Polar stratospheric ice clouds

This highly hygroscopic molecule readily combines with water molecules to form an acid aerosol droplet. Other aerosols are formed by nucleation around mineral particles injected as a result of volcanic activity. Under very cold conditions, such as at the poles in winter, these aerosols freeze to form polar stratospheric ice clouds (PSCs), the surfaces of which provide a substrate for important heterogeneous catalytic processes. An example of this is the well-known ozone hole effect. This arises because the steady state concentration of O3 is sustained by the series of reactions (5.1) and (5.21)-(5.25). As already discussed, the sink mechanism (5.24) and (5.25) requires the presence of catalyst X, of which Cl atoms are nowadays the most important, and which are provided, such as reaction (5.26), mainly by the photolysis of CFCs present at trace levels in the upper atmosphere and much of the Cl is temporarily locked up into the reservoir species such as HCl and ClOx-... 

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 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. 

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. 

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). 

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... 

Graham, J. D., and J. T. Roberts, Interaction of HCI with Crystalline and Amorphous Ice Implications for the Mechanisms of Ice-Catalyzed Reactions, Geophys. Res. Lett., 22, 251-254 (1995). Graham, J. D J. T. Roberts, L. A. Brown, and V. Vaida, Uptake of Chlorine Dioxide by Model Polar Stratospheric Cloud Surfaces Ultrahigh-Vacuum Studies, J. Phys. Chem.., 100, 3115-3120 (1996a). 

Middlebrook, A. M., B. S. Berland, S. M. George, and M. A. Tolbert, "Real Refractive Indices of Infrared-Characterized Nitric-Acid/Ice Films Implications for Optical Measurements of Polar Stratospheric Clouds, J. Geophys. Res., 99, 25655-25666 (1994). 

Hanson, D.R., and Mauersberger, K. (1988) Solubility and equilibrium vapor pressures of HQ dissolved in polar stratospheric cloud material ice and the trihydrate of nitric add, Geophys. Res. Lett 15,1507-1510. 

When it gets cold, small ice crystals form in the Antarctic stratosphere these are called polar stratospheric clouds. HC1 condenses onto these surfaces. Unfortunately for the ozone, there is a reaction of HCl(s) with C10N02(g), which is catalyzed by the surface. 

The reason for the dehydration and denitrification of the Antarctic stratosphere is the formation of the PSCs, whose chemistry perturbs the composition in the Antarctic stratosphere. Polar stratospheric clouds can be composed of small (< 1 pm diameter) particles rich in HNO3 or at lower temperatures (<190 K) larger (10 pm) mainly ice particles. These are often split into two categories, the so-called Type 1PSC, which contains the nitric acid either in the form of liquid ternary solutions with water and sulfuric acid or as solid hydrates of nitric acid, or Type II PSCs made of ice particles. The ice crystals on these clouds provide a surface for reactions such as... 

Peter T (1999) Physico-chemistry of polar stratospheric clouds. In Ice Physics and the Natural Environment. Wettlaufer JS (ed) p 143-167. Berlin Springer-Verlag Peters SJ, Ewing GE (1997a) Thin film water on NaCl(lOO) imder ambient conditions An infrared study. Langmuir 13 6345-6348... 

AIDA chamber, Ice nucleation. Polar stratospheric clouds. Soot aerosol, IR spectroscopy Introduction... 

Another type of particle is also present as shown by the secondary peak near 0.5 pm. Note that atmospheric particles such as those present in polar stratospheric clouds, and ice crystals in the tropical lower stratosphere, have large radii and/or non-spherical shapes. 

FIGURE 17.9 Polar stratospheric clouds containing ice particles can catalyze the formation of Cl atoms and lead to the destruction of ozone. 

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