Arctic ozone loss in 2011 unprecedented

Record ozone depletion over the Arctic early this year rivals what was observed in the Antarctic when holes in the protective atmospheric layer first appeared there during the 1980s.

The observation raises concerns that portions of the Northern Hemisphere might periodically begin experiencing potentially harmful levels of ultraviolet radiation during early spring, an international team of scientists reports online October 2 in Nature.

“It was significantly worse than anything we have ever seen,” says Geir Braathen of the World Meteorological Organization in Geneva, who was not one of the authors of the Nature paper. Typically, spring Arctic ozone depletion has maxed out at a drop of between 20 and 30 percent, the atmospheric chemist notes. “But in 2011, we had a loss of around 40 percent.”

In Antarctica, 70 percent of the ozone can disappear in springtime, Braathen says. Within a 5- to 7-kilometer–thick band of the stratosphere, ozone concentrations actually plummet to zero, he says.

Arctic conditions have not gotten nearly that bad, says Michelle Santee of NASA’s Jet Propulsion Laboratory in Pasadena, Calif., one of 29 authors of the new Arctic ozone analysis.

Although ozone can be found at any altitude over the Arctic, most accumulates between 14 to 21 kilometers up. There, concentrations hover around 4.5 parts per million much of the year. But in late March, “there was an approximately 2-kilometer altitude region where ozone fell to around 0.7 ppm,” Santee says — “meaning the ozone was pretty much gone.” In small regions, she adds, patches of the Arctic stratosphere saw ozone drop to 0.5 ppm.

It takes four things to destroy much of the stratosphere’s ozone: sunlight; very prolonged cold temperatures; a stable vortex of winds that prevents ozone losses inside it from being replenished with more from outside; and the presence of special clouds that foster the transformation of benign chlorine molecules into ozone-vanquishing types. For the first time in the Arctic, all of these conditions aligned for months, says JPL atmospheric scientist and coauthor Nathaniel Livesey, “making it the perfect storm.”

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