Stomatal ozone fluxes over a barley field in Italy. “Effective exposure” as a possible link between exposure- and flux-based approaches

Fluxes of ozone as well as of sensible and latent heat were measured over a barley field in Northern Italy from April to June 2002 with the eddy-correlation technique in order to determine the dose of ozone taken up by plants during the whole grain filling and maturation period. Stomatal ozone fluxes were then calculated by using the similarity between gaseous exchange processes occurring inside the stomata involving ozone and water vapour, whose flux was available from the measurements. The dose was then obtained by integrating the stomatal flux over time. On the average, the stomatal flux was found to be approximately 50% of the total flux. This approach was compared to the currently used ozone risk assessment procedure based on the use of the exposure index AOT40, calculated from routine ozone concentration records. Important differences between these two approaches appeared. In order to assess ozone risk to vegetation in a realistic way, a new concept of “effective exposure”, which combines both exposure and uptake approaches, is developed. A new risk assessment index is obtained by weighting exposure by a factor which includes stomatal conductance, obtained by a modelling procedure, which uses observational data available from routine monitoring stations. This new index, formally similar to AOT40, uses a lower ozone concentration threshold (<20 ppb instead of 40 ppb) and follows much more closely the stomatal ozone dose evolution than the original AOT40 index. This approach permits to calculate ozone exposure taking physiological aspects into account, for situations where only ozone concentrations and meteorological observations are available, as direct flux data are difficult to obtain on a routine basis.