News in Science

Sulphur find clouds climate predictions

Climate haze The cooling effect of clouds is overestimated in current climate change models, suggests new research.

The study, published today in Science looks at the life-span of cloud-forming sulphur aerosols, which play a major role in cooling the Earth.

Co-author Professor Stephen Foley, a geologist at Macquarie University, says atmospheric aerosols -- whether caused by pollution or through natural chemical reactions -- help clouds grow through the condensation of humidity on the microscopic particles.

How these particles reflect sunlight determines the lifetime and brightness of clouds, and their cooling impact on the atmosphere.

But it is unclear how the chemical composition of these particles change within clouds.

To find out exactly what happens, the international team led by Melbourne researcher Dr Eliza Harris, now at Massachusetts Institute of Technology, traced how aerosol particles change when a cloud forms.

They analysed sulphur compounds in air collected before the samples entered the cloud, after they left the cloud and while they were in the cloud.

By looking at the isotopes of the sulphur, Harris and colleagues could tell how the sulphate formed.

"The relative reaction rates of isotopes are like fingerprints," says Harris, who undertook the research while at the Max Plank Institute for Chemistry.

The study shows in the majority of cases so-called transition metal ions such as iron, manganese or titanium act as the catalyst for the oxidation of sulphur dioxide by oxygen.

Dust is the key

Critically for climate change modelling these transition metal ions are predominantly found on microscopic mineral dust particles.

"This means that wherever dust is carried in the atmosphere, it will gobble up the sulphur dioxide and fall out [of the cloud] easily because of its larger size," says Foley, of the ARC Centre of Excellence for Core to Crust Fluid Systems.

He says this reduced life span of sulphate in clouds means there is a shortened cooling effect.

Foley says only one of 12 climate models considers the role transition metal ions play in the oxidation of sulphur dioxide.

Instead the less common oxidation pathways via hydrogen peroxide or ozone are used with the result that the cooling effect of sulphate aerosols has been over-estimated.

Foley says it is not possible to say what impact including this new information in climate modelling will have.

However he says increasing industrialisation in China and India is expected to lead to a rise in sulphur emissions.

These are countries where wind-blown dust is also common, he says, suggesting cooling by sulphate aerosols is likely to be lower than current climate models predict in these regions.

Cirrus clouds

Also published today in Science, an international team led by Massachusetts Institute of Technology researchers has identified the major seeds on which cirrus clouds form.

Cirrus clouds --the thin wisps of vapour that trail across the sky -- cover nearly one-third of the globe.

The team sampled cirrus clouds using instruments aboard high-altitude research aircraft, analysing particles collected during multiple flights over a nine-year period. The team found the majority of cloud particles freeze, or nucleate, around two types of seeds: mineral dust and metallic aerosols.

The lack of biological particles such as black carbon and fungal spores has implications for climate modelling as many models assumed these to play a role in cloud formation.

The researchers say their work will help in a greater understanding of the climatic implications of these clouds.