NASA work helps better predict world's smoggiest days

Terra Daily: A research team led by NASA's Jet Propulsion Laboratory and the California Institute of Technology (Caltech), both in Pasadena, Calif., has fully characterized a key chemical reaction that affects the formation of pollutants in smoggy air in the world's urban areas. When applied to Los Angeles, the laboratory results suggest that, on the most polluted days and in the most polluted parts of L.A., current models are underestimating ozone levels by 5 to 10 percent.

The results-published this week in the journal Science-are likely to have "a small but significant impact on the predictions of computer models used to assess air quality, regulate emissions and estimate the health impact of air pollution," said Mitchio Okumura, professor of chemical physics at Caltech and one of the principal investigators on the research. "This work demonstrates how important accurate laboratory measurements are to our understanding of the atmosphere," said JPL senior research scientist Stanley P. Sander, who led the JPL team's effort. "This is the first time this crucial chemical reaction has been studied by two teams using complementary methods that allow its details to be understood."

The key reaction in question in this research is between nitrogen dioxide and the hydroxyl radical. In the presence of sunlight, these two compounds, along with volatile organic compounds, play important roles in the chemical reactions that form ozone, which at ground-level is an air pollutant harmful to plants and animals, including humans.

Until about the last decade, scientists thought these two compounds only combined to form nitric acid, a fairly stable molecule with a long atmospheric life that slows ozone formation.

Chemists suspected a second reaction might also occur, creating peroxynitrous acid, a less stable compound that falls apart quickly once created, releasing the hydroxyl radical and nitrogen dioxide to resume ozone creation.

But until now they weren't sure how quickly these reactions occur and how much nitric acid they create relative to peroxynitrous acid. The JPL team measured this rate using a high-accuracy, JPL-built, advanced chemical reactor. The Caltech team then determined the ratio of the rates of the two separate processes….

Smog masks in Los Angeles