Ozone, oxygen in the tri-atomic form, is one of the strongest oxidising agents, thus it is very reactive. There are only minute emissions of O3 to the atmosphere. A part of tropospheric ozone (may be 10-15%) is transported from the stratosphere where it is formed by UV radiation on O2. Most of the ozone in the troposphere is formed indirectly by the action of sunlight on nitrogen dioxide.
The precursor emissions concerning tropospheric photochemical production of ozone are NOx and VOC emissions. In the steady state, there is a photochemical equilibrium between NO, NO2 and O3. The presence of hygroxyl radicals and volatile organic compounds from anthropogenic or national origin causes a shift in the equilibrium towards higher concentrations of ozone. The production of ozone, and thus the concentrations reached, depend upon the input NO, NO2 and VOC concentrations, and also the ratios between them. There are favourable conditions for ozone production at VOC/NOx ratio between 4:1 and 10:1. Production of ozone is a fairly slow process (hours or more). Photochemical ozone occurs during unfavourable dispersion conditions in summer time (strong UV intensity) in large urban areas particularly in Southern Europe, and regionally in large parts of Europe, including Southern Scandinavia, due to prolonged reaction time in air parcels receiving continuous input of VOC and NOx through its transport.
O3 and other oxidants cause a range of acute effects including eye, nose and throat irritation, chest discomfort, cough and headache. These have been associated with hourly oxidant levels of about 200 µg/m3 . Pulmonary function decrements in children and young adults have been reported at hourly average O3 concentrations in the range 160-300 µg/m3. Increased incidence of asthmatic attacks and respiratory symptoms have been observed in asthmatics exposed to similar levels of O3. The non-ozone components of the photochemical mixture cause eye irritation at O3 levels of about 200 µg/m3.
Other environmental effects include damage to materials (including as a result of prolonged exposure to low concentrations), and vegetation effects.
EU Threshold values and WHO Guideline values for O3 is given in Table 6.18. All values are maximum values.
Table 6.18: EU Threshold values for O3 (µg/m3). EU Council Directive 92/72/EEC. WHO Guideline values (µg/m3).
1) Concentrations must be measured continuously.
0 and 9.00, 8.00 and 17.00, 16.00 and 1.00, 12.00 and 21.00.
For the information to be provided pursuant to Article 6 (1), first indent, the mean over 8 hours is a simple moving average, calculated each hour h from the 8 hourly values between h and h-9.
the number, date and duration of periods during which threshold values as presented in Table 6.18 are exceeded.
The following paragraphs on O3 concentrations, exceedances and trends are based on 1994 data reported by RIVM and NILU in "Exceedance of Ozone Threshold Values in the European Community in 1994" which again is based on an internal report to the Commission by the European Environment Agency Topic Centre on Air Quality (de Leeuw et al., 1995).
An overview of O3 data available for 1994 is presented in Table 6.19. For 1994 complete or nearly complete information on ozone concentrations (annual statistics and/or exceedance information) has been received for 461 monitoring stations. For the preceding five years data transmission is less extensive. For nearly all stations information on measurement methods, instruments and on calibration procedures has been submitted. The location of the stations and a description of the immediate and local environments of the monitoring station is less completely available.
Table 6.19: Overview of data received by ETC-AQ. For each item and for each year the number of Member States and the total number of stations for which data has been submitted, is indicated.
n.e.: not evaluated in this report due to time limitations.
The location of monitoring stations reporting 1994 data is presented in Figure 6.49. In total information for 461 stations and 14 Member States has been received. All Member States use the reference method (UV absorption) as prescribed in Annex V of the Ozone Directive.
Figure 6.49: Location of ozone monitoring stations as reported by Member States in the framework of the ozone Directive for the reference period 1994.
A summary of the maximum concentrations measured at any of the reporting stations in each country when exceedance of a threshold value is observed is presented in Table 6.20. When exceedances of a threshold has not been reported by a country this is indicated with a dash (-). In a few cases no conclusive answer whether exceedances have occurred could be given; in Table 6.20 this is indicated with a question mark. Exceedances are counted on a daily basis. A day on which at least one hourly or eight hourly values exceed the threshold values, is marked as exceedance.
Table .20: Maximum ozone concentrations measured in each country during a period of exceedance of threshold values (reference period 1 January-31 December 1994). A dash (-) indicates that no exceedances have been observed at any of the monitoring stations in the Member State. A question mark (?) indicates that no information on exceedances has been provided in computer readable form.
France 318 318 - ? ? ?
Greece (c) 400 400 400 ? ? ?
(a) Based on three non-overlapping eight hourly values between 0.00-8.00; 8.00-16.00; 16.00-24.00.
(b) Based on the eight hourly value between 12.00-20.00.
(c) Based on annual statistical information.
An overview of the reported 50 and 98 percentile values (based on hourly and moving eight hourly averaged concentrations) is presented in Table 6.21.
Table 6.21: Range in reported 50- and 98-percentile values (based on hourly and moving eight hourly averaged concentrations) observed at monitoring stations in Member States (in µg/m3), period 1 January-31 December 1994.
min max. min max. min max. min max.
France ? ? ? ? ? ? ? ?
The geographical distribution of 98-percentile values calculated on the basis of moving eight hourly averaged concentrations are presented in Figure 6.50 for background stations and in Figure 6.51 for urban stations and stations of unspecified type.
The 1994 data set does not allow a thorough assessment of geographical differences in Europe since spatial coverage is too fragmented and data are less useful because of the lack of knowledge on the representativeness of stations. From the available data no distinct regional differences are apparent in 50 percentile values (both hourly and 8 hourly) apart from the fact that coastal stations on the average report higher 50 percentile values then inland (urban) stations. Peak concentrations, as presented by the 98 percentile, in general show an increase from North-West to Central Europe; the elevated location of the monitoring stations may play a role her.
Figure 6.50: 98 percentiles (based on moving eight hourly concentrations; µg/m3) measured at background stations, calendar year 1994.
Figure 6.51: 98 percentiles (based on moving eight hourly concentrations; µg/m3) measured at urban and unspecified stations, year 1994.
Exceedances of the threshold value of 360 µg/m3 for hourly values has been observed at three stations in three member states, see Table 6.22.
Table 6.22: Location, date and maximum concentration (hourly value, in µg/m3) of all reported exceedances of the threshold value of 360 µg/m3 for the calendar year 1994.
The averaged number of observed exceedances per station of the other threshold values is summarised in Table 6.23. As the number of monitoring stations differ widely from country to country, the absolute numbers of exceedances are less suitable for comparison. Full details on the number of exceedances at the individual stations is presented in Table I.1 of Appendix E.
Table 6.23: Number of exceedances averaged over all reporting stations (reference period 1 January-31 December 1994).
France 7.3 3.5 ? ? ?
Greece >0 >0 ? ? ?
Sweden 0.5 0.2 ? ? ?
The threshold value for protection of human health is based on eight hourly values. According to the Ozone Directive four eight hourly periods have to be considered: 0.00-8.00, 8.00-16.00, 16.00-24.00 and 12.00-20.00.
Based on the averaged diurnal profile of ozone the highest eight hourly values are generally expected for the 12.00-20.00 period; only exceedances of the threshold values for this period are further processed here.
In 1994 exceedances of this threshold value has been observed in all 14 Member States providing data. Maximum concentrations up to 250 µg/m3 has been observed (see Table 6.20). Figure 6.52 shows the frequency distribution of eight hourly ozone concentrations in excess of the threshold value using so-called Box-Jenkins plots. For each Member State the Box-Jenkins plot indicates the minimum (here the minimum is of course 110 µg/m3), the maximum, the 25 percentile and the 75 percentile value of the exceedances. Although extreme peaks of more than 200 µg/m3 are observed in 7 out of 10 reporting Member States, Figure 6.52 shows that in each Member State for ca. 75% of all exceedances the concentrations are below 165 µg/m3 (that is, 150% of the threshold value).
The geographical distribution of the number of days the threshold value was exceeded is shown in Figure 6.53 for background stations in Figure 6.54 for the urban and unspecified stations.
Figure 6.52: Frequency distribution of ozone concentrations (eight hourly values; period 12.00-20.00) in excess of the 110 µg/m3-threshold for hourly values. For each country the total number of observed exceedances is given in row #Ex, the number of stations is given in row #St. Frequency distributions are presented as Box-Jenkins plots indicating the minimum, the 25-percentile, the 75-percentile and the maximum value. The data for Belgium did not allow for inclusion in this figure.
Figure 6.53: Number of exceedances of the threshold value for protection of human health (110 µg/m3 for eight hourly values) observed at background stations; calendar year 1994; eight hourly averaged values for the period 12.00-20.00.
Figure 6.54: Number of exceedances of the threshold value for protection of human health (110 µg/m3 for eight hourly values) observed at urban and unspecified stations; calendar year 1994; eight hourly averaged values for the period 12.00-20.00.
Exceedances most frequently occur in the summer months (April-August). In the southern countries (Italy, Spain) and in Austria exceedances are observed nearly the whole year: from February to October 1994. Note that the phenomenology of ozone peak concentrations strongly depends on the meteorological conditions. The behaviour as found here for 1994 might not be representative for other years.
The threshold values for warning the population (360 µg/m3, hourly value) has been exceeded in 1994 once in Greece, Italy and Portugal. All exceedances occurred in the Southern part of Europe. Two exceedance are observed at the end of May. The exceedance in Portugal is observed in January which is, in view of the general behaviour of ozone, quite surprising.
The geographical distribution of the number of exceedances of the threshold value for information of the public (180 µg/m3, hourly value) is presented in Figure 6.56 for background stations and in Figure 6.57 for urban and unspecified stations. Exceedances are observed in 12 of the 14 reporting Member States; only in Finland and Ireland the 180 µg/m3 level has not been reached.
Figure 6.55 shows the frequency distribution of concentrations in excess of the threshold value. Although incidentally the threshold value may be exceeded by more than a factor of 2.5 in almost all of the cases the exceedances are less extreme: The figure shows that on 75% of the days on which the threshold value was exceeded, the level of 225 µg/m3 (that is 125% of the threshold value) has not been reached.
Exceedances are observed during a large part of the year but most frequently and most widely spread geographically during the summer months.
Figure 6.55: Frequency distribution of ozone concentrations (hourly values) in excess of the 180 µg/m3 threshold for hourly values. For each country the total number of observed exceedances is given in row #Ex, the number of stations is given in row #St. Frequency distributions are presented as Box-Jenkins plots indicating the minimum, the 25-percentile, the 75-percentile and the maximum value. The data for Belgium did not allow for inclusion in this figure.
Figure 6.56: Number of exceedances of the threshold value for information of the population (180 µg/m3 for hourly values) observed at background stations; calendar year 1994.
Figure 6.57: Number of exceedances of the threshold value for information of the population (180 µg/m3 for hourly values) observed at urban and unspecified stations; calendar year 1994.
From the data now available it is clear that exceedances of the threshold value set for information to the public were observed in the southern European Member States (Greece, Italy, Spain) from March onwards. In May this region extended to the north (France, Austria, Luxembourg). Starting at the last week of June, exceedances were also frequently observed in Belgium, United Kingdom, and the Netherlands. More northern countries, Sweden and Denmark, only reported exceedances in the last week of July. This period with frequent exceedances across Europe ended at 5 August 1994. After this date, exceedances were less numerous and again confined to the southern part of Europe (Italy, Spain, Greece and France). These countries reported exceedances till half October.
Table 6.24 gives 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 1994 summer season. Please note that no exceedances were observed in Finland and Ireland. In this period some short periods of several days can be recognised on which exceedances are only observed at a limited number of stations (less than 50%) in one country. These periods with relatively low numbers of exceedances are followed with periods of 4 to 7 days with frequent exceedances in various countries. As an example the maximum hourly ozone concentrations in excess of 180 µg/m3 as measured on 27 July 1994 are geographically presented in Figure 6.58.
Table 6.24: Qualitative overview of exceedances of the 180 µg/m3 population information threshold value (1 h) during the period 26 June-8 August 1994. The number of asterisks refers to the percentage of stations in a country which reports and exceedance: *: less than 25%; **, between 25 and 50%, ***: between 50 and 75%; ****: more than 75%. Note that in Ireland and Finland no exceedances are observed in this period.
Figure 6.58: Example of an ozone smog episode: hourly ozone concentrations in excess of 180 µg/m3 as measured on 27 July 1994.
The data reports for the period 1989-1993 are available for 6 Member States and a detailed analysis of ozone exceedances can presently not be made.
The year-to-year variation in 50-percentile values are relatively small when compared to the variations in the 98-percentile values (see Figure 6.59 and Figure 6.60). The 98-percentile values are measured in the summer, typically during periods with hot weather. Ozone is strongly correlated with temperature mainly because the conditions leading to high temperatures (e.g. strong solar radiation, low wind speeds, continental flows) also trigger photochemical formation. Meteorological fluctuations may cause variations in peak ozone levels that are much larger than the variations due to changes in precursor emissions. A yearly fluctuation of ca. 15% in 98-percentile value is not exceptional, see Figure 6.60; according to model calculations a 15% reduction in ozone peak values is expected when commitments to the Sofia Protocol on NOx emissions and the Oslo Protocol on VOC emission to the international convention on Long-range transport of air pollution are fully implemented.
The yearly fluctuations differ from country to country. In Belgium and the Netherlands similar patterns are found: after two "high" year (1989 and 1990) the period 1991-1993 shows relatively low peak values but increased levels are again observed in 1994. For more northern countries (Denmark, Finland) the 1994-increase is less outspoken or even absent.
The available time series are too short to detect any possible trend in ground level ozone concentrations. Trends in ozone concentrations are expected as a result of trends in precursor emissions in Europe and as a result of the increasing trend in hemispheric background concentrations (Borrell and van den Hout, 1995). The magnitude and even the sign of a possible trend will differ from location to location. No conclusive answers can be given based on the data reported here.
Figure 6.59: Range in reported 50-percentile values (in µg/m3, based on hourly concentrations) in Austria, Belgium, Denmark, Finland, Great Britain and the Netherlands in the period 1989-1994.
Figure 6.60: Range in reported 98-percentile values (in µg/m3, based on hourly concentrations) in Austria, Belgium, Denmark, Finland, Great Britain and the Netherlands in the period 1989-1994.
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