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is a major factor in maintaining the high efficiency with which the temporary chlorine reservoirs are converted into photochemically active forms.

c. Heterogeneous Chemistry on PSCs and Aerosols

The reactions of C10N02, N205, and HOC1 with HC1 and H20 on solid and liquid surfaces relevant to PSCs have been the subject of numerous laboratory studies. The measured reaction probabilities depend on the nature (i.e., solid or liquid) and composition of the surface, the temperature and the relative humidity, and the concentrations of the gases. The dependence on the latter arises because of surface "saturation" effects that quickly arise at high reactant concentrations, as well as other effects such as surface "melting" and preactivation, which are less well understood.

Figure 12.25 summarizes some results of laboratory studies of the reaction probabilities for the reaction of C10N02 with HC1 and H20 and of HOC1 with HC1 on various surfaces that are believed to be present in the stratosphere under various conditions. Table 12.5 summarizes typical reaction probabilities for these heterogeneous reactions and for the reaction of N205 with HC1 and H20. It should be noted that in a number of studies of these reactions where the PSC is liquid (either a binary H2S04-H20 or ternary HN02-H2S04-H20 mixture), the laboratory data are better fit by a model that includes two reactions, one in the bulk and one at the surface (e.g., see Hanson, 1998).

A number of models of these heterogeneous reactions have been developed that are consistent with the laboratory observations. The reader is referred to papers by Elliott et al. (1991), Burley and Johnston (1992b), Mozurkewich (1993), Tabazadeh and Turco (1993), Henson et al. (1996), and Koch et al. (1997) for some illustrative approaches.

The hydrolysis of N205 on surfaces is important in that it provides a significant path for "denoxification" of the stratosphere, i.e., for the conversion of NO and N02 to other oxides of nitrogen such as HN03. This does not permanently remove NO, from the stratosphere (as is the case with denitrification), since HN03 can ultimately be photolyzed in the gas phase back to OH + N02. However, since this is relatively slow, denoxification at least helps to tie up NOx temporarily so that the chain destruction of 03 via reaction with CI can proceed more readily since less CIO is tied up in the form of C10N02.

The reactions tend to be fast on ice as well as on liquid solutions characteristic of the stratosphere. This indicates that they should occur on Type II PSCs as well as on H2S04-H20 mixtures characteristic of SSA and on HN03-H2S04-H20 ternary solutions which

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