Editing Ozone–oxygen cycle (section)

===[[Thermosphere]]===
For given relative reactants concentrations, The rates of ozone creation and oxygen recombination (reactions 2 and 5) are proportional to the air density cubed, while the rate of ozone conversion (reaction 4) is proportional to the air density squared, and the photodissociation reactions (reactions 1 and 3) have a linear dependence on air density. Thus, at the upper thermosphere, where air density is very low and photon flux is high, oxygen photodissociation is fast while ozone creation is low, thus its concentration is low. Thus the most important reactions are oxygen photodissociation and oxygen recombination, with most of the oxygen molecules dissociated to oxygen atoms.<ref>[https://scied.ucar.edu/learning-zone/atmosphere/thermosphere UCAR Center for Science Education: The Thermosphere]</ref>

As we go to the lower thermosphere (e.g. 100&nbsp;km height and below), the photon flux in the <170&nbsp;nm wavelengths drops sharply due to absorption by oxygen in the oxygen photodissociation reaction (reaction 1). This wavelength regime has the highest cross section for this reaction (10<sup>−17</sup> cm<sup>2</sup> per oxygen molecule), and thus the rate of oxygen photodissociation per oxygen molecule decreases significantly at these altitudes, from more than 10<sup>−7</sup> per second (about once a month) at 100&nbsp;km to 10<sup>−8</sup> per second (about once every few years) at 80&nbsp;km .<ref name="Photochemistry of Ozone">[https://slideplayer.com/slide/10904550/ Photochemistry of Ozone]</ref> As a result, the atomic oxygen concentration (both relative and absolute) decreases sharply, and ozone creation (reaction 2) is ongoing, leading to a small but non-negligible ozone presence.<ref>Mlynczak, M. G., Hunt, L. A., Mast, J. C., Thomas Marshall, B., Russell III, J. M., Smith, A. K., ... & Gordley, L. L. (2013). Atomic oxygen in the mesosphere and lower thermosphere derived from SABER: Algorithm theoretical basis and measurement uncertainty. Journal of Geophysical Research: Atmospheres, 118(11), 5724-5735.</ref>

Note that temperatures also drop as altitude decreases, because lower photon photodissociation rates mean lower heat production per air molecule.