Ammonium salts represent a major component of fine mode marine aerosol that play an important role in climate regulation, atmospheric pH control and nutrient supply to the oceans. We report here the results of analyses of aerosol samples collected in remote areas of the Atlantic Ocean. We show the isotopic abundance of aerosol ammonium varies with concentration, with isotopically light ammonium (-5 to -8?) associated with the lowest ammonium concentrations compared to isotopically heavy ammonium (+10?) associated with higher ammonium concentrations. We interpret the light ammonium signature as arising from marine emissions of ammonia. These results provide further evidence for the existence of a marine ammonia source and offers a method to define the relative contribution of marine and terrestrial ammonium to aerosols in remote areas and thereby investigate the role of these emissions in climate regulation.