Arctic ozone levels fell to a record low in March, 21% below the springtime average of previous years, scientists report in a suite of papers in the current issue of Geophysical Research Letters. The researchers blame the decline on longer winters that worsen the ozone-destroying ability of chlorine. The depletion still doesn't rival the famous ozone hole over Antarctica. But if it continues to worsen, they say, ozone-depleted air could move south, diluting the ozone over the northern United States.
Over the course of a normal winter, ozone builds up in the polar region to extremely high levels by March. This pattern has been tracked by satellite-based instruments such as the Total Ozone Mapping Spectrometer and by instruments on weather balloons. Over the past several years, average ozone levels have taken a nose dive in the late winter months, and this year's record low has researchers convinced that the lows are getting worse. "For the first time, we're seeing an unambiguous trend over the North Pole," says Paul Newman, an atmospheric physicist at NASA's Goddard Space Flight Center in Greenbelt, Maryland.
The culprit seems to be longer winters--prolonging the conditions that destroy ozone--that may result from significant weakening of large-scale weather systems above the northern mid-latitudes, says Newman. The winter cold forms polar stratospheric clouds that convert chlorine from pollutants such as Freon and other chlorofluorocarbons into activated molecules that destroy ozone with a vengeance. During these months--typically February and March--winds circulate around the pole, generally hemming in the clouds. The longer the so-called polar vortex persists, the greater the ozone loss. Once spring arrives and the vortex dissipates, the ozone-depleted air can head south--possibly as far as New York--potentially reducing the ozone cover, Newman says.
The new measurements of Arctic ozone loss make for "quite a stunning result," says Susan Solomon, an atmospheric chemist at the National Oceanic and Atmospheric Administration's Aeronomy Lab in Boulder Colorado, who adds that the real mystery is why the Arctic has had three long, cold winters in a row. Meanwhile, researchers hope that the suite of new ozone and weather measurements will help refine atmospheric and climate models, which in turn might reveal whether ozone levels in the Arctic will continue to drop.