Photochemical Production and Loss of Ozone in the Unperturbed Troposphere

5 Photochemical Production and Loss of Ozone in the Unperturbed Troposphere 

The tacit assumption made in the preceding discussion that ozone is photochemically stable in the troposphere is basically incorrect, and we must finally consider this aspect. It is true that the photodissociation of ozone as far as it leads to 0(3P) atoms causes no losses, because their subsequent attachment to molecular oxygen regenerates ozone. In Section 4.2 it was shown, however, that a part of the O( D) atoms produced in the 

The second of these is more important than the first one, since the OH radicals are converted to H02 mainly by reacting with methane and carbon monoxide rather than with ozone. All these losses are at least partially retrieved, however, by the formation of ozone from the photodissociation of N02 when NO is converted to N02 in smog-like reactions involving the oxidation of methane and carbon monoxide, as was first pointed out by Crutzen (1973). In fact, more ozone may be formed in this manner than is lost by the conversion of O( D) to OH radicals, if conditions are favorable. The oxidation of methane and carbon monoxide was discussed in Section 4.2. Here, we will reconsider it from the viewpoint of its ozone generating potential. 

If it is assumed that peroxidic radicals react only with nitrix oxide, the reaction of OH with methane induces the following reaction sequence  

In this scheme two molecules of ozone are generated for each molecule of methane consumed. The subsequent photodissociation of formaldehyde produces equivalent amounts of carbon monoxide, which also undergoes oxidation by OH radicals  

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