O 3 concentrations data

Data on O3 concentrations are required both to help develop and calibrate the models, and then to

test their validity. These data need to be representative of the study area and period, to be consistent, and to be as free as possible of measurement error.

Measured data on O3 concentrations for the study area countries were obtained from the AIRBASE

database. This database is maintained on behalf of the EEA, and draws together measurements from routine air pollution monitoring carried out by the EU member states, under obligations imposed by EU air quality directives. As mentioned in the draft final report about AIRBASE (Spangl et al., 2007), the European Topic Centre on Air Quality (ETC_AQ) is responsible for developing and maintaining both a European Air Quality Monitoring Network (EUROAIRTNET) and an air quality information system (AIRBASE database), in close collaboration with the European countries, to provide good information to support the work of the EEA. The purpose of this collaboration is to enable and produce air quality assessments on the European scale through adequate information on air quality.

AIRBASE is developed on the basis of this EUROAIRNET network. AIRBASE has been established, improved over time and made available on the Internet under the EC Exchange of information (EoI) Decision 97/101/EC11 in compliance with the EU air quality directives.

The objectives of the monitoring activities of the EU countries, as outlined in (Larsson, 1999), are:

 Monitoring of Member State compliance with the directives.

 Representative air quality surveillance monitoring to describe the state of and trends in air quality across Europe.

 To facilitate exposure/damage assessment with regards to health, vegetation, and materials.

 Public availability of monitoring data on-line, to inform and warn citizens, as well as to enable and inform short term abatement actions.

 Operational monitoring near specific sources to prevent undesirable pollution burdens on neighbouring areas.

 Monitoring programmes to support scientific research.

The selection criteria defining the specific areas to be monitored are intended to be representative either across the whole of Europe, or for separate regions of Europe to provide adequate spatial coverage of the air pollution situation. AIRBASE, however, comprises existing monitoring networks run at local, regional or national level, and many smaller geographic areas will not necessarily have a sufficient number of monitoring networks or sites to contribute.

Measured data on hourly O3 concentrations for the countries of interest were obtained for the

whole study period, from March 1st 2001 to February 28th 2007. Version 4 of the AIRBASE data products on the EEA data service website12 was used for this purpose.

O3 concentrations reported in AIRBASE are continuously monitored using automatic UV absorption

analysers, and in broad terms can thus be considered consistent in terms of measurement technique. For the 6-year time period time of this study, monitoring was available for a total of 1,463 sites across the study area. To ensure that data provided a reliable basis for modelling, several criteria were used to select from these 1,463 monitoring sites:

11

http://acm.eionet.europa.eu/databases/airbase/

12

 Sites should be located within the borders of the correct country as indicated by the metadata, Figure 3.3.

 Monitored data should be available for 4 years or more from the study period, and with measurements for at least 75% of the time in any monitoring period (daily and seasonally). Based on the EU regulations this equates to 18 hours during the day, and for 22 days of each month. For this purpose, two main seasons were defined: season 1 was the three summer months and season 2 was the three winter months. The histogram in Figure 3.4 illustrates the number of sites for each country. For example approximately 210 monitoring sites in France and German have six full years of data, while 60 and 40 monitoring sites, respectively, have 5 years of full data.

 The remote islands of Spain and Italy were excluded (e.g. Mallorca, Ibiza, Corsica, and Sardinia) because they are geographically far from mainland Europe, and thus affected by different meteorological conditions and air mass regimes.

Figure 3.4 Frequency of sites, by country, with 75% hourly data capture for 4 years and more

The hourly concentration data were cleaned by recoding hourly missing or negative values to 999. Sites with less than 75% data capture were also discarded. Sites meeting the other selection criteria were identified and maintained, reducing the overall number of sites to 1,211 as shown in Figure 3.5. The full descriptive statistics for these 1,211 sites are shown in Table 3.1.

Table 3.1 Descriptive statistics for the long-term concentration (hourly data from 1st March 2001 to 28th February 2007) at the 1,211 ozone monitoring sites

country No of sites (1211) a No. of measurement (54,685,277)b

Min Max Mean SD

Austria (AT) 107 5,067,880 0.00 336.20 58.29 36.68 Belgium (BE) 34 1,477,901 0.00 296.00 43.35 32.44 Germany (DE) 282 13,396,674 0.00 334.00 48.46 33.78 Denmark (DK) 6 267,315 0.00 195.10 47.34 26.21 Spain (ES) 232 10,154,233 0.00 470.00 48.56 32.98 France (FR) 354 16,210,237 0.00 417.00 50.85 34.31 Great Britain (GB) 63 27,03,792 0.00 327.00 42.28 26.95 Ireland (IE) 6 284,068 0.00 207.80 60.24 21.97 Italy (IT) 72 2,755,753 0.00 451.00 51.62 41.80 Netherlands (NL) 30 1,373,438 0.00 276.49 40.36 28.91 Portugal (PT) 25 993,986 0.00 358.00 48.73 30.69 a,b the total number

0 20 40 60 80 100 120 140 160 180 200 220 AT BE DE DK ES FR GB IE IT NL PT N o o f M o n ito ri n g si te s

Western Europe study area countries

4 5 6

Figure 3.5 Station locations in the AIRBASE data set and the mean O3 concentrations (6 year

average)

These sites are not evenly distributed across the study area, as indicated in Figure 3.5 (map of the final selected sites). This inequality is a problem that must be recognized and addressed in the modelling to be carried out in this study, and its implications are discussed in Chapter 8.