Catalytic cycles ozone depletion

Perfluorinated ethers and perfluorinated tertiary amines do not contribute to the formation of ground level ozone and are exempt from VOC regulations (32). The commercial compounds discussed above have an ozone depletion potential of zero because they do not contain either chlorine or bromine which take part in catalytic cycles that destroy stratospheric ozone (33). 

The catalytic cycle described earlier (reactions 8 and 9) cannot explain the rapid depletion of ozone over the South Pole, because reaction 9 requires free oxygen atoms, which are too scarce in the polar stratosphere to react at any appreciable rate with QO. Several catalytic cycles that do not require oxygen atoms have been suggested as being at work over Antarctica. 

Halogen atoms released by photolysis give rise to catalytic ozone depletion in cycles such as [1,14]... 

These catalytic cycles are largely responsible for the depletion of ozone. One chlorine atom may destroy more than 100,000 ozone molecules before it is transformed into a non-reactive species. Despite the substantial reduction of chlorine and bromine compounds released into the atmosphere as a result of the Montreal Protocol, this has not shown any significant impact on the reduction of the size of the ozone hole. If such a trend continues, it may still take some half a century for the recovery of ozone to the levels it had prior to 1984. 

Fluorine chemistry in the stratosphere has also been considered and attention has been drawn to the atmospheric chemistry of the FOx radicals. The compounds with O-F bonds have gained interest in connection with the ozone depletion problem. It has been suggested that FO and F02 radicals formed in the atmospheric degradation of hydrofluorocarbons (HFCs) could destroy ozone in chain reaction processes. Experimental studies of this hypothesis led to the conclusion that catalytic cycles involving F, FO, and F02 are irrelevant with respect to the chlorine cycle.8 However, kinetic investigations of the reactions of fluorine atoms with 02 and NOx provide useful information on the fluorine chemistry in the polluted atmosphere. 

Scheme 2.126 Preparation of trifluoromethyltrimethyl silane (MejSiCFj, Ruppert s Reagent). Ruppert s original method [2c] above) leads, after aqueous work-up, to the formation of stoichiometric amounts of the carcinogenic HMPA (OP(NMe2)3). In addition, ozone-depleting CFjBr is used as the starting material. A recent method beloiv), with potential for technical upscale, utilizes the inexpensive CHF, and depends on a catalytic cycle initiated by diphenyldisulfide [69].

As in the discussion of gas-phase chemistry, a complete understanding of ozone depletion requires consideration not only of how much CIO is present (i.e., C10/Cly), but also of the catalytic cycles in which CIO may engage. Solomon et al (1986) emphasized the catalytic ozone destruction initiated by the reaction between HO2 and CIO. However, this process cannot destroy enough ozone early enough in the spring season to be consistent with the seasonality of the ozone loss process as shown above in Figure 6.11. 

Stratospheric ozone depletion is one of the best-established phenomena arising from anthropogenic influence on the global environment. As chlorofluorocarbons and other chlorinated and brominated substances are emitted into the atmosphere, those that are not subject to attack in the troposphere may reach the stratosphere where UV radiation breaks the molecules apart, releasing their halogen atoms. These halogen atoms initiate catalytic cycles that destroy stratospheric ozone one chlorine atom can destroy as many as 100,000 ozone molecules before finally being removed from the stratosphere. 

It was initially suggested that the Antarctic ozone hole could be explained on the basis of solar cycles or purely atmospheric dynamics. Neither explanation was consistent with observed features of the ozone hole. Chemical explanations based on the gas-phase catalytic cycles described above were advanced. As noted, little ozone is produced in the polar stratosphere as the low Sun elevation (large solar zenith angle) results in essentially no photodissociation of 02. Thus catalytic cycles that require oxygen atoms were not able to explain the massive ozone depletion. Moreover, CFCs and halons would be most effective in ozone depletion in the Antarctic stratosphere at an altitude of about 40 km, whereas the ozone hole is sharply defined between 12 and 24 km altitude. Also, existing levels of CFCs and halons could lead at most to an 03 depletion at 40 km of 5-10%, far below that observed. 

It turns out that the HO NO, and CIO, cycles are all coupled to one another, and their interrelationships strongly govern stratospheric ozone chemistry. The NO, and CIO, cycles are coupled by reactions 4.34 and 4.35. For example, increased emissions of N2O would lead to increased stratospheric concentrations of NO and hence increased ozone depletion by the NO, catalytic cycle. Likewise, increasing CFC levels will lead to increased ozone depletion by the CIO, cycle. However, increased NO, will lead to an increased level of the CIONO2 reservoir and a mitigation of the chlorine cycle. Thus the net effect on ozone of si-... 

The existence of reservoir species is central to the ozone depletion cycles. In every cycle a reactive free radical can be temporarily sequestered as a relatively unreactive reservoir species. In fact, HCI and CIONO2 together store as much as 99% of the active chlorine. Thus only a small change in the abundance of reservoir species can have a profound effect on the catalytic efficiency of a cycle. The importance of relative concentrations in determining the predominance of different reactions in the ozone depletion cycles can be illustrated in the case of CIO, cycles. Above 20 km, CIO, Cycle 1 is a dominant contributor to ozone loss. At lower altitudes where atomic oxygen levels are significantly lower, other cycles, which involve coupling with HO, and NO, become important ... 

After discovery of the Antarctic ozone hole a number of field campaigns were mounted to measure concentrations of important species in the ozone depletion cycle. The key active chlorine species in the polar ozone-destroying catalytic cycle is CIO. Simultaneous measurement of CIO and O, as shown in Figure 4.19, provided conclusive evidence linking... 

Because of the absence of chlorine, HFCs pose no direct ozone depletion threat. However, some of their degradation products (i.e., the alkoxy radicals) possess unusual properties. The prime example is CF3O, which cannot undergo any of the usual chemical reactions that remove alkoxy radicals from the atmosphere. Another example is the FC(0)0 radical. Because of their unusual character, possibilities were raised that these species could partake in catalytic ozone depletion cycles. If true, these processes would render the relevant HFCs unsuitable. As demonstrated by a number of research groups, the chain propagation reactions are slow and the chain termination reactions are fast thus this is not the case. 

The chlorine chemistry is too slow for ozone depletion in the troposphere but bromine and, in particular, iodine provide - combining with heterogeneous chemistry (see below) - lead to ozone depletion in marine environments by the following catalytic cycles (Barrie and Platt 1995, Platt and Mortgaat 1999) A 5.398 < 5 10 cm molecule" s ... 

One example of a catalytic cycle that has had serious environmental consequences is the role that chlorofluorocarbons (CFCs) play in the phenomenon of ozone depletion in the upper atmosphere, particularly over the Earth s poles. Concern grew following the detection of the Antarctic ozone hole and the identification of the chlorofluorocarbons used in aerosols, refrigerators and air-conditioning plants as mediators of this depletion. 

The catalytic cycle involved in the depletion of the ozone layer... 

Earlier, in Chapter 6, we discussed the catalytic cycle involving chlorine free radicals generated by the effect of ultraviolet radiation on chlorofluorocarbons (CFCs). This cycle results in the phenomenon of ozone depletion and the consequent polar ozone holes . CFCs are not the only compounds that cause this depletion. Nitrogen oxides (NO, ) are also responsible. 

Towards the end of the CIAP programme some researchers had turned their interest to the potential input of reactive chlorine radicals on stratospheric ozone. In the most thorough of these studies, Stolarski and Cicerone [50] calculated substantial ozone depletions if inorganic chlorine were present in the stratosphere at a volume of mixing ratio of 1 nmol/mol of air. Odd oxygen destruction would take place by the catalytic reaction cycle (21) + (22). This reaction sequence is very similar to the... 

---