UK Smoke and Sulphur Dioxide Network

Results and Discussion

4.1 UNITS
Both smoke and sulphur dioxide concentrations are expressed in microgrammes per cubic metre (µg m^-3) in this report and on the disk.
(i) Smoke: in the UK, smoke concentrations are usually calculated according to the British Standard Smokestain (BS) Calibration. This report primarily uses the BS calibration, and all black smoke concentrations are in this form except where specified. However, elsewhere in Europe, the Organisation for Economic Co-operation and Development (OECD) Smoke Calibration Curve (OECD Publication no. 17913: 1964) is used. Concentrations given according to the BS calibration can be converted to OECD by dividing by 0.85. In any communication with the European Commission, it is normal to use the OECD calibration. OECD smoke concentrations have been included in this report where applicable, and are indicated as OECD and shown in italics.
(ii) SO₂ : In this report, SO₂ concentrations are given primarily in µg m^-3. However, concentrations of gaseous pollutants are sometimes expressed as parts per billion by volume (abbreviated to "ppb"). For SO₂, the conversion factor used by the EC is as follows: 1 ppb = 2.66 µg m^-3, at a temperature of 20^oC and 1013 mb pressure. This only applies to SO₂; conversion factors are different for other gaseous pollutants. SO₂ concentrations in ppb are included where applicable, are indicated as such and shown in italics.

4.2    SUMMARY OF RESULTS
Summary statistics for the year 1997 - 98 for each Network site is provided in Table 1. The sites are listed by region (Scotland, North, North West, Yorkshire and Humberside, East Midlands, West Midlands, Wales, East Anglia, London, South East, South West, and Northern Ireland). Within each region they are grouped by the Local or Unitary Authority in whose area they are situated. In most cases, this Authority will be responsible for the operation of the site, although a small number are operated by other organisations such as universities.

For sites in England, Wales and Scotland, Table 1 gives the location as an Ordnance Survey grid reference, to the nearest 100m. For sites in Northern Ireland, the Irish Grid is used. Again, the co-ordinates are given to the nearest 100m, but both the easting and northing are prefixed by a 9, to indicate that the Irish Grid has been used.

4.2    DATA CAPTURE
Table 1 shows summary data for all sites irrespective of the data capture. It is, therefore, important to check on the actual data capture for any site, as provided in the column "Valid Days", to assess the significance and reliability of the summary statistics presented. Pollution Year 1997 - 98 contained 364 days (see Appendix 1 for details of the Pollution Calendar), so this value appearing in the "Valid Days" column indicates 100% data capture. The minimum data capture requirement of the EC Directive is 75% - equivalent to 273 days.

The data capture statistics for the Network for 1997 - 98 (excluding sites which started or ceased operation part way through the year) were as follows: 20% of sites achieved 100% data capture, 47% of sites had over 95% data capture, and 64% of sites had over 90% data. 15% of sites had less than 75% data capture. The percentages of sites with 100%, 95%, and 90% data capture are the same as for last year; however, the percentage with less than 75% data capture has been reduced. Hopefully, the downward trend in data capture observed in the past few years (thought to be due to equipment failure and resource shortages) is beginning to reverse. Advice for site operators on dealing with equipment faults and maximising data capture are given in the Instruction Manual, which was fully updated and re-issued to all site operators in 1997. Additional copies of the Manual are available from AEA Technology if required.

4.3    NATIONAL AVERAGE SMOKE AND SO₂ CONCENTRATIONS
Mean UK concentrations in 1997 - 98, based on the core sites subset only, were as follows:
- Smoke: 9.4 µg m^-3 BS (11.1 µg m^-3 OECD).
- SO₂ : 20.6 µg m^-3 (7.7 ppb).
These values are slightly lower than the 1996-97 means of 9.9 µg m^-3 (11.6 µg m^-3 OECD) and 22.4 µg m^-3 (8.4 ppb) for smoke and SO₂ respectively.

4.5    IDENTIFICATION OF SITES WITH HIGH CONCENTRATIONS
Table 2 lists the sites identified as having the highest annual average smoke concentrations. All those with annual average smoke concentrations of 14 µg m^-3 or above have been included; these total 17 ( only sites with at least 75% data capture, ie. 273 days' data, for the year are shown). The sites with the highest smoke concentrations were predominantly in areas where the use of coal for domestic heating is still prevalent (eg. South and West Yorkshire, Northern Ireland), with some others in large towns and cities ( eg. Bradford, Manchester, London).

The highest annual average was 21 µg m^-3 BS (25 µg m^-3 OECD), recorded at the LONDONDERRY 11 site. This site was near the top of the same table last year, and has exhibited a small increase. In recent years, HALIFAX 16 has occupied the top of this table. It is missing this year, as just less than 273 day's data were available. However, an annual average of 20 µg m^-3 BS was recorded at HALIFAX 16, making it joint 2nd highest with BRADFORD 6. HALIFAX 16 has exhibited the highest annual average smoke concentration for six consecutive years, probably due to its location in the town centre, near a bus station, major roads, and several car parks. Eight of the sites in Table 2 are in the Yorkshire and Humberside region; specifically, in south and west Yorkshire.

Table 3 lists the sites identified as having the highest SO₂ concentrations. All those with annual mean SO₂ of 36 µg m^-3 (14 ppb) or above have been included (again, excluding any with less than 273 days' data). The highest annual average was 53 µg m^-3 (20 ppb) recorded at BELFAST 42 and LONDONDERRY 11. Eight sites in Northern Ireland appear in this list. DUNMURRY 2 occupied the top of this table in several previous years, but has now moved down. High SO₂ concentrations have frequently been observed in Northern Ireland (where natural gas is not available for domestic heating and use of coal is therefore higher), and the South Yorkshire and East Midlands areas historically associated with the coal industry, where again domestic use of coal is prevalent.

CARDIFF 12 appears in Table 3 this year, having exhibited a substantial increase in its measured concentrations of both pollutants, particularly SO₂ . Reasons for this are being investigated; the Cardiff Centre AUN site has not recorded a similar increase in SO₂ concentration, so a local source is suspected.

Three sites appear in both Table 2 and Table 3, showing relatively high concentrations of both smoke and SO₂. These are LONDONDERRY 11, (Northern Ireland), BRAMPTON 1, and MALTBY 2 (both near Rotherham). All of these sites are in areas where there is relatively high consumption of coal for domestic heating.

4.6    TRENDS
The timeseries of the annual mean smoke and sulphur dioxide concentrations for the UK as a whole is provided in Figure 2. The values shown in Figure 2 are averages from all sites in the network up to 1980, and thereafter from all sites in the core subset only. As explained in Section 2, this subset is intended to provide a representative selection of monitoring sites in urban areas throughout the UK. The resulting graph shows the rapid decrease in concentrations in the 1960s and 1970s and the more gradual decrease through the 1980s and 1990s.

4.7    SPATIAL DISTRIBUTION OF SMOKE AND SO₂
Figure 3 and Figure 4 show annual means of smoke and SO₂ respectively, for the entire Network. Only sites with less than 75% data capture have been excluded.

The data for 1997 - 98 shows a similar pattern to that observed in previous years. Highest values for black smoke and sulphur dioxide occurred not only in large conurbations, but also some smaller towns; most notably those in areas with a history of coal mining, where domestic use of coal is still relatively prevalent. In addition, relatively high concentrations of SO₂ were measured in Northern Ireland, particularly Belfast; natural gas is has very limited availability for domestic heating in this region, so greater use is made of coal and oil.

4.8    SEASONAL PATTERNS

It has long been known that smoke and SO₂ exhibit a seasonal variation. Concentrations of both pollutants, but particularly black smoke, are typically higher in the winter months (October to March). This is partly because a major source of both pollutants is the combustion of coal and oil; this increases during winter, mainly because of domestic and industrial heating requirements. In addition, cold, still weather conditions may prevent dispersion.

Figure 5 shows UK monthly mean concentrations of smoke and SO₂ , for the period April 1997 - March 1998. These are based on all sites in the Network. The seasonal variation for smoke is clearly visible, with monthly mean concentrations typically 5 to 8 µg m^-3 during the summer months, but rising to 8-16 µg m^-3 during the winter. For SO₂, the pattern is much less pronounced, with monthly means between 21 and 24 µg m^-3 (8-9 ppb) throughout the year, only rising slightly in winter.