ClO2g M Clg O2g Meqn 354

Equations 3.51-3.54 then sum to:

The reaction sequence (eqns. 3.49-3.54) is particularly fast at low temperature. Moreover, the square dependence on chlorine concentration implicit within equation 3.50 makes the reaction very sensitive to chlorine concentration. It is these low-temperature processes on particle surfaces that offer the best explana tion for the dramatic decrease in °3 observed over the Antarctic continent (Fig. 3.6). Future modelling of °3 depletion will have to allow increasingly for the heterogeneous aspect of its chemistry. It may well be that, in addition to solid surfaces for reactions, liquid droplets also provide an important medium for reaction.

In the 1990s it became clear that bromine-containing compounds (halons) also played an important role in stratospheric chemistry. Bromoform (CHBr3), a gas naturally released from the oceans, played some role in this but there were also significant emissions from human activities. For example, halons are used in some fire extinguishers, being non-toxic and leaving no residue after evaporation. This class of compounds is typified by the simple halon 1211 (CF2ClBr). These materials can supply both bromine and chlorine to the stratosphere, so they have also been regulated under protocols to reduce human impact on the ozone layer.

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