The Earth is a unique planet because it has an atmosphere that is capable of sustaining life. The atmosphere has been here for billions of years, yet humans have done a great deal of damage to it in the last century. The Industrial Revolution, harnessing electric energy, and the internal combustion engine have all done some damage to the atmosphere by creating smog. But some of the most damaging substances we have spewed into the air are chlorofluorocarbons (CFCs), hydrochlorofluorocarbons (HCFCs), and bromine. These chemicals destroy stratospheric ozone, which protects us and the animals from solar ultraviolet radiation. This dilemma,which was created by the solution to a problem for the refrigeration and air-conditioning industry, has been identified, the nations of the world have agreed to a plan to solve the dilemma, and the results, especially from the industry that caused the problem, have been mostly favorable.
Before CFCs were developed in the 1920s, dangerous chemicals such as sulfur dioxide and ammonia were used as refrigerants. After Thomas Midgeley developed CFCs, many uses were found for the new chemicals. These uses, such as refrigerants in refrigeration chillers and air-conditioning equipment, and propellant in aerosol cans, generally allowed the CFCs to escape into the air at some point during use. The chemicals end up in the stratosphere where they can destroy ozone, which allows more ultraviolet radiation to strike the Earth's surface. In the 1970s research started to piece together the link between CFCs and ozone depletion. By the late 1980s it became apparent that there was a connection, and the nations came together to solve the problem.
Several meetings have taken place to address the ozone depletion problem. The best known meeting was in Montreal in 1987, and the agreement formed is called the Montreal Protocol. The Montreal Protocol set a timetable for the phaseout of CFCs, as well as halons, which contain bromine. Subsequent meetings have added HCFCs and changed the dates for reductions and phaseouts. Fortunately the dates have been moved up instead of back.
The results of the meetings have been mostly successful. The phaseout of CFCs was accomplished on time, or early in some countries. Systems have been redesigned, making them more efficient and reliable. The best result is that the ozone layer is now being protected and will start healing itself in the decades to come.
Ozone Depletion and Chlorofluorocarbons
The Earth's atmosphere is composed of several layers. The lower layer is called the troposphere and extends to an altitude of approximately 36,000 feet. The next layer extends to 30 miles and is called the stratosphere. Above the stratosphere is the mesosphere, extending to 50 miles. The outermost layer is the thermosphere, which is made up of the ionosphere and the exosphere. The thermosphere ends at an altitude of 600 miles.
The part of the atmosphere referred to as the ozone layer exists in the stratosphere. The ozone layer starts at approximately 15 miles and extends to 30 miles. In this region is the highest concentration of ozone. Even though the concentration is high, the ozone only constitutes one-millionth of the atmosphere (Somerville, 1996). While ozone is rare, it has the important function of shielding the Earth from solar ultraviolet radiation. This shielding protects all the living organisms that inhabit the planet, including all animals and humans.
Ozone Formation and Destruction
Ozone is the triatomic form of oxygen, meaning it consists of three oxygen atoms, and is identified chemically as O3. In the troposphere, where we live, ozone is a form of pollution. Ground level ozone is an end product of private and industrial energy consumption.
Stratospheric ozone is formed naturally. Ozone is formed when single oxygen atoms combine with the diatomic, or normal, form of oxygen (O2). Single atoms are formed when diatomic oxygen absorbs solar ultraviolet radiation (Somerville, 1996). This process is called dissociation, and the chemical equation is written:
O2 + UV => O + O
The free oxygen atoms then combine with diatomic oxygen:
O + O2 => O3
Ozone is destroyed naturally by two different processes. The first is when a free oxygen atom combines with an ozone molecule to produce two diatomic oxygen molecules:
O + O3 => 2O2
The other process in which ozone is destroyed is when ozone molecules absorb ultraviolet radiation and form one diatomic oxygen molecule and one free oxygen atom:
O3 + UV => O2 + O
The time interval between the formation and natural destruction of ozone varies greatly. Ozone molecules can have a lifetime of only a few minutes, or they can survive for several months. The average life of ozone is eleven days (Rowland, 1997).
In 1928, Thomas Midgeley, Jr. was trying to develop a refrigerant that was non-toxic, non-corrosive, non-flammable, and chemically inert. Prior to Midgeley's research, substances such as sulfur dioxide and ammonia were used in domestic refrigerators. These refrigerants were toxic and/or flammable, creating a safety concern (Cox and Miro, 1997). Midgeley used the characteristics of the elements in the periodic table to create a new class of refrigerants. These new refrigerants were called chlorofluorocarbons (CFCs). CFCs made ideal refrigerants, and over time they have been used as propellants in aerosol cans and solvents in the electronics industry, as well as refrigerants for domestic, industrial, and automotive air-conditioning systems.
As CFCs were used, they were eventually discharged into the atmosphere. Whenever refrigeration systems were drained, the refrigerant was allowed to escape. The CFCs in aerosol cans were dispelled with the product. The production of CFCs during the mid-1960s had reached a level of 300,000 tons per year (Rowland, 1997).
Most molecules that are propelled into the atmosphere are removed within a few hours to a few weeks by three general processes (Rowland, 1997). The first process is called photolysis, which is when the molecule absorbs sunlight. The second process is dissolution in water, or rainout. The final way molecules are removed is through a reaction with the hydroxyl radical (HO) or ozone, a process called oxidation. CFC molecules differ from most molecules in that they are transparent to sunlight, are insoluble in water, and are chemically inert to oxidizing agents. Therefore the aforementioned processes will not remove them from the atmosphere. The molecules can remain in the lower atmosphere for a long time before being pushed into the stratosphere by powerful storms at the equator.
A British scientist named Jim Locklove used an electron capture detector (ECD), a device he invented, to test for the refrigerant CFC-11. Using air samples collected in Ireland, he showed that CFC-11 was present at a level of approximately 50 parts per trillion. Locklove then boarded a ship to test the air from England to Antarctica. This testing proved that CFC-11 was present everywhere in the atmosphere (Rowland, 1997).
In a paper published in 1974 in Nature magazine, Nobel Laureates F. Sherwood Rowland, Ph.D. and Mario Molina, Ph.D. hypothesized that molecules carrying chlorine and bromine up to the stratosphere were destroying the ozone at that level (Dugard, 1997). This article started the concern over stratospheric ozone. Molina and Rowland also estimated the lifetime of a CFC-11 molecule in the atmosphere to be 40 to 80 years, and 75 to 150 years for a CFC-12 molecule. Further studies proved the lifetimes to be 50 years for CFC-11 and 100 years for CFC-12 (Rowland, 1997).
CFCs and Ozone
In the stratosphere, CFC molecules absorb an intense, highly energetic solar ultraviolet radiation called UV-C (Rowland, 1997). UV-C is not present near the Earth's surface because it< is absorbed by the ozone layer in the stratosphere. The CFC molecule is destroyed as it absorbs the UV-C. The chemical equation for the reaction with CFC-11 is:
CCl3F + UV => Cl + CCl2F
The free chlorine atom then attacks an ozone molecule, and the reaction yields a chlorinemonoxide molecule (ClO) and a diatomic oxygen molecule:
Cl + O3 => ClO + O2
After destroying the ozone molecule, the chlorine monoxide molecule will react with a free oxygen atom to form a free chlorine atom and one diatomic oxygen molecule:
ClO + O => Cl + O2
These last two chemical processes can take place 100,000 times for each chlorine atom that is freed from a CFC molecule. Given the natural destruction of ozone, coupled with the destruction from man-made compounds, the average lifetime of an ozone molecule is reduced from eleven days to nine or ten days. Bromine atoms carried into the stratosphere in halon molecules go through the same type of process as chlorine atoms. The difference is that bromine is ten times more powerful an ozone destroyer than chlorine.
The Ozone Hole is Discovered
A group of scientists from the British Antarctic Survey measured the ozone level in Antarctica. The team, headed by Joseph Farman, took these measurements yearly at Halley Bay. In the spring of 1981 (September in Antarctica) they noticed the ozone level dropped by 20%. The ozone level then increased in the following months. In 1985 the ozone level decreased 50% in the spring. Farman and his group are credited with discovering the ozone hole over Antarctica. While it is called a hole, the ozone hole is actually a thinning of the ozone layer over the South Pole (Somerville,1996).
Written March 11, 1998
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