Opinion piece by Emma Ferranti with contributions by Joe Acton, Deepchandra Srivastava, Catherine Muller, Sarah Greenham, James Hall, Jian Zhong), University of Birmingham

Air pollution is the largest environmental threat to human health in the UK. According to Public Health England, 28,000-36,000 premature deaths occur annually in the UK due to exposure to air pollution.

Among the primary urban air pollutants particulate matter, small particles and droplets in the atmosphere, have the greatest impact on health.  Particulates are described by their diameter, with PM_2.5 indicating those particles with a diameter of less than 2.5 μm or approximately 30 times smaller than the diameter of a human hair.

In the UK, particulate matter emissions significantly decreased from the 1950s London smogs, thanks to measures like smoke control areas and a shift to cleaner fuels driven by the 1956 Clean Air Act and subsequent legislation. However, since the late 2000s, emission reduction progress has stagnated due to increased wood and solid fuel burning in domestic and industrial settings.

The Environment Act, 2021 introduced a more ambitious PM_2.5 target level for England, reducing it from an annual average of 20 μg m^-3 to 10 μg m^-3 to be achieved by 2040. This target is significantly higher than the World Health Organisation objective of 5 μg m^-3 but is exceeded in many UK urban areas and will therefore require action from local and national policy makers to achieve this goal. The nitrogen dioxide targets are set at an annual average concentration of 40 μg m^-3. Significant policy interventions have been implemented to reduce nitrogen dioxide levels (for example the London Ultra Low Emission Zone or Birmingham Clean Air Zone).

While nitrogen dioxide emissions in urban areas are mainly from traffic exhaust emissions, PM_2.5 has a much broader range of sources. PM_2.5 emissions from traffic stem from both exhaust and non-exhaust emissions (tyre/break wear and the resuspension of road dust) with non-exhaust emissions accounting for approximately 50% of total PM_2.5 emissions from traffic.

In addition to traffic, PM_2.5 can be emitted from many natural and man-made sources. It arises not only from direct emissions into the atmosphere (primary emissions) but also through chemical reactions involving other atmospheric pollutants (secondary sources). As a part of the WM-Air project, led by the University of Birmingham, the analysis of filter samples from two Birmingham background sites identified seven sources. This included biomass burning, traffic-related primary emissions, industrial activity, sea salt, anthropogenic secondary particulates (formed from pollutants emitted by traffic, industry, and agriculture), and biogenic secondary particulates (resulting from reactions between organic compounds from plants and urban pollution).

Among these sources, man-made pollution accounts for 81% of the PM_2.5 (primary and secondary) with the largest sources being biomass burning, traffic and anthropogenic secondary particles which are made up of a combination of industrial, traffic and agricultural pollutants.

PM_2.5, with its relatively long atmospheric lifetime (days-weeks), allows local concentrations in urban areas to be influenced by emissions across the surrounding region. This longevity, coupled with a diverse range of PM_2.5 sources presents three significant challenges for designing interventions to reduce PM_2.5 concentrations:

• PM_2.5 is emitted from a wide range of household, industrial and secondary sources, not limited to traffic Therefore, policy interventions must address sources beyond road traffic.
• Identifying the primary pollution sources in the region is essential for effective targeting, but this process can be complex and labour-intensive.

• Long atmospheric lifetimes of PM_2.5 necessitates considering sources across the surrounding region when addressing targets.

Non-transport sources

Unlike nitrogen dioxide, PM_2.5 is emitted from a broad range of sources. In most urban areas the main sources of direct PM_2.5­ emission are domestic/commercial biomass burning and road traffic. The transition to electric vehicles will eliminate nitrogen dioxide emissions from road traffic but non-exhaust emissions mean that while PM_2.5 emissions will reduce, the transition to electric vehicles will not eliminate PM_2.5 emissions from traffic.

Secondary sources, influenced by regional transport and biogenic emissions, account for 35% of the total PM_2.5 mass observed in Birmingham.  These sources are impacted by regional transport, with notable contributions from nitrate, sulphate and ammonium. Nitrate and sulphate are formed through the photochemical oxidation of sulphur dioxide and nitrogen oxides, primarily originating from combustion sources such as power generation, industrial combustion and traffic emissions, while ammonium emissions are mostly from agricultural activities.

Environmental policies must therefore reach beyond transport to consider public engagement or controls on domestic emissions as well as industrial and agricultural emissions.

Identifying the sources of PM_2.5

Diffusion tubes facilitate the setup of numerous nitrogen dioxide monitoring sites, yet most urban areas lack sufficient PM_2.5 monitoring stations. Despite this, the growing use of low-cost sensors enhances spatial resolution. To guide policy, understanding local emission sources is crucial. Source apportionment studies, based on filter analysis, offer the most comprehensive perspective but are labour-intensive and costly. The deployment of increasingly affordable instruments, such as aethalometers, allows the estimation of contributions from wood burning and traffic.

Resources such as the UK national atmospheric emissions inventory (NAEI) can also provide an estimate of direct emissions of PM_2.5­ but the contribution of secondary particulates is hard to quantify without complex measurements or modelling studies.

Regional pollution

The dispersion of pollutants in the atmosphere depends on their lifetime and weather conditions. Short-lived pollutants, like nitrogen dioxide, respond well to local emission controls. However, for longer-lived pollutants such as PM_2.5, consideration of upwind sources outside the region is essential.

In managing PM_2.5, an urban region cannot control its air quality in isolation. Collaboration within national frameworks is necessary, recognizing local, regional, and imported components to set objectives accordingly. Minimizing transboundary pollution relies on collaboration with neighbouring authorities, national government, and European contexts.

Meeting nitrogen dioxide targets has led to many high-profile air pollution inventions around traffic. However, addressing PM_2.5 requires considering its complex source profile. Designing policy to meet targets involves engaging with the public on issues such as domestic wood burning and working with industrial partners, the agricultural sector and neighbouring regions to reduce emissions.

Many of the sources of PM_2.5 also contribute significantly to carbon emissions and hence global warming. Addressing these emissions, therefore, offers a chance to deliver significant health benefits to urban areas while complementing net zero actions.