Fully halogenated CFCs

In 1976 the United States banned the use of CFCs as aerosol propellants. No further steps were taken until 1987 when the United States and some 50 other countries adopted the Montreal Protocol, specifing a 50% reduction of fully halogenated CFCs by 1999. In 1990, an agreement was reached among 93 nations to accelerate the discontinuation of CFCs and completely eliminate production by the year 2000. The 1990 Clean Air Act Amendments contain a phaseout schedule for CFCs, halons, carbon tetrachloride, and methylchloroform. Such steps should stop the iacrease of CFCs ia the atmosphere but, because of the long lifetimes, CFCs will remain ia the atmosphere for centuries. 

The phasing-out of fully halogenated CFCs is a success story of international cooperation in ameliorating an environmental hazard created by anthropogenic activities. In part, this success was achieved because chemical manufacturers found ready substitutes for the fully halogenated CFCs, the partially halogenated hydrochloro-fluorocarbons (HCFCs), for example HCFC-22,... 

Fully halogenated CFCs were eliminated in polystyrene foam food packaging and containers. Substitute blowing agents used are either no threat to the ozone or are a 95% improvement over fully halogenated CFCs. Action has been taken such as where PS foam cups now are 100% CFC-free, etc. 

Unlike the simple fully halogenated CFC s which can only be made in a single step, there are many potentially viable routes to the alternatives, several of which involve a catalytic hydrogenation step [4]. 

The primary atmospheric removal processes for halocarbons are photolysis and reaction with tropospheric hydroxyl radicals (OH). For the fully halogenated CFCs and halons, photolysis is the only important sink and their atmospheric lifetimes are dependent on their absorption cross-sections, the solar flux, and the surface to stratosphere transport time. As a general rule, the greater the number of Cl, Br, or I atoms on any one carbon atom, the larger the cross-section and the shorter the lifetime. For example, the lifetimes of CCIF3 (CFC-13), CCI2F2... 

In order to enable the fast discontinuation of the production and use of fully halogenated CFCs, hydro-chlorofluorocarbons were introduced in industry as a transition stage. The atmospheric fate and impact of these hydrochlorofluorocarbons and chlorinated solvents are described in [387]. The authors come to the conclusion, that these compounds, with the exception of 1,1,1-trichIoroethane, make a small or insignificant contribution to the stratospheric ozone depletion, global warming, photochemical smog , acid rain or chloride and fluoride levels in precipitations. The ozone depletion potentials are 10 to 50 times lower than that of CFCll or CFC12, mainly as a consequence of their shorter atmospheric lifetime—some months to 10 years—due to destruction in the atmosphere. 

The oxidation scheme for halomethanes not containing a hydrogen atom is similar to that for those which do, except that it is not initiated by tropospheric reaction with hydroxyl radicals, since the fully halogenated methanes are unreactive. Consequently, substantial amounts of CFCs and halons are transported intact up into the stratosphere, where they absorb UV radiation of short wavelength and undergo photodissociation (equation 36) to a halogen atom and a trihalomethyl radical. The halogen atom Y may enter into catalytic cycles for ozone destruction, as discussed in the introduction. 

International agreements (Montreal Protocol in 1987 and subsequent amendments), as well as national regulations, have strongly limited the production and the use of the CFCs. These chemical compounds have been gradually replaced by partially halogenated hydrocarbons, and specifically by hydrochlorofluorocarbons (HCFCs) and hydrofluorocarbons (HFCs). These alternative products are relatively easily destroyed in the troposphere and hence their lifetimes are substantially shorter than those of the CFCs (typically 1-10 yrs as opposed to 10-100 yrs). The ozone depletion potential of the HCFCs is about an order of magnitude smaller than that of the fully halogenated halocarbons. HFCs are not a threat to ozone because they do not contain any chlorine or bromine atoms. 

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