Reply to ASK-AN-EARTH-SCIENTIST

Subject: Polar Ozone Hole

Why is the Ozone Hole only over the North and South Poles, and not distributed evenly over the earth's atmosphere?

A number of processes and effects that occur simultaneously are believed to be causing seasonal polar ozone depletion known commonly as the "Ozone Hole". These include:

the natural chemical reactions responsible for the production and destruction of ozone in the stratosphere (middle atmosphere)
the seasonal variations in incident sunlight on the planet
"special" features of polar weather that isolates this region of the atmosphere from the rest of the planet during part of the year
the addition to the atmosphere of ozone depleting chemicals through human activities
the pressence of "active surfaces" in the polar statosphere upon which ozone destruction reactions can be enhanced. This includes volcanic aerosols and polar stratospheric clouds.

To understand its distribution in the atmosphere, you must first know what ozone is and how it is naturally created/destroyed. Ozone is a molecule composed of 3 atoms of oxygen (O₃). The "normal" oxygen we breath has 2 atoms of oxygen in it (O₂). Ozone is constantly produced and destroyed by chemical reactions in the upper atmosphere that involve the breaking of O₂ into two O atoms that then react with more O₂ to make O₃. Oxygen atoms are a type of "free radical", which is an atom or molecule with a very reactive electron on it. Light of ultra-violet wavelengths (around 300 nm) exites O₂ and O₃ molecules to the proper energy to allow both production and destruction reactions of O₂ and O₃ to proceed. These reactions normally absorb much of the UV radiation the sun directs towards us, protecting the Earth's surface from its harmful effects. The ozone reaction is actually more complex then the way I've just described it for two reasons: 1) it requires mediator molecules that help the reactions along by transferring some of the energy between the reacting molecules and 2) other chemical species can get involved, including other "free radicals" such as Cl and NO.
Ozone can be found in a number of locations in the atmosphere but here I restrict the discussion to statospheric ozone. Nature set up a balance for the production and destruction of ozone in the stratosphere by making available all the materials involved in the chemical reactions and keeping their concentrations relatively constant with time. One natural phenomenon that can disrupt this balance is a volcanic eruption that sends gasses and volcanic ash into the stratosphere. Not all eruptions do this but particularly violent ones (such as Mt. Pinatubo) can add particles to the upper atmosphere that will affect the ozone budget for some period of time.
Why does the amount of ozone vary in the statosphere? Under normal conditions, the amount of stratospheric ozone depends on the amount of sunlight reaching a certain geographic area of the atmosphere. The seasonal variation is low in low lattitudes because sunlight is fairly constant year-round. In high latitudes, sunlight goes way down in the winter months. Ozone typically "builds up" to higher values over the poles during the winter and early spring in each hemisphere. Because this season is offset by 6 months in the Northern and Southern hemispheres, the effect is seen at the North and South poles roughly 6 months apart.
How have humans affected the balance of ozone in the stratosphere? Human use of materials that themselves can destroy ozone or that can be transformed in the atmosphere into ozone depleting chemicals has upset the natural balance in the stratosphere such that degredation of ozone is favored relative to production. Sources of halogens (Cl, F, Br) such as widely used chloroflorocarbons (CFCs) have probably been an important factor. Freon is one class of CFC that became widely used in coolant systems soon after it was invented in the 1940s. Another pollutant we have added to the upper atmosphere that affects ozone are nitrogen oxides from air plane exhaust.
So, above and beyond, the seasonal variations in ozone that are more pronounced at the poles, why do the enhanced ozone holes due to human activities occur over the poles? The air masses above the poles become isolated from the rest of the atmosphere during their winter and early spring seasons due to a phenomenon known as the "polar vortex". In simplest terms, this vortex is a spinning, funnel shaped region of the atmosphere that forms in late fall and early winter over a pole, allowing chemical reactions in the enclosed air mass to be enhanced due to the lack of mixing with other, lower latitude, air masses. The effect of the pollutants we have added to the atmosphere are thus enhanced in these isolated regions of the atmosphere. The Antarctic vortex over the South Pole is more effective at isolating this region of the atmosphere during the austral winter than is the corresponding arctic vortex. A second feature of the polar stratosphere that is unique and probably aids the polar ozone depletion is polar stratospheric clouds. These very high altitude clouds are composed of ice crystals, sometimes greatly enriched in nitrogen oxide specis ("NO_x") that can enhance the ozone degredation reactions discussed above. These ice particles can react with various forms of Chlorine in the atmosphere and accumulate the molecule ClONO₂, which is a source of ozone depleting Cl radicals. Once spring time comes, this ClONO₂ decomposes and allows ozone degredation reactions can occur.
There is still much to be learned about the details of ozone distribution in the stratosphere. Undoubtedly, much of what we still don't know will be discovered as additional scientific studies are conducted and the data is collected and interpreted.

Dr. Ken Rubin, Assistant Professor
Department of Geology and Geophysics
University of Hawaii, Honolulu HI 96822