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Ozone Methodology: Current Conditions

Ground level O₃ is created through a complex series of atmospheric chemical reactions involving NO_x and volatile organic compounds (VOC) in the presence of specific climatic and weather conditions (Chameides and Lodge 1992). Ozone exposure levels are influenced by factors such as temperature, time of day, relative humidity, wind speed, wind direction, and spatial proximity of anthropogenic and biogenic emission sources (Schichtel and Husar 1999). Ozone can reduce foliage, stem, and root growth in trees by impacting leaf-cell photosynthesis and gas exchange.

Allen and Gholz (1996) revealed extensive spatial and temporal variation in O₃ concentrations across the region. For at least two reasons, accurate prediction of annual variability in O₃ levels for forested areas has not yet been achieved: (1) monitoring sites in rural, forested areas are lacking; and (2) modeling O₃ exposure is very difficult because of weather and human-related conditions that contribute to its annual variability (Allen and Gholz 1996). However, annual variation in O₃ at select monitoring sites has been analyzed.

Annual O₃ variability for the United States can be seen in figure 18.4, which shows 3-month maximum daily SUM06 O₃ exposure levels for 1988 through 1992. A SUM06 value is the sum of all mean hourly daytime O₃ concentrations that are at least 0.06 parts per million (ppm) over a continuous 3-month period (92 days) during the summer. The SUM06 exposure index represents the threshold ambient O₃ level (0.06 ppm-hours) below which many forms of vegetation can resist harmful cumulative O₃ effects. The SUM06 index may be particularly useful because negative effects of O₃ exposure, especially on tree photosynthetic capacity (Richardson and others 1992) and foliage production and retention (Kress and others 1992), may be cumulative and linear, extending over multiple growing seasons.