The European critical levels for ozone: improving their usage

The European critical levels (CLs) for ozone (O3) to protect crops, natural and semi-natural vegetation, as well as forest trees, are expressed as an Accumulated exposure Over a Threshold of 40 ppb (AOT40). In principle, this exposure index should represent the O3 concentrations at the upper boundary of the quasi-laminar layer of the plant canopy. However, in reality, those values cannot be measured and, therefore, must be estimated by micrometeorological models. Nevertheless, inappropriate calculation of AOT40 for ambient conditions using O3 levels actually measured at some reference height above the canopy leads to predictions of unrealistic crop yield losses. At the present time, CL to protect crops from long-term effects and yield losses, is based on open-top chamber experiments, mainly with spring wheat. In addition to concerns associated with the experimental methodologies used in these studies, a correct application of CL should include simulation of phenological stages of a representative wheat canopy. The present paper describes a model for simulation of leaf area index and canopy height development, based on algorithms adopted from a widely validated agrometeorological model of the German Weather Service. Because, O3 concentrations at the upper boundary of the quasi-laminar layer of the crop canopy are not unambiguouly connected with plant stomatal uptake, a correction of the actually simulated concentrations is needed to provide toxicologically effective O3 concentrations (effective AOT40). A comparison of results from the application of effective AOT40, with the observations of yield by the farmers, suggests that the estimated crop losses using the effective dose are within the bounds of probability. However, at the present time, for plants other than wheat, the data base is too small to derive meaningful and reliable effective dose–response relationships. Taking into account the definition of AOT40, soil–vegetation–atmosphere-transfer models must be generally applied. Future research efforts should address the important need for flux-orientated concepts which lead to a derivation of critical absorbed doses for O3 to protect vegetation (critical loads).