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 protective ozone layer.
Scientists have long known that reactive hydrogen gases destroy stratospheric ozone. Too little ozone may lead to unwelcome changes in climate and to more ultraviolet radiation reaching Earth's surface. Ideally, atmospheric scientists would like to make global maps of the distribution of these gases, because there is increasing concern that their abundances may be rising due to increases in stratospheric humidity. These gases - comprising hydroxyl (OH) and hydroperoxyl (HO2) -- cannot be easily measured from space, but a product of their reaction, hydrogen peroxide, is detectable.
However, a large, nagging discrepancy has existed between computer models of hydrogen peroxide abundance and actual atmospheric measurements, suggesting that a complete understanding of the chemistry has been lacking. Now scientists from NASA's Jet Propulsion Laboratory, Pasadena, Calif., the California Institute of Technology in Pasadena and the Harvard-Smithsonian Center for Astrophysics, Cambridge, Mass. have resolved much of this disparity. The results could ultimately allow concentrations of reactive hydrogen gas to be inferred by monitoring hydrogen peroxide from space or the ground.
"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 Dr. Stan Sander, one of the authors of the laboratory study performed at JPL. "This work provides a tool for better understanding what's going on in the climate system." In research published May 7 in the journal Geophysical Research Letters, the scientists found that previous measurements of the rate of hydrogen peroxide formation, which were based upon a model that used standard photochemical parameters, were too high—by a factor of two. Their finding largely reconciles previous measurements and model calculations of hydrogen peroxide in the upper atmosphere.
Atmospheric chemists had long puzzled over why models could not correctly predict hydrogen peroxide concentrations, but had not suspected the rate for forming hydrogen peroxide, 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, processes other than the central reaction—specifically a complication caused by the presence of methanol in laboratory tests—were compromising prior studies. The new information led to a change in a key rate parameter that provides input to the photochemical model used to examine aircraft, balloon and satellite data.
When the researchers applied the new laboratory rate for hydrogen peroxide formation to measured hydrogen peroxide levels from two different interferometer instruments flying aboard high-altitude research balloons as part of NASA's Upper Atmospheric Research Program, measured and modeled hydrogen peroxide levels were in agreement. The high degree of agreement between the two instrument measurements led the researchers to believe the discrepancy was not due to measurement error.
Dr. Mitchio Okumura, an associate professor of chemistry at Caltech and one of the authors of the study, said that while the new rate of hydrogen peroxide formation has no appreciable impact on stratospheric ozone loss rates, the finding does open the possibility for remote measurement of hydrogen peroxide to infer reactive hydrogen gas radicals.
"These gases are really central to the chemistry of the stratosphere and upper troposphere in understanding ozone depletion," he said. "Measurements of hydrogen peroxide will likely provide the best means of obtaining global maps of these gases in these regions of the atmosphere, because direct space-borne measurement of them below about 20 kilometers (12.4 miles) in altitude is quite challenging."
Dr. Ross Salawitch, an atmospheric chemist at JPL and a co-author of the study, said the research has important implications for future studies of ozone depletion. "The majority of observed ozone depletion over the past two decades was caused by the buildup of industrially-produced chlorofluorocarbons, he said. "As a result of the worldwide ban on chlorofluorocarbon production, Earth's atmosphere will cleanse itself of these gases over the next 50 to 100 years. Recently, however, scientists have become increasingly concerned that changes in Earth's climate could lead to increased levels of water in the stratosphere. This could lead to additional ozone depletion by reactive hydrogen gases, which are a byproduct of water. Our study addresses this concern, allowing scientists to monitor this process in the future."
In addition to Okumura, Sander, Christensen and Salawitch, the other authors include Drs. Geoffrey Toon, Bhaswar Sen, and Jean-Francois Blavier, all of JPL; and Dr. K.W. Jucks of the Harvard-Smithsonian Center for Astrophysics.
This research was funded as part of NASA's Earth Science Enterprise, a long-term research effort dedicated to understanding and protecting our home planet. Through the study of Earth, NASA will help to provide sound science to policy and economic decision makers so as to better life here, while developing the technologies needed to explore the universe and search for life beyond our home planet.
JPL is a division of the California Institute of Technology.
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