Ozone destruction by halogenated species

Natural chlorine in the stratosphere is mainly transferred there as methyl chloride (CH3Cl), which probably comes from marine and terrestrial biological sources (Section 3.4.2). This natural source, however, accounts for only 25% of the chlorine which is transported across the tropopause. By the early 1970s the CFCs used as aerosol propellants and refrigerants had become widely distributed through the troposphere. There appeared to be no obvious mechanism for the destruction of these highly stable compounds in the lower atmosphere. However, the knowledge that CFCs were being transported to the stratosphere raised concern over their effect on the O3 layer. These compounds, for example CFCl3 (Freon-11; Fig. 3.4b) and CF2Cl2 (Freon-12), absorb UV radiation in the 190-220 mm range, which results in the photodissociation reactions:

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