3.5. Radicals in the atmosphere

Radicals are defined as chemical species with an unpaired electron in the outer (valence) shell. Because of this unpaired electron, radicals have high free energies and are much more reactive than non-radical species whose electrons are all paired up. Because radicals have high free energies, their formation from non-radical species is in general endothermic; an external source of energy is required. In the atmosphere, this source of energy is supplied by solar radiation:



Generation of radicals by reaction (R3.7) provides the initiation step for radical reaction chains which are propagated by subsequent reactions of radicals with non-radical species:



Importantly, any reaction of a radical with a non-radical must always produce a radical in order to conserve the total odd number of electrons. The radical produced in (R3.8) goes on to react with another non-radical, propagating the chain, and in this manner a large number of non-radicals can be processed through the chain. During the propagation cycle, a non-radical species produced by a reaction of type (R3.8) may photolyze following (R3.7) to produce additional radicals; the photolysis is called a branching reaction as it accelerates (or "branches") the chain by augmenting the pool of radicals.



Termination of the chain requires reactions taking place between radicals:





Termination reactions are generally slower than propagation reactions because radicals are present at low concentrations and collisions between radicals are therefore relatively infrequent. In subsequent chapters we will encounter many types of radical-assisted chains following the general schematic (reactions R3.7-R3.10). Due to the critical importance of solar radiation in initiating radical-assisted chain mechanisms in the atmosphere, these mechanisms are often referred to as photochemical (Jacob, 1999).