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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.
How ozone is made
In the first step, an ozone molecule's life begins when intense ultraviolet solar radiation (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 2 ozone molecules.
- O2 + (radiation < 240nm) → 2O
- 2(O2 + O + M) → 2(O3 + M)
Here "M" is a so-called "third body collision partner", a molecule (usually nitrogen or oxygen) which carries off the excess energy of the reaction. Ozone forms slowly since there isn't a lot of solar energy at wavelengths less than 240 nm. This production process would require about one year to replace the amount of ozone that exists at around 20 km above Earth's surface.
How ozone works
When ozone in the upper atmosphere is hit by ultraviolet solar radiation, it quickly undergoes a chemical reaction. The triatomic ozone molecule becomes diatomic molecular oxygen plus a free oxygen atom:
- O3 + radiation → O2 + O
Free atomic oxygen then quickly reacts with other oxygen molecules and forms ozone again:
- O2 + O + M → O3 + M
The chemical energy released when O and O2 combine is thus converted into kinetic energy of molecular motion. The overall effect is to convert penetrating UV light into harmless heat. 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).
How ozone is removed
When an oxygen atom and an ozone molecule meet, they recombine to form two oxygen molecules:
O3 + O → 2O2
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 much slower than the period of the ozone-oxygen cycle.
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 the By-product of chlorofluorocarbons (CFCs) and halons, 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.
External links
"Ozone-oxygen cycle"