Editing Ozone–oxygen cycle (section)

===[[Stratosphere]]===
Absorption by oxygen in the mesosphere and thermosphere (in the oxygen photodissociation reaction) reduces photon flux at wavelengths below 200 nanometer, where oxygen photodissociation is dominated by [[Schumann–Runge bands|Schumann–Runge bands and continuum]], with cross-section of up to 10<sup>−17</sup> cm<sup>2</sup>.
Due to this absorption, photon flux in these wavelengths is so low in the stratosphere, that oxygen photodissociation becomes dominated by the Hertzberg band of the 200-240&nbsp;nm photon wavelength, even though the cross-section of this process is as low as 10<sup>−24</sup> - 10<sup>−23</sup> cm<sup>2</sup>. The ozone photodissociation rate per ozone molecule has a cross-section 6 orders of magnitude higher in the 220-300&nbsp;nm wavelength range. With ozone concentrations in the order of 10<sup>−6</sup>-10<sup>−5</sup> relative to molecular oxygen, ozone photodissociation becomes the dominant photodissociation reaction, and most of the stratosphere heat is generated through this procsees, with highest heat generation rate per molecule at the upper limit of the stratosphere ([[stratopause]]), where ozone concentration is already relatively high while UV flux is still high as well in those wavelengths, before being depleted by this same photodissociation process.

In addition to ozone photodissociation becoming a more dominant removal reaction, catalytic ozone destruction due to free radicals (mainly atomic [[hydrogen]], [[hydroxyl radical|hydroxyl]], [[nitric oxide]], [[chlorine]] and [[bromide]]) increases the effective ozone conversion reaction rate. Both processes act to increase ozone removal, leading to a more moderate increase of ozone relative concentration as altitude decreases, even though air density continues to increase.<ref name="Photochemistry of Ozone" />

Due to both ozone and oxygen growing density as we go to lower altitudes, UV photon flux at wavelengths below 300&nbsp;nm decreases substantially, and oxygen photodissociation rates fall below 10<sup>−9</sup> per second per molecule at 30&nbsp;km.<ref name="Photochemistry of Ozone" /> With decreasing oxygen photodissociation rates, odd-oxygen species (atomic oxygen and ozone molecules) are hardly formed de novo (rather than being transmuted to each other by the other reactions), and most atomic oxygen needed for ozone creation is derived almost exclusively from ozone removal by ozone photodissociation. Thus, ozone becomes depleted as we go below 30&nbsp;km altitude and reaches very low concentrations at the [[tropopause]].<ref name="NASA" />