4.2. Annual statistics, 1995

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Air pollution in the European Union - Exceedance of ozone threshold values in 1995 and summer 1996 - Part I

4.2 Annual statistics, 1995

The geographical distribution of 98-percentile values calculated on the basis of hourly concentrations is presented in Map 3 for background stations and in Map 4 for urban, street and other stations.

Similar to the observations in 1994 (de Leeuw et al., 1995) the 98-percentiles at background stations show in general low values in the Scandinavian countries, and an increase from North-West to Central Europe. In particular for the stations in Austria, the elevated location of the monitoring stations may play a role. For summer 1993 and 1994 similar patterns have been estimated from measurements made within the framework of EMEP (Hjellbrekke, 1996). However, the relatively low values observed in the EMEP data over the Iberian peninsula are not fully supported by the present observations which might be caused by differences in year, in averaging period and in set of reporting stations.

For urban and 'other' stations (Map 4) no large scale concentration gradient in 98-percentiles is recognized; high values are observed all over the continent. The local conditions (at stations downwind of the urban area relatively high ozone values might be observed whereas at stations with NOx sources such as traffic in their immediate surrounding relatively low ozone levels will be measured, see Annex II) seems to be more important than European-wide smog episodes. For example, in Athens the observed 98-percentile values of hourly average concentrations ranges from 79 on station Patission to 173 µg/m3 on station Marousi. The interaction with (local) NOx emissions (see also Annex II) is shown in the observed NO2 concentration: at Patission the highest yearly averaged NO2 concentration (95 µg/m3) is observed whereas at Marousi one of the lowest NO2 yearly averaged has been observed (36 µg/m3).

The 98-percentile values based on moving eight-hourly average concentrations show a strong correlation with the hourly 98-percentile: on the average, 8-h percentiles are about 5% lower, see Figure 1. The geographical distribution of the 8-h percentile values is very similar to the distribution of the 1-h percentile values.

Figure 1. Relation between 98-percentiles values based on hourly concentrations and 8-hourly concentration at all reporting stations (in µg/m3; 1 January-31 December 1995).