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

Fluxes of ozone as well as of sensible and latent heat were measured over a barleyfield in Northern Italyfrom April\r\nto June 2002 with the eddy-correlation technique in order to determine the dose of ozone taken up by plants during the\r\nwhole grain filling and maturation period. Stomatal ozone fluxes were then calculated byusing the similarity between\r\ngaseous exchange processes occurring inside the stomata involving ozone and water vapour, whose flux was available\r\nfrom the measurements. The dose was then obtained byintegrating the stomatal flux over time. On the average, the\r\nstomatal flux was found to be approximately50% of the total flux. This approach was compared to the currentlyused\r\nozone risk assessment procedure based on the use of the exposure index AOT40, calculated from routine ozone\r\nconcentration records. Important differences between these two approaches appeared.\r\nIn order to assess ozone risk to vegetation in a realistic way, a new concept of ‘‘effective exposure’’, which combines\r\nboth exposure and uptake approaches, is developed. A new risk assessment index is obtained byweighting exposure by\r\na factor which includes stomatal conductance, obtained bya modelling procedure, which uses observational data\r\navailable from routine monitoring stations. This new index, formallysimilar to AOT40, uses a lower ozone\r\nconcentration threshold (o20 ppb instead of 40 ppb) and follows much more closelythe stomatal ozone dose evolution\r\nthan the original AOT40 index. This approach permits to calculate ozone exposure taking physiological aspects into\r\naccount, for situations where onlyozone concentrations and meteorological observations are available, as direct flux\r\ndata are difficult to obtain on a routine basis.