Pollutant-H20 mix key to climate change

U. OREGON (US) — The impact that surface molecules of water have on airborne pollutants should be considered in climate modeling, given their significant effect on the future of global warming.

A new study examined the behavior of sulfur dioxide—a pollutant of volcanic gasses and combustion processes—as it approaches and adsorbs onto water at low temperatures that mimic high atmospheric conditions.

The role of sulfur dioxide in acid rain is well known, but how it reacts at the surface in the atmosphere to form acid rain is not, says Stephanie Ota, chemistry doctoral student at the University of Oregon.

Using a combination of short-pulsed infrared and visible laser beams, Ota monitored the interaction of sulfur dioxide with water as it is flowed over a water surface.

The results—detailed online 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 provides a doorway for sulfur dioxide to enter the water solution, the weakness of the surface-bonding interaction doesn’t guarantee it will be successful, says co-author Geraldine L. Richmond, professor of chemistry,

“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 says.

“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 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, Richmond says.

“In the past we presumed that most chemistry in the atmosphere occurred when gas molecules collide and react.

“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.”

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