improves knowledge of ozone depletion
Posted: May 10, 2002
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
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 highóby 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 reactionóspecifically
a complication caused by the presence of methanol in laboratory testsówere
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
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