Characterization of ozone at high elevation in the eastern United States: Trends, seasonal variations, and exposure

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Abstract
Ozone measurements at five high‐elevation (≳ 1000 m msl) Mountain Cloud Chemistry Program (MCCP) sites in the eastern United States for the period from May to October 1986–1988 have been analyzed. In order to characterize and compare the exposure to ozone at these sites, several ozone exposure indices are used. The sum of all hourly mean ozone concentrations equal to or greater than 0.07 ppm (SUM07) for 1988 indicates a north‐south gradient in ozone exposure: Southeastern sites (Mount Mitchell, North Carolina, 105 parts per million‐hours (ppm‐h), Whitetop Mountain, Virginia, 85.7 ppm‐h, Shenandoah Park, Virginia, 51.6 ppm‐h); Northeastern sites (Whiteface Mountain, New York, 49.2 ppm‐h, Mount Moosilauke, New Hampshire, 34.2 ppm‐h). This shows that forest areas in the southeastern high‐elevation sites experienced higher ozone exposure than their northeastern counterparts during the hot and dry 1988 measurement season. Ozone episodes (concentrations greater than 70 ppbv lasting 8 hours or more) were more frequent during the summer months at all sites. This suggests a correlation with the seasonal photochemical cycle and perhaps with local biogenic hydrocarbon emissions. Seasonal analysis suggests that ozone maximum occurs during spring coincident with spring maximum at Mauna Loa, Hawaii and at Alpine stations in Europe, suggestive of an underlying hemispheric phenomena. A reversed diurnal cycle (nighttime maximum) pattern was also observed from May to October at all the five high‐elevation sites. Back trajectory analysis, during high‐ozone episodes, indicates that most MCCP sites were influenced by urban and industrial sources from the midwestern region of the United States, suggesting large‐scale transport patterns.