Issues

The halocarbons are a group of compounds with halogen atoms (and possibly hydrogen) attached to a carbon skeleton. Most of the halocarbons are purely synthetic. However, there are significant natural sources of the methyl halides (CH3Cl, CH3Br and CH3I), chloroform (CHCl3) and, to a lesser extent, dichloromethane (CH2Cl2). The halocarbons can be regarded as derived from the alkane CnH2n+2 structure by substituting hydrogen atoms with halogens. An outline of the relevant chemical nomenclature is given in Box 16.1.

As polyatomic species, all these compounds will be likely to affect the infra-red radiative balance of the earth. However, the main concern about these gases has arisen from their role in the depletion of stratospheric ozone (see Box 16.2). The extent of depletion of ozone depends on the amount of chlorine and bromine released when halocarbon compounds are photolysed in the stratosphere and on the altitude at which the photolysis occurs. Chlorine and bromine participate in catalytic cycles that destroy ozone while leaving the halogens free to participate in further cycles of destruction of ozone.

Most of the compounds that are not fully halogenated, i.e. those with one or more hydrogen atoms remaining, react primarily with tropospheric OH. The products are removed by rainout and so relatively little of the chlorine or bromine reaches the stratosphere.

The main classes of compounds of concern are the following.