The 'green' fuel emits less carbon dioxide but could still boost pollution.
Widespread use of diesel engines, often thought of as 'greener' than the petrol alternatives, would make the air smoggier, according to new research.
Diesels typically emit less carbon dioxide than petrol (gasoline) vehicles, reducing their contribution to global warming. But that doesn't necessarily make diesel an environmentally friendly alternative, says Mark Jacobson, who models atmospheric pollution at Stanford University in California. Diesels produce a larger amount of nitrogen oxides than petrol-engined cars, he points out. And that can contribute to smog.
Smog is a complex soup of pollutants formed in the atmosphere from chemical reactions triggered by sunlight. Its main constituent is ground-level ozone, which is produced when hydrocarbons and nitrogen oxides from vehicle fumes react together. In the upper atmosphere, ozone shields us from harmful ultraviolet rays. But ground-level ozone is bad news.
"If you're pushing nitrogen oxides out of the tailpipe, you might as well be pushing out ozone," says Jacobson.
The increase in smog could contribute to health problems. "Smog is not good for humans," says Roy Harrison of Birmingham University, a UK government adviser on air pollution. "It reduces lung function, increases mortality rates, and increases hospital admissions for repiratory diseases."
Less smoggy in LA
Jacobson used a computer model of atmosphere quality to see what would happen if every vehicle in the United States ran on diesel. He found that smog levels increased significantly over most of the country1. In most areas, it was enough to push air quality from "moderate" to "very unhealthy", according to standards defined by the Environmental Protection Agency.
The southeast United States was the region hardest hit, probably because the air in this area is rich in hydrocarbons produced by plant life. That extra dose of hydrocarbons can mix with nitrogen oxides from diesel cars, making for worse smog, says Jacobson.
Diesel use caused a decrease in smog in only a few areas - mostly where there aren't any trees. Ironically, this includes Los Angeles, currently one of the smoggiest cities in the United States.
Diesel engines also produce more particulate matter than petrol-fuelled cars. These particles of soot can exacerbate health problems such as asthma and can contribute to global warming2.
New diesel car tailpipes are fitted with filters that trap both particulate matter and nitrogen oxide fumes. But these don't necessarily cure the problem, says Jacobson. "You have a trade-off - a particle trap reduces the efficiency of the nitrogen oxide filter, and vice versa," he says. According to Jacobson, cars fitted with filters still emit more nitrogen oxide than petrol vehicles.
Other technological fixes are being developed to reduce smog, including a paint that absorbs nitrogen oxides that is about to go on sale in Europe. The material, called Ecopaint, locks nitrogen oxides up as calcium nitrate, preventing it from reacting with hydrocarbons in the air.
But Jacobson argues that until such fixes can be put in place, current policies in the United States wrongly favour diesel as an environmentally friendly choice. "The latest energy bill gives diesels the same status as 'green' cars in terms of financial incentives," he points out.
While Jacobson admits that no one is seriously considering converting all the cars in the United States to diesel, he says that his model helps to prove a point - that diesel isn't necessarily a better option for the environment.
Jacobsen, M. Z. The effects on photochemical smog of converting the U.S. fleet of gasoline behicles to modern diesel vehicles. Geophysical Research Letters, 31, L02116, doi:10.1029/2003GL018448 (2004).
Jacobsen, M. Z. Control of fossil-fuel particulate black carbon and organic matter, possibly the most effective method of slowing global warming. Journal of Geophysical Research, 107, 4410, doi:10.1029/2001JD001376 (2004).