Editing Ozone (section)

====Location and production====
{{See also|Ozone–oxygen cycle|Ozone depletion}}

The highest levels of ozone in the atmosphere are in the [[stratosphere]], in a region also known as the [[ozone layer]] between about 10 and 50&nbsp;km above the surface (or between about 6 and 31 miles). However, even in this "layer", the ozone concentrations are only two to eight parts per million, so most of the oxygen there is dioxygen, O<sub>2</sub>, at about 210,000 parts per million by volume.<ref name="Hultman">{{cite book |last=Hultman |first=G. Eric |title=The Ozone Survival Manual |date=1980-01-01 |publisher=McGraw-Hill |isbn=978-0-915498-73-4}}</ref>

Ozone in the stratosphere is mostly produced from short-wave ultraviolet rays between 240 and 160&nbsp;nm. Oxygen starts to absorb weakly at 240&nbsp;nm in the Herzberg bands, but most of the oxygen is dissociated by absorption in the strong [[Schumann–Runge bands]] between 200 and 160&nbsp;nm where ozone does not absorb. While shorter wavelength light, extending to even the X-Ray limit, is energetic enough to dissociate molecular oxygen, there is relatively little of it, and, the strong solar emission at Lyman-alpha, 121&nbsp;nm, falls at a point where molecular oxygen absorption is a minimum.<ref>{{cite web |title=The MPI-Mainz UV/VIS Spectral Atlas of Gaseous Molecules of Atmospheric Interest: O2, Lyman-alpha |last=Keller-Rudek |first=Hannelore |url=http://joseba.mpch-mainz.mpg.de/spectral_atlas_data/cross_sections_plots/Oxygen/O2_Lyman%20alpha%20line_lin.jpg |archive-url=https://web.archive.org/web/20151117015120/http://joseba.mpch-mainz.mpg.de/spectral_atlas_data/cross_sections_plots/Oxygen/O2_Lyman%20alpha%20line_lin.jpg |archive-date=2015-11-17}}</ref>

The process of ozone creation and destruction is called the [[Chapman cycle]] and starts with the photolysis of molecular oxygen
: <chem>O2 -> [\ce{photon}] [(\ce{radiation}\ \lambda\ <\ 240\ \ce{nm})] 2O</chem>

followed by reaction of the oxygen atom with another molecule of oxygen to form ozone.
:<chem>O + O2 + M -> O3 + M</chem>

where "M" denotes the third body that carries off the excess energy of the reaction. The ozone molecule can then absorb a UV-C photon and dissociate
:<math chem>\ce{O3 -> O + O2} + \text{kinetic energy}</math>

The excess kinetic energy heats the stratosphere when the O atoms and the molecular oxygen fly apart and collide with other molecules. This conversion of UV light into kinetic energy warms the stratosphere. The oxygen atoms produced in the photolysis of ozone then react back with other oxygen molecule as in the previous step to form more ozone. In the clear atmosphere, with only nitrogen and oxygen, ozone can react with the atomic oxygen to form two molecules of {{chem2|O2}}:
:<chem>O3 + O -> 2 O2</chem>

An estimate of the rate of this termination step to the cycling of atomic oxygen back to ozone can be found simply by taking the ratios of the concentration of O<sub>2</sub> to O<sub>3</sub>. The termination reaction is [[catalysis|catalysed]] by the presence of certain free radicals, of which the most important are hydroxyl (OH), nitric oxide (NO) and atomic chlorine (Cl) and bromine (Br). In the second half of the 20th century, the amount of ozone in the stratosphere was [[ozone depletion|discovered to be declining]], mostly because of increasing concentrations of [[chlorofluorocarbon]]s (CFC) and similar [[haloalkane|chlorinated and brominated organic molecules]]. The concern over the health effects of the decline led to the 1987 [[Montreal Protocol]], the ban on the production of many [[ozone depletion|ozone-depleting]] chemicals and in the first and second decade of the 21st century the beginning of the recovery of stratospheric ozone concentrations.