A conceptual ozone dose-response model to develop a standard to protect vegetation

The present air quality standard to protect vegetation from ozone is based on a measured concentration (i.e., exposure) rather than on plant uptake rates (or dose). Proposed exposure-based standards have led to concerns about the appropriateness of chamber studies. There has also been some debate about the importance of the diel phase difference between plant conductance and ozone concentration in assessing the potential for plant damage. In this paper, we use physical reasoning based on (i) plant defenses and (ii) general resistance concepts of dry deposition to derive a suggested general form of a dose-based standard. The dose-based standard is then related to the more traditional exposure-based standard. Although we develop the model in terms of plant injury, we also discuss how the model can be extended to include damage, which historically has been the focus of air quality standards. With this new dose-based approach, we clarify some of the issues concerning chambers and the interaction of the daily cycles of ozone concentration and plant stomatal conductance. We further demonstrate that ii) weighted fluxes can be used as a surrogate for plant defenses, (ii) injury or damage to vegetation is more likely to be correlated with a dose-based index that differentially weights ambient ozone concentration or plant uptake rates than one which does not, (iii) the potential for ozone injury or damage to plants can occur throughout the day, and (iv) when assessing the potential for plant damage, a differentially weighted flux-based standard is likely to be more precise and more discriminating than a cumulative ozone-based exposure index. Finally, because our basic premise relies on plant defensive mechanisms, we outline areas of research that are necessary before a dose-based standard can be implemented. Published by Elsevier Science Ltd.