典型樹種揮發(fā)性有機(jī)物(VOCs)排放成分譜及排放特征

李玲玉,Alex B. Guenther,顧達(dá)薩,Roger Seco,Sanjeevi Nagalingam

典型樹種揮發(fā)性有機(jī)物(VOCs)排放成分譜及排放特征

李玲玉1*,Alex B. Guenther2**,顧達(dá)薩2,3,Roger Seco2,Sanjeevi Nagalingam2

(1.青島大學(xué)環(huán)境科學(xué)與工程學(xué)院,山東 青島 266071；2.加州大學(xué)爾灣分校地球系統(tǒng)科學(xué)系,美國 加利福尼亞州爾灣 92697；3.香港科技大學(xué)環(huán)境與可持續(xù)發(fā)展學(xué)部,香港 999077)

為研究典型樹種的揮發(fā)性有機(jī)物(VOCs)排放特征,并獲得基礎(chǔ)排放速率,應(yīng)用動態(tài)封閉式采樣系統(tǒng)對毛果楊、北美楓香和馬尾松的排放進(jìn)行了實驗室測量.利用熱脫附-氣相色譜-飛行質(zhì)譜儀對排放樣品進(jìn)行定性和定量分析,包括異戊二烯、單萜烯、倍半萜烯、烷烴和烯烴,計算獲得各樹種VOCs排放速率及其排放譜特征.研究表明:毛果楊、北美楓香和馬尾松的總VOCs排放速率分別為19.51、17.19和0.67μg/(g·h).毛果楊的異戊二烯排放最高(18.51μg/(g·h)),占其總排放的94.86%;馬尾松排放的異戊二烯僅占4.03%,單萜烯貢獻(xiàn)最高,為49.09%;北美楓香的單萜烯排放速率最高,為0.84μg/(g·h);3個樹種排放的倍半萜烯占各自VOCs總排放的比重均較小(<1.5%);各樹種的烷烴排放強(qiáng)度高于倍半萜烯,部分化合物甚至高于異戊二烯和單萜烯的排放強(qiáng)度.反式--羅勒烯是毛果楊排放最多的單萜烯化合物,占其單萜烯總排放的99.84%;北美楓香排放的單萜烯主要以香檜烯和-蒎烯為主;馬尾松以-蒎烯、香檜烯和-蒎烯為主.石竹烯、葎草烯、-杜松烯和-愈創(chuàng)木烯是主要的倍半萜烯物種.烷烴排放主要為C4和C5的化合物,特別是異丁烷和正丁烷;各樹種排放的烯烴中,1-丁烯排放占比最高.

揮發(fā)性有機(jī)物；天然源；排放速率；排放成分譜

植物排放的揮發(fā)性有機(jī)物(VOCs)對全球環(huán)境化學(xué)和氣候具有重要影響,是對流層臭氧和二次有機(jī)氣溶膠的重要前體物,如異戊二烯、單萜烯、倍半萜烯、丙酮等[1–6].研究不同樹種VOCs排放組成,獲得基礎(chǔ)排放速率,對準(zhǔn)確估算區(qū)域或全球天然源VOCs排放并評估其在大氣化學(xué)中的作用有重要意義.對此,國內(nèi)外學(xué)者開展了大量排放測量研究,建立了基礎(chǔ)排放速率數(shù)據(jù)庫并應(yīng)用于排放估算模式[7–8].然而,為獲得各樹種準(zhǔn)確的排放速率需要更多的定量觀測,且目前研究多集中于異戊二烯、單萜烯、倍半萜烯以及一些常見的VOCs物種.通過分析技術(shù)的改進(jìn),已有更多的未知化合物被檢出[9],但其基礎(chǔ)排放速率鮮有報道或具有較高不確定性,而這些化合物很可能導(dǎo)致“缺失的·OH自由基活性”[10–11].因此,需要對更多樹種識別更多的VOCs物種并定量測定排放強(qiáng)度,準(zhǔn)確評估排放量及其環(huán)境效應(yīng).

我國植被覆蓋面積廣且具有較高的生物多樣性,建立本地植被物種的排放速率數(shù)據(jù)庫尤為重要.對此,國內(nèi)學(xué)者開展了相關(guān)的測量研究工作[12–22],已有研究僅限于異戊二烯和單萜烯,且通常采用靜態(tài)封閉式采樣法,測量結(jié)果存在較大誤差[23].目前國內(nèi)外通常采用動態(tài)封閉式采樣法對植物VOCs排放進(jìn)行測量,由于氣體交換,封閉室內(nèi)環(huán)境處于平衡狀態(tài),該方法可在最大程度避免植物的異常排放、溫度、水分、CO2等的改變,測量結(jié)果更為準(zhǔn)確.

文獻(xiàn)報道,毛果楊()、北美楓香()和馬尾松()具有高的異戊二烯或單萜烯排放強(qiáng)度[16,24].毛果楊和北美楓香屬闊葉樹種,擁有相對較多的野外和實驗室測量研究[24–40],但多數(shù)研究僅限于對異戊二烯和單萜烯的測量,少量研究關(guān)注其他VOCs,如甲醇、丙酮、乙醛等羰基化合物[41–42].馬尾松是我國常見的優(yōu)勢針葉樹種并廣泛分布于南方地區(qū),覆蓋面積占南方森林覆蓋面積的47%[43],將對該地區(qū)天然源VOCs排放量以及大氣二次污染物的生成具有較大的貢獻(xiàn).然而,針對該樹種排放的定量測量相對較少且僅限于異戊二烯和總單萜烯排放[16,44–45].本課題組曾應(yīng)用半靜態(tài)封閉式采樣法測量了馬尾松異戊二烯、-蒎烯、-蒎烯和其他VOCs的排放速率[46],由于該方法具有較大的不確定性,未得出準(zhǔn)確的定量結(jié)果,但證明馬尾松是單萜烯高排放樹種.考慮到馬尾松的覆蓋面積大且具有高排放強(qiáng)度,對其進(jìn)行VOCs排放速率定量測定是十分必要的.

本研究主要目的包括:確定毛果楊、北美楓和馬尾松的VOCs排放速率;研究各樹種的VOCs排放組成,包括異戊二烯、單萜烯、倍半萜烯、烷烴和烯烴.

1 材料與方法

1.1 植物

盆栽毛果楊、北美楓香和馬尾松購買于美國Forest Farm nursery (www.forestfarm.com),每個樹種各有3個植株,樹齡為1~3a,植株生長信息見表1.植株置于實驗室的生長箱(2.5m×2.5m×2.5m)中,生長箱的環(huán)境條件:光合有效輻射1000μmol/(m2·s),溫度23℃,相對濕度60%,模擬白天夜晚分別為12h.

表1 植株生長信息

1.2 樣品的采集和分析

應(yīng)用動態(tài)封閉式采樣系統(tǒng)進(jìn)行VOCs排放測量(圖1),實驗在美國加州大學(xué)爾灣分校生物-大氣相互作用實驗室進(jìn)行.利用對VOCs無生成或吸附、且光合有效輻射透過率為100%的聚四氟乙烯采樣袋進(jìn)行封閉實驗,毛果楊和馬尾松使用的采樣袋體積為18L,北美楓香為110L.封閉時,用采樣袋罩住整個植株,并盡量減少對植株的擾動.封閉過程中,經(jīng)活性炭吸附去除VOCs后的零空氣以恒定流速(毛果楊和馬尾松:2.5L/min;北美楓香:8L/min)持續(xù)充入采樣袋內(nèi),同時袋內(nèi)氣體以相同流速經(jīng)采樣袋氣體出口流出,使袋內(nèi)氣體處于循環(huán)狀態(tài).采樣袋封閉后,平衡2~3h,用Tenax吸附柱采集袋內(nèi)氣體,采樣流速為200mL/min,時間為30min,體積為6L.對每個樹種,設(shè)3個平行樣品和1個空白樣品.采樣結(jié)束后,測量各植株葉片干重和葉面積,其中毛果楊和北美楓香的葉面積采用網(wǎng)格法測量[47–48],馬尾松的葉面積通過測量針葉長度和寬度計算獲得,單位為cm2.

樣品經(jīng)熱脫附-氣相色譜-飛行質(zhì)譜儀(Agilent GC 7890/Markes BenchTOF)進(jìn)行VOCs的測定.色譜柱型號為Agilent DB-5(30m),實驗設(shè)定條件:冷阱捕集溫度為-10℃,熱解吸溫度為285℃;載氣流速為1.2mL/min,分流比為1:5.2;柱箱初始溫度為-30℃,最高溫度為260℃,運行時間為35min.測定的化合物包括異戊二烯、單萜烯、倍半萜烯、烷烴和烯烴(不包括C3及以下的化合物).

圖1 動態(tài)封閉式采樣系統(tǒng)示意

1.3 排放速率計算

排放速率計算公式為:

式中:ER為排放速率[μg/(g·h);μg/(m2·h)];為零空氣流速(L/min);outlet和inlet分別為采樣袋出氣和進(jìn)氣樣品的VOCs濃度(μg/m3);為葉片干重(g)或葉面積(m2).

利用Guenther算法[49],將實際環(huán)境條件下的排放測量結(jié)果標(biāo)準(zhǔn)化為標(biāo)準(zhǔn)條件(光合有效輻射為1000 μmol/(m2·s),溫度為30℃)的排放速率.

2 結(jié)果與討論

2.1 VOCs排放速率

如表2所示,毛果楊、北美楓香和馬尾松的總VOCs排放速率分別為19.51,7.19,0.67μg/(g·h) (2086.91,562.35,104.03μg/(m2·h)).毛果楊作為闊葉樹具有較強(qiáng)的異戊二烯排放,排放速率最高,為18.51μg/(g·h)(1981.41μg/(m2·h)),馬尾松的異戊二烯排放速率最低,為0.027μg/(g·h)(4.1μg/(m2·h));北美楓香具有最高的單萜烯排放速率,為0.84μg/(g·h) (67.69μg/(m2·h)),而毛果楊的單萜烯排放強(qiáng)度最低,為0.13μg/(g·h)(14.14μg/(m2·h)).對倍半萜烯而言,毛果楊是高排放樹種,排放強(qiáng)度遠(yuǎn)高于其他兩個樹種(排放速率低于0.01μg/(g·h)).毛果楊的烷烴排放速率最高,為0.53μg/(g·h)(56.18μg/(m2·h)),北美楓香最低,為0.055μg/(g·h)(3.75μg/(m2·h)).總體而言,3個樹種排放烷烴的強(qiáng)度均高于倍半萜烯,烯烴的排放速率均低于0.05μg/(g·h).

表2 毛果楊、北美楓香和馬尾松的VOCs標(biāo)準(zhǔn)排放速率(均值±標(biāo)準(zhǔn)偏差)(溫度:30℃,光合有效輻射:1000μmol/(m2·s))

續(xù)表2

注: -為未檢出,下同.

2.2 排放成分譜

如圖2所示,所研究的3個樹種具有不同的VOCs排放組成.總的來說,異戊二烯和單萜烯占總VOCs排放的主導(dǎo)地位,而倍半萜烯的貢獻(xiàn)相對較低.毛果楊和北美楓香主要排放異戊二烯,分別占各自總VOCs排放量的94.86%和86.77%.對于針葉樹馬尾松而言,異戊二烯僅占總VOCs的4.03%,而單萜烯排放比例最高,為49.09%.北美楓香單萜烯的排放貢獻(xiàn)率為11.63%,低于馬尾松,但北美楓香的單萜烯排放速率卻是3個樹種中最高.倍半萜烯是總VOCs排放中比重相對較小的一類化合物,貢獻(xiàn)僅不到1.5%.除具有最高的單萜烯排放比重外,馬尾松釋放的烷烴占總VOCs排放的百分比也較高,為39.93%,遠(yuǎn)高于其他兩個樹種,但其排放速率低于毛果楊.對所有研究的樹種,在各自排放的VOCs中,烷烴比倍半萜烯和烯烴的貢獻(xiàn)百分比更大.

如表3所示,毛果楊檢出的單萜烯化合物種類最少,以反式-β-羅勒烯為主,其占比為99.84%,排放速率為0.13μg/(g·h).其余樹種檢出了更多的單萜烯化合物.香檜烯和-蒎烯是北美楓香排放最多的單萜烯化合物,共占其排放總單萜烯的62.18%,其次為-月桂烯和傘花烴.-蒎烯是馬尾松排放單萜烯的特征化合物,占總單萜烯排放的51.65%,其次為香檜烯和-蒎烯,貢獻(xiàn)百分比均為17.62%.毛果楊和馬尾松的VOCs排放均檢出了三環(huán)烯和3-蒈烯,而在北美楓香排放的樣品中未檢出;-側(cè)柏烯和-月桂烯僅在北美楓香中觀測到.環(huán)葑烯,2-龍腦烯和-葑烯僅在馬尾松中被檢出.

表3 各樹種各類化合物排放組成(%)

續(xù)表3

雖然石竹烯、葎草烯、δ-杜松烯和β-愈創(chuàng)木烯對總VOCs排放的貢獻(xiàn)較低,但它們是3個樹種排放的倍半萜烯中最主要的化合物. 3個樹種的倍半萜烯化合物檢出數(shù)量基本相同,但排放組成不盡相同.石竹烯是最主要的倍半萜烯化合物,分別占毛果楊、北美楓香和馬尾松倍半萜烯總排放的96.90%、56.38%和51.15%.葎草烯是馬尾松排放的另一種重要的倍半萜烯,在該類別中的比重為35.11%,北美楓香排放的倍半萜烯中,δ-杜松烯和β-愈創(chuàng)木烯是另外兩個主要的組成成分.

烷烴排放主要以C4和C5化合物為主,特別是異丁烷和正丁烷.馬尾松的異丁烷和正丁烷排放強(qiáng)度甚至高于異戊二烯;對毛果楊來說,異戊烷和2,2-二甲基丁烷能夠貢獻(xiàn)其烷烴總排放的近50%.1-丁烯是所有樹種排放的烯烴中占比最高的化合物,貢獻(xiàn)百分比為64.71%~92.90%,1-己烯僅在毛果楊排放的VOCs中被檢測到.

2.3 與其他研究比較

由表4可知,以往研究中,對毛果楊和北美楓香的研究最為廣泛,而對馬尾松的研究卻甚少.毛果楊作為闊葉樹具有很強(qiáng)的異戊二烯釋放能力,此前的研究也更多關(guān)注其異戊二烯的排放而很少有單萜烯和其他VOCs排放速率的報道.本研究中,毛果楊被確定為非常低強(qiáng)度的單萜烯排放樹種,排放速率為0.13μg/(g·h).對于北美楓香和馬尾松,僅異戊二烯和單萜烯的排放速率曾有報道,且未給出單萜烯化合物以及其他VOCs的排放組成.

表4 各研究測量結(jié)果比較

注:a背光葉片排放;b向光葉片排放;c實際環(huán)境條件且未獲得;”-”表示文章中未報道.

從表4得出,對同一樹種的測量結(jié)果間差異顯著,本研究結(jié)果低于文獻(xiàn)報道的排放速率,各研究結(jié)果間的差異主要是與植物的生長條件、生長階段、難以測量的脅迫影響、測量時環(huán)境條件和測量過程中不可避免的誤差等因素有關(guān).此外,本研究選取的國內(nèi)優(yōu)勢樹種馬尾松植株均產(chǎn)于美國,生長環(huán)境與國內(nèi)不同,VOCs排放特征可能有所差異,未來將在國內(nèi)對馬尾松排放進(jìn)行研究,比較二者的差異.

3 結(jié)論

3.1 毛果楊的總VOCs排放速率最高,為19.51μg/ (g·h)(2086.91μg/(m2·h)),馬尾松最低,為0.67μg/(g·h) (104.03μg/(m2·h)).

3.2 毛果楊異戊二烯排放速率為18.51μg/(g·h),對VOCs總排放的貢獻(xiàn)最為顯著,為94.86%,使其成為一種典型的異戊二烯排放樹種.針葉樹種馬尾松異戊二烯排放比例最低,為4.03%.

3.3 北美楓香檢測到了最高的單萜烯排放速率(0.84μg/(g·h)),雖然馬尾松的單萜烯排放速率并非最高(0.33μg/(g·h)),但在其排放的總VOCs中占比為最高(49.09%).

3.4 倍半萜烯對VOCs總排放的貢獻(xiàn)相對較小,均低于1.5%.烷烴排放強(qiáng)度普遍高于倍半萜烯,部分化合物甚至高于異戊二烯和單萜烯.

3.5 反式--羅勒烯是毛果楊的主要單萜烯類化合物,占其總單萜烯排放的99.84%,北美楓香排放的單萜烯主要為香檜烯和-蒎烯,-蒎烯、香檜烯和-蒎烯為馬尾松的主要單萜烯化合物.石竹烯、葎草烯、-杜松烯和-愈創(chuàng)木烯是所有樹種排放的主要倍半萜烯類化合物.烷烴主要以C4和C5化合物為主,特別是異丁烷和正丁烷. 3個樹種中最豐富的烯烴是1-丁烯.以上最主要的化合物能夠貢獻(xiàn)各VOCs類別的60%以上.

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Biogenic emission profile of volatile organic compounds from poplar, sweetgum, and pine trees.

LI Ling-yu1*, Alex B. Guenther2**, GU Da-sa2,3, Roger Seco2, Sanjeevi Nagalingam2

(1.College of Environmental Sciences and Engineering, Qingdao University, Qingdao 266071, China；2.Department of Earth System Science, University of California, Irvine, California 92697, USA；3.Division of Environment and Sustainability, Hong Kong University of Science and Technology, Hong Kong 999077, China)., 2019,39(12)：4966~4973

In order to study the characteristics of biogenic volatile organic compounds (BVOCs) emission from typical trees and obtain their basic emission rates for each BVOC compound, a dynamic enclosed system was used to conduct laboratory measurements on poplar, sweetgum, and pine trees. BVOC compounds including isoprene, monoterpenes, sesquiterpenes, alkanes, and alkenes were analyzed by TD-GC-TOFMS. The normalized species-specific BVOC emission rates of three tree species were calculated and their emission profiles were investigated. The total BVOC emission rates of,, andwere 19.51, 7.19, and 0.67μg/(g·h) (2086.91, 562.35, and 104.03μg/(m2·h)), respectively.had the highest isoprene emission rate of 18.51μg/(g·h), contributing 94.86% to the total BVOC emissions.had a lower isoprene contribution (4.03%), but the highest monoterpenes contribution (49.09%.had the highest monoterpenes emission rate of 0.84μg/(g·h). Sesquiterpenes contributed less than 1.5% to the total BVOC emissions for the three plants. The emission rates of alkanes for each tree species were generally higher than those of sesquiterpenes, and some were even higher than those of isoprene and monoterpenes. Trans--ocimene was the predominated monoterpene for, accounting for 99.84% of its total monoterpene emissions. The monoterpenes emitted by.was mainly composed by Sabinene and-pinene.-Pinene, sabinene, and-pinene were observed as the dominated monoterpenes for. Trans-caryophyllene, humulene,-cadinene, and-guaiene were prominent sesquiterpenes. Alkanes emitted from the three plants were mainly C4 and C5 compounds, of which particularly were isobutane and butane. 1-Butene was the most abundant alkene for all plants.

VOCs；biogenic；emission rate；emission profile

X511

A

1000-6923(2019)12-4966-08

李玲玉(1987-),女,山東濰坊人,博士研究生,副教授,主要從事植物揮發(fā)性有機(jī)物排放、大氣化學(xué)與污染控制等方面的研究.發(fā)表論文15篇.

2019-05-15

國家自然科學(xué)基金資助項目(41705098);山東省高等學(xué)?？萍加媱澷Y助項目(J17KA105);美國國家自然基金資助項目(AGS-1643042)

* 責(zé)任作者, 副教授, lilingyu@qdu.edu.cn; **, 教授, alex.guenther@uci.edu