Trends in tropospheric ozone concentrations and forest impact metrics in Europe over the time period 2000-2014

In Europe, tropospheric ozone pollution appears as a major air quality issue, and ozone concentrations remain potentially harmful to vegetation. In this study we compared the trends of two ozone metrics widely used for forests protection in Europe, the AOT40 (Accumulated Ozone over Threshold of 40 ppb) which only depends on surface air ozone concentrations, and the Phytotoxic Ozone Dose which is the accumulated ozone uptake through stomata over the growing season, and above a threshold Y of uptake (PODY). By using a chemistry transport model, we found that European-averaged ground-level ozone concentrations (-2%) and AOT40 metric (-26.5%) signifcantly declined from 2000 to 2014, due to successful control strategies to reduce the emission of ozone precursors in Europe since the early 1990s. In contrast, the stomatal ozone uptake by forests increased from 17.5 to 26.6 mmol O3 m-2 despite the reduction in ozone concentrations, leading to an increase of potential ozone damage on plants in Europe. In a climate change context, a biologically-sound stomatal fux-based standard (PODY) as new European legislative standard is needed.