h enough in the sky by that time for ozone-producing ultraviolet rays to penetrate the air.
The ozone hole over Antarctica exists because of the pole's unique atmosphere. Unlike the Northern Hemisphere, the South has no continents and mountain ranges extending toward the pole, an arrangement that would bring air currents to mix with the atmosphere at the pole. As a result, there are no ozone-laden currents to replenish the supply over the South Pole as it is depleted. Also, Antarctica is colder than the Arctic because of the absence of air currents bringing in warm air and because the ice-covered continent does not absorb the sun's rays very well. These conditions also favor the formation of nocturnal stratospheric clouds.
These special conditions do not explain how the hole forms or why the concentration of ozone during October has been rapidly decreasing from year to year. A number of researchers have suspected chlorine, which catalytically destroys ozone, because it has been on the rise since the use of CFCs began. Scientists have proposed that, during the polar night, hydrochloric acid and chlorine nitrate react to form molecules of chlorine gas. When the sun comes out, its visible light breaks down the chlorine gas into individual chlorine atoms, which destroy the ozone, even as the ultraviolet light is producing ozone. The reaction of HCL and CLONO3 has been observed on the surfaces of laboratory equipment; perhaps the nocturnal clouds produced over Antarctica also provide the surfaces necessary for such a reaction.
Still, chlorine may not be the sole villain. Other possibilities include dynamics--how the upwelling movement of air might create ozone lows--and the volcanic eruptions of aerosols. The chemistry of the ozone layer worldwide is also plagued by uncertainties. Researchers now have compelling observational evidence that trace gases other than CFCs, such as methane, nitrous oxide and carbon monoxi...