4.6 Data reported for 1989-1995.

The data reports for previous years are available for various Member States: for Austria for the period 1993-1995, for Denmark and Finland information for 1992-1995 is available. For the period 1989-1995 data reports are available for four Member States (Belgium, Luxembourg, the Netherlands and United Kingdom), see Figure 12. In discussing the time series of 50 and 98 percentile, the reader must be aware that the network (number and location of stations, measuring method etc.) might have been changed during the years.

The year-to-year variation in 50-percentile values are relatively small when compared to the variations in the 98-percentile values (see Figure 12). Peak values of ozone are strongly correlated with temperature mainly because the conditions leading to high temperatures (e.g. strong solar radiation, low wind speeds, continental flows) also trigger photochemical formation. Meteorological fluctuations may cause variations in peak ozone levels that are much larger than the variations due to changes in precursor emissions. A yearly fluctuation of ca. 15% in 98-percentile value is not exceptional.

The yearly fluctuations differ from country to country. In Belgium, Luxembourg and the Netherlands similar patterns are found: after two "high" years (1989 and 1990) the period 1991-1993 shows relatively low peak values but increased levels are again observed in 1994. In contrast to Luxembourg and the Netherlands where peak values in 1995 are lower than in 1994, in Belgium and the United Kingdom the 98-percentile value even further increased in 1995.

The available time series are too short to detect any possible trend in ground level ozone concentrations in the EU. Trends in ozone concentrations are expected to result from trends in precursor emissions in Europe and from the increasing trend in hemispheric background concentrations (Borrell and van den Hout, 1995). The magnitude and even the sign of a possible trend will differ from location to location. In a study of trends in concentrations of ozone and related species in the Netherlands and nearby countries Roemer (1996) concluded that the ground level oxidant (sum of ozone and NO2) concentrations have decreased significantly in the Netherlands from 1981 to 1994 with an average decrease of about 1% per year. For Germany a slightly downward (northern part) or slightly upward (southern part) trend was noted but probably none of these trends is significant at the 95% confidence interval (Roemer, 1996). Based on the data reported here in the framework of the Ozone Directive, no conclusive answers can be given.

Figure 12. Range in reported 50-percentile values (in µg/m3, based on hourly concentrations) and 98-percentile values (in µg/m3, based on hourly concentrations) in Belgium, Luxembourg, the Netherlands and United Kingdom in the period 1989-1995.



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