Editing Ozone–oxygen cycle (section)

==Photochemistry==
{{biogeochemical cycle sidebar|other}}
The Chapman cycle describes the main reactions that naturally determine, to first approximation, the concentration of ozone in the stratosphere. It includes four processes - and a fifth, less important one - all involving oxygen atoms and molecules, and UV radiation:<ref>Dütschکیر, H. U. (1968). The photochemistry of stratospheric ozone. Quarterly Journal of the Royal Meteorological Society, 94(402), 483-497.</ref>

===Creation===
An oxygen molecule is split ([[Photolysis|photolyzed]]) by higher frequency UV light (top end of [[UV-B]], [[UV-C]] and above) into two oxygen atoms (see figure):
:1. '''oxygen photodissociation''': O<sub>2</sub> + ℎν<sub>(<242&nbsp;nm)</sub> → 2 O
Each oxygen atom may then combine with an oxygen molecule to form an ozone molecule:
:2. '''ozone creation''': O + O<sub>2</sub> + A → O<sub>3</sub> + A
:where A denotes an additional molecule or atom, such as N<sub>2</sub> or O<sub>2</sub>, required to maintain the conservation of energy and momentum in the reaction. Any excess energy is produced as [[kinetic energy]].

===The ozone–oxygen cycle===
The ozone molecules formed by the reaction (above) absorb radiation with an appropriate wavelength between [[UV-C]] and [[UV-B]]. The triatomic ozone molecule becomes diatomic molecular oxygen, plus a free oxygen atom (see figure):
:3. '''ozone photodissociation''': O<sub>3</sub> + ℎν<sub>(240–310&nbsp;nm)</sub> → O<sub>2 </sub>+ O
The atomic oxygen produced may react with another oxygen molecule to reform ozone via the ozone creation reaction (reaction 2 above).

These two reactions thus form the ozone–oxygen cycle, wherein the chemical energy released by ozone creation becomes molecular kinetic energy. The net result of the cycle is the conversion of penetrating UV-B light into heat, without any net loss of ozone. While keeping the ozone layer in stable balance, and protecting the lower atmosphere from harmful UV radiation, the cycle also provides one of two major heat sources in the stratosphere (the other being kinetic energy, released when  O<sub>2</sub> is photolyzed into individual O atoms).

===Removal===
If an oxygen atom and an ozone molecule meet, they recombine to form two oxygen molecules:
:4. '''ozone conversion''': O<sub>3</sub> + O → 2 O<sub>2</sub>
Two oxygen atoms may react to form one oxygen molecule:
:5. '''oxygen recombination''': 2O + A → O<sub>2</sub> + A
:as in reaction 2 (above), A denotes another molecule or atom, like N<sub>2</sub> or O<sub>2</sub> required for the conservation of energy and momentum.
Note that reaction 5 is of the least importance in the stratosphere, since, under normal conditions, the concentration of oxygen atoms is much lower than that of diatomic oxygen molecules. This reaction is therefore less common than ozone creation (reaction 2).

The overall amount of ozone in the stratosphere is determined by the balance between production from solar radiation and its removal. The removal rate is slow, since the concentration of free O atoms is very low.

===Additional reactions===
In addition to these five reactions, certain [[free radical]]s - the most important being [[hydroxyl]] (OH), [[nitric oxide]] (NO), and atomic [[chlorine]] (Cl) and [[bromine]] (Br) -  [[Catalysis|catalyze]] the recombination [[Chemical reaction|reaction]], leading to an ozone layer that is thinner than it would be if the catalysts were not present.

Most OH and NO are naturally present in the stratosphere, but human activity - especially emissions of chlorofluorocarbons ([[Chlorofluorocarbon|CFC]]s) and [[Haloalkane|halons]] - has greatly increased the concentration of Cl and Br, leading to [[ozone depletion]]. Each Cl or Br atom can catalyze tens of thousands of decomposition reactions before it is removed from the stratosphere.