Photodissociation, free radical formation

In the case of NO2, for each photon absorbed below 400 nm, photodissodation occurs. For other photoabsorbers, HNOj and aldehydes, the photodissociation process leads to the formation of free radicals. 

The quantum yield of the formation of free radicals 0[Pg.250]

An additional but very important type of quenching process involves the photodissociation of weak bonds, such as found for tetraphenylhydrazine (Scheme 1) where the rate of dissociation into diphenylaminyl radicals shows a strong viscosity dependence. This reaction has been considered in terms of the high damping regime of the Kramers-Smoluchowski theory and it appears that, in addition to the activated process, there is an activationless dissociative pathway that favours formation of free radicals at infinite viscosity. 

At this time in the Earth s history the carbon dioxide abundance was higher than its present value since this gas accumulated in the absence of a biosphere. Rutten (1966) speculated that the atmospheric C02 level was 10 times PAL (present atmospheric level) about 3 x 109 years ago. On the other hand the presence of H20 in the atmosphere led, by photochemical dissociation, to the formation of free radicals and molecular oxygen. An estimate of the importance of these reactions is necessary to give some idea of the oxygen level in pre-biospheric times. The photodissociation of water vapour can be represented as follows (Suess, 1966) ... 

The photodissociation of nitrites, nitrates and peroxides is connected with the formation of free radicals ... 

It is important to note that certain cycles can lead to ozone formation this phenomenon is readily observed in the troposphere (see Box 5.4), which is rich in anthropogenic hydrocarbons and nitrogen oxides. In the free troposphere and lower stratosphere, the conversion of NO to NO2 by peroxy radicals (HO2 and CH3O2) produced by the oxidation of methane and carbon monoxide, followed by the photodissociation of NO2 leads to the formation of O3. The complete chains are the following (Crutzen, 1974) ... 

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