In the PTR-MS, m/Q 79 and m/Q 93 were taken to be benzene and toluene respectively in accordance with previous mass assignments . During both cruises, the PTR-MS was calibrated using a multi-component gas standard containing benzene and toluene (supplementary text). During the Southern Ocean cruise, the hourly measurement of outside air scrubbed by the Pt-catalyst was used as a blank for the benzene air measurements, while daily measurements of zero air from a gas standard were used as a blank for toluene air measurements. The air measurements were carefully filtered for ship stack contamination as laid out in Wohl et al. .