1.1 Polycyclic Aromatic Hydrocarbons

Polycyclic aromatic hydrocarbons (PAHs) are a group of persistent organic pollutant compounds they contain two or more benzene rings, they are generally produced through incomplete combustion or pyrolysis.

The International Agency for research on Cancer (IARC) has determined that B[a]P is carcinogenic to humans and is currently considered by IARC as the most carcinogenic PAH. Details of the assessment of the carcinogenicity of PAHs carried out by IARC can be found online.

Table 1 below shows the details of PAH that are required to be measured under the Fourth Daughter Directive (2004/107/EC) (EC, 2005) in the UK, plus benzo[ghi]perylene which was more recently included in the 2014 technical standard CEN/TS 16645:2014 (BSI, 2014).

Table 1: PAH structures of PAH that should be monitored according to the 4th Daughter Directive (2004/107/EC). The IARC Carcinogenic Classification of PAH can be found on the IARC website.

Compound	Structure
Benz[a]anthracene
Benzo[b]fluoranthene
Benzo[j]fluoranthene
Benzo[k]fluoranthene
Benzo[a]pyrene
Dibenz[a,h]anthracene
Indeno[1,2,3-cd]pyrene
Benzo[g,h,i]perylene

1.2 Air Quality Policy

In the UK there is a national air quality objective for B[a]P in ambient air, based on an annual mean concentration of 0.25 ng/m³. Details can be found in the UK Air Quality Strategy (Defra, 2007).

The EC Air Quality Framework Directive (Directive 96/62/EC) (EC, 1996) set a strategic framework to tackle air quality in a consistent way across Europe by setting limit and target values for air pollutants via a series of Daughter Directives. The Fourth Daughter Directive sets a target value for B[a]P of 1 ng/m³ (total content in the PM10 fraction averaged over a calendar year). Mandatory measurement requirements relating to the measurement of B[a]P can be found in the Fourth Daughter Directive (Directive 2004/107/EC).

B[a]P’s suitability as a marker for the PAH mixture in ambient air as stated in the EC Position Paper on PAH (EC, 2001) led to it being selected as the measure for monitoring in the Fourth Daughter Directive (Directive 2004/107/EC) and the more stringent UK National Air Quality Objective for PAH (annual mean of 0.25 ng/m³ B[a]P in ambient air). Measurements of B[a]P in ambient air are covered by the European standard EN 15549 (BSI, 2008), which has been adopted as the European reference method.

Measurements of PAH in deposition are covered by European standard EN 15980 (BSI, 2011) which details the measurement method sampling, sample preparation and analysis for benz[a]anthracene, benzo[b]fluoranthene, benzo[j]fluoranthene, benzo[k]fluoranthene, benzo[a]pyrene, dibenz[a,h]anthracene and indeno[1,2,3-cd]pyrene. There is no limit or target value related deposition of PAH in the UK or Europe.

The two rural background sites located at Chilbolton Observatory, Hampshire and Auchencorth Moss, Midlothian are used to support the European Monitoring and Evaluation Programme (EMEP) to Level 2. EMEP is a scientifically based and policy driven programme under the Convention on Long-range Transboundary Air Pollution (CLRTAP) which was signed by the UK in 1979. The convention aids international co-operation to solve transboundary air pollution problems, provides access to emission, measurement and modelling data and provides information on the effects of air pollution on ecosystems, health, crops and materials.

1.3 Sources of PAHs in the UK

Polycyclic aromatic hydrocarbons (PAHs) are a group of persistent organic pollutant compounds. They contain two or more benzene rings, they are generally produced through incomplete combustion or pyrolysis. The National Atmospheric Emissions Inventory (NAEI) has estimated the emission of PAH for the UK for many years. The inventory estimates the emissions of PAHs including benzo[a]pyrene. As with all emissions inventories there is some uncertainty in the estimates as the emissions are not based solely on measurements and require some estimation of emission factors and activities being required.

In recent years the Inventory indicates that residential and commercial combustion are the dominate emission sources of B[a]P in the UK. Defra’s Clean Air Strategy reports that the used of wood as a domestic fuel has been calculated to produce 78% of total national emission of Benzo[a]pyrene (B[a]P). Most recent information relating to UK Emissions of PAH and other pollutant can be accessed via the NAEI website. This data is updated on an annual basis.

2.1 Network Objectives

The objective of the PAH Network is to determine the ambient concentrations of PAHs in ambient air in the UK through monitoring and chemical analysis, and deliver or aid the delivery the following:

• A UK assessment of current concentrations of PAHs for assessment against the Fourth Daughter Directive and the UK Air Quality Strategy objectives, and provide measurement input for any future reviews.
• A Review of the measurements and trends of airborne concentrations of PAHs in representative UK industrial, urban and rural locations.
• Provide data and metadata to UK-AIR to enable demonstration of the UK’s compliance with the Fourth Daughter Directive, the OSPAR convention (OSPAR, 2017) and the UNECE Convention on Long Range Transboundary Air Pollutants (UNECE, 1979), to enable a better understanding of sources or potential sources of PAH.

2.2 Network Overview

The 33 monitoring stations operating in the UK PAH Network during 2020 are shown in Figure 1. Two monitoring sites where both Digitel particulate samplers and deposition samples are taken are marked with the drop marker (Auchencorth Moss and Chilbolton Observatory). Other sites where only Digitel particulate samples are taken are marked with red circles.

In 2020 there were 33 ambient air sampling sites operational in the network in the UK in addition to two deposition samplers. The majority of the sampling locations are urban background, but the network also includes urban industrial, two rural background sites located at Chilbolton Observatory, Hampshire and Auchencorth Moss, Midlothian and a single urban traffic site at London Marylebone Road. The rural background sites are used to support the European Monitoring and Evaluation Programme (EMEP).

2.3 Samplers in the PAH Network

The Network requires the sampling and analysis of two types of samples these are particulate and deposition samples.

‘PAH Digitel (solid phase)’ particulate samplers. These samples are in the PM₁₀ fraction of ambient air on a filter and are taken daily at all network stations using Digitel DHA-80 samplers with automatic filter changers. Each sample is taken for 24 hours with the sample changeover occurring at midnight (GMT). The samples are bulked into groups representing calendar months per location for analysis. The Digitel DHA-80 samplers (see Figure 2) used throughout the Network are considered to be equivalent to the requirements of the European Standard for sampling PM₁₀ matter (EN 12341) (BSI, 2014). The samplers are therefore valid for use with the European Standard method for the measurement of B[a]P in ambient air (EN 15549). The solid phase filter samples have a measurement period of 24 hours at a flowrate of approximately 30 m³/h.

Figure 2: Digitel DHA-80 sampler deployed to measure solid phase PAH in the UK Network.

‘PAH deposition’ samplers. These deposition samples are taken fortnightly at two rural stations within the network at Auchencorth Moss and Chilbolton Observatory (prior to 2016 the equipment at Chilbolton was located at Harwell). Each sample is taken for 14 days using a deposition sampler (Figure 3) that meets the requirement of the European Standard for the measurement of the deposition of PAHs (EN 15980). The deposition samplers itself consist of a glass funnel and a four litre brown glass collection bottle, which are located inside a protective tube in order to minimise photochemical reactions and the degradation of PAHs. The spikes seen on the image have been fitted to the top of the protective tubes to prevent damage and contamination by bird strikes.

Figure 3: Samplers to measure deposition of PAH in the UK Network.

2.4 Network Activities During 2020

2.5 Analytical Techniques and PAH reported

In 2020 all analysis was undertaken by TNO (Netherlands Organisation for Applied Scientific Research). The analytical method used to analyse for PAH in both sampling media is Gas chromatography–mass spectrometry (GC-MS). The PAH analysed and reported from deposition and particulate samples, typical detection limits and accreditation information TNO are shown in Appendix 2 (Table A2).

3.1 Comparison of B[a]P annual concentrations against EC target values and UK Air Quality Objective

The annual mean B[a]P concentration measured at all the PAH Digitel (solid phase) particulate samplers are shown in Figure 4.

No sites in the UK PAH Network measurement sites exceeded the EC target value of 1 ng/m³. Two sites exceeded upper assessment threshold (UAT) of 0.6 ng/m³. These were Scunthorpe Town (0.84 ng/m³) and Scunthorpe Low Santon (0.64 ng/m³). A further two sites exceeded the lower assessment threshold (LAT) of 0.4 ng/m³. These were Derry Brandywell (0.56 ng/m³) and Ballymean Ballykeel (0.47 ng/m³).

The more stringent UK Air Quality Objective for PAH (0.25 ng/m³ B[a]P) was exceeded at seven sites:

• Scunthorpe Town
• Scunthorpe Low Santon
• Derry/Londonderry Brandywell
• Port Talbot Margam
• Royston
• Ballymena Ballykeel
• Ruardean

Whilst some of the above sites have specific emission sources such as steel works (Scunthorpe sites and Port Talbot Margam), other urban sites may have solid fuel/wood use contributing to their exceedance of the UK Air Quality Objective for PAH.

3.4 Depositon (‘C’) samples

The 4 weekly bulked samples of Benzo[a]pyrene concentrations measured in deposition at the Auchencorth Moss and Chilbolton sites are displayed in Appendix 1. The levels of PAH at these rural sites in the UK are very low as reported in the previous annual reports. Whist the detection limits that TNO have are lower than the previous analytical laboratory the levels found are often within an order or magnitude of the Benzo[a]pyrene detection limit. There are not objectives or target values associated with the deposition measurements. However, the monitoring conducted at the two sites does enable the UK to meet its obligations under the Forth Daughter Directive. All deposition data is available on the UK-AIR website.

3.6 Trend Assessment at the UK PAH Network sites

To summarise the long-term each at each monitoring site, the slope of the trend for each site was calculated using TheilSen function in the openair R package (Figure 22). A positive slope means that there is an increasing trend of B[a]P annual mean and a negative slope means a decreasing trend. The slope of each site’s B[a]P concentration over time was also calculated with its 95% confidence interval, which indicates the uncertainty of the slope coefficient. Figure 22 shows separately the sites that had a significant trend and the sites that did not. Some sites are denoted as weakly significant, meaning that although the trend is statistically significant (p-value < 0.1), they do not show as strong trend as the other significant sites (p-value < 0.05). It should be noted that sites which have not been installed for very long have not been included due to the limited number of data points, the sites affected are: Bristol St Paul’s, Chilbolton, Nottingham and Ruardean.

Figure 22: B[a]P concentration trend (ng m^-3 yr^-1) from 2008 to 2020.

The trend calculated from many of the monitoring sites did not show a large decreasing or increasing trend. This was either because there was very little change in the annual concentrations (i.e. a slope was not significantly different from zero) or there were large variations between years but not in a consistent direction (as indicated by the large confidence intervals (red bars) at the two Scunthorpe sites). Sites showing large decreasing trend were Scunthorpe Low Santon, Ballymena Ballykeel, Royston, South Hiendley, Derry/Londondery Brandywell and Middlesbrough. These site all appear to have decreasing trends in B[a]P concentrations but do also have large uncertainties in the trends due to the inter-year variability in measured concentrations.