Trends in ground level ozone concentrations in the European Union

Following the Council Directive 92/72/EEC on air pollution by ozone the Member States of the European Union have to inform the European Commission on ozone concentrations and exceedances of threshold values within their territory. Using the available information covering the period of 5 years (1994–1998), the data has been analysed for a possible trend in statistical parameters (50- and 98-percentiles) and number and severity of exceedances. Time series are relatively short but the data suggest that there might be a small increasing trend in the 50 percentile values. The ozone peak values, expressed as 98-percentile values or as number of exceedance days tend to decrease. However, these conclusions must be interpreted carefully as on the short time scales considered here meteorological variations and inter-annual changes may play an important role. The decrease in peak values is most likely caused by the decrease in European ozone precursor emissions since 1990; insufficient data is available to explain the increasing 50-percentile values. Possible explanations are an increase in tropospheric ozone background values caused by a world-wide increase in CH₄, CO and NO_x emissions or a reduced ozone titration by reduced NO_x emissions on the local scale. The data submitted under the ozone directive is insufficient to provide firm conclusions on this point.