范行軍,操 濤,余旭芳,宋建中,王 艷,肖 新,謝 越,李飛躍

薪柴燃燒溶解性棕色碳排放特征及光學(xué)性質(zhì)

范行軍1,操 濤2,3,余旭芳1,宋建中2*,王 艷1,肖 新1,謝 越1,李飛躍1

(1.安徽科技學(xué)院資源與環(huán)境學(xué)院,安徽 鳳陽(yáng) 233100；2.中國(guó)科學(xué)院廣州地球化學(xué)研究所有機(jī)地球化學(xué)國(guó)家重點(diǎn)實(shí)驗(yàn)室,廣東 廣州 510640；3.中國(guó)科學(xué)院大學(xué),北京 100049)

對(duì)農(nóng)村薪柴(楊木和毛竹)燃燒排放的4類溶解性棕色碳(BrC)組分,包括水溶性有機(jī)物(WSOM)、水溶性類腐殖質(zhì)(HULISWS)、堿溶性有機(jī)物(ASOM)和堿溶性類腐殖質(zhì)(HULISAS)的組成特征和光學(xué)性質(zhì)進(jìn)行了初步研究.結(jié)果顯示,薪柴燃燒排放出大量的BrC,其中BrCT(WSOM + ASOM)占煙氣PM2.5質(zhì)量的46%~56%,排放因子為(7.5~16) g/kg.HULIS是薪柴燃燒排放BrC的重要組分,占BrCT的44%~46%.4類BrC的特征吸收指數(shù)(SUVA254)、光吸收效率(MAE365)和?ngstr?m指數(shù)(AAE)值分別為1.9~4.0m2/g、0.4~2.1m2/g和6.2~11.1,說明薪柴燃燒排放BrC具有較高的芳香度、較強(qiáng)的光吸收能力且其光吸收具有較強(qiáng)的波長(zhǎng)依賴性.三維熒光光譜分析結(jié)果顯示,薪柴燃燒排放BrC主要以類蛋白熒光物質(zhì)組成為主,這與雨水和大氣氣溶膠中水溶性BrC以類腐殖質(zhì)熒光物質(zhì)組成為主的特征存在顯著差異.相關(guān)性分析結(jié)果顯示,BrC的MAE365與HIX和SUVA254呈現(xiàn)顯著的正相關(guān)性,與2/3、FI、BIX和:呈現(xiàn)顯著的負(fù)相關(guān)性,說明薪柴燃燒排放BrC的光吸收特性與其芳香性、腐殖化程度、自生源貢獻(xiàn)和新鮮度等緊密相關(guān).本研究結(jié)果有助于進(jìn)一步認(rèn)識(shí)生物質(zhì)燃燒BrC的排放特征,為探索大氣BrC的來源和環(huán)境效應(yīng)提供數(shù)據(jù)基礎(chǔ)和科學(xué)依據(jù).

薪柴燃燒；棕色碳；排放特征；光吸收特性；熒光光譜特性

棕色碳(BrC)是大氣氣溶膠中一類在紫外-可見光波段具有較強(qiáng)光吸收,且其吸光特性呈現(xiàn)顯著的波長(zhǎng)依賴性的有機(jī)碳(OC)組分[1-3].BrC廣泛存在于云、霧、雨水和大氣氣溶膠中,可以直接和間接影響區(qū)域的輻射強(qiáng)迫[4-9].同時(shí),BrC對(duì)太陽(yáng)光的吸收還可提升區(qū)域大氣平均溫度[10],降低大氣光化學(xué)反應(yīng)速率[11],對(duì)全球氣候、大氣質(zhì)量及大氣光化學(xué)反應(yīng)等產(chǎn)生重要的影響.BrC已成為當(dāng)前國(guó)際大氣環(huán)境領(lǐng)域的研究熱點(diǎn)之一.

大氣BrC的光吸收特性與其來源和化學(xué)組成密切相關(guān).BrC的來源非常復(fù)雜,大量外場(chǎng)觀測(cè)的研究表明生物質(zhì)燃燒(BB)是其重要的來源[6-9,12-16].研究發(fā)現(xiàn)BB來源BrC占有機(jī)氣溶膠的比例達(dá)40%~ 92%[12,14-16],且貢獻(xiàn)了氣溶膠總吸光的17%~65%[6-9].基于熱-光分析法和溶劑(水和甲醇等)提取法等[12-13,17-20]對(duì)BB源BrC的排放因子和光學(xué)特征如?ngstr?m指數(shù)(AAE)和單位質(zhì)量吸收效率(MAE)等進(jìn)行觀測(cè),表明BB排放出大量的BrC,對(duì)大氣BrC的光吸收具有重要的貢獻(xiàn).另外,通過溶劑提取方法為研究不同類型BrC的化學(xué)特征和光學(xué)性質(zhì)等提供了可能[13,19].近年來,有關(guān)學(xué)者已對(duì)BB產(chǎn)生BrC的排放特征(如排放因子、光吸收特性、發(fā)色物質(zhì)組成等)開展了初步研究工作.結(jié)果表明生物質(zhì)的類型和燃燒方式對(duì)BrC的排放量和光吸收特性有重要的影響[13,19-20],同時(shí)BrC的光吸收特性與其發(fā)色物質(zhì)組成也存在著一定的聯(lián)系[12,19].總的來說,關(guān)于不同形式BB產(chǎn)生BrC的排放特征尤其是各特征之間的關(guān)聯(lián)性等的研究還十分有限.

盡管已有部分工作關(guān)注了薪柴燃燒BrC的排放特征[12,19,21],但是由于農(nóng)村地區(qū)薪柴具有使用量大和種類繁多等特點(diǎn),可能使其燃燒排放BrC在組成和性質(zhì)上表現(xiàn)出較大的差異性.本研究選用安徽農(nóng)村地區(qū)典型的薪柴(楊木和毛竹)燃料作為燃燒對(duì)象,采用有機(jī)碳、紫外-可見吸收和熒光光譜方法對(duì)實(shí)驗(yàn)室模擬薪柴燃燒排放煙氣顆粒中的BrC進(jìn)行定量和表征分析.通過對(duì)楊木和毛竹燃燒BrC的排放特征和光學(xué)特性的分析,以期為進(jìn)一步認(rèn)識(shí)大氣BrC的來源和環(huán)境行為以及為大氣環(huán)境控制提供可靠的數(shù)據(jù)基礎(chǔ)與科學(xué)依據(jù).

1 材料與方法

1.1 薪柴煙氣顆粒的采集

本研究中的燃料分別采集于安徽省滁州市和池州市農(nóng)村地區(qū).自然晾干后,將其切割成長(zhǎng)約10cm,直徑約0.5cm小塊樣本,便于進(jìn)一步燃燒處理.其理化性質(zhì)如表1所示.

表1 楊木和毛竹的元素組成分析

圖1 薪柴燃燒和煙氣細(xì)顆粒采集裝置

薪柴燃燒實(shí)驗(yàn)在搭建的生物質(zhì)燃燒-顆粒物采集系統(tǒng)中進(jìn)行.該實(shí)驗(yàn)裝置[19]如圖1所示.煙氣顆粒采集步驟如下:調(diào)節(jié)助燃?xì)饬骱拖♂寶饬鞣謩e為60和120L/min,然后點(diǎn)燃燃燒室內(nèi)薪柴樣本(10~30g),產(chǎn)生的煙氣在氣流作用下通過煙氣管道引入混合采樣室進(jìn)行稀釋和混合.利用中流量采樣器(天虹智能儀表廠,武漢)對(duì)煙氣顆粒(PM2.5)進(jìn)行采集,采樣流量設(shè)為80L/min.采樣所用石英濾膜(Φ90,Whatman, USA)使用前需在馬弗爐中400℃灼燒4h.每類薪柴燃燒實(shí)驗(yàn)重復(fù)4次,收集對(duì)應(yīng)濾膜樣品.采樣期間,收集4組空白濾膜樣品,用于分析結(jié)果校正.

1.2 BrC的分離與純化

薪柴燃燒排放BrC的分離和純化方法參考Fan等[19]的方法.利用純水和0.1M的NaOH對(duì)煙氣顆粒濾膜進(jìn)行連續(xù)萃取,進(jìn)而采用SPE對(duì)萃取液中HULIS組分進(jìn)行純化.

1.2.1 純水-NaOH連續(xù)萃取 根據(jù)采樣濾膜上的單位面積煙氣顆粒物負(fù)載量(1.7~5.7mg/cm2),取1.5~3cm2的顆粒濾膜,加入20mL超純水,置于超聲儀中25℃超聲30min.利用0.45μm水系濾頭進(jìn)行過濾,重復(fù)2次合并濾液得到40mL濾液,即水溶性有機(jī)物(WSOM).利用40mL的0.1M NaOH對(duì)殘余樣品膜片進(jìn)行萃取,超聲30min后滴加適量HCl調(diào)節(jié)萃取液pH值至2左右,最后利用0.45μm水系濾頭進(jìn)行過濾,得到40mL濾液,即堿溶性有機(jī)物(ASOM).

1.2.2 固相萃取(SPE) 利用SPE方法對(duì)上述WSOM和ASOM進(jìn)一步純化,分別得到水溶性和堿溶性腐殖質(zhì)(HOLISws,HULISAS)具體方法參見文獻(xiàn)[19].

1.3 儀器分析

WSOM、ASOM和HULIS物質(zhì)的TOC含量利用TOC-VCPN分析儀(Shimadzu,Japan)的NPOC模式進(jìn)行測(cè)定.利用紫外-可見光光譜儀(UV-2600, Shimadzu,Japan)對(duì)各類BrC吸收光譜進(jìn)行測(cè)定,掃描范圍為200~700nm,掃描間隔1nm.采用熒光光譜儀(F-4600,Hitachi,Japan)對(duì)BrC樣品進(jìn)行三維熒光光譜分析.激發(fā)和發(fā)射波長(zhǎng)范圍分別設(shè)置為200~400和280~520nm,掃描間隔均為5nm,掃描速度為2400nm/min.所有樣品信號(hào)最后利用空白樣品信號(hào)進(jìn)行校正.

1.4 數(shù)據(jù)處理與分析

1.4.1 排放因子(EF) 薪柴燃燒的BrC排放因子計(jì)算公式如下:

式中:EF表示類BrC的排放因子,g/kg;m表示類BrC的排放總量,mg;dry表示薪柴燃燒干基量,g;TOC表示類BrC的TOC測(cè)試含量,mg/L;V表示類BrC萃取或純化體積,L;0表示煙氣顆粒濾膜總面積;A表示類BrC分離或純化對(duì)應(yīng)濾膜面積,cm2;為采樣氣流稀釋比.

1.4.2 光譜參數(shù) 本研究中從紫外-可見光吸收光譜和熒光光譜中提取UV250、2/3、SUVA254、SUVA280、AAE、MAE365、FI、HIX、BIX和新鮮度指數(shù)(:)等光譜參數(shù),用以描述薪柴燃燒排放BrC的含量、分子特征和光學(xué)特性.

UV250是指250nm處的吸收值,用于指示W(wǎng)SOM和HULIS的吸光性物質(zhì)含量[13,19,23,25].2/3是250和365nm處UV-vis吸光度之比,該值大小與溶解性有機(jī)物的分子量和芳香度成反比[19,22-23]. SUVA254和SUVA280值與溶解性有機(jī)物的分子量和芳香度成正比[19,22],其計(jì)算公式[19,22-23]為:

式中:A表示波長(zhǎng)(本研究選擇254nm和280nm)處的吸光度;指光皿長(zhǎng)度(0.01m);表示樣品的TOC值,mg/L.

韓國(guó)高校教師學(xué)術(shù)道德教育實(shí)施主體呈現(xiàn)多元化特征。首先，為防范高校教師出現(xiàn)學(xué)術(shù)道德失范與不端等問題，在全社會(huì)形成良性的學(xué)術(shù)研究道德風(fēng)氣，韓國(guó)《學(xué)術(shù)振興法》第15條明確規(guī)定，教育部作為高校教師學(xué)術(shù)道德教育的最高決策管理機(jī)構(gòu)，全面負(fù)責(zé)資助、管理、監(jiān)督高校教師的學(xué)術(shù)道德教育工作①（韓）韓國(guó)國(guó)會(huì)，學(xué)術(shù)振興法（N）：第15條第2至4項(xiàng)。，是韓國(guó)學(xué)術(shù)道德教育的主管部門。而作為政府層面的學(xué)術(shù)道德教育工作則由韓國(guó)教育部下設(shè)“國(guó)家科學(xué)技術(shù)人力開發(fā)院”，以及韓國(guó)未來科技部下設(shè)“韓國(guó)研究財(cái)團(tuán)”共同組織實(shí)施。

Angstrom指數(shù)(AAE)表征BrC的光吸收強(qiáng)度的波長(zhǎng)依耐性[3,19,24],計(jì)算公式為:

A

l

=

K

′

l

-

AAE

(4)

本文范圍是330~400nm,為常數(shù).

質(zhì)量吸收指數(shù)(MAE365)是用以表征BrC光吸收能力的重要參數(shù)[13,19,25],計(jì)算公式為:

式中:365為波長(zhǎng)在365nm處的吸光度.

腐殖化指數(shù)HIX[16,25-26]為激發(fā)波長(zhǎng)為254nm時(shí),發(fā)射光譜435~480nm區(qū)間積分值與300~345nm區(qū)間積分值之比,該值越大表明溶解性有機(jī)質(zhì)(DOM)的腐殖化程度越高.熒光指數(shù)FI[16,26]為激發(fā)波長(zhǎng)為370nm時(shí),熒光發(fā)射光譜在470與520nm處的強(qiáng)度比值,該值越大表明芳香氨基酸對(duì)DOM熒光強(qiáng)度的貢獻(xiàn)率越大.自生源指數(shù)BIX[16,26]為激發(fā)波長(zhǎng)在310nm時(shí),發(fā)射光譜在380與430nm處熒光強(qiáng)度的比值,反映自生源貢獻(xiàn)的大小.新鮮度指數(shù)[26-27]為激發(fā)波長(zhǎng)為310nm時(shí),發(fā)射光譜在380nm熒光強(qiáng)度和420~435nm區(qū)間最大熒光強(qiáng)度的比值,反映新生DOM貢獻(xiàn)大小.

本研究采用Microsoft Excel 2016, Origin 2018和CorelDRAW X8進(jìn)行數(shù)據(jù)處理和作圖.本研究中數(shù)據(jù)結(jié)果均以平均值(±標(biāo)準(zhǔn)偏差)表示(=4).

2 結(jié)果與討論

2.1 薪柴燃燒排放BrC的組成和排放特征

楊木和毛竹燃燒排放出大量的BrC組分,其中WSOM、ASOM、HULISWS和HULISAS分別占對(duì)應(yīng)煙氣PM2.5質(zhì)量的(以C計(jì),下同)34%和37%、12%和19%、15%和17%、6%和7%.HULIS是大氣氣溶膠中重要的光吸收物質(zhì)[13,20,22-23],因此本研究對(duì)薪柴燃燒排放的HULIS組分進(jìn)行了觀測(cè).結(jié)果顯示,楊木和毛竹燃燒排放HULISWS(以TOC計(jì))分別占WSOM的44%和49%,而以UV250計(jì)[19,23],相對(duì)比例分別升高至60%和53%.同時(shí),HULISAS占薪柴燃燒排放ASOM的34%~57%(以TOC含量計(jì))和72%~ 74%(以UV250計(jì)).這些結(jié)果同F(xiàn)an等[13,19]報(bào)道的薪柴和農(nóng)作物秸稈燃燒排放HULIS相對(duì)含量類似,說明HULIS組分是BB排放WSOM和ASOM中重要組分,富集了大量的吸光性物質(zhì).另一方面,薪柴燃燒排放的總BrC總量(BrCT= WSOM + ASOM)約占煙氣PM2.5質(zhì)量的46%~56%,HULIS的總量(HULISWS+HULISAS)約占總煙氣PM2.5質(zhì)量的21%~24%,均高于本課題組前期研究的松木和杉木的BrCT相對(duì)含量(35.6%~39.9%)[19]和HULIS的相對(duì)含量(6.4%~ 16.9%)[13,19].這些差異可能歸因于薪柴燃料化學(xué)組成、燃燒條件和燃燒狀態(tài)等的差異[28-29].

圖2 楊木和毛竹燃燒BrC排放因子

2.2 紫外-可見光吸收特征

2.2.1 UV-vis光譜 本研究觀測(cè)了薪柴燃燒排放BrC經(jīng)TOC校正過的紫外-可見光吸收光譜特征,如圖3所示,各類BrC在200~700nm區(qū)間均呈現(xiàn)出光吸收強(qiáng)度隨波長(zhǎng)增大而減小的趨勢(shì),同典型的大氣BrC的吸收光譜特征類似[13,24].

盡管各類BrC的UV-vis吸收光譜特征類似,但是在吸收強(qiáng)度和峰位置方面存在顯著的差異.對(duì)比吸收強(qiáng)度發(fā)現(xiàn),楊木和毛竹排放HULISWS和HULISAS組分的特征吸收強(qiáng)度均高于對(duì)應(yīng)的WSOM和ASOM,說明HULIS是溶解性BrC中主要的吸光類物質(zhì),與前期報(bào)道的BB氣溶膠和大氣氣溶膠中HULIS的特征類似[13,19,23].其中,HULISAS的特征吸收光譜明顯高于其他類型BrC,說明HULISAS具有最強(qiáng)的芳香性和光吸收能力.另一方面,就UV-Vis吸收譜圖中峰位置而言,楊木和毛竹排放WSOM和HULISWS吸收光譜在250~270nm區(qū)間均呈現(xiàn)出明顯的吸收峰,尤其是WSOM組分.該結(jié)果與Fan等[13,19]研究中其它類型BB排放WSOM和HULIS的吸收光譜特征類似,這類峰的出現(xiàn)主要是由于有機(jī)物中C=C和C=O雙鍵π-π*電子躍遷所引起的[22-23,30].

2.2.2 UV-vis吸收光譜特征參數(shù) 本研究對(duì)薪柴燃燒排放BrC的23、SUVA254和SUVA280值進(jìn)行分析,用以表征各類BrC的分子特征.由表2可看出,楊木和毛竹燃燒排放HULISWS的2/3值分別為6.2和9.4,均低于對(duì)應(yīng)WSOM的值(6.6和15.9); HULISWS的SUVA254(以C計(jì),下同)為3.1m2/g,高于對(duì)應(yīng)WSOM值(2.4m2/g和3.0m2/g).SUVA280呈現(xiàn)特征與SUVA254類似.另一方面,相對(duì)于ASOM,薪柴燃燒排放HULISAS同樣表現(xiàn)出較低的2/3值和較高的SUVA254和SUVA280值.這些結(jié)果說明HULIS組分是WSOM和ASOM中重要的芳香性有機(jī)混合物,與Fan等[13,19]報(bào)道的BB排放HULIS的特征一致.同時(shí),HULISAS的2/3值均低于、SUVA254和SUVA280值均高于同類薪柴燃燒排放HULISWS的相關(guān)值,表明HULISAS比HULISWS芳香性更高.

圖3 楊木和毛竹燃燒排放BrC的校正UV-vis吸收光譜

表2 楊木和毛竹燃燒排放BrC的UV-vis光學(xué)特征參數(shù)

AAE和MAE365是表征BrC光吸收特性重要的參數(shù).從表2可看出,薪柴燃燒排放WSOM和HULISWS的AAE分別為10.5~11.1和7.4~8.9,說明薪柴燃燒排放水溶性BrC的光吸收具有較強(qiáng)的波長(zhǎng)依賴性.該結(jié)果同前期大量報(bào)道的BB[13,19-20]和大氣氣溶膠[7,24,31-32]中水溶性BrC的AAE值(6.2~9.2)類似.同時(shí)ASOM和HULISAS的AAE值分別為7.3~7.4和6.2~6.8,同F(xiàn)an等[19]報(bào)道的BB排放堿溶性BrC的AAE值(5.75~6.93)類似,亦說明堿溶性BrC的光吸收同樣具有強(qiáng)烈的波長(zhǎng)依賴性.另一方面,薪柴燃燒排放WSOM和HULISWS的MAE365分布為(以C計(jì),下同)(0.4~0.9)m2/g和(0.8~1.2)m2/g.觀測(cè)值基本在BB[13,19-20]氣溶膠中水溶性BrC的MAE365[(0.86~ 1.60)m2/g]和大氣氣溶膠[4-5,7,14,24,31-32]中水溶性BrC的MAE365[(0.13~1.79) m2/g]范圍內(nèi).由此說明BB排放BrC具有與大氣氣溶膠中BrC的類似的吸光能力.此外,ASOM和HULISAS的MAE值分別是0.8~1.2和1.7~2.1,分別高于對(duì)應(yīng)WSOM和HULISAS值,該結(jié)果類似于前期報(bào)道的其他類型BB排放特征[19],說明堿溶性BrC比水溶性BrC具有更強(qiáng)的光吸收能力.值得注意的是,HULIS是薪柴燃燒排放BrC中重要的吸光物質(zhì),其中HULISAS光吸收能力最強(qiáng),其MAE365值幾乎是HULISWS的2倍.

2.3 熒光光譜特征

2.3.1 三維熒光光譜 三維熒光光譜(3DEEM)技術(shù)可有效鑒別出DOM中不同類型的熒光發(fā)色團(tuán),根據(jù)其組成特征進(jìn)而判斷DOM的性質(zhì)和來源.近年來,3DEEM已廣泛應(yīng)用于雨水[16,26,33-35]、云霧[36]、BB氣溶膠[13]和大氣氣溶膠[37-39]中WSOM和HULISWS的研究.本研究對(duì)楊木和毛竹燃燒排放各類型BrC的3DEEM特征進(jìn)行了觀測(cè)和分析,具體譜圖見圖4.

從圖4可看出,楊木和毛竹燃燒排放WSOM、ASOM、HULISWS和HULISAS的3DEEM中均呈現(xiàn)出2類主要的熒光峰,分別為熒光峰T1(E/E= 220~ 225nm/340~360nm)和熒光峰T2(E/E= 255~260nm/ 330~340nm).查閱相關(guān)文獻(xiàn)發(fā)現(xiàn)[16,34,40],熒光峰T1和T2均可能歸屬為類色氨酸熒光峰.此外,熒光峰T2還可能歸屬為芳香類氨基酸和類酚類物質(zhì)[13,34].這些結(jié)果說明楊木和毛竹燃燒排放BrC中的熒光發(fā)色團(tuán)以類色氨酸物質(zhì)或芳香類氨基酸和類酚類熒光物質(zhì)組成為主.熒光峰T1和T2常在雨水[16,34,40]、BB氣溶膠[13]和大氣氣溶膠[39,41]的WSOM和HULISWS中被檢測(cè)到.相比而言,本研究中薪柴燃燒排放BrC的熒光峰T1和T2的強(qiáng)度均顯著高于雨水和氣溶膠樣品中的BrC,而后者的熒光峰以紫外光區(qū)類腐殖質(zhì)熒光峰(E/E=220~260nm/380~460nm)和可見光區(qū)類腐殖質(zhì)熒光峰(E/E=320~360nm/420~460nm)為主[16,26,33-35,37,39-41].這些熒光峰的差異表明薪柴燃燒排放BrC與雨水和氣溶膠BrC的熒光物質(zhì)組成存在明顯區(qū)別,而后者以類腐殖質(zhì)熒光物質(zhì)組成為主,說明雨水和氣溶膠中BrC的腐殖化程度較高.因此,大氣環(huán)境中BrC的類蛋白熒光峰可歸屬為BB源的貢獻(xiàn).相對(duì)而言,熒光峰T1是楊木和毛竹燃燒排放各類BrC中最主要的熒光峰.通過比較分析發(fā)現(xiàn),薪柴燃燒排放HULISWS熒光峰T1強(qiáng)度占WSOM的55%~ 72%,HULISAS熒光峰T1強(qiáng)度占對(duì)應(yīng)ASOM的99%~ 118%倍.這些數(shù)據(jù)明顯高于以TOC計(jì)的HULISWS/ ASOM和HULISAS/ASOM比例,說明HULIS組分是WSOM和ASOM重要的熒光發(fā)色物質(zhì).另一方面,突出的熒光強(qiáng)度百分比(特別是HULISAS)可能與SPE能有效分離WSOM和ASOM中的熒光淬滅物質(zhì),如鹽類和過渡金屬等有關(guān)[23].

2.3.2 熒光特征參數(shù)分析 為了進(jìn)一步分析楊木和毛竹燃燒排放BrC的光譜特征,本文對(duì)各類BrC的熒光特征參數(shù)進(jìn)行了分析,具體結(jié)果如表3所示.由表3可看出,楊木和毛竹燃燒排放WSOM的HIX值范圍是0.06~0.17,該值基本低于前期報(bào)道的雨水[16,26,36,38]和大氣氣溶膠[25,37,42]中WSOM的HIX值(0.74~6.79).說明薪柴燃燒排放WSOM的腐殖化程度要比大氣環(huán)境中的低,這與后者的復(fù)雜來源(包括大氣過程)有關(guān)[25,37].例如,具有顯著BB來源的冬季氣溶膠WSOM的腐殖化程度明顯低于夏季[37];經(jīng)過強(qiáng)烈老化過程的灰霾期氣溶膠WSOM的腐殖化程度明顯高于非灰霾期[25].其次,HULISWS的HIX值為0.24~0.43,高于對(duì)應(yīng)WSOM,說明前者具有更高的腐殖化程度,與其類腐殖質(zhì)物質(zhì)屬性一致.該結(jié)果與前期研究報(bào)道的廣州氣溶膠中HULISWS的HIX值高于WSOM結(jié)果一致[25].另外,HULISAS的HIX值(0.67~1.05)也明顯高于對(duì)應(yīng)ASOM值(0.62~0.8),同時(shí)也高于對(duì)應(yīng)HULISWS值,說明HULISAS是腐殖化程度最高的BrC.

FI、BIX和值可用以表征DOM的自生源貢獻(xiàn)大小[37,42]及其芳香性強(qiáng)弱[42-43].本研究中,楊木和毛竹燃燒排放WSOM的FI、BIX和值分別是2.10~2.28、2.17~2.40和1.89~2.02(表4),這些值要高于實(shí)際觀測(cè)的雨水和氣溶膠中WSOM的相關(guān)結(jié)果[26,36-37].例如,雨水和氣溶膠中WSOM的FI、BIX和值報(bào)道范圍分別是1.1~1.83[36-37]、0.64~ 1.4[36-37]和0.74~0.84[26].前期研究結(jié)果表明,FI>1.9和BIX>1均指示DOM的高自生源貢獻(xiàn)[16,26,37,42],然而此處薪柴燃燒WSOM具有較高的FI和BIX值并不是由于自生源的貢獻(xiàn).由此說明,大氣中WSOM呈現(xiàn)較高的FI和HIX值也可歸屬為生物質(zhì)燃燒的貢獻(xiàn).例如,Qin等[37]研究表明冬季大氣PM2.5中WSOM的FI和HIX值均高于夏季的,可能因?yàn)锽B對(duì)前者有重要貢獻(xiàn)造成的.而:反映了新產(chǎn)生的WSOM在整體WSOM中所占的比例,薪柴燃燒排放WSOM具有較高的:值說明其新鮮排放的本質(zhì).另一方面,薪柴燃燒排放HULISWS的FI、BIX和:值分別是1.90~2.11、1.32~1.66和1.21~1.47,均低于對(duì)應(yīng)WSOM值.同時(shí),HULISAS的此3類熒光光譜參數(shù)值也低于對(duì)應(yīng)ASOM值.這些結(jié)果說明HULIS比對(duì)應(yīng)WSOM或ASOM具有更高的芳香性,這與前面2/3、SUVA254和SUVA280反映結(jié)論一致.

2.4 光學(xué)參數(shù)相關(guān)性分析

通過對(duì)楊木和毛竹燃燒排放的4種類型BrC(=32)的9類光學(xué)參數(shù)進(jìn)行相關(guān)性分析,研究薪柴燃燒排放BrC的光學(xué)特性與物質(zhì)組成和分子結(jié)構(gòu)之間的關(guān)系,結(jié)果如表3所示.可以看出,MAE365與SUVA254和SUVA280(=0.744~0.806,<0.01)以及與HIX(= 0.798,<0.01)具有顯著正相關(guān)關(guān)系,同時(shí)與2/3值呈現(xiàn)顯著負(fù)相關(guān)性(=?0.687,<0.01).這些結(jié)果說明薪柴燃燒排放BrC的光吸收能力與其芳香性和腐殖化程度呈顯著正相關(guān).而AAE與2/3和HIX具有顯著相關(guān)性,其相關(guān)系數(shù)為0.786(<0.01)和?0.881(<0.01),說明薪柴燃燒排放BrC光吸收的波長(zhǎng)依賴性與其芳香性和腐殖化程度呈負(fù)相關(guān)關(guān)系.范行軍等[25]研究發(fā)現(xiàn)廣州氣溶膠中WSOM和HULISWS的HIX與MAE365和AAE亦分別呈現(xiàn)出顯著正相關(guān)和負(fù)相關(guān)關(guān)系,同樣表明大氣BrC的腐殖化程度與其光吸收特性具有顯著相關(guān)性.另外,本文中BrC的MAE365、FI、BIX和具有顯著的負(fù)相關(guān)(?0.448~?0.691,<0.05),而AAE與這些參數(shù)呈現(xiàn)顯著正相關(guān)(0.560~0.677,<0.01).因此,基于BrC的熒光發(fā)色團(tuán)組成和性質(zhì)的研究有助于對(duì)其光吸收特性的認(rèn)識(shí).

表3 薪柴燃燒排放BrC各光學(xué)參數(shù)之間的相關(guān)性

注:* *表示在0.01水平(雙側(cè))上顯著相關(guān),*表示在0.05水平(雙側(cè))上顯著相關(guān).

3 結(jié)論

3.1 薪柴燃燒可排放出大量的WSOM、ASOM、HULISWS和HULISAS組分,其含量分別占煙氣顆粒質(zhì)量的34%~37%,12%~19%,15%~17%和6%~7%,對(duì)應(yīng)排放因子分別為5.5~10.9、2.0~5.5、2.4~5.1和1.0~1.9g/kg,說明薪柴燃燒對(duì)大氣BrC具有重要的貢獻(xiàn).

3.2 薪柴燃燒排放HULIS的SUVA254和MAE365分別為3.1~4.0m2/g和0.8~2.1m2/g,均高于對(duì)應(yīng)WSOM和ASOM的相關(guān)值,說明HULIS是薪柴燃燒排放BrC中主要的芳香性有機(jī)物且具有較強(qiáng)的光吸收能力;相比而言,HULISAS的芳香度和光吸收能力最強(qiáng),說明堿溶性大分子有機(jī)物亦是不可忽視的一類吸光性物質(zhì).

3.3 3DEEM分析結(jié)果顯示薪柴燃燒排放BrC的發(fā)色團(tuán)均以類色氨酸為主,這與大氣BrC的熒光發(fā)色團(tuán)以類腐殖質(zhì)為主的組成特征差異明顯;與BrC的HIX值(0.06~1.05)基本低于實(shí)際大氣氣溶膠中WSOM的觀測(cè)結(jié)果一致.說明薪柴燃燒排放BrC腐殖化程度低,可能會(huì)經(jīng)過復(fù)雜的大氣過程改變其熒光物質(zhì)組成.

3.4 薪柴燃燒排放BrC的AAE和MAE365與2/3、SUVA254、HIX、FI、BIX和光學(xué)參數(shù)具有顯著相關(guān)性,說明薪柴燃燒排放BrC的光吸收特性與其芳香度、腐殖化程度和芳香族氨基酸的含量等有重要關(guān)聯(lián).

[1] 閆才青,鄭 玫,張遠(yuǎn)航.大氣棕色碳的研究進(jìn)展與方向 [J]. 環(huán)境科學(xué), 2014,35(11):4404-4414. Yan C Q, Zheng M, Zhang Y H. Research progress and direction of atmospheric brown carbon [J]. Environmental science, 2014,35(11): 4404-4414.

[2] 支國(guó)瑞,蔡 竟,楊俊超,等.棕色碳?xì)馊苣z來源、性質(zhì)、測(cè)量與排放估算 [J]. 環(huán)境科學(xué)研究, 2015,28(12):1797-1814. Zhi G R, Cai J, Yang J C, et al. Origin, properties, measurement and emission estimation of brown carbon aerosols [J]. Research of Environmental Sciences, 2015,28(12):1797-1814.

[3] Andreae M O, Gelencser A. Black carbon or brown carbon? The nature of light-absorbing carbonaceous aerosols [J]. Atmospheric Chemistry and Physics, 2006,6:3131-3148.

[4] Liu J, Bergin M, Guo H, et al. Size-resolved measurements of brown carbon in water and methanol extracts and estimates of their contribution to ambient fine-particle light absorption [J]. Atmospheric Chemistry and Physics, 2013,13(24):12389-12404.

[5] Yan C, Zheng M, Sullivan A P, et al. Chemical characteristics and light-absorbing property of water-soluble organic carbon in Beijing: Biomass burning contributions [J]. Atmospheric Environment, 2015, 121:4-12.

[6] Hecobian A, Zhang X, Zheng M, et al. Water-Soluble Organic Aerosol material and the light-absorption characteristics of aqueous extracts measured over the Southeastern United States [J]. Atmospheric Chemistry and Physics, 2010,10(13):5965-5977.

[7] Du Z, He K, Cheng Y, et al. A yearlong study of water-soluble organic carbon in Beijing II: Light absorption properties [J]. Atmospheric Environment, 2014,89:235-241.

[8] 段菁春,畢新慧,譚吉華,等.廣州灰霾期大氣顆粒物中多環(huán)芳烴粒徑的分布 [J]. 中國(guó)環(huán)境科學(xué), 2006,26(1):6-10. Duan J C, Bi X H, Tan J H, et al. The particle diameter distribution of polycyclic aromatic hydrocarbons (PAHs) in atmospheric particle during haze period in Guangzhou [J]. China Environmental Science, 2006,26(1):6-10.

[9] 崔 杰,黃曉鋒,袁金鳳,等.基于在線觀測(cè)的大氣PM2.5中棕色碳吸光貢獻(xiàn)估算 [J]. 中國(guó)環(huán)境科學(xué), 2017,37(2):401-406. Cui J, Huang X F, Yuan J F, et al. Estimation of light absorption by brown carbon in PM2.5based on on-line measurement [J]. China Environmental Science, 2017,37(2):401-406.

[10] Feng Y, Ramanathan V, Kotamarthi V R. Brown carbon: a significant atmospheric absorber of solar radiation? [J] Atmospheric Chemistry and Physics, 2013,13(17):8607-8621.

[11] Laskin A, Laskin J, Nizkorodov S A. Chemistry of Atmospheric Brown Carbon [J]. Chemical Reviews, 2015,115(10):4335-4382.

[12] Chen Y, Bond T C. Light absorption by organic carbon from wood combustion [J]. Atmospheric Chemistry and Physics, 2010,10(4): 1773-1787.

[13] Fan X, Wei S, Zhu M, et al. Comprehensive characterization of humic-like substances in smoke PM2.5emitted from the combustion of biomass materials and fossil fuels [J]. Atmospheric Chemistry and Physics, 2016,16(20):13321-13340.

[14] Cheng Y, He K B, Zheng M, et al. Mass absorption efficiency of elemental carbon and water-soluble organic carbon in Beijing, China [J]. Atmospheric Chemistry and Physics, 2011,11(22):11497-11510.

[15] Kirillova E N, Andersson A, Han J, et al. Sources and light absorption of water-soluble organic carbon aerosols in the outflow from northern China [J]. Atmospheric Chemistry and Physics, 2014,14(3):1413- 1422.

[16] 梁 儉,江 韜,魏世強(qiáng),等.夏、冬季降雨中溶解性有機(jī)質(zhì)(DOM)光譜特征及來源辨析 [J]. 環(huán)境科學(xué), 2015,36(3):888-897. Liang J, Jiang T, Wei S Q, et al. Absorption and fluorescence characteristics of dissolved organic matter (DOM) in rainwater and sources analysis in summer and winter season [J]. Environmental Science, 2015,36(3):888-897.

[17] 孫建中,支國(guó)瑞,陳穎軍,等.居民生活用煤和生物質(zhì)棕色碳排放因子研究 [J]. 環(huán)境科學(xué)與技術(shù), 2016,39(S1):338-345. Sun J Z, Zhi G R, Chen Y J, et al. Measurement of brown carbon emission factors for household use of coal and biomass in China [J]. Environmental Science & Technology, 2016,39(S1):338-345.

[18] 蔡 竟,支國(guó)瑞,陳穎軍,等.中國(guó)秸桿焚燒及民用燃煤棕色碳排放的初步研究 [J]. 環(huán)境科學(xué)研究, 2014,27(5):455-461. Cai J, Zhi G R, Chen Y J,et al. A preliminary study on brown carbon emissions from open agricultural biomass burning and residential coal combustion in China [J]. Research of Environmental Sciences, 2014,27(5):455-461.

[19] Fan X, Li M, Cao T, et al. Optical properties and oxidative potential of water- and alkaline-soluble brown carbon in smoke particles emitted from laboratory simulated biomass burning [J]. Atmospheric Environment, 2018,194:48-57.

[20] Park S S, Yu J. Chemical and light absorption properties of humic-like substances from biomass burning emissions under controlled combustion experiments [J]. Atmospheric Environment, 2016,136: 114-122.

[21] 韋思業(yè),蘇玉紅,沈國(guó)鋒,等.農(nóng)村室內(nèi)薪柴燃燒的顆粒物和炭黑排放因子 [J]. 生態(tài)毒理學(xué)報(bào), 2013,8(1):29-36. Wei S Y, Su Y H, Shen G F, et al. Emission factors of particulate matter and elemental carbon from rural residential wood combustion [J]. Asian Journal of Ecotoxicology, 2013,8(1):29-36.

[22] Baduel C, Voisin D, Jaffrezo J L. Seasonal variations of concentrations and optical properties of water soluble HULIS collected in urban environments [J]. Atmospheric Chemistry and Physics, 2010,10(9): 4085-4095.

[23] Fan X, Song J, Peng P. Comparison of isolation and quantification methods to measure humic-like substances (HULIS) in atmospheric particles [J]. Atmospheric Environment, 2012,60:366-374.

[24] Cheng Y, He K, Du Z, et al. The characteristics of brown carbon aerosol during winter in Beijing [J]. Atmospheric Environment, 2016,127:355-364.

[25] 范行軍,余旭芳,操 濤,等.廣州冬季氣溶膠中水溶性有機(jī)物和類腐殖質(zhì)的吸光性和熒光光譜特性 [J]. 環(huán)境科學(xué), 2019,40(2):532-539. Fan X J, Yu X F, Cao T, et al. Light absorption and fluorescence characteristics of atmospheric water-soluble organic compounds and humic-like substances during the winter season in Guangzhou. Environmental Science, 2019,40(2):532-539.

[26] 周石磊,張藝冉,黃廷林,等.基于UV-vis及EEMs解析周村水庫(kù)夏秋季降雨不同相對(duì)分子質(zhì)量DOM的光譜特征及來源 [J]. 環(huán)境科學(xué), 2019,40(1):172-184. Zhou S L, Zhang Y R, Huang T L, et al. Spectral characteristics and sources of dissolved organic matter with different relative molecular weight from rainwater from summer and autumn in the zhoucun reservoir based on UV-Vis and EEMs. Environmental Science, 2019, 40(1):172-184.

[27] Kothawala D N, von Wachenfeldt E, Koehler B, et al. Selective loss and preservation of lake water dissolved organic matter fluorescence during long-term dark incubations [J]. Science of the Total Environment, 2012,433:238-246.

[28] 祝 斌,朱先磊,張?jiān)獎(jiǎng)?等.農(nóng)作物秸稈燃燒PM2.5排放因子的研究 [J]. 環(huán)境科學(xué)研究, 2005,18(2):29-33. Zhu B, Zhu X L, Zhang Y X, et al. Emission factor of PM2.5from crop straw burning [J]. Research of Environmental Sciences, 2005,18(2): 29-33.

[29] Hong L, Liu G, Zhou L, et al. Emission of organic carbon, elemental carbon and water-soluble ions from crop straw burning under flaming and smoldering conditions [J]. Particuology, 2017,31:181-190.

[30] Peuravuori J, Pihlaja K. Isolation and characterization of natural organic matter from lake water: Comparison of isolation with solid adsorption and tangential membrane filtration [J]. Environment International, 1997,23(4):441-451.

[31] Yuan J F, Huang X F, Cao L M, et al. Light absorption of brown carbon aerosol in the PRD region of China [J]. Atmospheric Chemistry and Physics, 2016,16(3):1433-1443.

[32] Zhang X, Lin Y H, Surratt J D, et al. Sources, composition and absorption Angstrom exponent of light-absorbing organic components in aerosol extracts from the Los Angeles Basin [J]. Environmental Science and Technology, 2013,47(8):3685-93.

[33] Kieber R J, Whitehead R F, Reid S N, et al. Chromophoric dissolved organic matter (CDOM) in rainwater, southeastern North Carolina, USA [J]. Journal of Atmospheric Chemistry, 2006,54(1):21-41.

[34] Santos P S, Otero M, Duarte R M, et al. Spectroscopic characterization of dissolved organic matter isolated from rainwater [J]. Chemosphere 2009,74(8):1053-1061.

[35] Santos P S, Santos E B, Duarte A C. First spectroscopic study on the structural features of dissolved organic matter isolated from rainwater in different seasons [J]. Science of the Total Environment, 2012,426: 172-179.

[36] Birdwell J E, Valsaraj K T. Characterization of dissolved organic matter in fogwater by excitation–emission matrix fluorescence spectroscopy [J]. Atmospheric Environment, 2010,44(27):3246-3253.

[37] Qin J, Zhang L, Zhou X, et al. Fluorescence fingerprinting properties for exploring water-soluble organic compounds in PM2.5in an industrial city of northwest China [J]. Atmospheric Environment, 2018, 184:203-211.

[38] Zhang Y, Gao G, Shi K, et al. Absorption and fluorescence characteristics of rainwater CDOM and contribution to Lake Taihu, China [J]. Atmospheric Environment, 2014,98:483-491.

[39] Matos J T, Freire S M, Duarte R M, et al. Natural organic matter in urban aerosols: Comparison between water and alkaline soluble components using excitation–emission matrix fluorescence spectroscopy and multiway data analysis [J]. Atmospheric Environment, 2015,102:1-10.

[40] Santos P S, Otero M, Filipe O M, et al. Comparison between DAX-8 and C-18 solid phase extraction of rainwater dissolved organic matter [J]. Talanta, 2010,83(2):505-512.

[41] Duarte R M, Pio C A, Duarte A C. Synchronous scan and excitation- emission matrix fluorescence spectroscopy of water-soluble organic compounds in atmospheric aerosols [J]. Journal of Atmospheric Chemistry, 2004,48(2):157-171.

[42] Fu P, Kawamura K, Chen J, et al. Fluorescent water-soluble organic aerosols in the High Arctic atmosphere [J]. Scientific Reports, 2015, 5:9845.

[43] McKnight D M, Boyer E W, Westerhoff P K, et al. pectrofluorometric characterization of dissolved organic matter for indication of precursor organic material and aromaticity [J]. Limnology and Oceanography, 2001,46(1):38-48.

Emission characteristics and optical properties of extractable brown carbon from residential wood combustion.

FAN Xing-jun1, CAO Tao2,3, YU Xu-fang1, SONG Jian-zhong2*, WANG Yan1, XIAO Xin1, XIE Yue1, LI Fei-yue1

(1.College of Resources and Environment, Anhui Science and Technology University, Fengyang 233100, China；2.State Key Laboratory of Organic Geochemistry, Guangzhou Institute of Geochemistry, Chinese Academy of Sciences, Guangzhou 510640, China；3.University of Chinese Academy of Sciences, Beijing 100049, China)., 2019,39(8)：3215~3224

The study was aimed to investigate the emission characteristics and optical properties of primary brown carbon (BrC), including water-soluble organic matters (WSOM), water-soluble humic like substances (HULISWS), alkaline-soluble organic matters (ASOM), alkaline-soluble HULIS (HULISAS), emitted from household residential wood combustion. The results showed that residential wood combustion emitted large amounts of BrC. Among them, BrCT(WSOM + ASOM) were observed to make up 46%~56% of total smoke PM2.5mass, and the emission factors of them were 7.5~16g/kg. HULIS were important light absorber in primary BrC, which accounted for 44%~46% of BrCT. The SUVA254, MAE365and AAE values for primary BrC were 1.9~4.0m2/g, 0.4~2.1m2/gand 6.2~11.1, respectively, indicating that the primary BrC emitted from wood combustion exhibited high aromaticity, strong light absorbing capacity and its light absorption presented strong wavelength dependence. 3DEEM results showed that the protein-like substances were dominant fluorophores for primary BrC fractions. It was significantly different from the fluorescent characteristics for WSOM in rainwater and atmospheric aerosols, in which the humic-like substances were predominant fluorophores. The correlation analysis revealed that MAE365present strong positive correlations with HIX and SUVA254, while strong negative correlations were found with2/3, FI, BIX and:. It implied that the light absorption of primary BrC emitted from residential wood combustion were greatly related to their aromaticity, humification degree, autochthonous and freshness characteristics. The results obtained in this study are helpful to better understand the emission characteristics of BrC from BB, and also can provide such deep insight into the sources and environment effects of atmospheric BrC.

residential wood combustion；brown carbon；emission characteristics；light absorption；fluorescence spectral properties

X513

A

1000-6923(2019)08-3215-10

范行軍(1987-),男,安徽六安人,講師,博士,主要研究方向?yàn)榇髿鈿馊苣z中棕色碳的光學(xué)特征和來源解析.發(fā)表論文10篇.

2019-01-28

國(guó)家自然科學(xué)基金資助項(xiàng)目(41705107,41673117);安徽省科技重大專項(xiàng)(16030701102);安徽省教育廳重點(diǎn)項(xiàng)目(KJ2017A520);安徽省自然科學(xué)基金資助項(xiàng)目(1808085MB49,1708085QD85)

* 責(zé)任作者, 副研究員, songjzh@gig.ac.cn