In
atmospheric chemistry
Atmospheric chemistry is a branch of atmospheric science in which the chemistry of the Earth's atmosphere and that of other planets is studied. It is a multidisciplinary approach of research and draws on environmental chemistry, physics, meteoro ...
, a null cycle is a
catalytic cycle
In chemistry, a catalytic cycle is a multistep reaction mechanism that involves a catalyst. The catalytic cycle is the main method for describing the role of catalysts in biochemistry, organometallic chemistry, bioinorganic chemistry, materials s ...
that simply interconverts chemical species without leading to net production or removal of any component. In the
stratosphere
The stratosphere () is the second layer of the atmosphere of the Earth, located above the troposphere and below the mesosphere. The stratosphere is an atmospheric layer composed of stratified temperature layers, with the warm layers of air ...
, null cycles and when the null cycles are broken are very important to the
ozone layer
The ozone layer or ozone shield is a region of Earth's stratosphere that absorbs most of the Sun's ultraviolet radiation. It contains a high concentration of ozone (O3) in relation to other parts of the atmosphere, although still small in rela ...
.
One of the most important null cycles takes place in the stratosphere, with the
photolysis
Photodissociation, photolysis, photodecomposition, or photofragmentation is a chemical reaction in which molecules of a chemical compound are broken down by photons. It is defined as the interaction of one or more photons with one target molecule. ...
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.
[{{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] There is no net molecular or atomic change, however. Overall, the reaction converts UV photon energy into heat thereby warming the stratosphere.
["The Stratosphere - overview". ''scied.ucar.edu''. University Corporation for Atmospheric Research. Retrieved 1 November 2018.]
O
3 + hv (λ < 330 nm) → O
2 + O (
1D)
O (
1D) + M → O (
3P) + M
O (
3P) + O
2 → O
3
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
NOx emissions into the stratosphere. The NO
x reacts with both the atomic oxygen and ozone leading to a net decrease in ozone.
This is particularly important at night when NO
2 cannot photolyze.
NO + O
3 → NO
2 + O
2
NO
2 + O(
1D) → NO + O
2
Net: O
3 + O(
1D) → 2O
2 (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
x and ozone.
Tropospheric Null Cycle
O
3 + NO → O
2 + NO
2
NO
2 + hν → NO + O(
3P)
O (
3P) + O
2 + M → O
3 + M
Net: hv → H
This cycle links ozone to
NOx
In atmospheric chemistry, is shorthand for nitric oxide () and nitrogen dioxide (), the nitrogen oxides that are most relevant for air pollution.
These gases contribute to the formation of smog and acid rain, as well as affecting tropos ...
in the troposphere during daytime. In equilibrium, described by the
Leighton relationship
In atmospheric chemistry, the Leighton relationship is an equation that determines the concentration of tropospheric ozone in areas polluted by the presence of nitrogen oxides. Ozone in the troposphere is primarily produced through the photolysis ...
, solar radiation and the NO
2:NO ratio determine ozone abundance, maximizing around noon time.
References
Atmospheric chemistry
Ozone depletion