[1] Emissions of anthropogenic nitrogen (N) to the atmosphere have increased tenfold since preindustrial times, resulting in increased N deposition to terrestrial and coastal ecosystems. The current sources of N deposition to the ocean, however, are poorly understood. To investigate the sources of nitrate in rainwater deposited to the ocean, two years of daily rainwater samples were collected on the island of Bermuda in the western North Atlantic. Air mass back trajectories were computed for each sample and two dominant regimes were identified: slow moving events that originate over the ocean and occur all year, and fast moving events that originate over the continental USA and occur primarily during the cool season (October-March). Marine-influenced air masses result in rainwater nitrate with lower concentrations, higher average δ 15 N, and lower average δ 18 O (4.4 μM, À1.1‰ versus N 2 in air, and 69.0‰ versus Vienna SMOW, respectively) than those influenced by North American air masses (6.3 μM, À5.4‰, and 75.0‰). The δ 15 N decrease and concentration increase from marine to continental air masses are due to a change in NO x source, with increased anthropogenic inputs associated with continental air. We suggest that heterogeneous halogen chemistry in the marine boundary layer leads to isotopic fractionation. This causes higher δ 15 N-NO 3 À to be deposited near the coast and lower δ 15 N-NO x to be transported over the open ocean, yielding a low δ 15 N for anthropogenic NO 3 À deposition. It is possible that this process also contributes to variations in δ 15 N-NO 3 À from marine air masses. There is a negative linear correlation (r 2 = 0.58) between δ 15 N and δ 18 O which is driven by the seasonal change in trajectory influencing both the source NO x and the nitrate formation pathways.

[1] Identifying the dominant sources of atmospheric reactive nitrogen (N r ) is critical for determining the influence of anthropogenic emissions on N r deposition, especially in marine ecosystems. To test the influence of anthropogenic versus marine air masses, samples were collected in Bermuda, where seasonal atmospheric circulation patterns lead to greater continental transport during the cool season. -was consistently lower than or equal to the rainwater from the same sampling period, the opposite trend of that observed in polluted systems. We propose that this is due to HNO 3(g) uptake onto aerosol particles with a kinetic isotope effect, lowering the aerosol δ -was higher than that in rainwater during the cool season, but was not different during the warm season, which we tentatively attribute to the increased importance of heterogeneous halogen chemistry on the formation of NO 3 -during the cool season. Citation: Gobel, A.