Gaseous elemental mercury (GEM) and reactive gaseous mercury (RGM) were measured during an eight month circumnavigation to obtain knowledge of their worldwide distributions in the marine boundary layer (MBL). Background GEM concentrations were found to be 1.32 ± 0.2 ng/m(3) (summer) and 2.62 ± 0.4 ng/m(3) (spring) in the northern hemisphere and 1.27 ± 0.2 ng/m(3) (spring and summer) in the southern hemisphere. Radiation and relative humidity are shown to control diurnal cycles of RGM. During the cruise the ship passed areas of clean MBL air, air influenced by biomass burning (South Atlantic) and air with high concentrations of GEM and RGM of unknown origin (Antarctic). High GEM concentrations above the Atlantic indicate that emission from the ocean can be an important GEM source. Our data combined with data from earlier cruises provides adequate information to establish a seasonal cycle for the Atlantic. Results show a cycle similar to that found at Mace Head, Ireland but with larger amplitude. We have improved the basic knowledge of mean GEM and RGM concentrations in the MBL worldwide and shown how natural sources and reemissions can affect GEM concentrations in the MBL.