Volcanic fallout reveals secrets of past eruptions
IMPORTANT INFORMATION about a past volcanic eruption's impact on climate is provided by determining the height of the eruption.
Eruptions that reach higher, up to the stratosphere, have a greater influence on climate compared with the case when volcanic material only reaches the lower atmosphere wherein the effects are relatively local and short term because the material is washed out by rain.
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, has been developed by a team of American and French scientists.
To distinguish eruptions that made it to the stratosphere from those that did not, the researchers examined the isotopes of sulphur in fallout preserved in the ice in Antarctica.
Chemical fingerprint
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.
"In predictions about global warming, the greatest amount of error is associated with atmospheric aerosols," explained Mark Thiemens, Dean of UCSD's Division of Physical Sciences and professor of chemistry and biochemistry in whose laboratory the method, which is based on the measurement of isotopes — or forms of sulphur — was developed.
"Now for the first time, we can account for all of the chemistry involving sulphates, which removes uncertainties in how these particles are made and transported. "
Sulphuric acid droplets
"In the stratosphere, sulphur dioxide that was originally in the magma gets oxidized and forms droplets of sulphuric acid," said Joël Savarino, a researcher at the CNRS and the University of Grenoble.
Acts as a blanket
"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."
Sulphur 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 sulphur isotopes. Therefore the sulphur isotope signature in fallout reveals whether or not an eruption was stratospheric.
To develop the method, the team focused on two volcanic eruptions, according to a UCSD press release. Both eruptions, one in 1963 and the other in 1991, were stratospheric according to the isotope measurements.
Modern intruments
"Young volcanoes have the advantage of having been documented by modern instruments, such as satellites or aircraft," said Savarino.
"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 sulphur isotope fingerprint than the fallout from Pinatubo.
Chemical reactions
Data from eruptions in the recent past revealed what chemical reactions of sulphates occur in the upper atmosphere.
Sulphates can cause warming or cooling depending on how they are made. 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. — Our Bureau
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