The Central Baltic Air-Sea Exchange Experiment (CenBASE) was conducted from June to July 2022 on board the R/V Elisabeth Mann Borgese (research cruise EMB295), in an area of the central Baltic Sea east of Gotland, Sweden.

On 6 July 2022, ³He and SF₆ were injected at approximately 7 meters (m) depth in a hexagonal spiral pattern with a diameter of about 1 kilometer (km), centered at 57.263°N, 20.147°E over the course of 40 minutes. After the injection, discrete samples were taken approximately every 12 hours in the water column near the center of the SF₆ patch using a rosette with a conductivity, temperature, and depth (CTD) sonde and 13 5-liter (L) Niskin bottles. 250-milliliter (mL) syringes were used to obtain discrete SF₆ samples from the Niskin bottles. For discrete ³He samples, about 40 mL of seawater were collected in copper tubes placed in aluminum channels. Stainless steel clamps were used to seal the tubes at both ends for later shore-based measurements.

SF₆ concentrations were measured onboard using a purge-and-trap SF₆ analysis system (Bullister and Weiss, 1988; Gerke et al., 2024). This system separated SF₆ from other gases and measured its concentration by a gas chromatograph equipped with an electron capture detector (GC-ECD). Approximately 200 mL of the water sample were injected into a purge-and-trap unit. Nitrogen served as the carrier gas to purge the samples, and the gases were trapped on a 70-centimeter (cm) column filled with Heysep D (60/80 mesh). The trap was maintained at a temperature of approximately −70 degrees Celsius (°C) by suspending it over liquid nitrogen. The trapped analytes were then desorbed by heating the trap to 100°C. Separation was achieved using a 90-cm pre-column filled ⅓ with Porasil C and ⅔ with Molsieve 5A, and a 220-cm main column, packed 90% with Carbograph 1AC and 10% with Molsieve 5A.

The ³He samples were shipped to the laboratory at the Institute of Environmental Physics at the University of Bremen for analysis. In the laboratory, after being removed from the copper tube, the samples were released into glass bulbs. From there, they were transferred into glass ampoules, which were then sealed for analysis with a helium isotope mass spectrometer (MAP 215-50). δ³He precision for ocean samples is usually better than 0.5% (Sültenfuß et al., 2009).