"Johansson, Robert" <Robert_C._Johansson@omb.eop.gov> 

06/14/2007 05:11 PM

	

To

Lydia Wegman/RTP/USEPA/US@EPA

cc

Subject

prb footnote









Hi Lydia,

 

We have another comment for you:  

 

Can you move footnote 26 into the text and start a new paragraph with it
--- suggested edits also included here:

 

Background O3 concentrations used in the O3 risk assessment were defined
in chapter 2 of the Staff Paper as the O3 concentrations that would be
observed in the U.S. in the absence of anthropogenic emissions of
precursors (e.g., VOC, NOx, and CO) in the U.S., Canada, and Mexico.
During 1997, EPA estimated background concentration based on measured
air concentration in pristine areas.  For the current review, the
results of a global tropospheric O3 model (GEOS-CHEM) have been used to
estimate monthly background daily diurnal profiles for each of the 12
urban areas for each month of the O3 season using air quality data from
2001. Based on this model, the Criteria Document indicates that
background O3 concentrations are generally predicted to be in the range
of 0.015 to 0.035 ppm in the afternoon, and they are generally lower
under conditions conducive to man-made O3 episodes. In light of CASACs
critique of this model and the change in approach, EPA requests comment
on the use of modeled values versus the measured values used previously,
which were higher. (footnote CASAC comments from March 2007, page 2-3).

 

Thanks,

Rob

 

________________________________________________

Robert Johansson, Ph.D.

OMB/OIRA/NREA

725 17th St. NW, Room 10202

Washington, DC 20503

(P) 202-395-7873

(F) 202-395-7285

(E) RJohansson@omb.eop.gov

 

