秸秆烟尘中正构脂肪醇和甾醇的碳同位素组成

刘 刚,李久海,徐 慧,李中平,李立武(1.南京信息工程大学江苏省大气环境与装备技术协同创新中心,江苏 南京 210044；2.南京信息工程大学环境科学与工程学院,江苏 南京 210044；.中国科学院地质与地球物理研究所兰州油气资源研究中心,甘肃 兰州 70000)

秸秆烟尘中正构脂肪醇和甾醇的碳同位素组成

刘 刚1,2*,李久海1,2,徐 慧1,2,李中平3,李立武3(1.南京信息工程大学江苏省大气环境与装备技术协同创新中心,江苏 南京 210044；2.南京信息工程大学环境科学与工程学院,江苏 南京 210044；3.中国科学院地质与地球物理研究所兰州油气资源研究中心,甘肃 兰州 730000)

选取稻草、麦秸和玉米秸进行室内模拟燃烧试验,用气相色谱/稳定同位素在线分析系统(GC/C/IRMS)测定了烟尘中正构脂肪醇和甾醇的单体碳同位素比值.结果显示:在稻草明火烟尘中,正构脂肪醇(C20～C30)和甾醇(胆固醇、菜油甾醇、豆甾醇、β-谷甾醇)与稻草 δ13C值之差(Δ13C)的平均值分别为-1.3‰、+0.7‰.在稻草闷烧烟尘中,两类化合物的Δ13C平均值分别为-1.4‰、+1.1‰.在麦秸明火烟尘中,正构脂肪醇和甾醇单体的Δ13C平均值分别为+2.4‰、+0.5‰.在麦秸的闷烧烟尘中,两类化合物的Δ13C平均值分别为+3.0‰、+1.7‰.在玉米秸的明火烟尘中,两类化合物的Δ13C平均值分别为-2.5‰和+0.7‰.在玉米秸闷烧烟尘中,两类有机物的Δ13C平均值分别为-3.8‰、+1.8‰. 3类秸秆烟尘中的正构脂肪醇和甾醇均发生了明显的碳同位素分馏.秸秆闷烧比明火燃烧更有利于烟尘中此两类有机物发生碳同位素分馏.这对识别气溶胶中以秸秆燃烧为来源的这两类有机物有参考意义.

秸秆；烟尘；正构脂肪醇；甾醇；碳同位素

正构脂肪醇和甾醇是大气颗粒物中常见的有机物[1-3].植物生长过程、植物叶片风化、低成熟度煤的燃烧等均排放正构脂肪醇[4-7].生物质燃烧和烹饪过程也排放正构脂肪醇和甾醇[8-12].通常用分布模式、碳优势指数、最大碳数等指标识别大气中正构脂肪醇的来源[1-2,13-14]. C3植物中 C24～C34正构脂肪醇 δ13C值的分布区间为–27.9‰～ –40.3‰; C4植物中C22～C32正构脂肪醇δ13C值的分布区间为–16.7‰～ –27.8‰[15-16].C3和C4植物中甾醇 δ13C 值的变化范围分别是–39.2‰～–31.4‰、–18.6‰～–14.1‰[17].作物秸秆露天焚烧已成为收获季节大气中正构脂肪醇和甾醇的重要来源.虽然对秸秆烟尘中此 2类有机物的化学组成已有较多的研究[18-19],但对其碳同位素组成的研究工作还开展得比较少.笔者在模拟的明火燃烧和闷烧条件下,对稻草、麦秸和玉米秸等秸秆进行了燃烧试验,对烟尘中正构脂肪醇和甾醇的单体碳同位素组成开展了研究,以探讨不同燃烧条件对碳同位素分馏的影响,以期为识别大气气溶胶中此 3类秸秆燃烧为来源的 2类有机物提供支持.

1 材料与方法

1.1 样品采集与处理

2010～2012年共随机采集了6种稻草、6种麦秸、5种玉米秸. 稻草分别采自浙江(内2优6、扬两优6)、安徽(II优279、宁粳1)、江苏(红粮166、II优728).麦秸分别采自河南(矮抗58、温六、西农 979)、江苏(扬麦 16、扬麦 10、烟农10).玉米秸分别采自河南(豫丰 3358、掖单 4)、江苏(中甜9、中科11、东单60).秸秆样品的制备方法详见文献[20].

1.2 燃烧试验与烟尘采集

秸秆燃烧和烟尘采集的步骤详见文献[20].在一间房子内进行燃烧试验.明火燃烧时,取每种秸秆 1.0～1.5kg,自然堆放在一张铁盘上点燃.闷烧时取每种秸秆0.2kg,放入闷燃装置内点燃.用2台大气颗粒物采样器(Andersen, AH-200型)同时采集排放到室内的烟尘.采样所用的玻璃纤维滤膜预先在 500℃烘烤 2h.烟尘样品用铝箔(500℃烘烤2h)包裹, 在低温下保存.

1.3 有机物提取与同位素测定

样品中正构脂肪醇和甾醇的提取和分离方法是在参考前人工作的基础上改进而成的[21-23].具体步骤:称取秸秆粉末样4g或者取采集了烟尘的玻璃纤维滤膜0.5～1张,每次加入二氯甲烷/甲醇混合试剂(2:1,体积比)10mL,于室温下超声振荡 15min,共重复提取 3次.合并提取液,以3000r/min的转速离心 2min,用旋转蒸发器在40℃浓缩上清液至3mL,以氮吹仪在40℃吹至近干.加入2mL 1mol/L的KOH甲醇/水溶液(4:1,体积比)和 1mL去离子水,在 60℃皂化 2h.先后用3mL二氯甲烷提取皂化溶液内的中性组分各 3次.将提取液合并后用氮吹仪在40℃浓缩至1mL.用氧化铝/硅胶柱分离中性组分.把浓缩液滴加到层析柱上,依次用20mL正已烷、20mL正己烷/乙酸乙酯混合液(9:1,体积比)、20mL正己烷/乙酸乙酯混合液(8:2,体积比)洗脱烃、酮和醇.将醇馏分浓缩至近干,加入150µL双(三甲基硅烷基)-三氟乙酰胺(BSTFA),在70℃衍生2h.之后在室温下吹至近干,用正己烷定容.

用气相色谱/稳定同位素在线分析系统(GC/C/IRMS,Delta plus XP型)测定衍生物的单体碳同位素比值.色谱柱为DB-5(60m×0.32mm× 0.25µm)非极性石英毛细管色谱柱.载气流量1mL/min,气化温度300℃,进样量2µL.升温程序:初始温度 60 ,℃保持 2min,以 4 /min℃ 升温至300 ,℃保持35min.有机物经分离后在900℃燃烧炉中在线氧化,依次转化为CO2,经质谱计测定同位素比值. BSTFA的碳同位素比值用离线质谱仪(FinniganMAT-252型)测量.测量前在800℃氧化 BSTFA,生成的 CO2经净化后进行测定.根据衍生物中加入碳原子的多少,以同位素质量平衡方程计算每种醇的碳同位素比值. BSTFA衍生化对单体碳同位素组成无显著影响[24].每个烟尘样品只测定1次(无平行样), BSTFA测定2次.离线测量仪器的标准偏差＜0.3‰,在线测量的标准偏差＜0.5‰.

2 结果与讨论

2.1 正构脂肪醇

在稻草的明火烟尘和闷烧烟尘中, C20～C30正构脂肪醇的 δ13C 平均值分别为–38.1‰～–33.5‰、–38.8‰～ –35.8‰.烟尘中正构脂肪醇单体碳同位素组成的变化趋势虽然总体上与稻草相似,但二者不完全一致(图 1).这说明除了秸秆,燃烧条件也影响稻草烟尘中正构脂肪醇的碳同位素组成.与稻草相比,其烟尘中的正构脂肪醇亏损13C(图2).对于明火烟尘,二者之差(Δ13C)处于–2.1‰～+0.4‰之间,其平均值为–1.1‰.闷燃烟尘中 Δ13C值的波动范围为–2.5‰～–0.1‰,其平均值–1.4‰.明燃烟尘中C20的Δ13C值为正,可能与相对较少的δ13C值统计数有关.闷燃烟尘中的正构脂肪醇比明火烟尘稍微亏损13C.随着碳数的增加,两种烟尘中正构脂肪醇单体的 Δ13C均趋于变小.

图1 秸秆烟尘中正构脂肪醇和甾醇的δ13C值变化趋势Fig.1 Variability of average δ13C values for n-alkanols and sterols in smoke from crop residues

在麦秸的明燃和闷燃烟尘中, C20～C28正构脂肪醇的δ13C平均值分别为–35.0‰～–34.1‰、–37.4‰～–33.5‰.其变化趋势与麦秸差别较大(图1).说明燃料或燃烧条件对烟尘中此类有机物碳同位素组成的影响可能比稻草复杂.在烟尘和麦秸之间,正构脂肪醇单体的碳同位素组成存在系统性的差别.烟尘中的正构脂肪醇总体上比麦秸富集13C(图 2).在明燃烟尘中,其 Δ13C值为+0.1‰～+4.7‰, 平均为+2.4‰.在闷燃烟尘中,其Δ13C值为+1.1‰～+5.1‰,平均为+3.0‰.显然,麦秸烟尘中正构脂肪醇发生碳同位素分馏的方向与稻草烟尘完全相反.说明在同样的燃烧条件下,麦秸对同位素分馏的影响与稻草相反.此外,麦秸闷燃烟尘中的正构脂肪醇比明燃烟尘更加富集13C.表明闷烧条件更有利于13C的富集.

图2 秸秆烟尘中正构脂肪醇和甾醇的Δ13C值变化趋势Fig.2 Variability of Δ13C values for n-alkanols and sterols in smoke from straw combustion

玉米秸属于C4植物.在其燃烧排放的烟尘中,正构脂肪醇的碳同位素组成与麦秸(C3植物)烟尘和稻草(C3植物)烟尘均有明显的差别.在玉米秸明燃和闷燃烟尘中,其 δ13C平均值分别为–23.5‰～–21.5‰、–25.5‰～–19.5‰.两种烟尘间C20～C30正构脂肪醇单体的δ13C值变化趋势不同(图1).与玉米秸相比较也是如此.除了C24和C26,烟尘中的其他正构脂肪醇单体普遍比玉米秸亏损13C(图2).这种趋势大体上与稻草烟尘的类似.玉米秸烟尘中 C24的碳同位素组成比秸秆重.其原因尚不明了,有待进一步研究.在玉米秸的明燃烟尘中,正构脂肪醇的 Δ13C值为–9.1‰～+2.5‰,平均为-2.5‰.其闷燃烟尘中的Δ13C值为–9.0‰～+2.1‰,平均为-3.8‰.闷燃产生的正构脂肪醇似乎比明燃更加亏损13C.不过2种烟尘的Δ13C值基本上都随着正构脂肪醇单体碳数的增加而趋于减小.

2.2 甾醇

在稻草的明燃和闷燃烟尘中,四种甾醇(胆固醇(CH)、菜油甾醇(CA)、豆甾醇(ST)、β-谷甾醇(SI))的 δ13C 平均值变化区间分别为–32.7‰～–31.8‰、–32.3‰～–31.2‰.从胆固醇到β-谷甾醇,明燃和闷燃烟尘中甾醇δ13C值的变化趋势均与稻草极为相似(图1).说明烟尘中甾醇的碳同位素组成在很大程度上也继承了稻草.烟尘中的甾醇均比稻草富集13C.其中明火烟尘的Δ13C值变化于+0.4‰～+1.0‰之间,平均值为+0.7‰.闷燃烟尘的 Δ13C值变化于+0.6‰～+1.6‰之间,平均值为+1.1‰.由此可见,闷燃比明燃更有利于13C在甾醇中富集(图2).

在麦秸的明燃和闷燃烟尘中,甾醇的δ13C平均值变化区间分别为–34.8‰～–31.7‰、–32.0‰～–29.1‰.

从胆固醇到 β-谷甾醇, δ13C平均值的变化趋势大致上类似于麦秸(图1).此趋势在一定程度上也类似于稻草烟尘.烟尘中的甾醇比麦秸总体上要富集13C.其中明燃烟尘的Δ13C值为+0.1‰～+1.2‰,平均为+0.5‰.闷燃烟尘的 Δ13C 值为–0.1‰～+4.7‰,平均为+1.7‰.显然,闷燃所生成甾醇的碳同位素组成比明燃更重(图2).这与稻草烟尘的情形是类似的.

玉米秸烟尘中甾醇的碳同位素组成与麦秸烟尘和稻草烟尘均有明显的差别.在玉米秸的明燃和闷燃烟尘中,甾醇的δ13C值变化范围分别是–19.2‰～–13.6‰、–15.4‰～–14.7‰.与稻草烟和麦秸烟一样,玉米秸烟尘中不同甾醇之间δ13C值的变化趋势均与秸秆相似(图1).除了明燃烟尘中的胆固醇,玉米秸烟尘中的其他甾醇通常均比秸秆富集δ13C(图2).明燃烟尘中甾醇的Δ13C值变化于–2.6‰～+3.0‰之间,其平均值为+0.7‰.闷燃烟尘的 Δ13C值变化于+1.3‰～+2.3‰之间,其平均值为+1.8‰.相较于闷燃烟尘,明燃烟尘中的胆固醇和菜油甾醇是亏损13C的,而豆甾醇和β-谷甾醇则富集13C.这与稻草烟尘和麦秸烟尘的变化趋势不一致.由于只有 3套玉米秸明燃烟尘中甾醇的同位素数据,其统计误差可能比闷燃烟尘更大.因此,相对较少的测定数据可能是造成这一现象的原因.

2.3 同位素分馏机理初探

在陆生C3、C4植物中,正构脂肪醇通过乙酸途径合成,甾醇通过甲羟戊酸途径合成[25-27].植物中的脂类通常划分为游离态和结合态两类.以二氯甲烷/甲醇从植物中提取的是游离态脂,从氢氧化钾甲醇/水溶液皂化过的残余植物样中提取的是结合态脂[28].结合态脂在植物燃烧过程中会发生热解而释放到烟气中.以前述方法从秸秆中提取的正构脂肪醇属于游离态.从秸秆烟尘中提取的正构脂肪醇应该是两种形态的混合物.结合态正构脂肪醇的碳同位素组成比游离态重 0.4‰～0.5‰[28].在稻草、麦秸和玉米秸的排放烟尘中,正构脂肪醇的 Δ13C 平均值分别为–1.4‰、+3.0‰、–3.8‰.显而易见,这不能用秸秆中结合态和游离态正构脂肪醇之间的碳同位素组成差别来合理地解释.植物中结合态甾醇比游离态亏损13C达-0.5‰[28].以上述方法从秸秆中提取的4种甾醇是游离态有机物.在稻草、麦秸和玉米秸燃烧产生的烟尘中,甾醇的Δ13C平均值分别高达+1.1‰、+1.7‰、+1.8‰.表明在烟尘和秸秆的甾醇之间发生了更大的碳同位素分馏.这同样不能用秸秆中结合态和游离态甾醇之间同位素组成的差别来解释.

生物质的燃烧过程一般分为早燃、有焰燃烧和白热燃烧等 3个阶段.上述秸秆的明火燃烧过程包括了所有3个阶段.其燃烧温度高于600℃,持续时间短于5min.而秸秆的闷燃过程则仅包含了早燃和白热燃烧 2个阶段.其燃烧温度低于400℃,持续时间长于20min.秸秆在350℃就发生分解,并生成 80%的挥发份[29].在早燃阶段,秸秆中的游离态和结合态正构脂肪醇会发生气化,部分还会发生化学变化.正构脂肪醇受热脱水后主要转化为同碳数的烯烃和烷烃,以及其他短链的正构烯烃[30-32].这些转变过程可能伴随有碳同位素动力学分馏.这也许是稻草烟尘和玉米秸烟尘中正构脂肪醇比秸秆亏损13C的原因.此外,闷燃更有利于正构脂肪醇的挥发和分解,从而导致了在稻草和玉米秸的闷燃烟尘中正构脂肪醇的碳同位素组成均比明燃烟尘稍轻.然而,麦秸烟尘中正构脂肪醇的碳同位素分馏现象却不能用此机理解释.与其他2种秸秆相比,麦秸中含有更多的空气.因此,在其燃烧过程中就能提供较充足的氧气.结果使一部分挥发的正构脂肪醇被优先氧化,从而使烟尘中剩余的正构脂肪醇因同位素动力学效应而比麦秸富集13C.简言之,脱水过程和秸秆结构均有可能影响烟尘中正构脂肪醇的碳同位素分馏.此外,植物在燃烧过程中部分甾醇会转化成多种烯类和酮类有机物[18,32-33].在此过程中也会发生碳同位素动力学分馏,因而使残留于烟尘中的甾醇相对富集13C.闷燃条件下较低的燃烧温度和较长的燃烧时间更有利于甾醇转化为烯和酮.于是,闷燃烟尘中甾醇的碳同位素组成比明燃烟尘趋于偏重.不过这些推断均需要进一步的实验研究来验证.

3 结论

在稻草、麦秸、玉米秸等每种秸秆的明燃与闷燃烟尘之间,正构脂肪醇或甾醇的碳同位素组成存在明显的差别.甾醇比正构脂肪醇更加富集13C.与秸秆相比,烟尘中的这 2类有机物均发生了显著的碳同位素分馏.在稻草和玉米秸燃烧排放的烟尘中,正构脂肪醇总体上比秸秆亏损13C,而甾醇总体上富集13C.在麦秸烟尘中,正构脂肪醇和甾醇总体上均比麦秸富集13C.秸秆闷燃比明燃更有利于烟尘中 2类有机物发生碳同位素分馏.与正构脂肪醇相比,甾醇的碳同位素比值是更为可靠的源识别指标.

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致谢：袁静、孙丽娜、姚祁芳、张文杰、姚法来等同志参与了秸秆的采集工作,在此表示感谢.

The compound-specific carbon isotope compositions of n-alkanols and sterols in smoke from crop residue combustion.

LIU Gang1,2*, LI Jiu-hai1,2, XU Hui1,2, LI Zhong-ping3, LI Li-wu3(1.Jiangsu Collaborative Innovation Center of Atmospheric Environment and Equipment Technology, Nanjing University of Information Science & Technology, Nanjing 210044, China；2.School of Environmental Science and Engineering, Nanjing University of Information Science & Technology, Nanjing 210044, China；3.Lanzhou Center for Oil and Gas Resources, Institute of Geology and Geophysics, Chinese Academy of Sciences, Lanzhou 730000, China). China Environmental Science, 2017,37(5)：1735～1740

Crop residues of rice, wheat and maize were burned under conditions simulating open combustion. Compound specific carbon isotopic ratios (δ13C) of n-alkanols (C20-C30) and sterols (cholesterol, campesterol, stigmasterol, and β-sitosterol) in smoke from biomass burning were determined. The results showed that the mean distinction (Δ13C) values of δ13C ratios for n-alkanols and sterols between flaming smoke from rice straw and the fuel were –1.3‰ and +0.7‰, respectively. The mean Δ13C values in smoldering smoke from the fuel were –1.4‰ and +1.1‰, respectively. Moreover, the mean Δ13C values for the two groups of compounds in flaming and smoldering smoke from wheat straw were +2.4‰, +0.5‰, and +3.0‰, +1.7‰, respectively. The mean Δ13C values for the two types of compounds in flaming and smoldering smoke from maize stover additionally were –2.5‰, +0.7‰, and –3.8‰, +1.8‰, respectively. Significant isotopic fractionations occurred in both n-alkanols and sterols in smoke from all three sorts of crop residues. Smolder was propitious to isotopic fractionation in the compounds from smoke relative to flame burning. This may have significance for identifying n-alkanol and/or sterol derived from crop residue burning in atmospheric particles.

crop residue；smoke；n-alkanol；sterol；carbon isotope

X131.1,X511

A

1000-6923(2017)05-1735-06

刘 刚(1965-),男,甘肃定西人,教授,博士,主要研究方向为大气污染物组成与源识别.发表论文40余篇.

2016-10-21

国家自然科学基金项目(41073019)

* 责任作者, 教授, liugang650104@sina.com