Menace of CFCs: ?Where are we heading to?

General

Chlorofluoroalkanes [cfcs] were originally developed in the 1920s for use as refrigerant fluids. Cfcs (also known as Freon) are a family of chemicals based upon hydrocarbon skeletons (most often methane), where some or all of the hydrogens have been replaced with chlorine and/or fluorine atoms. These compounds are non-flammable, tasteless and odourless, and chemically stable. Their other important property is their volatility, having boiling points close to zero degrees centigrade. These physical properties make them ideal for use as refrigerant gases in air conditioners, freezers and refrigerators. Their nontoxic and non corrosive character made them much safer compared to other refrigerants, Sulphur dioxide and ammonia. They also found application as blowing agents in the manufacture of thermal foam insulation for buildings and foamed plastic cups and food packaging.

As more and more uses were found for these remarkable compounds, cfcs became big business, with hundreds of thousands tonnes being produced yearly. Now they are being phased out. These stable, nontoxic compounds are dangerous.

During all the time that the use of cfcs was increasing, no one thought about what would happen to the gases in the atmosphere. Because of their lack of reactivity and insolubility in water, there is no natural process for removing cfcs. In fact they drift up into the stratosphere , upper atmosphere, 25 to 50 km from the surface of the earth where they receive sufficient ultraviolet [UV] light to cause photolysis.  

Ozone depletion in the stratosphere

It is ironic that one of the properties that make cfcs so suitable for use in household items such as refrigerators, their chemical stability, should be the cause of their undoing. Most chemicals, when released into the atmosphere get rapidly broken down into smaller, harmless components by reactions in the lower atmosphere. The cfcs, however, are so stable and unreactive that they survive to reach the highest levels of the atmosphere, and become globally distributed in the stratosphere. At these high altitudes, the intensity of ultra-violet radiation is so great that even the stable cfcs are split apart to release a chlorine atom [radical].

CFCl3 [cfc]  ------- UV radiation ----  > CFCl2  +  Cl

CF2Cl2 [cfc]  ------- UV radiation ----  > CF2Cl  +  Cl

It is the atomic chlorine that does the damage by consuming ozone, since it can react with ozone (O3) to form oxygen at the stratosphere.

Cl + O3 ------ > ClO  +  O2

ClO  +  O3 ------ >  Cl  +  2O2

 2O3 ------ >  3O2  [overall reaction]

The Cl atom is regenerated in this reaction, and so the breakup of only one CFC molecule can initiate the subsequent removal of thousands of ozone molecules.

The rise in cfc concentration in the stratosphere will deplete the ozone layer, decreasing its ability to absorb UV light and increasing the incidence of skin cancer.

There are two ways in which cfcs may affect the climate. One is by depleting the ozone layer. Because  the ozone warms the stratosphere, a decrease in its thickness should cause a drop in the stratosphere temperature. What effect this would have on the troposphere, lower atmosphere, is not known: the relationship between troposphere and stratosphere is poorly understood.  A second effect of cfc is due to their strong  absorption of infrared radiation given off by the earth, cfcs contribute to the greenhouse effect. Cfcs are 10,000 times more effective than carbon dioxide in absorbing infrared radiation.

Alternate to cfcs

Alternate to cfcs have been developed, they include, hydrohalocarbons with at least one C – H bond per molecule, e.g., hydrochlorofluorocarbons, HCFs, such as CHCl2CF3, hydrofluorocarbons, HFCs, such as CF2FCF3, perfluorocarbons , which are completely fluorinated alkanes, e.g., CF4.

The notable features of these alternate molecules are as follows.

HCFCs destroy one third of the amount of ozone destroyed by cfcs. HFCs cause no damage to ozone layer. Although they are greenhouse gases.

Perfluorocarbons do not react with ozone. They are not involved in ozone depletion or photochemical SMOG formation. Their life time has been estimated as tens of thousands of years and they may add to greenhouse warming. Their very long life time makes this a serious threat.

SO, effectively no hundred percent solution.

What Governments are doing about Ozone Depletion?

194 nations, have signed an international agreement to end the production of chlorofluorocarbons (CFCs), halons and other ozone-depleting substances (ODS). The agreement is called the Montreal Protocol on Substances that Deplete the Ozone Layer (1987). The protocol has been amended several times, to speed up ODS phase out dates and to include more types of ODS.

Only, if all nations comply with the terms of Montreal Protocol, chlorine levels in the stratosphere will decline to a safe level of 2 ppb sometime in the middle of the 21st century