Collaborative Research: Characterization of Reactive Nitrogen in The North Pacific Atmosphere

氮是生命的基本元素，它的可用性会限制海洋表层浮游植物的生长。地表水中氮的一个来源是来自大气的沉积，这是人类（人为）活动和自然过程中活性氮排放的结果。自前工业化时代以来，人为向大气排放的氮和向海洋沉积的氮呈指数级增长。事实上，全球模拟研究表明，海洋中多达80%的氮沉积是人为的，而全球海洋的输入量与生物固氮的估计相匹配。与氮沉积增加相关的影响在陆地和海岸系统中都很明显，但对开放海洋生物地球化学的影响研究得较少。最近在北太平洋（NPO）的研究表明，即使在开阔的海洋中，也可以检测到人为大气沉积导致的氮增加，而其他工作可以根据自然生物和物理过程解释营养动力学。研究人员建议研究人为和自然来源对NPO中氮（如硝酸盐、铵态氮和有机氮）沉积的影响。这将基于在两个地点全年收集的气溶胶和雨水样本：(1)中国长岛岛，他们预计那里的人为氮输入较高；(2)夏威夷的瓦胡岛，他们希望在那里海洋资源占主导地位。他们还将在每个季节通过已经计划好的短期研究巡航收集海洋边界层的船载样本。除了取得科学成果外，该项目还将为团队成员（教师、研究生、本科生和技术人员）提供人力资源和专业发展，促进国际合作，并为教育和公众宣传做出贡献。确定氮沉积到公海的来源对于理解人类活动对生物地球化学循环的影响至关重要。一个主要的问题是，氮沉积是外部人为过程的结果，还是从海洋的角度来看，它代表了再循环的氮？该项目的具体目标是：描述大气成分和无机（铵和硝酸盐）和有机氮的来源，重点是季节性；诊断海气交换和人为氮源对大气沉积的影响；并通过在NPO的船载样品收集确定海洋边界层中气态和颗粒无机氮的同位素组成。利用活性氮的浓度和同位素测量，以及运输和化学箱模型，该研究旨在描述两个具有不同来源影响的地点的氮沉积特征。这项研究将首次对氮沉积强烈的NPO大气中活性氮的同位素值进行全面的季节性分析。最终，该项目将有助于更好地了解大气氮循环中的人为变化如何影响海洋表面的生物地球化学以及海洋大气的组成。该奖项反映了美国国家科学基金会的法定使命，并通过使用基金会的知识价值和更广泛的影响审查标准进行评估，被认为值得支持。

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.