秸稈煙塵中正構脂肪醇和甾醇的碳同位素組成

劉 剛,李久海,徐 慧,李中平,李立武(1.南京信息工程大學江蘇省大氣環境與裝備技術協同創新中心,江蘇 南京 210044；2.南京信息工程大學環境科學與工程學院,江蘇 南京 210044；.中國科學院地質與地球物理研究所蘭州油氣資源研究中心,甘肅 蘭州 70000)

秸稈煙塵中正構脂肪醇和甾醇的碳同位素組成

劉 剛1,2*,李久海1,2,徐 慧1,2,李中平3,李立武3(1.南京信息工程大學江蘇省大氣環境與裝備技術協同創新中心,江蘇 南京 210044；2.南京信息工程大學環境科學與工程學院,江蘇 南京 210044；3.中國科學院地質與地球物理研究所蘭州油氣資源研究中心,甘肅 蘭州 730000)

選取稻草、麥秸和玉米秸進行室內模擬燃燒試驗,用氣相色譜/穩定同位素在線分析系統(GC/C/IRMS)測定了煙塵中正構脂肪醇和甾醇的單體碳同位素比值.結果顯示:在稻草明火煙塵中,正構脂肪醇(C20～C30)和甾醇(膽固醇、菜油甾醇、豆甾醇、β-谷甾醇)與稻草 δ13C值之差(Δ13C)的平均值分別為-1.3‰、+0.7‰.在稻草悶燒煙塵中,兩類化合物的Δ13C平均值分別為-1.4‰、+1.1‰.在麥秸明火煙塵中,正構脂肪醇和甾醇單體的Δ13C平均值分別為+2.4‰、+0.5‰.在麥秸的悶燒煙塵中,兩類化合物的Δ13C平均值分別為+3.0‰、+1.7‰.在玉米秸的明火煙塵中,兩類化合物的Δ13C平均值分別為-2.5‰和+0.7‰.在玉米秸悶燒煙塵中,兩類有機物的Δ13C平均值分別為-3.8‰、+1.8‰. 3類秸稈煙塵中的正構脂肪醇和甾醇均發生了明顯的碳同位素分餾.秸稈悶燒比明火燃燒更有利于煙塵中此兩類有機物發生碳同位素分餾.這對識別氣溶膠中以秸稈燃燒為來源的這兩類有機物有參考意義.

秸稈；煙塵；正構脂肪醇；甾醇；碳同位素

正構脂肪醇和甾醇是大氣顆粒物中常見的有機物[1-3].植物生長過程、植物葉片風化、低成熟度煤的燃燒等均排放正構脂肪醇[4-7].生物質燃燒和烹飪過程也排放正構脂肪醇和甾醇[8-12].通常用分布模式、碳優勢指數、最大碳數等指標識別大氣中正構脂肪醇的來源[1-2,13-14]. C3植物中 C24～C34正構脂肪醇 δ13C值的分布區間為–27.9‰～ –40.3‰; C4植物中C22～C32正構脂肪醇δ13C值的分布區間為–16.7‰～ –27.8‰[15-16].C3和C4植物中甾醇 δ13C 值的變化范圍分別是–39.2‰～–31.4‰、–18.6‰～–14.1‰[17].作物秸稈露天焚燒已成為收獲季節大氣中正構脂肪醇和甾醇的重要來源.雖然對秸稈煙塵中此 2類有機物的化學組成已有較多的研究[18-19],但對其碳同位素組成的研究工作還開展得比較少.筆者在模擬的明火燃燒和悶燒條件下,對稻草、麥秸和玉米秸等秸稈進行了燃燒試驗,對煙塵中正構脂肪醇和甾醇的單體碳同位素組成開展了研究,以探討不同燃燒條件對碳同位素分餾的影響,以期為識別大氣氣溶膠中此 3類秸稈燃燒為來源的 2類有機物提供支持.

1 材料與方法

1.1 樣品采集與處理

2010～2012年共隨機采集了6種稻草、6種麥秸、5種玉米秸. 稻草分別采自浙江(內2優6、揚兩優6)、安徽(II優279、寧粳1)、江蘇(紅糧166、II優728).麥秸分別采自河南(矮抗58、溫六、西農 979)、江蘇(揚麥 16、揚麥 10、煙農10).玉米秸分別采自河南(豫豐 3358、掖單 4)、江蘇(中甜9、中科11、東單60).秸稈樣品的制備方法詳見文獻[20].

1.2 燃燒試驗與煙塵采集

秸稈燃燒和煙塵采集的步驟詳見文獻[20].在一間房子內進行燃燒試驗.明火燃燒時,取每種秸稈 1.0～1.5kg,自然堆放在一張鐵盤上點燃.悶燒時取每種秸稈0.2kg,放入悶燃裝置內點燃.用2臺大氣顆粒物采樣器(Andersen, AH-200型)同時采集排放到室內的煙塵.采樣所用的玻璃纖維濾膜預先在 500℃烘烤 2h.煙塵樣品用鋁箔(500℃烘烤2h)包裹, 在低溫下保存.

1.3 有機物提取與同位素測定

樣品中正構脂肪醇和甾醇的提取和分離方法是在參考前人工作的基礎上改進而成的[21-23].具體步驟:稱取秸稈粉末樣4g或者取采集了煙塵的玻璃纖維濾膜0.5～1張,每次加入二氯甲烷/甲醇混合試劑(2:1,體積比)10mL,于室溫下超聲振蕩 15min,共重復提取 3次.合并提取液,以3000r/min的轉速離心 2min,用旋轉蒸發器在40℃濃縮上清液至3mL,以氮吹儀在40℃吹至近干.加入2mL 1mol/L的KOH甲醇/水溶液(4:1,體積比)和 1mL去離子水,在 60℃皂化 2h.先后用3mL二氯甲烷提取皂化溶液內的中性組分各 3次.將提取液合并后用氮吹儀在40℃濃縮至1mL.用氧化鋁/硅膠柱分離中性組分.把濃縮液滴加到層析柱上,依次用20mL正已烷、20mL正己烷/乙酸乙酯混合液(9:1,體積比)、20mL正己烷/乙酸乙酯混合液(8:2,體積比)洗脫烴、酮和醇.將醇餾分濃縮至近干,加入150μL雙(三甲基硅烷基)-三氟乙酰胺(BSTFA),在70℃衍生2h.之后在室溫下吹至近干,用正己烷定容.

用氣相色譜/穩定同位素在線分析系統(GC/C/IRMS,Delta plus XP型)測定衍生物的單體碳同位素比值.色譜柱為DB-5(60m×0.32mm× 0.25μm)非極性石英毛細管色譜柱.載氣流量1mL/min,氣化溫度300℃,進樣量2μL.升溫程序:初始溫度 60 ,℃保持 2min,以 4 /min℃ 升溫至300 ,℃保持35min.有機物經分離后在900℃燃燒爐中在線氧化,依次轉化為CO2,經質譜計測定同位素比值. BSTFA的碳同位素比值用離線質譜儀(FinniganMAT-252型)測量.測量前在800℃氧化 BSTFA,生成的 CO2經凈化后進行測定.根據衍生物中加入碳原子的多少,以同位素質量平衡方程計算每種醇的碳同位素比值. BSTFA衍生化對單體碳同位素組成無顯著影響[24].每個煙塵樣品只測定1次(無平行樣), BSTFA測定2次.離線測量儀器的標準偏差＜0.3‰,在線測量的標準偏差＜0.5‰.

2 結果與討論

2.1 正構脂肪醇

在稻草的明火煙塵和悶燒煙塵中, C20～C30正構脂肪醇的 δ13C 平均值分別為–38.1‰～–33.5‰、–38.8‰～ –35.8‰.煙塵中正構脂肪醇單體碳同位素組成的變化趨勢雖然總體上與稻草相似,但二者不完全一致(圖 1).這說明除了秸稈,燃燒條件也影響稻草煙塵中正構脂肪醇的碳同位素組成.與稻草相比,其煙塵中的正構脂肪醇虧損13C(圖2).對于明火煙塵,二者之差(Δ13C)處于–2.1‰～+0.4‰之間,其平均值為–1.1‰.悶燃煙塵中 Δ13C值的波動范圍為–2.5‰～–0.1‰,其平均值–1.4‰.明燃煙塵中C20的Δ13C值為正,可能與相對較少的δ13C值統計數有關.悶燃煙塵中的正構脂肪醇比明火煙塵稍微虧損13C.隨著碳數的增加,兩種煙塵中正構脂肪醇單體的 Δ13C均趨于變小.

圖1 秸稈煙塵中正構脂肪醇和甾醇的δ13C值變化趨勢Fig.1 Variability of average δ13C values for n-alkanols and sterols in smoke from crop residues

在麥秸的明燃和悶燃煙塵中, C20～C28正構脂肪醇的δ13C平均值分別為–35.0‰～–34.1‰、–37.4‰～–33.5‰.其變化趨勢與麥秸差別較大(圖1).說明燃料或燃燒條件對煙塵中此類有機物碳同位素組成的影響可能比稻草復雜.在煙塵和麥秸之間,正構脂肪醇單體的碳同位素組成存在系統性的差別.煙塵中的正構脂肪醇總體上比麥秸富集13C(圖 2).在明燃煙塵中,其 Δ13C值為+0.1‰～+4.7‰, 平均為+2.4‰.在悶燃煙塵中,其Δ13C值為+1.1‰～+5.1‰,平均為+3.0‰.顯然,麥秸煙塵中正構脂肪醇發生碳同位素分餾的方向與稻草煙塵完全相反.說明在同樣的燃燒條件下,麥秸對同位素分餾的影響與稻草相反.此外,麥秸悶燃煙塵中的正構脂肪醇比明燃煙塵更加富集13C.表明悶燒條件更有利于13C的富集.

圖2 秸稈煙塵中正構脂肪醇和甾醇的Δ13C值變化趨勢Fig.2 Variability of Δ13C values for n-alkanols and sterols in smoke from straw combustion

玉米秸屬于C4植物.在其燃燒排放的煙塵中,正構脂肪醇的碳同位素組成與麥秸(C3植物)煙塵和稻草(C3植物)煙塵均有明顯的差別.在玉米秸明燃和悶燃煙塵中,其 δ13C平均值分別為–23.5‰～–21.5‰、–25.5‰～–19.5‰.兩種煙塵間C20～C30正構脂肪醇單體的δ13C值變化趨勢不同(圖1).與玉米秸相比較也是如此.除了C24和C26,煙塵中的其他正構脂肪醇單體普遍比玉米秸虧損13C(圖2).這種趨勢大體上與稻草煙塵的類似.玉米秸煙塵中 C24的碳同位素組成比秸稈重.其原因尚不明了,有待進一步研究.在玉米秸的明燃煙塵中,正構脂肪醇的 Δ13C值為–9.1‰～+2.5‰,平均為-2.5‰.其悶燃煙塵中的Δ13C值為–9.0‰～+2.1‰,平均為-3.8‰.悶燃產生的正構脂肪醇似乎比明燃更加虧損13C.不過2種煙塵的Δ13C值基本上都隨著正構脂肪醇單體碳數的增加而趨于減小.

2.2 甾醇

在稻草的明燃和悶燃煙塵中,四種甾醇(膽固醇(CH)、菜油甾醇(CA)、豆甾醇(ST)、β-谷甾醇(SI))的 δ13C 平均值變化區間分別為–32.7‰～–31.8‰、–32.3‰～–31.2‰.從膽固醇到β-谷甾醇,明燃和悶燃煙塵中甾醇δ13C值的變化趨勢均與稻草極為相似(圖1).說明煙塵中甾醇的碳同位素組成在很大程度上也繼承了稻草.煙塵中的甾醇均比稻草富集13C.其中明火煙塵的Δ13C值變化于+0.4‰～+1.0‰之間,平均值為+0.7‰.悶燃煙塵的 Δ13C值變化于+0.6‰～+1.6‰之間,平均值為+1.1‰.由此可見,悶燃比明燃更有利于13C在甾醇中富集(圖2).

在麥秸的明燃和悶燃煙塵中,甾醇的δ13C平均值變化區間分別為–34.8‰～–31.7‰、–32.0‰～–29.1‰.

從膽固醇到 β-谷甾醇, δ13C平均值的變化趨勢大致上類似于麥秸(圖1).此趨勢在一定程度上也類似于稻草煙塵.煙塵中的甾醇比麥秸總體上要富集13C.其中明燃煙塵的Δ13C值為+0.1‰～+1.2‰,平均為+0.5‰.悶燃煙塵的 Δ13C 值為–0.1‰～+4.7‰,平均為+1.7‰.顯然,悶燃所生成甾醇的碳同位素組成比明燃更重(圖2).這與稻草煙塵的情形是類似的.

玉米秸煙塵中甾醇的碳同位素組成與麥秸煙塵和稻草煙塵均有明顯的差別.在玉米秸的明燃和悶燃煙塵中,甾醇的δ13C值變化范圍分別是–19.2‰～–13.6‰、–15.4‰～–14.7‰.與稻草煙和麥秸煙一樣,玉米秸煙塵中不同甾醇之間δ13C值的變化趨勢均與秸稈相似(圖1).除了明燃煙塵中的膽固醇,玉米秸煙塵中的其他甾醇通常均比秸稈富集δ13C(圖2).明燃煙塵中甾醇的Δ13C值變化于–2.6‰～+3.0‰之間,其平均值為+0.7‰.悶燃煙塵的 Δ13C值變化于+1.3‰～+2.3‰之間,其平均值為+1.8‰.相較于悶燃煙塵,明燃煙塵中的膽固醇和菜油甾醇是虧損13C的,而豆甾醇和β-谷甾醇則富集13C.這與稻草煙塵和麥秸煙塵的變化趨勢不一致.由于只有 3套玉米秸明燃煙塵中甾醇的同位素數據,其統計誤差可能比悶燃煙塵更大.因此,相對較少的測定數據可能是造成這一現象的原因.

2.3 同位素分餾機理初探

在陸生C3、C4植物中,正構脂肪醇通過乙酸途徑合成,甾醇通過甲羥戊酸途徑合成[25-27].植物中的脂類通常劃分為游離態和結合態兩類.以二氯甲烷/甲醇從植物中提取的是游離態脂,從氫氧化鉀甲醇/水溶液皂化過的殘余植物樣中提取的是結合態脂[28].結合態脂在植物燃燒過程中會發生熱解而釋放到煙氣中.以前述方法從秸稈中提取的正構脂肪醇屬于游離態.從秸稈煙塵中提取的正構脂肪醇應該是兩種形態的混合物.結合態正構脂肪醇的碳同位素組成比游離態重 0.4‰～0.5‰[28].在稻草、麥秸和玉米秸的排放煙塵中,正構脂肪醇的 Δ13C 平均值分別為–1.4‰、+3.0‰、–3.8‰.顯而易見,這不能用秸稈中結合態和游離態正構脂肪醇之間的碳同位素組成差別來合理地解釋.植物中結合態甾醇比游離態虧損13C達-0.5‰[28].以上述方法從秸稈中提取的4種甾醇是游離態有機物.在稻草、麥秸和玉米秸燃燒產生的煙塵中,甾醇的Δ13C平均值分別高達+1.1‰、+1.7‰、+1.8‰.表明在煙塵和秸稈的甾醇之間發生了更大的碳同位素分餾.這同樣不能用秸稈中結合態和游離態甾醇之間同位素組成的差別來解釋.

生物質的燃燒過程一般分為早燃、有焰燃燒和白熱燃燒等 3個階段.上述秸稈的明火燃燒過程包括了所有3個階段.其燃燒溫度高于600℃,持續時間短于5min.而秸稈的悶燃過程則僅包含了早燃和白熱燃燒 2個階段.其燃燒溫度低于400℃,持續時間長于20min.秸稈在350℃就發生分解,并生成 80%的揮發份[29].在早燃階段,秸稈中的游離態和結合態正構脂肪醇會發生氣化,部分還會發生化學變化.正構脂肪醇受熱脫水后主要轉化為同碳數的烯烴和烷烴,以及其他短鏈的正構烯烴[30-32].這些轉變過程可能伴隨有碳同位素動力學分餾.這也許是稻草煙塵和玉米秸煙塵中正構脂肪醇比秸稈虧損13C的原因.此外,悶燃更有利于正構脂肪醇的揮發和分解,從而導致了在稻草和玉米秸的悶燃煙塵中正構脂肪醇的碳同位素組成均比明燃煙塵稍輕.然而,麥秸煙塵中正構脂肪醇的碳同位素分餾現象卻不能用此機理解釋.與其他2種秸稈相比,麥秸中含有更多的空氣.因此,在其燃燒過程中就能提供較充足的氧氣.結果使一部分揮發的正構脂肪醇被優先氧化,從而使煙塵中剩余的正構脂肪醇因同位素動力學效應而比麥秸富集13C.簡言之,脫水過程和秸稈結構均有可能影響煙塵中正構脂肪醇的碳同位素分餾.此外,植物在燃燒過程中部分甾醇會轉化成多種烯類和酮類有機物[18,32-33].在此過程中也會發生碳同位素動力學分餾,因而使殘留于煙塵中的甾醇相對富集13C.悶燃條件下較低的燃燒溫度和較長的燃燒時間更有利于甾醇轉化為烯和酮.于是,悶燃煙塵中甾醇的碳同位素組成比明燃煙塵趨于偏重.不過這些推斷均需要進一步的實驗研究來驗證.

3 結論

在稻草、麥秸、玉米秸等每種秸稈的明燃與悶燃煙塵之間,正構脂肪醇或甾醇的碳同位素組成存在明顯的差別.甾醇比正構脂肪醇更加富集13C.與秸稈相比,煙塵中的這 2類有機物均發生了顯著的碳同位素分餾.在稻草和玉米秸燃燒排放的煙塵中,正構脂肪醇總體上比秸稈虧損13C,而甾醇總體上富集13C.在麥秸煙塵中,正構脂肪醇和甾醇總體上均比麥秸富集13C.秸稈悶燃比明燃更有利于煙塵中 2類有機物發生碳同位素分餾.與正構脂肪醇相比,甾醇的碳同位素比值是更為可靠的源識別指標.

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致謝：袁靜、孫麗娜、姚祁芳、張文杰、姚法來等同志參與了秸稈的采集工作,在此表示感謝.

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

劉 剛(1965-),男,甘肅定西人,教授,博士,主要研究方向為大氣污染物組成與源識別.發表論文40余篇.

2016-10-21

國家自然科學基金項目(41073019)

* 責任作者, 教授, liugang650104@sina.com