1                    WASHINGTON, D.C. 20460
                                 Mav 11 1QQ6                  OFFICE CF THE ADMINISTRATOR
                                 Wldyoi, i^3U                   SCiENCc ADVISORY BOARD

        ^-CASAC-LTR-96-007 ^
 Honorable Carol M. Browner
 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

      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
that the final product is  sound. Our concern over the uncertainties has diminished
                                                              r\ A
              U.S. Environmental Protection Agency                  v~cr
              Region 5, Library (PL-12J)
              77 West Jackson Boulevard, 12th Flow
              Chicago, IL 60604-3590

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.
                                     Dr. Ge&rge T. Wolff, Chay
                                     Clean Air Scientific Advisory Committee