University of Oregon chemistry doctoral student Stephanie T. Ota examines the behavior of sulfur dioxide as it approaches and adsorbs onto water at low temperatures that mimic high-atmospheric conditions. |
High
in the sky, water in clouds can act as a temptress to lure airborne
pollutants such as sulfur dioxide into reactive aqueous particulates.
Although this behavior is not incorporated into today’s climate-modeling
scenarios, emerging research from the University of Oregon provides
evidence that it should be.
The
role of sulfur dioxide — a pollutant of volcanic gasses and many
combustion processes — in acid rain is well known, but how sulfur
dioxide reacts at the surface of aqueous particulates in the atmosphere
to form acid rain is far from understood.
In
National Science Foundation-funded laboratory experiments at the UO,
chemistry doctoral student Stephanie T. Ota examined the behavior of
sulfur dioxide as it approaches and adsorbs onto water at low
temperatures that mimic high-atmospheric conditions. Using a combination
of short-pulsed infrared and visible laser beams, she monitored the
interaction of sulfur dioxide with water as it is flowed over a water
surface.
The
results — detailed online ahead of regular publication in the Journal
of the American Chemical Society — show that as sulfur dioxide
molecules approach the surface of water, they are captured by the
top-most surface water molecules, an effect that is enhanced at cold
temperatures.
Although this reaching out, says co-author Geraldine L. Richmond, professor of chemistry,
provides a doorway for sulfur dioxide to enter the water solution, the
weak nature of the surface-bonding interaction doesn’t guarantee that
the water temptress will be successful.
“We
have found that that the sulfur dioxide bonding to the surface is
highly reversible and does not necessarily provide the open doorway that
might be expected,” Ota said. “For example, for highly acidic water,
the sulfur dioxide approaches and bonds to the water surface but shows
little interest in going any further into the bulk water.”
The
uptake of gases like sulfur dioxide has important implications in
understanding airborne pollutants and their role in global warming and
climate change. Sulfur dioxide that has come together with water,
becoming aqueous, reflects light coming toward the planet, while carbon
dioxide accumulating in the atmosphere traps heat onto the planet.
Understanding
the interaction of surface water molecules, such as those in clouds and
fog, with pollutants rising from human activity below may help
scientists better predict potential chemical reactions occurring in the
atmosphere and their impacts, said Richmond, who was elected May 3 as a
member of the National Academy of Sciences.
“In
the past we presumed that most chemistry in the atmosphere occurred
when gas molecules collide and react,” she said. “These studies are some
of the first to provide molecular insights into what happens when an
atmospherically important gas such as sulfur dioxide collides with a
water surface, and the role that water plays in playing the temptress to
foster reactivity.”
