UK Smoke and Sulphur Dioxide Network

Comparison with EC Directive Limits and Guide Values
In previous years, data from the UK Smoke and SO₂ Network have been compared with the European Council Directive² 80/779/EEC on sulphur dioxide and suspended particulates. However, this Directive is undergoing review, and in 1998 the European Commission published their proposal for the Daughter Directive³ on SO₂, oxides of nitrogen, particulate matter and lead.

The original Directive is still in force, and will remain so until the formal adoption of the Daughter Directive, expected in 2001. In addition, the limits relating to black smoke remain in force until 2005. The current report therefore compares results from the Smoke and SO₂ Network with the relevant sections of both the original and "new" Directives.

5.1   DIRECTIVE EEC/80/779 (THE SO₂ AND SMOKE DIRECTIVE)
The limit values are presented in Table 4, along with the non-mandatory guide values. It should be noted that the EC Directive cites smoke concentrations calculated using the OECD Smoke Calibration Curve (OECD Publication no. 17913: 1964).

Results from all sites operational in 1997 - 98 were examined for compliance with the EC Directive limit and guide values. Limit values are mandatory, whereas guide values are advisory only, and are intended to serve as long term precautions for health and the environment.
It should be noted that the EC require that percentile calculations (eg. median, 98th percentile) are calculated by their specified method. For checking formal compliance with the EC Directive, the specified Directive method has always been used. Hence, percentiles calculated by the EC Directive method are discussed in this section.

5.1.1   Exceedences of the EC Directive Limits
Analysis of the data for 1997 - 98 shows that for the fifth year running, no site exceeded any of the limit values of the EC Directive. The steadily decreasing number of sites at which exceedences have been recorded in previous years is shown in Table 5.

5.1.2        Sites "At Risk" of Exceeding EC Directive Limits
"At Risk" of exceeding a limit has been defined as being within 75% of the limit values. No sites were identified as being "At Risk" of exceeding the 98th percentile limit, the annual median limit, or the winter median limit.

The peroxide titration method of analysis used in the UK monitoring network is not the reference method of the Directive, but has been accepted as an equivalence method, under the terms of Article 10 of the Directive. However, under the terms of this acceptance as an equivalence method, it was agreed with the Commission in 1989, that a factor of 1.25 would be applied to the sulphur dioxide results to compensate for a possible underestimation of peak values. In practice, this is done by dividing the appropriate SO₂ limits by 1.25, and comparing measured data with this adjusted limit. No sites exceeded the adjusted limit values; also, 1997-98 was the first year in which no sites were "At Risk" of breaching any of the adjusted annual limits.

5.1.3        Comparison with EC Guide values
Numbers of sites exceeding the EC Directive guide values are given below:
- Smoke: Annual Arithmetic Means of Daily Values
  Sites with annual arithmetic mean smoke > 34 µg m^-3 BS: no sites
  Sites with annual arithmetic mean smoke > 51 µg m^-3 BS: no sites
  It has been rare for sites to exceed the lower smoke guideline in recent years.
- SO₂: Annual Arithmetic Means of Daily Values
  Sites with annual arithmetic mean > 40 µg m^-3 : 12 sites
  This is a decrease on last year's total of 19 sites.
  Sites with annual arithmetic mean > 60 µg m^-3 : No sites
  Last year, one site exceeded this guideline. This is the first year in which no sites have done so.
- Smoke: 24 hour means
  Sites with one or more 24 hour mean > 85 µg m^-3 BS: 39 sites
  This is a substantial reduction on last year's total of 52 sites, and continues the downward trend.
  Sites with one or more 24 hour mean > 128 µg m^-3 BS: 8 sites
  This is a small increase on last year's total of 6 sites.
- SO₂: 24 hour means
  Sites with one or more 24 hour mean > 100 µg m^-3: 41 sites
  Last year's total was 54 sites.
  Sites with one or more 24 hour mean > 150 µg m^-3: 11 sites
  This is a reduction on last year's total of 18 sites.
5.1 THE DAUGHTER DIRECTIVE
The proposed Daughter Directive on SO₂, NO₂, PM₁₀ and lead³ reached Common Position in the summer of 1998. This Directive will contain limit values for these pollutants, aimed at protection of human health and, in some cases, of ecosystems.

Only the parts of the Daughter Directive relating to SO₂ and particulate will be discussed here; the limits are given in Table 6 and Table 7. These limits are still at the proposal stage, and there may be some change before they come into force.

There is also an "alert threshold" for SO₂ of 500 µg m^-3 (188 ppb), measured over three consecutive hours at representative sites over at least 100 km² or an entire zone or agglomeration, whichever is smaller. Public warnings and advice are to be issued if this threshold is exceeded.

The proposed Directive limits are accompanied by "upper and lower assessment thresholds", which specify what type of monitoring is required.
- The upper assessment threshold is the level below which a combination of measurements and modelling techniques may be used to assess air quality;
- the lower assessment threshold is the level below which modelling alone, or with objective estimation techniques, is considered sufficient for assessment of air quality.
This ensures that monitoring resources are targeted where they are most needed. Exceedence of these assessment thresholds is to be determined on the basis of data from the previous five years where available.

It is clearly not possible to compare data from the Smoke and SO₂ Network with the proposed hourly limit. Nor is it relevant to compare data from this urban network with the annual and winter limits for protection of ecosystems, which are intended for protection of rural areas. However, it is possible to compare the daily data from the Network with the proposed 24 hour limit, for protection of human health.In the calendar year January - December 1997, there were 7 sites with more than three days over the proposed 24 hour limit of 125 µg m^-3 for SO₂: these were as follows:

BARNSLEY 8
BELFAST 13
BELFAST 42
BELFAST 44 BELFAST 45
BRAMPTON 1
NEWCASTLE UPON TYNE 24

The proposed particulate limits relate to PM₁₀, not black smoke, and therefore Network data cannot be directly compared with these. However, PM₁₀ is usually higher than black smoke except at very high "episode" concentrations (typically winter episodes of primary particulate), so if there are more than 35 days in the year on which mean black smoke concentration exceeded 50 µg m^-3, it is likely that this is also the case for PM₁₀, and the daily PM₁₀ limit would be exceeded. Likewise, if the annual mean black smoke concentration for the 12 month period April 1997 to March 1998 exceeds 40 µg m^-3, it is likely that the annual mean PM₁₀ concentration has also done so.

During the period April 1997 - March 1998, no sites had more than 35 daily mean black smoke concentrations greater than 50 µg m^-3. While this does not highlight any particular sites as being likely to exceed the 24 hour PM₁₀ limit, it does not mean that no exceedences will occur. Indeed, some exceedences are predicted, particularly at city centre and roadside locations, on the basis of data from automatic PM₁₀ monitoring sites.

In addition, no sites had annual mean black smoke concentrations higher than 40 µg m^-3 BS; the highest was 21µg m^-3 (see Table 3.) Therefore, black smoke data do not indicate any specific sites where exceedence of the annual limit is likely. Again, however, exceedence of PM₁₀ limits cannot be ruled out.

The black smoke limits of the Smoke and SO₂Directive will remain in force until 1 January 2005.