Atmospheric Fates of CFCs and Halons

In considering the atmospheric fate of CFCs and halons, it is useful to examine the total atmospheric lifetime of a compound X, rx. This is in effect the time required for a pulse emitted into the atmosphere to decay to /e of its initial value (see Chapter 5.A.lc). It can be calculated from 

For example, some may have short lifetimes and/or nonuniform distributions and hence be sensitive to such factors as the location of the emissions, sunlight intensity, season, etc.) 

The lifetime of CFCs in the atmosphere can also be estimated using a mass balance approach. Knowing the atmospheric concentrations of CFCs, one can calculate the total amount in the atmosphere. This amount must be the result of a balance between emissions into, and loss from, the atmosphere. If the emission rates are known, the loss rate required to give the observed atmospheric concentrations can be calculated, and from this, a lifetime obtained. Such calculations may be based on either the absolute atmospheric concentrations of CFCs or, alternatively, the observed relative rates of change in the concentrations. 

The chlorofluorocarbons (CFCs) have very long lifetimes in the troposphere. This is a consequence of the fact that they do not absorb light of wavelengths above 290 nm and do not react at significant rates with 03, OH, or N03. In addition to the lack of chemical sinks, there do not appear to be substantial physical sinks thus they are not very soluble in water and hence are not removed rapidly by rainout. While laboratory studies have shown that some of the CFCs decompose on exposure to visible and near-UV present in the troposphere when the compounds are adsorbed on siliceous materials such as sand (Ausloos et al., 1977 Gab et al., 1977, 1978), the lifetimes for CFC-11 and CFC-12 with respect to these processes have been estimated to be 540 and 1800 years, respectively (National Research Council, 1979). Similarly, an observed thermal decomposition when adsorbed on sand appears to be an insignificant loss process under atmospheric conditions. 

While there are a variety of other chlorinated organics such as methylchloroform (CH3CC13) that are emitted, these have relatively short tropospheric lifetimes because they have an abstractable hydrogen atom (e.g., see WMO, 1995). For example, while the stratospheric lifetime of methylchloroform is estimated to be 34 7 years (Volk et al., 1997), its overall atmospheric lifetime is only 5-6 years, primarily due to the removal by OH in the troposphere (toii 6.6 years), with a much smaller contribution from uptake by the ocean (roi i an 85 years) (WMO, 1995). 

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