Map of Antarctica.

Penguins have never used hairspray, nor is air conditioning terribly popular among their kind. So why is there an ozone hole forming over Antarctica? Why isn't the hole opening up over some place where lots of air conditioners are being used, like Florida for example?

The earth's atmosphere is a giant gas-mixing machine. Anything that gets into the air will eventually spread over the entire world. That explains how CFCs get to Antarctica in the first place. But why are they causing so much more of a mess down there than anywhere else? It is this sort of question that sent Susan Solomon on her polar odysseys in search of answers. With the evidence she gathered there she helped prove and refine a theory first put forth by Mario Molina.

The theory is simple: The hole forms over Antarctica because it's really cold there. Ok, it's a little more complicated than that. But the cold is the real culprit. Because of the extreme cold of the South Polar region, which is much colder than even the North Pole, an unusual kind of cloud forms in the stratosphere over Antarctica. These clouds are called polar stratospheric clouds (PSCs), and they are made of a mixture of water ice crystals, crystals of water ice mixed with nitric acid, as well as droplets of liquid water mixed with nitric acid and sulfuric acid.

Remember the chemistry that Mario Molina discovered, that we read about in Making and Destroying Ozone. CFCs break down ozone in a cycle of reactions, in which the radical chlorine oxide (ClO) is a key player.

In most parts of the stratosphere, ClO reacts with nitrogen dioxide (NO₂), a gas that is found in the upper atmosphere. NO₂ comes from natural sources but is also made by human activity. It is a radical, and often an environmental pollutant. The product of the reaction between ClO and NO₂ is ClONO₂.

This prevents ClO from reacting with more ozone, slowing down ozone depletion. But it turns out that the PSCs high above Antarctica are a catalyst for a chemical reaction between ClONO₂ and HCl. (HCl also exists as a gas in the high atmosphere. Some of it comes from natural sources and some comes from the breakdown of CFCs.)

This reaction produces nitric acid and Cl₂. Cl₂ will then break down into chlorine radicals when it gets hit by the sun's intense UV radiation. The chlorine radicals can then once again wreak havoc on ozone.

To summarize, over most of the globe, ClO is prevented from destroying ozone by reacting with NO₂ to form ClNO₂. But over Antarctica, PSCs catalyze the breakdown of ClNO₂ which in the end restarts the ozone destroying reaction cycle.

But don't think that because the worst ozone depletion happens at the South Pole that the rest of the world is safe. Ozone depletion still happens all over the world. Though much lesser in extent than the Antarctic ozone loss, global ozone loss can have harmful effects on all forms of life, including us.

After Dr. Solomon's Antarctic expeditions showed how CFCs really were to blame for dangerous ozone loss, the world responded prudently. The Montreal Protocol was drafted by an international conference in 1987, which restricted the use of CFCs worldwide. Over the years the treaty has been amended as new scientific evidence comes to light. Originally calling for reductions in CFC use, the protocol now calls for the eventual elimination of CFCs.

That's all well and good, but what will we use for refrigerants? That's what we'll explore in the next few readings.

Next: Swapping Atoms

CFC Sumggling (pdf format) — read about smuggling of CFCs made illegal by the Montreal Protocol in this news article, originally published in Environ Health Perspectives, 1996 December; 104(12): 1272.

The Montreal Protocol on Substances that Deplete the Ozone Layer (pdf format) — from the United Nations Environment Programme Ozone Secretariat.

Nobel Prize in Chemistry 1995 — featuring the science and autobiographies of Paul Crutzen, Mario Molina and F. Sherwood Rowland, from the Nobel Foundation.

Reference

Solomon, Susan. "Stratospheric Ozone Depletion: a Review of Concepts and History," Reviews of Geophysics, 1999, 37, 275-316.