4.1. Geographic coverage of monitoring stations

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Air pollution in the European Union - Exceedance of ozone threshold values in 1995 and summer 1996 - Part I

4. DISCUSSION

4.1 Geographic coverage of monitoring stations

For 1995 information for 858 stations has been received; for nearly all of them (855) geographical coordinates are available. For the interpretation of ozone data it is essential to have an indication of the direct surroundings of the station as the ozone concentration may be strongly influenced by local conditions. For example, the ozone concentrations may be scavenged by locally emitted nitrogen oxides or by enhanced dry deposition as might be the case under a forest canopy; see Annex II for a brief discussion on ozone phenomenology. For 725 stations (85%) information on immediate surroundings is available.

According to the submitted information on station type and characteristics, a distinction between background (or rural), urban, street and other stations has been made, see Table 6. Map 2 shows the location of background monitoring stations. Tentatively, the area for which the measurements at these stations are assumed to be representative is indicated with a circle using a radius of 100 km. This "radius of representativeness" might be different for the various regions in Europe - it might even depend on the wind direction - and should be based on more detailed analysis of the ozone phenomenology at the stations. Although the identification of the background stations is by far from complete and the radius of representativeness may differ from the assumed 100 km, Map 2 suggests that the present set of stations covers 40 to 50% of the land area of the EU. As is shown in Map 2 the geographical coverage of background stations is rather adequate in North-West Europe but in other regions, in particular in France, Italy and Greece, gaps are noted.

Table 6. Classification of ozone stations according to their direct surroundings as rural, urban, or street station;. 'other' indicates both other station types (e.g. in industrial areas) and stations for which no information of station type is available.

MS	Rural	Urban	Street	Other/Unknown

AT	59	34	12	5
BE	12	7	0	1
DE	95	166	47	34
DK	3	3	0	0
ES	13	6	52	15
FI	9	1	1	0
FR	0	0	0	110
GB	13	2	0	17
GR	0	6	4	0
IE	3	1	0	2
IT	7	33	6	22
LU	2	0	3	0
NL	26	3	8	0
PT	1	1	4	3
SE	6	0	0	0
EU	249	263	137	209

Map 2. Location of background ozone monitoring stations. Tentatively the area for which the ozone measurements might be representative is indicated with a circle with a radius of 100 km.

It has been attempted to quantify the coverage of the monitoring stations for urban areas as is relevant for population exposure. According to information obtained from the Eurostat population database, there are in the EU about 2000 cities with more than 25 000 inhabitants. It is estimated that from a total EU population of c. 362 million, c. 195 million people live in these cities with more than 25 000 inhabitants. Summation of the number of inhabitants of all the cities in which at least one ozone station is operational and has been reported to the EC in 1995, leads to a total of approximately 58 million, that is, only for 30% of the total urban population (equivalent to 16% of the total EU population) an estimate of ozone exposure can be made.

Although the uncertainties in the present set of station descriptions and the large number of stations for which a description of immediate surrounding is lacking preclude any firm conclusion, it is clear that the set of urban stations in the national networks is not representative for the total urban population in the EU. Conclusions concerning the exposure of urban population to high ozone levels should therefore be seen as tentative.