In any discussion of the atmosphere of planet Earth, we cannot exclude that group of gases known as the Halocarbons. These compounds combine carbon with one or more of the five elements called halogens: fluorine, chlorine, bromine, iodine, and astatine, but only the first three are relevant here.

Included in the family of halocarbons are the chlorofluorocarbons (better known as CFCs) and the hydrochlorofluorocarbons (or HCFCs), HCFC substitutes and also some others - carbon tetrachloride, halons, methyl chloride, methyl chloroform and methyl bromide.

Some of these gases, such as CFCs -11, -12, -113, -114 and -115 have had a lot of bad press. These were used, particularly in the USA and Europe as spray can propellants, or solvents, cleaners and also as coolants until the mid 1970s. Many of the world's nations agreed to control emissions of these chemicals in 1987 when they signed the Montreal Protocol on Substances that deplete the Ozone Layer. That agreement was tightened in 1990 and again in 1992.

The other role played by halocarbons, as greenhouse gases, is often overlooked but important to consider.

All of the halocarbons in the atmosphere result from human activities, except for methyl chloride and methyl bromide, which also have important natural sources. The concentration of methyl chloride does not appear to be growing, while the concentration of methyl bromide does. The concentration of halocarbons are much lower than those of other greenhouse gases, typically between .2 and 16.5 parts per trillion by volume.

Halocarbons generally raise some concern because of their greenhouse warming effect, on a molecule for molecule basis, is drastically greater than that of carbon dioxide. The five ozone depleting CFCs of most concern have warming effects ranging from 3000 to 13000 times that of carbon dioxide.

Another reason the halocarbons raise concern involves their longevity. Chemical engineers invented some of these chemicals specifically for their stability, a good thing given their intended purposes. But once on the atmosphere that very stability becomes a vice: they resist breakdown and removal for many decades. The atmospheric lifetimes of CFC-113 and CFC-115, the longest lived are about 400 years, so whatever harmful effects they may have will continue for centuries after we stop putting them into the atmosphere.

Reductions and elimination of CFC production under the international agreement known as the Montreal protocol will, as a beneficial side effect help slow the growing concentrations of greenhouse gases. But some halocarbons with greenhouse potential were not restricted under that protocol, and some of the CFC substitutes whose uses it will encourage are themselves halocarbons with greenhouse effects.

Related Articles:

- Gases Introduction
- Carbon Dioxide
- Halocarbons
- Methane
- Nitrous Oxide
- Ozone
- Water Vapour