Stratospheric chemistry halogen compounds

The residence time for retention of compounds within the stratosphere increases very rapidly from a low and highly variable quantity in the region just above the tropopause, to a period approaching a decade in the lower-middle stratosphere. Thus the altitude at which CFG photolysis occurs can have important consequences concerning the transfer of halogen compounds into the chemical inventory of the stratosphere, which is in turn a measure of the effectiveness of converting industrially produced halogen into active participants in the stratospheric chemistry. 

The chemistry involving NOx is closely intertwined with that of the halogens (CIO,. and BrOx) and of HO, so that the predicted effects of a given set of emissions from the HSCT depend on these species as well. Because halogen chemistry is treated in more detail in later sections, we shall focus here primarily on the reasons for the different effects of NO, emissions at different altitudes. How closely these chemistries are intertwined will be apparent in the treatment below of destruction of stratospheric ozone by chlorofluorocar-bons (CFCs) and brominated compounds. 

In short, the chemistry of the halogens, NOx, and HOx is intimately connected. As we saw earlier with respect to the HSCT, effects on one of these can affect the other cycles significantly as well, and indeed, the overall effects on stratospheric ozone may be due mainly to these secondary interactions involving other families of compounds. 

The stratospheric ozone-depleting potential of a compound emitted at the Earth s surface depends on how much of it is destroyed in the troposphere before it gets to the stratosphere, the altitude at which it is broken down in the stratosphere, and chemistry subsequent to its dissociation. Halocarbons containing hydrogen in place of halogens or containing double bonds are susceptible to attack by OH in the troposphere. (We will consider the mechanisms of such reactions in Chapter 6.) The more effective the tropospheric removal processes, the less of the compound that will survive to reach the stratosphere. Once halocarbons reach the stratosphere their relative importance in ozone depletion depends on the altitude at which they are photolyzed and the distribution of halogen atoms, Cl, Br, and F, contained within the molecule. 

Other secondary chlorine species (atomic Cl, CIO, ClOOCl etc.) have been made responsible for Arctic ozone depletion, whereas the sources of the chlorine atoms are poorly understood (Keil and Shepson 2006). The Cl atom reacts similarly to OH (e. g. in oxidation of volatile organic compounds Cai and Griffin 2006). However, the photolysis of HCl is too slow (even in the stratosphere) to provide atomic Cl. Thus, the only direct Cl source from HCl is due to its reaction with OH, but with a fairly low reaction rate constant (Rossi 2003). There are several chemical means of production of elemental Cl (and other halogens) from heterogeneous chemistry (see Chapter 5.8.2) in the troposphere the photolysis of chloroorganic is not very important, with a few exceptions (see Chapter 5.8.1). 

Halogenated organic substances are a potential risk to the stratospheric ozone, provided their residence times in the atmosphere are long enough for them to reach the stratosphere. The impact on the ozone chemistry increases with atomic number, i.e., bromine is more aggressive than chlorine. The atmospheric residence times of the most stable compounds are of the order of a hundred years, while others break down within a few days. Residence times are longer in seawater, except in anoxic waters Ballister and Lee, 1995 Tanhua et al., 1996). 

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