Measurement and Monitoring of Ozone

9. Ozone is measured at ground level by ultraviolet absorption spectrophotometry. Continuous measurements are made by the Department of the Environment at sites throughout the United Kingdom and the results are relayed by telemetry to central management units. Hourly ozone monitoring to an accuracy of +/- 10% and precision of 2 ppb¹ can be achieved with current instrumentation. Currently, 17 Rural Network Sites and 12 Enhanced Urban Network sites are being monitored in this way. Figure 4 shows the positions of these 29 monitoring stations currently in operation. Siting and sampling height are of crucial importance with ozone monitoring particularly in urban areas, where proximity to motor vehicle exhausts and boiler flues can dramatically reduce local ozone exposure levels. The information received from these sites, together with other information on air quality, is made available to the public by the Department of the Environment via the free telephone helpline (0800 556677), CEEFAX (Page 404), TELETEXT (Page 187) and the media. The detailed ozone data are also available from the Department of the Environment.
10. In addition to this national monitoring network, ozone has been measured in the British Isles since 1972 in a number of other sites, and information is available from 53 places for some or all of this time. These data are used in Figure 5, which shows the maximum concentrations recorded at each of these sites over the period 1972 to 1991. For reasons given later, these concentrations are averages over an 8-hour period. It can be seen that there is substantial variation in concentrations from place to place over the time period. Tn general, the lowest of these maximum concentrations were recorded in remote, northerly sites, while the highest (over 200 ppb) occurred in 1976 in rural southern England during a summer pollution episode.
11. Such differences are, however, trivial compared with the substantial variations in ozone concentrations occurring as a result of season and geography. Figure 6 illustrates the maximum concentrations of ozone occurring at monitoring sites in the British Isles between 1972 and 1991 (expressed as 8-hour running averages) for each month of the year. It can be seen that concentrations rarely exceeded 50 ppb in the winter months, but did so not infrequently from March to October. Concentrations above 100 ppb were almost confined to the months of May to August. Figure 2 is based on data from all sites operational in the United Kingdom between 1987 and 1990, and illustrates thenumber of hours in which the ozone concentrations rose above 80 ppb. Figure 2 and Figure 5 show that ozone concentrations are higher in rural areas in the south of England.
12. In determining an Air Quality Standard, it is important to consider the time over which the measurement is made. In theory, options could range from instantaneous measurements to ones averaged over as long as a year. Since ozone concentrations fluctuate over the day (falling during the hours of darkness), concentrations high enough to cause a risk to health usually only occur during daylight hours. The medical evidence, discussed later, suggests that exposure over several hours is usually necessary to provoke effects on people, and the Panel have been persuaded by the evidence that the proposed Standard should be based on a concentration averaged over eight hours. In order to ensure that maximum concentrations are not missed by arbitrary averaging periods from, say, midnight to 8 am, 8 am to 4 pm and 4 pm to midnight, we recommend that a running 8-hour average² should be used for the Standard.
13. From the foregoing, it may be seen that ozone pollution episodes arise as a consequence of several causes, often in combination. Some are not preventable by action taken in the United Kingdom - for example the occasional high concentrations occurring as a result of incursions of ozone-rich air from the stratosphere, which occur most frequently in the winter months, or the more common drift of air polluted by ozone and/or its precursors from continental Europe. The major factor, however, is summertime traffic pollution, and this is an area where both individuals and the Government can make a beneficial contribution. The widespread introduction of catalytic converters and of exhaust emission controls should bring about improvements, although these benefits have the potential eventually to be negated by increasing traffic density in and around cities.

²   Running 8-hour average ozone concentrations are calculated by first calculating the hourly average ozone concentrations over fixed periods from 00.00 to 00.59 onwards. These hourly averages are then taken consecutively in groups of eight and the 8-hour averages for 00.00 - 07.59, 01.00 - 08.59, etc onwards.