High-Precision Atmospheric ¹⁴CO₂ Measurement at the Rafter Radiocarbon Laboratory

Abstract

This article describes a new capability for high-precision 14 C measurement of CO 2 from air at the Rafter Radiocarbon Laboratory, GNS Science, New Zealand. We evaluate the short-term within-wheel repeatability and long-term between-wheel repeatability from measurements of multiple aliquots of control materials sourced from whole air. Samples are typically measured to 650,000 14 C counts, providing a nominal accelerator mass spectrometry (AMS) statistical uncertainty of 1.3‰. No additional uncertainty is required to explain the within-wheel variability. An additional uncertainty factor is needed to explain the long-term repeatability spanning multiple measurement wheels, bringing the overall repeatability to 1.8‰, comparable to other laboratories measuring air materials to high precision. This additional uncertainty factor appears to be due to variability in the measured 14 C content of OxI primary standard targets, likely from the combustion process. We observe an offset of 1.4‰ in our samples relative to those measured by the University of Colorado INSTAAR, comparable to interlaboratory offsets observed in recent intercomparison exercises.

Sixty years of radiocarbon dioxide measurements at Wellington, New Zealand: 1954–2014

“…Most flasks were collected during southerly, clean-air conditions (Stephens et al, 2013). CO 2 is extracted cryogenically (Turnbull et al, 2015b). For whole-air samples collected from 1984 to 1993, the extracted CO 2 was archived until 2012.…”

“…δ 13 C from all the remaining samples was in agreement with δ 13 C measured from separate flasks collected at Baring Head and measured for δ 13 C by the Scripps Institution of Oceanography close to the time of collection (http: //scrippsco2.ucsd.edu/data/atmospheric_co2/nzd, Keeling et al, 2001). Whole-air samples collected since 2013 are analysed for δ 13 C and other trace gases and isotopes at NIWA (National Institute of Water and Atmospheric Research, New Zealand) (Ferretti et al, 2000), and for the 14 CO 2 measurement, CO 2 is extracted from whole air at the Rafter Radiocarbon Laboratory (Turnbull et al, 2015b).…”

“…All measurements made since 1995, including recent measurements of flask samples collected in the 1980s and 1990s, were reduced to graphite, measured by accelerator mass spectrometry (AMS), and are identified by their unique "NZA" numbers. The LG1 graphitization system was used from 1995 to 2011 (NZA < 50 000; Lowe and Judd, 1987), and replaced with the RG20 graphite system in 2011 (NZA > 50 000; Turnbull et al, 2015b). Samples measured by AMS were stored for up to 3 years between sample collection and extraction, graphitization and measurement.…”

“…XCAMS measures all three carbon isotopes, such that the normalization correction is performed using the AMS-measured 13 C values (Zondervan et al, 2015). XCAMS measurements are made on single graphite targets measured to high precision of typically 1.8 ‰ (Turnbull et al, 2015b).…”

Tower measurement network of in-situ CO2, CH4, and CO in support of the Indianapolis FLUX (INFLUX) Experiment

“…These samples were collected in the mid-afternoon (1300-1500 LST, 1900-2000 GMT) when atmospheric mixing was most vigorous and were collected only on days when the wind was from the western sector (180-360°) so that Site 01 represented background conditions (Turnbull et al, 2015a (Turnbull et al, 2015a), a suite of halo-and hydrocarbons (Montzka et al, 1993) and stable isotopes of CO 2 , CH 4 and/or CO (Vaughn et al, 2004). The flask samples allowed partitioning of CO 2 into fossil and biogenic components (Turnbull et al, 2015b) and potentially among more detailed sectors or processes using the multiple species measured.…”