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UNITED STATES ENVIRONMENTAL PROTECTION AGENCY
WASHINGTON D.C. 20460
OFFICE OF THE ADMINISTRATOR
SCIENCE ADVISORY BOARD
November 29, 2006
EPA-COUNCIL-07-001
Honorable Stephen L. Johnson
Administrator
U.S. Environmental Protection Agency
1200 Pennsylvania Avenue, N.W.
Washington, DC 20460
Subject: Benefits and Costs of Clean Air Act - Air Quality Modeling Issues
Dear Administrator Johnson:
EPA's Office of Air and Radiation is developing the Agency's "Benefits and Costs of the
Clean Air Act 1990 - 2020: EPA's Second Prospective Analysis" (hereinafter Second
Prospective Study) pursuant to Section 812 of the Clean Air Act Amendments calling on EPA to
conduct periodic, scientifically reviewed studies to assess the benefits and the costs of the entire
Clean Air Act. The overall purpose of this Second Prospective Study is to estimate the net
health, welfare, ecological and economic benefits of the 1990 Clean Air Act Amendment
programs. As part of this effort, OAR has recently submitted to the Advisory Council on Clean
Air Compliance Analysis (Council) and its Air Quality Modeling Subcommittee (AQMS) its
draft emissions report, as well as materials describing its emissions inventories and its selection
of air quality models for the particulate matter and ozone analyses in the Second Prospective
Study. In July 2006, OAR requested input from the Council and AQMS on these draft products.
On August 7, 2006, the AQMS teleconferenced to discuss the OAR's charge questions
related to these air quality modeling issues. The Council and AQMS generally endorsed the
Project 812 Team's choices.
With respect to OAR's draft emissions report and emissions inventories, the Council and
AQMS generally agreed that the Project 812 Team's data and methodological choices were
sound. The Council and AQMS support the Team's technical choices for the base year
inventory, source of data and method for dealing with non-road equipment. With respect to
OAR's method for estimating the need for local controls to meet ozone and PM NAAQS in the
future, the Council and AQMS agreed that the Team's method was the most reasonable
approach, but suggested the Team clarify its assumptions more precisely, such as when its
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forecasts required unidentified measures. Moreover, the Council and AQMS cautioned that any
assumptions about costs should be drawn from the realm of observed practices.
With respect to OAR's model choices, the Council and AQMS support the 812 Project
Team's choice of the Community Multiscale Air Quality (CMAQ) modeling system (Version
4.5) for modeling particulate matter and ozone. Also deemed appropriate was the Team's choice
of the Comprehensive Air Quality Model with Extensions (CAMx) as part of its metamodeling
approach to aggregate numerous individual air quality modeling simulations into a multi-
dimensional air quality "response surface" for ozone.
While the Council and AQMS generally endorsed the Project 812 Team's choices, it is
important to underscore the context of analytic decisions for the Second Prospective Study which
can sometimes afford a greater license than would be appropriate in other contexts. The
objective of the Second Prospective Study is to estimate the difference in net benefits between
implementing the regulations issued under the Clean Air Act Amendments of 1990 (the "with
CAAA" case) and not having those regulations (the "without CAAA" case). In this context, a
number of technical choices were deemed acceptable in that they would not significantly affect
the difference between these two scenarios. An example is the Project Team's assumption about
30 year average temperatures. For other types of analyses, it would be more prudent to align 30
year temperature trends with the latest scientific projections incorporating the effect of
greenhouse gases. However, for the Second Prospective Study, this adjustment is less crucial
because it would have only a very small effect on the difference between the two scenarios. It is
important to bear this caution in mind when using the Second Prospective Study as a model for
future EPA analyses.
Detailed recommendations are included in the meeting minutes of the August 7
teleconference posted on the SAB web site. On behalf of the entire Council and the Air Quality
Modeling Subcommittee, we appreciate this opportunity to provide timely advice to the Agency.
We hope these comments are helpful to the Office of Air as it proceeds with this important work.
Sincerely,
/Signed/ /Signed/
Dr. Trudy Cameron, Chair Dr. David T. Allen, Chair
Advisory Council on Clean Air Air Quality Modeling Subcommittee
Compliance Analysis Advisory Council on Clean Air
Compliance Analysis
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