CFCs and their substitutes in stratospheric ozone depletion.
NOAA Earth System Research Laboratory, R/GMD1, 325 Broadway, Boulder, CO 80305-3328
What are the CFCs?
Chlorofluorocarbons (CFCs) are nontoxic, nonflammable chemicals containing atoms of carbon, chlorine, and fluorine. They are used in the manufacture of aerosol sprays, blowing agents for foams and packing materials, as solvents, and as refrigerants. CFCs are classified as halocarbons, a class of compounds that contain atoms of carbon and halogen. NOAA monitors these chemicals and other important halocarbons at twelve sampling sites using either continuous instruments or discrete flask samples (below).
Why were they used?
Refrigerators in the late 1800s and early 1900s used the toxic gases, ammonia (NH3), methyl chloride (CH3Cl), and sulfur dioxide (SO2), as refrigerants. After a series of fatal accidents in the 1920s when methyl chloride leaked out of refrigerators, a search for a less toxic replacement begun as a collaborative effort of three American corporations- Frigidaire, General Motors, and Du Pont. CFCs were first synthesized in 1928 by Thomas Midgley, Jr. of General Motors, as safer chemicals for refrigerators used in large commercial applications. In 1932 the Carrier Engineering Corporation used Freon-11 (CFC-11) in the world’s first self-contained home airconditioning unit, called the "Atmospheric Cabinet". During the late 1950s and early 1960s the CFCs made possible an inexpensive solution for air conditioning in many automobiles (CFC-12), homes, and office buildings. Later, the growth in CFC use took off worldwide with peak, annual sales of about a billion dollars (U.S.) and more than one million metric tons of CFCs produced.
CFCs can destroy stratospheric ozone.
Whereas CFCs are safe to use in most applications and are inert in the lower atmosphere, they do undergo significant reaction in the upper atmosphere or stratosphere. In 1974, two University of California chemists, Professor F. Sherwood Rowland and Dr. Mario Molina, showed that the CFCs could be a major source of inorganic chlorine in the stratosphere following their photolytic decomposition by ultra-violet (UV) radiation. Some of the released chlorine would become active in destroying ozone in the stratosphere. Ozone is a trace gas located primarily in the stratosphere. Ozone absorbs harmful ultraviolet radiation in the wavelengths between 280 and 320 nm of the UV-B band, which can cause biological damage in plants and animals. A loss of stratospheric ozone results in more harmful UV-B radiation reaching the Earth's surface.
The Antarctica “Ozone Hole”
A large springtime depletion of stratospheric ozone was getting worse each following year. This ozone loss was described in 1985 by British researcher Joe Farman and his colleagues. Others called it “the Ozone Hole” The ozone hole was different than ozone loss in the midlatitudes. The loss was greater over the Antarctic than the midlatitudes because of many factors: the cold temperatures of the region, the dynamic isolation of this “hole”, and the synergistic reactions of chlorine and bromine. Ozone loss also is enhanced in Polar Regions as a result of reactions involving polar stratospheric clouds (PSCs) and in midlatitudes following volcanic eruptions. Large amounts of reactive stratospheric inorganic chlorine in the form of chlorine monoxide (ClO) observed during the Antarctic “Ozone Hole” (example shown on the left), punctuated the urgent need for controlling the CFCs.
The Montreal Protocol
On September 16, 1987, 27 nations signed a global environmental treaty, the Montreal Protocol to Reduce Substances that Deplete the Ozone Layer that had a provision to reduce 1986 production levels of these compounds by 50% before the year 2000. This international agreement included restrictions on production of CFC-11, -12, -113, -114, -115, and the Halons (chemicals used as a fire extinguishing agents). An amendment approved in London in 1990 was more forceful and called for the elimination of production by the year 2000. The chlorinated solvents, methyl chloroform (CH3CCl3), and carbon tetrachloride (CCl4) were added to the London Amendment in 1992. Subsequent amendments added methyl bromide (with exemptions for specific uses), hydrobromofluorocarbons, and bromochloromethane.
The Substitutes: HCFCs and HFCs
Besides recycling, the demand for the CFCs has been further reduced by the use of substitutes. Some applications, for example degreasing of metals and cleaning solvents for circuit boards, which once used CFC- 113 now use halocarbon-free fluids, water (sometimes as steam), and diluted citric acids. Industry developed two classes of halocarbon substitutes-for other uses: the hydrochlorofluorocarbons (HCFCs) and the hydrofluorocarbons (HFCs). The HCFCs include hydrogen atoms in addition to chlorine, fluorine, and carbon atoms. The advantage of using HCFCs is that the hydrogen reacts with tropospheric hydroxyl (OH), resulting in a shorter atmospheric lifetime. HCFC-22 (CHClF2) has an atmospheric lifetime of about 12 years instead of 100 years for CFC-12 and has been used in low-demand home air-conditioning and some refrigeration applications since 1975. However, HCFCs still contain chlorine, which makes it possible for them to destroy ozone. The Montreal Protocol calls for their consumption to be eliminated by the year 2030 by non-article 5 nations (developed countries) and article-5 nations by 2040. The HFCs are considered one of the best substitutes for reducing stratospheric ozone loss because of their short lifetime and lack of chlorine, but have a large greenhouse warming potential and were included for reduction in emissions in the Kyoto Protocol. In the United States, HFC-134a (CF3CH2F) has been used in all new domestic automobile air conditioners since 1993. HFC- 134a is growing rapidly in 2008 at a growth rate of about 5 ppt/year with an atmospheric lifetime of 13.4 years.
Scientists hope that stratosphere ozone will continue on its track to recovery, but natural and manmade changes can influence the year-to-year variability of stratospheric ozone concentrations. For example, large stratospheric ozone depletion was seen over the populated Arctic Region in 2010-2011 because of very cold temperatures and still relatively high chlorine levels that contributed to large ozone losses from Type II PSCs forming. Exemptions for specific use, like those for methyl bromide, have been introduced in the Montreal Protocol; so monitoring these halocarbons are still important for recording the recovery of the stratospheric ozone layer back to pre-“Ozone Hole” levels that are expected to occur between 2040 and 2050.