Is diesel's dirty reputation for pollution undeserved?

Which is cleaner, diesel or gasoline? It's a complex question. While diesel emits more greenhouse gases per gallon, it is also more energy dense and can result in greater fuel economy, lowering emissions on a per-kilometre basis. A new analysis by a Canadian researcher indicates that the story may be even more complex, depending on what type of pollutants you care about.

"The topic has been controversial for the past five years or so," says Patrick Hayes, a chemist with Universite de Montreal. Hayes's own interest was piqued in 2010 when he took part in a major study led by the United States' National Oceanic and Atmospheric Administration and the California Air Resources Board to examine air quality in California and the nearby Pacific Coast. As part of this work, Hayes and other participants examined the respective impact of gasoline- and diesel-powered vehicles, specifically examining the formation of secondary organic aerosols (SOAs) generated by the interaction of emissions with sunlight, volatile compounds from natural sources and other agents in the atmosphere.

"Looking at the field data, especially the carbonaceous part that includes soot, unburned fuel and other products of incomplete combustion, it really looked like a lot of it was coming from gasoline rather than diesel," Hayes says. "That was surprising because at that time the paradigm was that diesel was creating all this soot, emitting all this particulate matter that you could see with your eyes." (Diesel engines--particularly those lacking particle filters--churn out high levels of particulate matter (PM) as well as nitrogen oxides, the notorious family of NOx compounds.)

Hayes and others found it hard to resolve the disagreement over which fuel contributed more to SOA production. Investigators are still grappling with this problem, as Hayes found in a new study. Researchers noted that previous analyses into this topic have adopted either a "bottom-up" approach--laboratory measurements of tailpipe emissions--or a "top-down" approach--field measurements of air quality. By combining both of these strategies, this latest work by Hayes offers a more detailed breakdown of the distinction between diesel and gasoline contributions to air pollution.

Published this past summer in Scientific Reports, the latest study points to gasoline engines as the predominant source of carbonaceous particulate matter and SO As. In fact, depending on the ambient temperature, gasoline engines were found to emit between 10 and 62 times more carbonaceous PM than diesels.

Diesel engines--particularly those lacking particle filters--churn out more than their fair share of PM as well as NOx compounds that are targeted by air pollution regulations. But these aren't the only pollutants being emitted, says Hayes. "If you look at most of the other pollutants, it's almost always higher for modern gasoline than modern diesel vehicles."