Tracer studies in the laboratory and in the atmosphere

Tritium atoms formed in neutron-induced nuclear reactions have initial kinetic energies (from 3He 192,000 e.v.) far in excess ofenergies, and must lose almost all of this energy before forming a stable chemical bond. However, in the chemical reaction region of 5 to 10 e.v., open-quotes hotclose quotes reactions are observed not found for thermal hydrogen atoms, especially substitution reactions in which T can replace, H, D, F, Cl, CH3, etc. These substitutions normally occur with retention of configuration in asymmetric carbon and often leave a molecule so excited that secondary decompositions follow, even in condensed phases. Photochemically produced T atoms of known energy have allowed determination of the threshold for T-for-H substitution in methane as 1.5 e.v. New mechanistic pathways were discovered when radioactive tracers were introduced into the study of ketene and methylene. Examples include the oxirene intermediate in the photolysis of ketene, The intramolecular elimination of molecular hydrogen when triplet methylene reacts with O2, and the radical-radical reactions observed for 3CH2 in methane. The authors' initial study of trace species in the atmosphere was directed toward the chlorofluorocarbon gases, and led the discovery that they can deplete stratospheric ozone. During the past decade, the authors have been using gas chromatography with greater sensitivity, resolution and precision to follow the atmospheric chemistry of the CFCs and other halocarbons, methane, CO and many non-methane hydrocarbons