THURSDAY, March 20 (HealthDay News) Scientists say they've identified a chemical reaction that's an important contributor to smog.

This chemical reaction -- which involves reactions between water vapor and nitrogen dioxide (NO2) in "excited" states -- was long assumed to be unimportant. But now chemists at the University of California, San Diego, conclude that it's actually a major contributor to urban ozone, the main component of smog.

They said their findings, published in the March 21 issue of the journal Science, may help air quality experts develop better ways to reduce ozone in hundreds of urban areas in the United States, and in other cities around the world that have serious air quality and smog problems.

Currently, more than 100 million people worldwide live in cities that don't meet international air quality standards, according to a news release on the study.

"This study provides us with additional insight into the chemistry of urban ozone production," team leader Amitabha Sinha, a professor of chemistry and biochemistry, said in a prepared statement.

"It shows us that the chemistry of urban ozone is even more complicated than we initially assumed. With improved knowledge of how ozone is produced, we should be in a better position to control the air quality of large urban areas across the United States as well as around the world," Sinha said.

Urban ozone levels, which peak in the afternoon, are caused by a series of complex chemical reactions involving the interaction of sunlight with hydrocarbons and nitrogen oxides from vehicle exhaust. Ozone is produced when hydroxyl radicals (OH) are produced from water vapor.

It was long believed that most OH radicals involved in urban ozone production were generated by the reaction of excited oxygen atoms with water vapor. But in their laboratory experiments, the UCSD team found that the reaction between excited nitrogen dioxide (NO2) and water vapor also plays a significant role in the production of OH radicals.

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The American Academy of Family Physicians discusses air pollution and health.

-- Robert Preidt

SOURCE: University of California, San Diego, news release, March 20, 2008

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