Clouds, polar stratospheric

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. 

An important implication of the fact that PSCs contain nitrate is that, if the particles are sufficiently large, they can fall out of the stratosphere and thereby permanently remove nitrogen from the stratosphere. The removal of nitrogen from the stratosphere is termed denitrification. If nitrate-containing PSCs sediment out of the stratosphere, then that could lead to an appreciably lower supply of nitrate for possible return to NO (and conversion of CIO to C10N02). 

Stratospheric NAT particles were first detected in situ in the 1999-2000 SAGE III ozone loss and validation experiment, carried out in the stratosphere over northern Sweden (Voight et al. 2000 Fahey et al. 2001). The particles identified were large enough (1-20 pm in diameter) to be able to fall a substantial distance before evaporating. The 

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Heterogeneous chemistry occurring on polar stratospheric cloud particles of ice and nitric acid trihydrate has been estabUshed as a dorninant factor in the aggravated seasonal depletion of o2one observed to occur over Antarctica. Preliminary attempts have been made to parameterize this chemistry and incorporate it in models to study ozone depletion over the poles (91) as well as the potential role of sulfate particles throughout the stratosphere (92). 

The discovery of ozone holes over Antarctica in the mid-1980s was strong observational evidence to support the Rowland and Molina hypothesis. The atmosphere over the south pole is complex because of the long periods of total darkness and sunlight and the presence of a polar vortex and polar stratospheric clouds. However, researchers have found evidence to support the role of CIO in the rapid depletion of stratospheric ozone over the south pole. Figure 11-3 shows the profile of ozone and CIO measured at an altitude of 18 km on an aircraft flight from southern Chile toward the south pole on September 21, 1987. One month earlier the ozone levels were fairly uniform around 2 ppm (vol). 

The stratosphere is very dry clouds do not form at lower latitudes because the temperature is not low enough. However, the stratosphere over Antarctica is distinctive the temperature can drop to below -90 Celsius during the winter and spring months, leading to the condensation of water vapor and nitric acid vapor, that is, to the formation of ice clouds (polar stratospheric clouds or PSCs). 

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. 

Toon, O. and Turco, R. (1991). Polar stratospheric clouds and ozone depletion. Scient. Am. 264, 68. 

Antarctic polar stratospheric clouds, effect on ozone depletion, 17 788-789 Anteiso acids, 5 28 Antenna effect, 8 263... 

Architectural coatings, 18 55-56 economic aspects of, 18 73-74 Architectural fabrics, 13 394 Architectural paints, 18 72 Archives, preservation of, 11 414 Arch Raschig process flow sheet, 13 578 Arc melting techniques, 25 522-523 ARCO process, 23 342 Arc-resistance furnace, 12 304 Arc resistance testing, 19 587 Arctic polar stratospheric clouds, effect on ozone depletion, 17 789-790 Arc vaporization, 24 738 Arc welding, copper wrought alloys,... 

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

You now have enough experience with DFT calculations to imagine how calculations could be performed that would be relevant for each of the three examples listed above. For instance, DFT calculations could be used to determine the relative energy of various kinds of lattice defects that could potentially exist in a solid material. Similar calculations could be used to determine the equilibrium positions of reactive molecules on the surfaces of ice crystals that could be thought of as mimics for polar stratospheric clouds. 

But these calculations only give information on minimum energy states. In the language introduced in Chapter 5, these calculations give only OK information. This is a bit too cold even for polar stratospheric clouds ... 

During the dark, polar winter the temperature drops to extremely low values, on the order of-80°C. At these temperatures, water and nitric acid form polar stratospheric clouds. Polar stratospheric clouds are important because chemical reactions in the stratosphere are catalyzed on the surface of the crystals forming these clouds. The chemical primarily responsible for ozone depletion is chlorine. Most of the chlorine in the stratosphere is contained in the compounds hydrogen chloride, HCl, or chlorine nitrate, CIONO. Hydrogen chloride and chlorine nitrate undergo a number of reactions on the surface of the crystals of polar stratospheric clouds. Two important reactions are ... 

Quinlan, M. A., C. M. Reihs, D. M. Golden, and M. A. Tolbert, Heterogeneous Reactions on Model Polar Stratospheric Cloud Surfaces Reaction of N205 on Ice and Nitric Acid Trihydrate, J. Phys. Chem., 94, 3255-3260 (1990). 

HSCT emissions may also interact with polar stratospheric clouds, PSCs, in much the same way as with particles (Pitari et al., 1993). That is, reaction of a number of nitrogenous species on PSCs leads to the formation of HN03, which can remain adsorbed on or in the PSC. The larger cloud particles sediment to lower altitudes in the stratosphere, redistributing NO, or into the troposphere, permanently removing NOr... 

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. 

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... 

Farman and co-workers (1985) suggested that the reaction between HC1 and C10N02 may play a key role if it were fast enough, which at the time did not seem to be the case for the gas-phase reaction. Subsequently, Solomon et al. (1986) proposed that enhancement of this reaction on the ice surfaces of polar stratospheric clouds could explain the development of... 

In short, the overall features of the chemistry involved with the massive destruction of ozone and formation of the ozone hole are now reasonably well understood and include as a key component heterogeneous reactions on the surfaces of polar stratospheric clouds and aerosols. However, there remain a number of questions relating to the details of the chemistry, including the microphysics of dehydration and denitrification, the kinetics and photochemistry of some of the C10x and BrOx species, and the nature of PSCs under various conditions. PSCs and aerosols, and their role in halogen and NOx chemistry, are discussed in more detail in the following section. 

These aerosols play a major role in stratospheric chemistry by directly providing surfaces for heterogeneous chemistry (discussed in more detail later) as well as serving as nuclei for polar stratospheric cloud formation. Figure 12.21 schematically shows the processes believed to be involved in PSC formation. The thermodynamic stability of the various possible forms of PSCs at stratospherically relevant temperatures and the transitions between them are discussed in detail by Koop et al. (1997a). 

FIGURE 12.21 Schematic of polar stratospheric cloud (PSC) formation. 

It is noteworthy that there is some laboratory evidence that HBr, in contrast to HC1, may form a hydrate, HBr-3H20, under polar stratospheric cloud formation conditions (Chu and Heron, 1995). 

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). 

Borrmann, S S. Solomon, J. E. Dye, D. Baumgardner, K. K. Kelly, and K. R. Chan, Heterogeneous Reactions on Stratospheric Background Aerosols, Volcanic Sulfuric Acid Droplets, and Type I Polar Stratospheric Clouds Effects of Temperature Fluctuations and Differences in Particle Phase, J. Geophys. Res., 102, 3639-3648 (1997b). 

Carslaw, K. S., M. Wirth, A. Tsias, B. P. Luo, A. Dombrack, M. Leutbecher, H. Volkert, W. Renger, J. T. Bacmeister, and T. Peter, Particle Microphysics and Chemistry in Remotely Observed Mountain Polar Stratospheric Clouds, J. Geophys. Res., 103, 5785-5796 (1998b). 

M. Loewenstein, G. V. Ferry, K. R. Chan, and B. L. Gary, Particle Size Distributions in Arctic Polar Stratospheric Clouds, Growth, and Freezing of Sulfuric Acid Droplets, and Implications for Cloud Formation, J. Geophys. Res., 97, 8015-8034 (1992). 

Elliott, S., R. P. Turco, O. B. Toon, and P. Hamill, Application of Physical Adsorption Thermodynamics to Heterogeneous Chemistry on Polar Stratospheric Clouds, J. Atmos. Chem., 13, 211-224 (1991). 

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