Chemical Cycles of Halogen Species

Once in the atmosphere, organic halogen molecules are broken down by direct photolysis or by hydroxyl radical attack. Either path leads to the release of atomic halogen. For example, OH reaction of methyl chloride leads to 

Halogen atoms are highly reactive toward hydrocarbons, leading to the formation of hydrogen halides through hydrogen abstraction. For Cl atoms, for example 

F and Cl atoms react readily in this manner. Br atoms can abstract hydrogen atoms only from H02 or aldehydes I atoms are even less reactive. The alternative to the Cl + RH reaction is oxidation of the halogen atom (X = Cl, Br, I) by ozone  

Because of the decreasing reactivity of the halogens, as they go from F to I, toward H-containing compounds to form HX, the fraction of free halogen atoms that react by path X + O3, as opposed to X + RH, is F, 0% Cl, 50% Br, 99% I, 100% (Platt, 1995). 

Reactions of the nitrogen oxides N02 or N205 with NaX contained in seasalt aerosol can lead to the formation of XNO or XN02, respectively, for example 

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