HFOs have C=C double bonds; the hydroxyl radical (OH) is the primary reactive agent in the atmosphere, reaction of OH radicals with C=C double bonds is very rapid; compared to a similar HFC the reaction of HFO is 2000 times faster. The HFCs are greenhouse gases because they absorb Earth’s outgoing infrared radiation in a spectral range where energy is not removed by CO2 or water vapour. The carbon-fluorine bond absorbs infrared in this range. The magnitude of this effect depends on:
• the number of C-F bonds in the molecule
• its atmospheric lifetime which is determined by how rapidly it breaks down
• its concentration in the atmosphere, which is determined by emissions and atmospheric lifetime
Minimizing the contribution to global warming can be achieved by reducing one or more of these factors, in particular by designing molecules that achieve the required technical and safety properties but with very short atmospheric lifetimes. However, retaining fluorine in a molecule has technical and safety benefits, for example it reduces flammability.
From 2005 IPCC/TEAP Special Report Safeguarding the Ozone Layer and the Global Climate System: Issues Related to Hydrofluorocarbons and Perfluorocarbons; Technical Summary
Why do HFO and HCFOs breakdown rapidly: The reaction of OH radicals with C=C double bonds is very rapid and dominates the atmospheric removal mechanism for HFOs and HCFOs. The inclusion of a double bond is a particularly effective method to increase the reactivity of organic molecules towards OH radicals.
Comparing the reactivity of HFC with HFO for reaction with OH radicals
CF3CF=CH2 (HFO-1234yf, GWP <1 AR5 value) is approximately 2000 times more reactive than CF3CF2CH3 (HFC-245cb, GWP 4620) [World Meteorological Organization, 2011. Scientific Assessment of Ozone Depletion: 2010, Global Ozone, Research and Monitoring Project – Report 5].
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