4.3. Exceedances of thresholds in 1995

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4.3 Exceedances of thresholds in 1995

Exceedances of threshold values for protection of human health

The threshold value for protection of human health (110 µg/m3) is based on eight-hourly values. According to the Ozone Directive, four eight-hourly periods have to be considered: 0.00-8.00; 8.00-16.00, 16.00-24.00 and 12.00-20.00.

Based on the average diurnal profile of ozone (see Annex II) the highest eight-hourly values are generally expected for the 12.00-20.00 period; only exceedances of the threshold values for this period have been considered here.

In 1995 exceedances of this threshold value have been observed in all Member States; for Portugal no information on exceedances is available but according to maximum reported values (Table 5) one or more exceedances must have been observed. In 7 Member States maximum concentrations exceeding twice the threshold value have been observed (see Table 3).

Figure 2 shows the frequency distribution of eight hourly ozone concentrations in excess of the threshold value using so-called Box-Jenkins plots. For each Member State the Box-Jenkins plot indicates the minimum (here the minimum is of course 110 µg/m3), the maximum, the 25 percentile and the 75 percentile value of the concentrations during exceedance. Although extreme peaks of more than 200 µg/m3 are observed in 9 out of 14 reporting Member States, Figure 2 shows that, in each Member State, less than 25% of all exceedance concentrations are above 165 µg/m3 (that is, 150% of the threshold value).

  AT BE DE DK ES FI FR GB GR IE IT LU NL PT SE
#ex 4254 607 11429 69 1035 53 1966 441 538 86 1273 155 945 n.a. 68
#st 110 20 342 6 80 11 97 32 10 6 44 5 37 4 6
n.a = number of exceedances not available

Figure 2. Frequency distribution of ozone concentrations (eight-hourly values; period 12.00-20.00) in excess of the 110 µg/m3 threshold for hourly values. 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. The data for Portugal did not allow for inclusion in this figure.

The geographical distribution of the number of days the threshold value was exceeded is shown in Map 5 for background stations and in Map 6 for urban, street and other stations. A comparison of Map 5 and 6 shows that exceedances are more frequently observed at rural stations. This is also demonstrated more clearly in Figure 3 where for each station type (rural, urban and street stations) the averaged number of exceedances is plotted. The average occurence of exceedances generally decreases in the order rural - urban - street. Note that ŽŽotherŽŽ stations and stations for which the type has not beenspecified are not presented in Figure 3.

Figure 3. The occurrence of exceedances of the threshold value for protection of human health (110 µg/m3 for eight-hourly values) averaged over all reporting rural, urban, and street stations.

Figure 4. Number of exceedances (in days) and frequency distribution of urban population exposed to eight-hourly concentrations exceeding 110 µg/m3, 1 January -31 December 1995. Note that information on ozone exceedances which has been made available within the framework of the ozone directive, is estimated to be representative for a total urban population of 58 million (that is, 30% of the EU population living in cities with more than 25 000 inhabitants and 16% of the total population in the EU).

Of the total EU urban population, about 58 million people live in cities with one or more ozone monitoring stations reporting in the framework of the Ozone Directive. The number of days on which this population is exposed to ozone levels exceeding the 110 µg/m3 threshold values ranges from zero days (for 18% of the population, see Figure 4) to more than 50 days (for 9% of the population). On the average the EU city population is exposed to concentrations above the threshold during 1-2 consecutive days. Maximum episode lengths of 5-8 days have been reported.

These results should be interpreted with great care. The observations at the present set of urban stations might not be representative for the total urban population; only 30% of the urban population lives in a city "covered" by a ozone monitoring station, and information on urban situation is scarce or even lacking in some member states (in nine Member States the number of operational urban monitoring stations is three or less). Although more quantitative conclusions can not be drawn at this stage, it is likely that a substantial fraction of the urban population is frequently exposed to high ozone levels.

In rural areas the number of exceedances is higher than in urban regions, see Figure 3. It is estimated that in c. 90 % of the area for which information is available (see Map 2) the threshold value is exceeded at least on one day in 1995. The fraction of the rural population living in this exposed area can not be quantified as yet.

Exceedances most frequently occur in the summer months (May - August). In the winter months (January-March and September - December) exceedances have been observed very occasionally.

Map 5. Number of exceedances of the threshold value for protection of human health (110 µg/m3 for eight hourly values) observed at background stations; 1 January - 31 December 1995; eight hourly average values for the period 12.00-20.00.
Map 6. Number of exceedances of the threshold value for protection of human health (110 µg/m3 for eight-hourly values) observed at urban, street and other stations; 1 January - 31 December 1995; eight-hourly average values for the period 12.00-20.00.

Exceedances of the threshold values for information and warning of the population.

The threshold values for warning of the population (360 µg/m3, hourly value) has been exceeded in 1995 once at the station Coimbra in Portugal on 26 April 1995. An explanation of this exceedance can not be given.

The geographical distribution of the number of exceedances of the threshold value for information of the public (180 µg/m3, hourly value) is presented in Map 7 for background stations and in Map 8 for urban, street and other stations. Exceedances are observed in 14 Member States; only in Finland the 180 µg/m3 level has not been reached. Similar to the exceedances of the threshold values of 8-hourly average concentrations, the threshold values for public information more frequently occur at rural stations than at urban or street stations. For the urban stations an overview of the number of exceedances weighted according to the population of the city, is presented in Figure 5. From these limited data, it seems likely that about 60% of the urban population is exposed to ozone concentrations exceeding 180 µg/m3 for one or more days.

Figure 6 shows the frequency distribution of concentrations in excess of the threshold value. Although incidentally the threshold value may be exceeded by more than a factor of 2, in almost all of the cases the exceedances are less extreme: Figure 6 shows that nearly all Member States on 75% of the days on which the threshold value was exceeded, the level of 250 µg/m3 (that is 150% of the threshold value) has not been reached.

Exceedances are observed during a large part of the year but occur most frequently and geographically most wide-spread during the summer months. Figure 7 shows the total number of exceedances (summed over all reporting stations) per day. Note that in Figure 7 a large number of exceedances does not necessarily imply a large geographical extent of a smog-episode. The differences in spatial density of the monitoring networks in the Member States are so large that there will be no clear relation between total number of exceedances and geographical extent. After four winter months with only occasionally some exceedances, the first smog episode is observed in the beginning of May (in particular 5-7 May). After a second episode -mainly covering Germany - in the third week of May and a low-ozone period in the beginning of June, there is a two month period with frequent exceedances of the 180 µg/m3. In September-December only a few exceedances are observed.


Figure 5. Number of exceedances (in days) and frequency distribution of urban population exposed to hourly ozone cocnentrations exceeding 180 µg/m3, 1 January - 31 December 1995.

  AT BE DE DK ES FI FR GB GR IE IT LU NL PT SE
#ex 128 211 1804 6 40 0 374 99 185 13 295 43 257 18 2
#st 110 20 342 6 86 11 110 30 10 6 54 5 37 8 6

Figure 6. Frequency distribution of ozone concentrations (hourly values) in excess of the 180 µg/m3 threshold for hourly values. 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.

Figure 7. Total number of exceedances (summed over all 858 operational stations) of the ozone threshold value for information (180 µg/m3); 1 January - 31 December 1995. Note the logaritmic scale.

Map 7. Number of exceedances of the threshold value for information of the population (180 µg/m3 for hourly values) observed at background stations;1 January - 31 December 1995.
Map 8. Number of exceedances of the threshold value for information of the population (180 µg/m3 for hourly values) observed at urban, street and other stations; 1 January - 31 December 1995.

Exceedance of daily threshold value for protection of vegetation

As Table 4 shows, exceedances of the daily threshold of 65 µg/m3 have been frequently observed in all reporting countries. Stations where the threshold has been exceeded on more than 100 days in 1995 are located all over the EU. The geographical distribution of the number of exceedance of the daily threshold is presented in Map 9 for the background stations and on Map 10 for the urban and other stations. In Map 9 an attempt has been made to quantify the area where exceedances are observed. For the background stations a representative radius of 100 km is assumed, see also Map 2. When the "representative areas" of two or more stations overlap, the number of exceedances in this location is estimated by a distance-weighted interpolation. In less than 1% of the total area "covered" by the rural stations (1 460 000 km2) the 65 µg/m3 level is not exceeded. In 19% of the area the 65 µg/m3 level is exceeded on 1-75 days and in 74% of the area on more than 75 days. For 6% of the "covered" area no information is available. Exceedances during more than 150 days are reported both by northern and southern Member States (see Map 9).

For exceedances of a daily average concentration the differences between rural and urban stations are more pronounced than is the case for hourly concentrations, see Figure 8. In urban areas the low concentrations during the night (caused by interaction with NOx emissions) reduce the daily average concentrations; in rural areas the decrease in ozone concentrations during the night is generally less strong.

Figure 9 shows the frequency distribution of daily values in excess of 65 µg/m3. Although extreme values of more than 200 µg/m3 are observed, in nearly all Member States (except Luxembourg) for 75% of the exceedances the daily averaged concentrations falls between 65 and 98 µg/m3 (that is 150% of the threshold value). Although the exceedances are most frequently observed in the period March to September, see Figure 10, daily values above 65 µg/m3 are observed in all reporting Member States also during the winter months (January-February and October-December 1995). The low number of exceedances in June is ascribed to the cold and rainy weather conditions in NW Europe; a similar dip in June is found for the exceedances of other threshold values, see Figure 7.

Figure 8. The occurrence of exceedances of the threshold value for protection of vegetation (65 µg/m3 as daily value) averaged over all reporting rural, urban and street stations.


  AT BE DE DK ES FI FR GB GR IE IT LU NL PT SE
#ex 12245 911 25052 623 4212 1276 3949 1868 1032 705 2008 389 1646 467 832
#st 110 20 347 6 80 11 98 32 10 6 44 5 37 9 6

Figure 9. Frequency distribution of ozone concentrations (24h values) in excess of the 65 µg/m3 threshold for daily values. 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.

Figure 10. Total number of exceedances (summed over all stations) of the threshold value for protection of vegetation (65 µg/m3 for daily averages).

Map 9. Number of exceedances of the threshold value for vegetation (65 µg/m3 for daily values) observed at background stations; 1 January - 31 December 1995. Data is interpolated using inverse distance weighting and a cut-off distance of 100 km.
Map 10. Number of exceedances of the threshold value for vegetation (65 µg/m3 for daily values) observed at urban and unspecified stations;1 January - 31 December 1995.

Exceedance of hourly threshold value for protection of vegetation

Exceedances of the hourly threshold values for protection of vegetation (200 µg/m3) are reported by all Member States with the exception of Finland, see Table 4. The geographical distribution of the number of days on which this hourly threshold value has been exceeded is presented in Map 11 for background stations and Map 12 for urban street and other stations. The maps show that above ca. 600 N this threshold value has not been exceeded in 1995; in England, BeNeLux and Germany frequent exceedances are observed. On the Iberian peninsula no exceedances are observed.

In 61% of the rural regions where data from representative monitoring stations is reported (see Map 2) no exceedances of the 200 µg/m3 threshold value has been observed; in 26% of the area an exceedance is observed during 1-5 days wheras exceedances during more than 5 days occur in 13% of the mapped area.

The frequency distribution of exceedances of the hourly threshold are presented in Figure 11. For ca. 75% of the exceedances the ozone levels falls between 200 and 250 µg/m3 (that is 125% of the threshold value).

  AT BE DE DK ES FI FR GB GR IE IT LU NL PT SE
#ex 37 98 745 2 17 0 154 40 102 6 166 13 136 12 1
#st 110 20 342 6 86 11 110 32 10 6 57 5 37 8 6

Figure 11. Frequency distribution of ozone concentrations (hourly values) in excess of the 200 µg/m3 threshold for hourly values. 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.

Map 11. Number of exceedances of the threshold value for vegetation (200 µg/m3 for hourly values) observed at background stations;1 January - 31 December 1995. Data is interpolated using inverse distance weighting and a cut-off distance of 100 km.
Map 12. Number of exceedances of the threshold value for vegetation (200 µg/m3 for hourly values) observed at urban, street and other stations; 1 January - 31 December 1995.

   
 


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