4. Main ozone episodes

Ozone formation and destruction is dependent on emissions, concentrations and ratios of precursors (mainly VOC, NOX, and CO), 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 may be found downwind of cities. Figure 6 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 1998 summer season.

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Figure 6: Qualitative overview of exceedances of the 180 µg/m3 population information threshold value (1h) during the period April - August 1998. 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 6 it is clear that the number of episodes covering extended areas of the European territory was limited during April-August 1998. 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.

More frequent exceedances were observed in May and August. The most widespread northern European episode this summer occurred between 9 and 12 August. Map 4 presents an overview of the sites where exceedances were observed on 9, 10, 11 and 12 August 1998. In the text box, the conditions leading to this episode are described in more detail. In contrast to this, in southern Europe exceedances are observed frequently throughout the reporting period.

The episode of 9-12 August 1998

On 7 and 8 August, the axis of a West-East oriented high-pressure cell was located over Central Europe resulting in a light Westerly circulation. Although the weather could be characterised as warm and sunny in large parts of Europe, the air mass dominant was relatively clean and exceedances were only reported from a limited number of sites. On 9 August, the core of the high pressure cell moved to the North Sea, as a result the circulation became Easterly over large parts of Western and Central Europe. Exceedances mainly were reported from the Southwestern part of Germany. This synoptical situation did not change markedly on 10 and 11 August. Very hot and polluted air in the lower atmosphere became more or less stagnant over the continent. Temperatures on 11 August rose to 37.7 degrees Celcius in Paris, and 41.6 in Braunberg (DE). On 10 August exceedances were reported from the Western part of Germany, France, Southeastern UK, Belgium, Luxembourg and the southern part of Germany. On 11 August, the area where exceedances were observed increased to include large parts of Germany and parts of Austria. On 12 August, an Atlantic depression started to move in from the west over Scotland, transporting relatively clean Atlantic air masses over the UK eastwards. This ended the episode in the Western parts of France, Belgium and the Netherlands. On 13 August, the Atlantic air masses reached most of the EU territory and ended the ozone episode.

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Map 4: Example of a smog episode: stations which reported an hourly ozone concentration in excess of 180 µg/m3, 9-12 August 1998 (all station types).

Figure 7 presents the maximum hourly ozone values recorded in the Athens conurbation (Greece) in July on days when the threshold for warning of the public (1h >360 µg/m3) was exceeded, as an example of a local ozone episode in the Mediterranean region. Note that not all stations presented reported exceedances on every day 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 7: Example of local ozone episodes, Athens July 1998. Maximum observed 1h values (µg/m3) on stations in the Athens conurbation which observed an exceedance of at least 180 µg/m3 (1h) on 2, 3 and 29 July.

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