Water vapour and water vapour isotopic composition measurements were performed with a cavity ring-down laser spectrometer (L2140-i, serial number HKDS2038, Picarro Inc., USA). The instrument is generally used for freshwater measurements in FARLAB, University of Bergen, and was deployed here for its test campaign. On the ship, the instrument was run in low-flow H2O Air mode and placed on a table inside a measurement container on the boat deck.

The inlet was mounted at 4 m above the boat deck on a ladder leading to the radar antennas. The total length of the inlet tubing was 5 m. A downward-facing steel can with diameter of 15 cm and height of 15 cm shielded the inlet from precipitation and sea spray. The uppermost 1.5 m of the tubing had a diameter of 3/8” (Stainless steel with Sulfinert coating and self-regulated heat trace, Thermon Inc), continued by 2.5 m of 1/4” stainless steel tubing connected to a T-piece. Here, the flow splitted into a line flushed by an inlet pump (KNF 622N) at a flow rate of 5-10 slpm, and 0.7 m of stainless steel tubing with 1/8” diameter leading to the CRDS analyzer, protected by a check valve (Swagelok Inc, USA) at 1/4” connection with appropriate fittings. The entire inlet was heated by a self-regulating heating wire, limited to 50 deg C.