For four of the five selected gases, we computed photolytic lifetimes in the Earth's present atmosphere. For SF6, we use the lifetime reported in ref. 16. Because the super greenhouse gases have no multiple bonds or hydrogen atoms that would make them readily susceptible to atmospheric degradation, the primary loss mechanism for these gases will, in general, be photolysis. Chemical losses by reactions with O(1D) in the atmosphere will probably be less important than photolysis (16), and OH is not expected to attack this class of molecules.
To estimate the UV-excited state spectra for the super greenhouse gases, vertical excitation energies were calculated by using the configuration interaction singles method (17). This method allows for systems with a large number of atoms and large number of basis functions to be investigated with reliability. The success of this method in predicting excited states depends on the choice of basis set, so we used the large 6-311 ++G(3dp,3pd) basis set. The UV absorption cross sections then were estimated, assuming that they have the same spectral shape as that of CF2Cl2, shifted to the proper wavelengths, and scaled by the oscillator strengths. The resulting spectra then were used to generate photolytic lifetimes by using the one-dimensional Caltech/Jet Propulsion Laboratory photochemical model for the terrestrial mesosphere and lower thermosphere (18, 19). The times for e1 folding are shown in Table 3. Given the crude way the lifetimes were computed, they must be regarded as order-of-magnitude results. The lifetime of the reference molecule CF2Cl2 computed by our model is 96 years.