PASADENA, Calif.—Scientists have unraveled a mystery about hydrogen peroxide that may lead to a more accurate way of measuring a gas that contributes to depletion of Earth's ozone layer.
Scientists have long known that the HOx radicals -- comprising hydroxyl (OH) and hydroperoxyl (HO2) – destroy stratospheric ozone. Too little ozone may lead to unwelcome changes in the climate and to more ultraviolet radiation reaching Earth's surface. The HOx radicals cannot be easily measured in the atmosphere, but a product of their reaction with each other, hydrogen peroxide, is detectable.
The issue is important because atmospheric scientists would like to make global maps of HOx distributions to better understand the health of the atmosphere, and knowing how much peroxide is in the atmosphere is helpful in doing so.
However, there has always been a large, nagging discrepancy between the distribution of hydrogen peroxide as it is modeled and as it is observed. This suggests that complete understanding of the chemistry has been lacking. Now scientists from NASA's Jet Propulsion Laboratory (JPL), the California Institute of Technology, and the Harvard-Smithsonian Center for Astrophysics have resolved much of the disparity.
In an upcoming issue of the journal Geophysical Research Letters, the scientists report a collaborative laboratory study performed at JPL and funded by NASA that revealed an error in the calculation of the rate that hydrogen peroxide is formed. They showed that improved knowledge of the reaction mechanism largely reconciles measurements of HOx radicals and hydrogen peroxide in the upper atmosphere. Moreover, these results could ultimately allow HOx concentrations to be inferred by monitoring hydrogen peroxide from space or the ground, assuming all the other photochemical reactions involving peroxide are well characterized.
"The importance is not so much the hydrogen peroxide itself, but the fact that it opens the possibility for remotely measuring hydrogen peroxide to infer the HOx radicals," says Mitchio Okumura, an associate professor of chemistry at Caltech and one of the authors of the study.
"The HOx radicals are central to the chemistry of the stratosphere and upper troposphere in understanding ozone depletion," he adds. Atmospheric chemists had puzzled over why models could not correctly predict hydrogen peroxide concentrations. However, they had not suspected that the rate for forming hydrogen peroxide from two hydroperoxyl radicals (HO2), the calculation of which was thought to be well known, could be in error.
Lance Christensen, a Caltech graduate student in chemistry working at JPL, and lead author of the paper, showed that at low temperatures relevant to the stratosphere, the actual reaction rate is slower than had been previously measured. The researchers found that earlier studies had neglected taking into account competing processes that could obscure the results, such as clustering and aggregation of the cold reactants.
"We're trying to improve our understanding of the atmosphere well enough to be able to model ozone depletion and climate change in general," says JPL researcher Stan Sander, one of the authors of the paper. "This work provides a tool for better understanding what's going on in the climate."
In addition to Okumura, Sander, Christensen, and Salawitch, the other authors are Geoffrey Toon, Bhaswar Sen, and Jean-Francois Blavier, all of JPL; and K.W. Jucks of the Harvard-Smithsonian Center for Astrophysics.
