3 SUMMARY OF DATA REPORTED FOR SUMMER 1996

3 SUMMARY OF DATA REPORTED FOR SUMMER 1996

The threshold for warning of the public (1h> 360 µg/m3) was exceeded at three stations during summer 1996 (Table 3):

Table 3: Observed exceedances of the threshold for warning of the public (1h concentration > 360 µg/m3) during summer 1996 (April-July).

Both the Athens and Firenze (Florence) episode will be described in more detail in Chapter 4. Table 4 presents a general overview of the observed exceedances of the threshold for information of the public during April-July 1996 on a country by country basis. As the number of stations differs widely from country to country, the absolute number of exceedances are less suitable for comparison. As in the annual ozone report [4], the concept of 'occurrence of exceedances' is used here. Occurrence of exceedances is defined as the total number of exceedances summed over all stations divided by the number of stations.

Table 4: Summary of exceedances of the threshold for information of the public (1h concentration > 180 µg/m3) during summer 1996 (April-July) on a country by country basis.

In northern and western Europe, the weather conditions during the 1996 summer season were on average less favourable for the formation of ozone then during the 1995 summer season. This year, cool and relatively clean Atlantic air masses prevailed on many days. In 1995 a number of heat-waves were recorded and especially July was very hot and sunny. As a result, less exceedances were reported this summer (April-July) than in the same period last year. In the Mediterranean region of southern Europe, frequent exceedances were observed during the stable, warm and sunny summer months.

Ireland was the only Member State where the threshold for the information of the public was not exceeded. In other countries, the number of days on which at least one exceedance was observed ranged from one in Denmark to 41 in France. 48% of all stations reported one or more exceedance. During the same period in 1995, 59% of all stations reported one or more exceedance. On average 3.1 exceedances occurred this year on stations which recorded at least one exceedance. During summer 1995 on average 5.0 exceedances were recorded. The average maximum hourly concentration during an exceedance of the threshold this year was 200 µg/m3 which is slightly lower than last year (EU average 1995: 203 µg/m3). An exception is Greece, where the average maximum rose from 219 to 235 µg/m3. The average length of an exceedance period was 2.9 hours (3.5 hours in 1995). However, from Table 4 it is clear that there is quite a large difference in the average episode length between the Member States.

Table 5 summarises the exceedances on a month by month basis. June had the highest number of stations reporting exceedances and the highest occurrence. This is mainly due to one ozone episode in the beginning of June in NW-Europe (see also Chapter 5).

Table 5: Summary of exceedances of the threshold for information of the public (1h concentration > 180 µg/m3) during summer 1996 (April-July) on a month by month basis.

In Figure 1 the number of days per month is presented on which at least at one station in a country was recorded an exceedance. Again, the relative abundance of exceedances in June is striking. Figure 2 presents the occurrence of exceedances per country on a month by month basis. No monthly pattern is apparent. Exceedances in June were observed on many stations, the occurrence in this month is not significantly different from that in other months. Exceedances in Greece and Italy were observed on a limited number of stations; this explains the relative high occurrence of exceedances in these countries.

Figure 1: Number of days on which at least one exceedance of the threshold value for information of the public (1h concentration > 180 µg/m3) was observed per country and per month during summer 1996.

Figure 2: Occurrence of exceedances (in days) of the threshold value for information of the public (1h concentration > 180 µg/m3) per country on a month by month basis during summer 1996.

The average occurrence of exceedances (in days) in each country of the threshold for information of the public by station type (rural, urban and street) is presented in Figure 3. The occurrence of exceedances generally decreases in the order rural-urban-street. Stations for which the type was not specified are not presented in this figure.

Figure 3: Average occurrence of exceedances (in days) of the threshold for information of the public (1h concentration > 180 µg/m3) by station type (rural, urban and street) and country.

Figure 4 shows the frequency distribution of hourly ozone concentrations in excess of the threshold value using Box-Jenkins plots. For each Member State the Box-Jenkins plot indicates the minimum (here the minimum is 180 µg/m3), the maximum, the 25 percentile and the 75 percentile value of the exceedances. The figure shows that during 25% of all observed exceedances, the maximum hourly concentration recorded was just above the 180 µg/m3 threshold. 75% of all maximum exceedances recorded were below 208 µg/m3. During the same period last year, the 75 percentile was ca. 6% higher.

Figure 4: Frequency distribution of ozone concentrations in excess of the 180 µg/m3 threshold for hourly values (April-July 1996). For each country the total number of observed exceedances is given in row '#Ex', the number of stations is given in row '#St'. Frequency distributions are presented as Box-Jenkins plots indicating the minimum, the 25-Percentile, the 75-percentile and the maximum value.

Geographical distribution

Maps 2 and 3 show the geographical distribution of the number of days on which the threshold value for information of the public was exceeded for urban and background stations, respectively. Exceedance data for urban stations are presented as dots. The exceedance data for rural stations are interpolated using simple inverse distance weighting and a tentatively estimated 'radius of representativeness' of 100 km. Note that this radius might be different for the various regions in Europe. Calculation should be based on more detailed analysis of the ozone phenomenology at the stations in future.

No simple spatial structure is visible as far as exceedances at urban stations are concerned. Note that information for July from Spain is missing, that the reporting period is only 4 months and stations of unspecified type are also plotted in this map. The interpolated exceedance map for rural locations is too fragmented to draw firm conclusions on spatial patterns. The rural stations cover approximately 47% of the EU territory. The area where exceedances were observed, calculated on basis of the interpolated map, is approximately 48% of the total area covered by background stations which is estimated at 23% of the total EU territory.

Map 2: Number of exceedances of the threshold value for the information of the public (1h > 180 µg/m3) observed at urban stations and stations of unspecified type. Summer 1996 (April-July).

Map 3: Number of exceedances of the threshold value for the information of the public (1h > 180 µg/m3) observed at background stations. Summer 1996 data (April-July), interpolated using inverse distance weighting and a cut-off distance of 100 km.