Assessing vegetation exposure to ozone: properties of the AOT40 index and modifications by deposition modelling

A discussion is presented on the application of micrometeorological deposit ion modelling principles to improve the characterisation of vegetation expo sure to ozone and thus the use of critical levels as the basis of targeted emission control. The AOT40 (accumulated exposure over a threshold of 40 pp b or nl l(-1)) ozone exposure index is shown to impose a differential weigh ting that results in a high sensitivity, by a factor of two to 10 depending on the pollution climate, with respect to concentration. This makes it nec essary to correct for systematic effects, such as the concentration profile below the measurement height, in order to justify a comparison with the bi ological data obtained from well-mixed exposure chambers. Available studies indicate a 50-70% lower AOT40 at the vegetation height. The resistance met hod for estimating the profile is extended to allow for stomatal effects th at potentially bias the plant response predicted with an exposure index. Th is integrated profile-uptake correction refines the current approach and se rves as a transitional step towards a real flux-based approach. For the lat ter, a new deposition parameterisation is tested against field observations . (C) 2000 Elsevier Science Ltd. All rights reserved.