The ozone–

oxygen cycle Oxygen cycle refers to the movement of oxygen through the atmosphere (air), biosphere (plants and animals) and the lithosphere (the Earth’s crust). The oxygen cycle demonstrates how free oxygen is made available in each of these regions, as wel ...

is the process by which

ozone Ozone (), or trioxygen, is an inorganic molecule with the chemical formula . It is a pale blue gas with a distinctively pungent smell. It is an allotrope of oxygen that is much less stable than the diatomic allotrope , breaking down in the lo ...

is continually regenerated in

Earth Earth is the third planet from the Sun and the only astronomical object known to harbor life. While large volumes of water can be found throughout the Solar System, only Earth sustains liquid surface water. About 71% of Earth's surfa ...

's stratosphere, converting

ultraviolet radiation Ultraviolet (UV) is a form of electromagnetic radiation with wavelength from 10 nm (with a corresponding frequency around 30 PHz) to 400 nm (750 THz), shorter than that of visible light, but longer than X-rays. UV radiation i ...

(UV) into

heat In thermodynamics, heat is defined as the form of energy crossing the boundary of a thermodynamic system by virtue of a temperature difference across the boundary. A thermodynamic system does not ''contain'' heat. Nevertheless, the term is ...

. In 1930 Sydney Chapman resolved the chemistry involved. The process is commonly called the Chapman cycle by atmospheric scientists. Most of the ozone production occurs in the tropical upper stratosphere and mesosphere. The total mass of ozone produced per day over the globe is about 400 million metric tons. The global mass of ozone is relatively constant at about 3 billion metric tons, meaning the Sun produces about 12% of the ozone layer each day.

Chemistry

#Creation: an oxygen molecule is split ( photolyzed) by higher frequency UV light (top end of

UV-B Ultraviolet (UV) is a form of electromagnetic radiation with wavelength from 10 nm (with a corresponding frequency around 30 PHz) to 400 nm (750 THz), shorter than that of visible light, but longer than X-rays. UV radiation i ...

UV-C Ultraviolet (UV) is a form of electromagnetic radiation with wavelength from 10 nm (with a corresponding frequency around 30 PHz) to 400 nm (750 THz), shorter than that of visible light, but longer than X-rays. UV radiation i ...

and above) into two oxygen atoms (see figure): #::O₂ + ℎν_(<242 nm) → 2 O #: Each oxygen atom then quickly combines with an oxygen molecule to form an ozone molecule: #::O + O₂ → O₃ #The ozone–oxygen cycle: the ozone molecules formed by the reaction above absorb radiation having an appropriate wavelength between

and

. The triatomic ozone molecule becomes diatomic molecular oxygen plus a free oxygen atom (see figure): #::O₃ + ℎν_{(240–310 nm)} → O₂+ O #:The atomic oxygen produced quickly reacts with another oxygen molecule to reform ozone: #::O + O₂ + A → O₃ + A #:where A denotes another molecule or atom, like N₂ or O₂ which is needed in the reaction as otherwise energy and momentum wouldn't be conserved: There is an excess energy of the reaction which is manifested as extra

kinetic energy In physics, the kinetic energy of an object is the energy that it possesses due to its motion. It is defined as the work needed to accelerate a body of a given mass from rest to its stated velocity. Having gained this energy during its acc ...

. These two reactions form the ozone–oxygen cycle, in which the chemical energy released when O and O₂ combine is converted into kinetic energy of molecular motion. The overall effect is to convert penetrating UV-B light into heat, without any net loss of ozone. This cycle keeps the ozone layer in a stable balance while protecting the lower atmosphere from UV radiation, which is harmful to most living beings. It is also one of two major sources of heat in the stratosphere (the other being the kinetic energy released when O₂ is photolyzed into O atoms). #Removal: if an oxygen atom and an ozone molecule meet, they recombine to form two oxygen molecules: #::O₃ + O → 2 O₂ #:And if two oxygen atoms meet, they react to form one oxygen molecule: #::2 O → O₂ The overall amount of ozone in the stratosphere is determined by a balance between production by solar radiation and removal. The removal rate is slow, since the concentration of free O atoms is very low. In addition to the reactions just mentioned, certain

free radical A daughter category of ''Ageing'', this category deals only with the biological aspects of ageing. Ageing Ailments of unknown cause Biogerontology Biological processes Causes of death Cellular processes Gerontology Life extension Metabo ...

s, the most important being

hydroxyl In chemistry, a hydroxy or hydroxyl group is a functional group with the chemical formula and composed of one oxygen atom covalently bonded to one hydrogen atom. In organic chemistry, alcohols and carboxylic acids contain one or more hydro ...

(OH), nitric oxide (NO) and atoms of

chlorine Chlorine is a chemical element with the symbol Cl and atomic number 17. The second-lightest of the halogens, it appears between fluorine and bromine in the periodic table and its properties are mostly intermediate between them. Chlorine i ...

(Cl) and

bromine Bromine is a chemical element with the symbol Br and atomic number 35. It is the third-lightest element in group 17 of the periodic table ( halogens) and is a volatile red-brown liquid at room temperature that evaporates readily to form a simi ...

(Br),

catalyze Catalysis () is the process of increasing the rate of a chemical reaction by adding a substance known as a catalyst (). Catalysts are not consumed in the reaction and remain unchanged after it. If the reaction is rapid and the catalyst recyc ...

the recombination

reaction Reaction may refer to a process or to a response to an action, event, or exposure: Physics and chemistry *Chemical reaction *Nuclear reaction * Reaction (physics), as defined by Newton's third law *Chain reaction (disambiguation). Biology and m ...

, leading to an ozone layer that is thinner than it would be if the catalysts were not present. Most of the OH and NO are naturally present in the stratosphere, but human activity, especially emissions of chlorofluorocarbons ( CFCs) and halons, has greatly increased the Cl and Br concentrations, leading to ozone depletion. Each Cl or Br atom can catalyze tens of thousands of decomposition reactions before it is removed from the stratosphere.

External links

Stratospheric Ozone: An Electronic TextbookThe Sun and the Earth's Climate

References

{{DEFAULTSORT:Ozone-oxygen cycle Cycle Oxygen Biogeochemical cycle Atmospheric chemistry