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.
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.
By Randolphe E. Schmid