Understanding this chemical reaction, which involves water vapor and nitrogen dioxide, could help reduce the smog that clogs the air in cities from Los Angeles to Beijing, scientists have said.
This finding shows that the chemistry of urban ozone is more complicated than previously thought, said Amitabha Sinha, professor of chemistry and biochemistry at UC San Diego and director of the study's research group. The study was published today in the journal Science.
"The bottom line is that if you want to model urban atmosphere that's polluted, you need to have all reactions that are important in your model," Sinha said.
This comes at a time when some Olympic athletes, including the marathon world-record holder Haile Gebrselassie, are opting out of the Summer Olympics in Beijing due to concerns about the smog that hovers over the city, thickening the air.
And at 75 parts per billion, the controversial new U.S. Environmental Protection Agency standards for ozone are tougher than before, but -- after a last-minute intervention by President Bush -- weaker than what the agency's own science advisers had recommended.
Ozone is a form of oxygen with three atoms instead of two. It is caused by pollution from man-made sources like power plants, drycleaners, car exhaust and industrial emissions. More than 300 U.S. counties have ozone levels that exceed EPA's new standards.
Thirty miles above the earth's surface, the gas is called "good ozone," and it serves as a shield, protecting the earth by filtering out dangerous ultraviolet light.. But ozone in the lowest layer of the atmosphere -- the troposphere -- causes lung disease, worsens asthma and heart disease, damages crops and slows plant growth.
Smog is formed when sunlight interacts with hydrocarbons and nitrogen oxides released into the air by cars and power plants. A highly reactive compound of oxygen and hydrogen called a hydroxyl radical kick starts that reaction. Sinha's paper focuses on this hydroxyl radical, known as OH.
Scientists have long known that ultraviolet light is associated with the formation of hydroxyl radicals. Ultraviolet light breaks apart the bonds holding the ozone molecule together and transfers energy, producing "excited" oxygen atoms -- which react with water vapor to produce OH, the main source of atmospheric OH radicals.
This study shows that another chemical reaction can also form OH radicals: visible light -- the light that can be seen by the human eye -- reacting with water vapor and nitrogen dioxide.
"The real [significance of this finding] is that there is an additional source of OH that's being produced in the atmosphere," said Joseph Francisco, professor of earth and atmospheric science, and chemistry at Purdue University.
When kicked into a higher energy state by visible light photons, nitrogen dioxide collides with water vapor to produce hydroxyl radicals, setting off a series of chain reactions to form ozone.
"In essence, you have more OH being generated by this source that can attack the hydrocarbons to make more ozone," Sinha said.
This new reaction is slower than the conventional one, but is thought to be important because visible light is much more prevalent in the troposphere than ultraviolet light. It's still unknown, however, exactly how fast the reaction occurs.
"Knowing that will determine how much hydroxyl radicals are being produced, and that in turn, will determine how much ozone being produced in urban atmosphere," Sinha said.
Sinha and his team detected the reaction using a laser technique that allowed them to monitor the formation of OH radicals.
The finding is surprising, Francisco said. "I don't think the community really thought that nitrogen dioxide reacts with water," he said. "It normally wouldn't do that. But with visible light, of which there's a lot, it can promote that chemistry."
Knowing more about the chemistry of smog should help scientists devise strategies curb the pollutant, said Jamie Matthews, a UC San Diego graduate student and co-author of the study.
"If you don't know what's contributing to the pollution then how can you stop it," he said. "If we know that nitrogen dioxide is a major contributor to smog, then in essence, we now can find ways to basically inhibit that. Now NO2 just became far more important to control."