UK Smoke and Sulphur Dioxide Network

Comparison with National Air Quality Standards

6.1 NATIONAL AIR QUALITY STANDARD FOR SO₂

The UK National Air Quality Strategy⁷ was published in March 1997. It includes a standard for SO₂, of 266 µg m^-3
(100ppb) as a 15 minute average concentration, and an objective that the 99.9th percentile of 15 minute means is within this value by 2005. The UK National Air Quality Strategy standard applies only to 15 minute means, and does not provide any equivalents relating to daily data. It is therefore not possible to compare Network data with this standard, although the Network data are useful in identifying areas of high SO₂ concentration which can then be targeted for automatic monitoring.

However, a recent study⁸ has found strong relationships between the 99.9th percentile of 15 minute means for SO₂ , and both the annual mean and the maximum daily mean. This study was based on data from automatic monitoring sites, largely urban background and urban centre sites. This study of surrogate statistics found that it was likely that the 99.9th percentile of 15 minute mean will exceed 100 ppb (267 µg m^-3) if -
- the annual mean exceeds 7ppb (19 µg m^-3),
- the maximum 24 hour mean exceeds 48 ppb (128 µg m^-3).
As the peroxide bubbler method of measurement used in this Network is known to underestimate peak SO₂ concentrations, a factor of 1.25 should be applied to data when using this relationship.

When applied to the Network data, these two surrogates predict very different numbers of exceedences. The surrogate for the annual mean, 19 µg m^-3, is less than the Network 1997-98 annual mean of 20.6 µg m^-3, and exceedence is very widespread; 117 sites had annual means greater than 19 µg m^-3 in 1997-98.

The surrogate for the maximum day, 128 µg m^-3, was exceeded by only 23 sites in 1997-98. Dividing the surrogate maximum day limit by 1.25 to account for under-reading by the method used gives 102 µg m^-3 for the maximum day; 39 sites had maximum daily means greater than this value in 1997-98.

Of the two surrogate relationships, the latter showed the stronger correlation⁸, and less uncertainty. In addition, both parameters relate to peak, rather than average, values. Therefore it would be expected that the latter relationship is the more reliable.

6.2 NATIONAL AIR QUALITY STANDARD FOR PM₁₀

There is also an objective for particulate; 50 µg m^-3 measured as the 99th percentile of running 24 hour means (this in effect means that no more than four days in a full year's monitoring should exceed this concentration). The particulate objective specifically applies to PM₁₀, not black smoke, and the UK National Air Quality Strategy acknowledges that these two techniques are not the same. However, as explained above, PM₁₀ is usually higher than black smoke except at very high "episode" concentrations, so if daily mean smoke exceeds 50 µg m^-3, it is likely that PM₁₀ has also done so. Table 1 shows the 98th percentile of daily black smoke values exceeded 50 µg m^-3 at many sites during 1997 - 98; in such cases it is likely that exceedence of the PM₁₀ standard may also have occurred.