Monitoring of boundary layer ozone in Norway from 1977 to 2002.

An evaluation of the Norwegian ozone monitoring data is presented. A detailed investigation of the technical monitoring history was required to prepare a subset of data with acceptable data quality for long-term trend studies. These data indicated a reduction in the 99-percentile of the daily (daytime) ozone data in the southern part of the country in the summer half year of the order of 1 ppbv/year during the 1990-ies for some sites. For sitesfurther north a statistically significant increase in the mean ozone concentration of 0.3-0.5 ppbv/year was found for the winter half year. An increasing trend in the mean ozone concentration was found also in summer but less clear. Based on trajectory analyses a significantly increasing ozone trend in background air masses was found.