A new high precision 14CO2 time series for North American continental air

[1] We develop a high precision Delta(CO2)-C-14 measurement capability in 2-5 L samples of whole air for implementation within existing greenhouse gas flask sampling networks. The long-term repeatability of the measurement is 1.8% (1-sigma), as determined from repeated analyses of quality control standards and replicate extraction and measurement of authentic field samples. In a parallel effort, we have begun a Delta(CO2)-C-14 measurement series from NOAA/ESRL's ( formerly NOAA/CMDL) surface flask sampling site at Niwot Ridge, Colorado, USA (40.05 degrees N, 105.58 degrees W, 3475 masl) in order to monitor the isotopic composition of carbon dioxide in relatively clean air over the North American continent. Delta(CO2)-C-14 at Niwot Ridge decreased by 5.7 parts per thousand/ yr from 2004 to 2006, with a seasonal amplitude of 3-5 parts per thousand. A comparison with measurements from the free troposphere above New England, USA (41 degrees N, 72 degrees W) indicates that the Delta(CO2)-C-14 series at the two sites are statistically similar at timescales longer than a few days to weeks (i.e., those of synoptic scale variations in transport), suggesting that the Niwot Ridge measurements can be used as a proxy for North American free tropospheric air in future carbon cycle studies.