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Environment

Atmospheric Nitric Acid Rate Constant Clarified

Scientists from multiple institutions collaborate to refine the value of a key formation constant used in modeling

by Jyllian N. Kemsley
November 1, 2010 | A version of this story appeared in Volume 88, Issue 44

The reaction of hydroxyl radicals with nitrogen dioxide to form gaseous nitric acid is critical in atmospheric chemistry because it sequesters the reagents and slows the catalytic cycle that produces air pollutants such as ozone. Researchers have struggled, however, to pin down the reaction’s rate constant, which is a key component of atmospheric modeling. A team of atmospheric scientists at multiple institutions and government labs in the U.S. has now used pulsed-laser photolysis to remeasure the overall rate constant and quantify the yields of both nitric acid and a competing product, peroxynitrous acid (Science 2010, 330, 646). The researchers found that the rate constant for nitric acid formation is 9.2×10–12 cm3 molecule–1 second–1 at 1 atm and 25 °C—as much as 20% lower than values currently recommended by data evaluation panels for atmospheric modeling. Incorporating the new constant into a model of summer ozone formation in the Los Angeles Basin, the researchers observed a 10% increase in computed peak ozone concentration, an amount that could change the assessed impact on human health, the researchers say.

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