From the busy highways of Britain to the icy waters of the Antarctic, environmental scientists at the University of Birmingham are exploring the impact of airborne particles on our planet’s atmosphere – whether this results in worsening air quality and ensuing health issues or rising atmospheric temperatures contributing to the effects of global warming.

Under normal circumstances, when the world’s roads are not strangely quiet as a result of lockdowns to tackle COVID-19, much of the global South swelters under a fug of air pollution largely caused by vehicle emissions from gridlocked city streets.

Yet, air quality in ‘western’ countries benefits from improvements in emissions technology associated with the latest vehicles and an increasingly urgent drive – particularly in Europe and North America - towards electric propulsion. Yet, even with the zero emissions technology promised by electric vehicles offering a vision free from nitrogen dioxide (NO2) and fine particulate matter (PM2.5) belching from internal combustion engines, there is one form of airborne pollutant that the switch to electricity cannot avoid, namely non-exhaust emissions (NEE) from road traffic.

Particles are released into the air from brake and tyre wear, road surface wear and road dust disturbed by a vehicle’s motion - regardless of the vehicle type or its mode of power. NEE contributes to ill-heath and premature mortality, yet there is no legislation in place to combat such emissions. Whilst legislation has driven down emissions of particles from internal-combustion vehicles, the NEE proportion of traffic emissions has increased in the UK. 

Professor Roy Harrison OBE is a member of the UK Government’s Air Quality Expert Group (AQEG) and contributed to a recent study which notes that particles from brake wear, tyre wear and road surface wear currently constitute 60% and 73% (by mass), respectively, of primary PM2.5 and PM10 emissions from road transport, and will become more dominant in the future. 

"Non-exhaust particles from road traffic are certainly a bigger source of pollution in the UK than tailpipe emissions and, as traffic volumes continue to grow,  it is worrying that there is no regulation in place to govern NEE particles," comments Professor Harrison. "We have a UK target of switching entirely to electric vehicles – in terms of new sales – by 2035. The aim of this is to reduce CO2 and air pollutant emissions, but there is currently a debate around electric and internal combustion vehicles regarding NEE particles. Battery-driven cars are heavier but generate power under braking and should emit fewer particles as regenerative braking does not rely on frictional wear of brake materials."

Professor Harrison is part of the team working on a study to help clarify some of the uncertainty surrounding NEE particles, as emissions vary according to brake, tyre and road-surface material, and driving style. These particles are also an important source of metals, such as copper and zinc, in the atmosphere - primarily associated with brake and tyre wear respectively. 

"There are a number of ways to mitigate the creation of NEE particles – for example, reducing the overall volume of traffic, lowering speed on trunk roads and motorways, and promoting driving behaviour that reduces braking and higher-speed cornering," explains Professor Harrison. "The resuspension of particles from the road surface can be reduced by road sweeping, street washing and applying dust suppressants to carriageways, although the impacts on airborne PM from trials of some of these approaches have so far proven inconsistent with short-lived benefits.

"The single most important step we can take, as recommended by the AQEG, is to recognise NEE as a source of ambient concentrations of airborne PM, even for vehicles with zero exhaust emissions of particles. We also need to work towards a consistent approach internationally for measurement of NEE and to update and narrow the uncertainties in emission factors." 

Yet, far away from the bustling sprawl of the world’s highways, Birmingham scientists are leading the drive to understand how the interaction of anthropogenic particles with radiation and clouds plays an important role in polar climate change. 

Surface temperatures are rising faster in the Arctic than the rest of the globe. Increased emissions of greenhouse gases are one of the driving factors, but air pollutants, such as aerosols, contribute significantly to climate change in the Arctic, affecting sea ice albedo – how much light that hits a surface is reflected without being absorbed – and the heat balance of the atmosphere. 

Sea salts, derived from the Arctic Ocean, are the dominant coarse particles in the Arctic atmosphere and are an important source of cloud condensation nuclei. However, certain organic aerosols, referred to as brown carbon, have been recognized recently as an important light-absorbing atmospheric particle contributing to Arctic warming and the ensuing erosion of the polar icecap.

Working on the Svalbard Archipelago – a cluster of islands lying between Norway and the Arctic Circle – Birmingham scientists and global research partners used a range of tools, including transmission electron microscope and Nanoscale Secondary Ion Spectrometer, to determine the size and mixing state of individual sulphate and carbonaceous particles collected in summer.

"We found that 74% of non-sea-salt sulfate particles were coated with organic matter and a fifth of these particles also had soot inclusions in the organic coating," explains Dr Zongbo Shi, Professor in Atmospheric Biogeochemistry. "We found the absorption cross section of individual organic matter-coated particles significantly increased when assuming the coating as light-absorbing brown carbon.

"Our studies discovered this organic matter coating on individual sulfate particles, which may affect reactions between reactive gases and sulfate particles in the Arctic air. It may also have a significant influence on the absorption properties of individual particles, depending on the optical properties of the organic matter.

"The key to unlocking Arctic climate change lies in examining particles created in the atmosphere by the chemical reaction of gases. These particles start tiny and grow bigger, becoming cloud condensation nuclei leading to more reflective clouds which direct outgoing terrestrial radiation back to earth and warm the atmosphere."

Professor Shi and his research partners plan further exploration of the impact of particulates on the Polar Regions, exploring the drivers of climate change in the Antarctic where melting ice favours the formation of trace gases that form atmospheric particles which bind to organic matter – a polar region less explored in atmospheric chemistry than the Arctic. They also plan to take advantage of scientific ‘cruises’ – shared exploration of the Arctic region aboard research vessels.

"Ongoing research into the particulate contribution to polar climate change is increasingly important to help drive global policy," adds Professor Shi. "The North-West Passage may open to shipping with a corresponding impact on particulate formation caused by emissions from vessels, whilst there is an increasing risk of Arctic being exploited for its resources."

Header image credit: Alamy.