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Human activities affect upper-atmospheric ozone (the stratospheric ozone layer)
We know that human-produced chemicals are responsible for the observed depletions of the ozone layer which protects the Earth from harmful UV radiation. The ozone-depleting compounds contain various combinations of the chemical elements chlorine, fluorine, bromine, carbon, and hydrogen. They are often described by the general term “halocarbons”. The compounds that contain only chlorine, fluorine, and carbon are called chlorofluorocarbons, usually abbreviated as CFC. CFCs have been used in many applications. Because they are so stable, they were ideal for use in refrigerators, air conditioners, foam blowing (making polystyrene foam containers such as Styrofoam™), cleaning electronics components, and as solvents (carbon tetrachloride was widely used in dry cleaning). CFCs are still used in “puffers” to treat asthma, but new chlorine-free propellants are being phased in.
Another important group of human-produced halocarbons is the halons, which contain carbon, bromine, fluorine, and (in some cases) chlorine and have been mainly used in fire extinguishers.
These compounds did not exist before chemists began synthesizing them. They have very long atmospheric lifetimes. The atmospheric concentrations of the CFCs rose, slowly at first, from zero before first synthesis in 1928, and then more rapidly in the 1960s and 1970s as more applications were found. The concentrations were rising in the 1980s at a rate of about 18 parts per trillion by volume (pptv) per year for CFC-12, 9 pptv/year for CFC-11, and 6 pptv/year for CFC-113. (pptv is the abbreviation for parts per trillion by volume.)
What actions have been taken to protect the ozone layer?
The Montreal Protocol on Substances that Deplete the Ozone Layer is a landmark international agreement designed to protect the stratospheric ozone layer. The treaty was originally signed in 1987 and amended in 1990 and 1992. In it governments decided to eventually discontinue production of CFCs, halons, carbon tetrachloride, and methyl chloroform. Their production was banned in the industrial countries as of January 1996 under the terms of the 1992 revision of the Montreal Protocol, and further emissions have almost stopped. Industry has developed more “ozone-friendly” substitutes.
The atmospheric concentrations of CFC-11 and CFC-113 are now slowly decreasing, and that of CFC-12 has been essentially level for the past several years. With adherence to the international agreements, the ozone layer is expected to recover over the next 50 years or so.
Greenhouse Gas Activity
CFCs and halocarbons are strong greenhouse gases, but are present in low concentrations. However, because of the century-long lifetimes of these CFC molecules, appreciable atmospheric concentrations of each will survive well into the twenty-second century. Because those that contain chlorine and bromine are involved in the depletion of the ozone layer, emissions are controlled by the Montreal Protocol and its amendments. As a result, the growth rate of CFCs and some other gases in the troposphere has slowed considerably.
Perfluorocarbons, (such as carbon tetrafluoride, CF4, and sulfur hexafluoride, SF6), also have technological utility and significant greenhouse gas capabilities. Their very long atmospheric lifetimes (over 1,000 years, much longer than that of other greenhouse gases) are a source of concern, even though their atmospheric concentrations have not yet produced large temperature effects.
Hydrofluorocarbons (HFCs) are being used as a substitute for CFCs. They also have a greenhouse effect from the fluorine, but the hydrogen in the molecule allows it to be broken down in the troposphere. This reduces both its atmospheric lifetime and the possible greenhouse effect. The atmospheric concentrations of all these gases, which are only very minor greenhouse contributors, need to be continuously monitored to ensure that no major sources have developed.
National Oceanic and Atmospheric Administration website
United Nations Development Program (UNDP) Montreal Protocol Unit website