Diesels: NO rises with altitude

While surveying emissions from heavy-duty diesel engines, an international team of researchers has made a surprising discovery: Exhaust pollution climbs with increasing altitude.

Rises in carbon monoxide and hydrocarbon emissions were small, report chemist Gary A. Bishop of the University of Denver and his colleagues in a forthcoming paper in Environmental Science & Technology. The increase in nitric oxide (NO), however, proved large–for each kilometer of higher altitude, NO output rose on average 4.1 grams per kilogram of fuel burned.

For 2 years, the researchers measured pollution spewed by a total of 5,772 heavy-duty diesel trucks at weighing stations. The sites–Anaheim, Calif.; San Marcos, Texas; Wassen, Switzerland; and Golden and Dumont, Colo.–ranged from 104 to 2,530 meters above sea level.

For each kilometer traveled, diesels emit more nitrogen oxides than gasoline-powered engines do. Indeed, California data indicate that although diesels are only 5 percent of the vehicles, they account for at least 27 percent of regional nitrogen oxides emissions, which contribute to ozone. The new data indicate that NO emissions from a tractor trailer traveling from Los Angeles to Denver would rise from some 16 g/kg of fuel to roughly 25 g/km, or almost 60 percent. The data clearly indicate that the diesel engines’ combustion chemistry changes with altitude, says Bishop, but the reason remains unknown.

There had been hints of altitude-linked increases in carbon monoxide and hydrocarbon emissions in previous studies, but those involved only a half-dozen or so diesel engines, Bishop says. What’s more, he adds, because “it’s hard to simulate altitude in the laboratory,” those studies missed the NO change entirely.

Still, he cautions against using the new data to bash diesel trucks. Per ton of cargo hauled, Bishop notes, “these remain the most fuel-efficient vehicles on the road,” which means they’re contributing less to such problems as global warming.