Ground level ozone concentrations and exposures from 2000 to 2013

Tropospheric ozone (O3) concentrations from passive samplers have been monitored according to harmonized methodologies on ICP Forests intensive monitoring (Level II) sites since the year 2000 with the aim to (i) quantify ozone concentrations during the vegetation period (April-September), (ii) estimate the related ozone exposures of forest ecosystems, and (iii) detect temporal and spatial trends across Europe. Season mean ozone concentrations ranged from 19 to 64 ppb with similar deviations from the median among the countries. A decreasing south-north gradient across Europe is apparent with the highest concentrations having been measured in Italy, southern Switzerland, the Czech Republic, Slovakia, and Greece. Mean AOT40 for 2000-2013 ranged from 2 to 67 ppm h. The AOT40 threshold of 5 ppm h set to protect forests from adverse ozone effects was exceeded in 18 out of 20 countries. An overall trend analyses, including all April–September data from 20 countries, revealed a significant decrease of 0.35 ppb ozone per year (n = 29 356; p = 0.000) from 2000 to 2013. When considering only sites with a data coverage of at least 4 years and 120 days (66%) between 1 April and 30 September, site-specific trend analyses did not reveal any uniform patterns across Europe.