Benzene and toluene were measured on both cruises using a Proton Transfer Reaction Mass Spectrometer (Ionicon PTR-MS, high sensitivity with a quadrupole) coupled to a Segmented Flow Coil Equilibrator (SFCE). Details on the installation on board for each cruise are provided elsewhere . Briefly, the PTR-MS and the SFCE were installed on the ship in a laboratory near an underway water tap. During the cruise in the Southern Ocean, an air pump was used to rapidly draw ambient air from about 16 m above sea level to the laboratory, where the PTR-MS subsampled from a tee piece upstream of the pump. Solenoids were used to set up an hourly cycle of measuring SFCE headspace, ambient air and ambient air passed through a Pt-catalyst, which acts as a blank measurement. During the Arctic cruise, SFCE headspace was sampled continuously. The SFCE sampled seawater from the underway water supply (Southern Ocean cruise: 4-7 m depth, Arctic cruise: 3-4 m depth) via an overflowing glass bottle. To make vertical profile measurements, waters were collected from CTD rosette into 900 cm 3 glass bottles and sampled with the SFCE.