May 31, 1996

EPA-SAB-CASAC-LTR-96-007
Honorable Carol M. Browner
Administrator
U.S. Environmental Protection Agency
401 M. Street S.W.
Washington, DC 20460

           RE:  CASAC Comments on Air Quality Modeling for the Section 812
                 Retrospective Study

Dear Ms. Browner:

      At their December 22, 1992 meeting, the Science Advisory Board's Advisory
Council on Clean Air Compliance Analysis (ACCACA, formally known as the Clean Air
Act Compliance Analysis Council, CAACAC) decided to refer review of the air quality
modeling part of the Retrospective Study of the impacts of the  Clean Air Act to the
Clean Air Scientific Advisory Committee (CASAC). The request was made to CASAC
in a March 22, 1993 memo from Mr. Jim DeMocker of the  Office of  Policy Analysis and
Review (OPAR)  in EPA's Office of Air and  Radiation. Subsequently, a Panel of CASAC
members (the Air Quality Models Subcommittee, AQMS) and SAB consultants was
constituted to review the air quality modeling.

      Three public conference calls on October 1, 21, and December 2, 1993 served
to brief the panel members on the charge and air quality methodology and for Panel
members to comment on the methodology.  The panel members provided EPA with
numerous comments during the conference calls and subsequently in writing.
These comments were summarized for you in a June 30, 1994 letter report
(EPA-SAB-CASAC-LTR-94-009). A major concern at the time was that the
uncertainties in the air quality estimates in the "no-Clean Air Act" scenarios would be
intolerable.

      The six member panel  conducted a final public conference call on April 26, 1996
to review the completed air quality modeling.  During the call and subsequently in
writing, the panel members provided comments to the Agency. All the panel members
expressed their satisfaction that the Agency followed the Panel's recommendations and

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that the final product is sound. Our concern over the uncertainties has diminished
considerably, but nevertheless, it still exists.  We have provided your staff with a
number of recommendations to explicitly articulate the uncertainties, to not overstate
the precision of the estimates, and to more carefully present the implications of the
uncertainties for the overall study  results.

      With the incorporation of these relatively minor changes, the Panel feels that air
quality estimates are reasonable and are appropriate to use to estimate the benefits of
the Clean Air Act. Please let me know if we can assist you any further.
                                     Sincerely,
                                                   /.
                                     Dr. GeoVge T. Wolff,
                                     Clean Air Scientific Advisory Committee

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                 U.S. ENVIRONMENTAL PROTECTION AGENCY
                           SCIENCE ADVISORY BOARD
                CLEAN AIR SCIENTIFIC ADVISORY COMMITTEE
                   AIR QUALITY MODELING SUBCOMMITTEE
CHAIR
Dr. George T. Wolff, General Motors Environmental & Energy Staff, Detroit, MI

MEMBERS
Dr. Philip Hopke, Clarkson University, Dept. Of Chemistry, Pottsdam, NY

Dr. Harvey Jeffries, Dept. of Env. Sciences & Engineering, University of North Carolina,
School of Public Health, Chapel Hill, NC

Dr. Paulette Middleton, Science Policy Associates, Boudler, CO

Dr. Peter K. Mueller, Electric Power Research Institute, Palo Alto, CA

Dr. James H. Price, Jr., Texas Natural Resource Conservation Commission, Austin, TX

SCIENCE ADVISORY BOARD STAFF
Dr. K. Jack Kooyoomjian, Designated Federal Official, USEPA, Science Advisory Board, 401
M Street, SW, Mail Code 1400F, Washington, DC 20460

Ms. Diana Pozun, Staff Secretary, USEPA, Science Advisory Board, 401 M Street, SW, Mail
Code 1400F, Washington, DC 20460

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                                   NOTICE

      This report has been written as part of the activities of the Science Advisory
Board, a public advisory group providing extramural scientific information and advice to
the Administrator and other officials of the Environmental Protection Agency.  The
Board is structured to provide balanced, expert assessment of scientific matters related
to problems facing the Agency.  This  report has not been reviewed for approval by the
Agency and, hence, the contents of this report do not necessarily represent the views
and policies of the Environmental Protection Agency, nor of other agencies in the
Executive  Branch of the Federal government, nor does mention of trade names or
commercial products constitute  a recommendation for use.

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