Many studies investigating the negative impacts of ozone on biomass production and physiological functions have demonstrated the relationships between ozone exposure and reductions in both growth and physiological gas exchange. Such studies have led to an increasing interest in effects of ozone exposure expressed as a critical cumulative exposure of 10'000 ppb hrs above the threshold of 40 ppb (AOT40). Ozone effects on plants mainly depend on atmospheric transport and stomatal uptake. Thus ozone risk assessments should not only use measured ozone concentrations, but should also account for the influence of atmospheric conditions and soil moisture on stomatal conductance and non-stomatal ozone deposition. Following the Level II approach, our on-going studies aim to provide a model to estimate the ozone flux for forest ecosystems throughout Switzerland.