Halogen Cycles

The existence of a chlorine cycle and the scattered evidence of biogeochemical cycles for halogenated hydrocarbons involve a wide range of environmentally relevant reaction mechanisms and pathways leading to their widespread distribution and matrix-dependent profiles. The extent to which biotic and abiotic reactions influence the chlorine (halogen) cycle depends on complex interactions between the intrinsic molecular properties of these compounds and characteristics of the environment. 

Ultimately, research in the halogen cycles and the interface between natural and anthropogenic fluxes is directed towards risk quantification and reduction, through research questions such as Do environmental transformation reactions... 

FIGURE 7.1. Schematic presentation of the halogen cycle in a halogen lamp [7.9]. (W... tungsten nX... halogen WXn... tungsten-halogen compound). The cycle is described in the text. 

Regeneration of the hydrogen halide is necessary to complete the halogen cycle. This is represented by ... 

Figure 5.28 shows the total ozone loss rate as a function of altitude. Chemical destruction of 03 in the lower stratosphere (< 25 km) is slow. In this region of the stratosphere, 03 has a lifetime of months. Rates exceeding 106 molecules cm-3 s 1 are achieved only > 28 km. Figure 5.29 gives the fractional contributions of the O, NO, HO, and halogen cycles to the total ozone loss rate. In many respects, Figures 5.28 and 5.29 represent the culmination of this chapter—the synthesis of how the various ozone depletion cycles interact at... 

Below 20 km, HO cycle 2 is dominant in 03 removal. Between 20 and 40 km, NO cycles dominate 03 loss NO cycle 2 in the lower portion of this range, and NO cycle 1 in the upper portion. Above about 40 km, HO cycles are again dominant (HO cycle 1). Because of the rapid release of Br in the lower stratosphere, BrO cycles are approximately comparable in importance to CIO cycles at 20 km. The coupled BrO /CIO cycle is itself responsible for 25% of the overall halogen-controlled loss in the lower stratosphere. The halogen cycles achieve two maxima (1) one in the lowermost stratosphere due to HOx/ClO cycle 1, HO /BrO cycle 1, and BrO /C10 cycles 1 and 2 and (2) one in the upper stratosphere due to CIO cycle 1. 

In my attempt to extend the halogen cycle principle to arc lamps my CRC Handbook really got a workout. The search led to the use of indium iodide in an arc at moderate pressure, producing a brilliant white light. This is the principle employed in the MarcSOO Lamp. The Marc Lamp was designed for use in 16mm movie projectors, now passe, but it is possibly the first example of a useful, continuous radiation, molecular arc lamp. 

PPA = polyphosphoric acid al = alkylation ac = acylation chlor = chlorination iso = isomerization halo = halogenation cycl = cyclization poly = polymerization for = formylation dehydr = dehydration nitr - nitration general = active in most Friedel-Crafts reactions deal = dealkylation rearr = rearrangement. 

In the lower stratosphere, because of the relative scarcity of oxygen atoms, the effective HO, cycles involve direct reactions of HO , and O3, HO, Cycles 2 and 4. NO, Cycles 1 and 2 account for less than 20% of the total O3 loss between 16 and 20 km. The halogen cycles account for about one-third of the total O3 loss. As in the case of the NO, cycles, the effective halogen cycles in the lower stratosphere do not explicitly involve the oxygen atom. The general halogen cycle is ... 

Pszeimy AAP, Moldanov J, Keene WC, Sander R, Maben JR, Martinez M, Crutzen PJ, Pemer D, Priim RG (2004) Halogen cycling and aerosol pH in the Hawaiian marine boundary layer. Atmos Chem Phys 4 147-168... 

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. 

Fig. 8.1 A simplified scheme of halogen cycling in the MBL. The transformations of compounds in the shaded ovals occur in the deliquesced sea-salt aerosol. Also given are the accommodation coefficients, a, and equilibrium constants, K, used in the model calculations...

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