Works for China!
Posted on 03/20/2008 10:10:08 PM PDT by neverdem
Hydroxyl radicals, nature's atmospheric scrubbers, are produced by nitrogen pollution too.
Some types of air pollution might be doing a good turn by creating extra doses of atmospheric cleaner, according to new research. A lab study has shown how nitrogen oxides, a largely agricultural pollutant, can help to make hydroxyl radicals — the natural cleaner-upper of our dirty atmosphere. But in doing so they can also produce more ozone, the major component of smog.
The work should help to improve models of atmospheric chemistry, and suggest better ways to control air pollution in big cities.
The hydroxyl radical is a very reactive and short-lived molecule that contains one hydrogen atom and one oxygen atom. It is known as the detergent of the lower atmosphere (troposphere), because it is involved in most reactions that break down volatile organic compounds (VOCs) — the hydrocarbon pollutants from urban life.
In the atmosphere, sunlight breaks up ozone to produce excited oxygen atoms, which then attack water to make hydroxyl radicals. These go on to react with hydrocarbons or carbon monoxide molecules, and break them up — scrubbing the atmosphere clean.
Hydroxyl isn't all good, however. In polluted skies with high levels of nitrogen oxides (NOx), a byproduct of the hydroxyl-scrubbing reaction can go on to create more ozone, a major component of smog.
So it might seem that policies aiming to reduce NOx pollution are very important, leaving nature's hydroxyl radicals to scrub up the VOCs. But things aren't so simple.
New source of soap Amitabha Sinha and colleagues at the University of San Diego, California, have found a previously overlooked but important part of hydroxyl chemistry: NOx pollution can help to make more atmospheric cleaner.
They recreated atmospheric reactions in the lab, and found that nitrogen dioxide, when excited by wavelengths of light similar to those seen when the Sun is low on the horizon, can split water in the same way that ozone does to produce the hydroxyl radical. “This is an important reaction,” says Sinha. At these certain times of day, this process could boost concentrations of the atmospheric scrubber by 50%, he says. His work is published in Science 1.
This also means that NOx pollution produces more ozone than thought, however. “In cities with large amounts of biogenic hydrocarbons, such as Atlanta, ozone concentrations will increase more quickly,” says Paul Wennberg, an atmospheric chemist from the California Institute of Technology, Pasadena.
“If you include this [reaction] in models it does profoundly change the way you view control measures for pollution,” says Wennberg. Policy makers need good models to determine what to do in different cities with different dominant pollutants, he says.
Low light “This is a very unexpected result,” says Dwayne Heard, an atmospheric chemist from the University of Leeds, UK.
Heard notes that Sinha’s result is based only on times when the Sun is near the horizon. This makes it hard to assess how levels of hydroxyl radicals and ozone will change in a city over the course of a day. “You need to look over 24 hours,” says Heard. “When the Sun is low in the sky, this process can produce a significant fraction of OH … at noon this process would have a very small contribution.”
But the result is very applicable to polar regions, where the Sun spends a long time hanging on the horizon. The new-found reaction could help explain discrepancies between models and measurements, says Heard, who has just returned from the Arctic.
The new reaction had been suggested previously, but dismissed as unimportant. In 1997, a group from the Max Planck Institute for Chemistry in Mainz, Germany, also suggested that this reaction might be happening, but at a much slower and therefore much less significant rate than Sinha is now suggesting.
Atmospheric models, which already have to take thousands of reactions into account, will be further complicated by this finding.
OH
Blasphemers!!
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