Atmospheric histories and growth trends of C₄F₁₀, C₅F₁₂, C₆F₁₄, C₇F₁₆ and C₈F₁₈

Abstract. Atmospheric observations and trends are presented for the high molecular weight perfluorocarbons (PFCs): decafluorobutane (C₄F₁₀), dodecafluoropentane (C₅F₁₂), tetradecafluorohexane (C₆F₁₄), hexadecafluoroheptane (C₇F₁₆) and octadecafluorooctane (C₈F₁₈). Their atmospheric histories are based on measurements of 36 Northern Hemisphere and 46 Southern Hemisphere archived air samples collected between 1973 to 2011 using the Advanced Global Atmospheric Gases Experiment (AGAGE) "Medusa" preconcentration gas chromatography-mass spectrometry systems. A new calibration scale was prepared for each PFC, with estimated accuracies of 6.8% for C₄F₁₀, 7.8% for C₅F₁₂, 4.0% for C₆F₁₄, 6.6% for C₇F₁₆ and 7.9% for C₈F₁₈. Based on our observations the 2011 globally averaged dry air mole fractions of these heavy PFCs are: 0.17 parts-per-trillion (ppt, i.e., parts per 10¹²) for C₄F₁₀, 0.12 ppt for C₅F₁₂, 0.27 ppt for C₆F₁₄, 0.12 ppt for C₇F₁₆ and 0.09 ppt for C₈F₁₈. These atmospheric mole fractions combine to contribute to a global average radiative forcing of 0.35 mW m⁻², which is 6% of the total anthropogenic PFC radiative forcing (Montzka and Reimann, 2011; Oram et al., 2012). The growth rates of the heavy perfluorocarbons were largest in the late 1990s peaking at 6.2 parts per quadrillion (ppq, i.e., parts per 10¹⁵) per year (yr) for C₄F₁₀, at 5.0 ppq yr⁻¹ for C₅F₁₂ and 16.6 ppq yr⁻¹ for C₆F₁₄ and in the early 1990s for C₇F₁₆ at 4.7 ppq yr⁻¹ and in the mid 1990s for C₈F₁₈ at 4.8 ppq yr⁻¹. The 2011 globally averaged mean atmospheric growth rates of these PFCs are subsequently lower at 2.2 ppq yr⁻¹ for C₄F₁₀, 1.4 ppq yr⁻¹ for C₅F₁₂, 5.0 ppq yr⁻¹ for C₆F₁₄, 3.4 ppq yr⁻¹ for C₇F₁₆ and 0.9 ppq yr⁻¹ for C₈F₁₈. The more recent slowdown in the growth rates suggests that emissions are declining as compared to the 1980s and 1990s.