be considered in determining source impacts and therefore requires
certain background sources to be fully modeled. The Guideline indicates
that ". . . all sources expected to cause a significant concentration
gradient in the vicinity of the source or sources under consideration for
emission limit(s) should be explicitly modeled." This guidance provides
considerable flexibility and requires judgment to be exercised by the
reviewing agency in identifying which background sources should be fully
modeled. The burden of collecting building dimension data may be mitigated
somewhat by application of this judgment. We are exploring the development
of additional guidance to better assist in this judgment. However, I
caution that it may not be possible to establish many objective "bright
line" tests that will eliminate the need for Regional Office judgment in
individual cases.
2
I realize that information needed to model background sources is
frequently not contained in the State's existing emission inventory. In
some cases the applicant will need the reviewing agency to assist in
collecting the data. However, I am not convinced that we must undertake a
national effort to issue regulations or to fund the States/Regional Offices
to collect the data. It is important to note that the PSD rules place this
burden primarily on the proposed source, not the regulatory agencies.
Your memorandum suggests that the PSD analyses could ignore building
downwash effects. I do not believe that the PSD rules and the Guideline
allow this alternative. Further, since it is not unusual to find a
national ambient air quality standards (NAAQS) violation caused by
downwash, the PSD analysis must carefully consider that possibility. If a
proposed source contributes to a NAAQS violation caused by downwash from a
background source, the permit cannot be issued. On the other hand, not
every source potentially subject to downwash must be evaluated. Therefore,
we are pursuing alternatives to better define the range within which
detailed modeling should be required.
In summary, please be assured that we are sensitive to the issues
raised in your memorandum and that we will coordinate with Region VI in
this effort. If you have any questions, please contact me or have your
staff contact Doug Grano at 629-5255.
cc: R. Bauman
D. deRoeck
E. Ginsburg
D. Grano
W. Laxton
E. Lillis
J. Tikvart
D. Wilson
J. Yarbrough |