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Ozone formation and destruction is dependent on emissions, concentrations and ratios of precursors (mainly VOC, CO and NOX), and on the amount and intensity of sunlight. Important in this respect is the role of nitrogen oxide emissions. In urban areas, the ozone concentrations may be lower then the rural ('background') concentrations due to chemical scavenging by local nitrogen oxide emissions (see for example Figure 3, which shows that the occurrences of exceedances are in general the highest at rural stations).

Episodes, periods with elevated ozone levels, will mainly occur during periods of warm sunny weather. In the Mediterranean countries, having prolonged spells of hot and sunny weather during the summer, ozone can quickly be formed and high levels can occur on many days and in the vicinity of urban centres. In northern Europe the build up of ozone is slower due to the more moderate weather conditions. Here, highest levels will generally be found outside cities.

Figure 5 presents a graphical representation of the percentage of stations in every Member State that reported exceedances of the threshold value for population information (180 µg/m3 for hourly values) during the 1996 summer season.

Figure 5: Qualitative overview of exceedances of the 180 µg/m3 population information threshold value (1h) during the period 1 April - 30 July 1996. The symbols represent the percentage of stations which observed at least one exceedance of the threshold for the information of the public during a particular day

From Figure 5 it is clear that the number of episodes covering extended areas of the European territory was limited during summer 1996. As already mentioned in Section 3, weather conditions in western and northern Europe were often unfavourable for the build-up of ozone. On many days cool and relatively clean Atlantic air masses prevailed in northern and western European countries. Frontal activity could often be found in the Alps region. One of the most widespread northern European episodes this summer occurred 5-8 June 1996. Map 4 presents an overview of the sites which observed an exceedance during this period. In the text box, the conditions leading to this episode are described in more detail. In contrast, in southern Europe frequent exceedances are observed throughout the reporting period.

The episode of 5-8 June 1996

On the first days of June, a high pressure cell formed over western Europe. On 4 June, this pressure cell dominated the weather north of the Alps. On that day, the air mass present north of the Alps could be characterised as relatively clean and cool Atlantic air. On 5 June, the centre of the high pressure cell was situated near the Baltic states. In western Europe, a light SE-circulation set in which started to transport the air mass, which had been present for a few days over the Continent, to the North Sea basin. Under the influence of the intense solar radiation, surface temperatures reached summerly values and exceedances were observed in the BeNeLux, northern France and southern UK. Hardly any exceedance was observed in Germany on the 5th; here still cool Atlantic air was present. On 6 June, temperatures reached 30 C at many places and the area where exceedances occurred extended to include the western part of Germany and parts of Denmark. The number of exceedances in the Eastern part of Germany was still limited. On 7 June a cold front, behind which cooler and cleaner air moved in and reached the western part of the UK; exceedances in the UK were limited to the south-eastern part of the country. In the hot air preceding the cold front (34 C was reached for example in the Netherlands), thunderstorms developed during the day in France and Belgium. Although there was no change in air mass, the thunderstorms and more general the build up of clouds ended the episode in France and Belgium. More east, the area where ozone levels reached the threshold now included eastern and northern parts of Germany and the southern tip of Sweden. On 8 June, the cleaner Atlantic air behind the cold front reached France, BeNeLux and SW-Germany and subsequently no exceedances were observed anymore in these regions. On 9 June, the cleaner and cooler air covered the whole of Europe north of the Alps and no wide-spread exceedances were reported anymore. Note that during the whole period the weather was warm and sunny in southern Europe and (occasional) exceedances were also reported from Spain, Italy and Greece.

Map 4: Example of a smog episode: stations which reported an hourly ozone concentration in excess of 180 µg/m3, 5-8 June 1996 (all station types).

Figure 6 and 7 present the maximum hourly ozone values recorded in the Athens conurbation (Greece) and in Firenze (Florence, Italy), as examples of 'local' ozone episodes in the Mediterranean region. Both in Athens and Firenze, the threshold for warning of the public (1h >360 µg/m3) was exceeded. Note that other stations in and around Athens did not report exceedances during this period. More information on the specific conditions leading to these particular episodes is not yet available to the authors of this report.

Figure 6: Example of a local ozone episode, Athens 18-22 May 1996. Maximum observed 1h values (µg/m3) on stations in the Athens conurbation which observed an exceedance of at least 180 µg/m3 (1h).

Figure 7: Example of a local ozone episode, Firenze (Florence) (I),7-13 June 1996. Maximum observed 1h values (µg/m3) on stations which observed an exceedance of at least 180 µg/m3 (1h).

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