The exchange of ozone between vegetation and atmosphere: micrometeorological measurement techniques and models

The European critical levels (CLs) to protect vegetation are expressed as a n accumulative exposure over a threshold of 40 ppb (nl l(-1)). In view of t he fact that these chamber-derived CLs are based on ozone (O-3) concentrati ons at the top of the canopy the correct application to ambient conditions presupposes the application of Soil-Vegetation-Atmosphere-Transfer (SVAT) m odels for quantifying trace gas exchange between phytosphere and atmosphere . Especially in the context of establishing control strategies based on flu x-oriented dose-response relationships, O-3 flux measurements and O-3 excha nge simulations are needed for representative ecosystems. During the last d ecades several micrometeorological methods For quantifying energy and trace gas exchange were developed, as well as models for the simulation of the e xchange of trace gases between phytosphere and atmosphere near the ground. This paper is a synthesis of observational and modeling techniques which di scusses measurement methods, assumptions, and limitations and current model ing approaches. Because stomatal resistance for trace gas exchange is param eterized as a function of water vapor or carbon dioxide (CO2) exchange, the most important micrometeorological techniques especially for quantifying O -3, water vapor and CO2 flux densities are discussed. A comparison of simul ated and measured O-3 flux densities shows good agreement in the mean. (C) 2000 Elsevier Science Ltd. All rights reserved.