ANNEX

1. The Department of the Environment will issue guidance to assist local authorities with their new duties under the Environment Act 1995 to review air quality in their areas. In the meantime, this Annex provides advice on how to use the Panel's recommended Air Quality Standard for sulphur dioxide to interpret air quality monitoring data which may have been collected in monitoring surveys which do not record 15-minute mean concentrations.
2. The Department of the Environment's Automatic Urban Network provides public information on air quality. We recommend that 15-minute mean sulphur dioxide concentrations are reported by the Network and that the bandings are adjusted so as to be consistent with the Panel's recommended Air Quality Standard at the earliest convenience.
3. In addition there are many sulphur dioxide monitoring stations that generate daily air quality data in compliance with the EC Directive on sulphur dioxide and suspended particulates (smoke). The Department of the Environment currently operates 225 sites using the peroxide bubbler method, measuring mean daily sulphur dioxide concentrations. If there were a straightforward relationship between air pollutant concentration and averaging period, then it would be possible to interpret exceedence of the Panel's recommended 15-minute Standard in terms of the corresponding daily mean value.
4. Clearly if the daily mean concentration exceeded 100 ppb, then it would be certain that at least one 15-minute mean concentration exceeded the Panel's recommended Standard. Daily mean concentrations significantly below 100 ppb could conceivably conceal excursions above the Standard.
5. In areas where significant quantities of coal and other sulphur-containing fuels are burnt in homes, sulphur dioxide concentrations have remained significantly elevated. In these areas, a reasonably well-defined relationship exists between sulphur dioxide concentrations and averaging time periods. On this basis, if either the maximum of the daily mean concentrations was below 28 ppb or 98% of the daily mean concentrations were below 19 ppb, then it is unlikely that the Panel's recommended 15-minute Standard would have been exceeded.
6. In the majority of urban areas these days, sulphur dioxide is not derived predominantly from domestic fuel combustion. Here, pollution by plumes from power stations and other large sources, often at some considerable distance, may be the cause of the sporadic excursions in sulphur dioxide concentration which lead to exceedences of the Panel's recommended 15-minute Standard.
7. In some surveys, even with modern instrumentation, the direct measurement and recording of 15-minute concentrations may prove to be too difficult or onerous. Under these circumstances, the Panel's recommended Standard would be considered to have been exceeded whenever the hourly mean sulphur dioxide concentration was greater than 50 ppb. This figure includes a substantial margin of error of up to a factor of two to allow for the possible presence of sporadic excursions. Clearly, the best measure of air quality with respect to sulphur dioxide is obtained using 15-minute, rather than hourly or daily, mean concentrations.