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 
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 multidimensional 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 

2