Interannual variations in the "( 17O) signature of atmospheric CO 2 at two mid-latitude sites suggest a close link to stratosphere-troposphere exchange

▵ (17O) measurements of atmospheric CO2 have the potential to be a tracer for gross primary production and stratosphere-troposphere mixing. A positive ▵ (17O) originates from intrusions of stratospheric CO2, whereas values close to -0.21 result from the equilibration of CO2 and water, which predominantly happens inside plants. The stratospheric source of CO2 with high ▵ (17O) is, however, not well defined in the current models. More, and long-term, atmospheric measurements are needed to improve this. We present records of the ?(17O) of atmospheric CO2 obtained 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) that cover 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 -0.34 to -0.12 , which is significantly higher than the range of -0.20 to -0.17 for the model simulation. The 100 hPa 60-90 N 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 ?(17O) variations over the period 2019-2021 from 0.40 to 0.82. As the ?(17O) of atmospheric CO2 seems to be dominated by stratospheric influx, its use as a tracer for stratosphere-troposphere exchange should be further investigated.