The so-called "hole," actually a thinner-than-normal area in the protective layer high up in the earth's atmosphere, has been forming at the end of Antarctic winter every year since the mid-1980s.
"The ozone hole is developing quite rapidly this year, in a very similar manner to the record-breaking year 2000," said Carine Richard-van Maele, spokeswoman of the World Meteorological Organization.
Reduction of the ozone layer can let harmful ultraviolet rays from the sun reach the earth's surface. Too much UV radiation can cause skin cancer and destroy tiny plants at the beginning of the food chain.
One cause of ozone depletion is the chlorine and bromine released by manmade chemical compounds such as chlorofluorocarbons, which were contained in some aerosols.
Although emissions of the chemicals have been curbed under a global accord, scientists predict it will take about 50 years for the ozone hole to close.
This year the ozone hole had developed "some time during the month of August," said Michael Proffitt, a leading expert on the ozone hole at WMO. "It's a gradual process."
The hole now appears to be 9.65 million square miles in area, about 10 percent below the record size recorded in mid-Sept. 2000, Proffitt said.
He said he was reluctant to put a figure on the record size because different ways used to measure it yield "slightly different numbers."
"They're all within a million square kilometers of each other."
U.S. government scientists have said the hole briefly approached 11.5 million square miles in 2000.
Proffitt told The Associated Press that because some of the hole remains in total darkness — the sun won't reach the South Pole until Sept. 21 — its total size wasn't clear.
He predicted that the hole would peak in size later this month.
The ozone hole forms in the polar vortex, the circular wind pattern that forms annually in the stratosphere over Antarctica, and this year the vortex is on the scale of 2000, with an area of about 13 million square miles.
"The ozone hole is always somewhat smaller than the vortex, but that's the size it could actually get to," Proffitt said. "It's possible."
One of the reasons the hole has been growing rapidly this year is because the vortex is elongated, exposing more of the ozone in the interior to the sun, Proffitt said.
Proffitt said WMO combines satellite measurements with data from balloons and Global Atmosphere Watch ground stations in Antarctica.
As usual, destruction of the ozone over Antarctica has been increasing since late August as the rising sun triggers the accelerated ozone loss after extremely low temperatures of winter.
By Alexander G. Higgins