A new flux-orientated concept to derive critical levels for ozone to protect vegetation

The current European critical levels for ozone (O-3) to protect crops, natu ral and semi-natural vegetation and forest trees are based on a relative sm all number of open-top chamber experiments with a very limited number of pl ant species. Therefore, the working group "Effects of Ozone on Plants" of t he Commission on Air Pollution Prevention of the Association of German Engi neers and the German Institute of Standardization reanalysed the literature on O-3 effects on European plant species published between 1989 and 1999. An exposure-response relationship for wild plant species and agricultural c rops could be derived from 30 experiments with more than 30 species and 90 data points; the relationship for conifer and deciduous trees is based on 2 0 experiments with nine species and 50 data points. From these relationship s maximum O-3 concentrations for different risk stages are deduced, below w hich the vegetation type is protected on the basis of the respective criter ia. Because it is assumed that the fumigation concentrations reflect the O- 3 concentrations at the top of the canopy, i.e. the upper surface boundary of the quasi-laminar layer if the micrometeorological big-leaf approach is applied, the application of these maximum O-3 concentrations requires the t ransformation of O-3 concentrations measured at a reference height above th e canopy to the effective phytotoxic concentrations at the top of the canop y. Thus, the approach described in this paper is a synthesis of the classic al concept of toxicology of air pollutants (critical concentrations) and th e more toxicological relevant dose concept. (C) 2000 Elsevier Science Ltd. All rights reserved.