1Center for Global Change Science, Massachusetts Institute of Technology, Cambridge, Massachusetts, USA
2Scripps Institution of Oceanography, Univ. of California San Diego, La Jolla, California, USA
3Centre for Australian Weather and Climate Research, CSIRO Marine and Atmospheric Research, Aspendale, Victoria, Australia
*now at: Atmospheric Chemistry Research Group, University of Bristol, Bristol, UK
**now at: Defence Science and Technology Organisation, Department of Defence, Melbourne, Victoria, Australia
Abstract. The first 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 38 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: 0.18 parts-per-trillion (ppt, i.e., parts per 1012) for C4F10, 0.12 ppt for C5F12, 0.28 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 3.6% of the total PFC radiative forcing. The globally averaged mean atmospheric growth rates of these PFCs during 1973–2011 are 4.58 parts per quadrillion (ppq, i.e., parts per 1015) per year (yr) for C4F10, 3.29 ppq yr−1 for C5F12, 7.50 ppq yr−1 for C6F14, 3.19 ppq yr−1 for C7F16 and 2.51 ppq yr−1 for C8F18. The growth rates of the heavy perfluorocarbons were largest in the early 1990s for C4F10 and C5F12 and in the mid-to-late 1990s for C6F14, C7F16 and C8F18. The more recent slow down in the growth rates of the high molecular weight PFCs suggests that emissions are declining as compared to the 1980s and 1990s. Nevertheless continued monitoring of these potent, extremely long-lived greenhouse gases is necessary to verify that global PFC emissions continue to decline.