The Policy Relevant Background (PRB) concentration
or range of concentrations represent what EPA believes would
be experienced if the United States and other countries in North
America were to initiate a zero anthropogenic emissions strategy,
which includes eliminating emissions associated with fertilizer.
The PRB concentrations define the level below which O3 standards
cannot be practicably set. In the 1996 ozone review, the EPA
used 0.04 ppm in its health risk assessment evaluations as the
level it expects as surface ozone background for an 8-hr daily
maximum concentration for clean sites. In the 2006 review of
the ozone standard, the EPA applied a model with 2 degree by
2.5 degree spatial resolution (i.e., great uncertainty) to define
ranges of concentrations for policy relevant background that
are much lower than the 0.04 ppm level (i.e., 0.015 to 0.035
ppm). At a monitoring site at Trinidad Head, California, which
meets Policy Relevant Background criteria many times during the
year, numerous occurrences of hourly average concentrations greater
than or equal to 0.05 ppm are measured. In a study, A.S.L.
& Associates characterized the daily maximum 8-hr ozone
concentrations for 3 clean sites in North America. A summary
figure shows that Custer National Forest in Montana, Theodore
Roosevelt National Park in North Dakota, and Yellowstone National
Park in Wyoming all experienced numerous occurrences of 8-hr
daily maximum concentrations greater than or equal to 0.04 ppm
during each of the years monitored. The U.S. EPA, by selecting
0.04 ppm or lower as its Policy Relevant Background level, more
than likely overestimates human health risk.
