Abstract
Abstract. A series of 12 high volume air samples collected from the S2 firn core during the North Greenland Eemian Ice Drilling (NEEM) 2009 campaign have been measured for mixing ratio and stable carbon isotope composition of the chlorofluorocarbon CFC-12 (CCl2F2). While the mixing ratio measurements compare favorably to other firn air studies, the isotope results show extreme 13C depletion at the deepest measurable depth (65 m), to values lower than δ13C = −80‰ vs. VPDB (the international stable carbon isotope scale), compared to present day surface tropospheric measurements near −40‰. Firn air modeling was used to interpret these measurements. Reconstructed atmospheric time series indicate even larger depletions (to −120‰) near 1950 AD, with subsequent rapid enrichment of the atmospheric reservoir of the compound to the present day value. Mass-balance calculations show that this change is likely to have been caused by a large change in the isotopic composition of anthropogenic CFC-12 emissions, probably due to technological advances in the CFC production process over the last 80 yr, though direct evidence is lacking.
Highlights
Before it was banned under the Montreal protocol and subsequent amendments, the use of chlorofluorocarbon-12 (CFC12, CCl2F2) as a refrigerant and aerosol propellant worldwide has resulted in significant atmospheric loading; at its peak in 2003, the mean mixing ratio of this compound in the troposphere was approximately 545 ppt, making it the most abundant anthropogenic halocarbon compound in the atmosphere (McCulloch et al, 2003; Forster et al, 2007; Montzka et al, 2011)
The atmospheric mixing ratio history of CFC-12 prior to this period has been reconstructed through the measurement of firn air samples combined with firn air modeling and emissions estimates
The large air samples used for the analysis presented here were dried, and compressed on-site to 120 bar pressure into 5 L Scott-Marrin aluminium cylinders using a threestage oil free piston compressor (Mak and Brenninkmeijer, 1994)
Summary
Before it was banned under the Montreal protocol and subsequent amendments, the use of chlorofluorocarbon-12 (CFC12, CCl2F2) as a refrigerant and aerosol propellant worldwide has resulted in significant atmospheric loading; at its peak in 2003, the mean mixing ratio of this compound in the troposphere was approximately 545 ppt (ppt = parts per trillion = pmol mol−1), making it the most abundant anthropogenic halocarbon compound in the atmosphere (McCulloch et al, 2003; Forster et al, 2007; Montzka et al, 2011). Due to the buildup of this and other anthropogenic halocarbons, total chlorine loading in the troposphere has increased from approximately 600 ppt (1900) to nearly 3400 ppt (2008) (Forster et al, 2007; Montzka et al, 2011). Zuiderweg et al.: Extreme 13C depletion of CCl2F2 in firn air samples
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