Update on the UK Automatic Urban & Rural Network (AURN)

A continuous real-time air quality network has been in operation in the UK for over 30 years providing valuable information to the publici on air pollutant concentrations for those pollutants that impact our everyday health, and the sensitive ecosystems within our lands. The Automatic Urban and Rural Network (AURN) (Figures 1 and 2) is funded by, and operated on behalf of, Defra and the Devolved Administrations of the UK and is the largest of the UK’s continuous compliance networks on air quality responding to the legislati ve demands of the Air Quality Standards Regulations 2010, and more recently the requirements of The Environment Targets (Fine Particulate Matter) (England) Regulations 2023.
The network structure is defined by the regulations with the UK split into zones and agglomerations for reporting purposes. The number of monitoring stations for each pollutant measured (i.e. ozone (O3), nitrogen dioxide (NO2), sulphur dioxide (SO2), particulate matter (PM10 and PM2.5 mass fractions) and carbon monoxide (CO)) are defined by the population within each zone and agglomeration set against the risk to compliance of achievement with the relevant air quality standards (as defined by the Ambient Air Quality Directive 2008/50/EC) upper and low assessment thresholds. Recent increases in the level of PM2.5 monitoring have been based solely on the size of the population in each zone/agglomerationii.
It is estimated that the AURN provides more than 4.2 million hourly measurements of data to the public information portal (UK-AIR Data Archive - Defra, UK) each year. The challenge of achieving data quality standards for each measurement are significant and the network manages to achieve this through scrutiny of the data by the Bureau Veritas data management team, and a regime of quality assurance provided by an independent QA/QC unit.  Other significant roles include those of the Local Site Operators, who work to standard operating procedures for routine calibration checks of instruments, and that of the Equipment Support Units (ESUs) who provide for emergency breakdown cover and routine service and maintenance of instruments across the network.
With increasing public attention and awareness of the impacts of poor air quality, the AURN is recognised as more than a compliance network with data being used in numerous research projectsiii (AQEG, 2015), and for Accredited Official Statistics (Figure 3)iv. In addition, the provision of reference method (or equivalent) data provides for a means by which performance of emerging technologies (such as low-cost sensors) for measurement uncertainty and data capture, can be evaluated.
Some sites on the AURN provide invaluable insights into pollutant occurrence over a long period of time. Long term data sets assist policy makers to understand what impact policies have had, and to help inform future policies that are aimed to reduce emissions further and improve the quality of the air we breathe. However, achieving a consistent quality in data over long periods of time is not without some challenges.  How do you ensure integrity of data for a pollutant trend when existing instruments are no longer available, and new technologies are deployed? Moreover, whilst general trends in recent decades have seen significant improvements in air quality how do instruments perform in a decreasing pollutant concentration atmosphere?
Particulate matter is one of the more challenging pollutants to monitor across the UK due to the variability in composition relative to the primary emissions sources and the chemical transformations that take place via gas to particle conversion (leading to secondary organic and inorganic components of the particulate mass). Such variability in composition does not arise for gaseous pollutants, which are stoichiometrically stable. Moreover, the need for immediate public information on pollutant occurrence and concentrations means that automated particulate matter instruments (BS EN16450:2017) are adopted more widely than the reference method (BS EN12341: 2023) which use filter-based measurements (gravimetric) which give rise to delays in reporting due to laboratory analysis. To show that automated particulate matter instruments are equivalent to the reference method, the UK has adopted a programme of ongoing equivalencev at a few sites, which vary in location and particulate matter characteristics.  Results show inter-year variability in the performance of the instruments relative to achievement of the measurement uncertainty requirements of the legislation (i.e. an expanded uncertainty of 25%). The results shed some light of the ongoing and future challenges for accurate particulate matter measurements at lowering concentrations in the atmosphere, and where adoption of tighter ambient air quality standards is set against World Health Organization (WHO) Air Quality Guidelines. Such challenges are giving rise to some wider investigationsvi  into the influencing factors that may affect the measurement uncertainty of instruments, and instrument manufacturers may do well to key into these elements of the legislative requirements when enhancing existing instrument performance or developing new technologies.   

i    https://uk-air.defra.gov.uk/networks/network-info?view=aurn
ii    Schedule 2 of The Environmental Targets (Fine Particulate Matter) (England) Regulations 2023
iii https://uk-air.defra.gov.uk/library/reports?report_id=830
iv     https://www.gov.uk/government/statistics/air-quality-statistics
v     https://uk-air.defra.gov.uk/assets/documents/reports/cat05/2409260917_2023_UK_Report_for_On-going_Particulate_Matter_Equivalence.pdf
vi     https://uk-air.defra.gov.uk/assets/documents/reports/cat05/2409260937_Evidence_Synthesis_Report_PM2.5_Equivalence_Group.pdf