"Chemical Equator" Divides Earth's Hemispheres

Matt Kaplan
for National Geographic News
October 2, 2008
A worldwide weather "barrier" that can block air pollution from traveling southward, has been discovered, a new study says.

Called a "chemical equator," the 31-mile-(50-kilometer) wide boundary separates the Northern Hemisphere's dirty air from that of the less polluted Southern Hemisphere.

Carbon monoxide, a toxic gas generated by forest fires and internal combustion engines, increased from 40 parts per billion south of the boundary to 160 parts per billion north of it, scientists found.

Aerosol particles, produced by the burning of fossil fuels, also shot up dramatically.

The finding is reported in the Journal of Geophysical Research - Atmospheres.

(Related: "Chinese Air Pollution Deadliest in World, Report Says" [July 9, 2007].)

Serendipitous Discovery

The chemical equator has long been thought to exist. But scientists expected to find it within the Intertropical Convergence Zone, a band of thunderstorms and clouds circling the globe near Earth's actual Equator.

Instead, the line was found in clear skies 621 miles (1,000 kilometers) north of the zone, showing that the chemical and meteorological divide between the hemispheres is not the same.

"One would expect to see some chemical isolation, but not to this degree and closer to the zone," said Peter May, an atmospheric scientist at the Centre for Australian Weather and Climate Research in Melbourne, Australia. He assisted with the logistics behind the research but was not involved in the study itself.

The group of climatologists who found the chemical equator didn't set out with that goal.

The team were studying how storms transported chemicals in Darwin, on the northern coast of Australia, when the weather suddenly became clear and windy.

"This was not the right weather for our study, so we decided to fly a survey flight northwards to find out what was happening," explained study lead author Jacqueline Hamilton of the University of York, England.

"It was then that we came across the chemical equator," she said.

As their aircraft, which was rigged with chemical detectors, traveled north, its sensors detected a tremendous difference in pollutant levels.

Not Impermeable

Not everything nasty is blocked, however.

"Eventually chemicals can cross this barrier if they remain intact in the air on a timescale of longer than around one year," cautioned lead author Hamilton.

While carbon monoxide and many aerosols will be kept out, chemicals that linger longer in the atmosphere, such as the greenhouse gas carbon dioxide, will not.

"This finding means that, [when the chemical equator was observed], pollution was not going into the upper atmosphere where it could alter the chemistry of both clouds and the ozone layer," May said.

This phenomenon could make mapping pollutants easier for researchers, but there are still many unknowns.

"[These are the] really early days—we still need to know a lot more about how atmospheric chemistry and weather interact before we can know what the implications of this discovery really are," May said.

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