# Null cycle

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In [atmospheric chemistry](/source/atmospheric_chemistry), a '''null cycle''' is a [catalytic cycle](/source/catalytic_cycle) that simply interconverts chemical species without leading to net production or removal of any component.<ref>{{Cite book|title=Atmospheric chemistry|last=M.|first=Holloway, Ann|date=2010|publisher=RSC Pub|others=Wayne, Richard P.|isbn=9781847558077|location=Cambridge|oclc=471801630}}</ref> In the [stratosphere](/source/stratosphere), null cycles and when the null cycles are broken are very important to the [ozone layer](/source/ozone_layer).

One of the most important null cycles takes place in the stratosphere, with the [photolysis](/source/Photodissociation) of ozone by ultraviolet photons with wavelengths less than 330 nanometers. This photolysis produces a monatomic oxygen that then reacts with the diatomic oxygen producing ozone.<ref name="Pitts 2000">{{Cite book|title=Chemistry of the upper and lower atmosphere : theory, experiments, and applications|last=Pitts|first=Barbara J.|date=2000|publisher=Academic Press|others=Pitts, James N.|isbn=9780080529073|location=San Diego|pages=661|oclc=162128929}}</ref> There is no net molecular or atomic change, however. Overall, the reaction converts UV photon energy into heat thereby warming the stratosphere.<ref>"The Stratosphere - overview". ''scied.ucar.edu''. University Corporation for Atmospheric Research. Retrieved 1 November 2018.</ref>

O<sub>3</sub> + hv (λ < 330 nm) → O<sub>2</sub> + O (<sup>1</sup>D)

O (<sup>1</sup>D) + M → O (<sup>3</sup>P) + M

O (<sup>3</sup>P) + O<sub>2</sub> → O<sub>3</sub>

Net: hv → H

The null cycle can be broken in the presence of certain molecules, leading to a net increase or decrease in ozone in the stratosphere. One important example is [NO<sub><var>x</var></sub>](/source/NOx) emissions into the stratosphere. The NO<sub><var>x</var></sub> reacts with both the atomic oxygen and ozone leading to a net decrease in ozone.<ref name="Pitts 2000" /> This is particularly important at night when NO<sub>2</sub> cannot photolyze.

NO + O<sub>3</sub> → NO<sub>2</sub> + O<sub>2</sub>

NO<sub>2</sub> + O(<sup>1</sup>D) → NO + O<sub>2</sub>

Net: O<sub>3</sub> + O(<sup>1</sup>D) → 2O<sub>2</sub> (net loss of ozone)

Null cycles can also occur in the troposphere. One example is the null cycle that occurs during the day between NO<sub><var>x</var></sub> and ozone.

Tropospheric Null Cycle

O<sub>3</sub> + NO → O<sub>2</sub> + NO<sub>2</sub>

NO<sub>2</sub> + hν → NO + O(<sup>3</sup>P)

O (<sup>3</sup>P) + O<sub>2</sub> + M → O<sub>3</sub> + M

Net: hv → H

This cycle links ozone to NO<sub><var>x</var></sub> in the troposphere during daytime. In equilibrium, described by the [Leighton relationship](/source/Leighton_relationship), solar radiation and the NO<sub>2</sub>:NO ratio determine ozone abundance, maximizing around noon time.

==References==
<references/>

Category:Atmospheric chemistry
Category:Ozone depletion

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Adapted from the Wikipedia article [Null cycle](https://en.wikipedia.org/wiki/Null_cycle) by Wikipedia contributors ([contributor history](https://en.wikipedia.org/wiki/Null_cycle?action=history)). Available under [Creative Commons Attribution-ShareAlike 4.0 International](https://creativecommons.org/licenses/by-sa/4.0/). Changes may have been made.