Simultaneous measurements of NH3 in the atmosphere and NH4 + in the ocean are presented from fieldwork spanning 10 years and 110 degrees of latitude, including the first such simultaneous measurements in the remote marine environment at >55°N. At high latitudes, fluxes were almost exclusively from air to sea, in contradiction with previous lower-latitude studies, which have suggested that the open oceans are predominantly sources of ammonia to the atmosphere. Sensitivity analysis demonstrates that the direction and magnitude of the ocean-atmosphere NH3 exchange is highly dependent on water temperature. This temperature effect is sufficiently strong to outweigh the effects of variability in concentrations in seawater and atmosphere in many parts of the (open) ocean. This is highlighted in data from the Atlantic oligotrophic gyres, where fluxes were found to be predominantly out of the ocean despite extremely low dissolved ammonium concentrations in surface waters.