Ozone-oxygen cycle
From Wikipedia, the free 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 energy. In 1930 Sidney Chapman resolved the chemistry involved.
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[edit] How ozone is made
In the first step, an ozone molecule's life begins when ultraviolet solar radiation (with high energy; less than 240 nm in wavelength) breaks apart an oxygen molecule (O2) into two oxygen atoms. These atoms react with other oxygen molecules to form ozone molecules.
- O2 + (radiation < 240nm) → 2 O
- O2 + O + M → O3 + M
In the second reaction, "M" is a so-called "third body collision partner", a molecule (usually nitrogen or oxygen) which undergoes no chemical change, but carries off the excess energy of the reaction. Ozone is formed primarily in the upper stratosphere, since the radiation at wavelengths less than 240 nm is absorbed efficiently already at about 30 km height.
[edit] How the ozone layer works
The ozone molecules formed by the above 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 + (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" once again 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).
[edit] How ozone is removed
If an oxygen atom and an ozone molecule meet, they recombine to form two oxygen molecules:
O3 + O → 2 O2
The overall amount of ozone in the stratosphere is determined by a balance between production by solar radiation, and removal by recombination. 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 halons, 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.
