1Max-Planck Institute for Chemistry, Mainz, Germany
2Moscow State University, Moscow, Russia
3Obukhov Institute of Atmosphere Physics RAS, Moscow, Russia
4International Center for Astronomical, Medical and Ecological Research NAS, Kiev, Ukraine
5Institute for Atmospheric and Climate Science, Swiss Federal Institute of Technology Zürich, Switzerland
6Paul Scherrer Institute, Villigen, Switzerland
Abstract. Long-term ozone measurements of two background mountain sites, namely the Kislovodsk High Mountain Station in Caucasus, Russia (KHMS, 43.70° N, 42.70° E, 2070 m a.s.l.) and the Jungfraujoch in Switzerland (JFJ, 46.55° N, 7.98° E, 3580 m a.s.l.) are compared. Despite of more than 1.5 km altitude difference ozone concentrations are comparable at JFJ an KHMS in the beginning of measurements (1990–1993) while the annually averaged levels at JFJ are around 15 ppb higher than the ones at KHMS for the most recent years (1997–2006). Averaged for different periods ozone concentrations at KHMS are comparable with the respective values observed at the elevated sites in the midlatitudes, situated in the altitude range 1600–2400 m a.s.l. Distribution function of the hourly concentrations has two peaks at JFJ and it is close to Gaussian distribution in the case of KHMS. Seasonality at both sites is characterized by double spring-summer maximum. Spring maximum at both stations is more pronounced for the air masses with the longest contact with upper free troposphere and stratosphere. Average concentrations increased at JFJ but decreased at KHMS for the period 1990–2006. Trends are more pronounced for the 1990s (+0.73±0.20 ppb/year at JFJ and −0.91±0.17 ppb/year at KHMS for the period 1991–2001) in comparison with later years (+0.04±0.21 ppb/year at JFJ and −0.37±0.14 ppb/year at KHMS for the period 1997–2006). Trends show a distinct seasonality, which is different for the different periods. To investigate possible reasons for this remarkable trends difference 3-D trajectories using LAGRANTO trajectory model are used. Effects of the horizontal and vertical transport on ozone trends are considered. In general we could not find any systematic changes in the transport patterns which could explain the significant changes of the trends between 1991–2001 and 1997–2006. It is likely that the position of the main emission source areas relative to the stations is among the main reason for the opposite surface ozone trends. During the 1990s the JFJ trend reflects increase of the ozone in the upper free troposphere/lower stratosphere. In contrary KHSM is much more influenced by dramatic emission decrease in the earlier 1990s in former USSR and emissions regulations in Western Europe. For later years ozone trends at KHMS are controlled by slight emission increase in the region, while trends at JFJ correspond to the scenario of European emissions control.