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. IARC monographs evaluate cancer hazards, these are provided online in reports relating to PAH (IARC, 2010) and chemical agents (IARC, 2012) including B[a]P.

Table 1 below shows the details of PAH that are required to be measured under the Air Quality Standard Regulations in the UK, plus benzo[ghi]perylene which was more recently included in the 2014 technical standard CEN/TS 16645:2014 (BSI, 2014a).

Table 1: PAH structures of PAH that should be monitored according to Air Quality Standard Regulations. 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-c,d]pyrene
Benzo[g,h,i]perylene

1.2 Air Quality Policy

The Ambient Air Quality Directive (2008/50/EC) and Fourth Daughter Directive (2004/107/EC) set standards such as statutory limit values and target values for the concentration of pollutants in ambient air. They also define monitoring and reporting obligations. These Directives were transposed by respective Air Quality Standard Regulations (as detailed below):

• The Air Quality Standards Regulations 2010 in England (UK Government, 2010), and their December 2016 (UK Government, 2016) and January 2019 (UK Government, 2019) amendments
• The Air Quality Standards (Scotland) Regulations 2010 in Scotland (Scottish Government, 2010), and their December 2016 amendment (Scottish Government, 2016)
• The Air Quality Standards (Wales) Regulations 2010 in Wales (Welsh Government, 2010) and their February 2019 amendment (Welsh Government, 2019)
• The Air Quality Standards Regulations (Northern Ireland) 2010 (DAERA, 2010), and their December 2016 (DAERA, 2017), December 2018 (DAERA, 2018) and November 2020 (DAERA, 2020) amendments

The Air Quality Standard Regulations (2010) and their amendments sets a target value for B[a]P of 1 ng/m³ (total content in the PM₁₀ fraction averaged over a calendar year) along with mandatory measurement requirements.

The respective Secretaries of States for England and the devolved administration responsible for the Environment have responsibility for meeting the limit values and target values as defined through the Air Quality Standards Regulations. Department for Environment, Food and Rural Affairs (Defra) coordinates assessment for the UK as a whole.

The UK Environment Act (2021) (UK Government, 2021) established a duty for the UK Government to set a legally mandatory target in England to reduce PM_2.5, alongside at least one further long-term target on air quality. Within this framework, the Environmental Targets (Fine Particulate Matter) (England) Regulations (2023) (Defra, 2023) came into force in January 2023. This set two PM_2.5 targets to be met by 2040, these provide:

• A legal target to reduce population exposure to PM_2.5 by 35% in 2040 compared to 2018 levels
• A legal target to require a maximum annual mean concentration of 10 micrograms of PM_2.5 per cubic meter (µg/m³) by 2040

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. The reference method for measurement of B[a]P in ambient air is detailed in a British Standard (BS 15549) (BSI, 2008).

Measurements of PAH in deposition are covered by another British Standard (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-c,d]pyrene. There is no limit or target value related to deposition of PAH in the UK.

International relations are reserved to the UK Government; therefore, Defra (with consultation with the Devolved Administrations) retains overall policy responsibility for the formulation of international air quality policy. Defra continues to represent the UK internationally, which reflects that while new domestic legislation is a devolved responsibility the overall compliance with international agreements will remain the responsibility of the UK Government.

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 and 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 B[a]P. 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 dominant emission sources of B[a]P in the UK. Defra’s Clean Air Strategy (Defra, 2019) reports that the use of wood as a domestic fuel has been calculated to produce 78% of total national emission of 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 Air Quality Standards Regulations (1ng/m³) 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 Air Quality Standards Regulations, 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 monitoring stations operating in the UK PAH Network during 2023 are shown in Figure 1. Two monitoring sites where both Digitel particulate samplers and deposition samples are taken are marked with blue circles (Auchencorth Moss and Chilbolton Observatory). Other sites where only Digitel particulate samples are taken are marked with red circles. More details can be found via the interactive map on (UK-AIR).

During 2023 the number of ambient air sampling sites changed due to site installations and closures but at the end of 2023 there were 36 sites in the network. 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 two urban traffic sites, London Marylebone Road and Armagh Roadside. 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 Standard for sampling PM₁₀ matter (EN 12341) (BSI, 2014b). The samplers are therefore valid for use with the 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. In 2021 and 2022 the samplers were upgraded by Ricardo to include improved communication to the samplers to allow improved remote monitoring of individual samplers.

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 Standard for the measurement of the deposition of PAHs (BS EN 15980). The deposition sampler itself consist of a glass funnel and a brown glass collection bottle, which are located inside a protective tube 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 Infrastructure Change

2.5 Analytical Techniques and PAH reported

In 2023 all analysis was undertaken by the Environment Agency, Monitoring Laboratory Service. 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 are shown in Appendix 2 (Table A2).

3.1 Comparison of B[a]P annual concentrations against AQSR target values

The annual mean B[a]P concentration measured at all the PAH Digitel (solid phase) particulate samplers are shown in Figure 4. The darkness of the shading provides an indication of data capture.

One site in the UK PAH Network measurement sites exceeded the annual mean Target Value for B[a]P of 1 ng/m³, this was Margam Youth Centre (1.19 ng/m³). A further site exceeded upper assessment threshold (UAT) of 0.6 ng/m³ this was Derry Brandywell (0.62 ng/m³). A further four sites exceeded the lower assessment threshold (LAT) of 0.4 ng/m³. These were Scunthorpe Town (0.48 ng/m³), Port Talbot Margam (0.46 ng/m³), Royston (0.46 ng/m³) and Armagh Roadside (0.41 ng/m³).

Whilst some of the above sites have specific emission sources such as steel works (Margam Youth Centre, Port Talbot Margam, Scunthorpe Town and Low Santon), other urban sites may have solid fuel/wood use contributing to the levels at the sites.

3.4 Depositon (‘C’) samples

The 4 weekly bulked samples of B[a]P concentrations measured in deposition at the Auchencorth Moss and Chilbolton Observatory 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.

There isn’t a Target Value associated with the deposition measurements. However, the monitoring conducted at the two sites does enable the UK to report and review trends in measured concentrations at these sites. 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 trend at each monitoring site, the slope of the trend for each site was calculated using TheilSen function in the openair (R package), which is a free and open-source programming language. The outputs are summarised in (Figure 22) which displays the significant trends on the right and insignificant trends on the left of the figure. Sites with 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 (shown as red bars), which indicates the uncertainty of the slope coefficient. 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.

Figure 22: B[a]P concentration trend (ng m^-3 yr^-1) from 2008 to 2023.

The slope of the trend calculated for 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 shown as red bars).

Sites showing significant decreasing trend are shown on the right-hand side plot of Figure 22 with the Scunthorpe Low Santon, Ballymena Ballykeel, Scunthorpe Town, Royston, Derry Brandywell and Middlesbrough showing the most significant decreases in concentrations. These sites all appear to have decreasing trends in B[a]P concentrations, but some sites have large uncertainties in the trends due to the inter-year variability shown by the length of the red bars.

Sites showing more significant decreasing trends are displayed in the plot on the right of Figure 22. The Scunthorpe Low Santon, Ballymena Ballykeel, Scunthorpe Town, Royston, Derry Brandywell, and Middlesbrough sites appear to exhibit the most significant decreases in concentrations of B[a]P. However, due to inter-year variability some sites have larger uncertainties which as indicated by the length of the red bars.