tion by OH, loss in the stratosphere, and the overall lifetime from both processes. Calculated lifetimes are in the range of 1-20 years. The model predicts that the oxidation products shown in Table 13.7 will not accumulate significantly in the troposphere, with concentrations typically 1% or less of their parent compound.

The reason that the ODPs of these CFC replacements are much smaller than those of the original CFCs is the presence of an abstractable hydrogen with which OH can react. However, this also means that they can also contribute to ozone formation in the troposphere. Hayman and Derwent (1997) have used their photochemical trajectory model to calculate tropospheric ozone-forming potentials of some of these CFC replacements. Table 13.10 summarizes these relative ozone-forming potentials, expressed taking that for ethene as 100. Clearly, although they react in the troposphere, their contribution to tropospheric ozone formation is expected to be very small.

While we have focused here on CFC replacements, similar chemistry applies to replacements for the bromine-containing halons. For example, CF2BrH is a potential halon substitute that will react with OH in the troposphere (DeMore et al., 1997). Through the subsequent reaction with 02 and then NO, the alkoxy radical CF2BrO is formed. This decomposes via scission of the weak C-Br bond to form COF2 (Bilde et ai, 1996).

Similarly, CF3I is a potential halon substitute. However, as for CH3I, it photolyzes rapidly to generate an iodine atom with an estimated lifetime of less than 2 days (Solomon et al., 1994a). While iodine in the stratosphere is expected to be very effective in ozone de-

TABLE 13.10 Calculated Ozone Formation Potentials for Some CFC Replacements"



Ozone formation potential6

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