The European critical levels for ozone: improving their usage

The European critical levels (CLs) for ozone (O-3) to protect crops, natura l and semi-natural vegetation, as well as forest trees, are expressed as an Accumulated exposure Over a Threshold of 40 ppb (AOT40). In principle, thi s exposure index should represent the O-3 concentrations at the upper bound ary of the quasi-laminar layer of the plant canopy. However, in reality, th ose values cannot be measured and, therefore, must be estimated by micromet eorological models. Nevertheless, inappropriate calculation of AOT40 for am bient conditions using O-3 levels actually measured at some reference heigh t above the canopy leads to predictions of unrealistic crop yield losses. A t the present time, CL to protect crops from long-term effects and yield lo sses, is based on open-top chamber experiments, mainly with spring wheat. I n 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 des cribes a model for simulation of leaf area index and canopy height developm ent, based on algorithms adopted from a widely validated agrometeorological model of the German Weather Service. Because, O-3 concentrations at the up per boundary of the quasi-laminar layer of the crop canopy are not unambigu ouly connected with plant stomatal uptake, a correction of the actually sim ulated concentrations is needed to provide toxicologically effective O-3 co ncentrations (effective AOT40). A comparison of results from the applicatio n of effective AOT40, with the observations of yield by the farmers, sugges ts that the estimated crop losses using the effective dose are within the b ounds of probability. However, at the present time, for plants other than w heat, the data base is too small to derive meaningful and reliable effectiv e dose-response relationships. Taking into account the definition of AOT40, soil-vegetation-atmosphere-transfer models must be generally applied. Futu re research efforts should address the important need for flux-orientated c oncepts which lead to a derivation of critical absorbed doses for O-3 to pr otect vegetation (critical loads). (C) 1999 Elsevier Science Ltd. All right s reserved.