RURAL OZONE ACROSS THE EASTERN UNITED-STATES - ANALYSIS OF CASTNET DATA, 1988-1995

A predominantly rural ozone monitoring network was operated under the auspices of the Clean Air Status and Trends Network (CASTNet) from 198 8 until 1995. Ozone data from sites in the eastern United States are p resented and several indices are used to describe the spatial and temp oral distribution of ozone concentration and exposure. These indices a re SUM06, W126, the 8-hour rolling average (MAX(8hr)>80), and the curr ent National Ambient Air Quality Standards (NAAQS) for ozone. Ozone in dices were selected to illustrate the spatial and temporal distributio n of ozone, and the sensitivity of this distribution to different repr esentations of concentration or exposure. CASTNet is unique in that a uniform set of site selection criteria and uniform procedures, includi ng traceability to a single primary standard, provide a high degree of comparability across sites. Sites were selected to avoid undue influe nce from point sources, area sources, or local activities. The sites r eflect a wide range of land use and terrain types including agricultur al and forested, in flat, rolling, and complex terrain from the easter n seaboard across the Appalachian Mountains to the Midwest. Results in dicate that ozone concentrations varied greatly in time and space acro ss the eastern United States. Sites in the upper northeast, upper midw est, and southern periphery subregions experienced relatively low ozon e during the years of record compared to sites in the northeast, midwe st, and south central subregions. Ozone exposures at an individual rur al site are dependent on many factors, including terrain, meteorology, and distance from sources of precursors. Relative to the current las of 1996) NAAQS, only a handful of CASTNet sites near major urban areas report exceedances. In contrast, the majority of CASTNet sites might exceed the proposed new primary standard for ozone. Sites at high elev ation (>900m) in the east exhibit relatively high exposure statistics (e.g., SUM06 and W126), but no exceedance of the current ozone standar d from 1988 through 1995. Terrain effects explain some of the variabil ity within subregions and are an important consideration in the design of monitoring networks for ozone and possibly other pollutants.