The term ozone depletion is used to describe two distinct, but related, observations: a slow, steady decline, of about 3% per decade, in the total amount of ozone in the earth's stratosphere during the past twenty years, and a much larger, but seasonal, decrease in stratospheric ozone over the earth's polar regions during the same period. (The latter phenomenon is commonly referred to as the "ozone hole".) The detailed mechanism by which the polar ozone holes form is different from that for the mid-latitude thinning, but the proximate cause of both trends is believed to be catalytic destruction of ozone by atomic chlorine and bromine. The primary source of these halogen atoms in the stratosphere is photodissociation of chlorofluorocarbon (CFC) compounds, commonly called freons, and bromofluorocarbon compounds known as Halons, which are transported into the stratosphere after being emitted at the surface. Both ozone depletion mechanisms strengthened as emissions of CFCs and Halons increased.

Since the ozone layer prevents most harmful UVB wavelengths (270- 315 nm) of ultraviolet light from passing through the Earth's atmosphere, observed and projected decreases in ozone have generated worldwide concern, leading to adoption of the Montreal Protocol banning the production of CFCs and halons as well as related ozone depleting chemicals such as carbon tetrachloride and trichloroethane (also known as methyl chloroform). It is suspected that a variety of biological consequences, including, for example, increases in skin cancer, damage to plants, and reduction of plankton populations in the ocean's photic zone, may result from the increased UV exposure due to ozone depletion.

Ozone cycle overview

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Ozone creation

Three forms (or allotropes) of oxygen are involved in the ozone-oxygen cycle: Oxygen atoms, O (atomic oxygen); oxygen molecules, O₂; and ozone, O₃. Ozone is formed in the stratosphere when oxygen molecules photodissociate after absorbing an ultraviolet photon whose wavelength is shorter than 240 nm. This produces two oxygen atoms. The atomic oxygen then combines with O₂ to create O₃. Ozone molecules absorb UV light between 310 and 200 nm, following which ozone splits into a molecule of O₂ and an oxygen atom. The oxygen atom then joins up with an oxygen molecule to regenerate ozone. This is a continuing process which terminates when an oxygen atom "recombines" with an ozone molecule to make 2 O₂ molecules. Prior to the beginning of the depletion trend, the amount of ozone in the stratosphere was kept roughly constant by a balance between the rates of creation and destruction of ozone molecules by UV light.

More on [ Ozone depletion ]

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