The North American Background (NAB) 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 NAB 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 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 background level, more than likely overestimates
human health risk.
