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Particulate Matter
Ambient suspended particulate matter consists of a “primary” component (ie. emitted directly into the atmosphere and therefore usually local to source), and a “secondary” component (formed in the atmosphere by chemical reactions, and therefore often a long-range pollutant). The primary component mostly consists of combustion related particles (emitted from sources such as vehicles, domestic and industrial coal and fuel oil burning), but includes other material such as entrained dust, and salt from sea spray. The secondary material consists mostly of sulphate and nitrate particles formed by oxidation of sulphur dioxide and oxides of nitrogen, and ammonium salts. Ambient particulate matter, when inhaled, can affect human health, particularly in sensitive individuals. The two particulate metrics most widely used in the UK are PM10 and Black Smoke. The term “PM10” refers to the mass fraction of particles collected by a sampler with a 50% cut-off at aerodynamic diameter 10 µm. PM10 is measured by automatic techniques, such as the Tapered Element Oscillating Microbalance (TEOM), gravimetric samplers and beta attenuation monitors. The term “black smoke” refers to any fine dark suspended particulate which can be measured by the smoke stain technique, not necessarily particulate resulting from combustion sources. Black smoke is defined by the ISO standard for the method (ISO 9835) as “strongly light absorbing particulate material suspended in the ambient atmosphere.... The major contributor to black smoke is soot particles; i.e. particles containing carbon in its elemental form”. Concentrations of particulate matter are expressed as microgrammes per cubic metre (µg m-3). PM10 Monitoring Particulate matter as PM10 is currently monitored at six sites in Northern Ireland using the techniques shown in Table 6.1 and Figure 6.1. The AUN sites at Belfast Centre and Londonderry, also the two Newry sites, use the Tapered Element Oscillating Microbalance (TEOM). A Beta Attenuation Monitor is located at Clara Street. Two gravimetric PM10 monitors (the KFG and Partisol) are also in use at Belfast Centre, as part of an investigation comparing PM10 monitoring techniques: however no ratified data are yet available from these. The reference method for PM10 is the gravimetric
technique, in which the ambient concentration of PM10 is calculated
from the mass of particulate matter collected on a filter. The more widely-used
TEOM has been found to underestimate relative to this reference method.
Therefore, by convention PM10 concentrations measured using the TEOM
(or using a Beta Attenuation Monitor, if it has a heated inlet) must be
multiplied by a factor of 1.3 to convert to gravimetric equivalent, before
comparison with EC Directive or AQS limit values. All TEOM measurements
in this report have been converted to gravimetric equivalent. Table 6.1
Automatic PM10 Monitoring Sites, 2000 and 2001
In earlier years, PM10 monitoring has also been carried out at the Shambles Market, Armagh (1999) and Dungannon (1998) using a TEOM housed in a mobile monitoring station. Smoke Monitoring The principle of the smoke stain method involves drawing air at a constant, measured flowrate through a paper filter. Suspended particulate matter is collected on the filter, forming a dark stain. An instrument known as a reflectometer is used to measure the darkness of the stain, and this reflectometer measurement is then used to calculate the concentration of particulate matter in the sampled air from a standard calibration. The sampler inlet funnel has a 50% cut-off at around 4.5 µm; thus black smoke can be considered an approximation to dark PM5. During 2000-2001, there were 31 sites measuring particulate as black smoke, 29 of which were part of the Smoke and SO2 Network. Black smoke is monitored using the same 8-port sampler apparatus as non-automatic SO2: site details are therefore identical to those presented in Table 5.2 and Figure 5.2 (for SO2) in Section 5.1. Many of these smoke monitoring sites have been in operation since the 1960s or 1970s: hence there is an extensive historical dataset for smoke. Limit Values and Objectives for PM10 Particulate matter, when measured as PM10, is covered by the EC 1st Daughter Directive (1999/30/EC), which contains a two-stage set of limit values. The UK Air Quality Strategy sets objectives for PM10 that are almost identical to the first stage limit values set by the EC Daughter Directive. These are outlined in Table 6.2. Table 6.2 Limit
Values and Objectives for Particulate Matter as PM10
All limit values refer to gravimetric equivalent
measurements. Limits and Guide Values for Black Smoke Before the 1st Daughter Directive and Air Quality Strategy set objectives for PM10, smoke was covered by EC Directive 80/779/EEC on sulphur dioxide and suspended particulates. This Directive has been superseded by the 1st Daughter Directive; however, the limits relating to smoke remain in force until 2005, and the new Daughter Directive deals only with PM10. The current report therefore compares results with the smoke limits and guidelines of Directive 80/779/EEC. The limit values are presented in Table 6.3 below, along with the non-mandatory guide values. Table 6.3 EC Directive
80/779/EEC Limit Values For Smoke (To be fully repealed in 2005)
NOTE: The Limit and Guide
Values given above for smoke according to the BS calibration are calculated
from the original OECD calibration figures given in the EC Directive using the
relationship: BS concentration = OECD concentration multiplied by 0.85 PM10 Results Table 6.4 presents data from automatic PM10 monitoring sites. TEOM data have been converted to gravimetric equivalent by multiplying by the appropriate factor of 1.3. Figures in bold italics indicate more than the permitted number of exceedences of the relevant limit or objective. Table 6.4 Results from Automatic PM10
Monitoring Sites
a)
Limited data capture: Londonderry 1997(60%) b)
Limited data capture: Lough Navar 1996 (22%) All the sites for which data are available currently meet the EC Directive Stage 1 limits and AQS Objectives. Only one site, Lough Navar, currently meets the EC Directive Stage 2 limit values: however, if confirmed, these need not be met until January 2010. Black Smoke Results A summary of smoke data for calendar years 2000 and 2001 for all Smoke and SO2 Network sites in Northern Ireland is shown in Appendix 2. Smoke in 2000: Annual mean smoke concentrations in Northern Ireland for calendar year 2000 ranged from 5µg m-3 to 17µg m-3 with one exception. An unusually high concentration of 27 µg m-3 was measured at STRABANE 2. This was investigated, and appears to be genuine; the site is located on a housing estate with considerable domestic coal and oil burning, and local topography may impede dispersion. The average for Northern Ireland was 11 µg m-3, compared with the average for the UK as a whole of 7.4 µg m-3. Maximum daily smoke concentrations at the majority of sites were less than 100 µg m-3. Smoke in 2001: Smoke concentrations in Northern Ireland were similar to those measured in the previous year, with all except one annual mean in the range of 5 µg m-3 to 17 µg m-3 . Again, the exception was STRABANE 2, with an annual mean smoke concentration of 34 µgm-3, substantially higher than at any other site. The average for all Network sites in Northern Ireland was again 11 µg m-3; slightly higher than the UK average of 8.2 µg m-3. Maximum daily smoke concentrations exceeded 100 µg m-3 at a large proportion of sites in 2001, with one site (BALLYMENA 3) recording a maximum daily value of 303 µg m-3. However, such high values only occur during short-term “episodes” of particularly high pollution and are rare: the 98th percentile of daily values was below 100 µg m-3 at all but one site. Two such episodes occurred in Northern Ireland during 2001: one during 18th-19th January 2001 and (in some areas) another around 11th-16th December 2001. The former also affected northern parts of England, the latter also affected Scotland. The two non-network sites, Downpatrick and Newtownards, recorded annual mean smoke concentrations of 4.5 µg m-3 and 10.8 µg m-3 respectively in 2001. Smoke concentrations, historically, have been higher in Northern Ireland than in most other parts of England, Wales and Scotland. This is because the limited availability of natural gas for domestic heating has necessitated greater use of coal and oil for this purpose. All the sites in Northern Ireland meet the limit values of 80/779/EEC for smoke. The last occasion on which any of the limit values for smoke were exceeded in Northern Ireland was in 1990. During 2000 and 2001, the annual arithmetic mean was well below the lower guide value of 34 µg m-3 at all sites in Northern Ireland with the exception of STRABANE 2. However, the maximum daily mean exceeded both the upper and lower 24-hour guide values at many sites in the UK, including several in Northern Ireland. In the Belfast area, the contribution to primary PM10 emissions from domestic contribution is around 26%2, a substantially higher proportion than in most UK cities. Therefore, trends in PM10 and black smoke emissions for Northern Ireland will not necessarily be the same as for other regions. However, NAEI data show a substantial decrease (of around 45%) in total UK PM10 emissions between 1990 and 2000, so it is not unreasonable to expect a decreasing trend for Northern Ireland. PM10 Trends Figure 6.2 shows annual mean PM10 concentrations for sites in Northern Ireland.
Figure 6.2 Annual
Mean PM10Concentrations at Automatic Monitoring Sites, Converted to
Gravimetric Equivalent where applicable. (Data capture at least 75%). The longest running site is Belfast Centre, which has been in operation since 1992. The annual mean PM10 concentration at this site has shown a steady decrease over this period. The urban sites at Londonderry and Monaghan Row, Newry have operated for shorter periods, but also appear to show decreasing annual mean PM10 concentrations. At the rural Lough Navar site, annual mean PM10 concentrations have remained stable and below 15 µg m-3. Regression analysis (Theil’s non-parametric regression analysis) identified a significant downward trend (with 95% confidence limit) in the annual mean PM10 concentrations at Belfast Centre and Londonderry, although not at Belfast Clara Street and Lough Navar. (At the two Newry sites, there is not enough data to assess trends). Three sites show a slight increase in 2001 relative to 2000: future years’ monitoring will be important to establish whether the downward trend resumes.
Black Smoke Trends The long-running historical dataset for the Smoke and SO2 Network gives an indication of how concentrations of fine suspended primary particulate, as measured by this technique, have decreased since the early 1960s. This trend is shown in Figure 6.3, a graph of the average smoke 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 24. The annual average concentration of smoke has fallen, from over 100 µg m-3 in the early 1960s to less than 10 µg m-3 in 2001. Figure 6.3 also shows the trend in annual mean for three particular sites in Belfast: BELFAST 12 (at the Royal Victoria Hospital), BELFAST 13 (the suburban Templemore Avenue) and BELFAST 33 (the industrial, city centre Dufferin Road), all of which have been in continuous operation since the early 1960s. These three individual sites show a similar pattern.
Network
average and 3 long-running Belfast sites. As well as the long-term trends it is also interesting to investigate the trend over the past five years. Regression analysis of the average annual mean smoke concentration based on all sites in Northern Ireland shows that there is a small downward trend for the period 1997 – 2001, which is not significant at the 95% confidence limit. However, further decrease in smoke concentration in Northern Ireland is expected in the next few years, as natural gas becomes more readily available in the region, and domestic solid fuel use declines. Estimated annual total UK emission data for black smoke are
available from the NAEI, for years 1970 to 2000. The correlation was
investigated between the total annual UK emission of black smoke, and the
average annual smoke concentration 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, the
correlation was found to be strong: R = 0.93. For the sample of 31 paired
values (for years 1970-2000) this is significant at the 99% confidence level.
The correlation is almost as strong as that found between total annual UK
emission of smoke, and the average UK annual smoke concentration as measured by
the Smoke and SO2 Network (R = 0.96). This is consistent with black
smoke emission trends for Northern Ireland over the past 31 years being similar
to those calculated for the whole UK. This is in contrast to the findings for
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