New research shows natural dust reduces cooling effect of sulphur in clouds

May 10th, 2013
An article published today in Science shows that natural dust reduces the cooling effect of sulphur in clouds – an important discovery in the role of pollutants and aerosols in climate change.

"The temperature of the atmosphere and its self-cleaning capacity depends a lot on clouds," says one of the article's authors, Professor Stephen Foley. "The lifetime and brightness of clouds is affected strongly by aerosols, which are currently thought to be the greatest single unknown factor in models of climate and climate change. Sulphur aerosols and their interactions with other elements, including dust, are thought to have an important cooling effect on the atmosphere, making them essential to climate models."

It is aerosols that form haze, which is often said to be due to industrial pollution. But aerosols have both natural and pollution origins, and it is essential that we know more about the balance between these, and about the microscopic chemical processes that take place on the surfaces of aerosol particles.

Lead author and Australian atmospheric scientist Dr Eliza Harris started this research in the Max Planck Institute for Chemistry in Mainz, Germany. Together with an international team of scientists, including Macquarie University's Professor Stephen Foley, Harris studied chemical reactions in the same cloud before, after, and on top of a mountain, which enabled changes in the chemistry of the cloud and its aerosols to be understood for the first time.

"Sulphur plays an important role in cloud formation, but the exact chemical reaction by which it becomes oxidized has been treated differently in the major climate models," Foley explains.

"It is thought to have an important cooling effect, but this depends on exactly which chemical reactions occur. By measuring the isotopes of sulphur, Eliza could show exactly which reaction pathways are important, distinguishing between natural and pollution-induced ones."

The study's results show that the oxidation of sulphur dioxide is influenced mostly by natural mineral dust, and not by pollutants. Transition metals in mineral dust aerosols, particularly titanium, are catalysts for the most important oxidation pathway for sulphur in clouds.

"This means that wherever dust is carried in the atmosphere, it will gobble up the sulphur dioxide and be removed quickly because of its larger size, which lets it settle out relatively easily," says Foley.

Harris and her colleagues conclude that the cooling effect is shortened and has, to date, been over-estimated.

Sulphur emissions in China and India are expected to rise in the next few years because of increasing industrialisation, and wind-blown dust is also high in these countries. Cooling by sulphur aerosols is likely to be significantly lower than predicted by climate models up to now, so this work is likely to have a significant impact on assessments of climate change.

Provided by Macquarie University

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