A collaboration among U of T chemistry professor James Donaldson and colleagues at the University of Colorado and the University of Otago in New Zealand is shedding new light on the role of photochemistry in the upper atmosphere.
Their research explains “how the absorption of sunlight can affect atmospheric chemistry in ways previously unconsidered,” Donaldson said.
“In order to understand the processes that take place in the atmosphere we need to have a good handle on mechanisms for chemical transformation.”
Donaldson’s work involves the role of ultraviolet (UV) light in the chemistry of our atmosphere. Sunlight is the major driving force in atmospheric chemistry, and light in the UV band is thought to be the most important. But there are occasions when chemical reactions occur without the high energy of UV radiation.
“This has been a mystery, and along with my collaborators we have developed a mechanism for chemistry to happen when there is insufficient UV light to explain it,” said Donaldson.
In some instances, molecules can absorb visible light and vibrate with enough energy that the vibrations can spread out over the molecule, causing a chemical bond to break. Although such indirect reactions are well understood in physical chemistry, according to Donaldson this method of delivering energy to molecules “has not been thought of in the context of atmospheric chemistry.”
The scientists’ study focused on using this type of indirect photochemistry to better understand the stratospheric aerosol layer found between 15 and 30 km up in the atmosphere. The layer contains highly dispersed aerosols (tiny droplets of liquid suspended in air) containing mostly sulphuric acid and water. It can have significant climate and chemical implications; for instance, the layer reflects some of the sun’s energy back into space, like its more famous cousin the ozone layer.
The formation of this atmospheric sulphate aerosol layer was not clearly understood. Donaldson’s group found that sulphuric acid does not absorb high-energy UV radiation, as was previously assumed. Instead, the scientists propose that if enough visible light were absorbed by sulphuric acid, it would cause molecular vibrations that supply enough energy to re-arrange the molecule and make it fall apart. They think this is a better explanation for the formation of the layer.
With some of the most pressing uncertainties in climate modelling depending on the interaction of light with aerosols, the need to understand the factors that control the formation of this sulphate aerosol layer is important. And, although the research was done on Earth, the work may provide insight into the atmospheres of other planets, such as the sulphur-rich gasses shrouding Venus.
The study was published earlier this month in Science.
Photograph by Simon Turnbull
