Ozone loss

This cycle accounts for 30—50% of the total photochemical ozone loss observed during spring in the lower stratosphere at mid-north latitudes (76). 

Halogen radicals account for about one-third of photochemical ozone loss observed in the spring in the lower stratosphere (below 21 km) at 15—60°N latitude (76). The following three cycles (4—6) are the most important. Rate constant data are given in Reference 11. 

Again, the myriad influences of human activity are usually viewed as separate effects (global warming, acid rain, ozone loss, urban pollution, etc.) However, these individual symptoms clearly have major interdependencies that must be understood if humans are to learn how to coexist with a stable Earth system. 

From equation (7) is also clear that the volume of the reactor V has only an effect on the ozone loss with the effluent. An increase in reactor volume results in an increase in residence time 6 and therefore in a decrease of the ozone loss. This means that large reactors are favourable. Also with respect to ozone supply a large reactor has advantages. However it has to be noted that large reactors require more mixing energy and are also more expensive. 

The consumption of ozone by component B is lower when using a PFR instead of a CFSTR, as can be calculated from the second terms in the right hand sides of equations (4) and (8). In Figure 3 the ratio of the ozone consumption by the conversion of component B in a PFR and a CFSTR is given in dependence on the ratio kA/kB and s. When a high conversion of component A is desired (e 1) and the kA and kB differ by more than one order of magnitude, which is mostly the case when only oxidation by molecular ozone is considered the ozone losses due to the conversion of component B can be reduced by more than 90%. 

Minimal losses are obtained in a PFR, while the losses are maximal for a CFSTR. From equation (4), it can be derived that, in the case of a CFSTR, the ratio of the ozone losses by decomposition and losses in the effluent is represented by k0e. Because in most cases fc0 l, ozone losses in the effluent of a CFSTR are inevitably high. 

All three terms which contribute to ozone losses are minimal in a PFR. The relative importance and the absolute values of these terms and the ozone consumption factor can be calculated from equations (7) and (16), provided the reaction rate constants and the reaction conditions are known. 

It is also interesting to compare the ozone losses due to the presence of ozone in the effluent. To that aim we consider the ideal situation that in both reactors the ozone concentration is distributed equally over the reactor volume. Further we assume that both reactor systems have the same average residence time, 0. The average concentration of ozone in the CFSTR and PFR is given by equation (5) and (12) respectively. From these two equations it can be derived that the ratio of the average ozone concentration in the PFR and the CFSTR, Re is given by ... 

Exposure of the air to sunlight brings about release of Cl and CIO. Rapid ozone loss occurs via the cycle shown in Figure 7.13. 

Figure 7.13 How heterogeneous reactions on polar stratospheric clouds result in rapid ozone loss...

It has been argued [235] by analogy with the case of molecules adsorbed on glassy n-hexane [232] that this enhancement is due to the electron transfer to CF2CI2 of an electron previously captured in a precursor state of the solvated electron in the water layer, which lies at and just below the vacuum level [300,301] and the subsequent. Similar results have been reported for HCl adsorbed on water ice [236]. It has been proposed that enhanced DEA to CF2CI2 via electron transfer from precursor-solvated states in ice [235] may explain an apparent correlation between cosmic ray activity (which would generate secondary LEE in ice crystals) and atmospheric ozone loss [11]. The same electron transfer mechanism may contribute to the marked enhancement in electron, and x-ray-induced dissociation for halo-uracil molecules is deposited inside water ice matrices [39]. 

Arctic at polar sunrise. The mechanism likely involves regeneration of photochemically active bromine via heterogeneous reactions on aerosol particles, the snow-pack, and/or frozen seawater. The source of the bromine is likely sea salt, but the nature of the reactions initiating this ozone loss remains to be identified. For a review, see the volume edited by Niki and Becker (1993) and an issue of Tellus (Barrie and Platt, 1997). 

Because the concentration of oxygen atoms increases with altitude, the reaction cycle represented by (26) and (27) is important primarily in the middle and upper stratosphere (e.g., Garcia and Solomon, 1994 WMO, 1995). For the lower stratosphere, however, it is only responsible for about 5% of the portion of the total ozone loss that is due to halogens at 15 km and 25% at 21 km (see Fig. 12.8 Wennberg et al., 1994). Most of the 03 loss associated with C10x and BrO at the relatively low altitudes in Fig. 12.8 is due to the following cycle (Solomon et al., 1986 Crutzen and Arnold, 1986) ... 

This cycle accounts for 30% of the ozone loss due to halogens in the lower stratosphere, and the corresponding cycle for bromine for 20-30% (Wennberg et al., 1994). Reaction of CIO with HOz, reaction (28), produces HOC1 + 02 with a yield >95% at temperatures from 210 to 300 K however, at the lowest end of this temperature range, there is evidence for the produc-... 

Atmospheric measurements of CIO, BrO, 03, and NzO confirm the importance of reactions (43a)-(45) in the destruction of 03. For example, Anderson et al. (1989) showed that this cycle is the largest contributor to ozone loss in the Antarctic vortex from 14-18 km. 

Evidence for the contribution of the CIO + BrO interaction is found in the detection and measurement of OCIO that is formed as a major product of this reaction, reaction (31a). This species has a very characteristic banded absorption structure in the UV and visible regions, which makes it an ideal candidate for measurement using differential optical absorption spectrometry (see Chapter 11). With this technique, enhanced levels of OCIO have been measured in both the Antarctic and the Arctic (e.g., Solomon et al., 1987, 1988 Wahner and Schiller, 1992 Sanders et al., 1993). From such measurements, it was estimated that about 20-30% of the total ozone loss observed at McMurdo during September 1987 and 1991 was due to the CIO + BrO cycle, with the remainder primarily due to the formation and photolysis of the CIO dimer (Sanders et al., 1993). The formation of OCIO from the CIO + BrO reaction has also been observed outside the polar vortex and attributed to enhanced contributions from bromine chemistry due to the heterogeneous activation of BrONOz on aerosol particles (e.g., Erie et al., 1998). 

Second, the northern polar vortex is much less stable and hence less isolated from mixing with external air masses compared to the Antarctic case events in January and February in which there was substantial mixing of air from midlatitudes into the vortex have been reported (e.g., see Browell et al., 1993 Plumb et al., 1994). This makes it particularly important to make both measurements and model predictions with sufficient resolution (Edouard et al., 1996). In addition, the Arctic polar vortex tends to break up earlier than the Southern Hemisphere polar vortex since ozone destruction is determined to a large degree by the extent of exposure to sunlight, the earlier breakup and mixing with air external to the vortex cuts the ozone loss short. 

For example, while the vortex-averaged 03 concentration at one altitude in the Arctic in the spring of 1994 was measured to decrease by 10%, the net chemical loss was estimated at 20% but this was partially compensated by an increase due to transport of air containing higher ozone concentrations from higher altitudes (Manney et al., 1995). Similar amounts of chemical ozone loss in the Arctic polar vortex have been calculated based on measurements of CIO, BrO, and 03 (e.g., Brune et al., 1991 Salawitch et al., 1993). 

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). 

However, although as much as 50% of the loss of O-, could be attributed to bromine chemistry at a Bry concentration of 25 ppt, a reduction in bromine did not give a proportional change in the total destruction of 03. Figure 12.46 shows the predicted cumulative O, loss as a function of the Br concentration as bromine decreases, the contribution of the BrO-CIO cycle decreases as well. However, the net effect on total ozone loss is quite small because near-total ozone destruction occurs even without a significant contribution from BrO. 

Anderson, J. G., D. W. Toohey, and W. H. Brune, Free Radicals within the Antarctic Vortex The Role of CFC s in Antarctic Ozone Loss, Science, 251, 39-46 (1991). 

Avallone, L. M D. W. Toohey, W. H. Brune, R. J. Salawitch, A. E. Dessler, and J. G. Anderson, Balloon-Borne in Situ Measurements of CIO and Ozone Implications for Heterogeneous Chemistry and Mid-Latitude Ozone Loss, Geophys. Res. Lett., 20, 1795-1798 (1993a). 

Deshler, T B. J. Johnson, D. J. Hofmann, and B. Nardi, Correlations between Ozone Loss and Volcanic Aerosol at Altitudes below f4 km over McMurdo Station, Antarctica, Geophys. Res. Lett., 23, 2931-2934 (1996). 

Hofmann, D. J S. J. Oltmans, W. D. Komhyr, J. M. Harris, J. A. Lathrop, A. O. Langford, T. Deshler, B. J. Johnson, A. Torres, and W. A. Matthews, Ozone Loss in the Lower Stratosphere over the United States in 1992-1993 Evidence for Heterogeneous Chemistry on the Pinatubo Aerosol, Geophys. Res. Lett, 21, 65-68 (1994b). 

McGee, T. J., P. Newman, M. Gross, U. Singh, S. Godin, A.-M. Lacoste, and G. Megie, Correlation of Ozone Loss with the Presence of Volcanic Aerosols, Geophys. Res. Lett., 21, 2801-2804 (1994). 

McIntyre, M. E., The Stratospheric Polar Vortex and Sub-Vortex Fluid Dynamics and Midlatitude Ozone Loss, Philos. Trans. R. Soc. London, A, 352, 227-240 (1995). 

Proffitt, M. H J. J. Margitan, K. K. Kelly, M. Loewenstein, J. R. Podolske, and K. R. Chan, Ozone Loss in the Arctic Polar Vortex Inferred from High-Altitude Aircraft Measurements, Nature, 347, 31-36 (1990). 

Randeniya, L. K P. F. Vohralik, I. C. Plumb, K. R. Ryan, and S. L. Baughcum, Impact of Heterogeneous BrONOj Hydrolysis on Ozone Trends and Transient Ozone Loss during Volcanic Periods, Geophys. Res. Lett., 23, f633-f636 (f996b). 

Schoeberl, M. R P. K. Bhartia, and E. Hilsenrath, Tropical Ozone Loss Following the Eruption of Mt. Pinatubo, Geophys. Res. 

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