Initial Results of an Intercomparison of AMS-Based Atmospheric ¹⁴CO₂ Measurements

Abstract

This article presents results from the first 3 rounds of an international intercomparison of measurements of Δ14CO2 in liter-scale samples of whole air by groups using accelerator mass spectrometry (AMS). The ultimate goal of the intercomparison is to allow the merging of Δ14CO2 data from different groups, with the confidence that differences in the data are geophysical gradients and not artifacts of calibration. Eight groups have participated in at least 1 round of the intercomparison, which has so far included 3 rounds of air distribution between 2007 and 2010. The comparison is intended to be ongoing, so that: a) the community obtains a regular assessment of differences between laboratories; and b) individual laboratories can begin to assess the long-term repeatability of their measurements of the same source air. Air used in the intercomparison was compressed into 2 high-pressure cylinders in 2005 and 2006 at Niwot Ridge, Colorado (USA), with one of the tanks “spiked” with fossil CO2, so that the 2 tanks span the range of Δ14CO2 typically encountered when measuring air from both remote background locations and polluted urban ones. Three groups show interlaboratory comparability within l% for ambient level Δ14CO2. For high CO2/low Δ14CO2 air, 4 laboratories showed comparability within 2%. This approaches the goals set out by the World Meteorological Organization (WMO) CO2 Measurements Experts Group in 2005. One important observation is that single-sample precisions typically reported by the AMS community cannot always explain the observed differences within and between laboratories. This emphasizes the need to use long-term repeatability as a metric for measurement precision, especially in the context of long-term atmospheric monitoring.

Detection of Fossil and Biogenic Methane at Regional Scales Using Atmospheric Radiocarbon

“…Uncertainty in measured Δ 14 CH 4 , contributing to uncertainty in Δ m and Δ bg , was ±5‰ to ±11‰ in the most recently reported observations from Townsend‐Small et al (). This is substantially higher than the uncertainty in measurements of Δ 14 CO 2 (±2‰ to ±3‰, Miller, Lehman, et al, ), indicating that additional sample processing or smaller sample size for CH 4 compared to CO 2 contribute substantial uncertainty to Δ 14 CH 4 measurements. Since there are very few measurements of Δ 14 CH 4 it is difficult to assess variability in Δ bg .…”

Changes to Carbon Isotopes in Atmospheric CO2 Over the Industrial Era and Into the Future

“…Reference material used for Δ 14 C measurements is typically oxalic acid (Stuiver, 1983), but whole air reference materials have also been used for atmospheric measurements (Graven et al, 2012b). Whole air and CO 2 have been used in intercomparisons between radiocarbon laboratories making atmospheric measurements and generally showed compatibility of 2‰ or better (Hammer et al, 2017;Miller et al, 2013), in addition to intercomparison activities using wood cellulose and other materials (e.g., Scott et al, 2010).…”

Optical Measurement of Radiocarbon below Unity Fraction Modern by Linear Absorption Spectroscopy

“…These applications in quantitative 14 C science require worldwide intercomparisons between measurement facilities, specifically for facilities performing liquid scintillation counting and accelerator mass spectrometry (AMS) . Recent AMS intercomparisons have focused on high-precision (≤1%) applications in atmospheric 14 C science, where agreed-upon standards for gas handling, sample preparation, and calibration are needed to realize ultimate sensitivities. , For many applications, however, a precision approaching 0.1% of modern 14 C (F 14 C) is not required. In this context, the emergence of alternative benchtop instruments could drive rapidly evolving interdisciplinary research and measurement applications, as well as reduce the number of samples processed by high-precision AMS laboratories, thus freeing more state-of-the-art laboratory time for the most demanding 14 C applications.…”

“…4 Recent AMS intercomparisons have focused on high-precision (≤1%) applications in atmospheric 14 C science, where agreed-upon standards for gas handling, sample preparation, and calibration are needed to realize ultimate sensitivities. 5,6 For many applications, however, a precision approaching 0.1% of modern 14…”