On the Mesospheric Removal of Very Long-lived Greenhouse Gases

The fluorinated gases SF6, NF3 and CFC-115 are chemically inert with atmospheric lifetimes of many centuries which, combined with their strong absorption of infrared radiation, results in unusually high global warming potentials. Very long lifetimes imply that potential mesospheric sinks could make important contributions to their atmospheric removal. In order to investigate this, the reactions of each species with the neutral metal atoms Na, K, Mg and Fe, which are produced by meteoric ablation in the upper mesosphere, were therefore studied. The observed non-Arrhenius temperature dependences of the reactions are interpreted using quantum chemistry calculations of the relevant potential energy surfaces. The absorption cross-section at the prominent solar Lyman-α solar emission line (121.6 nm) was also determined. In the second part of this study updated values for the infrared absorption cross sections of SF6, NF3 and CFC-115 were experimentally determined and used in two radiative transfer models in order to determine radiative forcing and efficiency values. These were carried out with thorough sensitivity analysis and included the effect of clouds and stratospheric adjustment. A three-dimensional chemistry climate model was used separately to determine updated atmospheric lifetimes of each species. Finally, we combined our results to determine updated global warming potentials over a 20, 100 and 500 year time period.