CFCs in the troposphere are extremely unreactive but in the stratosphere there are much higher levels levels of radiation energy which are needed to break down C-Cl bonds. This energy does not reach the troposphere because it is filtered out by the O2 and O3 molecules present in the stratosphere. Small concentrations of CFC 11 (CCl3F) were discovered in rural areas, wel away from any sources,and showed that it was able to diffuse to Antarctica. Such a stable gas would accumulate in the atmosphere. Scientists flew into the ozone hole and measured the concentrations of ClO radicals and O3. Figure 4 below (1) shows that the concentration of O3 fell dramatically at the point where the concentration of ClO radicals soared. This was conclusive proof that a catalytic cycle involving Cl radicals must be involved in O3 depletion. Also figure 1 (4) shows a severe depletion in the ozone layer over the Antarctic on October 1, 1999.
The rapid depletion of CFC’s in the stratosphere is due to the high levels of ultra-violet radiation which leads to the photodissociation of CFC molecules. Eg A CFC 11 molecule would absorb the high energy ultra-violet radiation and fragment to release chlorine radicals:
CCl3F ➔ CCl2F + Cl. (1)
The Cl radical could then destroy ozone in a catlytic cycle (1):
Cl + O3 ➔ ClO + O2
ClO + O ➔ Cl + O2
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overall: O + O3 ➔ 2O2 (1)
The chlorine atoms are not used up in these reactions, they are homogeneous catalysts. (4) The raction rate is fast and one chlorine molecule could destroy thousands of ozone molecules. The oxygen free radicals, O in the second equation, are formed continuosly in the stratosphere.
Ozone depletion is most severe over Antartica in the southern spring because of unique weather conditions, the sun disappears for six months. In the winter a vortex of cold air isolates the circulating air mass from the rest of the atmosphere (1), polar stratospheric clouds form allowing particles nitric acid frozen around the nuclei of sulphuric acid. The clouds provide a surface for the reaction of HCl with ClONO2. This reaction produces Cl2 which breaks down to form Cl radicals when the sun returns. These radicals lead to the destruction of Ozone. Figure 5 below (1) shows how polar stratospheric clouds help Cl radicals to destroy ozone.
As the chemistry of chlorine in the sratosphere is better understood it is shown that some gasses like NO2 and CH4 can react with ClO radicals and interrupt the catalytic cycle:
Cl + CH4➔ HCl + CH3
ClO + NO2 ➔ ClONO2
The chlorine atoms become bound up in the stable reservoir molcules, HCl and CIONO2. They remain chemically inactive until realeased (3).
CFCs have been used so widely for many reasons. It has the essential physical properties for a refigerent, appropriate boiling and freezing points (low enough to evapourate efficiently but high enough to liquefy by compression) (2). Its chemically stable, non toxic and cheap. (2) CFCs and the related HCFCs quickly became the refrigerantschioce for almost all applications. It had a wide range of uses, see table 2 below(2), and it was better than all previous refigerants.
Table 2 (2)
Scientists want to use HFC’s (hydrofluorocarbons) as a replacement for CFC’s because they contain fluorine as the only halogen (2). They also do far less damage to stratospheric ozone because HFC’s are broken down in the troposphere by OH radicals so very little reaches the stratosphere also the C-F bond is not broken in the stratosphere. HFC’s have no effect on O3 but they contribute to global warming. Also existing equipment will have to be modified or redesigned which could be very expensive.
References:-
- Article 1 – ‘Do CFCs destroy the ozone layer?’, taken from Chemistry Review, March 1993.
- Article 2 – ‘The rise and fall of CFCs’, taken from Chemistry Review, September 1996.
- Chemical Ideas pg 242 – Heinemann
- http://www.cis.ohio-state.edu/hypertxt/faq/usenet/ozone-depletion/top.html
