Diesel is cleaner than petroleum-powered cars, according to an international study published in the journal Scientific Reports. American chemist Patrick Hayes, now at the Université de Montréal, Canada, suggests that diesel engines have a poor reputation as dirty and a source of sooty nanoparticulates hazardous to human health and the environment, However, work by he and colleagues across six nations suggests that new regulations on diesel engines may be based on flawed evidence and that it is petroleum-powered cars that are the real problem when compared to modern, filtered diesel cars.

"Diesel has a bad reputation because you can see the pollution," Hayes says, "but it's actually the invisible pollution that comes from gasoline in cars that's worse." He says that vehicle technologists should focus on cleaning up petroleum, or gasoline, cars and removing older diesels from the road rather than applying blanket bans to diesels. After all, cradle-to-grave analyses of diesels will also show that diesel engines can last far longer than gasoline engines. Moreover, modern diesel vehicles have adopted new standards and are now very clean, so attention needs to now turn to regulating on-road and off-road gasoline engines more.

The latest study was led by teams in Switzerland and Norway and assisted by Hayes and colleagues in Italy, France and the USA. They investigated carbonaceous particulate matter (PM) emitted by cars. This sooty material is mostly black carbon, primary organic aerosol (POA), as well as secondary organic aerosol (SOA), which carry reactive oxygen species. Diesel particle filters (DPFs) significantly cut pollution.

The tam found that at low ambient temperatures, gasoline cars emit an average 10 times more carbonaceous PM at 22 degrees Celsius and 62 times more at -7 degrees Celsius compared to diesel cars. This is the well-known cold-start effect of gasoline engines whereas diesel engines are efficient from the off. "These results challenge the existing paradigm that diesel cars are associated, in general, with far higher PM emission rates, reflecting the effectiveness" of engine add-ons like DPFs to stem pollution," the team reports. Hayes et al., Sci Rep (2017) 7, 4926; DOI: 10.1038/s41598-017-03714-9]

The foundations of the new study are data obtained by Hayes while at the University of Colorado and carried out by him and new co-author Jose-Luis Jimenez. They obtained emissions data over the course of a month in a parking lot at the California Institute of Technology, Pasadena, and analyzing air from nearby traffic-heavy Los Angeles. Now, Hayes is analyzing data in Canada's Far North, which he calls "the final resting place of atmospheric pollution."

David Bradley blogs at Sciencebase Science Blog and tweets @sciencebase.