Trends and Emissions of Six Perfluorocarbons in the Northern and Southern Hemisphere.

Perfluorocarbons (PFCs) are potent greenhouse gases with Global Warming Potentials up to several thousand times greater than CO₂ on a 100-year time horizon. The lack of any significant sinks for PFCs means that they have long atmospheric lifetimes on the order of thousands of years. Anthropogenic production is thought to be the only source for most PFCs. Here we report an update on the global atmospheric abundances of the following PFCs, most of which have for the first time been separated according to their isomers: c-octafluorobutane (c-C₄F₈), n-decafluorobutane (n-C₄F₁₀), n-dodecafluoropentane (n-C₅F₁₂), n-tetradecafluorohexane (n-C₆F₁₄), and n-hexadecafluoroheptane (n-C₇F₁₆). Additionally, we report the first data set on the atmospheric mixing ratios of perfluoro(2-methylpentane) (i-C₆F₁₄). The existence and significance of PFC isomers has not been reported before, due to the analytical challenges of separating them. The time series spans a period from 1978 to the present. Several datasets are used to investigate temporal and spatial trends of these PFCs: time series of air samples collected at Cape Grim, Australia, from 1978 to the start of 2018; a time series of air samples collected between July 2015 and April 2017 at Tacolneston, UK; and intensive campaign-based sampling collections from Taiwan. Although the remote background Southern Hemispheric Cape Grim time series indicates that recent growth rates of most of these PFCs are lower than in the 1990s, we continue to see significantly increasing mixing ratios that are between 6% to 27% higher by the end of 2017 compared to abundances measured in 2010. Air samples from Tacolneston show a positive offset in PFC mixing ratios compared to the Southern Hemisphere baseline. The highest mixing ratios and variability are seen in air samples from Taiwan, which is therefore likely situated much closer to PFC sources, confirming predominantly Northern Hemispheric emissions for most PFCs. Even though these PFCs occur in the atmosphere at levels of parts per trillion molar or less, their total cumulative global emissions translate into 833 million metric tonnes of CO₂ equivalent by the end of 2017, 23% of which has been emitted in the last eight years. Almost two-thirds of the CO₂ equivalent emissions are attributable to c-C₄F₈, which currently also has the highest emission rates that continue to grow. Despite this, the sources of all PFCs covered in this work remain poorly constrained and reported emissions in global databases do not account for the abundances found in the atmosphere. [ABSTRACT FROM AUTHOR]