Atmospheric histories and growth trends of C 4F 10, C 5F 12, C 6F 14, C 7F 16 and C 8F 18
Atmospheric observations and trends are presented for the high molecular weight perfluorocarbons (PFCs): decafluorobutane (C 4F 10), dodecafluoropentane (C 5F 12), tetradecafluorohexane (C 6F 14), hexadecafluoroheptane (C 7F 16) and octadecafluorooctane (C 8F 18). 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 C 4F 10, 7.8% for C 5F 12, 4.0% for C 6F 14, 6.6% for C 7F 16 and 7.9% for C 8F 18. 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 10 12) for C 4F 10, 0.12 ppt for C 5F 12, 0.27 ppt for C 6F 14, 0.12 ppt for C 7F 16 and 0.09 ppt for C 8F 18. 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 10 15) per year (yr) for C 4F 10, at 5.0 ppq yr −1 for C 5F 12 and 16.6 ppq yr −1 for C 6F 14 and in the early 1990s for C 7F 16 at 4.7 ppq yr −1 and in the mid 1990s for C 8F 18 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 C 4F 10, 1.4 ppq yr −1 for C 5F 12, 5.0 ppq yr −1 for C 6F 14, 3.4 ppq yr −1 for C 7F 16 and 0.9 ppq yr −1 for C 8F 18. The more recent slowdown in the growth rates suggests that emissions are declining as compared to the 1980s and 1990s.