3. Summary of data reported for summer 1997

3. SUMMARY OF DATA REPORTED FOR SUMMER 1997

The threshold for warning of the public (1h> 360 µg/m³) was exceeded at one station during summer 1997 (Table 3):

Table 3: Observed exceedance of the threshold for warning of the public (1h concentration > 360 µg/m³) during summer 1997 (April-August).

During summer 1996 the threshold for warning of the public was also exceeded in the Athens conurbation, although at different stations. The Athens episode will be described in more detail in Chapter 5. Table 4 presents a general overview of the observed exceedances of the threshold for information of the public during April-August 1997 on a country by country basis¹. 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 (De Leeuw and van Zantvoort, 1997) 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 1997 (April-August) on a country by country basis.

^I Number of stations implemented in the framework of the Ozone Directive
^II The number of days on which at least one exceedance was observed
^III Left figure: averaged over all implemented stations, right figure: averaged over all stations which reported at least one exceedance.

Finland, Denmark, Ireland and Sweden did not observe exceedances of the population information threshold this summer. In other countries, the number of days on which at least one exceedance was observed ranged from 5 in Luxembourg to 49 in Italy. 41% of all stations reported one or more exceedance. On average 3.4 exceedances occurred this year on stations which recorded at least one exceedance. The average maximum hourly concentration during an exceedance of the threshold this year was 198 µg/m³.

Table 5 summarises the exceedances on a month by month basis. In the Mediterranean region of Southern Europe frequent exceedances were observed during the stable, warm and sunny summer months. April, May, June and July were on average unfavourable for the formation of ozone in Northern and Western Europe. August was a very hot and sunny month in large parts of Western and Northern Europe resulting in more favourable conditions for the formation of ozone. As a result, August had the highest number of stations reporting exceedances.

Table 5: Summary of exceedances of the threshold for information of the public (1h concentration > 180 µg/m³) during summer 1997 (April-August) on a month by month basis.

^I The theoretical maximum is 932 stations (all stations which are assumed to be operational during summer 1997 and for which data was transmitted).
^II Left figure: averaged over all implemented stations, right figure: averaged over all stations which reported at least one exceedance.

In Figure 1 the number of days per month is presented on which an exceedance was recorded at least at one station in a country. Again, the relative abundance of exceedances in August in Western Europe is striking. Spain, Italy, Greece and Portugal recorded most exceedance days in July. Figure 2 presents the occurrence of exceedances per country on a month by month basis. No monthly pattern is apparent. Exceedances in Greece and Portugal were observed at a limited number of stations; this explains the relatively 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 1997. Note that data from France is missing, Finland, Denmark, Sweden and Ireland did not report exceedances.

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 1997. Note that data from France is missing, Finland, Denmark, Sweden and Ireland did not report exceedances.

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 excluded from 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 during summer 1997. Note that data from France is missing, Finland, Denmark, Sweden and Ireland did not report exceedances.

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 206 µg/m³.

Figure 4: Frequency distribution of ozone concentrations in excess of the 180 µg/m³ threshold for hourly values (April-August 1997). 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.

3.1 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 urban² and background stations, respectively. Exceedance data for urban stations are presented as dots. Note that information for France is missing and that stations of unspecified type are also plotted in this map. 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.

The number of days on which exceedances were observed at urban stations and stations of unspecified type in Northern and Western Europe ranges from 0 in Finland, Sweden, Denmark, Ireland and large parts of Austria to more than 10 on some stations in South-Western Germany. No consistent spatial pattern is apparent in the Mediterranean region. Some stations did not report any exceedance, other stations in Italy and Greece reported more than 10 exceedances.

Map 2: Number of exceedances of the threshold value for the information of the public (1h > 180 µg/m³) observed at urban/street stations and stations of unspecified type. Summer 1997 (April-August).

Map 3: Number of exceedances of the threshold value for the information of the public (1h > 180 µg/m³) observed at background stations. Summer 1997 data (April-August), interpolated using inverse distance weighting, cut-off distance of 100 km.

3.2 Comparison with previous years

Exceedances observed during the 1997 summer period were compared to exceedances observed during the same period in 1996 and 1995³. France did not report exceedances of the population information threshold in 1997 and was left out of the analysis to ensure comparability between the years. Italy was not taken into account in the analysis because 1996 data was incomplete and inconsistent.

Figure 5.a presents the average exceedance duration⁴ of the population information threshold, Figure 5.b the average occurrence and Figure 5.c presents the average maximum concentration observed during exceedances.

All indicators show lowest values in 1997 compared to 1996 and 1995. Since most ozone stations are operational in Northern and Western Europe (ca. 70% in 1997) exceedance statistics are strongly influenced by the meteorological conditions in this area. For example, in 1995 Germany, Netherlands and Belgium reported 81% of all exceedances.

In 1995 a number of heat waves were recorded in northern and western Europe and conditions were favourable for high ozone levels on many days. During summer 1996, relatively clean and cool Atlantic air masses prevailed on many days in northern and western Europe. Subsequently, the number and duration of exceedances as well as the maximum concentrations observed were lower than in 1995. Relatively clean and cool Atlantic air masses also prevailed during April-July 1997 and hardly any exceedance was observed in northern and western Europe.

August 1997 was very hot and sunny in large parts of northern and western Europe⁵. The number and duration of exceedances as well as the maximum concentrations observed during exceedances were however much lower as those observed during other recent hot and sunny months. In Ireland, Finland, Sweden and Denmark the threshold was even not exceeded.

First analysis has shown that the relatively low number of exceedances in August might be explained by the position of the dominating high pressure cell. The high pressure cell was, on average, situated above Scotland - southern Norway resulting in a NE circulation above large parts of northern and western Europe. Relatively clean air masses originating from the Baltic area and western Russia dominated above northern and western Europe. Due to subsidence the air could be characterised as dry (sunny) and hot.

During other warm and sunny months in recent years, the dominating high pressure cell was located (on average) over the continent resulting in the transportation of relatively polluted air to western and northern Europe with a E-SE circulation and more situations in which air masses became stagnant.

Figure 5.a: Average duration in hours of exceedances during summer 1995, 1996 and 1997 (April-August, based on 13 Member States).

Figure 5.b: Average occurrence (nr) of exceedances at stations which reported at least one exceedance during summer 1995, 1996 and 1997 (April-August, based on 13 Member States).

Figure 5.c: Average maximum concentration (µg/m³) observed during exceedances (April-August, based on 13 Member States).

It is difficult to assess a possible trend in the number, duration and severity of exceedances of the threshold for information of the public which in its turn could indicate a possible trend in precursor emissions, because:

• High ozone levels (in this case exceedances of the population information threshold) are mainly observed during periods with warm and sunny weather. Especially in western and northern Europe, the year-to-year variations in meteorological conditions favourable for high ozone levels are large. The resulting variations in exceedance statistics can obscure a possible trend due to changes in precursor emissions. It is at the moment not possible to correct for this variability on a country by country basis nor for the complete EU territory;
• Exceedances statistics are available for only three years which is a very short time series to assess a possible trend;
• The number of stations implemented in the framework of the Ozone Directive increased by ca. 20% over the last three years. The increased territorial coverage can have implications for the number of exceedances observed. Also, a changing ratio between the number of rural and urban/street stations can have implications for the number of observed exceedances since peak ozone levels will on average be lower in urban areas than in rural areas.

¹ Note that in all tables and figures presented, information for France is not available.

²Exceedances reported from stations of unspecified type are also plotted in this map.

³ 1995, 1996: Validated exceedance statistics as transmitted by Member States were used for this purpose.

⁴ Averaged over all stations which reported at least one exceedance.

⁵ For example, August 1997 was the hottest August since the beginning of measurements in 1706 in the Netherlands.