Abstract

We develop a high precision ?14CO2 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%o (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 ?14CO2 measurement series from NOAA/ESRL's (formerly NOAA/CMDL) surface flask sampling site at Niwot Ridge, Colorado, USA (40.05°N, 105.58°W, 3475 masl) in order to monitor the isotopic composition of carbon dioxide in relatively clean air over the North American continent. ?14CO2 at Niwot Ridge decreased by 5.7‰/yr from 2004 to 2006, with a seasonal amplitude of 3-5%o. A comparison with measurements from the free troposphere above New England, USA (41°N, 72°W) indicates that the ?14CO2 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.