WASHINGTON -- The catalytic converter, an invention that has sharply reduced smog from cars, has now become a significant and growing cause of global warming, according to the Environmental Protection Agency.
Hailed as a miracle by Detroit automakers even today, catalytic converters have been reducing smog for 20 years. The converters break down compounds of nitrogen and oxygen from car exhaust that can combine with hydrocarbons, also from cars, and be cooked by sunlight into smog.
But researchers have suspected for years that the converters sometimes rearrange the nitrogen-oxygen compounds to form nitrous oxide, known as laughing gas. And nitrous oxide is a potent greenhouse gas, more than 300 times more potent than carbon dioxide, the most common of the gases, that is warming the atmosphere, according to experts.
This spring, the EPA published a study estimating that nitrous oxide now comprises about 7.2 percent of the gases that cause global warming. Cars and trucks, most fitted with catalytic converters, produce nearly half of that nitrous oxide, the study said. (Other sources of nitrous oxide include everything from nitrogen-based fertilizer to manure from farm animals.)
The EPA study also showed that nitrous oxide is one of a few gases for which emissions are increasing rapidly. Collectively known as greenhouse gases, they trap heat in the earth's atmosphere.
The increase in nitrous oxide, the study notes, stems from the growth in the number of miles traveled by cars that have catalytic converters. And the problem has worsened as improvements in catalytic converters, changes that have eliminated more of the nitrogen-oxygen compounds that cause smog, have conversely produced more nitrous oxide.
Wylie J. Barbour, an EPA official who worked on the recently published inventory, said that the problem created by the converter is classic. "You've got people trying to solve one problem, and as is not uncommon, they've created another."
Nitrous oxide, or N2O, is not regulated because the Clean Air Act was written in 1970 to control smog, not global warming. And no regulations exist to control gases that are believed to cause global warming.
The United States and the other industrialized nations agreed in Kyoto, Japan, last December to lower emissions of greenhouse gases to 5 percent below 1990 levels, over the next 10 to 15 years, but the agreement has not been approved by the Senate, and no implementing rules have been written.
"This hadn't really been on people's radar screen until climate change started becoming an issue," said one EPA official involved in reducing pollution from cars, who asked not to be identified by name.
The EPA has not proposed a solution at this point, and is seeking public comment on its study. Auto industry experts say they could solve the problem by tinkering with the catalytic converter, but some environmentalists suggest that the growing production of nitrous oxide is yet another reason to move away from gasoline-powered cars. The EPA's study estimated that nitrous oxide may represent about one-sixth of the global warming effect that results from gasoline use.
"It's like, clean is not green," said Sheila Lynch, executive director of the Northeast Alternative Vehicle Coalition, a public-private partnership that encourages non-traditional power sources.
Another expert, Christopher S. Weaver, an engineering consultant who wrote a study on the subject for the environmental agency, said, "We haven't cared enough to establish standards."
Precisely how much nitrous oxide the converters produce remains an issue. A report used by the EPA in preparing its greenhouse gas study, calculated that a car with a fuel economy of about 19 miles a gallon would produce .27 grams of nitrous oxide per mile. That represents an amount that is about one-third the limit of emissions for nitrogen oxide, the chemicals causing smog.
Steven H. Cadle, a research scientist at General Motors, said, "it's a huge number." In contrast, an older car without a catalytic converter produces much larger amounts of nitrogen oxides, but only about a tenth as much nitrous oxide, the greenhouse gas.
The EPA calculated that production of nitrous oxide from vehicles rose by nearly 50 percent between 1990 and 1996 as older cars without converters have neared extinction. Using a standard unit of measure for global warming gases, millions of metric tons of carbon equivalent, nitrous oxide emissions rose to 54.7 million tons from 36.7 million during those years, the study said.
The contradictory impact of the converter has not been lost on environmental officials or industry experts, who continue to debate not only the extent of the growing problem as well as how to reduce the emissions in future years.
Ned Sullivan, the head of the Maine Department of Environmental Protection, said the converter problem requires a "comprehensive" response. "This specific issue fits into a broader context that our regulatory system has tended to deal with pollutants on an individual, rather than a comprehensive, basis," he said.
He and others favor moving away from today's typical car design, a big gasoline engine driving the wheels, to electric cars. Maine would like electric cars. Another solution is hybrid cars, which use small, efficient engines running on gasoline to help turn the wheels and to charge batteries for electric motors that also run the wheels. Those have much higher fuel economy, and thus lower greenhouse gas emissions.
Car industry experts, however, favor less drastic changes. They propose cutting nitrous oxide production by adjusting catalytic converters in future models. They suspect that the gas is produced when the converter is warming up, and believe the converters could be redesigned to reach optimum temperature faster. That would also help them destroy other pollutants better.
Weaver said that measurements on more kinds of cars and light trucks would be needed to be certain about the size of the problem. But Weaver said, "It is quite clear that you produce nitrous oxide in a catalyst, in some circumstances."
At the Union of Concerned Scientists, an environmental group, an expert on transportation pollution, Roland Hwang, said, "We can't be pushing forward trying to reduce smog while making the global warming problem worse; we can't have programs that undercut each other." He said this was evidence that the transportation system would have to use something besides gasoline.
Cadle, of General Motors would not go that far. But, he said, "You have to be holistic and try and look at everything, which is obviously difficult."
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