We present a data set of tritium in snow collected during the Multidisciplinary drifting Observatory for the Study of Arctic Climate (MOSAiC, PS122) expedition, during which the R/V Polarstern drifted along with the Arctic sea ice from the Laptev Sea to Fram Strait, from October 2019 to September 2020. During the expedition, tritium data were collected occasionally in the snow, from the ship and from the ice.We took 9 samples from snow into 2x500 ml plastic bottles during leg 3. In the IUP Bremen noble gas lab the water samples were pre-processed with a gas extraction system for complete degassing and were then stored for several months. During that time, part of the tritium (3H) decayed by beta-decay to helium 3 (3He). The new produced 3He was then analysed by the same mass spectrometer system as for the noble gases. Tritium concentrations reported here are scaled to the 1st January 2020 and referred to as TU2020. Concentrations are given in TU (tritium unit), where 1 TU is the ratio of 1 tritium atom to 10^18 hydrogen atoms. Typical errors for this data set is 0.04TU or 3% whatever is larger.AcknowledgmentThese data were produced as part of the international Multidisciplinary drifting Observatory for the Study of the Arctic Climate (MOSAiC) with the tag MOSAiC20192020 (AWI_PS122_00). We thank all those who contributed to MOSAiC and made this endeavor possible, as listed in Nixdorf et al. (2021). CH was funded by Vetenskapsrådet grant number 2018-03859 awarded to CH, project Why is the deep Arctic Ocean Warming? (WAOW), and acknowledge support from the Swedish Polar Research Secretariat for berth fees onboard MOSAiC. MW gratefully acknowledge the funding by the Deutsche Forschungsgemeinschaft (DFG, German Research Foundation) – Project Number 268020496–TRR 172, within the Transregional Collaborative Research Center "ArctiC Amplification: Climate Relevant Atmospheric and SurfaCe Processes, and Feedback Mechanisms (AC)3.

The tracer data set was carefully checked for accurate measurements and outliers. According to the WOCE standards the following flags were applied to each measurement:flag 2 = good