The halocarbons are a group of partially halogenated organic compounds, in which carbon atoms link with halogen atoms by a covalent bond. (The halogen atoms include bromine, chlorine, fluorine, and iodine.) The most common type of halocarbon contains chlorine and belongs to the subclass of chlorocarbons, which includes substances such as carbon tetrachloride and tetrachloroethylene.

The other common subclass of halocarbon, the fluorocarbons, contains fluorine and includes polytetrafluoroethylene (Teflon) and perfluorocarbons (PFCs). Some of the Freons, though not all, are halocarbons. Halocarbons are produced and valued because of their nonflammability, low chemical reactivity, and low toxicity.

The halocarbons include several compounds that affect the environment, compounds such as chlorofluorocarbons (CFCs), hydrochlorofluorocarbons (HCFCs), hydrofluorocarbons (HFCs), halons, methyl chloride, and methyl bromide. Though it is possible for halocarbons to form naturally, from volcanic activity or by the interaction of halogen and plant matter, most halocarbons are produced by humans for industrial and chemical use and include substances used as refrigerants and fire extinguishants.

Many halocarbon compounds have a high global warming potential (GWP). Of these compounds, three (CFC-11, CFC-12, and CFC-113, all created by human activity) are the worst offenders. These gases are not reactive, so they are able to remain in the stratosphere for hundreds of years (Forster, 2005).

As light from the Sun enters the atmosphere, some of that light is scattered by molecules in the air or is reflected from clouds back into space. Some of the light that reaches Earth's surface is also reflected back into space, such as light that hits snow or ice. However, much of the light that reaches Earth is absorbed and retained as heat. The Earth's surface warms and emits infrared photons, which make several passes between the Earth and the atmosphere, warming the atmosphere and the Earth as they go back and forth (Middlebrook, 2000). Eventually, these infrared photons return to space.

The greenhouse gases (GHGs), which include many halocarbons, are able to absorb infrared photons, transferring their energy from the photons to gas molecules and thereby trapping thermal energy that would otherwise be released into space. Eventually, this absorption of energy causes a net change in the Earth's energy balance, increasing the overall amount of thermal energy held within the system. Different GHGs have different GWPs and lifetimes, measurements of the amount they contribute to global warming and the duration of that contribution, respectively. Both the GWP and the lifetime of halocarbons are unusually high. They may remain in the atmosphere for up to four hundred years, for example, continuing to affect the global climate long after their initial release.

Halocarbons containing chlorine and bromine also deplete the ozone layer. Until the 1970's, CFCs, perhaps the best known of the halocarbons, were used as propellants, solvents, and cleaners. Realizing that these compounds were depleting the ozone layer, many nations agreed to control halocarbon emissions when they signed the Montreal Protocol.

Ozone is a form of oxygen; it is formed of three atoms of oxygen bound together. Ozone in the stratosphere blocks harmful solar radiation. Halocarbons help speed the natural destruction of ozone. For example, CFCs are able to break down ozone when they combine with it in high-frequency ultraviolet (UV) light. During this breakdown, ozone molecules are destroyed, but chlorine reforms and is able to take part in the breakdown again. In this way, just one chlorine atom in the stratosphere is thought to be able to destroy about 100,000 ozone molecules.

Ozone depletion can have serious effects. The less ozone is available to shield the Earth, the more UV-B rays are able to penetrate the atmosphere and reach the planet's surface, where they cause skin cancer and cataracts in humans. Moreover, the disruption of the normal wavelengths of light reaching Earth's surface affects plants and their growth rates. It is also possible that the change in atmospheric radiation affects global wind patterns and therefore climate.

Halocarbons do not disintegrate easily and may become corrosive agents when burned, which makes them difficult to dispose of safely. Halocarbons are also implicated in liver disease, eye cataracts, and skin cancer, and they may affect the human immune system.

References

1. Forster, P., and M. Joshi. "The Role of Halocarbons in the Climate Change of the Troposphere and Stratosphere." Climatic Change 71, no. 1 (July, 2005): 249-266.

2. Middlebrook, Ann M., and Margaret A. Tolbert. Stratospheric Ozone Depletion. Sausalito, Calif.: University Science Books, 2000.

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