Different CFCs produce different effects of ozone destruction<\/p>\n
By Kardono<\/p>\n
JAKARTA (JP): Fluorinated chlorocarbons were first developed
\nin 1930 by the General Motor Research Laboratories in a search
\nfor a non-toxic and non-flammable refrigerant to replace sulfur
\ndioxide and ammonia then in use. Dichlorodifluoromethane, a
\ncompound consisting of one carbon atom, two fluorine atoms and
\ntwo chlorine atoms, is a typical member of the class of
\nfluorinated chlorocarbons, and is identified as CFC-12. The word
\nfreon is actually a trade name for the CFCs which are produced by
\nDu Pont, U.S.A. Also, we sometimes hear the word "Areton" which
\nis the same trade name for the CFCs developed by ICI, United
\nKingdom.<\/p>\n
CFCs are inert materials, especially in the troposphere, the
\nlowest layer of atmosphere, at about 11 kilometers above the
\nearth's surface. Their chemical inertness has made the CFCs
\nvaluable as aerosol propellants, as blowing agents for plastic
\nfoam production, and as solvents, in addition to their use as
\nrefrigerants.<\/p>\n
Vital<\/p>\n
Refrigerants are the vital working fluids in refrigeration
\nsystems. They absorb heat from a place where it is not wanted and
\ndispose of it in another area. Heat is removed from the system by
\nevaporating the liquid refrigerant; heat is disposed of by
\ncondensing the refrigerant vapor. Refrigeration system equipment
\nprovides liquid refrigerant to the place where cooling is
\ndesired. Because of these properties, refrigerants help people to
\nenjoy life. You can imagine working in a tall office building or
\ndriving during the dry season without air conditioning. All those
\ncomforts are possible thanks to the work of the CFCs.<\/p>\n
However, the use of CFCs ultimately leads to an atmospheric
\nrelease, as even hermetically sealed refrigerators and closed-
\ncell foams finally leak into the air. About 90 percent of all the
\nCFC-11 and CFC-12 produced is believed to have been released.<\/p>\n
Ozone destroyer<\/p>\n
It was in 1973 that the presence of halogenated hydrocarbons
\nin the troposphere was reported. It soon became apparent that the
\nquantities of the CFCs are about equal to the total amount ever
\nmanufactured. The tropospheric inertness of the CFCs was thus
\nconfirmed, and lifetimes of hundreds of years were indicated.
\nOnly one escape route is possible for the compounds, i.e.
\ntransport to the stratosphere, the second layer of the
\natmosphere, which is approximately between 18 and 50 kilometers
\nabove the earth's surface. In this stratospheric layer, the
\nphotolytic process for CFCs by ultraviolet radiation occurs --
\nthat is, a dissociation of one chlorine atom from the CFC
\nmolecule. CFC-12, more popularly known as Freon-12, which
\nconsists of one carbon atom, two fluorine atoms and two chlorine
\natoms, will undergo ultraviolet photolysis in the stratosphere
\nwith the result of a single chlorine atom separated from the
\nmolecule. Space shuttle-generated chlorine was initially
\npredicted as a source of chlorine atoms in the stratosphere.
\nHowever, after the discovery of the ultraviolet photolysis of the
\nCFCs mentioned above, it was quick to be seen that the chlorine
\natoms from the photolytic process were much more dominant.<\/p>\n
The presence of chlorine atoms in the stratosphere is
\nconsidered dangerous to the existence of ozone. Ozone, a molecule
\nconsisting of three oxygen atoms written structurally as O3, is a
\nvery unstable compound and is formed photochemically from the
\noxygen molecule (O2). It has an ability to absorb solar
\nultraviolet radiation which is liberated as heat. If there were
\nno ozone layers in the stratosphere, the solar ultraviolet
\nradiation would be transmitted to the earth's surface. Such
\nradiation is lethal to simple unicellular organisms, and to the
\nsurface cells of higher plants and animals. Ultraviolet radiation
\nin the wavelength ranges between 230 and 290 nanometers (so-
\ncalled UV-B), is also biologically active, and prolonged exposure
\nto it may cause skin cancer in susceptible individuals.
\nTherefore, any threat that could reduce the ozone layers in the
\nstratosphere should be avoided.<\/p>\n
As mentioned earlier, the presence of chlorine atoms in the
\nstratosphere will affect the existence of the ozone layers
\nthrough their action as catalyst. First, a chlorine atom combines
\nwith ozone to produce a molecule of oxygen (O2) and a molecule of
\nchloromonoxide (ClO). In the following step, chloromonoxide will
\nreact with a single oxygen atom (O), which is available in
\nabundance in the stratosphere to produce an oxygen molecule (O2)
\nand a chlorine atom (Cl). The final reaction is from the
\ncombination of ozone (O3) and a single oxygen atom (O) to two
\noxygen molecules (2O2). This means that in this reaction there is
\nozone degradation on the one hand and oxygen molecule development
\non the other.<\/p>\n
Let us identify the different categories of Freon. Freon is
\nusually followed by a coded two or three digit number, such as
\nFreon-11, Freon-12, Freon-115, etc. The hundred digit is the
\nnumber of carbon atoms in the molecules minus one. The ten is the
\nnumber of hydrogen atoms plus one. The unit is the number of
\nfluorine atoms, and the residue is devoted to chlorine atoms. If
\nthe first digit is zero, it is dropped. For example, Freon-11 is
\noriginally Freon-011. The hundredth digit of Freon-011 is zero
\nand adding zero to one is one, that is the number of the carbon
\natom. The second digit is one and subtracting one from one is
\nzero, which is the number of hydrogen atoms in this molecule. The
\nunit is devoted to the number of fluorine atoms, which is one. So
\nfar, there is one carbon atom which has four hands, no hydrogen
\natom, and only one fluorine atom with one hand. Therefore, there
\nare still three hands left unconnected to the carbon atom. As a
\nrule, these three hands should be connected to chlorine atoms
\nwhich have one hand each, and as a result the number of chlorine
\natoms should be three. Thus, Freon-11 consists of one carbon
\natom, one fluorine atom and three chlorine atoms, structurally
\nwritten as CFCl3. Similarly, Freon-12 consists of one carbon
\natom, two fluorine atoms and two chlorine atoms or CF2Cl2; Freon
\n115 consists two carbon atoms, five fluorine atoms and one
\nchlorine atom or CF3CF2Cl.<\/p>\n
Different types of chlorofluorocarbons (CFCs) will produce
\ndifferent effects of ozone destruction. For equal concentration
\nFreon-11 has a maximum photolysis rate at about 25 km high,
\nFreon-12 at 32 km, while Freon-115 does not produce its maximum
\ncontribution until 40 km high. Thus, the more heavily chlorinated
\nhalocarbons are, the more active in destroying the ozone they are
\nfor two reasons. First, they are photolyzed at lower attitudes
\nwhere their absolute impact is greater. Secondly, they can
\nrelease more chlorine atoms per molecule as a catalyst in ozone
\ndestruction.<\/p>",
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