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.