Banned pollutants still in use locally
By Dephanie Yeung
January 1996 was the deadline for banning chlorofluorocarbons, or CFCs, in the countries that signed the Copenha- gen Amendment in 1992. Although a year has passed, CFCs are still being used in Hong Kong and elsewhere.
In fact, everyday life relies heavily on CFCs.
"Nowadays you can't avoid using CFCs. There are no real and complete substitutes for them," said Mr. Allan S. C. Cheung, a senior lecturer of the Department of Chemistry at the University of Hong Kong.
"CFCs belong to a family of molecules which contain chlorine, fluorine and carbon," said Mr. Cheung. "And they serve as blowing media or solvents for cleaning purposes."
As blowing media, CFCs are compressed to liquids in a container such as a can of hair spray. Pressing the nozzle releases the pressure inside. Heated by the room temperature, CFCs instantly expand to become gases and blow other compounds in the hair spray out of the nozzle.
Added Mr. David Lee Phillips, assistant professor in the Department of Chemistry at the University of Hong Kong: "After the CFCs have come out, they have no use."
CFCs also serve as a refrigerant in air conditioners and refrigerators. When cooling, pressure is exerted on liquid. As it expands into gaseous state, it takes up heat from the surroundings and cools the environment.
"When we destroy or recycle fridges and air conditioners, CFCs are released into the atmosphere," said Mr. Cheung.
CFCs are inert gases, meaning they do not react chemically with other substances. So CFCs last a long time in the outermost reaches of the earth's atmosphere, the stratosphere.
"The life span of CFCs is 70 to 200 years," said Mr. Cheung.
"The chlorine atoms in CFCs are catalysts in chemical reactions," said Mr. Cheung. "This means chlorine is involved in chemical reactions, but it is not consumed. You still have it at the end of the reaction."
Said Mr. Phillips: "A single chlorine atom can destroy millions of ozone molecules."
"A lot of ozone molecules are being produced and destroyed in a regular cycle to reach an equilibrium. Now the destruction of ozone molecules is beyond the natural cycle due to the release of CFCs," said Mr. Cheung.
Scientists have experimented with different techniques to substitute for CFCs. There are mainly two types of substitutes: replacement of all the chlorine atoms in CFCs by hydrogen, or partial replacement of chlorine by hydrogen. This decreases the so-called ozone depletion potential. ODP is a measurement of the potential of different compounds to destroy the ozone layer.
Hydrogen is used because it is not reactive and nonflammable, has low toxicity and ODP rates, and is cheap.
"However, the substitutes for CFCs only serve 80 to 90 percent of the original function of CFCs. They are less effective and efficient," said Mr. Cheung.
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