CIONO

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 ... 

Although there are fewer studies of the heterogeneous chemisty of BrONOz and HOBr than of the corresponding chlorine compounds, it is clear from the laboratory studies that have been done that analogous chemistry occurs, and at least as fast as for the chlorine compounds. Table 12.8 shows some of the most important reactions and typical values of the reaction probabilities. On ice, the hydrolyses of CIONO, and BrONOz proceed at comparable rates (Tables 12.5 and 12.8). However, toward midlatitudes the particles are largely concentrated sulfuric acid-water mixtures, and on this surface the C10N02 hydrolysis reaction probability... 

At the same time, the PSCs are excellent sites for the conversion of chlorine compounds from the relatively inactive reservoir forms of HC1 and C10N02, which make up 99% of the chlorine budget in the lower stratosphere, to photolytically labile species such as Cl2, HOC1, and CIONO (30-32) ... 

Very recent results by Smith et al. (916a) indicate that the main primary process at 3025 A is CI0N02 CIONO + O rather than (VII-206). 

The release of man-made CFCs in the atmosphere has lead to an increase of chlorine containing reservoir molecules such as CIONO, (chlorine nitrate) and HC1 in the stratosphere. Under normal nonpolar conditions, the reaction between both species is extremenly slow. However in the presence of cold surfaces, the following reactions are believed to occur on the PSCs [33] ... 

Reactions (1), (2) and (4) convert stable chlorine reservoir species, CIONO, and HC1, into the more easily photolyzable species Cl, HOC1, and C1NO, (nitryl chloride), respectively. This unique chemistry of CIONO, and N,0, on the cold surfaces of the PSC-surfaces is taking place due to the low temperatures of 180 to 200 K encountered in the lower stratosphere at altitudes between 15 and 25 km in the polar vortex. At sunrise, after the polar winter, these photolabile species release Cl atoms that initiate the chain destruction of ozone according to the mechanism, which is responsible for the fast ozone depletion event occuring within a few days to several weeks [34,35] ... 

The uptake coefficient on liquid sulfuric acid is a strong function on the water activity, in analogy to the hydrolysis of CIONO, and therefore depends upon the composition of the mixture [92]. It was suggested [93] that the CIONO, uptake due to reaction with HC1 is dependent on both bulk and surface concentrations of HC1 y varies by more than two orders of magnitude (0.3>y>10°), and depends strongly on the HC1 partial pressure... 

Finlayson-Pitts, B.J., Ezell, M.J., and Pitts, J.N. Jr. (1989) Formation of chemically active chlorine compounds by reaction of atmospheric NaG particles with gaseous N,0, and CIONO, Nature 337, 241-244. 

Zhang, R Jayne, J.T., and Molina, M.J. (1994) Heterogeneous interactions of CIONO, and HQ with sulfuric arid tetrahydrate Implications for the stratosphere, J. Phys. Chem. 98,867-874. 

CIONO is either rapidly photolyzed or converted by isomerization into... 

A recent report suggests that NO, actually protects ozone by scavenging Ihe much more destructive CIO N02 + CIO------------ CIONO,. Chem. Eng. News 1992, 70(2). 4-5. 

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