Award: OCE-1233140

This project was part of a larger, collaborative research effort to study the biogeochemical cycling of nitrogen in the eastern tropical Pacific Ocean. Nitrate is the primary form of fixed nitrogen in the ocean and it is an essential macronutrient whose supply can limit primary production and carbon export from the surface ocean on seasonal, annual, and millennial timescales. The size of the oceanic nitrate reservoir is determined by rates of the microbial processes of nitrogen fixation, nitrification, denitrification, and anaerobic ammonia oxidation. In addition, atmospheric deposition of nitrate, largely sourced from land-based processes, adds additional fixed nitrogen to the surface ocean through rain, settling of particles, and dry deposition. These processes have a number of implications --- they determine nutrient supply and productivity in the oceans and they release gases such as nitrogen gas (N2) and nitrous oxide (N2O). The release of N2 represents a final end product that is unreactive and adds to the large, natural background of N2 in our atmosphere. Nitrous oxide (N2O), on the other hand, is a climatically important gas that adds to the greenhouse gas burden in the atmosphere. In the Pacific Ocean, water column oxygen deficient zones, where oxygen levels drop below 1 micromole per kilogram of seawater, the conversion of fixed nitrogen to nitrogen gas (N2) is catalyzed and represents an important loss of nitrogen from the ocean. While a great deal of research has focused on quantifying the sources and sinks of fixed nitrogen in the ocean, their distribution is patchy and sporadic in nature leading to large uncertainties. Uncertainties in the rates and controls of these ecologically and biogeochemically important processes weaken our ability to understand past ocean conditions and project future change in oceanic nitrogen and carbon cycles. In the atmosphere, the formation and loss of nitrate is not biologically controlled, but instead results from chemical reactions between oxidants (such as ozone and hydroxyl compounds) and nitrogen oxides. Nitrogen oxides, known as NOx (rhymes with "socks"), are primarily sourced from fossil fuel burning, fertilized soils, biomass burning, and lightning. NOx reacts with oxidants in the atmosphere to form atmospheric nitrate (NO3-) and because nitrate is very soluble, it is readily deposited via precipitation or on surfaces that contain water. This collaborative project sought to investigate ocean nitrogen biogeochemistry, including study of ocean processes and atmospheric particulate matter. A GEOTRACES program research cruise traveled from Peru to Tahiti during which seawater and atmospheric samples were collected. The report here will focus only on the atmospheric deposition component of the project. The primary tool used in this investigation was the isotopes of nitrate contained in particulate matter collected from the atmosphere during the research cruise. Nitrate contains both nitrogen and oxygen molecules. Isotopes of an element, such as nitrogen, represent the same chemical species with slightly different masses (e.g., 14N versus 15N, or 16O versus 17O or 18O) such that they exhibit different chemical and physical behaviors in the environment that can be quantified. The isotope results were combined with atmospheric transport modeling to determine the areas from which emissions would be sourced, and then interpreted in the context of known chemical reaction pathways in the atmosphere. In atmospheric nitrate, the oxygen isotopic composition is dependent upon the oxidants that controlled the conversion of NOx to nitrate prior to its collection on particulate matter. In this study, we were able to distinguish the influence of different reaction pathways that control the production of atmospheric nitrate, including investigating differences between samples that are influenced by air traveling over the South American continent compared to those influenced primarily by marine air chemistry. The nitrogen isotopic composition is dependent upon the NOx sources and the chemistry that occurs in the conversion of NOx to nitrate. The nitrogen oxygen isotopic composition matches best with continental source emissions from biomass burning and microbial processes in soils. The long-range transport across the ocean results in a change in the isotopic composition with the heavier 15N isotope tending to be lost compared to the 14N isotope. The results from this work were shared with the scientific community through presentations at national and international scientific conferences. A manuscript for submission to a peer-reviewed journal is also currently in preparation. Technical staff received additional training as part of this award, and continue to work at Brown University. An undergraduate was trained and participated in this research endeavor, including the manuscript in preparation, and is continuing on to graduate studies in atmospheric chemistry. Results from this work were also included in outreach presentations for third through fifth graders as part of a regular and ongoing relationship with local elementary schools in Providence, RI. The goal of this outreach is to share excitement, enthusiasm and realistic details of pursuing a career as an Earth scientist. Last Modified: 06/12/2017 Submitted by: Meredith G Hastings