Ozone concentrations measured by the Clean Air Status and Trends Network (CASTNet)

摘要

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.