Polar stratospheric clouds (referred to as "PSCs" by scientists and called nacreous clouds by the general public) form in the lower stratosphere between heights of 15 kilometers and 25 kilometers in the lee of high-latitude mountain ranges in winter. Ice crystals and particles composing the visible cloud are of uniform size, about 10 micrometers in diameter.

After sunset, light from the Sun below the horizon continues to illuminate particles within PSCs, producing iridescent bright red and blue coloration zones in the clouds. Colorful PSC displays are most frequently photographed before dawn and after sunset when they appear in Oslo, Norway, but photographs and videotapes have also been taken in Scotland, England, Scandinavia, Iceland, Alaska, and Antarctica.

Atmospheric scientists have divided PSCs into two "Types" as defined by chemical and thermal differences. Type I PSCs have temperatures of about -78œ Celsius. Type I(a) PSCs contain crystalline compounds of water and nitric acid; Type I(b) PSCs contain droplets of nitric (HNO3) and sulfuric (H2SO4) acids; Type I(c) PSCs contain nonspherical particles of metastable nitric acid in water; and Type II PSCs are composed of water ice crystals at temperatures of 85œ Celsius.

Discovery of the formation of the ozone hole in the stratosphere above the Antarctic continent each spring focused scientific attention on chemical reactions depleting ozone. Scientists deduced that ultraviolet light from the rising Sun promoted chemical reactions leading to ozone destruction, and investigations linked chemical reactions going on within the PSCs to regional ozone depletion above the Antarctic (Axelrod, 2004).

Particles within Antarctic PSCs act as catalytic surfaces for reactions involving the destruction of relatively stable atmospheric chlorine compounds (including artificial chlorofluorocarbons sometimes called Freons). Through a series of reactions, these chlorine compounds are transformed into highly reactive molecular chlorine gas (C12) and hypochlorous acid (HCl) molecules, which rapidly form chlorine radicals, which in turn destroy ozone molecules. During winter and early spring, there is a strong easterly flow of air in the Antarctic stratosphere, the Antarctic Polar Vortex. Little air from lower latitudes mixes with the ozone-depleted air within the vortex, allowing greater depletion of ozone to occur, creating the ozone hole (Botterill, 2003).

PSCs also catalyze other important chemical reactions, including denoxification, the conversion of oxides of nitrogen to nitric acid, which is found in all three Type I PSCs. Because decreasing the level of nitric oxide (NO2) allows high levels of ozone-removing hypochlorite (ClO) to remain in the stratosphere, this denoxification promotes development of the ozone hole. Denoxification leaves the waste product nitric acid behind in the cloud particles, which eventually exits the stratosphere, a process called denitrification.

References

1. Axelrod, Regina S., David Leonard Downie, and Norman J. Vig, eds. The Global Environment: Institutions, Law, and Policy. 2d ed. Washington, D.C.: CQ Press, 2004.

2. Botterill, Linda C., and Melanie Fisher, eds. Beyond Drought: People, Policy, and Perspectives. Collingwood, Vic.: CSIRO, 2003.

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