Environmental issues are an important challenge for science in the 21. century.  The role and fate of ozone in the atmosphere is definitely one of them.  Unlike stratospheric ozone which acts as a shield for UV radiation enabling  life on Earth's surface and is endangered by production of certain  chemicals ( Montreal Protocol ) ozone in the troposphere is considered a pollutant, greenhouse gas, and health hazard.

 Ozone in the troposphere has different sources: diffusion and intrusions from the stratosphere together with lightnings are more important for the free troposphere, whereas photochemistry of nitrogen dioxide is the main source in the planetary boundary layer.

In addition to the formation processes in all these layers, ozone destruction as well as ozone transport is also important. It is indicated that in  the past hundred years ozone concentration near the ground has more than doubled and is still rising. Long-term measurements on many locations are underway to assess this trend.

We report here various approaches to analysis of 11 years of continuous ozone monitoring data from the station Puntijarka near Zagreb. Station Puntijarka is one of the Tropospheric Ozone Research (TOR) network measurement  stations within the EUREKA environment project EUROTRAC (1988-2002). The site has a rural character (45.90^o N; 15.97^o E, 980 m a.s.l.) and is representative for the lower troposphere of the whole region.