Sunlight `scrambles pollutants`

Sunlight can rearrange the atoms of molecules to form new chemical substances.

Melbourne: Scientists have discovered a startling new mechanism where sunlight can rearrange the atoms of molecules to form new chemical substances, which they claim has implications for the extent that pollutants are dispersed across the Earth`s surface, and how quickly they are removed.

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

Now, a team at the University of Sydney has overturned this theory using a common, small pollutant molecule, known as acetaldehyde, in a lab-based experiment which substituted a laser light for the sun, the latest edition of the `Nature Chemistry` journal reported.

Professor Scott Kable, who led the study, said: "We chose a special variant of the acetaldehyde compound, where three of the four hydrogen atoms were replaced with `heavy hydrogen` (called deuterium).

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

According to the scientists, 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," Prof Kable said.

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

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


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