Ozone levels during summer 2009 were as low as during summer 2008 and according to several indicators were among the lowest since reporting of Europe-wide data commenced in 1997 (1).
Average temperatures in summer 2009 were generally higher than in 2008 and close to those measured in the extremely hot summer of 2003 when the highest number of exceedances in the last decade occurred. However, ozone formation is also influenced by other meteorological conditions and the chemical composition of the atmosphere. In Europe, a general observation is that the peak ozone concentrations decrease is very likely attributable to reductions in anthropogenic ozone
precursor gas emissions.
The number and spatial extent of exceedances of the information threshold (180 μg/m3, Directive 2002/3/EC) was lower than in any of the last ten summers — lower than the number of exceedances in summers 2007 and 2008, which were the third and second lowest respectively since 1997. As in most previous years, no exceedances of the information threshold value occurred in northern Europe. The highest one-hour ozone concentration of 284 μg/m3 was observed in France; for the first time since 1997, no concentration higher than 300 μg/m3 was reported.
As in all previous years, the Directive's long-term objective to protect human health (maximum ozone concentration of 120 μg/m3 over 8-hours) was exceeded in all EU Member States and other European countries. The target value for human health protection was also exceeded in a significant part of Europe. Nevertheless, both the percentage of Europe's population exposed to ozone levels above the target value and the number of occasions on which the long-term objective was exceeded were only slightly higher than in 2008 when the affected area and population were much more restricted than in the previous summers.
In contrast to previous summers, in 2009 there were no pan-European multi-day episodes. Summer 2009 was characterised by ozone episodes of two to five days followed by spells with few exceedances. A typical episode usually contained approximately 7–13 % of the total number of exceedances of the information threshold experienced during the summer.