Halocarbons

Introduction

Halocarbons are carbon-based compounds that also contain a halogen element, either chlorine, bromine, or fluorine. Halocarbons produced and released into the atmosphere over the last 50 years have contributed to the depletion of the ozone layer.

Historical Background and Scientific Foundations

The chlorofluorocarbons (CFCs) that were used as spray can propellants, cleaners, solvents, and refrigerants up until the mid 1970s remain stable in the troposphere, the layer of the atmosphere that is closest to Earth's surface. When the CFCs move upward into the stratosphere, the atmospheric layer just above the troposphere, they are broken down by the ultraviolet radiation from the sun to release free chlorine atoms. The chlorine then reacts with ozone to deplete the ozone layer.

Other ozone-depleting halocarbons include hydro-chlorofluorocarbons (HCFCs), halons, methyl bromide, and carbon tetrachloride. Halons are used in fire extinguishers and the other halocarbons are solvents frequently used by industrial manufacturers.

The contribution a particular chemical makes to climate changes is expressed as the Global Warming Potential (GWP). The GWP is the ratio of the warming caused by a substance compared to a similar mass of carbon dioxide

(CO₂). Carbon dioxide is not a halocarbon but it is considered the chief greenhouse gas as it is emitted in such large quantities (by both natural and human-generated processes) into the atmosphere.

Impacts and Issues

Although all the halocarbons contribute to global warming, not all contribute equally. Halocarbons are in much lower concentration than carbon dioxide in the atmosphere but their warming effect is between 3,000 and 13,000 times that of carbon dioxide. However, gases such as CFCs can remain in the atmosphere for hundreds of years before they are finally destroyed in the ozone layer.

WORDS TO KNOW

CARBON TETRACHLORIDE: A compound of carbon that has all four bonding positions each filled with a chlorine atom.

HALOCARBON: Compound that contains carbon and one or more of the elements known as halons (chlorine, fluorine, or bromine). Halocarbons do not exist in nature; all are manufactured. When released into the atmosphere, many halocarbons deplete the ozone layer and have high global warming potential.

HALOGEN: Any one of the halogen family of elements with similar chemical properties, including fluorine, chlorine, bromine, and iodine.

KYOTO PROTOCOL: Extension in 1997 of the 1992 United Nations Framework Convention on Climate Change (UNFCCC), an international treaty signed by almost all countries with the goal of mitigating climate change. The United States, as of early 2008, was the only industrialized country to have not ratified the Kyoto Protocol, which is due to be replaced by an improved and updated agreement starting in 2012.

METHYL BROMIDE: A compound of carbon that is made of a methyl group, which is made of three hydrogen atoms bonded to one carbon atom; and in a fourth bonding position on the carbon atom there is a bromine atom.

OZONE: An almost colorless, gaseous form of oxygen with an odor similar to weak chlorine. A relatively unstable compound of three atoms of oxygen, ozone constitutes, on average, less than one part per million (ppm) of the gases in the atmosphere. (Peak ozone concentration in the stratosphere can get as high as 10 ppm.) Yet ozone in the stratosphere absorbs nearly all of the biologically damaging solar ultraviolet radiation before it reaches Earth's surface, where it can cause skin cancer, cataracts, and immune deficiencies, and can harm crops and aquatic ecosystems.

OZONE HOLE: A term invented to describe a region of very low ozone concentration above the Antarctic that appears and disappears with each austral (Southern Hemisphere) summer.

IN CONTEXT: HALOCARBONS AND GREENHOUSE GASES

The current consensus among most scientists who study global warming is that recently measured increases in global temperature are not natural cyclic fluctuations, but rather reflect human alteration of the natural phenomena known as the greenhouse effect by increasing concentrations of greenhouse-related atmospheric gases. Although most greenhouse gases occur naturally, the evolution of an industrial civilization has significantly increased levels of these naturally occurring gases. In addition, new gases have been put into the atmosphere that magnify the greenhouse effect. Important greenhouse gases in the modern Earth atmosphere include water vapor and carbon dioxide, methane, nitrous oxides, ozone, halogens (bromine, chlorine, and fluorine), halocarbons, and other trace gases.

The sources of the greenhouse gases are both natural and human-generated. For example, ozone is a naturally occurring greenhouse gas found in the atmosphere. Ozone is constantly produced and broken down in natural atmospheric processes. In contrast, halocarbons enter the atmosphere primarily as the result of human use of products such as chlorofluorocarbons (CFCs). Water vapor and carbon dioxide are natural components of respiration, transpiration, evaporation, and decay processes. Carbon dioxide is also a by-product of combustion. Although occurring at lower levels than water vapor or carbon dioxide, methane is also a potent greenhouse gas. Nitrous oxides, enhanced by the use of nitrogen fertilizers, nylon production, and the combustion of organic material, including fossil fuels, have also been identified as contributing to strong greenhouse effects.

CFCs are considered one of the worst contributors to the depletion of the ozone layer. In 1987, the international community, as represented by more than 145 countries, signed the Montreal Protocol to ban the production of CFCs by the year 2000. One of the substitutes developed by industry, the hydrochlorofluorocarbons (HCFCs), still contain enough chlorine to interfere with atmospheric ozone chemistry, although in much lesser amounts than CFCs. The Copenhagen Amendment to the Montreal Protocol calls for the cessation of HCFC production by 2030. The European Union, through regulations passed in 2000, is attempting a stepwise reduction in the use of HCFCs.

In 1997, the Kyoto Protocol proposed greater controls for halocarbon emissions.

See Also Kyoto Protocol; Montreal Protocol.

BIBLIOGRAPHY

Web Sites

“Climate Change.” BBC Weather Centre. <http://www.bbc.co.uk/climate/evidence/halocarbons.shtml> (accessed August 14, 2007).

“European Union 2000 HCFC Regulations.” The Alliance for Responsible Atmospheric Policy, 2000. <http://www.arap.org/regs/euregs.html> (accessed August 14, 2007).

“Glossary of Climate Change Terms.” U.S. Environmental Protection Agency, November 13, 2006. <http://www.epa.gov/climatechange/glossary.html> (accessed August 14, 2007).