OZONE DESCRIPTORS FOR AN AIR-QUALITY STANDARD TO PROTECT VEGETATION

Exposure of plants to ozone (O3) causes injury and reduced growth. Des cribing the form and function of the O3 exposure in relation to plant response is important in the regulatory process. Research has shown th at plants show greater response to O3 as concentration increases. The duration of the O3 exposure is also important in the ability of vegeta tion to maintain O3 repair mechanisms. The O3 entering the leaf is imp ortant in plant response, thus O3 fluxes are more important than ambie nt concentrations. However, at this time an air quality stanard useful for the regulatory process should be based on ambient O3 exposures. T he selection of O3 exposure descriptors should incorporate factors per tinent to plant response. Research suggests that exposure descriptors which give greater weight to peak concentrations, and those which acco unt for cumulative exposure, show the closest relationship to plant re sponse. Ozone exposure summaries using concentration averages do not a dequately relate plant response with ambient exposures. Although the u se of cumulative exposure indices may be preferable to seasonal means, it appears that the use of a single-parameter exposure index will not guarantee that the most important components of exposure have been ca ptured. An appropriate alternative approach might use a combination of indices, such as a cumulative index and the number of hourly average concentrations above a threshold.