The emissions of anthropogenic nitrogen (N) to the atmosphere, and its subsequent deposition, have increased tenfold since preindustrial times [Galloway et al., 2004]. The impacts of increased N deposition to terrestrial and coastal systems are well studied [e.g., Elser et al., 2009; Paerl et al., 2002]; however, the implications for open ocean N biogeochemistry remain uncertain [Duce et al., 2008]. Both anthropogenic and natural processes impact the amount and form of N deposition to remote marine environments, and there have been clear increases in nitrate deposition as a result of increased anthropogenic emissions of N oxides [Elliott et al., 2007; Galloway et al., 2003; Hastings et al., 2009; Kim et al., 2011; Kodama et al., 2011]. The concentration of nitric acid (HNO3) in seawater is extremely low; therefore, deposition is a one-way process. Identifying the sources of N deposition to the open ocean is critical for understanding the biogeochemical impacts of human activities. If the N deposition is terrestrial in origin, it represents an external input to the open ocean as the reactive N chemistry over the continents is influenced heavily by anthropogenic activities; this input can be expected to change into the future [Duce et al., 2008]. In contrast, if the N originates from natural sources, then it might not be a N input to the ocean as a whole (e.g., if it derives from the upper ocean), and it is in any case less likely to undergo a marked change in the coming decades.
1.1 Nitrate in the Marine Atmosphere
 Nitrate (NO3−) is the ultimate sink for atmospheric nitrogen oxides (NOx = NO + NO2), and it is an increasingly significant component of acid rain as effective sulfur dioxide regulations lead to decreases in sulfuric acid concentrations. The lifetime of NOx is usually hours to days; thus, its conversion to longer lived reservoir species such as HNO3 or peroxyacetyl nitrate (PAN) is needed for reactive N to be transported long distances. The atmospheric cycle of NOx and the conversion of NOx to NO3− is complex, with different processes taking place during the day and night (Figure 1a). During the day, cycling between NO and NO2 is rapid –.
 The oxidation of NO to NO2 requires ozone , while the breakdown of NO2 back to NO is photolytic and produces ozone [–]. NO can also be oxidized to NO2 via peroxy radicals , which also ultimately leads to ozone production. At night, shuts down and dominates until [NO2] ≈ [NOx]. The dominant daytime sink of NOx is the oxidation of NO2 to HNO3 by the hydroxyl radical .
 During the night, when photolytic production of OH ceases, the concentration of OH decreases and NO2 reacts preferentially with ozone to form NO3, the dominant nighttime oxidant. NO2 and NO3 then react further to form N2O5, and NO2 and NO3 remain in thermal equilibrium with N2O5 [–]; M is an unreactive body, usually N2).
 NO3 can also be lost via gas phase reactions with volatile organic compounds (VOCs), including dimethyl sulfide (DMS) in marine areas [Stark et al., 2007].
 Recent studies have shown the potential for halogens to play a significant role in NOx and nitrate chemistry. In the presence of HCl, N2O5 can also react heterogeneously on aerosol particles to form both aqueous nitrate and ClNO2 which partitions to the gas phase [Thornton et al., 2010].
 It is hypothesized that the branching ratio between formation of HNO3(aq) in and ClNO2(g) and HNO3(aq) in is determined by the relative concentrations of chloride and water in the aerosol particles. In the polluted marine boundary layer where high concentrations of NOx and NaCl particles mix, i.e., in coastal regions, ClNO2 is produced in high yields, exceeding previously predicted values by a factor of 2 to 30 [Osthoff et al., 2008]. Maximum production of ClNO2 has been found thus far in polluted coastal regions of the North Atlantic, with the largest fluxes occurring in the northern hemisphere winter [Erickson et al., 1999]. The ClNO2 acts as a reservoir species, building up in concentration at night and regenerating NOx during the day through photolysis .
 The combination of and increases the lifetime of reactive N in the marine atmosphere by regenerating NOx, in contrast to the reaction without halogen chemistry , in which the conversion of N2O5 to aqueous nitrate aerosol ensures efficient removal of reactive N.
 First, acid displacement reactions occur when HNO3(g) in continental outflow reacts with NaCl in the marine boundary layer, releasing HCl and forming NaNO3(p). The HCl then reacts with OH to produce Cl radicals, which quickly form ClO by reaction with ozone; BrO is formed in an analogous manner. The ClONO2 (or BrONO2) formed can then combine with sea-salt aerosol to form coarse mode aerosol nitrate .
 The conversion of HNO3 to coarse mode NO3− should significantly decrease its lifetime as coarse mode particles are deposited preferentially through gravitational settling and precipitation scavenging via inertial impaction.
 In polluted environments, NOx can react with peroxyacetyl radicals to form PAN, a long lived NOx reservoir which can be transported long distances at high altitudes. When air masses subside, PAN thermally decomposes to again form NO2 [Fischer et al., 2011, and references therein].
 The chemical and physical processing of NOx and nitrate, along with the frequency and amount of precipitation as an air mass travels, controls how much reactive N will be transported to the remote marine atmosphere and the isotopic composition of reactive N.
1.2 Previous N Isotope Studies
 Inorganic N is the dominant form of N deposition in both polluted [Cornell et al., 2003; Russell et al., 1998] and remote sites [Duce et al., 2008; Galloway et al., 1996; Galloway et al., 1982; Galloway et al., 1989], and nitrate typically represents ~50% of inorganic N deposition. Previous studies have used N and O isotopes of nitrate as a tool for distinguishing nitrate sources and chemical formation pathways in polluted [Elliott et al., 2007; Freyer, 1978] and open ocean environments [Hastings et al., 2003; Morin et al., 2009]. In the above reactions section 1.1), as NOx is converted to nitrate, the N atom is conserved. As such, it is generally expected that the δ15N of nitrate will reflect the δ15N of the NOx source. Indeed, previous work suggests that the conversion of NOx to nitrate imparts little isotopic fractionation [Freyer et al., 1993]. In contrast, the O atoms of atmospheric NOx are rapidly exchanged with O3 in (–. NO2 can be converted to NO3− through multiple pathways, all resulting in the addition of one oxygen atom. Therefore, the δ18O of nitrate is set by the oxidants that convert NOx to nitrate (e.g., OH versus O3).
 Hastings et al.,  showed that nitrate in Bermuda (32.27°N, 64.87°W) rain has higher δ15N-NO3− and lower δ18O-NO3− in the warm season (April to September; −2.1‰ and 68.6‰) as compared to the cool season (October to March; −5.9‰ and 76.9‰; isotope ratios are reported using the delta (δ) notation in “per mil” (‰): δ15Nsample = [(15N/14N)sample/(15N/14N)N2 in air −1] * 1000‰ and δ18Osample = [(18O/16O)sample/(18O/16O)VSMOW −1] * 1000‰). However, the concentration of nitrate was not significantly different from the warm to the cool season (5.0 and 6.4 μM, respectively). They concluded that during the warm season there is substantial nitrate in Bermuda rain that has an isotopic signature distinct from the nitrate coming off North America, implying an alternative nitrate source over the North Atlantic. The rainwater nitrate originating from the south of Bermuda (i.e., marine air) has a δ15N of ~ 0‰, which Hastings et al.,  attributed to an increased contribution from lightning NOx. Two rain samples collected on research cruises in the Eastern Atlantic Ocean had δ15N values of −1.4 and −0.9‰ [Baker et al., 2007] consistent with rains at Bermuda.
 One interesting and heretofore unresolved result of the Hastings et al.  study is that during the cool season when North America is the air mass source region, the δ15N-NO3− in rain is lower in Bermuda than in the United States. The N and O isotopes of nitrate in weekly rainwater collections were measured in the northeastern USA at National Atmospheric Deposition Program (NADP) sites during the same time frame as the Hastings et al.,  study [Elliott et al., 2007; Elliott et al., 2009], and from October to March, the rainwater δ15N-NO3− varied from 0 to 3.5‰. In contrast, the Bermuda cool season average δ15N was −5.9 ± 3.3‰ (±1SD).
 To investigate the sources and chemistry that influence atmospheric nitrate deposition to the ocean, two years of event-based rainwater samples were collected on the island of Bermuda. Samples were analyzed for major ion concentrations and N and O isotopic ratios of nitrate. NOAA's Hybrid Single-Particle Lagrangian Integrated Trajectory model (HYSPLIT) was used to determine air mass history for each rainwater sample, and events were classified as originating over the continental USA or as marine in origin.