Since the 1980s, a seasonal ozone hole, characterized by severe loss of ozone, has appeared over the continent of Antarctica. However, scientists have not yet observed, on an annual basis, as severe a thinning of the protective ozone layer in the atmosphere over the Arctic.

The ozone layer shields life on Earth from harmful ultraviolet radiation. A northern ozone hole could be significant since more people live in Arctic regions than near the South Pole.

"A 'volcanic ozone hole' is likely to occur over the Arctic within the next 30 years," said Azadeh Tabazadeh, lead author of the paper and a scientist at NASA's Ames Research Center, located in California's Silicon Valley. Her co-authors are: Katja Drdla, also of Ames; Mark R.

Schoeberl of NASA's Goddard Space Flight Center, Greenbelt, Md.; Patrick Hamill of San Jose State University, Calif.; and O. Brian Toon from the University of Colorado, Boulder.

"If a period of high volcanic activity coincides with a series of cold Arctic winters, then a springtime Arctic ozone hole may reappear for a number of consecutive years, resembling the pattern seen in the Antarctic every spring since the 1980s," Tabazadeh said.

"Unlike the Antarctic, where it is cold every winter, the winter in the Arctic stratosphere is highly variable," Tabazadeh said. NASA satellite and airborne observations show that significant Arctic ozone loss occurs only following very cold winters, according to Tabazadeh.

Large volcanic eruptions pump sulfur compounds into the Earth's atmosphere. These compounds form sulfuric acid clouds similar to polar stratospheric clouds made of nitric acid and water.

The clouds of nitric acid and water form in the upper atmosphere during very cold conditions and play a major part in the destruction of ozone over Earth's poles. Following eruptions, volcanic sulfuric acid clouds would greatly add to the ozone-destroying power of polar stratospheric clouds, said Drdla.

"Volcanic aerosols also can cause ozone destruction at warmer temperatures than polar stratospheric clouds, and this would expand the area of ozone destruction over more populated areas," Tabazadeh said. "Nearly one-third of the total ozone depletion could be a result of volcanic aerosol effects at altitudes below about 17 kilometers

(11. 5 miles)," said the researchers.

"Volcanic emissions can spread worldwide," said Schoeberl. "Our Mt. Pinatubo computer simulation shows that the volcanic plume spread as far north as the North Pole in the lowest part of the stratosphere within a few months after the eruption."

Between about 15 and 25 kilometers (9 to 16 miles) in altitude, volcanic Arctic clouds could increase springtime ozone loss over the Arctic by as much as 70 percent, according to Drdla. "The combination of thick volcanic aerosols at lower altitudes and natural polar stratospheric clouds at higher altitudes could greatly increase the potential for ozone destruction over the North Pole in a cold year," Tabazadeh said.

"Both the 1982 El Chichon and 1991 Mt. Pinatubo eruptions were sulfur- rich, producing volcanic clouds that lasted a number of years in the stratosphere," Tabazadeh said. The Pinatubo eruption, as observed by NASA spacecraft, widely expanded the area of ozone loss over the Arctic.

Both of these eruptions did have an effect, however, over the South Pole, expanding the area and the depth of the ozone hole over the Antarctic, according to Tabazadeh. Computer simulations have shown that the early and rapid growth of the Antarctic ozone hole in the early 1980s may have been influenced in part by a number of large volcanic eruptions, she added.

"In 1993 the Arctic winter was not one of the coldest winters on record, and yet the ozone loss was one of the greatest that we've seen," Tabazadeh said. "This was due to the sulfurous Pinatubo clouds facilitating the destruction of additional ozone at lower altitudes where polar stratospheric clouds cannot form."

"Climate change combined with aftereffects of large volcanic eruptions will contribute to more ozone loss over both poles," Tabazadeh said. "This research proves that ozone recovery is more complex than originally thought."

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