Special Topic 6.21 Atmospheric photochemistry

Photochemical reactions have played a decisive role in the evolution of the atmosphere and of life on Earth. Such processes generally involve simple species, many of which are otherwise considered to be stable and unreactive. Special Topic 6.17 has already discussed photochemistry in a prebiotic atmosphere. Since geochemical inorganic processes cannot be responsible for the current level of oxygen in the atmosphere (21 %), it must be almost exclusively the product of biological activity (Special Topic 6.25). 

Ozone in the stratosphere is depleted by reactions with halogen atoms. Depletion of stratospheric ozone, commonly referred to as the ozone hole, usually occurs over the Earth s cold regions. The main source of chlorine atoms in the stratosphere is photodissociation of chlorofluorocarbon (CFC) compounds,1369 commonly called Freons, e.g.  

Nitrogen oxide, formed by photofragmentation of N02 or chlorine nitrate (C10N02), hydroxyl radicals and some other reactive species are also responsible for stratospheric ozone depletion. These compounds may have both natural and anthropogenic (combustion, etc.) origin. Atmospheric chemistry also takes place in aerosol particles, cloud droplets1370 and ice crystals.1371,1372 

Other harmful compounds, such as peroxyacyl nitrates (PANs), respiratory and eye irritants, are also produced in photochemical smog  

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Singlet oxygen is involved in many important chemical processes and photochemical applications, including photodynamic therapy (Special Topic 6.23), photocarcinogeneity (Special Topic 6.7) and phototoxicity (Special Topic 6.22), chemiluminescence (Section 5.6), atmospheric photochemistry (Special Topic 6.21), polymer degradation (Special Topic 6.13), photosynthesis1389 (Special Topic 6.25) or industrial organic synthesis (Special Topic 6.20). 

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