According to a new study, modern diesel vehicles emit less pollution in general than vehicles that run on petrol. This goes against the focus of current environmental regulators.

These are the findings of a new six-nation study published in ‘Scientific Reports’, whose groundwork was laid by an American chemist who currently works at the University of Montreal, in Canada.

“Given that diesel is a lot cleaner than it used to be, environmental regulators should increasingly shift their focus onto dirtier petrol cars and other sources of air pollution”, says the University of Montreal scientist, Patrick Hayes.

“Diesel has a bad reputation because you can see the pollution but in actual fact the worst is the invisible pollution that comes from petrol cars”, explains Hayes.

The study, conducted by researchers from Switzerland and Norway with the help of Hayes and colleagues in Italy, France and the United States, examined sooty particles (PM) emitted via the exhaust pipes of vehicles.

The sooty PM is composed of black carbon, primary organic aerosol (POA) and, in particular, secondary organic aerosol (SOA), which is known to contain harmful reactive oxygen species and can damage lung tissue.

In recent years, it has been a requirement that new diesel cars in Europe and North America be equipped with diesel particulate filters (DPF), which substantially reduce the pollution they emit.

In the laboratory, located in the Paul Scherrer Institute, near Zurich, in Switzerland, petrol vehicles emitted on average 10 times more carbon PM at 22°C and 62 times more at -7°C in comparison with diesel cars, according to the findings made by the researchers in their study.

“The increase in emissions at lower temperatures is related to a more pronounced cold-start effect, when a petrol engine is less efficient because it has not yet warmed up and its catalytic converter is not on yet”, they explain.

These results challenge the existing paradigm that diesel cars are, in general, associated with much higher rates of PM emission, which reflect the efficacy of engine add-ons such as DPFs to stop pollution, the researchers maintain.

The study also reveals, nevertheless, that older diesel cars pollute more than petrol vehicles because they do not have DPFs, and diesel cars in general emit much more nitrogen oxide, which cause smog and acid rain.

For their research, researchers used fieldwork about air pollution that Hayes carried out in California in 2010, and published in 2013, when he was a researcher at the University of Colorado and worked with José Luis Jiménez, also a co-author of the new study.

Over the course of four weeks in a parking lot of the California Institute of Technology, in Pasadena, Hayes analysed air coming from nearby traffic-heavy Los Angeles, drawn through a tube in the roof of a modified construction trailer.