Climate Metrics for C1-C4 Hydrofluorocarbons (HFCs).

Hydrofluorocarbons (HFCs) are potent greenhouse gases that are potential substitutes for ozone depleting substances. The Kigali amendment lists 17 HFCs that are currently in commercial use to be regulated under the Montreal Protocol. Future commercial applications may explore the use of other HFCs, most of which currently lack an evaluation of their climate metrics. In this work, atmospheric lifetimes, radiative efficiencies (REs), global warming potentials (GWPs), and global temperature change potentials (GTPs) for all saturated HFCs with fewer than 5 carbon atoms are estimated to help guide future usage and policy decisions. Atmospheric lifetimes were estimated using a structure activity relationship (SAR) for OH radical reactivity and estimated O(D) reactivity. Radiative metrics were obtained using theoretically calculated infrared absorption spectra that were presented in a previous work. Calculations for some additional HFCs not included in the previous work were performed in this work. The HFCs display unique infrared spectra with strong absorption in the Earth's atmospheric infrared window region, primarily due to the C-F stretching vibration. Results from this study show that the HFC global atmospheric lifetimes and REs are dependent upon their H atom content and molecular structure. Therefore, the HFC radiative metric evaluation requires a case-by-case evaluation. A thorough experimental evaluation of a targeted HFC's atmospheric lifetime and climate metrics is always highly recommended. However, in cases where it is experimentally difficult to separate isomers, the new results from this study should help guide the experiments, as well as provide relevant climate metrics with uncertainties and policy relevant data.