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17 hours ago
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Some vehicles are much more polluting than others and the design of exhaust pipes could affect how much air pollution we breathe when we walk along a busy road, research has found.
Diesel vehicles still dominate exhaust pollution 10 years after the International Council on Clean Transportation revealed that many diesel cars were highly polluting, emitting far more nitrogen oxides on the road than in official testing.
Prof David Carslaw, from the University of York, led a team that measured the exhaust from 38,000 vehicles in Milan and York.
In Milan, diesel vehicles were responsible for 81% of the nitrogen oxides from the local traffic. They produced 61% of the black carbon (tiny soot particles) and 55% of the small particle counts.
It is very difficult to measure air pollution from individual cars as they drive past. Carslaw said: “Exhaust plumes from passing vehicles tend to overlap with one another in most traffic situations, making it nearly impossible to determine the contributions made by different vehicles.”
Dr Naomi Farren, also at York and one of the researchers on the study, said: “We began by making second-by-second measurements right next to the kerb and developed a new data analysis approach. We realised there was huge potential to extend the approach to a broader suite of pollutants, different locations and new research questions.”
Carslaw said: “One application of our approach is ‘market surveillance’; ensuring that air pollutants remain well controlled under real driving conditions and checking that manufacturers are meeting their obligations.”
They found a big difference between vehicles. The most polluting 5% of cars produced twice as much nitrogen oxides as the median vehicle. For black carbon and small particles, the difference was more than seven times, suggesting problems with some particle filters, and there were also clear differences between manufacturers.
Several European countries have tightened their annual vehicle inspections to detect defective or missing exhaust filters, and there is pressure on the UK to follow.
Carslaw said: “The main surprise was that the position of the exhaust, on the left or right of a car, has an important and direct impact on air pollutant concentrations close to roads.”
A pedestrian on the pavement experiences about 40% more air pollution when a car with an exhaust pipe close to the kerb drives past, compared with a car that has an exhaust close to the centre of the road.
Carslaw said: “We were surprised to find that most diesel cars have their exhausts on the left – closest to the kerb in the UK where there is maximum impact on concentrations – and most petrol and petrol hybrid cars have their exhaust on the right.”
The team found that traffic pollution at the edge of UK roads could be reduced by 21% if all diesel cars had their exhaust pipes near the centre of the road.
The turbulence from electric vehicles, especially buses and vans, had a surprising beneficial effect, dispersing pollution without contributing their own exhaust.
Although diesel vehicles still dominated exhaust pollution, the effect of tighter legal limits was clear, with newer diesels cars producing similar nitrogen oxides to those powered by petrol and newer diesels produced much less particle pollution compared with older ones.
