Ozone-oxygen cycle

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Ozone_cycle.jpg
Ozone-oxygen cycle in the ozone layer.

The ozone-oxygen cycle is the process by which ozone is continually regenerated in the Earth's stratosphere, all the while converting ultraviolet radiation into heat. The chemistry was worked out by Sidney Chapman in 1930.

Contents

How ozone is made

In the first step, an ozone molecule's life begins when intense solar ultraviolet radiation (less than 240 nm) breaks apart an oxygen molecule (O2) into two oxygen atoms. These atoms react with other oxygen molecules to form 2 ozone molecules.

6O2 + (radiation<240nm) → 6(O + O) → O3 + O3

The rate at which ozone is formed is slow, since there isn't a lot of solar energy at wavelengths less than 240 nm. This production process would take about 1 year to replace the amount of ozone that exists at around 20 km.

How ozone works

When ozone in the upper atmosphere is hit by ultraviolet solar radiation, it quickly undergoes a chemical reaction. The tri-atomic ozone molecule becomes bi-atomic 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

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. 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 (i. e. infrared radiation). 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", forming 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 ozone-oxygen cycle rates.

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, most especially the release of chlorofluorocarbons (CFCs) and halons, greatly increased the Cl and Br concentrations, leading to ozone depletion.

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