The characterization of ozone exposures in rural West Virginia and Virginia

Ozone (O3) is suspected of being the most significant air pollutant affecting vegetation in the United States. Ozone concentrations measured at Bearden Knob and Parsons, located in a remote forested region of northcentral West Virginia, were characterized and compared with exposures experienced at other sites in the region. From 1988 to 1992, 1988 was one of the highest O3 exposure years in the region, while 1992 was the lowest. At almost all sites in 1992, few hourly average concentrations were > or = 0.10 ppm, while in 1988 several sites had more than 100 hourly average concentrations > or = 0.10 ppm. These instances occurred at both high- and low-elevation sites. In 1992, the high-elevation Bearden Knob site experienced a flat-type diurnal pattern, while the nearby low-elevation Parsons site experienced a changing diurnal pattern, indicative of scavenging. Using several indices, O3 exposures at Parsons were less than those at Bearden Knob. Evaluation of hourly averaged data for all sites in the region from 1988 to 1992 showed that Horton Station experienced the highest W126 O3 exposures. Horton Station is a high-elevation site in the mountains of southwestern Virginia. Square-wave exposures have been used under artificial conditions in vegetation experiments. If square-wave exposures are observed under ambient conditions, then the vegetation data collected under artificial conditions may be relevant for predictive purposes. Square-wave exposures were characterized for two high-elevation sites. In 1992, Horton Station experienced 25 episodes for which the hourly average concentrations remained near 0.05 ppm for eight hours or longer; there were 18 episodes for which the hourly average concentrations remained near 0.06 ppm, and three episodes for which the concentrations remained near 0.07 ppm. Bearden Knob experienced 31 episodes of eight hours or longer for hourly average concentrations near 0.05 ppm, 13 episodes at or near 0.06 ppm, and three episodes at or near 0.07 ppm. Until experiments are performed to determine the relative importance of hourly average concentrations above 0.03 ppm for assessing vegetation effects, using single-parameter exposure indices to predict effects may produce inconsistent results.