Interannual variations in the Δ(¹⁷O) signature of atmospheric CO₂ at two mid-latitude sites suggest a close link to stratosphere-troposphere exchange

Abstract. Δ(¹⁷O )measurements of atmospheric CO₂ have the potential to be a tracer for gross primary production and stratosphere-troposphere mixing. A positive Δ(¹⁷O) originates from intrusions of stratospheric CO₂, whereas values close to zero result from equilibration of CO₂ and water, predominantly happening inside plants. The stratospheric source of CO₂ carrying high Δ(¹⁷O) is, however, not well defined in the current models. More and long-time atmospheric measurements are needed to improve this. We present records of the Δ(¹⁷O) of atmospheric CO₂ conducted with laser absorption spectroscopy, from Lutjewad in the Netherlands (53° 24’N, 6° 21’E) and Mace Head in Ireland (53° 20’ N, 9° 54’ W), covering the period 2017–2022. The records are compared with a 3-D model simulation, and we study potential model improvements. Both records show significant interannual variability, of up to 0.3 ‰. The total range covered by smoothed monthly averages from the Lutjewad record is -0.065 to 0.046 ‰, which is significantly higher than the range of -0.009 and 0.036 ‰ of the model simulation. The 100 hPa 60–90° North monthly mean temperature anomaly was used as a proxy to scale stratospheric downwelling in the model. This strongly improves the correlation coefficient of the simulated and observed year-to-year Δ(¹⁷O) variations over the period 2019–2021, from 0.37 to 0.81. As the Δ(¹⁷O) of atmospheric CO₂ seems to be dominated by stratospheric influx, its use a as a tracer for stratosphere-troposphere exchange should be further investigated.