The uptake of gases like sulphur dioxide has important implications in understanding airborne pollutants and their role in climate change. Sulphur dioxide that has come together with water, becoming aqueous, reflects light coming towards the planet.

Chemistry doctoral student Stephanie Ota has examined the behaviour of sulphur 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 sulphur dioxide with water as it is flowed over a water surface. She found that as sulphur 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.

"We have found that the sulphur dioxide bonding to the surface is highly reversible and does not necessarily provide the open doorway that might be expected," says Ota. "For example, for highly acidic water, the sulphur dioxide approaches and bonds to the water surface but shows little interest in going any further into the bulk water."

"In the past we presumed that most chemistry in the atmosphere occurred when gas molecules collide and react," says Ota's colleague Geraldine Richmond. "These studies are some of the first to provide molecular insights into what happens when an atmospherically important gas such as sulphur dioxide collides with a water surface."

The researchers hope that 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

The research has been published in the Journal of the American Chemical Society.