Editing Ozone layer (section)

==Sources==
{{Main|Ozone–oxygen cycle}}
[[File:Ozone cycle.svg|thumb|upright=1.5|[[Ozone-oxygen cycle]] in the ozone layer]]
The Earth's ozone layer formed about 500 million years ago, when the [[neoproterozoic oxygenation event]] brought the fraction of oxygen in the atmosphere to about 20%.<ref>{{cite web|url=https://www.discovermagazine.com/environment/how-the-ozone-layer-evolved-and-why-its-important|title=How the Ozone Layer Evolved and Why It's Important |author=Paul M. Sutter |date=July 20, 2023 |publisher=Discover Magazine}}</ref>

The [[photochemical]] mechanisms that give rise to the ozone layer were discovered by the British physicist [[Sydney Chapman (mathematician)|Sydney Chapman]] in 1930. Ozone in the Earth's stratosphere is created by ultraviolet light striking ordinary [[oxygen]] [[molecule]]s containing two oxygen [[atom]]s (O<sub>2</sub>), splitting them into individual oxygen atoms ([[atomic oxygen]]); the atomic oxygen then combines with unbroken O<sub>2</sub> to create ozone, O<sub>3</sub>. The ozone molecule is unstable (although, in the stratosphere, long-lived) and when ultraviolet light hits ozone it splits into a molecule of O<sub>2</sub> and an individual atom of oxygen, a continuing process called the [[ozone–oxygen cycle]]. Chemically, this can be described as:
: <chem>O2{} + \mathit{h}\nu_{uv} -> 2O </chem>
: <chem>O + O2 <-> O3</chem>
About 90% of the ozone in the atmosphere is contained in the stratosphere. Ozone concentrations are greatest between about {{convert|20|and|40|km|ft}}, where they range from about 2 to 8 parts per million. If all of the ozone were compressed to the pressure of the air at sea level, it would be only {{convert|3|mm|in|abbr=off|frac=8}} thick.<ref>{{cite web |title=NASA Facts Archive |url=http://www.nasa.gov/facts/Earth/earth_facts_archives.html |access-date=June 9, 2011 |url-status=dead |archive-url=https://web.archive.org/web/20130406065500/http://www.nasa.gov/facts/Earth/earth_facts_archives.html |archive-date=April 6, 2013}}</ref>