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| Layers Of Atmosphere |
Ozone depletion describes two distinct but related phenomena
observed since the late 1970s: a steady decline of about 4% per decade
in the total volume of ozone in Earth's stratosphere (the ozone layer),
and a much larger springtime decrease in stratospheric ozone over
Earth's polar regions. The latter phenomenon is referred to as the ozone hole. In addition to these well-known stratospheric phenomena, there are also springtime polar tropospheric ozone depletion events.
The details of polar ozone hole formation differ from that of mid-latitude thinning, but the most important process in both is catalytic destruction of ozone by atomic halogens.[1] The main source of these halogen atoms in the stratosphere is photodissociation of man-made halocarbon refrigerants (CFCs, freons, halons). These compounds are transported into the stratosphere after being emitted at the surface. [2] Both types of ozone depletion were observed to increase as emissions of halo-carbons increased.
CFCs and other contributory substances are referred to as ozone-depleting substances (ODS). Since the ozone layer prevents most harmful UVB wavelengths (280–315 nm) of ultraviolet light (UV 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 that bans the production of CFCs, halons, and other ozone-depleting chemicals such as carbon tetrachloride and trichloroethane. It is suspected that a variety of biological consequences such as increases in skin cancer, cataracts,[3] 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
Three forms (or allotropes) of oxygen are involved in the ozone-oxygen cycle: oxygen atoms (O or atomic oxygen), oxygen gas (O2 or diatomic oxygen), and ozone gas (O3 or triatomic oxygen). Ozone is formed in the stratosphere when oxygen molecules photodissociate after absorbing an ultraviolet photon whose wavelength is shorter than 240 nm. This converts a single O2 into two atomic oxygen radicals. The atomic oxygen radicals then combine with separate O2 molecules to create two O3 molecules. These 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 two O2 molecules.
O + O3 → 2 O2 chemical equation
The overall amount of ozone in the stratosphere is determined by a balance between photochemical production and recombination.
Ozone can be destroyed by a number of free radical catalysts, the most important of which are the hydroxyl radical (OH·), the nitric oxide radical (NO·), the atomic chlorine ion (Cl·) and the atomic bromine
ion (Br·). All of these have both natural and man-made sources; at the
present time, most of the OH· and NO· in the stratosphere is of natural
origin, but human activity has dramatically increased the levels of
chlorine and bromine. These elements are found in certain stable organic
compounds, especially chlorofluorocarbons (CFCs), which may find their way to the stratosphere
without being destroyed in the troposphere due to their low reactivity.
Once in the stratosphere, the Cl and Br atoms are liberated from the
parent compounds by the action of ultraviolet light, e.g.
The Cl and Br atoms can then destroy ozone molecules through a variety of catalytic cycles. In the simplest example of such a cycle,[4]
a chlorine atom reacts with an ozone molecule, taking an oxygen atom
with it (forming ClO) and leaving a normal oxygen molecule. The chlorine
monoxide (i.e., the ClO) can react with a second molecule of ozone
(i.e., O3) to yield another chlorine atom and two molecules of oxygen. The chemical shorthand for these gas-phase reactions is:
