Why is the Ozone Hole Over Antarctica?

Since most ozone-depleting substances are released in the northern hemisphere, a common question is why the ozone hole occurs over the Antarctic. The first part of the answer is that even though most of these chemicals are heavier than air, regardless of where they're released, they mix throughout the troposphere over about a year, and then mix into the stratosphere in 2-5 years. The second part of the answer is that although the overall process is similar between global ozone depletion and the ozone hole, there are two different types of ozone depletion chemistry. Both types are important, but the ozone hole seems to grab most of the attention.

The first kind is called homogeneous depletion; resulting from reactions as gases mix together, it is responsible for the reduction in global ozone levels. The 5-10% drop in ozone over the US is an example of homogeneous chemistry.

The second kind of ozone depletion chemistry, called heterogeneous, causes the radical destruction of ozone over the Antarctic each spring that we call the ozone hole. It results from reactions on the surfaces of ice particles. The existence of these particles, and the seasonal and geographic location of the hole, all result from a combination of meteorological and other effects that are specific to Antarctica at that time of year:

1. The Antarctic Polar Vortex
2. Polar Stratospheric Clouds
3. Concentrations of Active Chlorine

The Antarctic Polar Vortex

Each winter, the air around the South Pole cools and begins circulating to the west. This vortex effectively isolates the air over Antarctica, with three effects:

1. Outside air, which is relatively ozone-rich, cannot mix in and sustain ozone levels.
2. Chemicals that tend to slow down the depletion reactions cannot mix with Antarctic air.
3. Heat from outside air is shut out, prolonging the period of very low stratospheric temperatures.

Because the air gets so cold over the Antarctic each winter, the vortex remains intact for several months, finally breaking up in December. The vortex is the reason for the timing and location of the hole; because such vortices do not form over more temperate regions, homogeneous gas-phase chemistry is the dominant global concern, producing long-term ozone depletion trends.

Polar Stratospheric Clouds

The Antarctic is a very cold place; temperatures in the lower stratosphere drop below -78° C (-108°F). Ordinarily, the stratosphere is so dry that it will not support clouds, but these cold temperatures do produce ice clouds. Some of the clouds are water ice, but more prevalent are clouds of nitric acid and water.

Like the wind vortex, the formation of PSCs has specific results:

1. In the absence of polar stratospheric clouds, most stratospheric chlorine is locked up in relatively inert compounds. However, the surfaces of the ice particles in the clouds allow these compounds to react, coverting the chlorine into ozone-destroying forms. The reactions are different from those occuring when gases mix over midlatitudes, and can only take place on cloud particles.
2. The nitric acid in the clouds comes from nitrogen oxides (NOx) (the process of removing NOx from the atmosphere is called denoxification). Nitric acid normally slows the ozone depletion reactions, so its removal allows ozone destruction to continue unabated.
3. This sequestering of NOx can become permanent if the cloud grows so large that it eventually sinks into the troposphere (called denitrification).

Concentrations of Active Chlorine

The forms of chlorine released from the clouds' surfaces cannot destroy ozone without the addition of UV light, which isn't available during the southern winter. Thus, their concentrations rise until the sun appears during the spring. When the sun does rise, the chlorine is rapidly converted to chlorine monoxide, and this is followed by a very rapid set of reactions, destroying up to 70% of the ozone in the lower stratosphere over a period of weeks.

Summary

The net effect of these factors is the ozone hole: an easily-measured, well-defined, seasonal phenemenon. The depth, area, and timing of the hole vary from year to year, but as the polar vortex breaks up and the stratosphere warms, the heterogeneous chemistry shuts down, and ozone levels over the Antarctic return to near normal. The ozone hole generally lasts from August to November, although the exact time period varies from year to year.

The ozone hole is the most obvious effect of the release of ozone-depleting substances into the atmosphere, and it is also the most extreme example of ozone depletion. However, the long-term downward trends in ozone levels over most of the globe also pose a serious threat. Although not as spectacular, homogeneous chemistry is a significant problem.

Much more detail about the ozone hole is available. The Ozone Hole Tour provides graphs and text to explain why the ozone hole occurs, and Robert Parson's Frequently Asked Questions site provides scientific citations.