Polar stratospheric clouds observations

Rosen, J. M., N. T. Kjome, and S. J. Oilmans, "Simultaneous Ozone and Polar Stratospheric Cloud Observations at South Pole Station during Winter and Spring 1991, J. Geophys. Res., 98, 12741-12751 (1993). 

Stefanutti, L M. Morandi, M. Del Guasta, S. Godin, G. Megie, J. Brechet, and J. Piquard, Polar Stratospheric Cloud Observations over the Antarctic Continent at Dumont D urville, J. Geophys. Res., 96, 12975-12987 (1991). 

Figure 6.10. Observations of the vertical profile of ozone observed at the South Pole in the Octobers of the late 1960s and early 1970s, contrasted with those of 1986 and 1997. Total ozone (DU) is indicated for each profile, from Hofmann et al. (1997). The right hand panel shows a typical polar stratospheric cloud observed at the South Pole from the observations of Collins et al. (1993). From Solomon (1999).

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). 

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. 

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... 

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). 

Santee, M. L A. Tabazadeh, G. L. Manney, R. J. Salawitch, L. Froidevaux, W. G. Read, and J. W. Waters, UARS Microwave Limb Sounder HNO, Observations Implications for Antarctic Polar Stratospheric Clouds, J. Geophys. Res., 103, 13285-13313 (1998). 

Toon, O. B E. V. Browell, S. Kinne, and J. Jordan, An Analysis of Lidar Observations of Polar Stratospheric Clouds, Geophys. Res. Lett., 17, 393-396 (1990a). 

More recent observations have detected a similar ozone hole in the Artie, but it is smaller and much more variable, largely because the temperatures vary more there. Volcanic activity that injects sulfur dioxide into the atmosphere also has an effect that depends on temperature and on the height of the SOs injection. The SO2 reacts with air to form SO3, which then reacts with water to form sulfuric acid aerosols. These volcanic aerosols, particularly at cold polar temperatures, reduce the nitrogen oxide concentration of the air and activate chlorine species that destroy ozone, as do the polar stratospheric clouds described earlier. Because these aerosols are stable at warmer temperatures ( 200 K) than the natural stratospheric clouds, and because they can exist at lower altitudes, they can have significant effects. Until the level of chlorine is reduced to preindustrial levels, low temperatures and volcanic activity are likely to create Arctic ozone holes each spring as a result of reactions during the winter. 

Ozone loss over the Arctic has been less dramatic than that over the Antarctic, mainly because the different distribution of land and sea in the Northern Hemisphere allows for only a weak vortex over the Arctic. There is more mixing of air with that from lower latitudes and temperatures do not become low enough for routine formation of polar stratospheric clouds. In years when the Arctic has been cold enough for cloud formation similar ozone destruction has been observed, but for less prolonged periods than over Antarctica. Trends in ozone over the rest of the globe have been small compared to those of the Antarctic, or even Arctic, and are quantified in section 2.4.1. 

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. 

Reactions taking place on the surface of solid or liquid particles and inside liquid droplets play an important role in the middle atmosphere, especially in the lower stratosphere where sulfate aerosol particles and polar stratospheric clouds (PSCs) are observed. The nature, properties and chemical composition of these particles are described in Chapters 5 and 6. Several parameters are commonly used to describe the uptake of gas-phase molecules into these particles (1) the sticking coefficient s which is the fraction of collisions of a gaseous molecule with a solid or liquid particle that results in the uptake of this molecule on the surface of the particle (2) the accommodation coefficient a which is the fraction of collisions that leads to incorporation into the bulk condensed phase, and (3) the reaction probability 7 (also called the reactive uptake coefficient) which is the fraction of collisions that results in reactive loss of the molecule (chemical reaction). Thus, the accommodation coefficient a represents the probability of reversible physical uptake of a gaseous species colliding with a surface, while the reaction probability 7 accounts for reactive (irreversible) uptake of trace gas species on condensed surfaces. This latter coefficient represents the transfer of a gas into the condensed phase and takes into account processes such as liquid phase solubility, interfacial transport or aqueous phase diffusion, chemical reaction on the surface or inside the condensed phase, etc. 

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. 

Browell, E.V., C.F. Butler, S. Ismail, P.A. Robinette, A.F. Carter, N.S. Higdon, O.B. Toon, M R. Schoeberl, and A.F. Tuck, Airborne lidar observations in the wintertime Arctic stratosphere Polar stratospheric clouds. Geophys Res Lett 17, 385, 1990. 

Drdla, K., A. Tabazadeh, R.P. Turco, M.Z. Jacobsen, J.E. Dye, C. Twohy, and D. Baumgardner, Analysis of the physical state of one arctic polar stratospheric cloud based on observations. Geophys Res Lett 21, 2475, 1994. 

Godin, S., G. Megie, C. David, D. Haner, C. Flesia, and Y. Emery, Airborne lidar observations of mountain-wave-induced polar stratospheric clouds during EASOE. Geophys Res Lett 21, 1335, 1994. 

Poole, L.R., and M.P. McCormick, Airborne lidar observations of Arctic polar stratospheric clouds Indications of two distinct growth stages. Geophys Res Lett 15, 21, 1988. 

Tabazadeh, A., O.B. Toon, B.L. Gary, J.T. Bacmeister, and M.R. Schoeberl, Observational constraints on the formation of type la polar stratospheric clouds. Geophys Res Lett 23, 2109, 1996. 

Toon, O.B., A. Tabazadeh, E.V. Browell, and J. Jordan, Analysis of lidar observations of arctic polar stratospheric clouds. J Geophys Res 105, 20,598, 2000. 

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