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Ozone
Ozone (O3) is a form of oxygen, with three atoms per molecule (unlike normal oxygen, O2, which has two). In the upper atmosphere it is beneficial, forming the “ozone layer” which protects living things from harmful UV radiation. However, at ground level it is a pollutant, having an irritant effect on the respiratory system. Ground level ozone is not emitted directly from source, but formed by chemical reactions involving the action of sunlight, where NO2 and volatile organic compounds (VOCs) are present. These reactions may happen over several hours, so the highest ozone concentrations may occur a long distance downwind of the sources of the primary pollutants. Also, O3 may persist for several days. Ozone pollution can therefore be a transboundary problem, and difficult to control by local action. O3 concentrations are usually therefore lowest in towns, and highest in the rural areas downwind of them. Because ozone formation requires sunlight, concentrations are highest in the summer, and during daylight hours. In this
report, concentrations of ozone are expressed as microgrammes per cubic metre (µg m-3).
To convert to parts per billion (ppb) if necessary, the following relationship
should be used: 1 ppb = 2.0 µg m-3
for ozone at 293K (20°C)
and 1013mb. Monitoring of O3 in Northern Ireland is carried out largely using continuous automatic analysers. Diffusion tubes are also available for this pollutant, but are used routinely at only one site, which is operated by Fermanagh District Council. Monitoring of ozone is carried out at the sites shown in Table 7.1 below, and in Figure 7.1. Table 7.1
Automatic O3 Monitoring Sites
Ozone is covered by the target values and objectives in Table 7.2. EC Directive 92/72/EC on air pollution by ozone will be superseded by a third Daughter Directive, 2002/3/EC. This Directive, which sets “target values” rather than limits, has not yet been transposed into Northern Ireland’s legislation. However, as this transposition is scheduled for 2003, this report compares ozone measurements with the 3rd Daughter Directive rather than the older Directive. The AQS objective is more stringent than the EC target value for protection of human health. Table 7.2 Target
Values and Objectives for Ozone
a)
AOT 40 is the sum of the differences between hourly concentrations
greater than 80 µg m-3 (=40ppb) and 80 µg m-3,
over a given period using only the 1-hour averages measured between 0800 and
2000. b)
Not included in the Air Quality Regulations. OZONE RESULTS Table 7.3 shows the annual maximum daily 8-hour running mean ozone concentration for each site, and also the number of days per year on which this parameter exceeded 100 µg m-3. Years in which the AQS objective was exceeded on more than 10 days are highlighted in bold italics. Annual mean ozone concentrations are also included in Table 7.3: although no limit values apply to this statistic, the annual mean may show long-term trends more clearly than short-term statistics. Annual data capture is at least 75% except where indicated. Table 7.3 O3 Results from Automatic Monitoring Sites
a)
Limited data capture: Londonderry 1997 (59%) b)
Limited data capture: Lough Navar 1987 (66%) The 2001 annual mean ozone concentration at the Fermanagh diffusion tube site was 83 µg m-3. This is very high compared with the automatic analyser results. Table 7.3 shows that: · In earlier years, peak concentrations of ozone were typically higher at the rural Lough Navar site than at the two urban sites. However, since the mid 1990s all three sites have had similar maximum daily 8-hour running mean. · In recent years, the AQS objective for ozone has been met at most sites. The exceptions are Lough Navar, 1995 and Londonderry, 2000. Ozone data from the rural Lough Navar site, for the five years 1997 to 2001, was compared with the EC 3rd Daughter Directive target value for protection of vegetation (based on the AOT40 statistic). This site currently meets the target value. Figure 7.2 shows a timeseries plot of the annual maximum daily 8-hour running mean for ozone. There appear to be no clear trends, although at Lough Navar this statistic appears slightly lower in recent years than in the late 1980s and early 1990s. If ozone concentrations remain at their current levels, occasional exceedences of the AQS objective may continue to occur at rural locations.
Figure 7.2 Maximum
Daily 8-hour Running Mean Ozone Concentration, µg m-3 Trends are sometimes more clearly identifiable in statistics based on longer sampling periods, such as the annual mean. Figure 7.3 shows a similar plot for the annual mean. Regression analysis (Theil’s non-parametric analysis) of the annual mean showed no statistically significant trends in the case of Londonderry or Lough Navar. However, in the case of Belfast Centre there was a positive trend in the annual mean ozone concentration from 1992 to 2001, significant at the 95% confidence level. Average ozone levels at Belfast Centre therefore appear to be increasing. However, it is possible that this is caused by decreasing concentrations of other pollutants (such as oxides of nitrogen), which “scavenge” ozone from the air, keeping levels low in urban areas.
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