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Hydrocarbons
There are many hydrocarbon compounds which have the potential to be pollutants when released into the atmosphere. Some occur naturally, others are man-made. (i)
Benzene and 1,3-butadiene A range of hydrocarbons are found in vehicle fuel, and occur in vehicle emissions. In most urban areas, vehicle emissions constitute a major source of hydrocarbons, including benzene and 1,3-butadiene. Also, there is the potential that they may be released to the air from facilities where fuels are stored or handled. Benzene is of most concern, as it is a known human carcinogen; long-term exposure can cause leukaemia. It is found in petrol and other liquid fuels, in small concentrations. In urban areas, the major source is vehicle emissions. 1,3-butadiene is also found in vehicle emissions: although
not actually present in petrol or diesel, it is formed as these fuels undergo
combustion. 1,3-butadiene is a suspected human carcinogen and therefore an air
quality objective has been set for it. In this report, concentrations of benzene and 1,3-butadiene are expressed as microgrammes per cubic metre (µg m-3). To convert to parts per billion (ppb) if necessary, the following relationships should be used: 1 ppb = 3.25 µg m-3 for benzene at 293K (20°C) and 1013mb. 1 ppb = 2.25 µg m-3 for 1,3-butadiene at 293K (20°C) and 1013mb. (ii) Polycyclic
Aromatic Hydrocarbons Another class of organic pollutants is the polycyclic aromatic hydrocarbons, (PAHs). These include the following compounds: Acenapthene, Acenapthylene, Anthracene, Benz(a)anthracene, Benzo(a)pyrene, Benzo(b)fluoranthene, Benzo(ghi)perylene, Benzo(k)fluoranthene, Chrysene, Dibenz(ah)anthracene, Fluoranthene, Fluorene, Indeno(1,2,3-cd)pyrene, Napthalene, Phenanthrene, Pyrene. They are all, to varying degrees, toxic or carcinogenic, and are therefore classified as Hazardous Air Pollutants. Concentrations of these hazardous compounds in ambient air are usually very small, and are reported as nanogrammes (i.e. 10-9 grammes) per cubic metre (ngm-3). The largest source of PAHs in the UK at present is road transport, which in 2000 contributed 53% to the total UK emissions. Non-ferrous metal processes such as aluminium smelting can also be a significant source, although there are no aluminium smelting plant in Northern Ireland at present. However, there is one significant source particularly relevant to parts of Northern Ireland: domestic solid fuel combustion. For this reason, it is important to continue monitoring PAH in areas where domestic solid fuel use is widespread. PAHs can be adsorbed onto the surface of fine particulate: therefore they are monitored in the particulate phase, by sampling the PM10 fraction and analysing for the compounds of concern. A range of 27 hydrocarbons including benzene and 1,3-butadiene were monitored at the Belfast South site from 1993 to 2000. This site ceased operation at the end of 2000; there are plans to replace it with an instrument monitoring a more limited range of compounds. A range of polycyclic aromatic hydrocarbons (PAHs) in the particulate phase are monitored at two sites in Northern Ireland as part of Defra’s Hazardous Air Pollutants (HAPS) Network. These are Belfast (at Clara Street) and Lisburn (at Dunmurry High School). The sites are shown in Table 8.1 below, and site locations are shown in Figure 8.1. “BTEX” diffusion tubes are also available for monitoring benzene, toluene, ethylbenzene and xylenes, but these were not used at any sites in Northern Ireland during 2000 – 2001. Table 8.1
Hydrocarbon Monitoring Sites
Within the European Community, benzene is covered by EC Directive 2000/69/EC (the 2nd Daughter Directive). The Air Quality Strategy initially covered benzene and 1,3-butadiene; in 2002, in England, Scotland and Wales, an objective was added for PAHs. Northern Ireland’s policy on an objective for PAH has still to be determined. However, the objective is discussed in this section because of its relevance to domestic solid fuel use. EC limits and AQS objectives for these three hydrocarbon pollutants are summarised in Table 8.2: Table 8.2 Limit
Values and Objectives for Hydrocarbons
* Footnote to
Table 8.2 No policy decision has yet been taken on the inclusion
of the PAH objective for Northern Ireland. PAHs are to be covered by a fourth Daughter Directive, currently at the draft stage. This specifies a range of PAHs to be monitored, but proposes a target value for just one PAH compound, benzo(a)pyrene, which will be used as a marker of carcinogenic risk from PAHs in ambient air. The proposed target value for benzo(a)pyrene is 1 ngm-3 for the annual mean total benzo(a)pyrene in the PM10 particulate fraction, to be met by 1st January 2010. However, this limit has not yet been finalised and may change. Benzene and 1,3 Butadiene Table 8.3 shows calendar year annual mean, maximum running
annual mean and maximum hourly concentrations of benzene and 1,3-butadiene at
Belfast South. Table 8.3
Concentrations of benzene and 1,3-butadiene at Belfast South
a)
Very limited data capture for benzene, 1993 (27%). nm
= no measurement. Annual mean benzene concentration has been below 5 µg m-3 since 1994: hence the EC 2nd Daughter Directive limit value for this pollutant is currently met at this site. Also, the maximum running annual means for this site show that neither benzene nor 1,3-butadiene has exceeded the applicable Air Quality Strategy objective (16.25 µg m-3 and 2.25 µg m-3 for benzene and 1,3-butadiene respectively) during the period 1994 – 2000. The Air Quality Strategy objectives for benzene and 1,3-butadiene have therefore been met at this site throughout its operational period. Polycyclic Aromatic Hydrocarbons Annual mean benzo(a)pyrene concentrations at Lisburn were 0.93 ngm-3 in 2000, and 0.96 ngm-3 for 2001. Annual mean benzo(a)pyrene concentration at Belfast Clara Street was 0.37 ngm-3 in 2001. Both sites are therefore within the proposed EC target value of 1.0 ngm-3 for this PAH (although in the case of Lisburn by a very small margin). The new AQS objective of 0.25 ngm-3 for benzo(a)pyrene does not apply to Northern Ireland at the present time. However, both sites currently exceed this value; in the case of Lisburn, the exceedence is substantial. To put these values into context, they should be compared with results from other sites in the HAPS Network in England, Wales and Scotland – see Table 8.4 and Figure 8.2. The two sites in Northern Ireland are shown in bold italics. Table 8.4 Annual Mean Benzo(a)pyrene Concentrations at
all HAPS Network Sites
*Kinlochleven site (Scotland) was near an aluminium
smelter, which closed in July 2000 Annual mean benzo(a)pyrene concentration for 2000-2001 at rural sites (Hazelrigg, High Muffles, Stoke Ferry), ranged from 0.04 to 0.09 ngm-3, well within the Air Quality Strategy’s new 2010 objective of 0.25 ngm-3. Urban sites typically had higher annual mean benzo(a)pyrene concentrations for 2000 and 2001 ranging from 0.11 to 2.28 ngm-3: nine of the 19 sites exceeded the AQS objective of 0.25 ngm-3 in 2000 or 2001. Typically, the highest values were measured at sites near industrial areas, such as Kinlochleven, Scunthorpe, Middlesbrough, and Port Talbot. Only three sites approached or exceeded the proposed EC limit value for benzo(a)pyrene of 1.0 ngm-3 : Kinlochleven (2000 annual mean 2.28 ng m-3), Lisburn (annual means 0.93 ng m-3 and 0.96 ng m-3 in 2000 and 2001 respectively) and Scunthorpe (2000 annual mean 1.18 ng-3). Kinlochleven was near an aluminium smelter, which closed in June 2000 (hence the much lower 2001 annual mean). The Scunthorpe site is near a major steel works and a coal tar processing plant. The Lisburn site, by contrast, is in a non-industrial residential area. In the case of Lisburn, it is likely that domestic fuel burning is the dominant source of PAHs. As there are only two years data for PAHs in Northern
Ireland, it is not possible to assess trends. A study4 carried out
by netcen for Defra has predicted that the continued high use of solid fuels in Northern Ireland for domestic
purposes may result in some urban areas exceeding the AQS objective of 0.25
ngm-3 for benzo(a)pyrene in 2010. However, no exceedences of the proposed EC
target value of 1.0ngm-3 were
predicted.
ng m-3 2000 & 2001 Figure 8.3 shows annual mean benzene and 1,3-butadiene concentrations at the Belfast South hydrocarbon monitoring site. Average benzene concentrations at the site have clearly decreased over the 8 years of monitoring. A regression analysis (Theil’s non-parametric analysis) confirms that the downward trend in the annual mean is statistically significant at the 95% confidence level. For 1,3-butadiene, by contrast, regression analysis confirms there is no statistically significant trend in the annual mean. However, levels are low and do not appear to be rising.
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