引用本文
张介霞, 詹力扬, 陈立奇. 南大洋N2O研究进展及测量新技术展望. 地球科学进展, 2013, 28(11): 1201-1208[Zhang Jiexia, Zhan Liyang, Chen Liqi. Review on researchesof nitrous oxide inthe Southern Oceanand relative new technologies. Advance in Earth Science, 2013, 28(11): 1201-1208]  
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南大洋N2O研究进展及测量新技术展望*
张介霞1,2, 詹力扬2, 陈立奇2,*
1. 厦门大学海洋与地球学院,福建厦门 361005
2. 国家海洋局海洋#cod#x02014;大气化学与全球变化重点实验室,国家海洋局第三海洋研究所, 福建厦门 361005
陈立奇(1945-),男,福建晋江人,研究员,主要从事海洋大气化学与全球变化科学研究. E-mail:lqchen@soa.gov.cn

张介霞(1986-)，女，四川乐山人，博士研究生，主要从事海洋N循环研究.Email:zhangjiexia1986@sina.com

基金:[HT6SS][ZK(]中国极地科学战略青年基金项目#cod#x0201c;普里兹湾N2O分布特征及可能形成机制#cod#x0201d;(编号：20120310)资助.
摘要

氧化亚氮(N2O)是大气最重要的温室气体之一,能在大气滞留120年,其增温潜势是CO2的310倍。工业革命以来,大气中N2O浓度已增加了20%,但海洋仍然是大气N2O重要的自然源,占总自然源的1/3左右。南大洋是全球海洋系统重要组成部分,具有独特的水文特征。模式研究表明其是大气N2O的重要源。然而,受制于恶劣的海况条件,南大洋航次观测数据至今十分稀少。简单综述了南大洋N2O研究的重要性、研究进展及现场观测技术和模型应用的结果及其存在的问题。强调稳定同位素方法的运用将为海洋N2O形成机制研究提供新的强有力手段,另外,研发高精度的走航观测技术也是今后N2O研究取得突破的必须努力方向。展望激光光谱技术的运用对南大洋N2O研究的突破以及海冰中N2O浓度测量技术的发展,将对评估海冰在形成和融化过程中可能存在的源汇提供直测证据。

关键词: 氧化亚氮; 温室气体; 南大洋; 新技术
Review on researchesof nitrous oxide inthe Southern Oceanand relative new technologies
Zhang Jiexia1,2, Zhan Liyang2, Chen Liqi2,*
1.Ocean and Earth Science College of Xiamen University, Xiamen 361005,China
2.Key Laboratory of Global Change and MarineAtmospheric Chemistry of State Oceanic Administration (SOA),Third Institute of Oceanography, SOA, Xiamen 361005,China
Abstract

Nitrous oxide is an important greenhouse gas,which has a long lifetime of about 120 years and has a 310 times greenhouse effect than CO2. Since the industrial revolution, the atmospheric N2O concentration has increased significantly by 20%. Ocean is a net source, about 1/3 of total oceanic souce. Southern Ocean is an important part of the global ocean system, has a unique hydrological characteristics. So far it is regarded as a significant natural source to the global N2O flux according to the model studies. However,the field work is very limit, due to the fierce in situ conditions. The importance of N2O reseaches of Southern Ocean, progresses of nitrous oxide researches, especially new technologies applied to underway N2O measurements in the Southern Ocean are reviewed. The advance of field, model studies and their problems or uncertainties that need to be resolved are also discussed. The using of stable isotope methods will provide powerful tools for marine N2O mechanism. Development of highprecision monitoring technology is generally the driving force of future research. Developing techniques of laser spectroscopy in marine N2O studies and measurment of N2O in sea ice will provide powerful tools to differeciate the N2O source sink characteristic, constrain their budget and formation mechanism in region such as Southern Ocean.

Keyword: Nitrous oxide; Greenhouse Gas; Southern Ocean; New Technology
1 引言

氧化亚氮(N2O)是一种重要的温室气体(GHG),虽然它的辐射效应只占6%,但其温室效应是同等浓度下二氧化碳(CO2)的310倍左右[ 1]。同时N2O还能在平流层中通过光化学过程与臭氧反应,从而消耗大气中的臭氧[ 2]。氟氯烃化合物(CFCs)被禁止使用后,N2O成为年排放量最大的大气臭氧破坏物质[ 3]。自工业革命以来,人类活动(如氮肥的使用、农业的发展等)不断改变着全球氮(N)循环模式[ 4]。大气中N2O浓度迅速升高,目前以0.26%的速率稳定增长,预计到2100年,该浓度值将超过400×10-9[ 5]。

长期观测研究认为海洋是大气中N2O净源,其自然源占总源的22-25%左右,是全球自然源的1/3[ 6],其中大部分源于沿岸上升流[ 7]和缺氧区[ 8, 9]如大洋东边界和阿拉伯海等区域[ 10, 11]。研究显示,在海洋次表层,存在N2O来源的区域可通过涡动扩散过程,将N2O被输送到混合层,形成大气的源。对于开阔大洋区域,表层海水与大气N2O平衡。然而,由于表层温度的季节性迅速变化,可能导致表层海水表现为暂时性的源或汇特征[ 5]。Nevision等[ 12]根据表层N2O分压,在北大西洋观察到N2O汇。对于表现为弱汇的海区[ 13],深层水的形成又可能将大气中的N2O输送到深层水中,但从千年尺度来看,海洋热盐环流最终会将其输送到大气中。然而,全球海洋N2O收支估算的不确定性高达50%[ 5],这主要是由于：①运用模型的方法不同；②采用不同海—气交换参数；③对沿岸区域的划分不同。

海洋中N2O被认为主要由细菌硝化和反硝化过程[ 14]产生,在低溶解氧条件下,很大程度是由硝化菌反硝化过程(nitrifier-denitrification)中NO还原产生,很小程度上是氨氧化过程的中间产物羟氨和传统的反硝化过程产生[ 15]。然而,在最近的研究中指出[ 16, 17],古菌不仅能产生N2O,而且氨氧化过程主要是由古菌而非细菌执行的,该发现很大程度上改变了过去对海洋硝化过程的认识。由于共同利用NO,NO2-等中间物质,不同生物过程可能重叠或相互作用[ 18～ 20],因此硝化菌和反硝化菌对N2O的产生都很重要,难以区分。到20世纪末期,稳定同位素方法引入到海洋N2O研究中,该方法以生物活动过程对稳定同位素15N-N2O,O18-N2O的分馏[ 21, 22]为基础,为揭示N2O形成机制及全球N2O收支提供了有用的新信息,但仍无法定量区分硝化反硝化机制的贡献[ 23, 24]。Toyoda等[ 25]对稳定同位素质谱仪进行了改装,使稳定同位素异构体的15N14NO和14N15NO的测定可以被区分。迄今为止,海洋中N2O的形成机制一直是学者们讨论的焦点。

随着海洋脱氧(deoxygenation)及海洋酸化等[ 26, 27]全球变化过程受到关注,有关N2O与溶解氧之间关系的研究进一步深入,Bianchi等[ 28]根据获得溶解氧的数据对海洋脱氧过程进行模拟,结果显示反硝化过程增强,而N2O的产量没有变化。海洋酸化对海洋N2O可能产生的影响也开始引起人们的关注。Beman等[ 27]提出海洋酸化可能减弱海洋硝化过程而最终导致N2O产量的下降；Gehlen等[ 29]则持与Beman相反的观点。尽管目前还没有大量的实测数据,海洋N2O循环过程势必对全球变化过程产生某种程度的响应和反馈,与此相关的问题也将成为海洋N2O研究的重要内容。

南大洋是地球系统的重要组成部分,影响着全球气候变化和生物地球化学以及生态循环[ 30]。随着气候变暖,极区海域区域性变暖[ 31, 32]、局部海冰和冰架的迅速变化[ 33, 34, 35],因此南大洋上下水层结构分支的变化需要重新审视[ 32]。近期研究对南大洋N2O源强的评估不断下调[ 36],其贡献从早期的1.7 Tg N/a[ 37]下调到0.9 Tg N/a[ 37]左右。同时,也有相关研究观察到南大洋N2O不饱和现象[ 38, 39],但由于这些现场数据只是在局部海区发现,因此科学家们仍坚持南大洋为全球N2O源的主流说法。南大洋海域受物理和生物耦合过程影响较大,加强南大洋N2O温室气体的研究,可更好揭示南大洋在全球N2O循环过程中扮演的角色。

2 南大洋N2O研究进展

南大洋占全球海洋面积的1/6,是极具重要研究价值的洋区之一。从水文学角度来看,这个洋区是温盐环流(Thermohaline circulation, THC)的调控枢纽中心[ 40]；从生物地球化学角度来看,南大洋的营养盐在全球海洋生物过程中扮演着极为重要的角色[ 41]；研究表明,南大洋对全球C、N循环过程也具有极为重要的调控作用,因此精确观测和准确评估温室气体对南大洋源汇是十分重要的[ 41]。

遗憾的是,由于南大洋及南极海域的极端环境使得成为地球大洋最难以接近和到达的海域,因此时至今日在南大洋进行过的N2O调查研究工作十分有限。Priscu等[ 42]对Ross冰架水体(Ross Ice Shelf 或 RIS)NH4+,N2O开展研究工作,其研究结果显示RIS水体中N2O和大气基本持平,即非大气N2O的源区也非汇区。Weiss等[ 43]对南印度洋进行调查发现近南极某些区域表层海水具有较高的N2O饱和度；Rees等[ 38]在德雷克海峡观察到表层海水N2O分压与大气平衡；别林斯高晋海表层海水在季节融冰水稀释的作用下,略呈不饱和状态。根据这一研究结果,Rees等[ 38]认为,Bouwman等[ 36]提出南大洋每年向大气释放N2O占海洋总源45%左右的源强可能需要重新评估。Law等[ 44]在南大洋施铁肥试验中(SOIREE)的49°～61°S 纬度范围调查了水体中N2O分布,结果显示调查区域表层海水N2O分压和大气平衡,在施放铁肥后观测到80 m处水深N2O浓度显著升高。然而,Walter等[ 45]对大西洋扇形区域南大洋的施铁肥试验中得出与Law等不同的结论。Zhan 等[ 46]对南大洋印度洋和普里兹湾表层海水进行调查,提出物理因素为南大洋表层海水N2O分布主控因素。Chen等[ 39]近期的研究模拟了近岸融冰过程对表层海水N2O分布的影响,提出融冰水过程可能是南大洋近岸水体N2O不饱和现象的主要原因。虽然目前有关南大洋N2O现场调查工作屈指可数,但是这些有限的研究成果还是为之后的研究提供了重要的线索。

模型研发和运用使大尺度评估取得进展。Nevison等[ 47]利用现场调查共60000个数据估算得到全球大洋的表层海水大气△ pN2O,并利用美国国家大气研究中心(National Center for Atmospheric Research,NCAR)获得的气象数据估算海气传输系数(gas transfer coefficiency),最后得到4 TgN/a的大气海洋年通量数据(范围1.2～6.8 TgN/a之间)。运用海洋综合环流模型(ocean general circulation model, OGCM),结合海洋中△N2O和AOU关系随深度的变化,对N2O在海水中的垂直分布进行模拟,得到与Nevison相类似的结果,得出全球海气通量值3.85 TgN/a(范围在2.7～8 Tg N/a)[ 20],其中,南大洋的释放通量却惊人地占到全球海洋总通量的43％左右。在他们的研究中,均提出南大洋是全球N2O强源区。然而,Nevison等[ 37]对塔斯马尼亚岛大气观测数据分析得出南大洋N2O源强为0.9 TgN/a,较之前估算值低。Suntharalingam等[ 20]的研究也提出模型结果可能高估南大洋对全球N2O收支的贡献。Hirsch等[ 48]和Huang等[ 49]分别对全球N2O释放通量进行评估,模拟结果均显示南大洋区域对大气N2O储库的贡献在0～4 TgN/a范围内[ 50, 51]。由此可见,模型通过数据外推,在估算大面积的海洋N2O通量,仍存在很大的不确定性,其中包括交换系数计算的不确定性、年际异常气候引起的不确定性(如ENSO事件)、根据热点研究区域获取的数据外推引起的不确定,尤其对强烈变化的近岸和快速融冰的高纬度区域影响较大等[ 47]。因此,高分辨率的现场数据的获取仍尤为重要。

Zhan等[ 46]利用中国第22次南极科学考察航渡对南大洋和普里兹湾表层海水进行现场采样。Chen等[ 39]根据表层温盐以及N2O饱和度异常将普里兹湾表层水进行区域性划分,认为不同水体除受融冰水的稀释影响外,还受阳光辐射的影响。根据历史研究,可以推测夏季普里兹湾生物生产力高,有机物再矿化过程使次表层海水中N2O增加,但实际并非如此,普里兹湾表层海水中N2O浓度大多只接近大气水平甚至不饱和,这可能与海冰稀释和海水强烈的分层作用有关。整个南大洋表层海水南向呈现N2O不饱和度加剧的趋势,根据实时气象数据计算结果显示,夏季南大洋可能是全球N2O潜在的汇区,粗略估算实时通量为-3.44±1.17 μmol /(m2 d)[ 39, 46, 50]。这些现象与早期的研究结果相类似[ 38],虽然覆盖范围零散,且均在夏季进行,尚不能肯定该现象是否出现在整个南大洋或其他季节,然而,这也说明南大洋是大气N2O重要源区的观点需要更为谨慎的评估。

海洋N2O的产生和消耗受微生物活动的影响,而生物活动的进行与水体中溶解氧浓度的高低密不可分[ 6～ 8]。南大洋混合层以下水体溶解氧浓度较高[ 50],不太可能存在强烈的N2O产生和消耗过程。此外,Walter等[ 13]的研究结果显示,北大西洋水体中N2O浓度较低,不存在高值现象,这可能于低温环境下N2O产生的生物过程受到抑制有关,如果这种现象同样存在于低温的南大洋水体。那么,南大洋高纬度地区的N2O可能主要源于如绕极深层流等的外来输入贡献。

综上,可以看见,南大洋N2O研究存在以下现状：①由于南大洋恶劣的现场条件,研究相对缺乏,研发和应用具有更高的数据精度和分辨率的稳定同位素光谱分析的走航在线观测技术,成为今后N2O研究取得突破的必须努力的方向,是南大洋N2O深入研究的重要手段。②从现象上可以猜测南大洋N2O形成速率较低,进一步的研究可以从其他角度区分南大洋N2O的生物过程,从而证实生物形成机制对南大洋N2O形成机制的贡献是否可以忽略。

以下将对N2O研究技术进行综述和展望。

3海洋N2O新测量技术展望

3.1 走航技术展望

自上个世纪以来,大气中N2O、CO2等温室气体的浓度以史无前例的速度持续上升,被认为是引起全球变暖的主要原因[ 6]。为了提高对海洋对大气化学组成和气候的影响的认识,海洋对温室气体(如CO2、N2O)的调控作用已成为科学家们所关注的焦点。目前国际上多数研究者采用气相色谱法对海洋N2O进行走航观测[ 43, 44, 51],表层海水管路连接Weiss等[ 43]制作的水汽平衡器,利用 GC-ECD或GC-MS进行检测。但对于盛行西风的南大洋来说,长时间的颠簸震动可能影响气相色谱的稳定性,因此船载气相色谱的稳定性和安全性存在不确定性。除此之外,南大洋复杂的水文动力学特征,使得各系统中水体的稳定性存在显著不同,导致海水中微生物的分布以及空间运移上存在差异,直接影响海水中N2O的形成和分布。南大洋季节性融冰水的稀释作用,使得表层海水略不饱和,整个海区的源汇特征受物理水文特征的影响较大[ 38, 46]。早期的走航技术单样分析时长为6～10 min[ 52],该分析周期可以满足开阔大洋的研究,但对于快速融冰的南大洋来说,无法高效的捕获南大洋N2O表层海水独特特征,难以清晰描述调查区域N2O的分布特征和通量。20世纪末,傅里叶红外分析仪的运用使N2O的连续观测成为可能,但其方法运行条件较为苛刻(需液氮制冷),分析精确度也相对较差,仪器基线漂移严重[ 53]。陆续采用的离散或半连续等研究方法,在时空分布的分辨率上仍较低[ 47, 54]。由于上述原因,直至目前尚未有相关文献报道。

近期,研究者利用激光光谱技术[ 55, 56](离轴积分腔积分光谱,OA-ICOS),结合水汽平衡器,实现连续测量大气和海水中N2O。该类仪器分析精度可达到0.3×10-9 Hz-1/2,响应时间为2 mins 左右,最高采样分析频率为1Hz,可分辨小尺度的变化,且基线漂移十分微小,易于维护校正,实现长时间序列自动观测,如此高的分析精度是气相色谱法难以企及的。若将该仪器运用到了南大洋N2O的现场观测,对南大洋的研究来说将会是突破性的的进展。

3.2 稳定同位素方法的运用展望

20世纪80年代初,根据稳定同位素分馏原理,氮同位素比值引入海洋N2O研究,该分析方法的提出为N2O形成机制的研究提出新的途径[ 21]。15N-N2O、18O-N2O和18O-O2等参数分析方法的建立将N2O形成机制的研究推动到新的台阶。但由于N2O中的同位素组成依赖于基底物质,因此对形成机理的区分仍比较模糊。20世纪末Toyoda等[ 25]对稳定同位素质谱仪进行一定的改装后,使稳定同位素异构体的15N14NO和14N15NO可以被区分。这一惊人的突破在区分硝化机制和反硝化机制具有相当的优势[ 57]。此外,结合稳定同位素异构分析方法的最新实验室模拟实验[ 58]提出与Goreau等[ 59]报道不尽相同的结论,即低氧环境下氨氧化N2O的增量没有原先预计的高。Santoro等[ 17]将该技术与实验室生物培养技术结合,提出古细菌是海洋中N2O主要生物来源。相继的培养实验发现不同过程产生的SP之间存在明显的差别,证明了新测量技术的价值[ 57, 60]。

南大洋整个洋区生产力水平较低,大部分海区都呈现一种典型的高营养盐、低叶绿素(HNLC)海区[ 63]。但与开阔大洋相比,在高生产力的陆架区,并没有观察到高N2O浓度分布特征[ 39, 46],由于南极高纬度海区水团混合复杂,单纯地根据溶氧,硝酸盐、水文特征等参数探讨形成N2O的驱动机制仍受到限制。因此稳定同位素方法在南大洋的运用,可为南大洋N2O生物机理的提供更好的证据。

随着技术的发展,轻便型的高精度稳定同位素异构体的光谱分析仪器的现场应用,不仅可以实现现场走航,还可以测定海水中N2O分压、15N-N2O、18O-N2O以及15N-N2O稳定同位素异构体位置优势参数(15Nα,15Nβ)。该种仪器分析除了分析精度最高可达0.05×10-9,稳定同位素比值分析精度优于1‰,更好的量化N2O的源汇。与稳定同位素异构体分析技术相比,弥补了其分析过程中N2O标准制备差异的问题[ 62]。尽管其精度略逊于稳定同位素质谱仪,但其现场原位观测和高采样频率和无损检测的特点使该类仪器独具优势,尤其是对条件恶劣的南大洋来说,该方法在南大洋现场工作中的运用势必为N2O研究提供更多令人振奋的新成果。

3.3海冰中N2O浓度测量技术

海冰是南大洋特殊的环境因素之一。过去30年,南大洋海冰平均变化范围为2.0×106～14.6×106 km2,是全球海洋海冰覆盖面积最广的洋区[ 63]。海冰被认为是阻碍海洋-大气物质交换的屏障,但目前已有研究证明气体物质可在海洋—海冰—大气之间进行交换[ 64]。然而测定海冰中N2O浓度定量研究过程中很少涉及的问题。早期相继有研究者对海冰中的CO[ 65]、O2以及CO2[ 66]进行测量,直到2012年Randall等[ 67]首次实现了对北极海冰中N2O的测量。样品用氦气在70 ℃的管内吹出,通过冷凝除水,由于海冰中N2O浓度较低,使用检测器为脉冲式火焰光度检测器(PFPD)。研究发现海冰中N2O浓度相当低(约6 nmol/L),是表层海水饱和度的40%,大气的30%,证明了海冰融化对表层海水起到稀释作用[ 67]。而当海冰在形成时,部分N2O同盐卤水一起被排挤出,此时N2O可能通过海冰释放到大气中,或者在海冰形成过程中直接释放,因此海水在形成和融化过程中可能是一种潜在的源或汇[ 67]。这对南北极区域N2O研究来说是一个新的突破,同时也是了解南大洋N2O循环行为的一个重要参数。其将为对早期研究的猜测提供更为直接的证据。

4 结语

自Craig等[ 70]1963年首次对海洋中溶解态N2O开展研究以来,海-气N2O通量观测和评估研究一直受到人们的关注[ 71, 72, 73]。但是到上个世纪九十年代中期为止,相关研究主要集中在太平洋,大西洋和北印度洋[ 10, 13, 18, 19, 70, 74, 75, 76, 77, 78, 79, 80, 81, 82, 83, 84]等区域。国内研究起步较晚,在2000年后,相关文章陆续发表,且研究重点主要集中在近岸水体[ 87, 88, 89, 90, 91]。

分析检测技术的发展通常是科学研究发展的重要驱动力。海洋N2O研究方法也经历了一系列的突破。Craig等[ 70]开创了海洋N2O研究,其后20年间,溶解态N2O的分析方法不断改进[ 74, 76, 90]。Zhan等[ 91]利用CTC自动进样器,将该方法的效率进一步提高,在保证分析精确度的前提下,极大的缩短的样品分析所需的时间,同时极大的减少了水汽对样品分析的干扰。稳定同位素和稳定同位素异构体分析方法的运用[ 25]则为N2O形成机制的研究提供新的科学工具,极大推动相关研究的进展。

然而,时至今日,由于某些限制因素,相关研究仍然存在值得进一步深入探讨的问题,例如：历史数据覆盖范围不足所引发的的问题。极区在大洋环流过程中扮演着源头和驱动力的角色[ 40, 92],其对大洋能量和物质的输运均起至关重要的作用,对N2O在其深层水体的分布循环过程进行研究是准确了解水体中N2O循环过程的关键内容。

模型研究显示,南大洋是大气N2O潜在源区[ 47]。由于南大洋极端的环境,使得 N2O相关研究相对其他洋区滞后。高精密度稳定同位素异构体的检测技术的发展可满足南大洋N2O研究更高的数据精度和分辨率的要求,同时获取稳定同位素数据,探讨南大洋N2O形成机制。另外,海冰存在和季节性变化是南大洋的重要特征之一,海冰在形成和融冰过程中可能是一种潜在的源或汇变化过程[ 67～ 69],研发海冰测定技术将有助改善对南大洋N2O的源汇评估的不确定性。

The authors have declared that no competing interests exist.

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