Ambient flux-based critical values of ozone for protecting vegetation: differing spatial scales and uncertainties in risk assessment

The current European critical levels for ozone (O3) to protect crops, natural and semi-natural vegetation and forest trees (Level I) are based on the accumulated exposure over a threshold of 40 ppb exposure index. For the revision of the 1999 UN-ECE protocol (Protocol to the 1979 Convention on Long-Range Transboundary Air Pollution to Abate Acidification, Eutrophication and Ground-Level Ozone. Gothenburg, 30 November), critical levels are partly replaced by critical cumulative stomatal uptake (critical absorbed dose or CAD). Here, we discuss possible improvements in deriving CAD. At present, the main problems with the flux approach can be attributed to uncertainties in the parameterisation of, (1) stomatal conductance, (2) non-stomatal deposition, and (3) the representativeness of species used in flux-effect studies. Although in general, the flux approach is well accepted by plant effects scientists in North America, concerted research efforts have not taken place in that direction. Furthermore, because of the differences in the approach to setting ambient air quality standards, it appears very doubtful that policy makers and air quality regulators in the US and Canada will readily accept the overall philosophy.