A very important issue for vegetation is the definition of natural background for ozone. This is important because the ozone exposures used in some control chambers in both the United States and Europe for vegetation experiments were much below the ozone levels occurring in today's environment. The resulting effect of applying ozone concentrations much lower than observed at the cleanest sites in the world will probably result in overestimating vegetation growth losses using the current models. Theoretical values have been estimated to be 0.025 ppm. However, hourly average concentrations of ozone exceed 0.025 ppm even at the most isolated sites (e.g., South Pole, North Pole, Spitzbergen Norway). Thus, we are left with the dilemma of deciding whether to accept at face value a hypothetical 0.025 ppm value or accept the fact that natural background is actually a range of hourly average concentrations that are affected by natural organic and nitrogen emissions characteristic of the local area. Altshuller and Lefohn (1996) discuss O3 background levels at coastal and inland sites in the United States and Canada. See our publications list for more information. The ranges suggest that the background O3 is somewhat dependent on a number of conditions such as the nature of upwind flow, lack of pollution sources, and terrain conditions and natural emissions associated with forest and agricultural areas.

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