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Chlorofluorocarbons (CFCs) are ozone-depleting substances\udas well as strong greenhouse gases, and the control of their production and\uduse under the Montreal Protocol has had demonstrable benefits to both\udmitigation of increasing surface UV radiation and climate forcing. A global\udban on consumption came into force in 2010, but there is evidence of\udcontinuing emissions of certain CFCs from a range of sources. One compound\udhas received little attention in the literature, namely CFC-114\ud(C2Cl2F4). Of particular interest here is the differentiation\udbetween CFC-114 (CClF2CClF2) and its asymmetric isomeric form\udCFC-114a (CF3CCl2F) as atmospheric long-term measurements in the\udpeer-reviewed literature to date have been assumed to represent the sum of\udboth isomers with a time-invariant isomeric speciation. Here we report the\udfirst long-term measurements of the two isomeric forms separately, and find\udthat they have different origins and trends in the atmosphere.\ud \udAir samples collected at Cape Grim (41° S), Australia, during\udatmospheric background conditions since 1978, combined with samples\udcollected from deep polar snow (firn) enable us to obtain a near-complete\udrecord of both gases since their initial production and release in the\ud1940s. Both isomers were present in the unpolluted atmosphere in comparably\udsmall amounts before 1960. The mixing ratio of CFC-114 doubled from 7.9 to\ud14.8 parts per trillion (ppt) between the start of the Cape Grim record in\ud1978 and the end of our record in 2014, while over the same time CFC-114a\udtrebled from 0.35 to 1.03 ppt. Mixing ratios of both isomers are slowly\uddecreasing by the end of this period. This is consistent with measurements\udof recent aircraft-based samples showing no significant interhemispheric\udmixing ratio gradient.\ud \udWe also find that the fraction of CFC-114a mixing ratio relative to that of\udCFC-114 increased from 4.2 to 6.9 % over the 37-year period. This\udcontradicts the current tacit assumption used in international climate\udchange and ozone depletion assessments that both isomers have been largely\udco-emitted and that their atmospheric concentration ratio has remained\udapproximately constant in time. Complementary observations of air collected\udin Taiwan indicate a persisting source of CFC-114a in South East Asia which\udmay have been contributing to the changing balance between the two isomers.\ud \udIn addition we present top-down global annual emission estimates of CFC-114\udand CFC-114a derived from these measurements using a two-dimensional\udatmospheric chemistry-transport model. In general, the emissions for both\udcompounds grew steadily during the 1980s, followed by a substantial\udreduction from the late 1980s onwards, which is consistent with the\udreduction of emission in response to the Montreal Protocol, and broadly\udconsistent with bottom-up estimates derived by industry. However, we find\udthat small but significant emissions of both isomers remain in 2014.\udMoreover the inferred changes to the ratio of emissions of the two isomers\udsince the 1990s also indicate that the sources of the two gases are, in\udpart, independent

Tropospheric observations of CFC-114 and CFC-114a with a focus on long-term trends and emissions