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 todays 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 doesnt 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.”