OZONE LAYER
The Earth is unique amongst the planets of the solar system due to its oxygen-rich atmosphere.
The atmosphere contains two forms of oxygen, di-oxygen (O2) and tri-oxygen, or ozone (O3). Both of these allotropes of oxygen help protect life on the Earth’s surface and in the lower atmosphere by absorbing harmful ultraviolet (UV) radiation from the sun.
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The stratosphere is the layer of atmosphere between 12 and 50 kilometres above the surface of the Earth. The lower regions of the stratosphere contain around 90% of the ozone in the atmosphere, at a concentration of less than 10 ppm (known as the ozone layer).
This stratospheric ozone is in a dynamic equilibrium with oxygen and is continually being formed and destroyed. Ozone levels are maintained naturally by a continuous cycle of synthesis and breakdown reactions by the action of energy from UV-C and UV-B radiation.
A free radical, or radical, is a highly reactive species due to the presence of an unpaired electron. The unpaired electron is represented by a dot (•).
This reaction also removes high-energy UV radiation and prevents it from reaching the Earth's surface. It also results in this level of the stratosphere having a higher temperature than the lower region – the normal trend is for the temperature of the atmosphere to decrease with height.
Note that the breakdown of oxygen requires UV radiation of shorter wavelength than the breakdown of ozone. The strong double bond in oxygen is disrupted by the Sun's high energy UV-C radiation to form atoms, (O•), which are free radicals since they have an unpaired electron. Such oxygen radicals can then react with an oxygen molecule to form ozone. The bonds in ozone, being weaker, can then be broken by the less energetic UV-B radiation (of longer wavelength) to reform oxygen and an oxygen free radical.
The key feature here is that the surface of the Earth is protected by these reactions from the very damaging effects of UV-B and UV-C radiation. This means that the majority of the ultraviolet radiation reaching the Earth’s surface is the least harmful UV-A form. Consequently, the ozone layer protects life on Earth from that radiation which would be most harmful to living tissues.
OZONE DEPLETION
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Chlorofluorocarbons (CFCs) once released into the atmosphere remain chemically unchanged for years. They diffuse upwards and once in the stratosphere are exposed to the sun's higher-energy ultraviolet rays where the carbon-chlorine bonds are broken.
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This reaction forms chlorine free radicals, which break down ozone and disrupt the equilibrium between ozone formation and ozone destruction.
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***Information taken from Kognity.