Table of Contents

• Coverage
• Dataset Description
  • Methods & Sampling
  • BCO-DMO Processing Description
• Related Publications
• Parameters
• Instruments
• Project Information
• Funding

Aerosol sampling was conducted from March 24, 2021 to June 30, 2022. A high-volume aerosol sampler operating at 1,200 L min^-1 with 25 × 20 cm filters was used. Aerosol samples were for seven days for each filter, with a total of 66 samples collected.

We measured for ammonium isotopes (¹⁵N), and a suite of ion concentrations (calcium (Ca²⁺), magnesium (Mg²⁺), sodium (Na⁺), and potassium (K⁺), Chloride (Cl^-) and sulfate (SO₄^2-)). All measurements were conducted at the Department of Earth, Environmental, and Planetary Sciences at Brown University in Providence, RI (USA).

The filter was extracted in a pre-cleaned bottle with ~30 mL of MQ water (exact amounts were recorded via weight) and sonicated for 1 h. After sonication, the solutions were filtered via a vacuum pumped. Samples were frozen then at -20 ◦C until further analyses.

 The concentration of ammonium, nitrate, and nitrite were determined colorimetrically (indophenol blue) using a discrete UV–Vis analyzer (Westco Smartchem 200). U.S. EPA Compliant Methods 350.1 (O’Dell, 1993), 353.2 (Revision 2.0), and 354.1 were followed for ammonium, nitrate, and nitrite, respectively. Standard lab protocols were followed, including calibration to standards as well as blanks and in-house quality control measurements.

The azide method (Zhang et al., 2007) was used to complete ammonium (¹⁵N /¹⁴N) isotope analyses on a continuous flow isotope ratio mass spectrometer. Values were reported in ‰ relative to standards (IAEA-N₂ and USGS-25).

Concentrations of calcium, magnesium, sodium, potassium, chloride, and sulfate were measured on an Ion Chromatography (IC) instrument (Dionex Integrion HPIC) using suppressed conductivity detection. Anions were determined using a Dionex AS19-4 μm guard (4 × 50 mm) and analytical column (4× 250 mm) with 20 mM KOH as eluent with a flow rate of 1 mL min^-1. Cations were analyzed using a Dionex CS-16 guard column (5 × 50 mm) analytical column (5 × 250 mm). Eluent of 30 mM methane sulfonic acid was used to detect cations with a pump flow rate of 1 mL min⁻¹. All samples were filtered with 0.20-μm filters before being analyzed. In-house quality control standards were run approximately every six samples.

* Adjusted parameter names to comply with database requirements
* Added sampling latitude and longitude to dataset
* Converted date to ISO notation (Y-m-d)

NSF Award Abstract:
Nitrogen is an essential element for life, and its availability can limit the growth of phytoplankton in the surface waters of the oceans. One source of nitrogen to surface waters is deposition from the atmosphere, which is the result of reactive nitrogen emissions from both human (anthropogenic) activities and natural processes. Anthropogenic nitrogen emissions to the atmosphere and nitrogen deposition to the oceans have increased exponentially since preindustrial times. In fact, global modeling studies have suggested that as much as 80% of total nitrogen deposition to the oceans is anthropogenic in origin, and that the magnitude of input to the global oceans rivals estimates of biological nitrogen fixation. The impacts associated with this increased nitrogen deposition are clear in both terrestrial and coastal systems, but the implications for open ocean biogeochemistry are less well studied. Recent work in the North Pacific Ocean (NPO) has suggested that increased nitrogen due to anthropogenic atmospheric deposition is detectable even in the open ocean, while other work can explain nutrient dynamics based upon natural biological and physical processes. The investigators propose to study the influence of both anthropogenic and natural sources on the deposition of nitrogen (as nitrate, ammonium, and organic nitrogen) in the NPO. This will be based on collection of aerosol and rainwater samples year-round at two sites: (1) Chang-Dao Island, China where they expect high anthropogenic nitrogen inputs; and (2) Oahu, Hawaii where they expect marine sources to dominate. They will also collect ship-based samples in the marine boundary layer via already planned short-term research cruises in each season. In addition to the scientific outcomes, this project will provide for human resources and professional development of the team members (faculty members, a graduate student, undergraduate student, and technicians), advance international collaborations, and contribute to education and public outreach. Identifying the sources of nitrogen deposition to the open ocean is crucial for understanding anthropogenic impacts on biogeochemical cycles. A primary question is, is nitrogen deposition the result of external, anthropogenic processes or does it represent recycled nitrogen from the ocean's point of view? The specific objectives of this project are to: characterize the atmospheric composition and sources of inorganic (ammonium and nitrate) and organic nitrogen with an emphasis on seasonality; diagnose the influence of air-sea exchange versus anthropogenic sources of nitrogen on atmospheric deposition; and determine the isotopic composition of gaseous and particulate inorganic nitrogen in the marine boundary layer via ship-based sample collections in the NPO. Using concentration and isotopic measurements of reactive nitrogen species, in addition to transport and chemical box modeling, the study aims to characterize nitrogen deposition in two locations with very different source influences. This study will provide the first comprehensive, seasonal analysis of the isotopic values of reactive nitrogen species in the NPO atmosphere where nitrogen deposition is considered intense. Ultimately this project will lead to a better understanding of how anthropogenic changes in the atmospheric nitrogen cycle may influence the biogeochemistry of the surface ocean as well as the composition of the marine atmosphere.This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.