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Ozone Hole Splits in Two

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Ozone hole splits in two

ASSOCIATED PRESS

WASHINGTON, Sept. 30 — The ozone hole over Antarctica is markedly smaller this year than in the last few years and has split in two, government scientists reported Monday. The so-called “hole,” actually an area of thinner than normal ozone, was measured at 6 million square miles in September. That compares with around 9 million square miles on September measurements over the last six years, according to researchers at the National Oceanic and Atmospheric Administration and National Aeronautics and Space Agency.

WHILE OZONE at ground level is considered a pollutant, the layer of ozone high in the stratosphere is vital to life because it blocks dangerous radiation coming from the sun. Thinning of the ozone layer could lead to a rise in skin cancer, experts warn. Aerosols and other chemicals are blamed for the thinning, and treaties banning those ingredients are expected to help the layer recover over time.

This year’s improvement was attributed to warmer than normal temperatures around the edge of the polar vortex, or circular wind pattern that forms annually in the stratosphere over Antarctica, according to Paul Newman, a lead ozone researcher at NASA’s Goddard Space Flight Center in Greenbelt, Md.

Craig Long, a meteorologist at the NOAA Climate Prediction Center, said the stratosphere over the southern hemisphere was unusually disturbed this year by the wind, causing the hole to split into two separate holes.

In 2001 the Antarctic ozone hole reached a maximum size of more than 10.2 million square miles, larger than the entire area of North America, including the United States, Canada and Mexico combined. In the year 2000, it briefly approached 11.5 million square miles. The last time the ozone hole was as small as it is this year was in 1988, and that was also due to warm temperatures.

Newman explained that while “chlorine and bromine chemicals cause the ozone hole, the temperature is also a key factor in ozone loss.”

The coldest temperatures over the South Pole occur in August and September. Thin clouds form in these cold conditions, and chemical reactions on the cloud particles help chlorine and bromine gases to rapidly destroy ozone. By early October, temperatures typically start to warm and the ozone layer starts to recover.

The ozone is an atmospheric gas which filters out a harmful form of solar radiation known as ultraviolet radiation. About 90% of the ozone in our atmosphere is contained in the stratosphere (the region from about 30,000 feet to 180,000 feet above the Earth's surface).

Ozone is produced by intense ultraviolet (UV) radiation in the upper stratosphere, which causes oxygen molecules (O2) to reform as ozone molecules (O3). The ozone molecule spends most of its life absorbing UV.

Ozone is destroyed when it reacts with one of a variety of chemicals in the stratosphere such as chlorine, nitrogen, bromine or hydrogen. Such chemicals come primarily from man-made chlorine and bromine compounds produced for refrigerants, aerosol sprays, and solvents.

A number of factors found only at the poles increase the likelihood of ozone depletion. As the air in the stratosphere cools and descends during the winter, strong winds create a vortex around each pole. This air is effectively isolated the rest of the atmosphere. Within this vortex form polar stratospheric clouds (PSCs). Studies indicate that the chemical composition of these stratospheric clouds - and the extremely low temperature - provide the perfect environment for breaking ozone molecules.

Ozone levels over Antarctica fall to abnormally low values between August and late November, destroying about 95% of the ozone in the lower stratosphere. In 1987, and from 1989-95, the hole covered the entire continent and part of the surrounding ocean. There is currently no ozone hole at the North Pole, but a recent study determined that ozone losses of over 60 percent occurred in the Arctic stratosphere at an elevation near 60,000 feet during the winter of 1998/1999.

An Australian study published two weeks ago reported that chlorine-based chemical levels in the atmosphere are falling, and the hole in the ozone layer should close within 50 years. Although the ozone layer has not yet begun to repair itself, the hole would probably start closing within five years, said Paul Fraser, of the Australian government-funded Commonwealth Scientific and Industrial Research Organization, or CSIRO.

Ozone in the air overhead is measured in Dobson Units. The ozone hole is the area with total column ozone below 220 Dobson Units, as measured by satellites and balloon-borne ozone measuring instruments launched from the South Pole station.

In the tropics, ozone levels are typically between 250 and 300 Dobson Units year-round. In temperate regions, seasonal variations can produce large swings in ozone levels, reaching as high as 475 Dobson Units in some areas, and as low as 300.

A reading of 100 Dobson Units means that if all the ozone in the air above a point were brought down to sea-level pressure and cooled to freezing it would form a layer 1 centimeter thick. At that scale a reading of 250 Dobson Units translates to a layer about an inch thick.

© 2002 Associated Press. All rights reserved. This material may not be published, broadcast, rewritten or redistributed.

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