Different CFCs produce different effects of ozone destruction
Different CFCs produce different effects of ozone destruction
By Kardono
JAKARTA (JP): Fluorinated chlorocarbons were first developed in 1930 by the General Motor Research Laboratories in a search for a non-toxic and non-flammable refrigerant to replace sulfur dioxide and ammonia then in use. Dichlorodifluoromethane, a compound consisting of one carbon atom, two fluorine atoms and two chlorine atoms, is a typical member of the class of fluorinated chlorocarbons, and is identified as CFC-12. The word freon is actually a trade name for the CFCs which are produced by Du Pont, U.S.A. Also, we sometimes hear the word "Areton" which is the same trade name for the CFCs developed by ICI, United Kingdom.
CFCs are inert materials, especially in the troposphere, the lowest layer of atmosphere, at about 11 kilometers above the earth's surface. Their chemical inertness has made the CFCs valuable as aerosol propellants, as blowing agents for plastic foam production, and as solvents, in addition to their use as refrigerants.
Vital
Refrigerants are the vital working fluids in refrigeration systems. They absorb heat from a place where it is not wanted and dispose of it in another area. Heat is removed from the system by evaporating the liquid refrigerant; heat is disposed of by condensing the refrigerant vapor. Refrigeration system equipment provides liquid refrigerant to the place where cooling is desired. Because of these properties, refrigerants help people to enjoy life. You can imagine working in a tall office building or driving during the dry season without air conditioning. All those comforts are possible thanks to the work of the CFCs.
However, the use of CFCs ultimately leads to an atmospheric release, as even hermetically sealed refrigerators and closed- cell foams finally leak into the air. About 90 percent of all the CFC-11 and CFC-12 produced is believed to have been released.
Ozone destroyer
It was in 1973 that the presence of halogenated hydrocarbons in the troposphere was reported. It soon became apparent that the quantities of the CFCs are about equal to the total amount ever manufactured. The tropospheric inertness of the CFCs was thus confirmed, and lifetimes of hundreds of years were indicated. Only one escape route is possible for the compounds, i.e. transport to the stratosphere, the second layer of the atmosphere, which is approximately between 18 and 50 kilometers above the earth's surface. In this stratospheric layer, the photolytic process for CFCs by ultraviolet radiation occurs -- that is, a dissociation of one chlorine atom from the CFC molecule. CFC-12, more popularly known as Freon-12, which consists of one carbon atom, two fluorine atoms and two chlorine atoms, will undergo ultraviolet photolysis in the stratosphere with the result of a single chlorine atom separated from the molecule. Space shuttle-generated chlorine was initially predicted as a source of chlorine atoms in the stratosphere. However, after the discovery of the ultraviolet photolysis of the CFCs mentioned above, it was quick to be seen that the chlorine atoms from the photolytic process were much more dominant.
The presence of chlorine atoms in the stratosphere is considered dangerous to the existence of ozone. Ozone, a molecule consisting of three oxygen atoms written structurally as O3, is a very unstable compound and is formed photochemically from the oxygen molecule (O2). It has an ability to absorb solar ultraviolet radiation which is liberated as heat. If there were no ozone layers in the stratosphere, the solar ultraviolet radiation would be transmitted to the earth's surface. Such radiation is lethal to simple unicellular organisms, and to the surface cells of higher plants and animals. Ultraviolet radiation in the wavelength ranges between 230 and 290 nanometers (so- called UV-B), is also biologically active, and prolonged exposure to it may cause skin cancer in susceptible individuals. Therefore, any threat that could reduce the ozone layers in the stratosphere should be avoided.
As mentioned earlier, the presence of chlorine atoms in the stratosphere will affect the existence of the ozone layers through their action as catalyst. First, a chlorine atom combines with ozone to produce a molecule of oxygen (O2) and a molecule of chloromonoxide (ClO). In the following step, chloromonoxide will react with a single oxygen atom (O), which is available in abundance in the stratosphere to produce an oxygen molecule (O2) and a chlorine atom (Cl). The final reaction is from the combination of ozone (O3) and a single oxygen atom (O) to two oxygen molecules (2O2). This means that in this reaction there is ozone degradation on the one hand and oxygen molecule development on the other.
Let us identify the different categories of Freon. Freon is usually followed by a coded two or three digit number, such as Freon-11, Freon-12, Freon-115, etc. The hundred digit is the number of carbon atoms in the molecules minus one. The ten is the number of hydrogen atoms plus one. The unit is the number of fluorine atoms, and the residue is devoted to chlorine atoms. If the first digit is zero, it is dropped. For example, Freon-11 is originally Freon-011. The hundredth digit of Freon-011 is zero and adding zero to one is one, that is the number of the carbon atom. The second digit is one and subtracting one from one is zero, which is the number of hydrogen atoms in this molecule. The unit is devoted to the number of fluorine atoms, which is one. So far, there is one carbon atom which has four hands, no hydrogen atom, and only one fluorine atom with one hand. Therefore, there are still three hands left unconnected to the carbon atom. As a rule, these three hands should be connected to chlorine atoms which have one hand each, and as a result the number of chlorine atoms should be three. Thus, Freon-11 consists of one carbon atom, one fluorine atom and three chlorine atoms, structurally written as CFCl3. Similarly, Freon-12 consists of one carbon atom, two fluorine atoms and two chlorine atoms or CF2Cl2; Freon 115 consists two carbon atoms, five fluorine atoms and one chlorine atom or CF3CF2Cl.
Different types of chlorofluorocarbons (CFCs) will produce different effects of ozone destruction. For equal concentration Freon-11 has a maximum photolysis rate at about 25 km high, Freon-12 at 32 km, while Freon-115 does not produce its maximum contribution until 40 km high. Thus, the more heavily chlorinated halocarbons are, the more active in destroying the ozone they are for two reasons. First, they are photolyzed at lower attitudes where their absolute impact is greater. Secondly, they can release more chlorine atoms per molecule as a catalyst in ozone destruction.