United States Environmental Protection Agency
          Office of Air Quality Planning and Standards
          Research Triangle Park, North Carolina 27711

                        February 4, 1986



MEMORANDUM

SUBJECT:  Current EPA Modeling Guidance

FROM:     Joseph A. Tikvart, Chief /s/
          Source Receptor Analysis Branch (MD-14)

TO:       Bruce P. Miller, Acting Chief
          Air Programs Branch, Region IV

     In answer to your inquiry of December 24, 1985, you are
correct in assuming that the current official guidance on air
quality modeling is primarily contained in:  (1) Guideline on Air
Quality Models, EPA 450/4-78-027 (1978) and (2) Regional Workshops
on Air Quality Modeling: A Summary Report, EPA 450/4-82-015 (1981),
with revisions 8/82, 9/82 and 10/83.  However, your request for
material constituting the totality of all generic EPA policy
regarding air quality modeling for SIP revisions, implies an
unrealistic expectation.  Considerable guidance material and policy
memoranda have been issued that specify or implicitly affect
modeling procedures, e.g., GEP stack height, PSD, emissions
trading, etc.  None of this guidance is contained or even
referenced in the above two documents.  To a large extent the
regulatory issues for which modeling input is required are
discussed in Sections 7 and 11 of the proposed revisions to the
Guideline on Air Quality Models, which will be promulgated by mid-
year.  Additional information may also be found in the Air Programs
and Guidelines Index and its update (EPA 450/2-82-016).

     Your request for other memoranda constituting national
guidance appears to refer to Model Clearinghouse memoranda, as we
discussed in our meeting on December 16, 1985.  Decisions and
guidance provided by the Model Clearinghouse in response to
inquiries from the Regions on modeling specific sources or groups
of sources do not constitute national guidance.  However, these
activities do set precedents for future modeling of closely similar
situations.  In most cases these precedents are so narrow, i.e.,
so geared to the specific aspects of the individual modeling
application, that it is uncommon for another situation sufficiently
similar to occur.  In a few cases the Clearinghouse has dealt with
sufficiently broad issues that the precedents set are applicable
to many regulatory modeling exercises.  The decisions in these
cases should still not be considered as new national guidance but
rather as an interpretation of existing guidance with fairly broad
applicability.  Recent Clearinghouse memoranda of this nature are:

     1.  February 27, 1983, memorandum from Sheldon Meyers to Air
and Waste/Air Management Division Directors, "Emissions Trading-
-Technical Clarifications";

     2.  November 9, 1983, memorandum from Joseph Tikvart to Jack
Divita, Region VI, "Use of Composite Quarterly Meteorological Data
with ISCLT Modeling";

     3.  January 20, 1984, memorandum from John O'Connor to Thomas
Devine, Region IV, "PSD Increment Consumption Calculations";

     4.  November 28, 1984, memorandum from Richard Rhoads to
Robert Duprey, Region VIII, "Correcting Atmospheric Dispersion
Model Results to Standard Temperature and Pressure"; and

     5.  December 24, 1984, memorandum from Richard Rhoads to James
Wilburn, Region IV, "Regional Implementation of Modeling Guidance."

Lew Nagler of your staff should have copies of all these memoranda. 
If any are missing, please have him contact Dean Wilson at FTS 629-
5681.

     In regard to your second request, we understand that, when
providing guidance on the use of dispersion models and data bases,
Region IV plans to interpret the words "should," "recommended," and
"preferred" to mean "required."  These terms were carefully chosen
through consensus of all 10 Regions as appropriate language for
ensuring that the most effective procedure is used to select the
best available technique or data base in every situation.  The
terms are generally interpreted by the Regions as "required" for
the preponderance of regulatory analyses; but the terms are used
in a way so as not to preclude the Agency from acting prudently and
reasonably when situations arise where it would not make good sense
to interpret them as "required."  One way of accomplishing this end
is to interpret them in the sense you describe in your memorandum
dated December 24, 1985.  Thus, we concur that the terms may be
interpreted as "required" by Region IV, but that Region IV may also
allow States to ask for an exemption from any of these
requirements.  The  State should document to  Region IV's
satisfaction why an alternative approach is adequate for
determining that the source poses no threat to the PSD increments
or the NAAQS.  After review, Region IV may wish to propose the
procedure to the Model Clearinghouse for concurrence.  

     If you have further questions, please contact me.  

cc:  T. Helms
     S. Reinders
     R. Rhoads