Interannual variations in the Δ17O signature of atmospheric CO2 at two mid-latitude sites suggest a close link to stratosphere-troposphere exchange

We present the first records of the triple oxygen isotope signature Δ17O 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 average uncertainty of 0.07 ‰ is ∼3 times smaller than the total observed variability. A positive Δ17O originates from intrusions of stratospheric CO2, whereas values close to 0.0 result from equilibration of CO2 and water, predominantly happening inside plants due to photosynthesis. A biosphere driven seasonal signal is, however, not observed in the records. 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 ‰ that was simulated with a 3-D transport model. One of the major model uncertainties is the representation of the stratospheric influx of Δ17O. We modified the model using the 100 hPa 60-90° North monthly mean temperature anomaly as a proxy to scale stratospheric downwelling. This results in a strong improvement in the simulated and the observed year-to-year Δ17O variations at Lutjewad. The correlation coefficient over the period 2019-2021 improved from 0.37 to 0.81. To infer terrestrial carbon fluxes, the contribution of the stratosphere to the observed signal should therefore be considered. In fact, as the Δ17O of atmospheric CO2 seems to be dominated by stratospheric influx, it might be used as a tracer for stratosphere-troposphere exchange.