Atmospheric residence time
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Description
The atmospheric residence time or folded-e lifetime is a measure of the time it takes for a gas present in the air to decay exponentially. It is sometimes referred to wrongly as the half-life.
Detailed description
The atmospheric residence time is a measure of the length of time a gas remains in the atmosphere before it rains out or is decomposed by, for example, hydrolysis or photolysis. Hydrolysis usually occurs in the troposphere. Some of the water vapour molecules split into H+ and OH- free radicals. These radicals are very reactive and may break the covalent bonds within the molecules of the gas, forming new compounds, some of which may or may not be water-soluble. Photolysis occurs when molecules are split by the sun's radiation and may occur at any altitude, depending on the absorption spectrum of the gas.
As the decay of concentration is exponential and asymptotic to zero, it is defined as the time to reach 1/e (~36.79%) of the original concentration. It is also known as the folded-e lifetime. It is incorrect to use the term half-life. It is difficult to give categorical figures for gases with multiple uptake mechanisms, especially carbon dioxide with atmospheric residence times that can vary from about 5 to 300 years and methane from about 19 to 50 years. Some gases, such as tetrafluoromethane, may have atmospheric residence times exceeding 50,000 years.
For example, if 1 tonne of a gas with an atmospheric residence time of 100 years is emitted, about 368 kg will remain in the atmosphere after 100 years, 135 kg after 200 years, 50 kg after 300 years ... 1 kg after 700 years and so on.
If a gas has an atmospheric residence time exceeding a few weeks or months, some of it will be transported to the tropopause or even into the stratosphere where intense ultraviolet radiation may photolyse it. This is the case with ozone-depleting substances, where the photolysis will cause chlorine or bromine atoms to be released, to chain-react with ozone molecules, thus destroying them.
