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Converting US gasoline fleet to diesel may increase ozone levels, suggests new study

10 February 2004

Replacing gasoline cars with modern diesels would result in increased ozone levels in most areas in the USA, suggests a new theoretical study published in the January 30 issue of Geophysical Research Letters by a group of researchers led by Mark Jacobson of Stanford University.

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Diesels, due to their superior thermal efficiency, consume less fuel than gasoline engines and produce less emissions of carbon dioxide—the major greenhouse gas linked to global warming. Dieselization of the passenger car fleet has been chosen in Europe as an important strategy to combat the climate change and has also been considered, according to Jacobson, in California.

Compared to gasoline vehicles, diesel cars emit higher levels of nitrogen oxides (NOx) but less hydrocarbons (HC), both gases being important precursors of ground level ozone and, thus, contributors to smog. The study, carried out using a computer pollution/weather/climate model developed by Jacobson, yielded results indicating that replacing gasoline cars with diesels would increase ozone levels in 75% of the US territory, while decreased ozone would be seen over the remaining 25% of the USA.

It was assumed in the study that diesel cars were equipped with particulate filters. The simulation was carried out for diesel NOx emissions 25%, 50%, and 100% higher than those from gasoline, diesel HC and CO emissions half of those from gasoline, and diesel PM emissions twice as high as those from gasoline.

Dieselization of the US car fleet would bring undisputed environmental benefit only if the NOx emissions from diesel engines be reduced to levels from modern gasoline engines, concluded the study.

Source: Stanford University