Identifying the Molecular Origin of Global Warming

by Dr. Partha P. Bera (NASA Ames Research Centre, USA)

Monday, August 29, 2011 from 11:30 to 12:30 (Asia/Kolkata)
at Colaba Campus ( AG-80 )

Description

The greenhouse gases (GHG) in the Earth’s atmosphere play a central role in global warming. Carbon dioxide, methane, water and halocarbons are the main greenhouse gases. Some molecules are inherently more potent than others. For example, the global warming potential (GWP) of hydrofluorcarbon HFC-23 (CHF₃) is is 14800, i.e., it is almost fifteen thousand times more potent as a greenhouse gas compared to carbon dioxide, and the GWP of SF₆ is 22800. Halocarbons are in general extremely potent greenhouse gases. In this study we investigate the physical characteristics of greenhouse gases to understand the key molecular properties that govern their potency as global warming agents. We find that their molecular radiative efficiency - ability to absorb infrared radiation - is the most important factor. Long lifetime and concentration in the atmosphere also contribute. In our studies a molecular radiative efficiency, or effectiveness of absorption of IR radiation, was developed by means of systematic theoretical quantum mechanical calculations. The origin of large radiative efficiency and large IR absorption for several classes of molecules was traced to their large bond dipole derivatives. Polarity of the bonds inside the GHGs and unique position of these absorption bands in the electromagnetic spectrum are major contributing factors. Subsequently, we proposed a design strategy to screen the industrially used halocarbons. We investigated various classes of halocarbons, including hydrofluorocarbons, perfluorocarbons, fluoro-ethers, thioethers, and olefins, and we found significant differences in their infrared absorption abilities. Perfluoroethers are most intense absorbers followed by perfluorothioethers, sulfur fluorides and perfluorocarbons. Even within the same class of molecules, a strategic design can reduce the radiative efficiencies of fluorocarbons by a factor of two. Finally, we propose a group increment scheme for estimating molecular radiative efficiencies.