Polar regions stratospheric clouds

It now appears that both the extreme magnitude and geographic limitations of the Antarctic ozone depletion are due to meteorologic patterns peculiar to the South Polar regions. The large decrease beyond the small reduction in the rest of the stratosphere apparently involves the circulation of the polar vortex, a complex interaction of Cl with oxides of nitrogen, their physical trapping in extremely cold (T < — 80°C) clouds and preferential removal of some species by precipitation. 

Through a variety of studies, it is now generally accepted that the observed losses are associated with chlorine derived from CFCs and that heterogeneous chemistry on polar stratospheric clouds plays a major role. The chemistry in this region is the result of the unique meteorology. As described in detail by Schoeberl and Hartmann (1991) and Schoeberl et al. (1992), a polar vortex develops in the stratosphere during the winter over Antarctica. The air in this vortex remains relatively isolated from the rest of the stratosphere, allowing photochemically active products to build up... 

The finding that the heterogeneous chemistry that occurs on polar stratospheric clouds also occurs in and on liquid solutions in the form of liquid aerosol particles and droplets in the atmosphere provided a key link in understanding the effects of volcanic eruptions on stratospheric ozone in both the polar regions and midlatitudes. As discussed herein, the liquid particles formed from volcanic emissions are typically 60-80 wt% H2S04-H20, and hence the chemistry discussed in the previous section can also occur in these particles (Hofmann and Solomon, 1989). We discuss briefly in this section the contribution of volcanic emissions to the chemistry of the stratosphere and to ozone depletion on a global scale. For a brief review of this area, see McCormick et al. (1995). 

During the winter season in the South Pole, a strong circumpolar wind develops in the middle to lower stratosphere. Such strong winds, known as the polar vortex, isolate the air over the polar region. In the winter there is no sunlight and the air contained in the vortex becomes very cold (i.e., temperatures below — 80°C) leading to the formation of the polar stratospheric clouds (PSC). These PSC are composed not only of water but also of nitric acid trihydrate. 

The atmosphere is not only a mixture of gases. It also contains a variety of tiny liquid or solid particles, commonly referred to as aerosols. Atmospheric particulate matter may consist of a large variety of species in the lower stratosphere, however, the most abundant aerosol particles are composed of highly concentrated aqueous sulfuric acid droplets. In polar regions during winter, very sparse clouds, called polar stratospheric clouds (PSCs) are also observed. 

In many cases, the NOx family is formed as the sum of NO and N02, and accounts for the most reactive nitrogen species. The NOx/ NOy concentration ratio, which is often reported from field observations, is an indicator of the reactivity of odd nitrogen and its ability to destroy stratospheric ozone (or to affect other chemical families including chlorine and bromine compounds). The value of this ratio increases with altitude above 30 km to reach a value of nearly one in the upper stratosphere and mesosphere. It decreases substantially when the stratospheric aerosol load is enhanced, for example, after large volcanic eruptions (Fahey et al, 1993), and substantial amounts of nitrogen oxides are converted to nitric acid by heterogeneous reaction (5.152). It is also low in the polar regions, especially in air masses processed by polar stratospheric clouds. 

When high CIO concentrations (0.5-2 ppbv) are present in the stratosphere (e.g., in polar regions during winter when air masses are processed by polar stratospheric clouds), the three-body reaction... 

For more than a century, the presence of thin clouds has been reported at high altitudes in the polar regions. Because of their colorful appearances, especially at low solar elevation, these clouds have been named mother-of-pearl or nacreous clouds, or more recently in the context of polar ozone research -polar stratospheric clouds (PSCs). 

Figure 5.68. Reaction scheme responsible for rapid ozone destruction in polar regions when large amounts of chlorine and bromine are activated by polar stratospheric clouds. Concentrations (cm-3) and fluxes (cm-3s-1) associated with chemical reactions are calculated for Antarctic springtime conditions (lower stratosphere). After Zellner (1999).

The stratosphere is very dry and generally cloudless. The long polar night, however, produces temperatures as low as 183 K (-90°C) at heights of 15-20 km. At these temperatures even the small amount of water vapor condenses to form polar stratospheric clouds (PSCs), seen as wispy pink or green clouds in the twilight sky over polar regions. 

We see the distribution of generally lower minimum temperatures in the Antarctic versus the Arctic and the overall lower frequency of PSC formation in the Arctic. Thus polar stratospheric clouds are less abundant in the Arctic and, where they do form, they tend to disappear several weeks before solar radiation penetrates the Arctic stratosphere. Also, the Arctic polar vortex is less stable than that over the Antarctic, because Antarctica is a land mass, colder than the water mass over the Arctic. As a result, wintertime transport of ozone toward the north pole from the tropics is stronger than in the Southern Hemisphere. All these factors combine to maintain relatively higher levels of ozone in the Arctic region. Ample evidence for perturbed ozone chemistry does, however, exist over the Arctic including observed CIO levels up to 1.4 ppb (Salawitch et al., 1993 Webster et al., 1993). CIO concentrations are routinely as high in February in the Arctic as in September in the Antarctic but the PSCs disappear sooner in the Arctic. As a result, denitrification is the most important difference between the Antarctic and Arctic the Arctic experiences only a modest denitrification, whereas denitrification in the Antarctic is massive. The reason for the difference is the sufficient persistence of NAT PSCs in the Antarctic to allow time to settle out of the stratosphere. Figure 4.21 shows evidence of denitrification in both the Arctic and... 

The boundary between the troposphere and the stratosphere (about 8 km in polar regions and about 15 km in tropical regions), usually characterized by an abrupt change of lapse rate. The regions above the troposphere have increased atmospheric stability than those below. The tropopause marks the vertical limit of most clouds and storms, troposphere... 

Chlorine CI2, and bromine monochloride BrCl are formed in the reactions of CIONO2, Br0N02, HCl, HBr, HOCl, HOBr in the heterogeneous reaction in the polar stratospheric clouds (see Sect. 6.5), and their photolyses play an important role in the chain reactions of the ozone hole formation. In the troposphere, CI2 is known to be produced in the heterogeneous reactions on sea salts, but observational data is still limited. Bromine Bra is known to be produced by the heterogeneous chain reactions in the tropospheric ozone destruction in the arctic region. Meanwhile, iodine I2 is released from sea weeds in coastal regions. 

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