Chemistry of atmosphere more complicated than previously understood

Chemists from the University of Sydney have shed light on the sun's role in mixing up molecules, with implications for the extent that pollutants are dispersed across the Earth's surface and how quickly they are removed.

The research by Professor Scott Kable, Dr Meredith Jordan and collaborators at the School of Chemistry on the startling new mechanism where sunlight can rearrange the atoms of molecules to form new chemical substances is published in a recent issue of Nature Chemistry.

Until now, chemical models of the atmosphere assumed a molecule emitted into the atmosphere stays fixed as that molecule, until it is either photolysed (broken up) by sunlight or attacked by other molecules.

Professor Kable and Dr Jordan have now overturned this theory using a common, small pollutant molecule, acetaldehyde, in a lab-based experiment that substituted a laser light for the sun.

"We chose a special variant of the acetaldehyde compound, where three of the four hydrogen atoms were replaced with 'heavy hydrogen' (called deuterium)," Professor Kable explains.

"While not changing any of the chemical or photochemical properties to any significant extent, this subtle chemical change did allow us to follow the photochemical reactions with much more detail."

Professor Kable says conventional atmospheric models predicted that acetaldehyde should simply break in half when it absorbs light.

"Our experiments showed that the atoms in the molecules were instead extensively scrambling - specifically the hydrogen and deuterium atoms were scrambling - before the acetaldehyde broke apart."

Acetaldehyde is converted into various other chemical compounds during the scrambling process. The most important of these is an alcohol (vinyl alcohol) which has very different photochemical properties to acetaldehyde and is removed from the atmosphere by different processes.

"Our research shows that compounds such as acetaldehyde, when emitted to the atmosphere, will transform into other substances before the sun has a chance to destroy them," Professor Kable says.

"If molecules are being transformed by sunlight, then the chemistry of the atmosphere is much more complicated than we have considered up until now."

Although this work changes scientific understanding of how pollutants are dispersed through the atmosphere, Professor Kable is careful to note it won't change global warming models. "Nearly all carbon-based compounds in the atmosphere end up as CO₂ eventually. It won't change models of CO₂ loading in the atmosphere," he says.

The article 'Near-threshold H/D exchange in CD3CHO photodissociation', by Meredith Jordan and Scott H Kable et al, is published in Nature Chemistry, 23 May 2011.