Observations of the novel boundary will provide important clues to help scientists to model simulations of the movement of pollutants in the atmosphere more accurately, and to assess the impact of pollution on climate, the researchers say.

A scientific paper about the chemical equator is to be published in the Journal of Geophysical Research - Atmospheres, a publication of the American Geophysical Union (AGU).

Scientists had previously thought that a meteorological feature-- the Intertropical Convergence Zone (ITCZ)--formed the boundary between the polluted air of the Northern Hemisphere and the clearer air of the Southern Hemisphere.

The ITCZ is a cloudy region circling the globe where the trade winds from each hemisphere meet. It is characterized by rapid vertical uplift and heavy rainfall, and acts as a meteorological barrier to pollutant transport between the hemispheres.

But, in cloudless Western Pacific skies well to the north of the ITCZ, Jacqueline Hamilton of York University and her colleagues found evidence for an atmospheric chemical equator around 50 kilometers (31 miles) wide.

Across that newfound borderline, air quality differed dramatically. For instance, carbon monoxide, a tracer of combustion, increased from 40 parts per billion to the south, to 160 parts per billion in the north.

The difference in pollutant levels was increased by extensive forest fires to the north of the boundary and very clean air south of the chemical equator being pulled north from the Southern Indian Ocean by a land based cyclone in northern Australia.

The scientists discovered evidence of the chemical equator using sensors on a specially equipped airplane during a series of flights north of Darwin, a city on the northern coast of Australia. At the time, the ITCZ was situated well to the south over central Australia.

Researchers from the universities of York, Manchester, Cambridge, Leicester, and Leeds--all in the United Kingdom--collaborated in the study.

"The shallow waters of the Western Pacific, known as the Tropical Warm Pool, have some of highest sea surface temperatures in the world, which result in the region's weather being dominated by storm systems," says Hamilton, lead author of the scientific paper. "The position of the chemical equator was to the south of this stormy region."

"This means that 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," Hamilton notes. "To improve global simulations of pollutant transport, it is vital to know when the chemical and meteorological boundaries are in different locations."

This research was funded by the United Kingdom's Natural Environment Research Council (NERC) as part of the ACTIVE project (Aerosol and Chemical Transport in Tropical Convection). Other partners include the Australian Bureau of Meteorology and Flinders University in Adelaide, Australia. Flights were carried out onboard the NERC Airborne Research and Survey Facility Dornier 228 aircraft.

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