Reported by : Karen Andrea Weitemeyer December 1, 1998
CFCs are members of the halocarbon family; which consist of a chemical compound of the element carbon, and one or more of the halogens bromine, iodine, chlorine, or fluorine (Encyclopedia Brittannica, 1982). CFCs were first produced in 1928 by Thomas Midgley of the Frigirdaire Division of General Motors, to obtain a safer substitute to the previous toxic coolants such as ammonia and sulfur dioxide used in refrigeration and cooling systems. Since then CFCs have been used domestically and industrially in refrigeration, air conditioning, aerosol sprays and in many other applications. Due to CFCs desirable properties for use in these industries little was thought of the eventual presence in the stratosphere and the resulting consequences. However, in 1974, Rowland and Molina, released a report about the problems CFCs posed on ozone depletion and the Greenhouse effect (Roan, 1989). This report will discuss the various problems with CFCs.
INDUSTRIAL AND DOMESTIC USES
The commercial production of CFCs began in 1931 by E.I.du Pont de Nemours & Co.(Buxton, 1989). In particular CFC-12 was produced for the replacement of obsolete cooling systems. By 1940, Dow Chemical used CFCs as a blowing agent in the production of an insulator called Styrofoam (Buxton, 1989).This further increased the industrial demand in using CFCs. CFC propelled aerosol sprays where first used in the second world war as pesticides to control mosquitoes which caused Malaria (Buxton, 1989). These same propellants were once again sought after for aerosol spray products such as hairspray and toiletry products. The commercial boom of these products increased the production of CFCs further (Buxton, 1989). With more consumerism, industries such as refrigeration, air conditioning, propellants for aerosol sprays, blowing agents in foam and furniture production , cleaning fluids and solvents in electronics and precision cleaning (see table 2), demanded an increase in the use of CFCs up to the mid 1980s.
TABLE 2: Uses of CFCs (Buxton, 1989)
CFC-12 aerosols, refrigeration, air conditioning, polyurethane foams
CFC-11 aerosols, air conditioning
CFC-113 solvent in electronics and precision cleaning
CFC-114 commercial chillers propellant in hairsprays, deodorants, spray paints
Refrigeration, Air conditioners, heat pumps:
CFCs are used as the gas which is compressed and expanded in these systems. Thus, when CFC gas is compressed, it heats up and when CFC gas is expanded, it cools (Zumdahl, 1995); thus, CFCs are used in the vapor compression cycle. They were desirable because of their non-flammability, low toxicity, thremophysical properties and normal boiling point (CCME, 1992).
Rigid and Flexible Foams:
CFC-11, 12, 113, 114 are used in the production of plastics, building and application insulation, cushion foams, packaging materials, flotation devices, and shoe soles (Buxton 1989, p.58). In 1986 the foam industry was responsible for the production of 25-30% of the total global CFC (Buxton 1989). Once again CFCs were used as a blowing agent because of there chemical properties: characteristic boiling point, vapor pressure, low toxicity, non-flammable, non reactive, and low thermal conductivity. This allowed foams with CFCs to have an enhanced insulating ability (Buxton 1989). The use of CFCs also allowed manufacturers to create flexible and rigid foam.
Electronics and dry Cleaning Solvents:
CFC-113 is used because of its low flammability and low toxicity, it is used to clean electronics, delicate instruments, cloths, and fabrics, and metals. The electronics industry use CFC-113 to remove flux after soldering without warming solvent-sensitive components on the circuit board which allows for better electrical performance by coating it and preventing corrosion and electro-migration (Buxton 1989). Precision cleaning uses CFC-113 to clean instruments like gyroscopes, computer disk drives, and optical components (Buxton 1989). CFC-113 is an excellent solvent, degreasing agent with low toxicity, is nonflammable, low surface tension, high stability.
The numerous industrial uses for CFCs generates an enormous release and leakage of CFCs into the atmosphere. When Rowland and Molina discovered the effect of CFCs in the upper atmosphere in 1974, government regulations and in particular, the Montreal protocol sought and continues to plan for the eventual removal of CFC in industrial usage. However, it will be quite some time before CFCs will no longer have a major influence in stratospheric chemistry as the expected life time of CFCs in the atmosphere is at least one century (see table 3).
TABLE 3: Lifetime of CFCs (Prather et al., 1990)
CFC-11 CFCl3 60 years
CFC-12 CF2Cl2 120 years
CFC-113 C2F3Cl3 90 years
CFC-114 C2F4Cl2 200 years
CFC-115 C2F5Cl 400 years
CHEMISTRY OF CFCs IN THE STRATOSPHERE
CFCs are released into the atmosphere where they accumulate. Their accumulation is due to their stability and their inability to be removed by the usual physiochemical cleaning mechanisms that remove trace impurities from the atmospheric and tropospheric sinks; such as solar photolysis, rainout or oxidation (Rowland, 1991, and Molina, 1996). Unfortunately, this allows CFCs to mix and be dispersed up into the stratosphere, which is where CFCs decompose and release halogens which will affect the ozone layer (Molina, 1996). Essentially, CFCs accumulate in the middle stratosphere at approximately 25km altitude (Rowland, 1991) and photodissociate by short wavelength solar UV radiation, in the 190 to 225 nm range (Drake, 1995). These wavelengths will not penetrate passed the lower stratosphere because oxygen and ozone absorbs these wavelengths; which means CFCs can only be photochemically decomposed in the stratosphere and not below it, For example in CCl2F2 is broken down in the stratosphere by the following:
CCl2F2 + UV (< 220nm) => Cl + CClF2 (1)
this reaction with UV radiation will continue for the entire CFC molecule until the it has decomposed releasing chlorine, fluorine and carbon. The release of the chlorine atom will effect the ozone layer significantly.
The stratosphere contains about 90% of the total atmospheric ozone (Molina, 1996). Ozone is continuously being created and destroyed through the Chapman cycle:
Formation:
O2 + UV(200nm) => O + O (2)
O + O2 + M => O3 + M (3)
Destruction:
O3 + UV => O2 + O (4)
O + O3 => O2 + O2 (5)
O + O +M => O2 + M (6)
where M is a catalyst (Drake, 1995 , Rowland, 1991, and Molina, 1996). Each of these chemical reactions cycle into each other. Reaction 3 dominates which allows for a net gain in ozone levels (Molina, 1996). However, when Chlorine atoms are introduced into the atmosphere naturally or from anthropogenic sources (see table 4)
TABLE 4: Sources of Chlorine
CFC-11 22%
CFC-12 25%
CFC-113 3%
CFC-114 <1%
CFC-115 <1%
HCFC-22 3%
carbon tetrachloride 13%
methyl chloroform 13%
Natural Sources **
NaCl, HCl from oceans
CH3Cl from biologic activity and biomass burning 20%
* Most of the natural sources disappear before reach stratosphere therefore most Chlorine comes from human activities.
**Most of these sources are removed by rainfall long before they reach the stratosphere.
it will promote the destruction of the ozone cycle through the following reactions:
Cl + O3 => ClO + O2 (7)
this occurs within a half a second(Rowland, 1991), and is followed by
ClO + O => Cl + O2 (
which takes a few minutes (Rowland, 1991) and all the while you have the reaction which occurs naturally with ozone and UV radiation as seen in equation 5. This gives a net effect of:
2O3 => 302 (9)
(Molina, 1996). Reactions 7 and 8 contain the ClOx chain which is a free radical catalytic chain reaction which repeats over and over again before it stops (Rowland, 1991). This makes the ClOx chain a very efficient ozone destroyer. The altitudes of predominate ozone depletion from chlorine occurs predominately between 35 to 45 kilometers into the stratosphere and some lower at about 20 km altitude (Drake, 1995). However, some of the chlorine released will bond with some other chemicals found within the stratosphere, such as CH4 and NO2, to produce a temporary inert reservoir of Chlorine, such as HCl , ClNO2, HOCl, by these reactions:
ClO + HO2 => HOCl + O2 (10)
ClO + NO2 + M => ClONO2 + M (11)
Cl + CH4 => HCl + CH3 (12)
(Rowland, 1991, and Molina, 1996).These reservoirs will eventually break down through reactions with other free radicals or through the absorption of solar radiation which releases chlorine to catalytically become a component in depleting ozone again (Molina, 1996). In some cases the reservoir will only last one hour and chlorine will be released by photolysis of ClONO2 and HCl (Rowland, 1991).
Fluorine is also released in the breaking apart of CFCs. This chemical will rapidly react with Hydrogen to form HF which is a permanent fluorine reservoir. Thus, it will hardly ever be a free radical and thus a component in aiding in the depletion of ozone (Molina ,1996).
PROBLEMS
CFCs have two main effects on the atmosphere, ozone depletion and the Greenhouse effect, which are two serious problems.
Ozone depletion has greatest influence in the Arctic and Antarctic . Chlorine released from the destruction of CFCs has a predominate role as the catalyst in these high latitudes. The inert reservoir of chlorine can exist in the high latitudes. In the winter months over Antarctica the poles stratospheric air is trapped without sunlight, and temperatures range from -85oC to -90oC (Rowland, 1991). At this temperature clouds are precipitated, called polar stratospheric clouds. These clouds have crystals of Nitric acid and trihydrate, which allows for a heterogeneous nitrogen and chlorine chemistry (Rowland, 1991). The N2O5 is converted into nitric acid and is trapped in the clouds (Rowland, 1991). Thus, the air has no nitrogen oxides, which frees up the chlorine reservoir and chlorine is released into the air to react with ozone. Other chlorine reservoirs are similarly trapped in the polar stratospheric clouds and also release chlorine through the following examples:
HCl + ClONO2 => Cl2 + HNO3 (13)
ClONO2 + H2O => HOCl + HNO3 (14)
HOCl + HCl => Cl2 + H20 (15)
(Buxton, 1989 and Rowland, 1991).
In the spring time sunlight hits the Cl2 and HOCl to photodissociate and release atomic Cl which attacks the ozone. Because there is no NO2 and no ClNO2 the concentration of the ClO chain increases allowing for larger amounts of reactions with ozone, as in the following reactions:
2[Cl + O3 => ClO + O2] (16)
ClO + ClO => ClOOCl (17)
ClOOCl + UV => Cl + ClO2 => 2Cl + O2 (18)
(Molina, 1996). This has a net effect of ozone becoming an oxygen molecule as in reaction 9. This reaction will continue until there is no ozone left of the clouds evaporate causing chlorine to go back into there temporary reservoirs (Rowland, 1991). There are also cycles of Cl and Br which contribute to ozone depletion (Molina, 1996).
The effect of the ozone depletion due to chlorine from the destruction of CFCs are numerous. The hole will allow an increased intensity of UV-B to penetrate to the lower atmosphere (Drake, 1995). This is harmful to life and causes an increase in melanoma and nonmeloanoma, and an increase in cataracts. It also causes a suppressed human immune system, it will effect plant growth and photosynthesis will reduce, as well some insects could be effected (Drake, 1995). This will also cause a change in the climate, as with less ozone heat absorption will change and cause changes in wind patterns.
The greenhouse effect is the second main problem with CFCs in the atmosphere. CFCs are considered to be strong absorbers of long wavelength radiation (Houghton, 1980), which makes them a greenhouse gas. Thus, CFCs and other greenhouse gases will cause the earth to heat up as more radiation is absorbed and less is reflected back into outer space. This heating up on the earth will cause dramatic changes to climate through out the world, such as melting of polar ice caps.
OTHER PROBLEMS
The greenhouse effect and ozone depletion problems are major reasons why humans should prevent the release of CFCs into the atmosphere, as little can be done to stop CFCs from interacting with the stratosphere once they get there. Thus, measures should be taken to solve problems which exist in release rates of CFCs. Measures should be taken in preventing leakage in the equipment which uses them and in the recycling methods.
Other problems exist in the uncertainty of the amounts of CFCs actually in the atmosphere, the uncertainty in the exact release rate, and limitations in modeling of the chemistry in the stratosphere (Houghton, 1980). These problems in combination with the extremely long lifetime of CFCs will cause the CFC problem to continue will into the twenty-first century (Rowland, 1991).
CONCLUSION
CFCs have been a solution to old chemicals previously used in refrigeration. There use in this industry revealed their excellent chemical properties which resulted in the use of CFCs in air conditioning, as a blowing agent, and as a cleaning agent. In 1974, Rowland and Molina discovered the impact CFCs had on the ozone layer in the stratosphere. It was found that CFCs photochemically dissociate to release chlorine. This chlorine will react with ozone to produce a chloroxide chain which cycles through continuously decreasing the amount of ozone. This is significant as most of the stratospheric ozone is concentrated exactly where CFC photodissociate and release chlorine. As a result , these reactions will significantly deplete the ozone layer. Chlorine can also be stored in temporary reservoirs until they are released again to aid in the ozone depletion process. Thus, the longevity of chlorine is increased an it’s effects can last for a very long time. Moreover, the longevity of CFCs will cause humans to be aware of there affects on the ozone layer and the Greenhouse effect well into the next century. For these reasons, industry has to find better ways to prevent the leakage of CFCs into the environment, and find alternatives.
YAZAN
موضوع: PROBLEMS WITH HLOROFLUOROCARBONS 
الإثنين نوفمبر 02, 2009 2:58 am
