生物質(zhì)燃燒源類腐殖質(zhì)的臭氧老化特征

操 濤,宋建中,范行軍

生物質(zhì)燃燒源類腐殖質(zhì)的臭氧老化特征

操 濤1,2,4,宋建中1,2,范行軍1,3*

(1.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；2.中國(guó)科學(xué)院深地科學(xué)卓越創(chuàng)新中心,廣東 廣州 510640；3.安徽科技學(xué)院資源與環(huán)境學(xué)院,安徽 蚌埠 233100；4.中國(guó)科學(xué)院大學(xué),北京 100049)

針對(duì)生物質(zhì)燃燒排放類腐殖質(zhì)(BC-HULIS)的臭氧(O3)氧化開(kāi)展模擬研究,利用總有機(jī)碳分析儀(TOC)、紫外-可見(jiàn)吸收光譜(UV-vis)、熒光激發(fā)發(fā)射矩陣光譜結(jié)合平行因子分析(EEM-PARAFAC)以及傅里葉變換紅外光譜(FTIR)表征老化前后HULIS的光學(xué)性質(zhì)和化學(xué)結(jié)構(gòu)變化. 研究表明,經(jīng)臭氧氧化后BC-HULIS占相應(yīng)的水溶性有機(jī)碳(WSOC)的比例降低,表明部分HULIS發(fā)生降解生成水溶性小分子化合物. 此外經(jīng)O3老化后,HULIS的質(zhì)量吸收指數(shù)(MAE365)和芳香性指數(shù)(SUVA254)分別從1.8～2.7和4.2～5.0m2/gC下降到1.1～1.3和3.7～4.1m2/gC,表明老化后HULIS的吸光能力和芳香度均呈現(xiàn)降低. EEM-PARAFAC解析結(jié)果顯示,BB-HULIS中熒光發(fā)色團(tuán)主要由類蛋白熒光組分(C2)和類腐殖質(zhì)熒光組分(C1、C3、C4)組成. O3老化后,BB-HULIS的總熒光強(qiáng)度顯著降低,兩種性質(zhì)的熒光組分相對(duì)含量和熒光參數(shù)發(fā)生顯著變化,如老化后HULIS中類腐殖質(zhì)熒光組分的相對(duì)含量和腐質(zhì)化指數(shù)(HIX)均顯著高于老化前樣品,表明老化過(guò)程發(fā)生類蛋白的降解和類腐殖質(zhì)的聚合. 另外,FTIR分析結(jié)果顯示O3老化后含氧官能團(tuán)含量顯著增強(qiáng),表明了O3老化對(duì)HULIS官能團(tuán)的影響.

生物質(zhì)燃燒；類腐殖質(zhì)；臭氧氧化；光學(xué)性質(zhì)；化學(xué)結(jié)構(gòu)

大氣類腐殖質(zhì)(HULIS)是由一系列具有復(fù)雜性質(zhì)的有機(jī)混合物組成,具有與陸源和水生腐殖酸相似的紫外-可見(jiàn)、熒光和紅外光譜特征[1-2].HULIS普遍存在于大氣氣溶膠、雨、云和霧水中,是水溶性有機(jī)碳(WSOC)的重要吸光組成,占比達(dá)9%～72%[3]. HULIS具有顯著的吸濕性和表面活性,對(duì)云凝結(jié)核的形成和氣溶膠的吸濕生長(zhǎng)有重要作用[4].HULIS能夠吸收太陽(yáng)光,對(duì)輻射強(qiáng)迫以及大氣化學(xué)過(guò)程也有一定的影響[5].毒性研究表明,HULIS是大氣PM2.5中能夠催化產(chǎn)生活性氧化物(ROS)的重要載體,對(duì)人體健康亦產(chǎn)生重要危害[6-7].

HULIS的來(lái)源主要包括一次排放(生物質(zhì)燃燒和化石燃料燃燒等)[8-10]和二次生成(揮發(fā)性有機(jī)物的光化學(xué)反應(yīng)、煙炱顆粒的非均相反應(yīng)等)[11-12].在這些來(lái)源中,生物質(zhì)燃燒(BC)被認(rèn)為是重要來(lái)源之一[5,9,13].已有研究表明,BC產(chǎn)生的HULIS(記為BC-HULIS)占細(xì)顆粒物質(zhì)量的11.2%～23.4%[1],且具有較強(qiáng)的光吸收能力;不同燃燒條件對(duì)BC HULUS的產(chǎn)生有一定影響[14].事實(shí)上,直接排放的BC- HULIS釋放到大氣中后也會(huì)與大氣氧化劑,如O3、NO等,發(fā)生一系列復(fù)雜的大氣化學(xué)過(guò)程[12],改變其含量、光學(xué)和化學(xué)性質(zhì).其中,O3是一種重要的強(qiáng)氧化劑,也是當(dāng)前大氣中重要的污染物.因此開(kāi)展BC-HULIS的O3老化研究具有重要的意義. Pillar等[15-16]研究表明生物質(zhì)燃燒排放煙氣顆粒中鄰苯二酚的O3老化過(guò)程會(huì)促進(jìn)HULIS的生成. Vione等[17]研究發(fā)現(xiàn)O3老化會(huì)導(dǎo)致BC氣溶膠中發(fā)色團(tuán)的漂白和降解.對(duì)BC煙氣顆粒在O3老化過(guò)程中WSOC的演化規(guī)律研究中發(fā)現(xiàn),O3老化過(guò)程中有機(jī)碳呈現(xiàn)先升高再降低的趨勢(shì),而樣品的吸光性則呈現(xiàn)顯著的漂白[18].然而該研究主要是針對(duì)WSOC的研究,對(duì)重要吸光性組分如HULIS的臭氧老化特征尚未有深入認(rèn)識(shí). HULIS是重要的棕色碳組分,因此,需要對(duì)老化過(guò)程中BC煙氣顆粒HULIS的組成和性質(zhì)發(fā)生的變化進(jìn)行系統(tǒng)研究.

本研究采集水稻、玉米秸稈以及松木作為燃料進(jìn)行燃燒,收集煙氣顆粒,在實(shí)驗(yàn)室進(jìn)行O3氧化模擬研究.利用超純水萃取結(jié)合固相萃取方法獲得氧化前后顆粒中的HULIS,采用總有機(jī)碳分析儀(TOC)、紫外-可見(jiàn)光譜(UV-vis)、熒光激發(fā)發(fā)射矩陣光譜(EEM)和傅里葉變換紅外光譜(FTIR)對(duì)HULIS的有機(jī)碳(OC)含量、光學(xué)性質(zhì)和官能團(tuán)進(jìn)行了表征,討論了O3氧化前后HULIS的光吸收能力、發(fā)色團(tuán)的演化和化學(xué)官能團(tuán)的變化,進(jìn)一步加深對(duì)BC-HULIS臭氧氧化機(jī)制的認(rèn)識(shí).

1 材料與方法

1.1 BC煙氣顆粒采集

水稻和玉米秸稈取自安徽省蚌埠市周邊,松木樣品取自安徽省六安市.樣品收集后自然晾干并按一定大小分好,便于進(jìn)一步的燃燒實(shí)驗(yàn).燃燒實(shí)驗(yàn)在實(shí)驗(yàn)室自制的燃燒和采樣系統(tǒng)中進(jìn)行,詳細(xì)運(yùn)行過(guò)程見(jiàn)文獻(xiàn)[9].簡(jiǎn)單來(lái)說(shuō)就是將切成小塊的實(shí)驗(yàn)樣品放進(jìn)燃燒爐內(nèi),點(diǎn)火后封閉爐子,打開(kāi)助燃和稀釋氣泵,雖然是封閉燃燒,但是底部供氧充足,生物質(zhì)在燃燒爐內(nèi)處于明火燃燒狀態(tài).使煙氣顆粒順著稀釋冷卻管進(jìn)入混合倉(cāng),經(jīng)細(xì)顆粒物收集器(武漢天虹智能儀表廠,武漢)以80L/min的流速收集在提前焙燒的干凈石英濾膜上(Whatman,90mm).采集好的石英濾膜放冰箱冷凍備用,同時(shí)采集大氣顆粒物空白膜片和操作空白膜片.

1.2 反應(yīng)裝置

利用自制的O3氧化裝置進(jìn)行BC煙氣顆粒的O3氧化模擬研究.該裝置主要由氣瓶、O3產(chǎn)生裝置和加濕裝置、混合罐、主反應(yīng)器、檢測(cè)器和尾氣處理裝置組成,具體構(gòu)造和描述見(jiàn)文獻(xiàn)[18].參照Fan等[18]的方法開(kāi)展O3氧化模擬,具體為:打開(kāi)氣瓶,干燥空氣進(jìn)入O3產(chǎn)生裝置,O3是由干燥空氣在波長(zhǎng)185nm紫外燈下光解產(chǎn)生,形成混合氣體;超純氮?dú)獗灰爰訚衿髦?隨后與氧氣和O3等混合氣體充分混合后進(jìn)入玻璃反應(yīng)器中,主要起稀釋O3濃度和增加濕度作用.O3濃度使用便攜式臭氧檢測(cè)儀測(cè)定,同時(shí)測(cè)定溫度和濕度.在本研究中O3濃度、溫度和濕度控制在(114±6) mg/m3(高O3濃度暴露以達(dá)到短時(shí)間快速反應(yīng)的效果),26℃和(42±1)%.生物質(zhì)燃燒細(xì)顆粒物樣品膜片按所需要的面積切好,反應(yīng)時(shí)放在石英玻璃板上,在反應(yīng)器內(nèi)與混合氣體充分暴露,O3氧化時(shí)間為12h,分別在0和12h從反應(yīng)器中取出樣品使用鋁箔紙包好并存放在冰箱中待分析.

在反應(yīng)過(guò)程中需要使用鋁箔紙將O3產(chǎn)生裝置和O3反應(yīng)裝置密封,避免光參與反應(yīng).另外所有分析和實(shí)驗(yàn)均采用空白校正.

1.3 HULIS分離純化

根據(jù)Song等[19]的分離純化方法得到HULIS.流程為:臭氧氧化前后的顆粒物膜片經(jīng)超純水超聲萃取(共60mL,分2次各30min超聲萃取),聚四氟乙烯濾頭過(guò)濾(0.22μm,津騰,天津)后得到WSOC,一半體積的WSOC加鹽酸酸化(pH=2),過(guò)甲醇活化洗凈后的ENVI-18柱(200mg,Supelco,美國(guó)),冷凍干燥后用甲醇洗脫,氮吹干燥后加相同體積超純水超聲定容.

1.4 儀器分析

采用島津總有機(jī)碳分析儀(TOC-VCPN,島津,日本)對(duì)老化前后的WSOC和HULIS的有機(jī)碳含量測(cè)定. UV-vis光譜使用島津UV-2600(UV-2600,島津,日本)測(cè)定.掃描范圍200～700nm,間隔1nm,采用超純水做空白和基線.為了更好地表征HULIS的光學(xué)性質(zhì),對(duì)一些光學(xué)參數(shù)如254和280nm處的特征吸收值(SUVA254和SUVA280),250和365nm處吸光度的比值(2/3),Angstrom指數(shù)(AAE)和質(zhì)量吸收指數(shù)(MAE365)分別進(jìn)行計(jì)算,參數(shù)介紹和計(jì)算過(guò)程參考文獻(xiàn)[9-10,20].

采用F-4600(Hitachi,日立,日本)測(cè)定氧化前后HULIS和WSOC的EEM,光譜激發(fā)范圍為200～ 400nm,發(fā)射范圍為290～520nm,掃描間隔均為5nm,掃描速度為2400nm/min.為了避免濃度效應(yīng)和內(nèi)濾效應(yīng),熒光強(qiáng)度使用TOC濃度和拉曼峰面積較正. PARAFAC分析使用MATLAB R2014a (MathWorks. Inc,美國(guó))和DOMFluor工具包[21-22]. PARAFAC計(jì)算采用非負(fù)性約束的2～7個(gè)組分模型,并通過(guò)殘差分析、分半檢驗(yàn)驗(yàn)證熒光組分的數(shù)量,對(duì)BC-HULIS的36樣本建立了4組分模型,使用最大熒光強(qiáng)度(max)來(lái)估計(jì)單個(gè)熒光團(tuán)的相對(duì)水平[23].從x= 254nm處的熒光發(fā)射光譜中提取腐殖化指數(shù)(HIX),用435～480nm處的熒光強(qiáng)度積分面積除以300～345nm處的熒光強(qiáng)度積分面積計(jì)算得到[24].

采用傅里葉變換紅外光譜儀(FTIR,Thermo,美國(guó))在室溫下測(cè)定HULIS的紅外光譜,測(cè)量前,在所有樣品中加入溴化鉀(KBr)并對(duì)樣品進(jìn)行冷凍干燥,取出干燥后的粉末樣品壓制成片狀,扣除KBr空白.每個(gè)FTIR光譜經(jīng)過(guò)64次掃描,分辨率為4cm-1.

2 結(jié)果與討論

2.1 HULIS含量

為了探究O3老化前后HULIS相對(duì)含量的變化,以有機(jī)碳含量(TOC)和250nm的吸光度(UV250)進(jìn)行計(jì)算和比較.如圖1所示,以TOC計(jì)算的水稻、玉米和松木燃燒排放物質(zhì)在老化前的HULIS/WSOC比值分別為51%、53%和42%,該值與之前報(bào)道的生物質(zhì)燃燒直接排放的HULIS/WSOC比值接近[1,9].但是,經(jīng)O3老化后,同一樣品的HULIS/WSOC比值下降至50%、49%和39%;以UV250計(jì)算的O3老化前水稻、玉米和松木的HULIS/WSOC比值為72%、64%和53%,O3老化后3種樣品的HULIS/WSOC比值下降至67%、58%和51%.兩種參數(shù)計(jì)算下O3老化后的HULIS的相對(duì)含量均表現(xiàn)為下降,但UV250比值下降幅度更大,表明O3老化導(dǎo)致了HULIS中碳質(zhì)吸光組分的減少,可能是其中的大分子吸光性組成分解成水溶性小分子不吸光物質(zhì).具體來(lái)說(shuō),在O3老化HULIS的過(guò)程中,HULIS可能存在生成與降解兩種反應(yīng)途徑;有研究報(bào)道煙炱顆粒與臭氧的非均相氧化會(huì)產(chǎn)生HULIS[25];而臭氧對(duì)HULIS氧化會(huì)打破HULIS內(nèi)部的C-C鍵,產(chǎn)生低分子量的有機(jī)化合物和氣態(tài)化合物,如一氧化碳和二氧化碳,導(dǎo)致碳含量的減少[26].

圖1 O3老化前后生物質(zhì)燃燒排放顆粒的HULIS/WSOC比值

2.2 HULIS的吸光性質(zhì)

2.2.1 UV-vis光譜 為了更簡(jiǎn)單地對(duì)UV-vis光譜進(jìn)行比較,所有光譜使用相應(yīng)HULIS的TOC含量進(jìn)行校正.如圖2所示,老化前后的HULIS吸收光譜吸收強(qiáng)度和吸收峰都發(fā)生了明顯變化.首先,老化后HULIS的吸收強(qiáng)度相對(duì)于老化前的樣品明顯降低,表明HULIS的吸光能力顯著降低.老化前HULIS的吸收光譜在250～300nm范圍內(nèi)有一個(gè)明顯的吸收峰,表明雙鍵和芳香族結(jié)構(gòu)(如酚類衍生物和苯甲酸等)中存在π-π*電子躍遷[1,27].吸收峰在O3老化后消失了,這說(shuō)明O3老化后HULIS中這些具有含苯環(huán)芳香類物質(zhì)發(fā)生了降解和轉(zhuǎn)化[28].燃燒狀態(tài)對(duì)產(chǎn)物吸光性影響較大,以往研究指出燜燒狀態(tài)下的有機(jī)氣溶膠吸光性要弱于明火燃燒狀態(tài)排放[29],且明火燃燒狀態(tài)能更好地模擬田間供氧充足的開(kāi)放式燃燒過(guò)程.

圖2 O3老化前后BC-HULIS的UV-vis光譜

2.2.2 光譜學(xué)參數(shù) 如表1所示,老化前BC- HULIS的2/3值在4.4～5.9范圍內(nèi),顯著低于老化后的數(shù)值7.2～7.9.老化前水稻、玉米和松木燃燒直接排放HULIS的SUVA254的數(shù)值分別為(4.4±0.1),(4.2±0.1)和(5.0±0.1)m2/gC,老化后降至(3.7±0.2),(3.9±0.1)和(4.1±0.2)m2/gC.已有研究表明,2/3值與樣品的芳香性和分子量成反比,而SUVA254與有機(jī)質(zhì)的芳香性成正比[8]. O3老化導(dǎo)致2/3數(shù)值增加表明HULIS芳香性和分子量降低,而SUVA254數(shù)值的顯著降低更是直接表明老化后樣品的芳香性下降,因此可以認(rèn)為O3老化反應(yīng)對(duì)BC-HULIS的芳香性和分子量產(chǎn)生明顯影響[9].

表1 O3老化前后BC-HULIS的光學(xué)參數(shù)

注: SUVA254和MAE365的單位是m2/gC.

AAE和MAE365是評(píng)價(jià)HULIS吸光性質(zhì)的重要參數(shù)[1,9,14].如表1所示,水稻、玉米和松木燃燒排放HULIS老化前的AAE值分別為8.3±0.1,6.4±0.1,7.1±0.1,與以往文獻(xiàn)報(bào)道相近[8,18].對(duì)于老化后的樣品,AAE的值增加至8.7±0.1,7.8±0.02,8.1±0.1.表明O3老化后的BC-HULIS具有更強(qiáng)的波長(zhǎng)依賴性,AAE的變化與前人研究結(jié)果一致[30].但是由于實(shí)際煙羽中顆粒物狀態(tài)以及顆粒物粒徑隨大氣氧化時(shí)間變化較大[30-32],HULIS的AAE變化與粒徑的關(guān)系在臭氧氧化中需要進(jìn)一步實(shí)驗(yàn)探究.水稻、玉米和松木燃燒直接排放HULIS的MAE365值分別為(1.8±0.03),(2.3±0.04)和(2.7±0.05)m2/gC;O3老化后,HULIS的MAE365下降至(1.1±0.1),(1.3±0.03)和(1.2±0.1)m2/gC.相較于老化前HULIS的MAE365值,O3老化后MAE365值下降了39%、43%和56%.這表明臭氧氧化會(huì)破壞HULIS中具有強(qiáng)吸光能力的物質(zhì),導(dǎo)致HULIS的吸光能力降低;生物質(zhì)燃燒WSOC大氣光化學(xué)反應(yīng)同樣會(huì)導(dǎo)致其吸光能力減弱[28].

2.3 摩爾吸光系數(shù)、分子量和芳香碳豐度

摩爾吸光系數(shù)(280)是指單位物質(zhì)的量碳HULIS在280nm的吸光能力[27],具體計(jì)算式為:

(280) = Abs280×12/(×) (1)

式中: Abs280為280nm處吸收值;12為碳原子的摩爾質(zhì)量(g/mol);為HULIS的有機(jī)碳含量(mg/L);為吸收池寬(cm).

選擇280nm作為光學(xué)參數(shù)是因?yàn)镠ULIS前體物中芳香組分的π-π*電子躍遷發(fā)生在這一波長(zhǎng)范圍內(nèi)[27].如表2所示,老化后HULIS的平均(280)在317～341L/(molC·cm),全部低于老化前425～518L/ (molC·cm),結(jié)果表明O3老化會(huì)降低HULIS的摩爾吸光系數(shù),但是無(wú)論是老化前還是老化后的HULIS摩爾吸光系數(shù),均要高于大氣氣溶膠HULIS的摩爾吸光系數(shù)173L/(molC·cm)((布達(dá)佩斯地區(qū))[27,33],137L/(molC·cm)(廣州地區(qū)).這表明,BC-HULIS具有比環(huán)境氣溶膠HULIS更高的摩爾吸收率,表明BC-HULIS具有更強(qiáng)的吸光能力.

芳香碳豐度(Ar,%)可以根據(jù)摩爾吸光系數(shù)計(jì)算[27,34]:

Ar(%)=6.47+(280)′0.05 (2)

如表2所示,老化前BC-HULIS的芳香碳豐度約為28%～33%,O3老化后下降到23%～25%,表明在臭氧老化過(guò)程中部分芳香性物質(zhì)發(fā)生了分解.

另外摩爾吸光系數(shù)還可以用來(lái)估算平均分子量的大小[27,34]:

=534+1.33′(280) (3)

如表2所示,水稻、玉米和松木燃燒排放HULIS老化前的分子量大約為1133,1100和1222Da,O3老化后下降至987,1024和956Da,表明O3老化使HULIS中大分子物質(zhì)發(fā)生分解,特別是松木樣品HULIS平均分子量下降幅度最大.

綜上,O3老化使HULIS的芳香性降低,吸光能力下降,通過(guò)計(jì)算摩爾吸光系數(shù)、分子量和芳香碳豐度等參數(shù),發(fā)現(xiàn)O3老化后HULIS的摩爾吸光系數(shù)降低,分子量和芳香碳豐度也相應(yīng)降低,這一變化特征和前面用光學(xué)參數(shù)得出來(lái)的HULIS吸光能力和分子量以及芳香性的變化一致.

表2 O3老化前后BC-HULIS的摩爾吸光系數(shù)ε(280),平均分子量和芳香碳豐度

2.4 EEM-PARAFAC分析

2.4.1 三維熒光光譜 如圖3所示,老化前和老化后BC-HULIS的EEM譜圖中均有2個(gè)熒光峰,但熒光峰的位置和強(qiáng)度有明顯的區(qū)別.老化前HULIS的兩個(gè)熒光峰分別位于ex/em=230～240nm/350～ 370nm(A)和ex/em=270～280nm/350～360nm(B);老化后,這2個(gè)熒光峰位置發(fā)生紅移至ex/em=220～ 240nm/400～420nm(C)和ex/em=270～290nm/390～ -410nm (D). A峰通常被劃分為類蛋白或多酚物質(zhì),如多酚化合物和類色氨酸物質(zhì)或類似這些物質(zhì)的分子結(jié)構(gòu)[35];B峰可能屬于激發(fā)波長(zhǎng)較長(zhǎng)的蛋白類化合物,通常被認(rèn)為是類色氨酸化合物,該峰在表層水、大氣氣溶膠以及雨水的研究中也被檢出[23];C和D峰位于典型的Fulvic-like區(qū),可能與DOM中的Fulvic-like具有相似的化學(xué)結(jié)構(gòu). Fulvic-like的官能團(tuán)主要包括含苯羧基、-C=C-和酚OH,以及含有O、S、P原子的官能團(tuán)[35]. O3老化使HULIS的最大熒光峰(峰A和B)位置向長(zhǎng)波產(chǎn)生一定偏移,表明HULIS中主要的熒光發(fā)色團(tuán)物質(zhì)化學(xué)結(jié)構(gòu)發(fā)生變化,而且老化后HULIS的最大熒光峰熒光強(qiáng)度遠(yuǎn)低于老化前,這說(shuō)明O3老化使HULIS中的原有熒光發(fā)色團(tuán)物質(zhì)結(jié)構(gòu)被破壞且新產(chǎn)生的熒光基團(tuán)熒光量子產(chǎn)率低,導(dǎo)致最大熒光峰強(qiáng)度遠(yuǎn)不如老化前.

HIX能反映天然有機(jī)質(zhì)的腐殖化程度,同時(shí)也被廣泛應(yīng)用于對(duì)大氣氣溶膠和雨水中WSOC中類腐殖質(zhì)類熒光發(fā)色團(tuán)性質(zhì)的表征[18,24].總的來(lái)說(shuō),高的HIX值與WSOC的高芳香性和高縮聚程度有關(guān),如表1所示,老化前水稻、玉米和松木HULIS的HIX值分別為1.1、0.8和1.2,老化后分別增加至3.8、2.1和1.7,老化后HULIS的HIX值顯著增加,表明老化后HULIS中類腐殖質(zhì)類熒光組分的相對(duì)含量顯著增加,可能與產(chǎn)生更多的低聚合芳香結(jié)構(gòu)物質(zhì)有關(guān).

圖3 O3老化前后BC-HULIS的EEM譜圖

2.4.2 EEM-PARAFAC分析 EEM-PARAFAC解析出4個(gè)獨(dú)立的熒光組分.如圖4所示,這4個(gè)熒光組分可以歸屬于長(zhǎng)波類腐殖質(zhì)熒光組分(HULIS-1,C1),類蛋白/多酚類熒光組分(PLOM,C2),短波類腐殖質(zhì)熒光組分(HULIS-2,C3)和高氧化腐殖質(zhì)熒光組分(HULIS-3,C4)[21,24,36].在大氣氣溶膠和燃燒源排放煙氣顆粒物WSOC和HULIS的熒光光譜中也有報(bào)道過(guò)相近位置的熒光組分[14,24,36-39].

圖4 EEM-PARAFAC分析得到的BC-HULIS熒光組分

由圖5可知,在老化前BC-HULIS中,類蛋白/多酚類熒光組分(C2)相對(duì)含量最大(34%～47%),而在老化后的HULIS中,類蛋白/多酚類熒光組分(C2)顯著減少至9%～22%,結(jié)果表明,PLOM熒光發(fā)色團(tuán)對(duì)O3老化產(chǎn)生的響應(yīng)最大.此外,腐殖質(zhì)類熒光組分(C1、C3和C4),尤其是長(zhǎng)波類腐殖質(zhì)熒光組分(C1)和高氧化類腐殖質(zhì)熒光組分(C4),其相對(duì)含量在老化后上升,成為老化后HULIS中主要的熒光組分(78%～91%)[16],該結(jié)果與HIX值上升相一致.

圖5 老化前后BC-HULIS中4個(gè)熒光組分含量的相對(duì)分布

2.5 紅外光譜分析

如圖6所示,所有樣品的紅外光譜與從雨水和大氣氣溶膠中分離純化出的HULIS具有類似的光譜特征,這些光譜的解釋參考了前人關(guān)于天然有機(jī)質(zhì)、直接排放顆粒物和非均相反應(yīng)生成吸光性物質(zhì)的研究[1,15-16,18,40-41].在3420cm-1附近集中的寬而強(qiáng)的吸收峰一般是由于酚、羧基和羥基的OH伸縮振動(dòng);2930cm-1處的肩峰被認(rèn)為是脂肪族C-H伸縮;1710cm-1左右的被認(rèn)為是C=O伸縮振動(dòng),一般認(rèn)為是羧基、羰基,醛的C=O基團(tuán)也對(duì)該波數(shù)附近的吸收峰有貢獻(xiàn).在1700～1000cm-1范圍內(nèi)也出現(xiàn)了較多的集中峰,例如,1608和1515cm-1可以歸結(jié)為芳香環(huán)和C=C雙鍵的伸縮振動(dòng);1454和1384cm-1可能由脂肪族CH、CH2和CH3貢獻(xiàn);1275和1115cm-1表示芳香族C-O或酚羥基的伸縮和環(huán)型C-O的伸縮振動(dòng)[1,18].這些結(jié)果表明,無(wú)論是老化前還是老化后的BC-HULIS都普遍由羥基、羰基、不飽和芳香環(huán)等多種官能團(tuán)組成.

如圖6所示,老化前和老化后在4000～2000cm-1波長(zhǎng)范圍內(nèi)沒(méi)有看到明顯的光譜變化,說(shuō)明該范圍內(nèi)的官能團(tuán)對(duì)O3暴露不敏感[16,18],而在2000～ 1000cm-1之間存在顯著差異. O3老化后,1710cm-1的C=O伸縮振動(dòng)增加,尤其是在水稻秸稈和玉米秸稈燃燒排放HULIS中,說(shuō)明O3老化可以使HULIS中形成更多的C=O伸縮振動(dòng)官能團(tuán),O3老化生物質(zhì)燃燒排放中的酚類化合物使紅外光譜也產(chǎn)生了類似的變化[18].老化后,1515,1454,1275cm-1的峰均減少,說(shuō)明在HULIS內(nèi)存在芳香C=C、C-O的分解. 這些結(jié)果證實(shí)了芳香骨架中的C=C和木質(zhì)素骨架中的C=C (1515cm-1)以及脂肪鏈和芳香環(huán)(1454cm-1)易于降解[16,18].

圖6 O3老化前后BC-HULIS的紅外光譜

本研究聚焦于O3老化前后BC-HULIS的發(fā)色團(tuán)、光吸收能力和官能團(tuán)的變化,有助于進(jìn)一步了解BC顆粒物進(jìn)入大氣的O3老化過(guò)程.研究發(fā)現(xiàn),在O3老化過(guò)程中BC-HULIS的吸光能力有一定的下降,這意味著B(niǎo)C-HULIS的吸光能力可能不像之前報(bào)道的那么高,也就是說(shuō),生物質(zhì)排放顆粒物氣溶膠的輻射強(qiáng)迫效應(yīng)隨其在大氣中的停留時(shí)間而變化,在這里僅考慮的是O3的非均相氧化,大氣化學(xué)過(guò)程中的液相和氣相反應(yīng)同樣可能會(huì)導(dǎo)致源排放棕色碳的吸光性產(chǎn)生變化[42-44],因此基于源排放物質(zhì)的模式估算可能導(dǎo)致結(jié)果過(guò)高,需要更詳細(xì)的工作來(lái)揭示排放到大氣中的顆粒物的氧化過(guò)程.

3 結(jié)論

3.1 O3老化后的HULIS中吸光性物質(zhì)含量顯著降低,導(dǎo)致其吸光能力顯著減弱.

3.2 經(jīng)過(guò)O3氧化反應(yīng)后,HULIS的吸收光譜變得更加平滑,芳香性和吸光能力降低,具體表現(xiàn)在SUVA254和MAE365降低和2/3增加;經(jīng)O3氧化后HULIS的AAE值顯著增大,表明老化的HULIS具有更高的波長(zhǎng)依賴性;O3老化前后BC-HULIS的摩爾吸光系數(shù)、芳香度和平均分子量均顯著降低.

3.3 O3老化改變了生物質(zhì)燃燒排放HULIS中熒光團(tuán)的主導(dǎo)地位,EEM-PARAFAC分離鑒定出的熒光發(fā)色團(tuán)百分比改變明顯,老化后長(zhǎng)波類腐殖質(zhì)熒光組分相對(duì)增多,這與FTIR光譜1710cm-1C=O伸縮振動(dòng)信號(hào)增加相一致.

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Changes in optical properties and chemical functional groups of humic substances emitted from biomass combustion with O3oxidation.

CAO Tao1,2,4,SONG Jian-zhong1,2,FAN Xing-jun1,3*

(1.State Key Laboratory of Organic Geochemistry,Guangzhou Institute of Geochemistry,Chinese Academy of Sciences,Guangzhou 510640,China；2.CAS Center for Excellence in Deep Earth Science,Guangzhou 510640,China；3.College of Resource and Environment,Anhui Science and Technology University,Bengbu 233100,China；4.University of Chinese Academy of Sciences,Beijing 100049,China).,2022,42(8)：3483～3491

Biomass combustion (BC) is an important source of humic-like substances (HULIS) in atmospheric aerosol,and the oxidation process has a significant impact on the optical properties and chemical structures of BC-HULIS. This study focused on the changes in primary BB-HULIS due to ozone (O3) oxidation and the optical properties and chemical structure of HULIS before and after oxidation were characterized with the total organic carbon (TOC) analyzer,UV-Vis spectroscopy,Excitation-Emission Matrix coupled with parallel factor analysis (EEM-PARAFAC) and Fourier transform infrared spectroscopy (FTIR). The results showed that the relative contents of HULIS in the corresponding water-soluble organic carbon (WSOC) decreased with O3oxidation,suggesting the transformation of HULIS into water-soluble low molecular weight compounds. Furthermore,mass absorption efficiency (MAE365) and aromatic index (SUVA254) of HULIS decreased from 1.8～2.7m2/gC and 4.2～5.0m2/gC to 1.1～1.3m2/gC and 3.7～4.1m2/gC,respectively,indicating that both the absorption capacity and aromaticity of HULIS declined with O3oxidation. The fluorescent components in BC-HULIS were mainly composed of protein-like compounds (C2) and humic-like substances (C1,C3,C4). After O3oxidation,the total fluorescence intensities of BC-HULIS weakened greatly,and the relative contribution of two types of fluorophores and fluorescence index were all significantly changed. For instances,the relative contents of humic-like components and the humidification index (HIX) of BC-HULIS after O3oxidation were obviously higher than those before O3oxidation,suggesting the degradation of protein-like compounds and the aggregation of humic-like substances during the O3oxidation process. In addition,FTIR results showed that the oxygen-containing functional groups were markedly enhanced after O3oxidation,indicating effects of O3oxidation on the chemical functional groups of BC-HULIS.

biomass burning；HULIS；ozone oxidation；optical properties；chemical structure

X513

A

1000-6923(2022)08-3483-09

2022-01-17

國(guó)家自然科學(xué)基金資助項(xiàng)目(41977188);有機(jī)地球化學(xué)國(guó)家重點(diǎn)實(shí)驗(yàn)室開(kāi)放基金資助項(xiàng)目(SKLOG202101);安徽省自然科學(xué)基金資助項(xiàng)目(2108085MD140)

* 責(zé)任作者,副教授,fanxj@ahstu.edu.cn

操 濤(1995-)男,安徽安慶人,中國(guó)科學(xué)院廣州地球化學(xué)研究所博士研究生,主要從事生物質(zhì)和煤燃燒排放棕色碳等吸光性物質(zhì)的研究.發(fā)表論文1篇.