News in Science

'Chemical equator' keeps southern air clean

A boundary of air, wrapped like a belt around the earth's equator, is keeping the polluted atmosphere of the Northern Hemisphere separate from the relatively pristine south, say UK researchers.

The team, led by Dr Jacqueline Hamilton of the University of York, have dubbed the peculiar wall of air the "chemical equator."

And while scientists have known about the feature for decades, Hamilton's team has just discovered an odd new wrinkle in its behavior.

Typically, where the northern and southern tropical trade winds come together they form a region of stormy updrafts called the Intertropical Convergence Zone (ITCZ).

Instead of mixing, the trade winds rise from the ocean or land surface high into the stratosphere, where they diverge again.

The circulation pattern quarantines the Northern and Southern Hemispheres, keeping the pollutant-laden northern air dirty and the southern air clean.

There is some exchange between the two, but the process takes a year or two, versus about a week for air to circulate within a hemisphere.

The team discovered that in January and February, the chemical equator splits off from the ITCZ north of Australia.

As hot air rises quickly off the sweltering Australian continent, storms proliferate and the ITCZ is shifted over land, well south of the geographical equator.

Clean air from the Indian Ocean rushes in to fill the gap, pushing north of Australia and into the South Pacific, where warm waters create a second stormy area.

There, in a region of balmy ocean called the Tropical Warm Pool, the team found the chemical equator thousands of kilometres north of the ITCZ.

'Striking boundary'

Flying on a research plane from south to north, they saw carbon monoxide levels jump from 40 parts per billion to 160 parts per billion in the space of just 50 kilometre.

"It's a really striking boundary," says Hamilton, adding that the high concentrations of carbon monoxide in the north are the result of biomass burning in Thailand and Sumatra, as well as general pollution from across the hemisphere, where most of the world's industry and population reside.

Carbon monoxide and other pollutants that remain in the atmosphere for a few months all display this kind of separation between hemispheres.

Tracking them is important because carbon monoxide is a chemical precursor to low-level ozone, a greenhouse gas that is hazardous if breathed in.

Aerosols from fires and incompletely combusted gasoline can also cause health problems and absorb heat from the sun, increasing the effects of global warming.

"I never noticed anything like a 'chemical equator' before," says John Gilles of the National Center for Atmospheric Research.

Gille, who leads an ongoing study to track carbon monoxide circulation across the globe, says seasonal variation in atmospheric conditions is probably behind the phenomenon. "It's something I'd like to go back and look for in our eight years of data."