Isotopic composition of rainwater nitrate at Bermuda: The influence of air mass source and chemistry in the marine boundary layer

Abstract

Abstract 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, −1.1‰ versus N 2 in air, and 69.0‰ versus Vienna SMOW, respectively) than those influenced by North American air masses (6.3 μM, −5.4‰, 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 − 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 − deposition. It is possible that this process also contributes to variations in δ 15 N‐NO 3 − 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.