During the Polarstern cruise ANT-XXV/1 in November 2008, surface seawater from below the ship's hull was continuously pumped through a shower head into a 20 L equilibrator bottle and drained at the bottom. Equilibrium between the supplied water and the headspace air (Hg0 equ) was achieved in about 1 h. The equilibrated air was transferred to the TEKRAN 2537A after closing the drain and elemental mercury (Hg0) was preconcentrated by amalgamation on gold traps and finally analyzed by cold vapor atomic fluorescence spectrometry. The detection limit was 0.1 ng L-1. Wind speed was taken from the Polarstern Meteorological station and surface water salinity and temperature from the thermosalinograph. Hg0 equil. was a duplicate measurement of the headspace and provided at standard reference condition of 0°C by the Tekran instrument. The value was corrected to the in situ temperature and divided by Henry's Law constant at respective sea surface water temperature.