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Journal of Tropical Oceanography >

2025 , Vol. 44 >Issue 3: 167 - 178

DOI: https://doi.org/10.11978/2024194

Marine Environmental Science

Tracing the sources and formation mechanisms of marine atmospheric nitrate using stable isotopes

  • CHEN Tianshu , 1 ,
  • XIAO Hongwei , 1 ,
  • GUAN Wenkai 2 ,
  • XIAO Huayun 1
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  • 1. School of Agriculture and Biology, Shanghai Jiao Tong University, Shanghai 200240, China
  • 2. School of Oceanography, Shanghai Jiao Tong University, Shanghai 200230, China;
XIAO Hongwei. email:

Copy editor: LIN Qiang

Received date: 2024-10-15

  Revised date: 2024-11-08

  Online published: 2024-11-13

Supported by

National Key R&D Program of China(2023YFF0806001)

National Natural Science Foundation of China(42373083)

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Abstract

Nitrate ($\mathrm{NO}_{3}^{-}$) in the atmosphere, a key product formed from nitrogen oxides (NOx) through reactions with multiple oxidants such as ozone (O3) and hydroxyl radicals (·OH), is one of the main atmospheric pollutants, impacting air quality, climate, and ecosystems. This paper reviews the formation mechanisms, oxidation pathways, and global distribution of nitrogen and oxygen isotopic (such as δ15N and δ18O) signatures in marine atmospheric $\mathrm{NO}_{3}^{-}$, focusing on the roles of various oxidants like O3 and ·OH. Notably, the hydrocarbon/dimethyl sulfide (HC/DMS) pathway, the heterogeneous reaction of N2O5 with chlorine-containing (Cl2) aerosols, and the reaction of NO2 with reactive halogen compounds significantly impact the formation mechanisms of $\mathrm{NO}_{3}^{-}$ in the marine atmosphere and result in elevated δ18O values. Based on global observational data, the δ15N and δ18O composition of $\mathrm{NO}_{3}^{-}$ shows significant variations across different oceanic regions and coastal cities, probably reflecting regional differences in pollution sources, photochemical conditions, and atmospheric reaction pathways. Additionally, $\mathrm{NO}_{3}^{-}$ deposition into marine systems affects the nitrogen cycle within the oceans. Future research should prioritize long-term monitoring and data collection across diverse global regions to enhance quantitative assessments of oxidant contributions, thereby providing a more systematic understanding of atmospheric $\mathrm{NO}_{3}^{-}$ formation mechanisms and their implications for marine ecosystems and climate change.

Key words： stable isotopes; nitrate; marine atmosphere; marine atmospheric nitrate deposition; NOx

Cite this article

CHEN Tianshu , XIAO Hongwei , GUAN Wenkai , XIAO Huayun . Tracing the sources and formation mechanisms of marine atmospheric nitrate using stable isotopes[J]. Journal of Tropical Oceanography, 2025 , 44(3) : 167 -178 . DOI: 10.11978/2024194

氮元素是大气的重要成分, 自然界中的氮化合物种类繁多, 包括: 氮气(N2, 占氮库的99%以上), 氧化态氮: 硝酸盐($\mathrm{NO}_{3}^{-}$)、亚硝酸盐($\mathrm{NO}_{2}^{-}$ )和气态硝酸(HNO3), 还原态氮: 铵离子($\mathrm{NH}_{4}^{+}$)和气态氨分子(NH3), 以及有机氮(organic nitrogen, ON)等形式(朱艳宸 等, 2020)。其中, $\mathrm{NO}_{3}^{-}$和$\mathrm{NH}_{4}^{+}$是大气气溶胶中主要的无机氮物质, 而$\mathrm{NO}_{2}^{-}$由于性质不稳定, 容易被氧化, 其含量较少。HNO3和$\mathrm{NO}_{3}^{-}$作为大气中氮氧化物的最终氧化形式是光化学反应的典型产物(李佩霖 等, 2016), 它们不仅是大气中的主要污染物, 而且能够促进细颗粒物的生成, 从而影响气候系统和人类健康(董鑫媛 等, 2020)。
氮氧化物(NOx)包括一氧化氮(NO)和二氧化氮(NO2)等, 主要来源于化石燃料燃烧、生物质燃烧、微生物过程和闪电等人为和自然活动(Hastings et al, 2003)。$\mathrm{NO}_{3}^{-}$主要通过NOx的氧化反应生成, 然后通过干湿沉降方式返回至陆地与海洋生态系统(Altieri et al, 2013), 因此$\mathrm{NO}_{3}^{-}$浓度与排放强度、形成机制和沉降等密切相关(Zong et al, 2022)。在大气中, NOx通过与羟基自由基(·OH)、臭氧(O3)以及其他氧化剂的反应生成$\mathrm{NO}_{3}^{-}$ (Li et al, 2022)。这些反应路径和环境条件的变化导致了$\mathrm{NO}_{3}^{-}$中氮和氧同位素组成的变化, 大气环境中氮氧同位素值反映了不同来源氮的同位素值和大气物理化学转化过程的同位素分馏效应(宋韦 等, 2024), 自然状态下, N存在14N(99.634%)和15N(0.366%)两种同位素, 氧存在三种同位素: 16O(99.762%)、17O(0.038%)和18O(0.200%), 在同位素地球化学中, 通常用δ值表示同位素组成。通常根据δ15N、δ18O和Δ17O的值来解析大气中$\mathrm{NO}_{3}^{-}$的来源和生成途径(Alexander et al, 2020; Berhanu et al, 2012; Miller, 2002; Zong et al, 2022)。不同来源的$\mathrm{NO}_{3}^{-}$具有不同的δ15N值(Galloway et al, 2008), 例如, 生物土壤排放和车辆排放的δ15N值往往偏负, 生物质燃烧和闪电产生的δ15N-NOx的值接近零, 而煤燃烧具有更偏正的同位素值(Elliott et al, 2019)。同时, 大气中不同的氧化剂也具有不同的氧同位素特征(Michalski et al, 2003), 如O3具有很高的氧同位素值(Δ17O值为+25‰~+40‰, δ18O值为+80‰~+130‰), 而水光解产生的·OH的同位素值则为负或接近于零(Δ17O值接近零, δ18O值为-15‰~0‰), δ18O-OH的同位素值偏负或接近于零(Δ17O值接近零, δ18O值为-89‰~-56‰)(Johnston et al, 1997; Michalski et al, 2011; Vicars et al, 2014)。
大气气溶胶(包括其对云的影响)对气候变化的总辐射强迫为-0.9(-1.9~0.1)W·m-2, 其中$\mathrm{NO}_{3}^{-}$气溶胶的辐射强迫为-0.11(-0.3~0.03)W·m-2(李佩霖 等, 2016), 大气$\mathrm{NO}_{3}^{-}$可通过影响颗粒物负荷和地球的辐射热平衡对空气质量和气候产生影响(Burger et al, 2023)。此外, 大气$\mathrm{NO}_{3}^{-}$作为海洋氮输入的重要外部来源之一, 其沉降过程对海洋生态系统的氮循环产生了显著影响(Jickells et al, 2017; Kim et al, 2011)。近年来, 由于人类活动的增加, 如化石燃料的燃烧和农业施肥, 大气中NOx的排放大幅度增加, 导致$\mathrm{NO}_{3}^{-}$的沉降通量显著上升(Doney et al, 2007; Duce et al, 2008, Magnani et al, 2007)。$\mathrm{NO}_{3}^{-}$沉降作为一种外部营养物输入, 不仅改变了海洋营养物的平衡, 还可能对海洋生态系统产生深远的影响, 并影响气候变化。尤其是在氮限制或低氮环境下, $\mathrm{NO}_{3}^{-}$沉降被认为是一种重要的氮源, 能够促进初级生产者(如浮游植物)的生长(de Leeuw et al, 2003, Zhao et al, 2015)。然而, 这种外部输入的增加也可能打破海洋中长期存在的氮磷平衡, 带来一系列生态影响(Kim et al, 2011)。因此, 研究海洋大气$\mathrm{NO}_{3}^{-}$的来源和形成机制对于理解大气化学反应、气候变化和全球氮循环尤为重要。本文总结了海洋大气$\mathrm{NO}_{3}^{-}$的产生、$\mathrm{NO}_{3}^{-}$的同位素及测定方法、全球$\mathrm{NO}_{3}^{-}$的δ15N和δ18O分布特征以及$\mathrm{NO}_{3}^{-}$沉降对海洋生态系统的影响, 并对如何更加系统地开展海洋大气$\mathrm{NO}_{3}^{-}$的研究进行了展望。

1 海洋大气中$\mathrm{NO}_{3}^{-}$产生途径

大气中的$\mathrm{NO}_{3}^{-}$主要通过NOx的氧化反应产生(Seinfeld et al, 2016)。NOx氧化为$\mathrm{NO}_{3}^{-}$有多种途径, 这些途径因纬度、光照时间和大气化学条件的差异而有所不同(Davidson et al, 1997)。在海洋大气环境中, NOx的来源主要包括人类活动(如燃烧化石燃料、船舶排放、生物质燃烧)(Li et al, 2022; van Der A et al, 2008)和自然过程(如闪电和海洋中的微生物活动)(Altieri et al, 2021; Zong et al, 2020)。一旦释放到大气中, 这些NOx会经历一系列的氧化过程, 最终生成$\mathrm{NO}_{3}^{-}$ (Chen et al, 2022; Wei et al, 2024)。在大气中, $\mathrm{NO}_{3}^{-}$的生成和转化相对复杂, 不同的氧化剂(如O3和·OH)对$\mathrm{NO}_{3}^{-}$的生成途径有显著影响, 海洋中的气溶胶不仅是$\mathrm{NO}_{3}^{-}$生成的重要场所, 还通过氯化钠(NaCl)、氯化钙(CaCl2)、硫酸盐($\mathrm{SO}_{4}^{2-}$)和碳酸盐($\mathrm{CO}_{3}^{2-}$)等常见的无机盐和其他有机物质的存在影响$\mathrm{NO}_{3}^{-}$的物理化学性质。海洋大气中$\mathrm{NO}_{3}^{-}$的主要生成途径如图1所示。
图1 海洋大气中硝酸盐的主要生成途径

hv表示光解反应; HC/DMS表示碳氢化合物/二甲基硫(hydrocarbon/dimethyl sulfate, HC/DMS)途径

Fig. 1 Major generation pathways of nitrate in the marine atmosphere

当NO排放时, 会迅速被O3(反应R1)、过氧自由基(RO2或HO2)(反应R2)和(或)卤素氧化物(XO, 其中X=Br、Cl或I)(反应R3)氧化为NO2(Burger, 2023)。一般来说, NO与NO2在光化学平衡中(R1和R4)(Xiao et al, 2015), 然后NO2进一步与其他物质发生反应生成$\mathrm{NO}_{3}^{-}$, 此反应过程为R5—R11。白天, 大气中的NO2可以被·OH氧化生成HNO3(反应R5)。夜间, 反应R5停止, NO2被O3进一步氧化形成NO3(反应R6)。NO3可以与NO2通过热平衡反应生成N2O5(反应R7)(Xiao et al, 2015), 然后在夜间通过水解形成HNO3(反应R8)(Altieri et al, 2013; Fang et al, 2011)。此外, HC/DMS途径(即NO3与碳氢化合物或二甲基硫的反应)也是NOx转化为$\mathrm{NO}_{3}^{-}$的重要途径(反应R9), 尤其是在海洋边界层的高纬度地区(Song et al, 2020)。
在海洋大气中, $\mathrm{NO}_{3}^{-}$生成的反应路径受到海洋环境中特有的化学成分的影响。N2O5可以与含氯气溶胶发生非均相反应, 生成硝酰氯(ClNO2)和$\mathrm{NO}_{3}^{-}$ (反应R10)(王海潮 等, 2020)。此外, 在某些卤素含量较高的地区, NO2还可以与活性卤素化合物(如溴氧化物BrO)反应, 生成BrNO2, 再水解为HNO3和HOBr(反应R11、R12)(Lao et al, 2024)。这种卤素的参与在沿海和富含海盐气溶胶的环境中尤其重要, 显著影响了海洋大气中$\mathrm{NO}_{3}^{-}$的生成路径, 海洋大气$\mathrm{NO}_{3}^{-}$的形成路径如下:
NO + O3 → NO2 + O2 (R1)
NO + RO2 (或HO2) → NO2 + RO (或·OH) (R2)
NO + XO → NO2 + X (R3)
NO2 + O2+ hv → NO + O3 (R4)
NO2 + ·OH + M → HNO3 + M (R5)
NO2 + O3 → NO3 + O2 (R6)
NO3 + NO2 + M ↔ N2O5+ M (R7)
N2O5 (g) + H2O (l) → 2HNO3 (R8)
NO3 + HC或DMS → HNO3 +产物(R9)
N2O5 + Cl- → ClNO2 + $\mathrm{NO}_{3}^{-}$ (R10)
NO2 + BrO → BrNO2 (R11)
BrNO2 + H2O → HNO3 + HOBr (R12)

2 硝酸盐氮氧同位素测定方法

稳定同位素分析是一种精确且可靠的分析技术, 主要包括质谱、光谱、核磁共振谱、气相色谱和中子活化等方法, 常用于大气环境监测, 以评估、预防和控制大气污染(李亲凯 等, 2016)。δ15N可被用于确定NOx的来源(Elliott et al, 2007), 而δ18O(或Δ17O)在NOx向$\mathrm{NO}_{3}^{-}$转化过程中会发生显著变化, 这种变化可能与氧化过程相关。因此, δ18O(或Δ17O)可以作为$\mathrm{NO}_{3}^{-}$形成和迁移转化过程的指示标志(Savarino et al, 2007), $\mathrm{NO}_{3}^{-}$同位素组成的长期记录能够详细描述其生成的来源和化学过程, 并加深对大气化学变化的认识(Altieri et al, 2021; Savarino et al, 2007; Wankel et al, 2010)。
$\mathrm{NO}_{3}^{-}$中的同位素的测试前处理方法主要包括以下几种类型: 蒸馏法、扩散法、热解法、离子交换树脂法、细菌法和化学法(也称为两步还原法)(张雯淇 等, 2019)。蒸馏法利用还原剂(如戴氏合金或铜锌合金)将样品中的$\mathrm{NO}_{3}^{-}$还原为$\mathrm{NH}_{4}^{+}$, 然后通过蒸馏富集$\mathrm{NH}_{4}^{+}$。扩散法首先将$\mathrm{NO}_{3}^{-}$还原为$\mathrm{NH}_{4}^{+}$并收集, 然后在玻璃消解管中加热扩散, 收集的$\mathrm{NH}_{4}^{+}$通过高温灼烧或加入还原剂转化为N2, 之后用质谱仪测定氮同位素(Brooks et al, 1989; 孙文青 等, 2019; 肖红伟 等, 2012)。热解法是一种将硝酸盐样品在高温条件下分解, 后将产生的气态产物传输到质谱仪进行同位素比率分析的方法(Michalski et al, 2002)。离子交换树脂法包括阳离子交换法和阴离子交换法, 该方法将水中的溶解硝酸盐转化为硝酸银晶体, 随后用质谱仪测定氮同位素, 部分硝酸银晶体与石墨在高温下燃烧, 生成二氧化碳气体, 用于氧同位素组成的测定(Chang et al, 1999; Silva et al, 2000)。细菌法则利用一种特定的反硝化细菌(致金色假单胞菌, Pseudomonas aureofaciens), 将$\mathrm{NO}_{3}^{-}$还原为一氧化二氮(N2O)后进行氮氧同位素的测定(Sigman et al, 2001; 尹希杰 等, 2023)。化学法(两步还原法)首先使用海绵状镉将$\mathrm{NO}_{3}^{-}$还原为$\mathrm{NO}_{2}^{-}$, 然后在醋酸缓冲液中使用叠氮化钠将$\mathrm{NO}_{2}^{-}$进一步还原为N2O, 水样中的N2O被自动系统低温捕获, 并随后释放至气相色谱柱中。分离出的N2O随后通过开放分流进入连续流动同位素比质谱仪进行分析(McIlvin et al, 2005)。目前, 离子交换树脂法、细菌法和化学法被认为是海洋大气硝酸盐样品的首选前处理方法, 因其能够较好地处理低浓度硝酸盐, 同时提供精确的氮氧同位素数据。硝酸盐氮氧同位素在测试中主要的前处理方法汇总如表1所示。
表1 硝酸盐氮氧同位素在测试中主要的前处理方法

Tab. 1 Main pretreatment methods for nitrogen and oxygen isotopic analysis of nitrate

预处理方法 可测定同位素 优点 缺点 文献
蒸馏法 N 可应用于不同类型的样品, 不需要昂贵的设备; 适用低浓度样品 步骤较繁琐; 容易受到有机物和其他含氮化合物的干扰、容易发生同位素分馏, 导致精度和灵敏度有限 Diaconu et al, 2005
扩散法 N 所需设备和试剂相对简单和廉价; 可批量处理样品; 耗时较短 样品需求量较大; 耗时较长; 不适合低浓度样品的同位素分析; 不完全扩散可能引起同位素分馏, 导致较大的误差 Brooks, 1989;
孙文青 等, 2019
热解法 N、O 避免了样品与反应管氧同位素的交换, 适用于微量样品的高精度同位素分析 设备昂贵, 对操作条件要求严格, 需要经验丰富的技术人员 Michalski et al, 2002
离子交换树脂法 N、O 具有高选择性, 能去除干扰离子; 适用于低浓度样品 需要的样品量较大, 耗时较长;交换和洗脱过程中可能会导致部分样品损失; 费用较高 Chang et al, 1999;
Silva et al, 2000
细菌法 N、O 检出限较低; 前处理方法简单; 成本较低; 可以同时分析氮和氧同位素 需要细菌培养和较长的反应时间; 需要无氧条件操作, 易受氧气污染; 细菌的代谢特性可能影响同位素分析的结果, 导致结果不稳定 毛绪美 等, 2005;
尹希杰 等, 2023
化学法 N、O 反应速度快; 检出限低; 结果稳定 使用剧毒试剂; 试剂纯度和质量要求高, 需要精确控制反应条件(如温度、酸碱度)以确保结果准确 McIlvin et al, 2005;
张雯淇 等, 2019

3 海洋大气系统的同位素示踪

3.1 氮氧同位素在不同过程中的特征

在自然界中, 氮的同位素分布会因不同过程而发生变化。一般情况下, 14N较轻且在大气和生物过程中更常见, 如大气NOx和微生物反应中14N优先反应(Zong et al, 2020)。而在化石燃料燃烧和高温反应过程中, 15N(较重的氮同位素)则更容易富集, 因此这些来源的NOx通常富含15N。此外, 农业肥料和污水处理过程中的氮同位素分馏也会导致水体或土壤中残留物富集15N。其中, 生物质燃烧可能产生δ15N范围为-7‰~+12‰的NOx(Fibiger et al, 2016), 而土壤释放的NOx具有相对较低的δ15N特征(-44.2‰~-14.0‰)(Miller et al, 2018), 平均值为 (-(35.4±10.7)‰)(Zong et al, 2020), 闪电产生的NOx的δ15N特征约为0‰(Hoering, 1957)。总的来说, 偏轻的氮(14N)多见于自然生物过程, 偏重的氮(15N)则常见于燃烧和人类活动相关的排放(Xiao et al, 2015; Zong et al, 2022)。对于氧同位素, 已有研究表明δ18O-H2O的数值范围为-25‰~0‰, 与δ18O-O3(+90‰~+122‰)有显著差异(Fang et al, 2011)。通常, 通过·OH途径生成的$\mathrm{NO}_{3}^{-}$具有较低的δ18O-$\mathrm{NO}_{3}^{-}$特征, 因为生成的$\mathrm{NO}_{3}^{-}$中有1/3的氧原子来自·OH。相反, 通过O3途径生成的$\mathrm{NO}_{3}^{-}$的δ18O较高, 因为$\mathrm{NO}_{3}^{-}$中有5/6的氧原子来自O3(Krankowsky et al, 1995; Vicars, 2014)。通过BrO、DMS或HC途径生成的$\mathrm{NO}_{3}^{-}$具有最高的δ18O, 因为这些过程中的氧原子来源于O3的末端氧原子(约+130‰)(Lao et al, 2024)。
根据Walters等(2016)等的模型计算结果(图2), 在白天, 假设当地δ15N-NOx的范围为-15‰~0‰, 在25oC下, 当${{f}_{\text{NO}}}_{_{2}}$(定义为NO2的浓度除以总NOx的浓度)=0.70时, 由(R1)和(R4)过程产生HNO3时, δ15N由δ15N-NO的-40.2‰~-25.2‰变为δ15N-NO2和δ15N-HNO3(1)的-4.2‰~+10.8‰是由于NO和NO2之间的氮同位素交换倾向于将15N分配到NO2中。夜间, NOx的光化学循环停止, 几乎所有的NOx都以NO2形式存在。因此, δx-NOx≈δx-NO2。假设反应R8中动力学同位素分馏忽略不计, 当地δ15N-NOx的范围为-15‰~0‰, δ18O-H2O范围为-25‰~0‰, N2O5和HNO3(2)的δ15N估算范围为+10.5‰~+25.5‰, 比NOx的δ15N约高+25.5‰, 原因是N2O5/NO2交换(25°C)。假设N2O5与H2O之间的氧质量平衡, 估算在25°C时, δ18O-HNO3(2)范围为97.0~109.5‰, 且随着温度降低, N2O5/NO2的N和O分馏增加, 这将导致δx-N2O5相对于局部NOx增大。通过反应R9生成的夜间硝酸盐[记为HNO3(3)], 其O和N同位素组成等同于NO3同位素组成, 假设同位素质量平衡且忽略反应中的动力学同位素分馏, 根据这一机制, 估计NO3和HNO3(3)的δ15N范围为-33.1‰~-18.1‰, 比δ15N-NO2低约18.4‰。此外, NO3和HNO3(3)的δ18O范围估计为+102.9‰~+112.9‰, 比δ18O-NO2低约9.7‰。NO3和NO2之间的同位素交换导致HNO3(3)的δ15N和δ18O较低, 因为交换倾向于将15N和18O分配到NO2中(25℃), 且随着温度降低, 由NO3/NO2同位素交换引起的N和O分馏会增加, 从而导致δx-HNO3(3)值上升。
图2 不同硝酸盐生成途径同位素值的变化

改自Walters等(2016)

Fig. 2 Isotopic variations in different nitrate formation pathways. Modified from Walters et al (2016)

3.2 全球δ15N-$\mathrm{NO}_{3}^{-}$和δ18O-$\mathrm{NO}_{3}^{-}$分布存在差异

在大气中, NOx转化为$\mathrm{NO}_{3}^{-}$的主要过程包括·OH和O3等氧化路径, 不同的产生路径对δ15N-$\mathrm{NO}_{3}^{-}$和δ18O-$\mathrm{NO}_{3}^{-}$的值及其分布特征产生了显著影响(Alexander et al, 2020)。图3根据全球海洋以及沿海地区的δ15N-$\mathrm{NO}_{3}^{-}$、δ18O-$\mathrm{NO}_{3}^{-}$和$\mathrm{NO}_{3}^{-}$数据(Altieri et al, 2013; Beyn et al, 2014; Chuang et al, 2022; Costa et al, 2011; Elliott et al, 2009; Fang et al, 2011; Felix et al, 2014, 2015; Gobel et al, 2013; Jung, 2015; Katsura, 2012; Kawashima et al, 2011; Kojima et al, 2011; Koszelnik et al, 2013; Lao et al, 2024; Lin et al, 2022; Liu et al, 2018; Luo et al, 2016; Morin et al, 2009, 2012; Occhipinti et al, 2008; Proemse et al, 2012; Proemse et al, 2013; Savarino et al, 2007, 2013; Shi et al, 2014; Smirnoff et al, 2012; Spoelstra, 2004; Sun et al, 2022; Xiao et al, 2015; Xiao et al, 2023; Yan et al, 2019; Yang et al, 2014; Yeatman et al, 2001; Zhang et al, 2022; Zhang et al, 2024; Zhao et al, 2021; Zong et al, 2022)汇总了全球不同海洋地区的δ15N-$\mathrm{NO}_{3}^{-}$和δ18O-$\mathrm{NO}_{3}^{-}$的分布。
图3 全球不同海洋地区和沿海城市的δ15N-$\mathrm{NO}_{3}^{-}$ (a)和δ18O-$\mathrm{NO}_{3}^{-}$ (b)分布

该图基于自然资源部标准地图服务网站下载的审图号GS(2016)1665的标准地图制作, 底图无修改

Fig. 3 Distribution of δ15N-$\mathrm{NO}_{3}^{-}$ (a) and δ18O-$\mathrm{NO}_{3}^{-}$ (b) in different ocean regions and coastal cities around the world

δ15N-$\mathrm{NO}_{3}^{-}$随着纬度的增加呈现出微弱的上升趋势(图4a), 但部分数据点偏离回归线较大, 表明除了纬度以外, δ15N-NO– 3的变化可能还受其他因素(如人类活动、气候条件、污染源等)影响。由图3a可知, 在北美、欧洲和部分亚洲(如日本和中国)等北半球中纬度和高纬度地区, δ15N-$\mathrm{NO}_{3}^{-}$值普遍较高[平均(+2.93±5.36)‰, 最大值+12.88‰, 最小值-7‰], 这些地区通常有密集的人类活动, 特别是化石燃料的燃烧和工业排放(Mcduffie et al, 2020)。燃煤发电厂、机动车尾气和工业过程通常会释放出偏正氮同位素的NOx, 导致较高的δ15N值。在热带地区, 如赤道附近的大西洋[平均(-1.22±4.23)‰, 最大值+5.94‰, 最小值-10.95‰]和南半球的南美洲、非洲部分地区[平均(-14.54±10.84)‰, 最大值-1.81‰, 最小值-42.75‰], δ15N-$\mathrm{NO}_{3}^{-}$值普遍较北半球中高纬度低。这可能意味着这些区域的$\mathrm{NO}_{3}^{-}$主要来源于自然源, 如海洋排放(海洋生物固定的N2)、闪电和生物排放等。同时由于这些地区可能由于人类活动相对较少, 受到较少的工业和交通污染影响, 因此δ15N-$\mathrm{NO}_{3}^{-}$值普遍较低。南半球大致出现随着纬度升高δ15N-$\mathrm{NO}_{3}^{-}$值普遍降低的趋势(图4a), 主要与极地大气中的光化学分馏和大气输送过程有关。紫外线辐射和低温条件下, 氮同位素发生分馏, 使轻的14N更容易留在$\mathrm{NO}_{3}^{-}$中, 导致δ15N-$\mathrm{NO}_{3}^{-}$值降低。此外, 远距离大气输送将低δ15N的$\mathrm{NO}_{3}^{-}$带入南极地区。这些因素共同作用, 使得南极的$\mathrm{NO}_{3}^{-}$可能出现偏负的δ15N值(Burger et al, 2023)。总体来说, 由于工业化、城市化和化石燃料燃烧的影响, 北半球的大气δ15N-$\mathrm{NO}_{3}^{-}$值通常较高, 尤其是在工业和人口密集的地区, 氮同位素(15N)偏正。南半球的大气δ15N-$\mathrm{NO}_{3}^{-}$值较北半球低(图4a), 主要受到自然氮循环过程的影响, 如雷电和海洋气溶胶, 这些来源通常含有较多的轻氮同位素(14N)。在北美、欧洲以及东亚(如中国和日本)等工业发达地区δ18O-$\mathrm{NO}_{3}^{-}$值较高[平均(+73.74±12.34)‰, 最大值+91.93‰, 最小值+29.51‰](图4b), 这些地区通常具有较强的光化学反应环境(例如城市污染上空), 强烈的光化学反应导致NOx与大气中的O3和过氧化物(如HO2)反应生成$\mathrm{NO}_{3}^{-}$, 这种过程往往富集18O, 从而提高δ18O-$\mathrm{NO}_{3}^{-}$值。在北半球的低纬度地区, δ18O-$\mathrm{NO}_{3}^{-}$值较低[平均(+63.26±8.52)‰, 最大值+86.1‰, 最小值+50.91‰], ·OH途径生成的$\mathrm{NO}_{3}^{-}$可能占主导地位, 因为·OH是光解产物, 同时由于低纬度地区光化学反应条件相对稳定, 日光强度相对平均, 不同于高纬度地区的光化学反应波动较大(Lao et al, 2024; Savarino et al, 2013), 这种稳定性可能导致该地区的δ18O值的区域一致性较强, 但整体偏低。此外, 溴氧化物(BrO)、碳氢化合物(HC)和二甲基硫化物(DMS)其他氧化剂在全球某些地区也很重要, Morin等(2008)通过测量北极大气中$\mathrm{NO}_{3}^{-}$中的氮和氧稳定同位素组成, 发现春季光化学反应驱动的活性氮从积雪中释放到大气中, 使得溴的氧化作用成为NOx局部氧化生成$\mathrm{NO}_{3}^{-}$的主导作用, 这导致北极地区出现δ18O-$\mathrm{NO}_{3}^{-}$异常高值(图4b)。在南半球, δ18O-$\mathrm{NO}_{3}^{-}$值在低纬度地区通常较低, 而在高纬度地区较高, 表现出随纬度显著增加的趋势, 与北半球的规律相反。
图4 δ15N-$\mathrm{NO}_{3}^{-}$和纬度的相关性分析(a)及不同地域的δ18O-$\mathrm{NO}_{3}^{-}$分布(b)

Fig. 4 Correlation analysis between δ15N-$\mathrm{NO}_{3}^{-}$ and latitude (a) and distribution of δ18O-$\mathrm{NO}_{3}^{-}$in different regions (b)

4 海洋大气硝酸盐沉降及其对海洋生态的影响

大气中的$\mathrm{NO}_{3}^{-}$沉降是指通过气溶胶干沉降和湿沉降过程, 将NOx转化为$\mathrm{NO}_{3}^{-}$并沉降到海洋表面的现象(Seok et al, 2021; Shi et al, 2021; Zhao et al, 2015)。大气$\mathrm{NO}_{3}^{-}$主要集中在1~5µm粒径的颗粒上, 作为海洋氮输入的重要外部来源之一, 其沉降过程对海洋生态系统的氮循环产生了显著影响(Jickells et al, 2017; Kim et al, 2011)。
在氮限制或低氮环境下, $\mathrm{NO}_{3}^{-}$沉降被认为是一种重要的氮源, 能够促进初级生产者(如浮游植物)的生长(de Leeuw et al, 2003; Zhao et al, 2015)。然而, 这种外部输入的增加也可能打破海洋中长期存在的氮磷平衡, 带来一系列生态影响。首先, 大气硝酸盐沉降为海洋提供了额外的氮源, 有助于增加浮游动植物的生物量和生产力(Steinberg et al, 2008), 提高光合作用以增强海洋的碳汇能力, 尤其是在偏远的海洋区域(Anderson et al, 1994; Pahlow et al, 2000; Park et al, 2008)。另一方面, 长期的大气硝酸盐沉降可能会导致海洋生态系统的营养失衡(Bange, 2008; Scanlan et al, 2008; Steinberg et al, 2008), 海洋中的浮游植物通常依赖固定的氮磷比例(Redfield比通常为117:16:1)来维持正常物种的竞争力(Kim et al, 2014b; Yang et al, 2020), 过量的$\mathrm{NO}_{3}^{-}$沉降会使水体中的氮浓度相对于磷浓度增加, 导致氮磷比失衡, 这种转变对不同海域的营养平衡和生态系统结构产生了多样化的影响, 改变近海和边缘海的营养状况的同时影响了海洋氮循环的整体动力学(Shi et al, 2021)。
全球范围内, 气溶胶$\mathrm{NO}_{3}^{-}$浓度变化较大(0.2~467.2 nmol·m-3), 沿海地区尤其是亚洲和欧洲的边缘海域浓度较高, 开放海洋和极地地区浓度较低。一般来说, 北半球海洋大气边界层中的气溶胶$\mathrm{NO}_{3}^{-}$浓度较高, 可达(23.9±46.3)nmol·m-3, 是南半球浓度[约为(10.9±18.3)nmol·m-3]的2倍多(Lao et al, 2024)。研究表明, 大气氮沉降存在很强的空间梯度, 与南半球相比, 北半球海洋的输入要大得多, 尤其是在亚洲等大型人口中心的下风向的海洋区域, 沉降量更高(Duce et al, 2008; Jickells et al, 2017)。在南半球高纬度地区, 大气$\mathrm{NO}_{3}^{-}$的浓度约比北半球低一个数量级(Shi et al, 2021)。北半球较高的大气$\mathrm{NO}_{3}^{-}$浓度更多地受到化石燃料燃烧和农业活动等人为源的影响(Duce et al, 2008; Shi et al, 2021), 这与大气$\mathrm{NO}_{3}^{-}$的分布一致(图5)。例如, 北太平洋上层海洋中观察到的N增加是由于最近大气氮沉降的增加, 主要是东北亚人为氮排放的急剧增加(Kim et al, 2014a); 在热带大气中, $\mathrm{NO}_{3}^{-}$主要通过·OH通道生成; 在南半球高纬度地区, BrO和DMS通道对$\mathrm{NO}_{3}^{-}$生成的作用逐渐增加(Yang et al, 2005); 在南极沿岸, 大气$\mathrm{NO}_{3}^{-}$的极低δ15N值表明其主要来源是南极雪层在光解作用下释放的$\mathrm{NO}_{3}^{-}$/NOx。
图5 全球不同海洋地区和沿海城市$\mathrm{NO}_{3}^{-}$的分布

该图基于自然资源部标准地图服务网站下载的审图号GS(2016)1665的标准地图制作, 底图无修改

Fig. 5 Distribution of $\mathrm{NO}_{3}^{-}$ in different ocean regions and coastal cities around the world.

大气$\mathrm{NO}_{3}^{-}$沉降在一些沿海水域(如中国沿海水域)对海洋氮循环具有显著影响(Kim et al, 2014a; Zhang et al, 2011), 但在更大范围的开阔海洋中, 其贡献相对较小, 与海水中大量存在的$\mathrm{NO}_{3}^{-}$库相比几乎可以忽略不计, 在表层海水中, 高浓度的$\mathrm{NO}_{3}^{-}$出现在沿海海域和南大洋, 尽管河口水体的大气沉降通量非常高, 但Δ17O≈0, 表明大气中的$\mathrm{NO}_{3}^{-}$被表层生物迅速利用并转化, 或者大气信号被其他$\mathrm{NO}_{3}^{-}$来源(如河流)淹没(Park et al, 2008; Shi et al, 2021)。大气 $\mathrm{NO}_{3}^{-}$沉降在不同的地理区域和生态系统中的影响各异, 这种差异性在全球范围内促进了海洋生态系统的多样化变化, 未来的研究应进一步探讨大气沉降如何在不同环境条件下改变海洋的营养动力学和生物地球化学过程, 以全面理解其对海洋生态系统的长远影响。

5 展望

尽管近年来对海洋大气$\mathrm{NO}_{3}^{-}$的同位素研究取得了显著进展, 但仍存在一些不足和挑战, 未来需要从以下方面加强研究: (1)加强采样的空间覆盖范围, 尤其是偏远海域; (2)完善氧化过程贡献的定量评估, 虽然δ18O和Δ17O同位素分析可以提供氧化剂信息, 但对不同氧化剂(如·OH、O3、BrO等)在$\mathrm{NO}_{3}^{-}$生成中的相对贡献的定量评估仍然不够精确; (3)加强同位素法极地大气的实用性, 为了提高氮和氧同位素在极地大气中的实用性, 需要更多关于区域源(如海冰上的积雪)和区域过程(如来自HONO和海冰氧化剂排放的·OH)的同位素组成的测量; (4)精确对低NOx环境中大气氧化剂的定量评估, 在低NOx环境中, 大气氧化剂预算存在较大不确定性, 这可能影响未来O3减少政策的有效性; (5)深入探究大气$\mathrm{NO}_{3}^{-}$沉降对海洋生态系统的影响。随着全球人类活动的加剧, 大气中的NOx排放预计将继续增加, 这将进一步加大$\mathrm{NO}_{3}^{-}$沉降对海洋营养物平衡和生物地球化学过程的影响。

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