Abstract. Since 2002 in situ airborne measurements of atmospheric CO₂ mixing ratios have been performed regularly aboard a rental aircraft near Bialystok (53°08´ N, 23°09´ E), a city in northeastern Poland. Since August 2008, the in situ CO₂ measurements have been made by a modified commercially available and fully automated non-dispersive infrared (NDIR) analyzer system. The response of the analyzer has been characterized and the CO₂ mixing ratio stability of the associated calibration system has been fully tested, which results in an optimal calibration strategy and allows for an accuracy of the CO₂ measurements within 0.2 ppm. Besides the in situ measurements, air samples have been collected in glass flasks and analyzed in the laboratory for CO₂ and other trace gases. To validate the in situ CO₂ measurements against reliable discrete flask measurements, we developed weighting functions that mimic the temporal averaging of the flask sampling process. Comparisons between in situ and flask CO₂ measurements demonstrate that these weighting functions can compensate for atmospheric variability, and provide an effective method for validating airborne in situ CO₂ measurements. In addition, we show the nine-year records of flask CO₂ measurements. The new system, automated since August 2008, has eliminated the need for manual in-flight calibrations, and thus enables an additional vertical profile, 20 km away, to be sampled at no additional cost in terms of flight hours. This sampling strategy provides an opportunity to investigate both temporal and spatial variability on a regular basis.