Editing Ozone–oxygen cycle (section)

==Main reactions in different atmospheric layers==

===[[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.

===Below thermosphere: Reaction rates at steady state===
Odd oxygen species (atomic oxygen and ozone) have net creation rate only by oxygen dissociation (reaction 1), and net destruction by either ozone conversion or oxygen recombination (reactions 4 and 5). At steady state these processes are balanced, so the rates of these reactions obey:
:(rate of reaction 1) = (rate of reaction 4) + (rate of reaction 5).

At steady state, ozone creation is also balanced with its removal. so: 
:(rate of reaction 2) = (rate of reaction 3) + (rate of reaction 4).

It thus follows that:
:(rate of reaction 2) + (rate of reaction 5) = (rate of reaction 3) + (rate of reaction 1).
The right-hand side is the total photodissociation rate, of either oxygen or ozone.

Below the thermosphere, the atomic oxygen concentration is very low compared to molecular oxygen.<ref>Richter, H., Buchbender, C., Güsten, R., Higgins, R., Klein, B., Stutzki, J., ... & Hübers, H. W. (2021). Direct measurements of atomic oxygen in the mesosphere and lower thermosphere using terahertz heterodyne spectroscopy. Communications Earth & Environment, 2(1), 19.</ref> Therefore, oxygen atoms are much more likely to hit oxygen (diatomic) molecules than to hit other oxygen atoms, making oxygen recombination (reaction 5) far rarer than ozone creation (reaction 2). Following the steady-state relation between the reaction rates, we may therefore approximate:<ref>Hingane, L. S. (1984). Ozone in the mesosphere and lower thermosphere. Proceedings of the Indian Academy of Sciences-Earth and Planetary Sciences, 93, 91-103.</ref>
:(rate of reaction 2) = (rate of reaction 3) + (rate of reaction 1)

===[[Mesosphere]]===

In the mesosphere, oxygen photodissociation dominates over ozone photodissociation, so we have approximately:<ref name="Photochemistry of Ozone" />
:(rate of reaction 2) = (rate of reaction 1) =  (rate of reaction 4)

Thus, ozone is mainly removed by ozone conversion. Both ozone creation and conversion depend linearly on oxygen atom concentration, but in ozone creation an oxygen atom must encounter an oxygen molecule and another air molecule (typically nitrogen) simultaneously, while in ozone conversion an oxygen atom must only encounter an ozone molecule. Thus, when both reactions are balanced, the ratio between ozone and molecular oxygen concentrations is approximately proportional to air density.

Therefore, the relative ozone concentration is higher at lower altitudes, where air density is higher. This trend continues to some extent lower into the stratosphere, and thus as we go from 60&nbsp;km to 30&nbsp;km altitude, both air density and ozone relative concentration increase by ~40-50-fold.<ref name="NASA">[https://ozonewatch.gsfc.nasa.gov/facts/SH.html NASA Ozone Watch]</ref><ref>Bakhmetieva, N. V., Kulikov, Y. Y., & Zhemyakov, I. N. (2020). Mesosphere ozone and the lower ionosphere under plasma disturbance by powerful high-frequency radio emission. Atmosphere, 11(11), 1154.</ref><ref>[https://www.engineeringtoolbox.com/standard-atmosphere-d_604.html#google_vignette  The Engineering Toolbox: U.S. Standard Atmosphere vs. Altitude ]</ref>

===[[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" />

===[[Troposphere]]===
In the troposphere, ozone formation and destruction are no longer controlled by the ozone-oxygen cycle. Rather, tropospheric ozone chemistry is dominated today by industrial pollutants other gases of volcanic source.<ref name="Photochemistry of Ozone" />