Cleaner engine standards and a gradual uptake of electric vehicles have been credited with helping improve Auckland’s air quality over recent years.
But for policymakers working to tackle city traffic pollution, another problem remains: not all of it comes from vehicle exhaust pipes.
Cars, trucks, vans and buses all generate and stir up dust and tiny particles as they move along roads and motorways, regardless of what powers them.
As exhaust emissions slowly decline, these other sources of traffic pollution – particularly road dust – are becoming increasingly important to understand.
Our new research suggests they’re also likely to require a different set of solutions.
What we found on Auckland’s roads
Traffic creates tiny particles as tyres and brakes wear down, road surfaces erode and passing vehicles kick dust back into the air. As some EVs and SUVs are heavier than conventional cars, this can increase wear on tyres and roads.
This all creates what are known as non-exhaust emissions – and they don’t disappear when petrol and diesel cars are replaced by EVs. But relatively little has been known about the scale of this type of pollution in Auckland.
To investigate, we analysed data from 2006 to 2019 to measure the contribution of road dust to PM₁₀ – the technical abbreviation for airborne particles with a diameter of 10 micrometres or less.
Our study covered three monitoring sites: Queen Street in the central city, Khyber Pass on a major arterial route, and Penrose near a key motorway corridor. We then combined this air-quality data with information on traffic and weather conditions.
Even after accounting for factors such as rainfall, wind, temperature and traffic, road dust was more strongly linked to PM₁₀ levels than pollution from vehicle exhausts.
Perhaps surprisingly for a relatively wet city, road dust remained a major contributor to Auckland’s particulate pollution.
Heavy vehicles – those weighing more than 3.5 tonnes – also stood out. While accounting for just 4% of traffic in our dataset, a higher proportion of heavy vehicles was associated with increased PM₁₀ levels.
Our modelling suggests part of that effect may come from heavy vehicles disturbing soil and other loose material, which can then become road dust.
Weather also influenced the picture. Stronger winds were associated with higher PM10 levels when road dust concentrations were elevated, while rainfall had no statistically significant effect in the relevant model.
Overall, the findings suggest what makes up Auckland’s traffic can matter as much as how much traffic there is.
Why road dust is a regulatory headache
For regulators, exhaust pollution is relatively straightforward to tackle. It comes from a clear source and vehicle-emission standards can be designed to reduce it.
Road dust is much harder to deal with. It comes from multiple sources, can be stirred up repeatedly by traffic, and falls largely outside conventional vehicle-emission standards.
All the same, this pollution can have serious health effects. Non-exhaust emissions have been linked to cardiovascular and respiratory problems, including increased hospital admissions in New Zealand.
Reducing those risks will require a better understanding of where road dust builds up, what sends it back into the air and who is most exposed.
Possible policy responses include better containment of soil and loose material near roads, improved management of road dust, and targeted controls on heavy-vehicle movements at pollution hotspots.
Reducing traffic volumes could help, too. Better public transport and infrastructure for walking and cycling would mean fewer vehicle movements generating and stirring up particles.
There is still much more to learn.
Our study covers only three Auckland sites, over a 13-year period up to 2019. Wider and more recent monitoring could show how newer vehicle technology, heavier vehicles and changing travel patterns are affecting non-exhaust pollution.
But the broader challenge is already apparent. As tailpipe emissions decline, transport and air-quality policies will increasingly need to account for the pollution generated simply by vehicles moving on roads.
Otherwise, a significant source of Auckland’s particulate pollution risks remaining overlooked.
