Chlorofluorocarbons and related compounds

Chlorofluorocarbons (CFCs) are useful as refrigerants, solvents, and aerosol propellants, but they can act as catalysts for the destruction of stratospheric ozone. These compounds also contribute directly to global warming by acting as GHGs.

Background

Developed in the 1930s as refrigerants, chlorofluorocarbons (CFCs) gained rapid acceptance in the 1940s. New uses were found for them as aerosol propellants, blowing agents, solvents, fire suppressants, and inhalation anesthetics. Production climbed, reaching a high of 566,591 metric tons in the United States by 1988. In 1971, it was shown that CFCs had accumulated in the atmosphere, and by 1974, a relationship was demonstrated between atmospheric CFCs and depletion of the ozone layer. Over the next twenty years, the manufacture and use of CFCs were drastically reduced to protect the ozone layer from further harm.

Fate of Halocarbons in the Atmosphere

CFCs, although denser than air, mix throughout the atmosphere and eventually reach the stratosphere (10 to 50 kilometers in altitude). Although CFCs have low chemical reactivity (and hence long lifetimes) in the lower atmosphere, in the stratosphere they encounter and absorb energetic ultraviolet radiation, resulting in their chlorine atoms being set free. These chlorine atoms can act as catalysts for the destruction of stratospheric ozone molecules. Because of the catalytic process, each chlorine atom can destroy thousands of ozone molecules. Other volatile chlorine compounds, such as methyl chloroform and carbon tetrachloride, can also form destructive chlorine atoms. Bromine atoms are also destructive of ozone, and bromine-containing compounds include the halons, used as fire suppressants and inhalation anesthetics, and methyl bromide, a soil fumigant and natural product of sea organisms.

In 1971, James E. Lovelock, using a sensitive detector, found traces of CFCs in air samples from different parts of the world. F. Sherwood Rowland and Mario Molina realized the potential ozone destructiveness of CFCs and stirred up concern among industrialists and politicians, which led to the signing of the Montreal Protocol in 1987. Because of the long lifetimes of CFCs, however, even in the absence of further production, it will take years for the existing pollutants to dissipate and allow the ozone layer to recover.

Substitutes for CFCs

Phasing out CFCs meant that substitutes were needed for applications in refrigeration, air conditioning, and aerosols. The ideal substitute would be nontoxic, nonflammable, noncorrosive (like a CFC), and of suitable physical properties (such as boiling point and heat of vaporization), but without the ozone destructiveness of the CFCs. Attention naturally focused on the related compounds, hydrochlorofluorocarbons (HCFCs) and hydrofluorocarbons (HFCs). The HCFCs, although they contain chlorine, tend to be destroyed by chemical reactions in the lower atmosphere before they reach the ozone layer. HFCs, which contain no chlorine, have negligible ozone destructiveness even should they reach the stratosphere. In addition to adopting these new compounds, the existing stocks of CFCs in abandoned equipment had to be trapped and either recycled or disposed of in an environmentally acceptable manner, rather than being vented into the atmosphere.

Context

The impact of CFCs and their related compounds is mainly on the ozone layer because of their catalytic effect. CFCs are also potent greenhouse gases, but are present in the atmosphere at such low levels (0.1-0.5 parts per billion) that their contribution to the total greenhouse effect is relatively small. Loss of ozone in the stratosphere also affects temperature in complex ways, warming some parts of the atmosphere and cooling others. Nevertheless, the sheer numbers of individual halocarbons, even at low levels individually, add up to a warming potential that is worth controlling. Atmospheric levels of substances controlled by the Montreal Protocol have declined appreciably, but, not surprisingly, the levels of their substitutes have risen. This trade-off is good for the ozone layer, but it leaves much to be accomplished on the global warming front.

Key Concepts

• Freon: trade name for CFCs made by DuPont chemical company
• greenhouse effect: a phenomenon in which gases trap solar heat within Earth’s atmosphere, preventing it from radiating away into space
• halocarbons: the general family of compounds that includes CFCs, HCFCs, HFCs, and other molecules in which carbon bonds with halogen atoms
• halon: a compound containing bromine, carbon, chlorine, and fluorine
• Kyoto Protocol: a 1997 international agreement to limit greenhouse gas emissions
• Montreal Protocol: a 1987 international agreement to phase out the manufacture and use of ozone-depleting chemicals
• ozone layer: the portion of the Earth’s stratosphere (10-50 kilometers high) where ozone has formed and absorbs dangerous ultraviolet radiation from the Sun

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