Chlorine ozone destruction

Chlorine atoms are also very efficient ozone destruction catalysts, as noted originally by Stolarski and Cicerone (4) ... 

Since the recognition of the role of chlorine in catalytic ozone destruction, increasing effort has been devoted to finding replacements. In most cases reported so far, the replacements are partially halogenated molecules that retain one or more hydrogen atoms (HCFCs and HFC s). The presence of H-atoms gives HO a handle (via H-atom abstractions such as R4) for their tropospheric... 

Molina, M. J. and Rowland, F. S. (1974). Stratospheric sink for chlorofluoromethanes chlorine-catalyzed destruction of ozone. Nature 249, 810-812. 

Figure 8-17 The chain reaction by which small concentrations of organic chlorine compounds produce ozone destruction in the stratosphere.

Chlorine and CIO cause ozone destruction in the stratosphere by catalyzing the reaction... 

There are numerous natural contributors of chlorine to the stratosphere, for example, volcanic eruptions. The main concern regarding ozone destruction in recent years is associated with human activities that have increased chlorine and other synthetic chemical input into the stratosphere. At the top of the list of such chemicals are chlo-rofluorocarbons, or CFCs. CFCs are compounds that contain carbon, chlorine, and fluorine they were first developed in 1928. Common CFCs are called Freons, a trade name coined by the DuPont chemical company. CFC compounds are nonreactive, nontoxic, inflammable gases. Because of their... 

F. Sherwood Rowland (1927-) and Mario Molina (1943-) predicted the destruction of stratospheric ozone in 1974. Rowland and Molina theorized that inert CFCs could drift into the stratosphere, where they would be broken down by ultraviolet radiation. Once in the stratosphere, the CFCs would become a source of ozone-depleting chlorine. The destruction of ozone by CFCs can be represented by the following series of reactions ... 

In 1974, Cicerone and Stolarski suggested that if there were sources of atomic chlorine in the stratosphere, the following catalytic ozone destruction cycle... 

After the first reports of this phenomenon, major field campaigns were launched, which clearly established a relationship between ozone destruction and chlorine chemistry. For example, Fig. 1.8 shows simultaneous aircraft measurements of ozone and the free radical CIO as the plane flew toward the South Pole. As it entered the polar vortex, a relatively well-contained air mass over Antarctica, 03 dropped dramati-... 

Chlorine and bromine nitrate serve as temporary reservoirs for chlorine and bromine, taking them out of their ozone destruction cycles. (While theoretical studies suggest that other forms of bromine such as 02Br0N02 could in principle also act as reservoirs (Lee et al., 1999a, 1999b), there is no evidence at the present time that these are important under atmospheric conditions.)... 

Subsequent reactions of the CF2C1 radical also release the chlorine atoms tied up in this fragment, so that all of the chlorine in the original molecule becomes available for ozone destruction. 

In short, the net effect of fluorine atom chemistry on ozone destruction is very small, lCL-lO4 times smaller than the effect of chlorine on a per-atom basis (Sehested et al., 1994 Ravishankara et al., 1994 Li et al., 1995b Lary, 1997). 

Analogous to chlorine chemistry, the formation of bromine nitrate represents the major short circuit in its ozone destruction cycle ... 

There are several reasons for the dramatic ozone destruction (see Fig. 2.17) low temperatures may have prolonged the presence of polar stratospheric clouds, which play a key role in the ozone destruction, the polar vortex was very stable, there were increased sulfate aerosols from the 1991 Mount Pinatubo volcanic eruption, which also contribute to heterogeneous chemistry, and chlorine levels had continued to increase. These issues are treated in more detail shortly. 

In summary, reactions (43a)-(45) have generally been taken to represent the chemistry occurring in the ozone hole. However, reduced efficiency of chlorine atom production in the photolysis of (C10)2, reaction (44), and hence ozone destruction, needs to be modeled and tested against the atmospheric observations. 

In Section C.3, we saw that gas-phase chlorine chemistry in the stratosphere is inextricably intertwined with bromine chemistry. Because of this close interrelationship, altering the concentrations of only one of the halogens (e.g., through controls) may not have the proportional quantitative result that might be initially expected. We explore in this section in more detail the role of brominated organics in stratospheric ozone destruction and the interrelationship with chlorine chemistry. 

Indeed, these reactions play an important role in the Antarctic ozone hole and they have important implications for control strategies, particularly of the bromi-nated compounds. For example, Danilin et al. (1996) examined the effects of ClO -BrO coupling on the cumulative loss of O-, in the Antarctic ozone hole from August 1 until the time of maximum ozone depletion. Increased bromine increased the rate of ozone loss under the denitrified conditions assumed in the calculations by converting CIO to Cl, primarily via reactions (31b) and (31c) (followed by photolysis of BrCl). Danilin et al. (1996) estimate that the efficiency of ozone destruction per bromine atom (a) is 33-55 times that per chlorine atom (the bromine enhancement factor ) under these conditions in the center of the Antarctic polar vortex, a 60 calculated as a global average over all latitudes, seasons, and altitudes (WMO, 1999). 

Thus, the effect of heterogeneous bromine chemistry is primarily to amplify the chlorine-catalyzed destruction of ozone through the more rapid conversion of the reservoir species HC1 back into active forms of chlorine (Lary et al., 1996 Tie and Brasseur, 1996). This becomes particularly important under conditions of enhanced aerosol particles, e.g., after major volcanic eruptions. 

Note that the HFCs in Table 13.3 have such small ODPs that only upper limits can be placed on them. That is, their ODPs are essentially zero for all practical purposes. The reason is the obvious one, that they do not contain chlorine and, as discussed in Chapter 12.C, fluorine does not participate in ozone destruction cycles (Molina and Rowland, 1974 Rowland and Molina, 1975 Stolarkski and Rundel, 1975 Cicerone, 1979 Ravishankara et al., 1993 Sehested et al., 1994 Li et al., 1995). 

Heterogeneous chemistry leading to ozone destruction can also lead to ozone-temperature correlations since many of the important reactions forming active chlorine are faster at lower temperatures (see Chapter 12). In addition, there is more PSC formation at the poles and hence more ozone destruction in these regions associated with lower temperatures (see Fig. 13.14 and associated discussion below). 

In 1974, F. Sherwood Rowland and Mario Molina, who shared the 1995 Nobel Prize in Chemistry with Crutzen, showed that chlorine from photolyzed chlorofluorocarbons (CFCs) such as CF2C12 and CFCI3, which were used as supposedly inert refrigerants, solvents for cleaning electronic components, plastic foam blowing agents, and aerosol spray propellants, can also catalyze ozone loss. Subsequently, the chlorine monoxide molecule CIO, which is involved in the chlorine-catalyzed ozone destruction cycle, has been shown to be present in the holes in the ozone layer and to correlate inversely with... 

After the discovery of the Antarctic ozone hole" in 1985, atmospheric chemist Susan Solomon led the first expedition in 1986 specifically intended to make chemical measurements of the Antarctic atmosphere by using balloons and ground-based spectroscopy. The expedition discovered that ozone depletion occurred after polar sunrise and that the concentration of chemically active chlorine in the stratosphere was 100 times greater than had been predicted from gas-phase chemistry. Solomon s group identified chlorine as the culprit in ozone destruction and polar stratospheric clouds as the catalytic surface for the release of so much chlorine. 

Li, and Cal profiles ai altitudes of Xrt to I00 km. The method also has been useful lor studying ihe hydroxyl free radical (OH), This radical is of principal inlerest because or ihe cataly tic role which it exerts in atmospheric chemistry. The OH radical, along with chlorine and nitrogen oxides, is involved in the ozone destruction cycle. 

Urban, J., A.H P. Goede, H. Kullmann, K. Ktlnzi, G. Schwaab, N. Whybom and J. Wohlgemuth (1998) Chlorine activation and ozone destruction in the Arctic winter stratosphere 1996, as measured by an Airbome-Submillimetre SIS Radiometer. Geophysical Research Letters (in press). 

Daniel JS, Solomon S, Portmann RW, Garcia RR (1999) Stratospheric Ozone Destruction The Importance of Bromine Relative to Chlorine. J Geophys Res 104 23871... 

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