Atmospheric Chemistry and Photochemical Reactions

One notable aspect of atmospheric chemistry is that it occurs largely in the gas phase where molecules are relatively far apart, so a molecule or a fragment of a molecule (a radical) may travel 

A second major aspect of atmospheric chemistry is the occurrence of photochemical reactions that are initiated when a photon (essentially a packet of energy associated with electromagnetic radiation) of UV radiation is absorbed by a molecule. The energy of a photon, E, is given hy E = hv, where h is Planck s constant and v is the frequency of the radiation. Electromagnetic radiation of a sufficiently short wavelength breaks chemical bonds in molecules, leading to the formation of reactive species that can participate in reaction sequences called chain reactions. 

An important example of a photochemical reaction is the photochemical dissociation of O2 molecules in the stratosphere at altitudes above about 10 km that occurs when the molecules absorb highly energetic UV radiation from the sun 

These two reactions are responsible for the essential stratospheric ozone layer, which is discussed in more detail in Section 6.7. 

These are chain reactions in which CIO and Cl- are continually reacting and being regenerated, the net result of which is the conversion of O3 and O in the atmosphere to O2. One Cl atom can 

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