Transient tracers (CFC-12 and SF6), noble gases (He and Ne isotopes), and Tritium measurements from POLARSTERN cruise PS122 (MOSAiC, 2019-2020)

We present a data set of oceanic trace gases 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, trace gases from anthropogenic origin chlorofluorocarbon 12 (CFC-12), sulfur hexafluoride (SF6), and tritium, along with noble gases helium and neon and their isotopes were collected at a weekly or higher temporal resolution throughout the entire water column and occasionally in the snow, from the ship and from the ice. This trace gases data set can be used for process studies and water mass tracing throughout the Arctic in subsequent analysis.

Transient tracers (CFC-12, SF6) The CFC-12 and SF6 water samples from the CTD-bottle systems were stored in ~220 ml glass ampoules by avoiding contact to the atmosphere during the tapping by a dedicated tubing and rinsing procedure. After sampling, the ampoules are flame sealed after a headspace of pure nitrogen had been applied. The determination of CFC-12 and SF6 concentrations in the IUP Bremen gas chromatography lab is accomplished by purge and trap (cryogenic trapping at -65°C) sample pre-treatment of a precise water volume of 140 ml followed by gas chromatographic separation on a capillary column and electron capture detection (ECD). After thermal desorption the released gases are separated on a pre-column of type Aluminia Bond/CFC, 0.54 mm ID x 3m, and a main column of type Aluminia BOND/CFC, 0.54 mm ID x 30 m. SF6 and CFC-12 are then detected on a micro-ECD. The analytical system is calibrated frequently by analyzing different volumes of known standard gas concentrations. The loss of CFCs and SF6 into the headspace is considered by equilibration between liquid and gas phase under controlled conditions before the sealed ampoules are opened and the volume of the headspace was precisely measured. A more detailed description of the measurement system is given by Bulsiewicz et al. (1998). CFC-12 concentrations are reported in pmol/kg and SF6 in fmol/kg, both reported on SIO98 scale (Prinn et al., 2000). 271 samples were analyzed successfully, including 43 pairs of replicate samples that were each averaged for the final data set. The precision of the measurement, based on the comparison of the replicate samples, is 1% or 0.003 pmol/kg for CFC-12 (whichever is greatest) and 2% or 0.02 fmol/kg for SF6 (whichever is greatest). The accuracy for CFC-12 is 2% or 0.005 pmol/kg (whichever is greatest) and for SF6 is 3% or 0.03 fmol/kg (whichever is greatest), including errors of calibration, linearity, standard-gas, gas volumes for calibration, water volume, gas loss into the head-space, and calibration scale.

Noble gases (3He, 4He, 20Ne, 22Ne) The water samples were stored from the CTD/water bottle systems (ship and Ocean City) without contact to atmospheric air into 40 ml gas tight copper tubes, which are clamped of at both sides. In the IUP Bremen noble gas lab the samples were pre-processed with a UHV (ultra-high vacuum) gas extraction system. Sample gases are transferred via water vapour into a glass ampoule kept at liquid nitrogen temperature. For analysis of the noble gas isotopes the glass ampoules are connected to a fully automated UHV mass spectrometric system equipped with a two stage cryogenic system and a quadrupole and a sector-field mass spectrometer. Regularly, the system is calibrated with atmospheric air standards (reproducibility < 0.2%). Measurement of line blanks and linearity are done as well. The performance of the Bremen facility is described in Sültenfuß et al. (2009). Noble gas concentrations are reported in nmol/kg for He and Ne; δ 3He is reported in %. The precision for He is 0.4%, 0.7% for Ne and 0.8% for δ3He (based on the 25 pairs of replicate measurements).

Tritium (3H) The samples were stored in 500 ml plastic water bottles from the CTD/water bottle systems (ship and Ocean City). Additionally, 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.

Acknowledgment These 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.