VIA EMAIL AND FIRST CLASS MAIL 

March 14, 2011 

Members of the Ozone Review Panel 

Clean Air Scientific Advisory Committee 

C/o Dr. Holly Stallworth 

Designated Federal Officer (DRO) 

EPA Science Advisory Board (1400R) 

U.S. Environmental Protection Agency 

1300 Pennsylvania Avenue, NW 

Washington DC 20004 

Email: stallworth.holly@epa.gov 

RE: Policy Relevant Background Ozone 

 EPA Science Advisory Board Notification of a Public Teleconference of CASAC 

 Ozone Review Panel for the Reconsideration of the 2008 Ozone NAAQS 

 76 Fed. Reg. 10895 (Feb. 28, 2011) 

Dear Panel Members: 

The American Gas Association, founded in 1918, represents 199 local energy companies that 

deliver clean natural gas throughout the United States. There are more than 70 million 

residential, commercial and industrial natural gas customers in the U.S., of which 91 percent — 

more than 64 million customers — receive their gas from AGA members. AGA is an advocate 

for natural gas utility companies and their customers and provides a broad range of programs 

and services for member natural gas pipelines, marketers, gatherers, international natural gas 

companies and industry associates to promote operational excellence in the safe, reliable and 

efficient delivery of natural gas. 

The Clean Air Scientific Advisory Committee (CASAC) was established pursuant to the Clean 

Air Act Amendments of 1977 and is charged with providing advice, information and 

recommendations to the Administrator of the Environmental Protection Agency (EPA) on the 

scientific and technical aspects of issues related to the criteria for air quality standards, research 

related to air quality, and strategies to attain and maintain air quality standards. The CASAC 

Ozone Review Panel, in particular, is responsible for conducting reviews of EPA’s scientific 
AGA Policy Relevant Background Ozone 

March 14, 2011 

Page 2 of 2 

assessments of the health and welfare effects of ground level ozone, pursuant to EPA’s recent 

decision to reconsider the 2008 ozone national ambient air quality standards (NAAQS) to 

ensure that they are scientifically sound and protective of public health and the environment. 

Most recently, the EPA Administrator transmitted additional charge questions to the CASAC 

Ozone Review Panel regarding its reconsideration of the 2008 ozone NAAQS. 

Pursuant to the CASAC’s call for public input and comment in the February 28, 2011 Federal 

Register and the additional charge questions submitted by EPA, the AGA hereby submits the 

attached questions pertaining to policy relevant background (PRB) ozone and the extent to 

which these variable PRB ozone levels should be incorporated into the CASAC Ozone Review 

Panel’s response to the additional charge questions submitted by the EPA Administrator. 

Please do not hesitate to contact me should you have any comments or questions with respect 

to this letter or the attached. Thank you. 

Very truly yours, 

/s/ 

Pamela A. Lacey 

Senior Managing Counsel 

American Gas Association 

400 North Capitol Street N.W. 

Washington, D.C. 20001 

placey@aga.org

(202) 824-7340 

Enclosure 

cc: Erbin Keith 

 
March 14, 2011

Questions for Consideration by the CASAC Ozone Review Panel

For the Public Teleconference March 23, 2011

1. What exact procedures should be used in calculating policy relevant background (PRB) ozone

levels?

a. Should PRB ozone levels be determined through modeling, or through observation‐

based analyses, or some hybrid of both?

b. How can monitor‐derived estimates of PRB ozone levels assure that there is no

influence of North America anthropogenic emissions?

c. When performing air quality modeling to estimate PRB ozone levels, how should global

and regional models be used?  What grid resolution is sufficient?  How can we verify

that we are accounting for all of the natural sources of ozone and proper treatment of

stratospheric ozone intrusion?  How should we evaluate the modeled estimates of PRB

ozone levels?

d. For example, for Los Angeles the South Coast Air Quality Management District

(SCAQMD) estimates a summer background ozone level of 48 ppb using urban/regional‐

scale modeling with a fine grid, whereas EPA’s 2007 Staff Paper estimates PRB ozone

levels in the 15‐25 ppb range using a global chemistry model with a coarse grid.

2. How is the variability in PRB ozone levels – including, but not limited to spatial, temporal and

methodological variability – taken into account in setting the level of the new 8‐hour ozone

National Ambient Air Quality Standard (NAAQS)?

a. Some of the monitor‐derived estimates of PRB ozone range from 30 to 50 ppb for April‐

June with values as high as 66 ppb in the spring time (Lefohn, 2007), whereas EPA’s

model‐derived PRB ozone in the 2007 Staff Paper range from 15 to 35 ppb (EPA, 2007).

b. Given the effects of intercontinental transport, we would expect PRB ozone levels to be

higher in the western US.

c. Stratospheric ozone intrusion can result in higher PRB ozone levels in high terrain.

d. PRB ozone levels may be expected to be highest in the spring when favorable conditions

for intercontinental transport couple with higher frequencies of stratospheric ozone

intrusion.

3. How is the decadal increase in ozone transport from Asia being accounted for in estimating

PRB ozone levels and in the NAAQS setting process?

a. Parrish and co‐workers (2009) estimate ozone coming into the US from the western

Pacific is increasing by 5 ppb per decade.

b. What is the potential role for increases in slower oxidizing pollutants such as CO and

methane to build up PRB ozone over the next several decades?