June 16, 1989



MEMORANDUM

SUBJECT:  Response to Region VI Position on PSD Modeling Issue

FROM:     Edward Lillis, Chief
          Noncriteria Pollutant Programs Branch (MD-15)

TO:       Gerald Fontenot, Chief
          Air Programs Branch, Region IV (6T-A)


     The purpose of this memorandum is to respond to your
February 15, 1989 memorandum requesting guidance on the
consideration of background sources in PSD increment and NAAQS
analyses.  In discussions with Jim Yarborough of your staff, it
appears that your request was intended to focus on the question
of whether sources outside the proposed project's impact area
need to be modeled for short-term effects, given the general
statements made in the 1980 PSD Workshop Manual.  I have reviewed
your position and, as explained more fully below, I am in basic
agreement with you that a short-term analysis should not
automatically exclude emission contributions outside the impact
area.

     The "Prevention of Significant Deterioration Workshop
Manual" (EPA-450/2-80-081, October 1980) (the PSD Manual)
describes the extended area of concern, which goes out 50
kilometers beyond the defined impact area, as the "annular ring." 
The Manual suggests that this ring may have greater utility for
annual impact determinations, but it should not be so strictly
interpreted to preclude the potential usefulness of the area
within this ring for short-term analyses.  For example,
stationary sources located just beyond the area of impact could
be important components of a short-term NAAQS or PSD increment
analysis.   Therefore, a blanket exclusion from short-term
modeling analyses of sources located outside the impact area is
unacceptable.

     It is reasonable for States to establish a method for
determining which sources may not need to be modeled.  Both the
PSD Manual and the "Guideline on Air Quality Models (Revised)"
provide some considerations for developing such a method.  Any
method for excluding sources from modeling should include
flexibility for case-by-case judgments, as well as assurances for
accountability.  The bottom line is that the basis for excluding
a source from the required modeling analysis should be consistent

with EPA guidance, which includes that any exclusions be clearly
documented under the required ambient impact analysis.  Until a
State establishes an exclusion method that your office can
accept, your position that all increment-consuming sources (for
increment analyses) and all major sources (for NAAQS analyses) in
the annular ring be modeled is the best approach.   


cc:  J. Calcagni        
     J. Tikvart
     D. Wilson
     D. Grano
     New Source Review Contacts
     Regional Modeling Contacts

bcc: D. deRoeck
     G. McCutchen
     E. Lillis
     Sect. RF