Polar stratospheric clouds , role

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). 

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). 

While the sulfuric acid is key nucleation precursor in the low troposphere, its contribution to the polar stratospheric chemistry is a lot more modest. Another strong acid-nitric-plays a major role as the dominant reservoir for ozone destroying odd nitrogen radicals (NOj) in the lower and middle polar stratosphere. Nitric acid is an extremely detrimental component in the polar stratosphere clouds (PSCs), where nitric acid and water are the main constituents, whose presence significantly increases the rate of the ozone depletion by halogen radicals. Gas-phase hydrates of the nitric acid that condense and crystallize in the stratosphere play an important role in the physics and chemistry of polar stratospheric clouds (PSCs) related directly to the ozone depletion in Arctic and Antarctic. 

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. 

The existence of aerosol surfaces in the stratosphere has been known for many decades, but it was not until the discovery of the Antarctic ozone hole that their role in surface chemistry was recognized. Here the stratospheric sulfate layer and polar stratospheric clouds will be... 

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... 

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. 

While the many heterogeneous reactions in the troposphere so far described plays a complementary role to the homogeneous gas phase reactions, heterogeneous reactions on polar stratospheric cloud (PSC) are of primary importance for the formation of stratospheric ozone hole. 

While PSCs form in the winter lower polar stratosphere, there is a second class of high altitude clouds that are formed during summertime at mesopause levels, when the temperature observed at these heights drops below approximately 150 K (see Plate 13). The chemical role of these mesospheric clouds is not yet well understood, but it has been suggested that the frequency of appearance of such clouds could increase in the future in response to human-induced cooling of the middle atmosphere (associated with enhanced levels of CO2 and other greenhouse gases in the atmosphere). 

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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