IRIS analyser assessment reveals sub-hourly variability of isotope ratios in carbon dioxide at Baring Head, New Zealand's atmospheric observatory in the Southern Ocean

• Published in: Atmospheric Measurement Techniques
• Main Authors: P. Sperlich, G. W. Brailsford, R. C. Moss, J. McGregor, R. J. Martin, S. Nichol, S. Mikaloff-Fletcher, B. Bukosa, M. Mandic, C. I. Schipper, P. Krummel, A. D. Griffiths
• Format: Article in Journal/Newspaper
• Language: English
• Published: Copernicus Publications 2022
• Subjects: Environmental engineering; TA170-171; Earthwork. Foundations; TA715-787; Southern Ocean; New Zealand
• Online Access: https://doi.org/10.5194/amt-15-1631-2022; https://doaj.org/article/d9c14f0c31bf4b34ac2694450d7d5ad2

We assess the performance of an isotope ratio infrared spectrometer (IRIS) to measure carbon ( δ 13 C) and oxygen ( δ 18 O) isotope ratios in atmospheric carbon dioxide (CO 2 ) and report observations from a 26 d field deployment trial at Baring Head, New Zealand, NIWA's atmospheric observatory for Southern Ocean baseline air. Our study describes an operational method to improve the performance in comparison to previous publications on this analytical instrument. By using a calibration technique that reflected the principle of identical treatment of sample and reference gases, we achieved a reproducibility of 0.07 ‰ for δ 13 C-CO 2 and 0.06 ‰ for δ 18 O-CO 2 over multiple days. This performance is within the extended compatibility goal of 0.1 ‰ for both δ 13 C-CO 2 and δ 18 O-CO 2 , which was recommended by the World Meteorological Organization (WMO). Further improvement in measurement performance is desirable to also meet the WMO network compatibility goals of 0.01 ‰ for δ 13 C-CO 2 and 0.05 ‰ for δ 18 O-CO 2 , which is needed to resolve the small variability that is typical for background air observatories such as Baring Head. One objective of this study was to assess the capabilities and limitations of the IRIS analyser to resolve δ 13 C-CO 2 and δ 18 O-CO 2 variations under field conditions. Therefore, we selected multiple events within the 26 d record for Keeling plot analysis. This resolved the isotopic composition of endmembers with an uncertainty of ≤ 1 ‰ when the magnitude of CO 2 signals is larger than 10 ppm. The uncertainty of the Keeling plot analysis strongly increased for smaller CO 2 events (2–7 ppm), where the instrument performance is the limiting factor and may only allow for the distinction between very different endmembers, such as the role of terrestrial versus oceanic carbon cycle processes.