Stratosphere, upper, ozone decomposition

The net reaction for this two-step mechanism is the conversion of an O3 molecule and an oxygen atom into two O2 molecules. In this mechanism, chlorine atoms catalyze ozone decomposition. They participate in the mechanism, but they do not appear in the overall stoichiometry. Although chlorine atoms are consumed in the first step, they are regenerated in the second. The cyclical nature of this process means that each chlorine atom can catalyze the destruction of many O3 molecules. It has been estimated that each chlorine atom produced by a CFC molecule in the upper stratosphere destroys about 100,000 molecules of ozone before it is removed by other reactions such as recombination CF2 Cl -b Cl CF2 CI2... 

Ozone forms in the upper stratosphere from molecular oxygen under the influence of UV solar radiation. In the lower stratosphere and troposphere, the source of ozone is the decomposition of nitrogen dioxide under the influence of UV and visible radiation. The formation of the vertical profile of ozone concentration is connected with its meridional and vertical transport. The general characteristic of this profile is the total amount of ozone measured by the thickness of its layer given in Dobson units (1 DU = 0.001 cm). 

Side Note 1.1. Decomposition of Ozone in the Upper Stratosphere... 

Certain undesired reactions can take place via radical intermediates. Examples are the autox-idation of ethers (see Figure 1.38) or the decomposition of ozone in the upper stratosphere. This decomposition is initiated by, among other things, the chlorofluorohydrocarbons ( HCFCs ), which form chlorine radicals under the influence of the short-wave U V light from the sun (Figure 1.12). They function as initiating radicals for the decomposition of ozone, which takes place via a radical chain. However, this does not involve a radical substitution reaction. 

The scheme described for the formation and decomposition of ozone molecules accounts for some, but not all, of the facts about the ozone layer. Many chemical reactions occur that involve substances other than just oxygen. In addition, the effects of turbulence and winds that mix up the stratosphere must be considered. A very complicated picture results. The overall result of ozone formation and removal reactions, coupled with atmospheric turbulence and other factors, is to produce an ozone profile in the upper atmosphere as shown in Figure 18.3 T. 

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