Editing Ozone (section)

===Ozone layer===
{{Main|Ozone layer}}

====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.

====Importance to surface-dwelling life on Earth====
[[File:Ozone altitude UV graph.svg|thumb|upright=1.25|Levels of ozone at various altitudes and blocking of different bands of ultraviolet radiation. Essentially all UVC (100–280&nbsp;nm) is blocked by dioxygen (at 100–200&nbsp;nm) or by ozone (at 200–280&nbsp;nm) in the atmosphere. The shorter portion of this band and even more energetic UV causes the formation of the ozone layer, when single oxygen atoms produced by UV [[photolysis]] of dioxygen (below 240&nbsp;nm) react with more dioxygen. The ozone layer itself then blocks most, but not quite all, sunburn-producing UVB (280–315&nbsp;nm). The band of UV closest to visible light, UVA (315–400&nbsp;nm), is hardly affected by ozone, and most of it reaches the ground.]]

Ozone in the ozone layer filters out sunlight wavelengths from about 200&nbsp;nm UV rays to 315&nbsp;nm, with ozone peak absorption at about 250&nbsp;nm.<ref>{{cite journal |title=Photolysis of Atmospheric Ozone in the Ultraviolet Region |year=2003 |last1=Matsumi |first1=Yutaka |last2=Kawasaki |first2=Masahiro |journal=Chemical Reviews |volume=103 |issue=12 |pages=4767–82 |pmid=14664632 |doi=10.1021/cr0205255}} See the graphical absorption of ozone in two of its absorption bands, as a function of wavelength.</ref> This ozone UV absorption is important to life, since it extends the absorption of UV by ordinary oxygen and nitrogen in air (which absorb all wavelengths < 200&nbsp;nm) through the lower UV-C (200–280&nbsp;nm) and the entire UV-B band (280–315&nbsp;nm). The small unabsorbed part that remains of UV-B after passage through ozone causes sunburn in humans, and direct DNA damage in living tissues in both plants and animals. Ozone's effect on mid-range UV-B rays is illustrated by its effect on UV-B at 290&nbsp;nm, which has a radiation intensity 350 million times as powerful at the top of the atmosphere as at the surface. Nevertheless, enough of UV-B radiation at similar frequency reaches the ground to cause some sunburn, and these same wavelengths are also among those responsible for the production of [[vitamin D]] in humans.

The ozone layer has little effect on the longer UV wavelengths called UV-A (315–400&nbsp;nm), but this radiation does not cause sunburn or direct DNA damage. While UV-A probably does cause long-term skin damage in certain humans, it is not as dangerous to plants and to the health of surface-dwelling organisms on Earth in general (see [[ultraviolet]] for more information on near ultraviolet).