The Montreal Protocol
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.
Sources:
National Oceanic and Atmospheric Administration website
http://www.ncdc.noaa.gov/oa/climate/gases.html
United Nations Development Program (UNDP) Montreal Protocol
Unit website
http://www.undp.org/seed/eap/montreal/
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