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The U.S. Environmental Protection Agency’s Clean Air Status and Trends Network (CASTNet),
operated in partnership with the National Park Service, is considered the principal source of
information on rural ozone concentrations in the United States. The monitoring network is
currently comprised of 79 sites of which 75 measure atmospheric ozone concentrations. The
CASTNet ozone data set provides a 14-year record of hourly mean ozone concentrations, rolling
8-hour mean concentrations, and seasonal SUM06 values. Analysis of the data set reveals no
significant long-term trend in rural ozone levels in the United States over the past eleven years.
Despite these results, ozone concentrations measured during 2000 in the eastern United States
were among the lowest during the period of record and were significantly lower than
concentration values recorded during 1999 due to cooler, cloudier meteorological conditions.
CASTNet QA procedures are different from SLAMS QA requirements, and therefore ozone data
measured at CASTNet sites cannot be used to gauge compliance with ozone National Ambient
Air Quality Standards (NAAQS). However, it is possible to use the current and proposed
standards as threshold levels for determining the extent of ozone pollution at rural locations in
the United States. Comparing hourly ozone concentrations with the current standard shows that
several sites within the network measured values in excess of the 125-ppb, 1-hour standard
established by EPA. If EPA were to officially adopt the proposed 8-hour standard, approximately
50% of the sites within the network would be classified as nonattainment.
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CNumber:
ACE 2002 Baltimore, Maryland June 23-27, 2002
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Publication Type:
Proceedings
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Publication Date:
June 2002
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