Making atmospheric measurements of difficult-to-separate isomers of CFCs routine: a case study of CFC-113 and CFC-113a that refines our understanding of recent atmospheric changes for both of these chemicals.

Ethane-based chlorofluorocarbons and hydrochlorofluorocarbons can have multiple isomers that are difficult to separate using gas chromatography with detection by electron capture (GC-ECD) or even mass spectrometry (GC-MS). Examples include CFC-113 and CFC-113a; CFC-114 and CFC-114a; CFC-112 and CFC-112a; and HCFC-124 and HCFC-124a.  As a result, atmospheric histories reported in the past by most laboratories for the more abundant isomer can represent an ill-defined combination of both chemicals.  This is especially true for CFC-113 over the past decade, as mole fractions of CFC-113a have increased rapidly during that time and the contribution of this isomer to our understanding of atmospheric changes measured ostensibly for CFC-113 has not been known. Being able to accurately and routinely determine atmospheric abundances for each of these isomers separately from one another is important as these isomers have different environmental impacts (ozone-depleting potentials and global warming potentials) and different sources likely contribute to their emissions.  Here we demonstrate a technique for quantifying the abundances of co-eluting isomers using GC-MS even when ions unique to the different isomers are unavailable.  Initial results for CFC-113 and 113a will be presented that allow a reassessment of the atmospheric decline and global emission rate of CFC-113 over the past decade, and they confirm the rapid increases in the atmospheric abundance of CFC-113a.  Co-variations between the measured CFC-113a atmospheric mole fractions and other gases are observed at particular sites (e.g., Mauna Loa, Hawaii) that help identify regions where CFC-113a emissions are currently substantial, contributing to its rapid global increase.