Ozone uptake by various surface types: a comparison between dose and exposure

The results obtained during several campaigns conducted at various places in Southern Europe and focused on ozone deposition on vegetation are analyzed and interpreted with the main scope of determining the relative contribution of stomatal uptake to the total ozone flux. The stomatal ozone fluxes are calculated using the analogy with water vapour fluxes inside the stomata. By integrating stomatal fluxes over time, ozone doses received by the plants are calculated. At most sites, the stomatal ozone flux accounts <50% of the total flux, except for those sites located at the northern edge of the geographical domain studied. Sites characterized by a rather dry Mediterranean climate generally show reduced stomatal fluxes, even in the case of high ozone concentrations. The role of water supply, influencing stomatal activity, appears essential. This approach, using time integration of ozone stomatal fluxes, yielding ozone doses to vegetation, is then compared with the use of the exposure index AOT40, widely used in ozone risk assessment work. A fairly good correlation exists between both approaches, but the latter gives no indication about the physiological uptake processes.