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Sulphur Dioxide
Sulphur dioxide (SO2) is formed during the combustion of fuels containing sulphur. The most significant source of this pollutant is fossil fuelled power generation, although diesel engines and a number of chemical processes also produce SO2. In Northern Ireland, domestic solid fuel and oil burning is a major source of SO2. Sulphur dioxide is a respiratory irritant, and is toxic at high concentrations. It is also damaging to ecosystems and a major precursor in the formation of acid rain. In this report, concentrations of sulphur dioxide are expressed as microgrammes per cubic metre (µg m-3). To convert to parts per billion (ppb) if necessary, the following relationship should be used: 1 ppb = 2.66 µg m-3
for sulphur dioxide at 293K (20°C) and 1013mb. Monitoring of SO2 is carried out by three methods: continuous automatic analysers, the non-automatic Net Acidity method (using the 8-port sampler) and diffusion tubes. Automatic analysers (based on the UV Fluorescence method, which is the reference method for the EC 1st Daughter Directive, 1999/30/EC) provide continuous monitoring of SO2, and the data can be compared with air quality limit values and objectives based on short-term and longer averaging periods. Automatic SO2 monitoring sites in Northern Ireland are listed in Table 5.1 and shown in Figure 5.1: Table 5.1
Automatic SO2 Monitoring Sites
A more widespread method of measuring SO2 is by the 8-port sampler apparatus. This also measures suspended particulate matter as black smoke (see Section 6). This technique technically measures total net acidity rather than sulphur dioxide. However, it is widely used for low-cost indicative measurement of SO2. This non-automatic method samples on a 24-hour basis, so results are not suitable for comparison with air quality objectives based on shorter periods. However, it has been in widespread use since the early 1960’s, so there is an extensive historical dataset which can be used to assess trends. During 2000 and 2001, there were 31 smoke and SO2 monitoring sites operating in Northern Ireland, all but two belonging to the Smoke and SO2 Network. A further three sites (Bentra, Cam Forest and Fermoyle which replaced the latter in 2001) were part of the Rural SO2 Network, monitoring SO2 only. These are shown in Table 5.2 and Figure 5.2. Table 5.2
Non-Automatic Smoke and SO2 Monitoring Sites
(Rural SO2 sites monitor SO2 only, not smoke.)
Diffusion tubes are also used to monitor SO2. These passive samplers are similar to those used for NO2 and described in Section 4. However, it should be noted that SO2 diffusion tubes are considered less accurate than NO2 diffusion tubes, and they are considered unsuitable for use for Review and Assessment purposes. Table 5.3 and Figure 5.3 show the SO2 diffusion tube monitoring sites in use during 2000 and 2001. Site details are given in Appendix 1. Table 5.3
Diffusion Tube SO2 Monitoring Sites
Sulphur dioxide is covered by the following limit values and objectives as shown in Table 5.4. All these limits are for protection of human health except where stated. Table 5.4 Limit
Values and Objectives for Sulphur Dioxide
Before the 1st Daughter Directive came into force, SO2 was covered by EC Directive 80/779/EEC on Sulphur Dioxide and Suspended Particulates. This Directive has been superseded by the 1st Daughter Directive. Although the limits of this older Directive remain in force until they are fully repealed in 2005, they are less stringent than those in the later 1st Daughter Directive and have been fully met in Northern Ireland since the early 1990s. The current report therefore compares current SO2 results with the limit values of the 1st Daughter Directive, rather than Directive 80/779/EEC. Table 5 shows results from the automatic SO2 monitoring sites. Annual data capture is at least 75% except where stated. Figures in bold italics indicate more than the permitted number of exceedences of the relevant limit value. Table 5.5 SO2 Results from Automatic Monitoring Sites
a) Limited data capture: Londonderry 1997 (26%) * Data from these two Newry sites are awaiting full
ratification. In most cases, the requirements of the 1st Daughter Directive and the objectives of the Air Quality Strategy are met at these sites. However, at Belfast East the objective for the 15-minute average is still consistently exceeded more than the permitted 35 times per year. Also, in 2001, the 24-hour limit value was exceeded more than the permitted three times. It would appear that the area in the vicinity of this site (a residential suburb where domestic solid fuel use is prevalent) is likely to have difficulty in meeting the AQS objectives for sulphur dioxide. The same is likely to be true of similar locations. A summary of results from all Smoke and SO2 Network sites in Northern Ireland is provided in Appendix 2, for years 2000 and 2001. Relatively high concentrations of SO2 have been measured in Northern Ireland, particularly Belfast, for many years. The limited availability of natural gas in previous years has led to greater domestic use of solid fuels and oil. This has led to higher concentrations of pollutants such as SO2, particularly in residential areas. Data from 8-port samplers (i.e. daily 24-hour averages) cannot be directly compared with air quality limits based on shorter averaging periods. Nor is it relevant to compare data from urban sites with the annual and winter mean limit values set for the protection of ecosystems. However, both the EC 1st Daughter Directive, and the Air Quality Strategy, set an objective of 125 µg m-3 for the 24-hour mean, not to be exceeded more than 3 times per calendar year. In 2000, the overall average SO2 concentration for all Smoke and SO2 sites in Northern Ireland was 27 µg m-3; considerably higher than the annual mean of 15 µg m-3 obtained for the whole UK. Only one Smoke and SO2 site in Northern Ireland recorded more than three days when the 24-hour average for SO2 was greater than the limit of 125 µg m-3: DUNMURRY 2. However, DUNMURRY 2 was suspected to be directly affected by emissions from a nearby boiler chimney, and has since been replaced by DUNMURRY 3. The annual mean SO2 concentration at DUNMURRY 2 was unusually high (82µg m-3) for the same reason. At other sites, annual mean SO2 concentrations ranged from 8µg m-3 in Portadown, to 42µg m-3 at BELFAST 33, a site in an industrial area of Belfast. In 2001, the overall annual mean SO2 for all sites in
Northern Ireland was 20 µg m-3.
This value is lower than the previous year but still considerably higher than
the annual mean for the whole UK, which was again 15 µg m-3. Three Smoke and SO2 sites in Northern Ireland exceeded the 24-hour limit value of 125 µg m-3 on more than
three days; BELFAST 13, BELFAST 33 and BELFAST 42. These exceedences related to
episodes of high SO2 during January and February 2001. Annual mean
SO2 concentrations for 2001 ranged from 3µg m-3 at the small town of Keady near Armagh, to
43 µgm-3 at BELFAST 33. Annual mean SO2 concentrations at the two non-network sites,
Downpatrick and Newtownards, were as follows: Downpatrick, 10.7 µg m-3 in both 2000 and
2001: Newtownards, 8.6 µg m-3
in 2000. Annual and winter mean sulphur dioxide concentrations at the three Rural
SO2 Network sites (Bentra,
Cam Forest and Fermoyle) were less than 5 µg m-3
: well within the limit of 20 µg m-3
set for protection of vegetation in rural areas. As stated above, SO2 diffusion tubes produce data of limited accuracy and are considered unsuitable for use for Review and Assessment purposes. However, some limited SO2 diffusion tube data are available for years 2000–01. Where annual means are available, these are shown in Table 5.6. Annual mean SO2 concentrations, as measured using diffusion tubes, ranged from 2 µg m-3 to 15 µg m-3. These values appear typically lower than those measured using other techniques. Table 5.6 Annual
Mean Sulphur Dioxide as Measured at Diffusion Tube Sites
nm = not measured. For reasons outlined in Sections 1.1 and 5.4, widespread reliance on solid fuels and oil for domestic heating has continued throughout the 1970s, 1980s and 1990s. Domestic combustion accounts for an estimated 28% of total annual SO2 emission in the Belfast area2, compared with 4% of total annual SO2 emission in the UK as a whole. Sulphur dioxide emissions and source distribution have therefore been different in Northern Ireland than in other parts of the UK for many years, and it cannot be assumed that UK trends in either emissions or ambient concentrations are necessarily representative of Northern Ireland. However, UK emission data from the NAEI shows two interesting features: (i) a decrease of almost 70% between 1990 and 2000, and (ii) a levelling-off of the downward trend in the most recent years 1999 and 2000. Trends at Automatic Sulphur Dioxide Sites Trends in SO2 concentration, as monitored at the automatic sites, are illustrated by Figure 5.4. Regression analysis (Theil’s non-parametric regression analysis) identified a significant downward trend (with 95% confidence limit) in the annual mean SO2 concentrations at Belfast Centre and Belfast East, though not at Londonderry. It is interesting to note the apparent levelling-off of the downward trend in 2000-2001: it is possible that this may reflect a levelling-off in total emissions, similar to that evident in the UK data.
(data capture at
least 75%) Trends at Non-Automatic Sulphur Dioxide sites Most of the non-automatic (8-port sampler) sites belong to the Smoke and SO2 Network, and many have a long-running historical dataset. Thus, it is possible to identify how concentrations of sulphur dioxide, as measured by the net acidity technique, have decreased since the early 1960s. This trend is shown in Figure 5.5, a graph of the average SO2 concentration at all Network sites in Northern Ireland since 1962. For historical reasons the annual averaging periods run April -March. The annual mean is based only upon sites with at least 75% data capture for the year, which in most years totalled between 14 and 27. The annual average concentration of SO2 has fallen, from over 80 µg m-3 in the 1960s to around 30 µg m-3 in 1980. From 1980 – 1987, average concentrations rose, before the downward trend continued again from the late 1980s. A possible explanation for the rise in the early 1980s is that it may have resulted from a rise in coal and oil burning, as the use of town gas was phased out. Town gas use decreased through the late 1970s and early 1980s, and the eventual shut-down of the supply began in 1984 and was completed in 1988. Figure 5.5 also shows the trend in annual mean for three particular sites in Belfast: BELFAST 12 (at the Royal Victoria Hospital), BELFAST 13 (at Templemore Avenue, co-located with the Belfast East automatic monitoring site) and BELFAST 33 (Dufferin Road, an industrial area of the city centre). All of these have been in continuous operation since the early 1960s. These three individual sites show a similar pattern to the average, including the period during the early 1980s when the general downward trend appeared to be reversed.
Figure 5.5 Annual
Mean SO2 Concentration at Smoke & SO2 Sites in
Northern Ireland. Network average
and 3 long-running Belfast sites. Although this network has provided a long-term historical dataset, it is also useful to examine trends for the most recent five years. Regression analysis for annual (pollution year) means 1997 to 2001 identified a downward trend, significant at the 95% confidence level, over this period. Therefore, it appears that on average, SO2 concentrations in Northern Ireland, as measured by the non-automatic Smoke and SO2 Network, are still decreasing. Further decrease is expected in the next few years, as natural gas becomes more readily available in the region. The estimated total annual UK emission of SO2 is available from the NAEI, for years 1970 onward. The correlation was investigated between this parameter and the average annual SO2 in Northern Ireland, as measured by the Smoke and SO2 Network. The correlation between the two parameters can be expressed in terms of the correlation coefficient R. The closer this value is to 1, the stronger the correlation between them. In this case, correlation coefficient R = 0.61; for the sample of 31 paired values (for years 1970-2000) the correlation is significant, at the 99% confidence level. However, this should be compared with the much stronger correlation (R = 0.94) between the NAEI estimate of total annual UK emission of SO2, and UK annual mean SO2 concentrations, as measured by the same network. This is consistent with Northern Ireland’s SO2 emissions over the past 31 years having followed a different trend to those of the whole UK. | ||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
