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

“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

“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 authoThis 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.