State space analysis of changing seasonal ozone cycles (1988-1997) at Jungfraujoch (3580 m above sea level) in Switzerland

The 10-year (1988-1997) surface ozone record at Jungfraujoch (JFJ; 46 degrees -33 'N, 07 degrees 59 'E) in the Swiss Alps is analyzed with regard to changes in the seasonal cycle. For comparison, the surface ozone time series (1988-1996) at Zugspitze (ZUG; 47 degrees 25 'N, 10 degrees 59 'E, 2960 m above sea level) on the northern fringe of the Alps in Germany is also examined. The applied structural model in state space form is flexible enough to isolate a statistically significant decrease in the estimated seasonal component for May ozone concentrations over the last 10 years, in both the original JFJ and ZUG ozone records. A negative trend in May is also displayed by ordinary linear regression but is not statistically significant, demonstrating the advantage of the employed methodology over simple regression approaches in the analysis of changing seasonal cycles of tropospheric ozone records. We attempt to isolate potential mechanisms underlying the observed changes in the seasonal ozone cycles at JFJ by analyzing filtered ozone records. Sieving was carried out with wind direction, wind speed, and time windows to exclude the confounding influences from the atmospheric boundary layer and outflow from southern Europe. Taking only nighttime (2400-0500 central European time (CET)) ozone values into account, the seasonal ozone cycle at JFJ is characterized by a broad spring-summer maximum. This filter is believed to represent the lower free troposphere over the Alps and also shows a significant decrease in May ozone concentrations. It is suggested that the results may reflect ozone changes in the free troposphere over continental Europe.