Reactions on Polar Stratospheric Cloud PSC

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. 

Particle names Chemical composition Shape Phase Particle diameter ()tm) Threshold emperature (K) 

Stratospheric sulfate aerosols(SSA) H2SO4/H2O Fine liquid droplet 0.1-5 T 261 

Sulfuric acid tetrahydrate(SAT) H2SO44H2O Fine solid crystals 1 T 213 

Nitric acid trihydrate (NAT) HNO3 3H2O Solid crystals 1-5 T 196 

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This book covers homogeneous gas-phase kinetics important in the atmosphere, which has been almost established, and provides the solid scientific bases of oxidation of trace gases and oxidant formation. Nevertheless, unresolved problems remain, for example, unsatisfactory reproduction of observed OH/HO2 mixing ratio by model simulation under certain conditions, and oxidation mechanisms involving isoprene, terpenes and other biogenic hydrocarbons, and anthropogenic aromatic hydrocarbons. Therefore, descriptions of these topics are not completed in the book. Heterogeneous reaction chemistry is not covered well except for the chemistry on polar stratospheric clouds (PSCs) and reactive uptake coefficients of selected... 

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... 

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. 

Heterogeneous reaction rates on particles in polar stratospheric clouds (PSCs) are more difficult to estimate because of the uncertainties in the type of PSC particles present in the lower stratosphere. The particle composition, volume density and radius must be derived from a thermodynamic model (e.g., Carslaw cl at, 1994 1997). Condensation of HNO3 occurs when the partial pressure of nitric acid in air exceeds the equilibrium vapor pressure Pjjnq F°r example, in the case of nitric acid trihydrates (NAT), if partial pressures are expressed in Torr and the temperature T in Kelvin, we have (Hanson and Mauersberger, 1988)... 

Ice particles found within polar stratospheric clouds (PSCs) and upper tropospheric cirrus clouds can dramatically impact the chemistry and climate of the earth s atmosphere. The formation of PSCs and the subsequent chemical reactions that occur on their surfaces are key components of the massive ozone hole observed... 

As for the photolytic process, reaction (8.47) is known to be the main rout but reaction (8.48) also proceeds partly under the stratospheric conditions (Atkinson et al. 2004 Sander et al. 2011). The heterogeneous reactions of N2O5 on ice particle of polar stratospheric clouds (PSC) (Sects. 6.1.5 and 6.5.1),... 

The preceding reactions occur throughout the stratosphere and are responsible for depletion of ozone on a global scale. The extreme ozone depletion at the South Pole, however, did not at first seem to be consistent with the kinetics of the knovm ozone-destroying reactions and the known free chlorine concentrations. The discrepancy turned out to be due to heterogeneous chemistry (Fig. 4.47). Under wintertime conditions when polar stratospheric temperatures drop below approximately —78 °C, polar stratospheric clouds (PSCs) are formed. These clouds can form from H2SO4/H2O droplets, which take up HNO3 under cold temperatures from ice crystals formed from the condensation of water and from solid... 

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. 

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... 

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... 

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