Ozone, O3, is a strong photochemical oxidant which may cause serious health problems and damage to materials and ecosystems. Human exposure to elevated levels of ozone concentrations can give rise to inflammatory responses and decreases in lung function. Symptoms observed are coughing, chest pain, difficulty in breathing, headache and eye irritation. Both laboratory and epidemiological data indicate large variations between individuals in response to episodic ozone exposure; the effects seem to be more pronounced in children than in adults. Studies indicate that exposure to ozone concentrations in the range 160-360 µg/m3 for a period of 1-8 hours, often observed in Europe, reduces various pulmonary functions.

Exposure of ecosystems and agricultural crops to ozone results in visible foliar injury and in reductions in crop yield and seed production. Within the framework of the UN-ECE Convention on Long-Range Transboundary Air Pollution the critical level1 for ozone is expressed as the accumulated ozone exposure above a threshold of 40 ppb (corresponding to 80 µg/m3). Guideline values for this accumulated ozone exposure of 3000 ppb.h and 10,000 ppb.h are given for crops and forest, respectively.

In view of the harmful effects of photochemical pollution in the lower levels of the atmosphere, the Council of Ministers adopted in 1992 Directive 92/72/EEC on air pollution by ozone. The Directive came into force in March 1994. It established procedures for harmonized monitoring of ozone concentrations, for exchange of information, for communication with and alerting of the population regarding ozone and optimizing the action needed to reduce ozone formation.

Article 6 of the Directive specifies how the information on monitoring results must be provided by the Member States to the Commission. Regarding the time frame, two main types of reporting can be distinguished. Information on exceedances of the so-called information threshold (article 6 sub 2) and warning threshold (article 6 sub 3) for the ozone concentration is to be provided within one month after occurrence. Information on exceedances of all threshold values given in Article 6 must be provided within six months following the annual reference period (article 6 sub 1). Article 7 of the Directive stipulates that the Commission shall at least once a year evaluate the data collected under the Directive. The present report gives an overview of ozone monitoring results of 1996. Similar overviews of the 1994 and 1995 annual data have been prepared by the European Topic Centre on Air Quality (de Leeuw et al., 1995; de Leeuw and van Zantvoort, 1996). Prior to the current report an overview on ozone threshold exceedances during Summer 1996 (April-July) was also presented to the Commission (Sluyter and van Zantvoort, 1996).

The data reported here do not cover all ozone monitoring stations in the European Union. For inclusion in this report, the data must satisfy certain criteria stipulated in the Directive, concerning inter alia measuring methods, sampling methods, station siting, quality assurance and documentation. Formats on the transfer of data have been defined by the Expert Group on Photochemical Pollution. This group, established by the Commission following Article 7 of the Directive, had several meetings to co-ordinate the work within the Member States and the Commission in the framework of the Directive.

Background information on the current experience and knowledge concerning photochemical air pollution, dealing in particular with the phenomenology of ozone, the scientific understanding as based on experiments and theory, and the insights from modelling studies on the relation between ozone levels and precursor emissions, may be found in Borrell and Van den Hout (1995), Derwent and Van den Hout (1995), Barrett and Berge (1996) and in the Consolidated report on the status of tropospheric ozone pollution in the European Community (Beck and Krzyzanowski, 1997) which is currently in preparation following Article 8 of the Directive. Some aspects of the ozone phenomenology are briefly discussed in Annex II.

1 Critical levels are defined as concentrations of pollutants in the atmosphere above which direct adverse effects on receptors, such as plants, ecosystems or materials may occur according to present knowledge (Bull, 1991).

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