Q. How is the ozone hole doing?

A. Currently, the ozone hole is not as large as it was in 2011, but it is larger than it was in 2010. The ozone hole refers to the rapid depletion of stratospheric ozone over Antarctica. This ozone is located in a layer about 15 miles above the surface.

Human activity has contributed to the deterioration of the ozone layer by adding chlorofluorocarbons, or CFCs, to the atmosphere. These chemicals were invented by chemists to be used as propellants in spray cans, as Styrofoam puffing agents and as coolants. These chemicals are very stable and once injected into the atmosphere can remain for decades and are transported all over the globe, including over Antarctica. Once they are high over Antarctica, CFCs can significantly deplete the stratospheric ozone layer.

During Antarctica's winter, a vortex of winds develops around the pole and isolates the polar stratosphere. Very cold temperatures can then develop (109 degrees below zero Fahrenheit), which helps to form thin clouds made of ice, nitric acid and sulphuric acid mixtures. Stratospheric temperatures in the mid-latitudes, where we live, do not get this cold. As the sun rises in the Southern Hemisphere in springtime (e.g., late September), a crucial ingredient is added — UV radiation. The combination of UV radiation, stratospheric clouds and chemicals results in a rapid destruction of ozone.

This September we celebrate the 25th anniversary of the Montreal Protocol, an international treaty to end the production of ozone-destroying chemicals. The Montreal Protocol is working, as ozone depletion due to human effects is starting to decrease. Unfortunately, even with the Montreal Protocol, the ozone hole is expected to stay around until about 2050.

Steve Ackerman and Jonathan Martin, professors in the UW-Madison department of atmospheric and oceanic sciences, are guests on WHA radio (970 AM) at 11:45 a.m. the last Monday of each month.

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