Polar ozone depletion

A detailed analysis of the atmospheric measurements over Antarctica by Anderson et al. (19) indicates that the cycle comprising reactions 17 -19 (the chlorine peroxide cycle) accounts for about 75% of the observed ozone depletion, and reactions 21 - 23 account for the rest. While a clear overall picture of polar ozone depletion is emerging, much remains to be learned. For example, the physical chemistry of the acid ices that constitute polar stratospheric clouds needs to be better understood before reliable predictions can be made of future ozone depletion, particularly at northern latitudes, where the chemical changes are more subtle and occur over a larger geographical area. 

M. J. Molina, Polar Ozone Depletion. 1995 Nobel Lecture. Angewandte Chemie. International English Edition, 35 (Sept. 6, 1996) 1786-1785. 

Prather, M. J., More Rapid Polar Ozone Depletion through the Reaction of HOCI with HCI on Polar Stratospheric Clouds, Nature, 355, 534-537 (1992). 

R. S. Stolarski, The Antarctic Ozone Hole, Scientific American, January 1988. The 1995 Nobel Prize in Chemistry was shared by Paul Crutzen, Mario Molina, and F. Sherwood Rowland for their work in atmospheric chemistry, particularly concerning the formation and decomposition of ozone. Their Nobel lectures can be found in P. J. Crutzen, My Life with 03, NO, and Other YZO Compounds, Angew. Chem. lnt. Ed. Engl. 1996,35, 1759 M. J. Molina, Polar Ozone Depletion, ibid., 1779 F. S. Rowland, Stratospheric Ozone Depletion by Chlorofluorocarbons, ibid., 1787. 

Bromine containing species, introduced from Man s release of halons, are also believed to play a significant role in the polar ozone depletion, despite the fact that the total inorganic bromine concentration in the stratosphere is typically two orders of magnitude lower than the inorganic chlorine. This is manifested in the presence of Br0N02 and HOBr, which however are less stable than the chlorine reservoirs, so that relatively more BrO, is in the active fotm [36,37]. There is a synergism between the chlorine and bromine species the oxides radicals GO and BrO react with each other to produce a series of products, G, Br, BrCl and OCIO. The latter compound is an indicator of the elevated levels of both BrO and GO [38]. 

The Henry s law solubility of trace species such as HNO, HC1, HBr and HOC1 in sulfuric acid solutions is an important issue. Reactions (1) to (4) generate HNO, and its solubility determines whether the product dissolves or is released into the gas phase. As expected from Van t Hoff law, the solubilities of HNO, and HC1 have found to increase with decreasing temperature. In addition, the solubilities for both HC1 and HNO, increase as the concentration of sulfuric acid decreases [49,80]. Both of these factors will work together to predict that the highest solubilities for HC1 and HNO, in stratospheric sulfate aerosols will occur at low temperatures, where the sulfate particles will be most dilute. The measured solubility of HNO, in sulfuric acid is small enough that most of the stratospheric nitric acid will be in the gas phase. Thus the denitrification, which contributes to polar ozone depletion, will not occur on the global sulfate aerosol. 

According to these researchers, this could decrease the lower stratospheric temperature at the polar vortex by about 0.2°C, which in turn could trigger additional polar ozone losses of up to 8% (polar ozone depletion is very sensitive to small temperature changes). Another model, however, showed a much weaker effect on stratospheric temperatures and ozone loss. As discussed above, hydrogen levels are more likely to increase by 20% than by 400% in the coming decades (Tromp et al., 2003). 

FIGURE 5.20 Schematic of photochemical and dynamical features of polar ozone depletion [WMO (1994), as adapted from Webster et al. (1993)]. Upper panel shows the conversion of chlorine from inactive reservoir forms, CIONO2 and HC1, to active forms, Cl and CIO, in the winter in the lower stratosphere, followed by reestablishment of the inactive forms in spring. Corresponding stages of the polar vortex are indicated in the lower panel, where the temperature scale represents changes in the minimum temperatures in the lower polar stratosphere. 

Gas-phase chemistry associated with the ClOj, and NO cycles is not capable of explaining the polar ozone hole phenomenon. Heterogeneous reactions occurring on PSCs play the pivotal role in polar ozone depletion (McElroy et al., 1986 Solomon et al., 1986 Molina, 1991). The ozone hole is sharply defined between about 12 and 24 km altitude. Polar stratospheric clouds occur in the altitude range 10 to 25 km. Ordinarily, liberation of active chlorine from the reservoir species HCl and CIONO2 is rather slow, but the PSCs promote... 

Whereas the major features of polar ozone depletion are now understood, a number of issues remain to be resolved. The composition of PSCs and their formation and growth... 

It has become clear only recently that the atmospheric sierosol plays an important role for the climate on earth. It is common to distinguish between direct and indirect effects of the aerosols on the climate. Aerosols effect directly the radiation balance of the earth due to scattering and absorption of electromagnetic radiation (radiative forcing). On the other hand they influence the physics and chemistry of the atmosphere as condensation nuclei for cloud droplets and their chemical reactions with atmospheric trace gases. Though these indirect aerosol effects are difficult to quantify, they are at least as important as the direct radiative forcing. An especially important and complex example for the indirect influence of aerosols on the chemistry and radiation balance of the earth is the role of stratospheric aerosol particles on the polar ozone depletion, which is discussed in more detail below. 

In the Southern Hemisphere, chlorine activation leads to a remarkable springtime decrease in ozone that has come to be known as the Antarctic ozone hole on the other hand, polar ozone depletion is considerably smaller in the Arctic. The difference arises from the weaker wave driving of the stratospheric circulation of the Southern Hemisphere. As seen in Fig. 9, wave amplitudes are smaller in the the Southern Hemisphere, and so is wave driving, to the point that the southern polar jet persists much longer into the spring than its northern counterpart. The delayed breakdown of the southern polar vortex allows ozone loss to continue throughout the months of September and October. In the Northern Hemisphere, on the other hand, the conditions necessary for ozone loss usually disappear in mid- to late March, when the northern polar vortex breaks down. 

Is the depletion of ozone over the poles a foreshadowing of what may happen worldwide Although not as dramatic as the decreases seen in Antarctica, global stratospheric ozone levels have also fallen. A United Nations Environment Program Study called Scientific Assessment of Ozone Depletion concludes that ozone in the mid-northern latitudes has decreased about 6% since 1979. The closer to the equator, the smaller the observed decrease in ozone becomes. These decreases are more troublesome than polar ozone depletion because they occur over populated areas. The evidence for ozone depletion has brought worldwide cooperation for the phaseout of chlorofluorocarbons and other ozone-depleting compounds. This cooperation, which is explained in the next section, has halted any further decreases in global ozone. 

Later, polar ozone depletion was found to occur not oidy over the Antarctica but also the Arctic circle (McElroy et al. 1986 Muller et al. 1997). However, in the Arctic, different from the Antarctic, a polar vortex does not sufficiently develop due to the larger inhomogeneity of the earth surface along the latimde, and higher temperatures than Antarctica, the size of ozone hole is smaller in general, but the similar ozone depletion due to CFCs etc. occurs by similar chemical reactions to those in the Antarctic. Particularly, ozone depletion comparable to the Antarctic ozone hole for density and spatial size was observed over the Arctic in 2011 (Manney et al. 2011). 

Peter, T. Cmtzen, P.J., 1993 The Role of Stratospheric Cloud Particles in Polar Ozone Depletion. An Overview , in Journal of Aerosol Science, 24, Suppl. 1 119-120. 

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