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- The same devices credited with clearing up vehicle air
pollution around the world may in fact be polluting the atmosphere.
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- Researchers at the University of California, (UC) Berkeley,
say that catalytic converters -- considered so essential in reducing auto
emissions -- appear to produce large quantities of ammonia, which can result
in the heavy layers of brownish haze that blanket many urban areas.
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- The findings are based on a sampling of emissions from
more than 60,000 vehicles in a California highway tunnel over eight days.
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- The samplings showed that while cars equipped with modern
catalytic converters emit about 50 percent less hydrocarbons, carbon monoxide
and other harmful gases than cars in earlier studies, the typical vehicle
emitted 0.28 ounces of ammonia -- the equivalent of a teaspoon -- per hundred
miles. Findings appear in the Sept. 1 issue of Environmental Science &
Technology.
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- The amount of ammonia emitted from the cars is much less
than the other pollutants, but it's not an insignificant amount, says lead
researcher Robert Harley, an assistant professor at UC Berkeley.
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- "The ammonia emissions are about 27 percent of things
like nitric oxide, so it is a significant amount of ammonia," he says.
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- Besides addinig to atomospheric haze, Harley says ammonia
is the source of health concerns.
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- "It's not really the ammonia itself that cause the
problems, but its indirect products. It reacts with atmospheric acids,
like nitric and sulfuric acids, to make airborne particles that cause haze.
And because those particles can be deposited in people's lungs, you have
a real health concern," he says.
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- Other major sources of ammonia include animal waste,
fertilizers and sewage treatment plants.
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- Health concerns about ammonia pollution have prompted
the U.S. Environmental Protection Agency to propose regulating ammonia
emission under the Clean Air Act. The proposal is being challenged, however,
and the case is now in the hands of the U.S. Supreme Court.
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- Ironically, one expert says the problem may be that catalytic
converters work too well.
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- When nitric oxide from an engine's exhaust is "over-reduced,"
the researchers say a complex chemical reaction ensues. Ammonia gas (NH3)
forms in the catalytic converter and subsequently is emitted via the tailpipe.
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- Matthew Fraser, an assistant professor of environmental
science and engineering at Rice University, says the process is seen only
in newer catalytic converters.
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- "This comes from any catalytic converter with a
reducing agent," in other words, newer ones, Fraser says.
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- The first catalytic converters mandated nationally in
1981 didn't emit ammonia, but they also didn't work as well, he says.
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- "The older catalytic converters were purely oxidizing,
and you don't see any reduced gases coming off of them," Fraser says.
"But they didn't control for nitric oxide, and since that's so important
in the ozone formation, all new cars have switched to the newer, three-way
catalysts to reduce those emissions."
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- "The problems with the ammonia emissions are an
unintended consequence of that switch," Fraser says.
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- But before any corrective action is considered, Fraser
says we need to determine which vehicles are emitting how much ammonia.
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- "The studies on this are still preliminary, and
it's not clear if there are a small number of vehicles emitting at a very
high rate or if all vehicles are emitting at a moderate rate," he
says. "So we need to answer that before we can determine whether it
will be a simple thing to fix on a small number of cars, or if it's a larger
problem with more widespread consequences."
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