Indentifying the Molecular Origin of Global Warming

Indentifying the Molecular Origin of Global Warming Partha P. Bera, Joseph S. Francisco and Timothy J. Lee NASA Ames Research Center, Space Science and Astrobiology Division, Moffett Field, California 94035, and Department of Chemistry and Department of Earth and Atmospheric Sciences, Purdue University, West Lafayette, Indiana 47907-1393 Abstract The physical characteristics of greenhouse gases (GHGs) have been investigated to assess which properties are most important in determining the radiative efficiency of a GHG. Chlorofluorcarbons (CFCs), hydrofluorocarbons (HFCs), perfluorocarbons (PFCs), hydrofluoroethers, fluoroethers, nitrogen fluorides, and various other known atmospheric trace molecules have been included in this study. Compounds containing the halogens F or Cl have in common very polar X-F or X-Cl bonds, particularly the X-F bonds. It is shown that as more F atoms bond to the same central atom, the bond dipoles become larger as a result of the central carbon atom becoming more positive. This leads to a linear increase in the total or integrated X-F bond dipole derivatives for the molecule, which leads to a non-linear (quadratic) increase in infrared (IR) intensity. Moreover, virtually all of the X-F bond stretches occur in the atmospheric IR window as opposed to X-H stretches, which do not occur in the atmospheric window. It is concluded that molecules possessing several F atoms will always have a large radiative forcing parameter in the calculation of their global warming potential. Some of the implications for global warming and climate change and a new design strategy for more environmentally friendly industrial materials from a molecular quantum chemistry perspective will be discussed.