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CFC Replacements:- HCFCs and HFCs

    In order to phase out the production of CFCs alternative chemicals that didn't cause such severe damage to the environment had to be found. 

 

Hydrochlorofluorocarbons

    Hydrochlorofluorocarbons are one type of chemical that is being used as a substitute for CFCs. They are being used as foam blowing agents, solvents and refrigerants. However these chemicals still contain chlorine and consequently have ozone-depleting potential and they will contribute to global warming. The ozone-depleting potential of HCFCs is estimated at being between 1 and 12% of CFC-11. It has been decided that the production of HCFCs is to be phased out by 2030 and limited from 1996. At present the mixing of HCFCs in the atmosphere is relatively low but this is hard to assess accurately due to there still being a high abundance of CFCs in the atmosphere. HCFCs have shorter lifetimes than CFCs as they are removed in the troposphere by reaction with OH radicals. 

    However it has been discovered that the stratospheric lifetime of HCFCs is very high. HCFC-142b has a lifetime of 21 years but in the stratosphere it is 121 years. As a result HCFCs have much higher ozone depleting potentials than originally thought. HCFCs have been moving from the troposphere to the stratosphere, across the tropopause in small air parcels. Continued use of HCFCs and growth in the atmosphere will complicate the situation in the stratosphere still further.

 

Hydrofluorocarbons

    Hydrofluorocarbons are the second main ‘drop-in’ substitute for CFCs currently being employed. HFCs are currently being viewed as a more long-term replacement than HCFCs. This is because these chemicals have an ozone depleting potential of zero as they contain no chlorine. It has also been established that CF3, found in many HFCs has a negligible ozone-depleting potential. However it is still uncertain as to whether there are other problems arising from the use of these compounds. One of the major concerns is linked with the degradation products of HFCs. The mechanisms for loss of HFCs in the atmosphere include decomposition and loss with O2. One of the major by-products produced is Trifluoroacetate, (trifluoroacetic acid). This is a stable by-product but there are concerns about accumulation in certain aquatic environments and the effect this will have on aquatic organisms. It is thought that Trifluoroacetate will build up in wetlands where there is poor drainage, such as in marshes. Detailed three-dimensional models of Trifluoroacetate in the environment have been done, but it is still uncertain how closely concentrations are linked to emissions of HFCs. The situation is complicated further by Trifluoroacetate being produced from degradation of HCFCs. There is also recent evidence that Trifluoroacetate is produced from other sources apart from HFCs and HCFCs. One alternative source of Trifluoroacetate is 3-trifluoromethyl-4-nitrophenol, which is used to control the sea lamprey in four of the Great Lakes.

HFCs and HCFCs both contribute to global warming by absorbing long wave radiation. Model calculations to establish the global warming potential of CFC replacements have been done. The global warming potentials of these compounds have been found to be about an order of magnitude less than the CFCs that they replace. This is mainly due to their lifetimes being a lot shorter. The contribution to global warming by these chemicals depends on the concentrations present in the atmosphere. Currently there are no restrictions on the use of HFCs and if high concentrations are allowed to build up in the atmosphere in the future, then significant radiation absorption's will result. It is difficult to forecast the contributions to global warming of these chemicals and it is quite likely there will be deviations from the global warming predictions.

 

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