Stratospheric chemistry bromine reactions

The reactions of halogen atoms and radicals are of fundamental importance in stratospheric chemistry (see Sects. 4.4 and 8.2, 8.3, and 8.4), and the halogen cycle is also of interest in the marine boundary layer in the troposphere (see Sect. 7.5). In this section, among the atmospheric reactions of halogen atoms and radicals, fundamental homogeneous reactions of Cl atoms and CIO radicals are described, and the reactions of bromine and iodine atoms and radicals are discussed in the more phenomenological discussions in Chaps. 7 and 8. 

While gas phase chemistry leads to much higher levels of active bromine compared to chlorine, this is even more the case for iodine. Solomon et al. suggested that iodine might be of importance in ozone depletion. At present there is no information on the amounts of iodine in the stratosphere, but heterogeneous reactions will probably not play a significant role. Conversely, fluorine is almost completely in its deactivated form HF, and also heterogeneous reactions have been found to be immeasurably slow. Hence, fluorine species are not expected to influence stratospheric chemistry. 

These data also demonstrate the impact of bromine chemistry on the stratosphere (see Chapter 12.D). The initial ODP for methyl bromide is 15, due primarily to the large a factor associated with bromine chemistry. However, since it is removed by reaction with OH in the troposphere as well as by other processes such as hydrolysis in the oceans and uptake by soils and foliage (see Chapter 12.D), it has a short atmospheric lifetime of 1.3 years and hence the ODP decreases rapidly with time, toward a long-term steady-state value. 

An important aspect of stratospheric bromine chemistry is the possibility of synergistic interactions between bromine and chlorine cycles via the following reaction,... 

If propagation reactions compete favorably with termination reactions, the formation of two chlorine radicals could result in the reaction of many molecules of methane. It should be noted that this effect is important in the loss of stratospheric ozone resulting from the production of chlorine or bromine radicals from freons. Free radicals are very important in biological processes involving oxygen and their production is involved in some toxicological mechanism. In organic chemistry, free radicals are a major factor in polymerization processes. 

The solubility of HBr in sulfuric acid has been studied as well [37]. Bromine radicals contribute to ozone destruction through a catalytic reaction cycle involving BrO and CIO. Thus heterogeneous chemistry of bromine containing species merits some attention, even though most of the stratospheric bromine is already present in active species. Table 1 shows these results in a manner analogous to the HCl and HNO3 results. 

Chlorine nitrate CIONO2 and bromine nitrate B1ONO2 are important reservoir molecules formed by the chain termination reactions, CIO + NO2 and BrO + NO2, in the CIO and BrOx cycles in the stratosphere, respectively. Iodine nitrate IONO2 plays a similar role in the in the iodine chemistry in the troposphere. 

Ozone, in turn, can be destroyed by interaction with another photon that breaks it into an oxygen molecule (O2) and an oxygen atom (O). Stratospheric ozone also can be destroyed by reaction with other species, such as nitric oxide (NO), as shown in Eq. (4.42), and halogen atoms, such as chlorine and bromine. Chlorine and bromine atoms are released into the stratosphere from the photodegradation of haloalkanes, often called halons. Classes of haloalkanes that impact ozone chemistry include CFCs and hydrochlorofluorocarbons (HCFCs). The net concentration of ozone in the stratosphere is established by the rates of both the production and the destruction reactions. 

---