New research from the University of York has managed to quantify the benefits to roadside air quality in the UK if all diesel cars had their exhausts positioned on the offside (right) of the vehicle.
While it might seem intuitive that this would be the case, it has hitherto been a very difficult thing to measure. 
The availability of high-speed (fast-response) pollution sensors, which take measurements several times per second, has allowed the researchers to use a technique called ‘plume regression’ to analyse how vehicle exhaust pollution spreads and how much each type of vehicle contributes to air pollution near roads.
Plume regression monitors how pollution levels change over time as many different vehicles pass by. Instead of trying to detect individual plumes, it treats the data as a continuous stream of overlapping plumes, then analyses pollution changes against known information about the vehicles – fuel type, exhaust location, size and speed – gleaned via automatic license plate readers.
It was found that the position of passenger car’s exhaust – whether on the left or right side – results in a 40% difference in pollutant concentration contribution at the curbside.
In the UK, 80% of diesel vehicles have their exhausts on the left, closest to the pavement. The team calculate that moving these to the right would reduce their contribution to roadside pollutants by a third. Clearly, overall emissions would not be reduced but their impact on human health would be reduced.
The situation is particularly bad in the UK because so many of the cars on our roads have been manufactured in countries that drive on the right. In this research, 76% of observed vehicles fell into this category.
An unexpected finding was that plume regression produced negative coefficients for EVs. In other words, EVs improved roadside air quality. The team speculate: ‘a likely explanation of this observation is that EVs disperse the plumes of nearby fossil-fueled vehicles through vehicle-induced turbulence, which is influenced by the EV’s aerodynamic properties (frontal area and drag coefficient), speed and existing traffic TRAP concentrations due to fossil-fueled vehicles.’
Lead author of the study, Professor David Carslaw from the Department of Chemistry at the University of York said: ‘These findings demonstrate that even seemingly small details in vehicle design can have a substantial impact on air pollution close to roads.
‘The data on tailpipe placement offers a tangible and relatively simple modification that manufacturers could implement to reduce roadside pollution. Furthermore, the ‘accidental’ benefit of EVs in diluting existing pollution adds another compelling reason to accelerate their adoption.
‘This study highlights the complex connections between all the different elements that impact air quality in our cities, And that’s important, because it gives us new ways of thinking for’ both our politicians and the car industry to tackle the health challenges we face from traffic pollution.’
The full research can be read here.
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