Free tropospheric ozone changes over Europe as observed at Jungfraujoch (1990-2008): An analysis based on backward trajectories

High-altitude ozone measurements at the Jungfraujoch observatory, Switzerland (JFJ, 3850 asl), covering the period 1990-2008 are investigated in this study. Anthropogenic ozone precursor emissions decreased over Europe and North America since the early 1990s. However, ozone concentrations at JFJ over the 19 year period show significant positive trends in the 1990s and no significant trends after 1999. Ozone trends were further studied with respect to air mass origin using 20 day back trajectories. The ozone increase during winter was particularly large in air masses with recent contact with the European planetary boundary layer (PBL), most probably in response to the steady decrease in European NO emissions leading to less ozone titration. On the other hand, the corresponding summer ozone increase was small, possibly attributable to the balancing effects of decreasing European ozone precursor emissions and increasing baseline ozone concentrations. Ozone from all source regions other than European PBL has a similar temporal pattern, exhibiting an increase for about the first 10 years, then leveling off to either no trend or insignificant decrease. This suggests that the physical processes determining the trend are beyond the time scale of the used backward trajectories or not described adequately by the simplified transport description in individual trajectories.