Measuring, modelling and testing ozone exposure, flux and effects on vegetation in southern European conditions what does not work. A Review from Italy.

Abstract Ozone (O 3 ) exposure at Italian background sites exceeds UN/ECE concentration-based critical levels (CLe c ), if expressed in terms of AOT40. Yet the occurrence of adverse effects of O 3 on forests and crops is controversial. Possible reasons include (i) ability of response indicators to provide an unbiased estimate of O 3 effects, (ii) setting of current CLe c in terms of cut-off value and accumulation level, (iii) response functions adopted to infer a critical level, (iv) environmental limitation to O 3 uptake and (v) inherent characteristics of Mediterranean vegetation. In particular, the two latter points suggest that critical levels based on accumulated stomatal flux (CLe f ) can be a better predictor of O 3 risk than CLe c . While this concept is largely acknowledged, a number of factors may limit its applicability for routine monitoring. This paper reviews levels, uptake and vegetation response to O 3 in Italy over recent years to discuss value, uncertainty and feasibility of different approaches to risk assessment.