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.