Measurement and Monitoring of Sulphur Dioxide

10. Following the introduction of the 1956 Clean Air Act, the existing network for the measurement of smoke and sulphur dioxide was expanded to form a national survey. The measurements were made on a 24-hour average basis by local authorities and other organisations, using a standardised peroxide bubbler method which, strictly speaking, measures net gaseous acidity. At the same time, air pollution by particles was monitored by the black smoke method which involved measuring the darkness of a black stain produced when air was drawn through a filter paper. At its peak in 1966, this national survey collected data from no fewer than 1200 sites around Britain, mostly in urban locations. The Department of Trade and Industry's Warren Spring Laboratory was responsible for the collection, quality control and publication of these data.
11. As the 1956 Clean Air Act took effect, there was a progressive fall in the concentrations of sulphur dioxide, which allowed a progressive reduction in the number of monitoring sites as more towns complied with the legislation. There are currently 225 such sites. Figure 1 shows the reduction in the levels of sulphur dioxide measured at 2 sites, Lambeth (in London) and Belfast, between 1961 and 1993. European Council Directive (80/779/EEC), which came into force in 1982, set limit and guide values for sulphur dioxide and suspended particulates (smoke). In compliance with the Directive, 155 of these sites are now in the EC Directive Network, and the others, together with a selection of sites from the Directive Network, comprise the Basic Urban Network. The numbers of Directive sites have also been progressively reduced as concentrations of smoke and sulphur dioxide have declined in Britain.
12. The medical evidence, as discussed in the next section of this report, indicates that immediate effects on health may follow relatively short-term exposures to sulphur dioxide. The World Health Organization (WHO) in 1987 recommended, in the Air Quality Guidelines for Europe, a guideline for sulphur dioxide expressed as a 1-hour average concentration. In the light of this recommendation, the Department of the Environment has been reporting sulphur dioxide concentrations on the basis of 1-hourly measurements and has described these in bands ranging from 'Very Good' to 'Very Poor'. The Department is currently considering amendments to this descriptive scheme. The different limits, guidelines, and bands currently in use in Britain are shown in Table 2. It should be noted that the EC Directive's limit values, which are the only ones to have the force of law, differ according to the concurrent concentration of smoke particles. This reflects the research on which the EC Directive was based, namely that the two pollutants together were found to be more harmful than either alone.
13. In order to measure sulphur dioxide concentrations over short periods, continuously recording automatic analysers are necessary. Measurements using such instruments have been made on a small scale in the United Kingdom since the 1970s, but since 1992 the number of sites has been increased with the establishment of the Department of the Environment's Automatic Urban Network. In March 1995 there were 20 stations operating in this network. together with another 3 rural sites at which sulphur dioxide is continuously monitored. The locations of these sites are shown in Figure 2.
14. The control of urban smoke emissions has succeeded in reducing the number of exceedences of the EC Directive limit values to rare events, largely confined to areas where the burning of fossil fuel continues to be an important source of pollution. However, the 1-hour WHO Guideline is exceeded more often, and the maximum hourly-averaged levels of sulphur dioxide observed each month from January 1988 to March 1993 for 6 of the monitoring sites are summarised in Figures 3A and 3B. It can be seen that exceedences have occurring at a number of locations throughout the year, the highest concentrations occurring in the winter months. The highest hourly figures, of over 500 ppb¹, occurred in Belfast.
15. A summary of the sulphur dioxide data obtained frorn the 18 operational automatic monitoring sites for 1993 is given in Table 3. Table 3 also gives the number of occasions when the concentration of sulphur dioxide exceeded 100 ppb, averaged over a 15-minute period. In general, exceedences of this level have tended to occur at sites in areas where solid fuels have been burned for domestic space heating, such as Belfast or Featherstone~Barnsley. The Ladybower site in rural Derbyshire has experienced a number of exceedences, probably as a consequence of its location between several large fossil-fiielled power stations, which suggests that many other areas in central England near large power stations may also experience transient high concentration events.
16. A further conclusion to be drawn from Figure 3a is that exceedences of the WHO Guideline also occur at the Cromwell Road kerbside site in west London. In general, the annual average concentration beside Cromwell Road is consistently some 6 to 10 ppb higher than that recorded at the central London background site in Bridge Place, as shown for the 1993 data in Table 3. This difference is a consequence of the sulphur dioxide in diesel exhaust; however, the peak concentrations tend to occur simultaneously at the two sites, suggesting that other sources such as industry and power stations in the Thames estuary also make an important contribution.
17. As will be seen in the next section, the medical evidence shows that the effects of sulphur dioxide on health may occur within a very short time after exposure to a sufficient concentration. This suggests that it would be desirable for any standard to take account not of average concentrations over periods of an hour or a day, but rather of shorter periods down to a few minutes. This reasoning was used in deriving the WHO Guideline, for which the 1-hour value is calculated on the basis of likely 10-minute peaks within the hour. Figures 4 and 5 illustrate the relationships between the hourly and the maximum 15-minute means at two United Kingdom monitoring sites, Belfast, which has multiple low level sources, and Ladybower, which is exposed to plumes from a few high-level power station stacks. On average, the 15-minute maxima are approximately 17% higher than the hourly means in Belfast and 20% higher at Ladybower.

Table 2 Air quality standards and guidelines for sulphur dioxide (ppb) 6

Source/ Organisation	Annual averages (April - March)	98th percentile (daily measurements April - March)	24-hour average	1-hour average	10-minute average
EC Directive
Limit values	30 (smoke > 34 µm3)	94 (smoke > 128 µm3)	-	-	-
	45 (smoke < or = to 34 µm3)	131 (smoke < or = to 128 µm3)	-	-	-
Guide values	15-23	-	38-56	-	-
World Health Organisation	-	-	-	130	188
DoE Very Good Good Poor Very Poor	- - - -	- - - -	- - - -	< 60 60-124 125-399 >or = 400	-  - - -

Notes regarding EC D rect ve 801779/EEC:

• refers to daily measurements;
• annual average limit value is expressed as the median value; annual average guide values are arithmetic means;
• two limit values quoted depending upon associated smoke concentrations, (given in brackets as µ/m³);
• values for sulphur dioxide are given in ppb, the EC Directive specifies values in µ/m³ and they have been converted in this table using a factor of 1 µm³ = 0.376 ppb. (assuming a temperature is 20^oC)
• values for black smoke are given in µg/m³ for the BSI method used in the UK. The values stated in the EC Directive relate to the OECD method, where OECD = BSI/0.85

Table 3 1993 Calendar year statistics for sulphur dioxide and the number of exceedences of the Panel's recommended Air Quality Standard

Site	Annual mean (ppb)	Max. hourly average (ppb)	Max. 15-minute average (ppb)	Number of 15 minute periods >100 ppb	Number of days on which the recommended standard was exceeded
Strath Vaich (North Scotland)	0.4	9	10.7	0	0
Edinburgh (Centre)	8	171	218	50	16
Newcastle (Centre)	8	139	189	34	10
Sunderland	6	132	139	34	6
Belfast (Centre)	18	288	326	447	48
Belfast (East)	25	432	476	852	81
Leeds (Centre)	10	206	269	97	18
Barnsley 12	26	350	614	1160	93
Liverpool (Centre)	15	190	269	185	38
Ladybower (Derbyshire)	7	107	182	19	7
Birmingham (Centre)	10	196	206	46	10
London (Bloomsbury)	11	283	306	121	26
London (Bridge Place)	8	202	237	71	16
London (Cromwell Road)	14	199	225	67	15
Cardiff (Centre)	6	144	169	10	4
Bristol (Centre)	8	124	197	14	10
Lullington Heath(East Sussex)	2	48	68	0	0
Stevenage	6	124	128	12	3