The Institute of Technology is in a strong position to help tackle many of the most vexing scientific questions about climate change
Peter McMurry: Solving the aerosol puzzle
by Richard Broderick
In his office, mechanical engineering professor
Peter McMurry pulls up a satellite image on his computer
screen.
Over the thin mid-section of Central America, a
large cloud tails off to the west, obscuring a swath of
land and ocean far out into the Pacific.
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| Mechanical engineering professor Peter McMurry is researching how
and why clouds form, what role anthropogenic forcing plays in cloud
formation, and the secrets of aerosol systems and their role in the
formation of nanoparticles that go on to serve as cloud seeds. |
“See that?” he asks. “That’s smoke from a forest
fire. The fact that you can see it on this image means
it’s reflecting light back into space.”
The net result of that reflection, he points out, is a
cooling effect: the same as caused by the haze tapering
upwind into clouds on another image he calls up
off the East Coast of the United States. Cloud cover, in
short, is a critical component of the globe’s system of
climate regulation. How and why clouds form, what
role anthropogenic forcing (caused by humans) plays
in cloud formation, and in particular the secrets of
aerosol systems and their role in the formation of
nanoparticles that go on to serve as cloud seeds are
some of the primary research focuses carried on by
McMurry and collaborators at the University, the National
Center for Atmospheric Research (NCAR), and
other institutions.
In turn, that research has led McMurry and his
colleagues to develop a number of highly sophisticated
instruments to measure the size and composition
of aerosol particles. These instruments are
being used to understand the chemical processes
responsible for high rates of particle nucleation and
growth that are observed in the atmosphere.
“New particle formation occurs as a result of photochemical
reactions in the atmosphere that lead to
the production of condensable vapors,” he explains.
“Those vapors can go on to nucleate and form molecular
clusters that grow into sizes that are eventually
big enough to serve as seeds for cloud formation. We
are trying to understand the mechanism by which
these particles are produced.”
One of the earliest instruments McMurry created
measures aerosol particles, individually counting
nanoparticles as small as three nanometers while
also evaluating the distribution of particles by size
in any given sample.
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| Mechanical engineering
professor
Peter McMurry and his
colleagues built the
apparatus shown above
to measure the moisture
absorption and
volatility of atmospheric
nanoparticles formed
by nucleation. |
Currently the team, which includes University
of Minnesota chemistry professor Jeffrey Roberts
and NCAR researchers Fred Eisele and James Smith,
is working on other instruments designed to reveal
why, under certain conditions, nucleation and
growth rates are so high. For example, in places like
Mexico City that are covered in dense smog, nanoparticles
can grow to 100 nm—big enough to seed
clouds—in less than a day.
“Typically, particles are produced at rates that
are orders of magnitude greater than current models
predict. Once formed, freshly nucleated particles
typically grow about 10 times faster than can be explained.
Clearly current models don’t take into account
all of the complex processes taking place in
the atmosphere,” he said. “Our goal is to identify
and understand those processes so they can be included
in global climate models.”
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