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January 4, 2007
CHEMISTRY
OF VOLCANIC
FALLOUT REVEALS SECRETS OF PAST ERUPTIONS
A team of American
and French scientists has
developed a method to determine the influence of past volcanic
eruptions on
climate and the chemistry of the upper atmosphere, and significantly
reduce
uncertainty in models of future climate change.
In the January 5 issue of the journal Science,
the researchers from the University of California, San Diego, the
National
Center for Scientific Research (CNRS) and the University of Grenoble in
France
report that the chemical fingerprint of fallout from past eruptions
reveals how
high the volcanic material reached, and what chemical reactions
occurred while
it was in the atmosphere. The work is particularly relevant
because the
effect of atmospheric particles, or aerosols, is a large uncertainty in
models
of climate, according to Mark Thiemens, Dean of UCSD's Division of
Physical
Sciences and professor of chemistry and
biochemistry.
"In predictions about global warming, the greatest amount of error is
associated with atmospheric aerosols," explained Thiemens, in whose
laboratory the method, which is based on the measurement of isotopes-or
forms
of sulfur-was developed. "Now for the first time, we can
account for
all of the chemistry involving sulfates, which removes uncertainties in
how
these particles are made and transported. That's a big deal
with climate
change."
Determining the height of a past volcanic eruption provides important
information about its impact on climate. If volcanic material
only
reaches the lower atmosphere, the effects are relatively local and
short term
because the material is washed out by rain. Eruptions that
reach higher,
up to the stratosphere, have a greater influence on climate.
"In the stratosphere, sulfur dioxide that was originally in the magma
gets
oxidized and forms droplets of sulfuric acid," said Joël
Savarino, a
researcher at the CNRS and the University of Grenoble,
who led the
study. "This layer of acid can stay for years in the stratosphere
because
no liquid water is present in this part of the atmosphere. The layer
thus acts
as a blanket, reflecting the sunlight and therefore reducing the
temperature at
ground level, significantly and for many years.
To distinguish eruptions that made it to the stratosphere from those
that did
not, the researchers examined the isotopes of sulfur in fallout
preserved in
the ice in Antarctica.
The volcanic
material is carried there by air currents. Thiemens, Savarino
and two of
their students traveled to Antarctica
and
recovered the samples by digging snow pits near the South Pole and Dome
C, the
new French/Italian inland station.
Sulfur that rises as high as the stratosphere, above the ozone layer,
is
exposed to short wavelength ultraviolet light. UV exposure
creates a
unique ratio of sulfur isotopes. Therefore the sulfur isotope
signature
in fallout reveals whether or not an eruption was
stratospheric.
To develop the method, the team, which also included Mélanie
Baroni, the first
author on the paper who is a postdoctoral fellow working with Savarino,
and Robert
Delmas, a research director at the CNRS, focused on two volcanic
eruptions. Both eruptions, the 1963 eruption of Mount
Agung in
Bali and the 1991 eruption of
Mount Pinatubo in the Philippines,
were stratospheric according to the isotope measurements.
"Young volcanoes have the advantage of having been documented by modern
instruments, such as satellites or aircraft," said Savarino, who began
his
investigations into sulfur isotope measurements when he was a
postdoctoral
fellow working with Thiemens. "We could therefore
compare our
measurements on volcanic fallout stored in snow with atmospheric
observations."
Not only did their isotope measurements match the atmospheric
observations,
they were also able to distinguish the Pinatubo eruption from the
eruption of
Cerro Hudson that occurred the same year. Cerro Hudson did
not send
material as high as the stratosphere and the fallout had a different
sulfur
isotope fingerprint than the fallout from Pinatubo.
Volcanic material from more ancient eruptions is preserved in Antarctica,
but the older, deeper seasonal layers of ice are extremely thin as a
result of
the pressure from the overlying ice. Therefore, it is not
currently
feasible to extract enough fallout from the ice to apply the isotope
method to all
past volcanoes. However, data from eruptions in the recent
past reveal
what chemical reactions of sulfates occur in the upper atmosphere.
Some scientists have proposed that if global warming becomes severe,
sulfates
could be injected into the stratosphere in order to block some of the
incoming
solar radiation and reduce the temperature. Thiemens
explained that
understanding the chemical reactions of sulfates in the stratosphere is
critical to determining if this approach would be effective.
"Sulfates can cause warming or cooling depending on how they are
made," he said. "They are usually white particles, which tend
to reflect sunlight, but if they are made on dark particles like soot,
they can
absorb heat and worsen warming."
The study was funded by the French Polar Institute (IPEV) and the
National
Science Foundation Office of Polar Programs.
##
Contact:
Sherry
Seethaler
University of
California-San
Diego
858-534-4656
sseethaler@ucsd.edu
This text derived from:
http://ucsdnews.ucsd.edu/newsrel/science/volcano07.asp
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