Radical Source

RESEARCHERS HAVE FOUND that nitrogen dioxide molecules, when excited by visible light, react with water vapor in the atmosphere to form substantial amounts of hydroxyl radicals (Science 2008, 319, 1657). They say this reaction, which has been long considered too insignificant to include in atmospheric models, could account for up to 50% of tropospheric hydroxyl radicals under certain conditions.

The researchers add that the reaction probably contributes to urban ozone formation and smog. "The reaction of hydroxyl radicals with hydrocarbons, after going through a series of reaction steps, ultimately leads to urban smog formation in the presence of NO and NO₂," says professor of chemistry and biochemistry Amitabha Sinha of the University of California, San Diego, who led the study. "Understanding the underlying factors that control smog production will ultimately allow us to set guidelines on how to control it."

The primary source of tropospheric hydroxyl radicals is the reaction of water with excited oxygen atoms dissociated from ozone by UV light. But the new study indicates that hydroxyl formation from excited NO₂ reacting with water, although a thousand times slower, is significant because it relies on visible light, which is more abundant in the troposphere.

Scientists in Germany first suggested that atmospheric hydroxyl radicals could be produced from excited NO₂ and water in 1997. Their measurements, however, did not detect formation of hydroxyl radicals.

With a sensitive laser technique, Sinha and his colleagues have now directly observed the process. They suggest the rate is 10 times faster than the German group had estimated.

Ronald C. Cohen, an atmospheric chemist at UC Berkeley, says this 10-fold increase "takes this source from a minor player to near domination" of hydroxyl radical formation in the troposphere.

In a Science commentary, Caltech chemist Paul Wennberg and UC Irvine chemical engineer Donald Dabdub raise concerns that the rate constant reported by Sinha's group may be too large. They say further investigation is needed, given the high sensitivity of atmospheric models to the reaction of excited NO₂ with water and the potential importance of the chemistry.