Tracking isotopic signatures of CO₂ at the high altitude site Jungfraujoch with laser spectroscopy: analytical improvements and representative results

Abstract. We present the continuous data record of atmospheric CO₂ isotopes measured by laser absorption spectroscopy for an almost four year period at the High Altitude Research Station Jungfraujoch (3580 m a.s.l.), Switzerland. The mean annual cycles derived from data of December 2008 to September 2012 exhibit peak-to-peak amplitudes of 11.0 μmol mol⁻¹ for CO₂, 0.60‰ for δ¹³C and 0.81‰ for δ¹⁸O. The high temporal resolution of the measurements also allow us to capture variations on hourly and diurnal timescales. For CO₂ the mean diurnal peak-to-peak amplitude is about 1 μmol mol⁻¹ in spring, autumn and winter and about 2 μmol mol⁻¹ in summer. The mean diurnal variability in the isotope ratios is largest during the summer months too, with an amplitude of about 0.1‰ both in the δ¹³C and δ¹⁸O, and a smaller or no discernible diurnal cycle during the other seasons. The day-to-day variability, however, is much larger and depends on the origin of the air masses arriving at Jungfraujoch. Backward Lagrangian particle dispersion model simulations revealed a close link between air composition and prevailing transport regimes and could be used to explain part of the observed variability in terms of transport history and influence region. A footprint clustering showed significantly different wintertime CO₂, δ¹³C and δ¹⁸O values depending on the origin and surface residence times of the air masses.

Several major updates on the instrument and the calibration procedures were performed in order to further improve the data quality. We describe the new measurement and calibration setup in detail and demonstrate the enhanced performance of the analyzer. A measurement precision of about 0.02‰ for both isotope ratios has been obtained for an averaging time of 10 min, while the accuracy was estimated to be 0.1‰, including the uncertainty of the calibration gases.