Road dust is a significant source of particulate air pollution in Tāmaki Makaurau, at times having a greater impact than vehicle exhaust, research suggests.
A 13-year study of Auckland air quality found road dust was a significant contributor to PM₁₀, tiny airborne particles associated with respiratory and cardiovascular problems.
The study, published in the Journal of the Royal Society of New Zealand, analysed data collected at three Auckland air-quality monitoring sites: Queen Street in the CBD, Khyber Pass, a major arterial route near the city centre, and Penrose, close to a large motorway.
Lead researcher Dr Mingyue Selena Sheng, from the University of Auckland Business School’s Energy Centre, says the findings highlight the importance of looking beyond exhaust emissions when considering the impact of road transport on air quality.
PM₁₀ data from the three sites was used to identify contributions from soil, fuel, biomass burning, secondary sulphate, marine aerosol and road dust. The researchers combined this with traffic data and information on temperature, wind speed and rainfall, using a series of models to examine factors influencing PM₁₀ concentrations.
“Our findings show that road dust is, at times, a dominant and statistically significant contributor,” says Sheng.
“Much of the focus has been on reducing what comes out of the tailpipe, but road dust and other non-exhaust emissions need our attention.”
Non-exhaust emissions include particles from the wear of brakes, tyres, clutches and road surfaces, as well as dust and other material deposited on roads that is “resuspended” or lifted back into the air by moving vehicles, tyres and atmospheric conditions.
In one analysis, road dust had a larger estimated effect on PM₁₀ than fuel emissions, and the researchers’ modelling suggests a 1 percent increase in road dust is likely to cause an 8.8 percent increase in PM₁₀ concentration.
Co-author, Business School economist Dr Le Wen, says non-exhaust emissions are closely linked to the physical movement of vehicles, so the number and type of vehicles on our roads matters.
Heavy vehicles, defined as those weighing more than 3.5 metric tonnes, represented about 4 percent of traffic on average in the dataset, and the researchers found evidence of an association between a higher proportion of heavy vehicles and increased PM₁₀ levels.
Although the percentage of heavy vehicles is relatively low, Wen says their role in resuspending road dust and contributing to PM₁₀ levels plays a disproportionate role in air pollution.
The findings are particularly relevant as transport shifts towards electric vehicles which typically weigh more due to their batteries.
“While electric vehicles offer a pathway to reducing tailpipe emissions, their environmental benefits may be offset by increased non-exhaust emissions, especially from heavier vehicle fleets.
“Moving to electric vehicles has important benefits, but it doesn’t make particulate pollution from road transport disappear. If we want cleaner urban air, non-exhaust emissions need to be a key part of the conversation.”
The researchers say tackling non-exhaust emissions will require targeted measures alongside efforts to cut tailpipe pollution. The paper suggests transport policy should require vehicles carrying soil and other loose material to keep it contained in urban areas and consider time-based restrictions to reduce air pollution during peak periods.
The article, Impact of road dust on local air pollution: a case study of Auckland, New Zealand, is by Dr Mingyue Selena Sheng, Dr Le Wen, Quan Le Truong and Dr Perry Davy.
