©Hamilton et al./AGU
An invisible barrier separates the carbon-monoxide-rich air of South-East Asia from the pristine air of the Southern Ocean

Scientists have discovered a "chemical equator" just north of Australia that divides polluted air from South-East Asia from the largely uncontaminated atmosphere of the Southern Ocean that surrounds Antarctica.

The discovery will help researchers create accurate simulations of how pollutants move in the atmosphere and assess their impact on climate.

The polluted air of the northern hemisphere and the clearer air of the southern hemisphere tend not to mix. A mobile cloudy belt known as the Intertropical Convergence Zone that runs around the globe roughly at the level of the equator is thought to form a meteorological barrier between the two.

But a team of researchers have found an additional barrier high up above the Western Pacific that prevents the pollution from forest fires in countries such as Thailand and Sumatra contaminating the pristine skies above the Southern Ocean.

While flying a plane equipped to detect chemicals in the atmosphere, the team led by Jacqueline Hamilton of the University of York, UK, found a 50 kilometre wide "chemical line" north of Australia.

'Hidden' barrier

Levels of carbon monoxide - a by-product of burning - were four times higher to the north of the line than they were to the south.

At the time, the Intertropical Convergence Zone lay much further south, over Central Australia. So the researchers concluded that they must have come across another "hidden" barrier.

Hamilton explains that the shallow waters of the Western Pacific may help form the barrier.

The surface waters are amongst the hottest in the world, fuelling strong storm systems. "These powerful storms may act as pumps, lifting highly polluted air from the surface to high in the atmosphere where pollutants will remain longer and may have a global influence," she says.

To the south, cyclones above Australia suck in pristine maritime air. The two systems do not mix, creating an invisible chemical barrier.

Hamilton's study was carried out in January and February 2006, during the Australian-Indonesian monsoon, and the effect may be seasonal, she says.