南海大气铵沉降及其来源的氮同位素示踪研究

Reduced reactive nitrogen (NHx, NH3 / NH4+) was very important for the marine (especial for marginal sea) nitrogen cycle, the ecological system of the community structure and its corresponding biogeochemical activity. Atmospheric deposition is the major removal mechanism of marine atmospheric NHx, including dry deposition and wet deposition. The current research of NH4+ deposition in marine atmosphere lack of systematic and accurate study, the source and chemical formation pathway of NH4+ in marine atmospheric deposition is not clear. The stable nitrogen isotope has been proved to be an effective tool in identifying the sources and chemical formation of nitrogen deposition. In this project, Yongxing Island and Fiery Cross Island were chosen as the observation stations, which were located at the northwest and south of the South China Sea respectively. The atmospheric deposition samples including dry deposition (weekly aerosol) and wet deposition (evently rain) will be collected. Total nitrogen and NH4+ concentration in the samples and the corresponding nitrogen isotope of NH4+ will be analyzed, aiming for studying the atmospheric deposition of total nitrogen and NH4+, and the seasonal variation of the concentration of NH4+ and its nitrogen isotopic composition. Combined with the nitrogen isotope analysis model and backward trajectory, possible chemical formation pathways of NH4+ in atmospheric deposition were studied. Additionally, the nitrogen isotopic fractionation effects of NH4+ in dry deposition and wet deposition, like rainfall, sunshine, temperature, etc. in the marine system were also discussed. The results of this project will be helpful to further study of NH4+ formation in marine atmospheric deposition, and to provide the basic information to further evaluation of nitrogen input in ocean system.

海洋大气中还原态活性氮(NHx)对海洋(尤其是贫营养的边缘海)氮循环、生态系统的群落结构及相应的生物地球化学活动具有重要的影响。大气沉降是大气NHx主要的去除过程，目前海洋大气铵的沉降研究缺乏系统性、研究结果存在较大不确定性，铵的来源和化学形成机制尚不清晰。氮同位素能很好地研究大气沉降中铵的不同来源及形成过程。为此，本项目拟以南海西沙永兴岛和南沙永暑岛作为观测点，收集每场雨水及以周分辨率采集气溶胶样品，分析样品的总氮、铵离子浓度及铵氮同位素值。目的在于获得南海西沙和南沙海域大气总氮及铵的年沉降量、铵浓度和铵氮同位素变化特征，同时结合同位素模型及后向轨迹等，探讨海洋大气沉降中铵的来源、化学形成机制，以及海洋大气环境参数对铵氮产生的同位素分馏。本研究将有助于提高对海洋大气铵的来源和形成过程的认识，为评估海洋氮输入提供研究基础。

本项目拟以南海西沙群岛和南沙群岛(分别位于南海西北部和南部)作为观测点，采集为期1年的每场降雨及以周分辨率采集气溶胶样品，分析样品的总氮含量、NH4+离子浓度及铵氮同位素的季节变化特征。目的在于：1) 获得南海西沙和南沙海域总氮及铵氮的年干湿沉降量，评估NH4+在南海海域大气氮沉降中的贡献；2)分析海陆气团条件下大气沉降中NH4+的浓度和δ15N-NH4+的差别及其形成原因；3) 利用气团来源的聚类算法、稳定同位素质量守恒定律及同位素混合模型，得到南海西沙和南沙海域大气沉降中NH4+的海陆贡献及其他来源；4) 通过分析海洋系统各环境参数如光照、温度、湿度、降雨量等对海洋大气沉降NH4+的浓度和δ15N-NH4+的影响，探索海洋大气沉降NH4+的化学形成机制及产生氮同位素变化机理。项目于2020年11月份在西沙永兴岛和南沙美济礁同时安装安装降雨降尘采样设备，进行双点采样，降雨采集模式为每天采集一次，降尘样品采集为每个月采集一次，截止2021年11月，采集已满12个月，共获得同时段对比降雨样品155个，降尘样品24个。并获得采样期间连续的气象参数数据（包括温湿度、风速风向、后向轨迹）。采集的样品运回实验室后，分析测定样品中的总氮浓度分析、NH4+浓度分析、NH4+的氮同位素分析。数据分析表明，在陆地气团和海洋气团主导下的雨水铵氮同位素比值分别为-2.7±2.2‰和-5.5±2.1‰，有显著差异；春季较低的δ15N-NH4+值(-4.9‰)可能源于附近东南亚地区同期增加的农业排放气团迁徙；在环境参数相关影响方面，湿沉降过程中的雨水冲刷、温度及日照时间对铵氮的同位素值均有一定的影响；南海海域铵沉降量方面，湿沉降大于干沉降，约占总沉降的92%，湿沉降中铵的沉降量大于硝酸盐的沉降量，而在干沉降量上，二者相当。研究表明铵沉降在南海海域对大气氮沉降的贡献随着远离陆地逐渐增大，其贡献高于硝酸盐的贡献，使用铵氮稳定同位素技术可以溯源和解析海域铵氮沉降的来源。