5.1 Monitoring stations + 5.2 Summary of data reported

5. Information reported to the Commission under Council Directive 92/72/EEC in 1994-1996

5.1 Monitoring stations

Table 5 presents a summary of the number of sites from which data was transmitted by the Member States covering their 1995 measurements (de Leeuw and van Zantvoort, 1996). The table also consists of an overview on meta-information attached to the data reports. Figure 9 shows the locations of the stations and Figure 10 presents the different types of stations. Data for 1994 were submitted by 15 Member States for 782 monitoring sites. However, the data was complete from 10 countries only (de Leeuw et al., 1995; van Zantvoort and Sluyter, 1995). For 1996 data from 836 stations within 13 countries was reported (De Leeuw and van Zantvoort, 1997). From Figure 9 it is obvious that the spatial coverage of the network needs improvement in several countries and Figure 10 reveals that gaps in the geographical coverage can be noted in both urban and rural monitoring. Some countries have not submitted data from rural sites, whereas others interpret the concept of urban monitoring to address only 'street' stations.

Table 5: Overview of the parameters and number of sites from which ozone data for 1995 were reported by Member States (de Leeuw and van Zantvoort, 1996)

These data are intended to support the Commission in its aim to:

• assess regions with (un)acceptable air quality;
• promote the state of knowledge concerning the harmful effects of ozone and its impact on human health and the environment;
• build up knowledge on effective and appropriate measures to reduce pollution by ozone;
• monitor trends in ozone and the impact of national and Community provisions to reduce ozone precursors.

These objectives all centre around one basic issue: "Does the network sufficiently cover the areas of high and typical air pollution exposure of the population and vegetation?" We have performed three calculations to give an indication of the spatial representativeness of the stations from which data and classification were reported by the countries on their 1995 ozone measurements. This analysis is based on measurement stations only and it does not include supplementary information from assessment methods other than monitoring which may be used in several countries. These additional assessments may significantly enlarge the representativeness of the various ozone networks. In two calculations each rural site was given a radius of spatial representativeness of 50 km and 100 km respectively. The land cover from broad-leaved and coniferous forest and arable land within 50- and 100-km radius circles centred on each station were calculated and summed per country. In a third calculation a representative radius of 10 km was used for urban/street sites to estimate the number of people living in a city where ozone monitoring is taking place. This 'radius of spatial representativeness' is expected to vary among different regions in Europe. It is probably dependent on local conditions such as the wind direction, land-use, the vicinity of NO_x emissions and possibly other factors. These calculations should therefore be considered as tentative estimates.

The computations indicate that the spatial coverage of the 1995 ozone monitoring network is insufficient to appraise the ozone situation over all Member States. The current subset of rural/background stations in the EU15 countries is estimated to cover a maximum of 20 - 40% of forests and 30 - 50% of crops, both depending on the chosen radius of representativeness of the observatories. Even if a radius of 100 km is used, the coverage of arable land is below 25% for four Member States. This number increases to eight countries if a 50 km radius is used. The subset of urban/street sites covers no more than 12% of all EU15 residents and approximately 25% of EU15 city dwellers (city defined as more than 50,000 inhabitants).

Although the Directive states that: "Member States shall provide the Commission with [...] a description of the area covered by the stations, and the site-selection criteria [...]" this has not resulted in effective provision of information. The current labelling as 'rural', 'urban', 'street' and 'other' without any spatial indication or explanation on a station's local environment is insufficient. In future our estimates may be improved if better documentation on the spatial representativeness of individual stations is made available by the Member States. Countries may consider to supplement their data reports with information on locations where similar air pollution situations may be expected.

5.2 Summary of data reported

Table 9 to 13 in Appendix 2 present the occurrence of exceedances of the various thresholds during 1994, 1995 and 1996. The tables are compiled from annual ozone reports prepared for the Commission (De Leeuw et al., 1995; De Leeuw and van Zantvoort, 1996, 1997). These tables present the average number of exceedances observed per country; i.e. the total number of exceedances summed over all monitoring stations in a country divided by the total number of sites in that country. The exceedances over the population warning threshold are not included, because this threshold is only exceeded a handful of times annually. The tables in Appendix 2 show that most thresholds are exceeded in all EU15 Member States every year. However, hourly peak concentrations in excess of the population information and the vegetation protection thresholds are rarely, or never, observed at Finnish, Danish and Irish stations during the period covered by the data. The health protection threshold, in particular 8 hr average for the period 12:00 and 20:00 hours, has on the average been exceeded during more than 4 weeks (not necessarily consecutively) in the networks of 3-6 countries annually.

Table 6 and 7 present reported percentile statistics and maximum observed concentrations, based on 1-h and 8-h moving averages in 1994, 1995 and 1996. In these tables the lowest and highest values observed at individual stations with a data coverage of 75% or more is shown. Note that the networks in various countries may have changed during the three years covered. The minimum and maximum values do not necessarily refer to the same sites.

5.2.1 Interannual variation

The time series reported in the framework of the Ozone Directive (1994 - 1996) are too short to draw conclusions on trends and the magnitude of interannual variation. Furthermore, co-located data on NO_x are not available, which makes it hazardous to resolve trends in ozone. The quantity of O_x (NO₂ + O₃) is often a convenient parameter to overcome the disturbing influence of local emissions of NO.

Percentile data from 7 north-western Member States covering a longer period (maximum 7 years) were available to De Leeuw and van Zantvoort (1997) and these allow us to briefly consider the interannual variation. De Leeuw and van Zantvoort (1997) concluded that the interannual variation in the 50th percentile values was relatively small (2-7 μg.m^-3 on a 50th percentile value varying between 14 and 67 μg.m^-3) in comparison to the variation in the 98th percentile values (variation of 11-18 μg.m^-3 for 98th percentile values ranging from 76 to 145 μg.m^-3). In north west Europe peak concentrations in ozone, are reflected by the 98th percentile values, show a strong correlation with high temperatures, low wind-speed and anticyclonic conditions. Due to the considerable interannual variability in the occurrence of these conditions in north-west Europe the 98th percentile values also show a large year-to-year variation.

Figure 9: Location of monitoring sites from which the data were reported for 1995 in the framework of the Ozone Directive. Source: de Leeuw and van Zantvoort, 1996

Figure 10: Classification of rural, urban, street or other/unspecified stations in each Member State as reported for 1995

Table 6: Range in 50-, 98- and 99.9-percentile values and maximum measured concentrations (based on 1-h average) observed at individual monitoring stations in Member States (μg.m^-3) in 1994, 1995 and 1996 (na = no information available; ? = data not submitted)

^*Additional information submitted on a voluntary basis.

Table 7: Range in 50-, 98- and 99.9-percentile values and maximum measured concentrations (based on moving 8-h average) observed at individual monitoring stations in Member States (μg.m^-3) in 1994, 1995 and 1996 (na = no information available; ? = statistics on 8-h average concentrations not submitted)