玉龍雪山PM2.5穩(wěn)定碳同位素特征及來(lái)源分析

肖揚(yáng)寧,肖紅偉*,陳振平,黃莉磊,馬 艷,李智滔,冷 泉,金學(xué)武,肖化云

玉龍雪山PM2.5穩(wěn)定碳同位素特征及來(lái)源分析

肖揚(yáng)寧1,2,肖紅偉1,2*,陳振平1,2,黃莉磊1,2,馬 艷1,3,李智滔1,2,冷 泉4,金學(xué)武4,肖化云5

(1.東華理工大學(xué),江西省大氣污染成因與控制重點(diǎn)實(shí)驗(yàn)室,江西 南昌 330013；2.東華理工大學(xué)水資源與環(huán)境工程學(xué)院,江西 南昌 330013；3.東華理工大學(xué)地球科學(xué)學(xué)院,江西 南昌 330013；4.玉龍雪山省級(jí)自然保護(hù)區(qū)管護(hù)局,云南 麗江 674199；5.上海交通大學(xué)環(huán)境科學(xué)與工程學(xué)院,上海 200240)

于2020年4～8月在青藏高原東南部玉龍雪山進(jìn)行PM2.5采樣,共采集44個(gè)樣本,測(cè)定其水溶性離子成分、水溶性有機(jī)碳(WSOC)濃度、總碳(TC)濃度及其穩(wěn)定碳同位素組成(13CTC).結(jié)果表明,玉龍雪山春夏季TC濃度分別為(7.1±3.8)μg/m3和(2.9±0.7)μg/m3,WSOC濃度分別為(3.3±2.1)μg/m3和(1.5±0.4)μg/m3,均呈現(xiàn)春高夏低的變化趨勢(shì).春夏季13CTC值分別為(-24.7±1.0)‰和(-26.0±0.6)‰,春季較夏季偏正,表明可能受到不同來(lái)源影響.通過(guò)對(duì)非海鹽鉀離子(nss-K+)相關(guān)性、NASA火點(diǎn)圖及后向軌跡分析可知,東南亞地區(qū)春季生物質(zhì)燃燒可能是主導(dǎo)原因.利用貝葉斯模型計(jì)算玉龍雪山PM2.5中TC來(lái)源貢獻(xiàn),結(jié)果表明春季主要來(lái)源于生物質(zhì)燃燒和煤燃燒,貢獻(xiàn)比分別為60.6%和23.5%;夏季主要來(lái)源于生物質(zhì)燃燒、植物蒸發(fā)和機(jī)動(dòng)車排放,同時(shí)二次有機(jī)氣溶膠形成對(duì)TC的貢獻(xiàn)也不可忽視.

玉龍雪山；TC；WSOC；穩(wěn)定碳同位素；源解析

大氣氣溶膠對(duì)空氣質(zhì)量、降雨、能見度造成影響,且可能引起氣候變化.化石燃料、生物質(zhì)燃燒和自然排放等是氣溶膠的主要來(lái)源.PM2.5是氣溶膠中的重要組成,且成分復(fù)雜,主要包括碳質(zhì)組分、硫酸鹽和硝酸鹽等[1].其中TC是PM2.5的重要組成部分,主要包括有機(jī)碳(OC)、元素碳[2].OC又可分為水溶性有機(jī)碳(WSOC)和不可水溶性有機(jī)碳.木材燃燒、工業(yè)過(guò)程和自然排放等是WSOC主要來(lái)源[3-5].

TC穩(wěn)定同位素比值(13C)可以用來(lái)示蹤大氣含碳組分來(lái)源,目前δ13C示蹤法已經(jīng)成功運(yùn)用在城市、郊外、近遠(yuǎn)海海域等區(qū)域[6-9].研究表明不同來(lái)源的13C差異較大,如:機(jī)動(dòng)車尾氣的13CTC值范圍是(-25.3±0.5)‰,煤燃燒的13CTC是(-23.6±0.7)‰[10]; C3、C4植物燃燒所釋放的13CTC范圍分別是(-26.8±0.5)‰和(-12.4±0.6)‰[11].國(guó)內(nèi)外學(xué)者對(duì)δ13CTC進(jìn)行了大量的研究,研究發(fā)現(xiàn)北京、西安、南昌、上海和廣州氣溶膠TC主要來(lái)源是化石燃料、機(jī)動(dòng)車排放、生物質(zhì)燃燒和二次有機(jī)氣溶膠等[12-14].研究發(fā)現(xiàn),印度尼西亞的森林大火可能使東亞地區(qū)的TC濃度升高.其影響范圍包括城市、城郊和農(nóng)村等地區(qū),甚至是偏遠(yuǎn)海域南海永興島[15-18].目前,TC的研究主要集中在城市與農(nóng)村,對(duì)青藏高原研究較少[19-21].

玉龍雪山位于青藏高原東南部,是中國(guó)西南橫斷山脈的最南端,有研究表明受人為等影響,玉龍雪山白水1號(hào)冰川正在加速消融[22-23].森林大火所產(chǎn)生的大氣污染物已嚴(yán)重影響周邊區(qū)域空氣質(zhì)量,且可能通過(guò)遠(yuǎn)距離運(yùn)輸跨越大洋[17,24].對(duì)玉龍雪山TC研究,不僅能了解人為因素對(duì)冰川生態(tài)環(huán)境的影響,還可以分析森林大火產(chǎn)生的污染物氣團(tuán)傳輸、區(qū)域變化途徑.本研究結(jié)合水溶性離子、WSOC與TC濃度及13CTC值等研究方法,討論其季節(jié)變化特征和主要污染來(lái)源與貢獻(xiàn).進(jìn)一步揭示生物質(zhì)燃燒對(duì)周邊區(qū)域貢獻(xiàn)程度,同時(shí)為玉龍雪山的生物多樣性與生態(tài)質(zhì)量保護(hù)提供科學(xué)依據(jù).

1 材料與方法

1.1 采樣點(diǎn)和樣品采集

于2020年4～8月在青藏高原東南部玉龍雪山云杉坪(100°14￠25.772E, 27°8￠10.852N)設(shè)置1個(gè)采樣點(diǎn)進(jìn)行樣品采集,采樣點(diǎn)周邊以森林為主,無(wú)明顯污染源.該地區(qū)氣候雨季、旱季區(qū)分明顯,6～10月為雨季,主要受到西南和東南季風(fēng)影響,降水充沛,其中降雨主要集中在7～9月;11月～次年5月為旱季,主要受到西風(fēng)環(huán)流南支和青藏高原冬季季風(fēng)影響,日照充足[25].本研究使用配備PM2.5切割器的大流量采樣器(青島嶗山電子儀器廠生產(chǎn), KC-1000型)進(jìn)行采樣,采樣時(shí)間為早上9:00～3d后早上8:30(持續(xù)71.5h),平均采樣體積為2769L.共采集44份有效樣品.采樣前,將濾膜置于設(shè)溫425℃的馬弗爐內(nèi)烘烤4h,去除有機(jī)碳等雜質(zhì).在完成采樣之后,樣品被儲(chǔ)存在-20℃冰箱內(nèi).

1.2 樣品處理分析

1.2.1 水化學(xué)分析 水溶性離子分析使用賽默飛(Thermo Scientific公司生產(chǎn)的Dionex-AQUION型高效離子色譜儀)測(cè)定,最低檢出限為1×10-9.剪取1/8濾膜,將濾膜置于50mL聚四氟乙烯離心管內(nèi),加入50mL去離子水定容,將離心管置于超聲儀內(nèi)超聲30min,期間每15min將離心管倒置搖晃,置于4200r/min離心機(jī)內(nèi)離心30min,用0.22μm直徑微孔濾膜過(guò)濾上清液后待測(cè).所測(cè)試水溶性陰陽(yáng)離子分別為F-、Cl-、NO3-、SO42-、草酸(C2O42-)和Na+、NH4+、K+、Mg2+、Ca2+.WSOC使用uniTOC總有機(jī)碳分析儀(德國(guó)MEMBRAPURE公司)測(cè)定,最低檢出限為1×10-9.前處理方法與水溶性離子一致,由于上清液濃度高于儀器檢測(cè)上限,將2mL上清液加入50mL去離子水稀釋后待測(cè).

1.2.2 同位素分析 TC濃度及δ13CTC值使用穩(wěn)定同位素質(zhì)譜儀(EA MAT-253Plus)測(cè)定,使用打孔器在濾膜樣品分布均勻區(qū)域,打7～8個(gè)孔裝入錫杯,用百萬(wàn)分之一天平稱重并記錄,包好置干燥器內(nèi)保存等待上機(jī)分析.使用國(guó)際參考標(biāo)準(zhǔn)物質(zhì)USGS-41a (13C: 37.6‰ VPDB)及工作標(biāo)準(zhǔn)尿素(Thermo,13C: -41.3‰)和高粱粉(13C: -13.8‰) 對(duì)13C進(jìn)行校正,各種標(biāo)準(zhǔn)進(jìn)行多次測(cè)量分析,確定分析精度和誤差分別為0.03‰和0.2‰[8].

1.2.3 后向軌跡和火點(diǎn)分析 后向軌跡和火點(diǎn)分別來(lái)源于美國(guó)國(guó)家海洋和大氣管理局(NOAA)研發(fā)的HYSPLIT后向軌跡模型(起始高度為1000m,起始時(shí)間為16:00,并向后推 3d(72h))和美國(guó)國(guó)家航空航天局(NASA)火災(zāi)信息資源管理系統(tǒng)(https://firms. modaps.eosdis.nasa.gov/).

1.2.4 貝葉斯穩(wěn)定同位素混合模型 貝葉斯穩(wěn)定同位素混合模型(MixSIAR)可用于量化TC的源貢獻(xiàn)[13],本研究基于同位素質(zhì)量守恒方法定量計(jì)算采樣區(qū)潛在污染源對(duì)TC的貢獻(xiàn)百分比(),公式如下:

MixSIAR可以根據(jù)研究者的數(shù)據(jù)結(jié)構(gòu)和研究類型來(lái)建立混合模型,通過(guò)源數(shù)據(jù)類型、先驗(yàn)和誤差項(xiàng)等選項(xiàng)對(duì)研究數(shù)據(jù)進(jìn)行分析.其中C3、C4植物13CTC范圍分別為(-20‰～-34‰)和(-9‰～-19‰),通過(guò)先驗(yàn)數(shù)據(jù)(C3,=4908;C4,=1353)確定C3、C4植物的13CTC分別是(-26.8±0.5)‰和(-12.4±0.6)‰[11]; 陳穎軍等[10]對(duì)5種不同的燃煤煙塵進(jìn)行分析,確定了煤燃燒的13CTC是(-23.6±0.7)‰;對(duì)4種不同的柴油車尾氣進(jìn)行分析,得出柴油車尾氣的13CTC(-25.23‰±0.35‰),汽油車尾氣的13CTC則為(-25.41)‰,而機(jī)動(dòng)車尾氣的13CTC具有空間差異性,中國(guó)南方大氣13CTC平均值在(-25.63‰和-25.92‰),略低于北方(-25.98)‰,因玉龍雪山位于中國(guó)西南部,最終確定機(jī)動(dòng)車的13CTC為(-25.3± 0.5)‰[10].

MixSIAR在R Studio上運(yùn)行,結(jié)果以占比形式輸出(需多次計(jì)算,使數(shù)值收斂至最優(yōu)解).

2 結(jié)果與討論

2.1 TC、WSOC濃度水平與WSOC/TC分析

如圖1所示,采樣期間玉龍雪山TC平均濃度為(4.6±3.1)μg/m3,春、夏季TC平均濃度分別為(7.5± 3.5)μg/m3(分布范圍為1.9～15.1μg/m3)、(2.9±0.7)μg/ m3(分布范圍1.8～4.4μg/m3);WSOC平均濃度為(2.2±1.6)μg/m3,春、夏季W(wǎng)SOC平均濃度分別為(3.3±2.1)μg/m3(分布范圍為1.5～9.7μg/m3)、(1.5± 0.4)μg/m3(分布范圍為1.0～2.5μg/m3).春季TC與WSOC濃度分布范圍較夏季更廣,且春季TC與WSOC濃度均高于夏季,平均濃度分別是夏季的2.4和2.2倍.春高夏低的季節(jié)特征可能和春季東南亞地區(qū)森林大火頻發(fā)相關(guān),生物質(zhì)燃燒產(chǎn)生的污染物通過(guò)氣團(tuán)運(yùn)輸?shù)竭_(dá)玉龍雪山,且春季采樣點(diǎn)降水少,靜穩(wěn)天數(shù)較多,污染物難以沉降.夏季受到西南和東南季風(fēng)影響,降水充足,受濕沉降影響,污染物濃度顯著降低.

圖1 WSOC、TC濃度和δ13CTC和WSOC/TC比值的季節(jié)性變化

如表1所示,玉龍雪山與城市站點(diǎn)相比,TC平均濃度明顯偏低,如天津(22.6±6.3)μg/m3[14]、南昌(12.1±2.1)μg/m3[13];與中國(guó)南海背景點(diǎn)永興(3.6± 1.6)μg/m3[26]、捷克背景點(diǎn)(3.8±2.0)μg/m3[18]接近,但高于西北太平洋的沖繩島(2.0±0.6)μg/m3[7]. WSOC平均濃度低于北京(7.20±5.5)μg/m3[27],韓國(guó)光州(3.7±2.5)μg/m3[28];高于日本筑波(1.2±0.4)μg/ m3[16]和日本由利本莊(0.8±0.5)μg/m3[16];與華北平原背景點(diǎn)(3.2±1.8)μg/m3[29]相似,表明玉龍雪山可能受到人為污染影響較小.春夏季W(wǎng)SOC/TC比值分別為46%、52%,高于日本前橋37%[30]、低于日本赤城59%[30],與日本由利本莊46%[16]接近.已有研究表明,WSOC主要來(lái)源于生物質(zhì)燃燒、海洋源和揮發(fā)性有機(jī)物等前體物的二次轉(zhuǎn)化[31-33].夏季W(wǎng)SOC/TC高于春季,表明夏季TC可能更多來(lái)源于二次氣溶膠,與日本由利本莊[16]、日本赤城[30]得到的結(jié)果相似.

表1 本研究與前人研究PM2.5中TC和WSOC濃度及δ13CTC值對(duì)比

2.2 離子相關(guān)性分析

nss-K+可用于示蹤生物質(zhì)燃燒[16],分析發(fā)現(xiàn)春季TC與nss-K+之間存在顯著相關(guān)(2=0.84,<0.01);春季W(wǎng)SOC與nss-K+之間也存在顯著相關(guān)(2=0.87,<0.01).玉龍雪山春夏季PM2.5中nss-K+濃度分別為(0.3±0.2)和(0.04±0.00)μg/m3,春季是夏季7倍,季節(jié)變化特征明顯,表明春季TC可能主要來(lái)源于生物質(zhì)燃燒.玉龍雪山春、夏季草酸濃度分別為(0.2±0.1)μg/m3和(0.1±0.1)μg/m3,春季草酸濃度明顯高于夏季,有研究表明,印尼森林大火也引起了草酸濃度升高[17].夏季TC、WSOC與nss-K+之間相關(guān)性相對(duì)春季較弱(分別為2=0.44,<0.01;2=0.53,< 0.01),表明夏季生物質(zhì)燃燒貢獻(xiàn)相對(duì)較小,可能主要來(lái)源于二次有機(jī)物形成.

圖2 TC濃度、WSOC濃度與nss-K+相關(guān)性

2.3 δ13CTC的變化特征以及來(lái)源解析

玉龍雪山PM2.5中13CTC值的變化特征如圖1(a)所示,采樣期間13CTC值為(-25.6±1.0)‰(分布范圍為-27.6‰～-21.8‰),春夏季13CTC值分別為(-24.7± 1.0)‰和(-26.0±0.6)‰.與瑞典南部背景點(diǎn)(-26.7～ -25.6)‰[9]、捷克森林背景點(diǎn)(-28.9～-25.4)‰[18]相似,但玉龍雪山分布范圍更廣,表明玉龍雪山TC來(lái)源可能更加復(fù)雜.玉龍雪山春季13CTC值與札幌(-24.8± 0.7)‰[34]相似,Pavuluri等[34]認(rèn)為札幌TC主要來(lái)源是生物質(zhì)燃燒、化石燃料燃燒,與本研究觀點(diǎn)一致.貝葉斯模型計(jì)算結(jié)果如圖4所示,春季TC主要來(lái)源于C3植物、煤燃燒,貢獻(xiàn)比分別為53.9%和23.5%,貢獻(xiàn)含量分別為4.0和1.8μg/m3;其次來(lái)源于機(jī)動(dòng)車排放和C4植物,貢獻(xiàn)比分別為15.9% 和6.7%,貢獻(xiàn)含量分別為1.2和0.5μg/m3.夏季主要來(lái)源于C3植物和機(jī)動(dòng)車排放,貢獻(xiàn)比分別為60.8%和24.7%,貢獻(xiàn)含量分別為1.8和0.7μg/m3;其次來(lái)源于煤燃燒和C4植物,貢獻(xiàn)比分別為12.1%和2.4%,貢獻(xiàn)含量分別為0.4和0.07μg/m3.春季C3植物、煤燃燒和機(jī)動(dòng)車排放貢獻(xiàn)含量分別是夏季的2.2、4.5和1.7倍.春季煤燃燒占比較高,且春夏貢獻(xiàn)含量相差4.5倍,可能是因?yàn)榇杭揪挼榕c云南局部氣溫較低,存在燃煤供暖需求.夏季機(jī)動(dòng)車排放貢獻(xiàn)占比更大,可能是因?yàn)闄C(jī)動(dòng)車排放的一次污染物在夏季更容易發(fā)生光化學(xué)反應(yīng)生成二次氣溶膠,同時(shí)夏季燃煤供暖需求降低,這與Cao等[14]得到的結(jié)果相似.

圖4 采樣點(diǎn)春夏季不同來(lái)源對(duì)δ13C的相對(duì)貢獻(xiàn)

后向軌跡和火點(diǎn)圖分析結(jié)果如圖 3 所示,玉龍雪山地處北半球副熱帶高壓,在大氣環(huán)流和高原季風(fēng)的共同影響下,春夏季東南亞和南亞地區(qū)的氣團(tuán)源源不斷的向玉龍雪山匯集,結(jié)合后向軌跡歐拉聚類可知,氣團(tuán)主要來(lái)源于緬甸北部的短距離傳輸,春季東南亞地區(qū)火點(diǎn)分布極其密集,緬北地區(qū)的污染物通過(guò)氣團(tuán)爬升到達(dá)采樣點(diǎn).其次是云南的中距離氣團(tuán)以及南亞印度地區(qū)和印度洋遠(yuǎn)距離傳輸.夏季(6～8)氣團(tuán)主要來(lái)源于緬甸北部及云南的短距離傳輸,少數(shù)來(lái)源于南亞、印度洋、四川和廣西等地的遠(yuǎn)距離傳輸.夏季東南亞地區(qū)火點(diǎn)較少,與春季形成鮮明對(duì)比.受氣團(tuán)傳輸和多種污染物疊加影響,可推斷在采樣期間東南亞地區(qū)森林大火產(chǎn)生的生物質(zhì)燃燒對(duì)玉龍雪山TC具有重要貢獻(xiàn).研究表明,二次有機(jī)氣溶膠(SOA)的13C值更偏負(fù),同時(shí)二次有機(jī)碳(SOC)與溫度成正比,溫度低于15℃時(shí),SOC難以形成[35-36].因此夏季13C相較春季偏負(fù)可能是由于夏季光照強(qiáng),溫度高,形成的SOA比例相對(duì)較高.研究表明,植物釋放的揮發(fā)性有機(jī)物(VOCs)遠(yuǎn)高于人為排放[37].觀測(cè)期間春夏季C3、C4植物和機(jī)動(dòng)車排放含量占TC的比值分別為76.5%和87.9%,與春季相比,夏季明顯提升了11.4%,結(jié)合夏季TC和WSOC與nss-K+相關(guān)性較弱,WSOC/TC比值高于春季,表明夏季TC可能主要來(lái)源于生物質(zhì)燃燒、生物揮發(fā)和機(jī)動(dòng)車排放等前體污染物經(jīng)過(guò)光化學(xué)反應(yīng)所生成的二次污染物.

3 結(jié)論

3.1 玉龍雪山研究期間PM2.5中TC與WOSC濃度分別為(4.4±3.1)和(2.2±1.6)μg/m3,其中春季濃度分別為(7.1±3.8)和(2.9±0.7)μg/m3,夏季分別為(3.3± 2.1)μg/m3和(1.5±0.4)μg/m3.春季TC與WSOC濃度分布范圍較夏季更廣,且春季TC與WSOC濃度均高于夏季,春季TC與WSOC平均濃度分別是夏季的2.4、2.2倍.

3.2 結(jié)合TC、WSOC和nss-K+相關(guān)性、WSOC/TC、后向軌跡及火點(diǎn)圖分析表明,采樣點(diǎn)TC、WSOC濃度呈現(xiàn)春高夏低的季節(jié)變化趨勢(shì),可能受到不同季節(jié)氣象因素和來(lái)源變化影響.春季東南亞地區(qū)火點(diǎn)密集,與nss-K+顯著相關(guān),表明TC可能主要直接來(lái)源于森林大火一次排放.夏季光照強(qiáng),溫度高, WSOC/TC比值更高,表明TC可能更多來(lái)源于二次氣溶膠.

3.3 玉龍雪山13CTC值為(-25.6±1.0)‰,春夏季13CTC值分別為(-24.7±1.0)和(-26.0±0.6)‰,春季相較夏季偏正.結(jié)合貝葉斯模型計(jì)算結(jié)果表明春季TC可能主要來(lái)源于生物質(zhì)燃燒、煤燃燒,貢獻(xiàn)比分別為60.6%、23.5%;夏季TC可能主要來(lái)源于生物質(zhì)燃燒、植物釋放和機(jī)動(dòng)車排放等前體污染物經(jīng)過(guò)光化學(xué)反應(yīng)所生成的二次污染物.

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Composition characteristics of chemical and stable carbon isotopes in PM2.5of Yulong Snow Mountain.

XIAO Yang-ning1,2, XIAO Hong-wei1,2*, CHEN Zhen-ping1,2, HUANG Li-lei1,2, MA Yan1,3, LI Zhi-tao1,2, LENG Quan4, JIN Xue-wu4, XIAO Hua-yun5

(1.Jiangxi Province Key Laboratory of the Causes and Control of Atmospheric Pollution, Nanchang 330013, China；2.School of Water Resources and Environmental Engineering, East China University of Technology, Nanchang 330013, China；3.School of Geosciences, East China University of Technology, Nanchang 330013, China；4.Yulong Snow Mountain Provincial Nature Reserve Management and Protection Bureau, Lijiang 674199, China；5.School of Environmental Science and Engineering, Shanghai Jiaotong University, Shanghai 200240, China)., 2022,42(5)：2034～2040

In this study, PM2.5samples were collected in the Yulong Snow Mountain in the southeastern Qinghai-Tibet Plateau from April to August 2020. A total of 44 samples were taken to measure the composition of water-soluble ions, the concentration of water-soluble organic carbon (WSOC), the concentration of total carbon (TC), and alongside the composition of stable carbon isotope (13CTC). As measured, the TC concentrations of Yulong Snow Mountain in spring and summer were (7.1±3.8) μg/m3and (2.9±0.7) μg/m3respectively, and the WSOC concentrations (3.3±2.1) μg/m3and (1.5±0.4) μg/m3, showing a trend as high in spring and low in summer. The13CTCvalues in spring and summer were measured as (-24.7±1.0)‰ and (-26.0±0.6) ‰ respectively, which were more positive in spring than in summer, indicating that the value difference may be affected by different sources. As suggested by analysis of nss-K+correlation, FIRMS of NASA, and backward trajectory, biomass burning in Southeast Asia in spring may serve as the dominant contributor. Besides, the Bayesian model was also utilized to calculate the contribution of TC sources to PM2.5in Yulong Snow Mountain. The results pointed that biomass burning and coal combustion accounted for the main sources in spring, with contribution rates of 60.6% and 23.5% respectively. Whereas in summer, TC mainly comes from biomass burning, plant evaporation, and vehicle emissions, together with the formation of secondary organic aerosol whose contribution cannot be ignored.

Yulong Snow Mountain；TC；WSOC；stable carbon isotope；source analysis

X513

A

1000-6923(2022)05-2034-07

肖揚(yáng)寧(1997-),男,貴州天柱人,碩士研究生,從事大氣地球化學(xué)研究.發(fā)表論文1篇.

2021-10-21

國(guó)家自然科學(xué)基金資助項(xiàng)目(42063001,41663003);江西省大氣污染成因與控制重點(diǎn)實(shí)驗(yàn)室開放基金資助項(xiàng)目(AE2108)

* 責(zé)任作者, 研究員, xiaohw@ecit.cn