Ozone-oxygen cycle
Encyclopedia
The ozone-oxygen cycle is the process by which ozone
is continually regenerated in Earth
's stratosphere
, all the while converting ultraviolet radiation into heat
. In 1930 Sydney Chapman
resolved the chemistry
involved. The process is commonly called the Chapman cycle by atmospheric scientists.
Most of the ozone production occurs in the tropical upper stratosphere and mesosphere. The total mass of ozone produced per day over the globe is about 400 million metric tons. The global mass of ozone is relatively constant at about 3 billion metric tons, meaning the Sun produces about 12% of the ozone layer each day.
O3 + (240 nm < radiation < 310 nm) → O2 + O
The atomic oxygen produced immediately reacts with other oxygen molecules to reform ozone:
O2 + O + M → O3 + M
where "M" denotes the third body that carries off the excess energy of the reaction. In this way,
the chemical energy released when O and O2 combine is converted into kinetic energy of molecular motion. The overall effect is to convert penetrating UV light into heat, without any net loss of ozone. This cycle keeps the ozone layer in a stable balance while protecting the lower atmosphere from UV radiation, which is harmful to most living beings. It is also one of two major sources of heat in the stratosphere (the other being the kinetic energy released when O2 is photolyzed into O atoms).
O3 + O· → 2 O2
And if two oxygen atoms meet, they react to form one oxygen molecule:
2 O· → O2
The overall amount of ozone in the stratosphere is determined by a balance between
production by solar radiation, and removal. The removal rate is slow, since the concentration of O atoms is very low.
Certain free radicals, the most important being hydroxyl
(OH), nitric oxide
(NO), and atoms of chlorine
(Cl) and bromine
(Br), catalyze
the recombination reaction
, leading to an ozone layer that is thinner than it would be if the catalysts were not present.
Most of the OH and NO are naturally present in the stratosphere, but human activity, especially emissions of chlorofluorocarbons (CFCs) and halon
s, has greatly increased the Cl and Br concentrations, leading to ozone depletion
. Each Cl or Br atom can catalyze tens of thousands of decomposition reactions before it is removed from the stratosphere.
Ozone
Ozone , or trioxygen, is a triatomic molecule, consisting of three oxygen atoms. It is an allotrope of oxygen that is much less stable than the diatomic allotrope...
is continually regenerated in Earth
Earth
Earth is the third planet from the Sun, and the densest and fifth-largest of the eight planets in the Solar System. It is also the largest of the Solar System's four terrestrial planets...
's stratosphere
Stratosphere
The stratosphere is the second major layer of Earth's atmosphere, just above the troposphere, and below the mesosphere. It is stratified in temperature, with warmer layers higher up and cooler layers farther down. This is in contrast to the troposphere near the Earth's surface, which is cooler...
, all the while converting ultraviolet radiation into heat
Heat
In physics and thermodynamics, heat is energy transferred from one body, region, or thermodynamic system to another due to thermal contact or thermal radiation when the systems are at different temperatures. It is often described as one of the fundamental processes of energy transfer between...
. In 1930 Sydney Chapman
Sydney Chapman (astronomer)
Sydney Chapman FRS was a British mathematician and geophysicist. His work on the kinetic theory of gases, solar-terrestrial physics, and the Earth's ozone layer has inspired a broad range of research over many decades....
resolved the chemistry
Chemistry
Chemistry is the science of matter, especially its chemical reactions, but also its composition, structure and properties. Chemistry is concerned with atoms and their interactions with other atoms, and particularly with the properties of chemical bonds....
involved. The process is commonly called the Chapman cycle by atmospheric scientists.
Most of the ozone production occurs in the tropical upper stratosphere and mesosphere. The total mass of ozone produced per day over the globe is about 400 million metric tons. The global mass of ozone is relatively constant at about 3 billion metric tons, meaning the Sun produces about 12% of the ozone layer each day.
Chemistry
The ozone molecules formed by the below reaction absorb ultraviolet radiation having wavelengths between 240 and 310 nm. The triatomic ozone molecule becomes diatomic molecular oxygen plus a free oxygen atom:O3 + (240 nm < radiation < 310 nm) → O2 + O
The atomic oxygen produced immediately reacts with other oxygen molecules to reform ozone:
O2 + O + M → O3 + M
where "M" denotes the third body that carries off the excess energy of the reaction. In this way,
the chemical energy released when O and O2 combine is converted into kinetic energy of molecular motion. The overall effect is to convert penetrating UV light into heat, without any net loss of ozone. This cycle keeps the ozone layer in a stable balance while protecting the lower atmosphere from UV radiation, which is harmful to most living beings. It is also one of two major sources of heat in the stratosphere (the other being the kinetic energy released when O2 is photolyzed into O atoms).
Removal
If an oxygen atom and an ozone molecule meet, they recombine to form two oxygen molecules:O3 + O· → 2 O2
And if two oxygen atoms meet, they react to form one oxygen molecule:
2 O· → O2
The overall amount of ozone in the stratosphere is determined by a balance between
production by solar radiation, and removal. The removal rate is slow, since the concentration of O atoms is very low.
Certain free radicals, the most important being hydroxyl
Hydroxyl
A hydroxyl is a chemical group containing an oxygen atom covalently bonded with a hydrogen atom. In inorganic chemistry, the hydroxyl group is known as the hydroxide ion, and scientists and reference works generally use these different terms though they refer to the same chemical structure in...
(OH), nitric oxide
Nitric oxide
Nitric oxide, also known as nitrogen monoxide, is a diatomic molecule with chemical formula NO. It is a free radical and is an important intermediate in the chemical industry...
(NO), and atoms of chlorine
Chlorine
Chlorine is the chemical element with atomic number 17 and symbol Cl. It is the second lightest halogen, found in the periodic table in group 17. The element forms diatomic molecules under standard conditions, called dichlorine...
(Cl) and bromine
Bromine
Bromine ") is a chemical element with the symbol Br, an atomic number of 35, and an atomic mass of 79.904. It is in the halogen element group. The element was isolated independently by two chemists, Carl Jacob Löwig and Antoine Jerome Balard, in 1825–1826...
(Br), catalyze
Catalysis
Catalysis is the change in rate of a chemical reaction due to the participation of a substance called a catalyst. Unlike other reagents that participate in the chemical reaction, a catalyst is not consumed by the reaction itself. A catalyst may participate in multiple chemical transformations....
the recombination reaction
Chemical reaction
A chemical reaction is a process that leads to the transformation of one set of chemical substances to another. Chemical reactions can be either spontaneous, requiring no input of energy, or non-spontaneous, typically following the input of some type of energy, such as heat, light or electricity...
, leading to an ozone layer that is thinner than it would be if the catalysts were not present.
Most of the OH and NO are naturally present in the stratosphere, but human activity, especially emissions of chlorofluorocarbons (CFCs) and halon
Halon
Halon can refer to:* Haloalkane, or halogenoalkane, a group of chemical compounds consisting of alkanes with linked halogens. In particular, bromine-containing haloalkanes.* Halomethane fire extinguishing systems...
s, has greatly increased the Cl and Br concentrations, leading to ozone depletion
Ozone depletion
Ozone depletion describes two distinct but related phenomena observed since the late 1970s: a steady decline of about 4% per decade in the total volume of ozone in Earth's stratosphere , and a much larger springtime decrease in stratospheric ozone over Earth's polar regions. The latter phenomenon...
. Each Cl or Br atom can catalyze tens of thousands of decomposition reactions before it is removed from the stratosphere.