Abstract

Abstract. We present 20-year flask sample records of atmospheric CO2, δ(O2/N2), and atmospheric potential oxygen (APO) from the stations Lutjewad (the Netherlands) and Mace Head (Ireland), and a 3-year record from Halley station (Antarctica). We include details of our calibration procedures and the stability of our calibration scale over time, which we estimate to be 3 per meg over the 11 years of calibration, and our compatibility with the international Scripps O2 scale. The measurement records from Lutjewad and Mace Head show similar long-term trends during the period 2002–2018 of 2.31 ± 0.07 ppm yr−1 for CO2 and −21.2 ± 0.8 per meg yr−1 for δ(O2/N2) at Lutjewad, and 2.22 ± 0.04 ppm yr−1 for CO2 and −21.3 ± 0.9 per meg yr−1 for δ(O2/N2) at Mace Head. They also show a similar δ(O2/N2) seasonal cycle with an amplitude of 54 ± 4 per meg at Lutjewad and 61 ± 5 per meg at Mace Head, while the CO2 seasonal amplitude at Lutjewad (16.8 ± 0.5 ppm) is slightly higher than that at Mace Head (14.8 ± 0.3 ppm). We show that the observed long-term trends and seasonal cycles are in good agreement with the measurements from various other stations, especially the measurements from the Weybourne Atmospheric Observatory (United Kingdom). However, there are remarkable differences in the progression of annual trends between the Mace Head and Lutjewad records for δ(O2/N2) and APO, which might in part be caused by sampling differences, but also by environmental effects, such as North Atlantic Ocean oxygen ventilation changes to which Mace Head is more sensitive. The Halley record shows clear trends and seasonality in δ(O2/N2) and APO, the latter agreeing especially well with continuous measurements at the same location made by the University of East Anglia (UEA), while CO2 and δ(O2/N2) present slight disagreements, most likely caused by small leakages during sampling. From our 2002–2018 records, we find a good agreement with Global Carbon Budget 2021 (Friedlingstein et al. (2021) for the global ocean carbon sink: 2.1 ± 0.8 PgC yr−1, based on the Lutjewad record. The data presented in this work are available at https://doi.org/10.18160/qq7d-t060 (Nguyen et al., 2021).

Highlights

  • The global carbon cycle is a dynamic system that comprises the exchanges of carbon between various reservoirs and is important for studying human-induced climate change and its impacts (Ciais et al, 2013)

  • Due to the sensitive nature of oxygen measurements, we conducted an extensive and intensive calibration procedures, which demonstrated long-term stability for δ(O2/N2) of 3 per meg in 11 years based on our own internal cylinders and 8.6 per meg in 10 years based on our Scripps primary standards

  • Measurements of the global primary standard cylinders and inter-comparison cylinders confirm the stability, quality, and comparability of our calibration procedure, there are some indications that our calibration scale might not be entirely stable over the past 20 years

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Summary

Introduction

The global carbon cycle is a dynamic system that comprises the exchanges of carbon between various reservoirs and is important for studying human-induced climate change and its impacts (Ciais et al, 2013). By combining the decadal trends of atmospheric CO2 and O2, we can quantify the global land and ocean carbon sinks (Bender et al, 1996; Keeling and Shertz, 1992; Manning and Keeling, 2006; Tohjima et al, 2019). This is because CO2 and O2 cycles are closely coupled – in most processes, there is an anti-correlation in the changes of their mole fraction, except for the oceanic uptake of CO2 (Manning and Keeling, 2006). To quantify the various components of the global carbon cycle, the changes in atmospheric mole fraction of the two species can be used in combination with their stoichiometric exchange ratio (ER), which is the ratio of CO2 and O2 exchanged (consumed / produced) in a process. The ER value varies depending on the process, and is close to 1.1 for photosynthesis / respiration (Severinghaus, 1995) and on average 1.38 for the global mix of fossil fuels (Keeling and Manning, 2014)

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