accidental breakthrough on the puzzle of atmospheric acids

The blue haze which gives the Blue Mountains their name is a result of plants releasing a wide variety of oils.
The blue haze which gives the Blue Mountains their name is a result of plants
releasing a wide variety of oils.

In a classic example of an important, accidental scientific breakthrough a research team from the School of Chemistry may have solved the mystery of why there is more non-industrial acid in air than anyone can explain.

When Sydneysiders remark on the haze that gives the Blue Mountains their name, what they are seeing is a result of plants releasing a wide variety of oils. Once sunlight acts on the oils it produces a haze that contains twice the amount of organic acid than atmospheric models would predict from a corresponding volume of plants. Professor Scott Kable and his School of Chemistry colleague Dr Meredith Jordan are lead authors of the study published in Science on 7 September.

"We did not set out to solve the long-standing question of why the atmosphere has twice as many acids produced by plants – that is organic acids – than scientists have previously been able to account for," said the University's Professor Scott Kable. "It is a very happy accidental result, in the best traditions of science, that our pure research on photochemistry has produced this outcome."
The puzzle has been solved because of experimental research led by Professor Kable being supported by the theoretical modelling led by Dr Meredith Jordan.

"The compound we concentrated on was acetaldehyde, which is very common in the atmosphere, simply trying to understand what happens to it when irradiated with light. Deuterated acetaldehyde was synthesised by Dr Richard Payne's group in the Chemistry School and experiments on these compounds allowed us to follow the chemistry at a level not previously attempted.

"We used a laser to imitate the action of the sun. Excitingly, the resulting products were not what we had predicted but because these were laboratory results, conducted in a vacuum, we had to find out what would happen in the real world, in atmospheric conditions."

These 'real world' results confirmed that their chemical transformation was producing high yields; 25 percent of acetaldehyde molecules were being transformed into vinyl alcohol, which in turn forms organic acid under the action of the sun. The chemical process is called keto-enol tautomerization where a hydrogen atom is transferred to a different part of a molecule to create vinyl alcohol.
"Organic acids in the atmosphere have an impact. They dissolve in water and acidify droplets, changing its pH value. Plant growth is very sensitive to water and soil pH. Understanding the mechanism different organic acids are produced by is the key to understanding their apparent overabundance in the atmosphere.

"For rural regions and especially for regions with a lot of plant material, whether the Amazon Basin our own hazy Blue Mountains this a major step forward in understanding how plants interact with the atmosphere."



L-R: Brianna Heazlewood, Meredith Jordan, Duncan Andrews, Alan Maccarone and
Scott Kable at a conference in Madison, Wisconsin.

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