We’ve
all benefited from catalytic converters and their reduction of
automobile emissions. Now UCSD chemists have discovered
that chemical processes in the atmosphere itself are removing
hydrocarbons at a faster rate than once believed. In the May 24 issue of the Proceedings
of the National Academy of Sciences, they report that naturally
occurring atmospheric chemicals
react with sunlight
more effectively than scientists previously thought, breaking down
smog and other pollutants after they absorb energy
from sunlight.
Many different kinds of air molecules
have been known to behave in this way—producing natural air
cleaners called OH radicals. But a sensitive laser technique developed
at UCSD allowed the scientists
to observe reactions that produce smog-destroying OH radicals at
wavelengths that were previously difficult to observe.
“This study is important because it shows that the atmosphere could be
generating far more OH radicals than previously thought and accounted for by
current models,” says Amitabha Sinha, an associate professor of chemistry
and biochemistry, who conducted the study with graduate student Jamie Matthews
of UCSD and Joseph Francisco of Purdue University. “It could imply that
the atmosphere is more effective at breaking down pollution than models have
shown.”
The atmosphere has three main ways
to cleanse itself of hydrocarbon pollutants. Two are relatively
direct: water droplets
in clouds absorb and rain them out of the atmosphere or sunlight breaks the
molecules apart.
“The third way is the one we are concerned with here, the way that involves
breaking these hydrocarbons down chemically,” says Francisco. “For
that, the atmosphere relies on a reactive group of chemicals called OH radicals
that attach themselves to hydrocarbons and rip them into inert pieces.
“One of the biggest questions in our field concerns the amount of OH radicals
the atmosphere holds,” he says. “It’s tough to get a handle
on them because they are so reactive—which means they vanish fast—and
also because we don’t have complete knowledge of all the sources that produce
them yet.”
Despite the contribution of this previously unknown source of OH
radicals, Sinha emphasized that the results do not mean we can
ignore atmospheric
pollution.
“What it means,” he says, “is that we need to do a much more
careful job with our measurements in order to accurately account for all sources
of OH radicals present in the air.” — Kim McDonald
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