[en] Among air pollutants, ozone is the most important stressor to vegetation, which undergoes damage and biomass reduction after penetration of ozone molecules into the leaf tissues through the stomata. Stomatal ozone fluxes are considered the governing factor needed to assess risk to plant health due to ozone. Although this parameter may be calculated by modeling, direct measurements are scarce. Moreover, southern European situations, especially regarding Italy, require special attention due to the decoupling between ozone concentrations and fluxes. This work reviews ozone flux measurements made during the last 15 years through Italy. - The dependence of ozone fluxes on environmental factors such as water supply is stronger than the dependence on the surface covering species.

[en] This paper presents the physical mechanisms that govern the dispersion and transport of air pollutant; the influence of the state of the 'carrying fluid', i.e. the role of meteorology; and finally, outlines the different techniques of assessing the process. Aspects of physical mechanisms and meteorology covered include: fate of an air pollutant; turbulence and dispersion; transport; wind speed and direction; atmospheric stability; and the role of atmospheric water. Assessment techniques covered are: concentrations measurements; modelling meteorological observations; and tracer releases. It is concluded that the only way to reduce air pollution is to pollute less. 10 refs., 12 figs., 2 tabs

[en] Stomatal O₃ fluxes to a mixed beech/spruce stand (Fagus sylvatica/Picea abies) in Central Europe were determined using two different approaches. The sap flow technique yielded the tree-level transpiration, whereas the eddy covariance method provided the stand-level evapotranspiration. Both data were then converted into stomatal ozone fluxes, exemplifying this novel concept for July 2007. Sap flow-based stomatal O₃ flux was 33% of the total O₃ flux, whereas derivation from evapotranspiration rates in combination with the Penman-Monteith algorithm amounted to 47%. In addition to this proportional difference, the sap flow-based assessment yielded lower levels of stomatal O₃ flux and reflected stomatal regulation rather than O₃ exposure, paralleling the daily courses of canopy conductance for water vapor and eddy covariance-based total stand-level O₃ flux. The demonstrated combination of sap flow and eddy covariance approaches supports the development of O₃ risk assessment in forests from O₃ exposure towards flux-based concepts. - Combined tree sap flow and eddy covariance-based methodologies yield stomatal O₃ flux as 33% in total stand flux.

[en] Upcoming decades will experience increasing atmospheric CO₂ and likely enhanced O₃ exposure which represents a risk for the carbon sink strength of forests, so that the need for cause-effect related O₃ risk assessment increases. Although assessment will gain in reliability on an O₃ uptake basis, risk is co-determined by the effective dose, i.e. the plant's sensitivity per O₃ uptake. Recent progress in research on the molecular and metabolic control of the effective O₃ dose is reported along with advances in empirically assessing O₃ uptake at the whole-tree and stand level. Knowledge on both O₃ uptake and effective dose (measures of stress avoidance and tolerance, respectively) needs to be understood mechanistically and linked as a pre-requisite before practical use of process-based O₃ risk assessment can be implemented. To this end, perspectives are derived for validating and promoting new O₃ flux-based modelling tools. - Clarifying and linking mechanisms of O₃ uptake and effective dose are research challenges highlighted in view of recent progress and perspectives towards cause-effect based risk assessment

[en] Ozone (O₃) 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 O₃ on forests and crops is controversial. Possible reasons include (i) ability of response indicators to provide an unbiased estimate of O₃ 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 O₃ 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 O₃ 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 O₃ in Italy over recent years to discuss value, uncertainty and feasibility of different approaches to risk assessment. - While the flux-based approach is scientifically sounder, a more practical concentration-based approach is still necessary for routine monitoring