CIRES researchers propose answer to basic atmospheric chemistry question
By Annette Varani
CIRES Fellow and Professor of Chemistry and Biochemistry, Veronica Vaida |
In
a spring 2003 issue of Science magazine, CIRES scientists proposed
a long-sought answer to how atmospheric sulfate aerosols are formed in
the stratosphere.
CIRES fellow Veronica
Vaida, chair of the CU-Boulder chemistry and biochemistry department,
in collaboration with CIRES visiting fellows Henrik Kjaergaard from the
University of Otago in New Zealand and Jamie Donaldson from the University
of Toronto, and CIRES doctoral candidate Paul Hintze, showed how a fundamental
molecular process driven by sunlight may play a significant role in determining
the planet's energy budget.
Atmospheric sulfates
gather in a stratospheric region called the Junge layer that surrounds
Earth's surface at altitudes between nine and 21 miles, said Vaida. The
Junge layer reflects sunlight back into space and radiation to Earth,
affecting the planet's energy budget.
The Junge layer is thought to be composed primarily of sulfuric acid and
water molecules, she said. Because sulfates have important chemical and
climate effects, scientists have wanted to understand how atmospheric
sulfuric acid breaks down, releasing sulfur oxides in the upper stratosphere
where concentrations have been measured.
When high-altitude
air descends in the cold polar vortex each spring, the gases recombine
and form the Junge layer, Vaida said. Sunlight can be absorbed by sulfuric
acid molecules and in some instances decompose them.
"It was thought
that solar radiation could break the bonds of sulfuric acid molecules
at very high energies in the ultraviolet spectrum." Vaida said. But
high-energy radiation is present only at the top of and above the atmosphere
because the atmosphere effectively absorbs ultraviolet radiation.
"We ruled out
the standard hypothesis that had been proposed but never observed,"
Vaida said. Instead, she said, the CIRES team sought ways that the sulfates
could be breaking down within the visible range of light.
In order to explain
the measured and modeled concentrations of sulfates found in the upper
stratosphere and mesosphere, "The mechanism we proposed was really
the only game in town," she said.
Using spectroscopy,
the team investigated the effect of visible light on sulfuric acid molecules
to prove that molecular rearrangements could be induced to explain the
observed sulfate layer. "We found visible radiation at much lower
energies than previously thought could accomplish the molecular breakdown,"
Vaida said.
"Understanding
the fundamental properties of sulfuric acid, we now know what affects
formation of the sulfate layer, and can predict its formation by looking
at the altitude, temperature and solar flux," she said. "The
work allows us to model chemical properties of the Earth's atmosphere."
Support for the research
was provided by the National Science Foundation, the Marsden Fund administered
by the Royal Society of New Zealand and NSERC of Canada. Vaida credits
the success of the team's discovery to the CU-NOAA partnership at CIRES
that unites university academic departments with eight NOAA laboratories.
The collaboration produces an increased flow of ideas and additional access
to specialized expertise.
"We had a lot
of help from NOAA people uniquely qualified in the areas that we needed
- the connection fostered by CIRES was key," she said. "We could
bring together fundamental chemistry with atmospheric science in a way
that can't be done anywhere else - it was rather magical."
The next step is "to
quantify the yield with which sulfur oxides are going to be released
and refine our knowledge of related processes," Vaida said.
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