7. Discussion and conclusions

7.1 The Ozone Directive and 5EAP

This report deals with the status of pollution by tropospheric ozone in the European Union in the perspective of the EU Council Directive on air pollution by ozone (92/72/EEC) and the Fifth Environment Action Programme (5EAP). The report was announced under Article 8 of the above Directive which states that "the Commission shall submit [...] a report on the information collected and on the evaluation of photochemical pollution in the Community". The discussion and conclusions in this chapter will therefore mainly address these two issues.

7.2 Information collected

The Ozone Directive defined concentration thresholds for ozone, it established a harmonised procedure for monitoring and exchanging data and it also arranged to inform and warn the population if certain thresholds were exceeded. Member States were encouraged to report earlier information. In addition the Directive required measurements of NOx at particular stations, whereas it only recommended VOC measurements at these sites.

In addition to the above arrangements the Directive aimed:

  1. to assess the individual risk of exposure of the population to values in excess of the health protection thresholds of ozone;
  2. to assess the exposure of vegetation to the vegetation related thresholds of ozone;
  3. to optimise the action needed to reduce ozone formation;
  4. to obtain exceedance time series to assist the progress of assessment of trends;
  5. to obtain information on concentrations of VOCs and NOx.

In order to facilitate collection of data to support production of these assessments the Directive put forward station siting criteria: i.e. ozone monitoring stations located at geographically and climatologically representative sites where the risk of approaching or exceeding threshold values is highest and where it is likely that either the population or vegetation is exposed.

In relation to 1), 2), we conclude from the information reported so far that the risk and exposure assessment objectives can be fulfilled only partially. This should not mask the fact that the information made available under the Directive is a major leap forward in comparison to the pre-Directive period. However, the spatial coverage of the ozone monitoring network still requires improvement in many countries. Gaps in geographical coverage can be noted in both urban and rural monitoring. The current subset of rural/background stations 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 observations. 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 having more than 50,000 inhabitants). This conclusion does not necessarily imply a call for 100% coverage of all people, crops and ecosystems by very dense monitoring networks in the EU15. It does, however, emphasise that knowledge on the representativeness of current stations at the European level needs to be improved. Although the Directive states that: "Member States shall provide [...] a description of the area covered by the stations, and the site-selection criteria [...]" this has not resulted in an adequate level of information provided in this matter. In the future assessments of risk and exposure may be improved if better documentation on the spatial representativeness of individual sites is made available by the Member States. This could be supplemented by information on the number and extent of locations where a similar situation and air quality occurs.

In relation to 4), for compliance with legal requirements of the Directive and also for the scientific community there is a need for good quality data in order to address the question of trends in ozone and ozone episodes. Current trends in concentrations are uneven: there appears to be increases in some rural zones, stability or reductions in others. The question rises whether reductions in concentrations can be directly linked to decreases in emissions of ozone precursors. The 1994 - 1996 time series reported in the framework of the Directive are too short to assess any trend. However, information covering a longer time period has been acquired from seven Member States. Four countries reported information covering 1989 - 1996. This is very valuable data despite the fact that the composition of networks has changed in these countries and monitoring practices may also have been altered.

The issue of linking decreases in ozone to abatement of emissions of precursors is hampered because coincident data on concentrations of NOx are not available. Due to the rapid reaction between ozone and NO a change in the ozone concentration can easily be caused by a change in NO. The quantity of Ox (= O3 + NO2) is often a convenient parameter to assess the influence of fresh emissions of NO through the years. We therefore recommend co-located measurement of ozone and NOx, in particular at stations closely situated to sources of NOx.

In relation to 5), statistics on VOCs were reported by only a few countries. Although this limited amount of VOC data helps in the description of the formation and transport of ozone and its source species, it is nevertheless insufficient to conclude on the situation in all Member States. NOx and NO2 statistics were transferred by about half of the Member States. This data archive provides information relevant to health effects, but helps only marginally in understanding the climatology of ozone.

The experiences with public information required by the Directive are rather positive in many countries. The Directive defined air quality information and warning thresholds, obliging Member States to disseminate information to the public as soon as any exceedance was observed. The information that was issued received considerable attention from the media, and the increased public awareness has brought the ozone problem higher on the list of political priorities.

The question whether the population information threshold is of any real benefit to the public is difficult to answer. We addressed this question by comparing how often the health protection threshold was exceeded without the population information threshold being reached. (Note that the exceedances of these thresholds are not necessarily exceeded at the same sites during the same period). The data indicates that the ratio between the number of exceedances of the two thresholds has been higher than 3 in all EU15 countries and higher than 10 in Sweden, Denmark and Finland.

The quality and quantity of data reporting has improved in the period 1994 - 1996. The improved formats for reporting have resulted in good harmonisation of the data reports although deviations from the format still occur. Unfortunately, it still happens that particular countries completely fail to transmit their data within the time schedule set in the Directive.

7.3 Evaluation of photochemical pollution and emissions of precursors

From the measured data it is concluded that both the EU and WHO thresholds set for the protection of human health are exceeded substantially and in all Member States. We estimate that more than 90% of EU citizens were exposed to an exceedance of the threshold at least once in 1995. More than 80% of these people experienced more than 25 exceedances.

The current data set derived from monitoring locations in the urban environment is representative of only a limited fraction of the European population; we estimate 41 million people.

The number of additional hospital admissions attributable to ozone exposure in concentrations exceeding the threshold for protection of human health is estimated to be over 80 cases per year. If this pattern is extrapolated to full country populations, and to all 15 EU countries then we assess that about 700 hospital admissions could have been attributed to exceedances of this ozone threshold (110 mg.m-3 8-h mean). However, the real impact of the pollution is greater because many effects arise below the current health protection threshold. A conservative estimate, suggests that these impacts may have led to over 3000 additional admissions in the full population of 15 EU countries.

The threshold value for providing information to the public has been exceeded in almost all Member States during a limited number of days, but rarely or never at Finnish, Danish or Irish stations. In 1995 this concerned about 31 million Europeans, which approximates 45% of the urban population living in cities with operational monitoring in that year. The warning threshold is reached occasionally, in particular in the southern Member States.

The EU15 threshold value for the protection of vegetation was exceeded substantially, by up to a factor of 3, widely and frequently. The full EU15 area of coniferous forest and arable land experienced exceedances of this threshold. In less than 1% of the area of broad-leaved forest were exceedances not observed. Recent work, however, indicates that the 65 mg.m-3 (24-h average) threshold may be of little relevance in assessing the potential for ozone effects on vegetation. Exceedances of this current threshold occurred even at the end of the 19th century. It was exceeded during close to 1% of the time between 1876 and 1911 near Paris.

The WHO guideline for protection of crops is exceeded in all Member States except Finland. On average over all EU15 countries only a small fraction (about 6%) of arable land is not exposed to exceedances of this critical level. The magnitude of exceedance of this critical level differs from year to year. Even in the low ozone years the area of exceedance covers almost the entire EU15 surface.

In the case of forests the situation is slightly better. In Scandinavia, Ireland and the United Kingdom forests seldom experience exceedances. On average over Europe about 35% of coniferous forest is estimated to experience exceedances. This is in contrast to the broad-leaved forest, from which we estimate that approximately 70% is not protected.

The 5EAP set emission reduction targets for the precursors of ozone The 5EAP includes emission abatement targets of the ozone source species. The VOC target requires a 30% reduction in the year 2000 from the 1990 emission levels. The NOx target also uses 1990 as its reference year, and 2000 as the year to achieve a reduction of 30%. It also aims at a stabilisation in 1994 at the 1990 levels.

Emissions of both VOC and NOx increased until the late 1980s but declined in the 1990s. Between 1990 and 1994 VOC emissions from the EU15 countries decreased by approximately 9%. Similar emission reductions occured for NOx, which showed a 8% drop between 1990 and 1994. However, given the current reduction rate, it is most unlikely that the remaining reductions of more than 20% will be met until 2000.

In 1990 as an average over EU15, the transport sector accounted for about 45% of total anthropogenic VOC emissions. Similarly, for NOx the largest fraction originated from transport with an almost stable share of 64% between 1990 and 1994. The second largest source sector is industry (approximately 35%) for VOC, whereas it is the energy sector contributing about 19% for NOx. Increased traffic is an important obstacle to attaining emission reductions necessary to meet air quality objectives of ozone.

7.4 Conclusions from the meteorological perspective

All wide-spread ozone episodes observed in North and Central Europe from 1994 occurred in spring and summer with dry, sunny weather conditions in stagnant air, in which ozone precursors such as nitrogen oxides, carbon monoxide and volatile organic compounds accumulate. Ozone episodes are shown to be systematically associated with high pressure systems and elevated daily maximum temperatures. On the first/second days of the episodes, the exceedances can be mostly attributed to local/regional photochemical formation, whereas, on the following days, long-range transport plays a major role in the cross-border redistribution of ozone concentrations.

The different situation with the occurrence of exceedances observed in different Member States are to a large part explained by the direction of the predominant wind in situations of anticyclonic circulation and, are related in a large part to the dependence of ozone levels on the meeting of the air masses with areas of high emissions of precursors. The lower occurrence of exceedances in Portugal and Ireland can be mostly attributed to the influence of clean oceanic air masses.

Meso-scale meteorology and interaction between European and local scale weather situations are of major influence on air pollution in South Europe. However, only more dense ozone monitoring networks would allow the assessment of the special meteorological scenarios, such as the land-sea-breeze-circulations which induce particularly high ozone levels in coastal urban/industrial Mediterranean areas.

Downward transport of ozone from the upper troposphere/lower stratosphere may also contribute to observed surface ozone concentrations during spring and summer.


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