Catalytic converters fix one pollution problem, cause another

August 16, 2000

Pollution control devices reduce air pollution, increase ammonia emissions

The same catalytic converters credited with reducing harmful pollution from automobiles may themselves produce large quantities of haze-causing ammonia, according to a report in the September 1 issue of Environmental Science & Technology, a peer-reviewed journal of the American Chemical Society, the world's largest scientific society.

A separate study that will be presented August 20 at the Society's 220th national meeting in Washington, D.C. indicates the main source of airborne ammonia emissions.

Ammonia plays a role in the formation of very small airborne particles. The U.S. Environmental Protection Agency recently targeted such particles for regulation under Clean Air Act standards on the grounds that they endanger human health. Opposition to EPA's proposed regulation led to a legal case that the U.S. Supreme Court will hear this term.

The researchers, led by Robert Harley, Ph.D., a professor at the University of California-Berkeley, found unexpectedly high levels of ammonia in the vehicles' exhaust. The most likely cause is the catalytic converter, he said. Engines that are not tuned properly also can contribute to ammonia formation, he added.

Harley explained that the root of the problem may be catalytic converters that work too well. When nitrogen oxide from the engine's exhaust is "over-reduced," a complex chemical reaction ensues. Ammonia gas (NH3) forms in the catalytic converter, which is then emitted from the vehicle's tailpipe.

Researchers used air particle sampling to measure the levels of ammonia and other emissions from more than 60,000 vehicles passing through a California highway tunnel over an eight-day period. They found that the typical car emitted 0.28 ounces of ammonia - the equivalent of a teaspoon - per hundred miles.

They also found that cars equipped with modern catalytic converters emit approximately 50 percent less hydrocarbons, carbon monoxide and other harmful gases than cars in earlier studies. The researchers note, however, that the current study included more vehicles with state-of-the-art pollution controls - catalytic converters used with low-sulfur reformulated gasoline - than previous studies.

"Catalytic converters have played a major part in reducing air pollution caused by automobiles. Unfortunately, this research suggests that while fixing one problem, the converters have caused an unexpected secondary problem," said Andrew Kean, a graduate student in Harley's laboratory.

A catalytic converter is a ceramic structure coated with a combination of platinum, rhodium and/or palladium. As exhaust passes through the device, it converts hydrocarbons, carbon monoxide and nitrogen oxides into harmless compounds.

"I am confident that if the catalytic converters were not being used, we wouldn't see the high ammonia emissions that we saw," Kean said. "Though they have been very helpful to reducing air pollution from cars, catalytic converters have caused ammonia levels to increase."

Catalytic converters, introduced in 1981, have become standard equipment on cars and light-duty trucks.

Animal waste, fertilizers and sewage treatment plants also are major sources of ammonia emissions. Diesel vehicles do not contribute significantly to ammonia emissions, according to previous studies.

The University of California Transportation Center and the Bay Area Air Quality Management District funded the research.
The online version of the research paper cited above was initially published July 18 on the journal's Web site. Journalists can arrange access to this site by sending an email to or calling the contact person for this release.

Dr. Robert Harley, Ph.D., is a professor in the department of civil and environmental engineering at the University of California-Berkeley.

Andrew Kean is a graduate student in the department of mechanical engineering at the University of California-Berkeley.

A nonprofit organization with a membership of 161,000 chemists and chemical engineers, the American Chemical Society publishes scientific journals and databases, convenes major research conferences, and provides educational, science policy and career programs in chemistry. Its main offices are in Washington, D.C., and Columbus, Ohio.

American Chemical Society

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