From critical levels to critical loads for ozone: a discussion of a new experimental and modelling approach for establishing flux–response relationships for agricultural crops and native plant species

Abstract

Present critical levels for ozone (O 3) for protecting vegetation against adverse effects are based on exposure–response relationships mainly derived from open-top chamber experiments and are expressed as an Accumulated exposure Over a Threshold of 40 ppb (AOT40). In that context with a revision of the UN (United Nations)-ECE (Economic Commission for Europe) Gothenburg protocol, AOT40 values should be replaced by flux-oriented quantities, i.e. in the end by critical loads. At present, the database for the derivation of critical loads for O 3 is extremely inadequate. Furthermore, the currently available flux–response relationships are also derived from open-top chamber experiments. The use of a relationship for spring wheat in a risk assessment for an agricultural site in Hesse, Germany, demonstrates in principle, the applicability of the critical load concept for O 3. Comparisons of diurnal variation of stomatal uptake and AOT40 showed that a major part of toxicologically effective stomatal uptake occurred before noon whereas the AOT40 values were dominated by the O 3 concentrations during afternoon. In other words, the AOT40 exposure index do not adequately address the O 3 burden during hours when plants are sensitive to O 3 uptake. However, due to the differences in radiation, air temperature and humidity between the chamber and the ambient microclimates, a derivation of flux-response relationships from chamber experiments is likely to be questionable, especially for species rich ecosystems: Here, without any changes in the pollution climate, significant modifications of species composition as well as an earlier beginning of the growing season has been previously observed. To overcome the problems associated with chamber-derived flux–response relationships, a new experimental and modelling concept, was developed. The approach, briefly described in this paper, combines methods in air pollution toxicology and micrometeorology. As an analogy to the free-air fumigation concept, O 3 is released into the air by an injection system above the plant canopy. The assessment of dispersion and surface deposition of O 3 released is based on Lagrangian trajectory modelling. Depending on wind direction and velocity, atmospheric stratification and surface roughness, without any disturbance of the microclimate and micrometeorology, several sub-areas can be identified around the source position with differing deposition rates above the ambient level. Taking into account the actual O 3 background deposition, deposition rates and vegetation responses observed in these sub-areas can easily be used to derive flux–effect relationships under ambient conditions and more realistic limiting values to protect our environment.