Perfluorocarbons in the global atmosphere: tetrafluoromethane, hexafluoroethane, and octafluoropropane

Abstract

We present atmospheric baseline growth rates from the 1970s to the present for the long-lived, strongly infrared-absorbing perfluorocarbons (PFCs) tetrafluoromethane (CF₄), hexafluoroethane (C₂F₆), and octafluoropropane (C₃F₈) in both hemispheres, measured with improved accuracies (~1–2%) and precisions (<0.3%, or <0.2 ppt (parts per trillion dry air mole fraction), for CF₄; <1.5%, or <0.06 ppt, for C₂F₆; <4.5%, or <0.02 ppt, for C₃F₈ within the Advanced Global Atmospheric Gases Experiment (AGAGE). Pre-industrial background values of 34.7&plusmn;0.2 ppt CF₄ and 0.1&plusmn;0.02 ppt C₂F₆ were measured in air extracted from Greenland ice and Antarctic firn. Anthropogenic sources are thought to be primary aluminum production (CF₄, C₂F₆, C₃F₈), semiconductor production (C₂F₆, CF₄, C₃F₈) and refrigeration use (C₃F₈). Global emissions calculated with the AGAGE 2-D 12-box model are significantly higher than most previous emission estimates. The sum of CF₄ and C₂F₆ emissions estimated from aluminum production and non-metal production are lower than observed global top-down emissions, with gaps of ~6 Gg/yr CF₄ in recent years. The significant discrepancies between previous CF₄, C₂F₆, and C₃F₈ emission estimates and observed global top-down emissions estimated from AGAGE measurements emphasize the need for more accurate, transparent, and complete emission reporting, and for verification with atmospheric measurements to assess the emission sources of these long-lived and potent greenhouse gases, which alter the radiative budget of the atmosphere, essentially permanently, once emitted.