崔 杰,黃曉鋒,袁金鳳,曹禮明,王 川,蘭紫娟,何凌燕

(北京大學(xué)深圳研究生院,環(huán)境與能源學(xué)院,城市人居環(huán)境科學(xué)與技術(shù)實(shí)驗(yàn)室,廣東 深圳 518055)

基于在線觀測的大氣PM2.5中棕色碳吸光貢獻(xiàn)估算

崔 杰,黃曉鋒*,袁金鳳,曹禮明,王 川,蘭紫娟,何凌燕

(北京大學(xué)深圳研究生院,環(huán)境與能源學(xué)院,城市人居環(huán)境科學(xué)與技術(shù)實(shí)驗(yàn)室,廣東 深圳 518055)

使用三波長光聲黑碳光度計(jì)(PASS-3)和氣溶膠質(zhì)譜儀(AMS),于2011年秋季在浙江城市點(diǎn)金華和2014年夏季河北區(qū)域點(diǎn)望都進(jìn)行了大氣在線觀測,采用改進(jìn)的光吸收Angstrom指數(shù)(AAE)的方法統(tǒng)計(jì)外推黑碳AAE值,估算這2個(gè)典型地區(qū)PM2.5中棕色碳的吸光貢獻(xiàn).結(jié)果表明:經(jīng)統(tǒng)計(jì)外推獲得的金華和望都的黑碳AAE在405nm下的基準(zhǔn)值分別是1.12和0.77,532nm下的基準(zhǔn)值分別是0.87和0.63;進(jìn)而估算金華和望都PM2.5中的棕色碳在405nm下的吸光貢獻(xiàn)分別為9.8%和22.3%;532nm下的吸光貢獻(xiàn)分別為5.9%和15.2%,表明我國大氣中氣溶膠棕色碳在短波段下的吸光貢獻(xiàn)和輻射強(qiáng)迫影響不可忽視.

棕色碳(BrC)；黑碳(BC)；光吸收系數(shù)；光吸收Angstorm指數(shù)(AAE)

吸光性碳質(zhì)氣溶膠包括黑碳(BC)和棕色碳(BrC),是大氣PM2.5中主要的吸光物質(zhì).近年來,棕色碳對氣候變化的輻射強(qiáng)迫效應(yīng)逐漸受到重視[1-4].棕色碳為吸光性有機(jī)碳,主要來源有生物質(zhì)及生物燃料的燃燒、大氣環(huán)境中的類腐殖酸和揮發(fā)性有機(jī)物的光化學(xué)反應(yīng)[5-7],其對光的吸收從長波段到紫外波段呈現(xiàn)明顯的增加趨勢[8-9].

近期全球模型研究中,發(fā)現(xiàn)美國、歐洲和東亞的棕色碳在440nm下對總氣溶膠吸收光學(xué)厚度貢獻(xiàn)分別占到25%、20% 和15%[10].美國南加州夏季外場觀測的數(shù)據(jù)顯示棕色碳在短波段440nm下的吸收占到了顆粒物總吸收的40%,而在中波段675nm下的吸收僅占到總吸收的10%[11].我國有利用沉積膜方法對北京地區(qū)黑碳?xì)馊苣z的質(zhì)量吸收效率及有機(jī)碳的吸光性進(jìn)行研究[12],膜沉積采集到的有機(jī)組分在632nm波長下的吸光率高達(dá)1.4×104cm2/g,顯然棕色碳的存在影響了膜帶中黑碳的測量.我國對于棕色碳吸收譜特征的研究還遠(yuǎn)遠(yuǎn)不夠,所以評估我國棕色碳吸光貢獻(xiàn)對改善氣候模型有著重要的意義.

目前有兩種估算棕色碳吸光貢獻(xiàn)的方法:一種是基于實(shí)際大氣的光學(xué)和化學(xué)觀測結(jié)果,結(jié)合米散射模型進(jìn)行復(fù)雜的計(jì)算[13];一種是基于光學(xué)觀測結(jié)果,結(jié)合吸收Angstrom指數(shù)(AAE)的方法,根據(jù)黑碳AAE基準(zhǔn)值(純黑碳吸收時(shí)的AAE值,AAE_BC)推算黑碳的吸收系數(shù),從而獲得棕色碳的吸收貢獻(xiàn)[14-16],第二種方法因其考量因素少從而比較常見.然而,以往研究運(yùn)用AAE方法時(shí),通常不區(qū)分實(shí)際采樣地點(diǎn)和季節(jié)對AAE的影響,僅從理論上將AAE_BC取為1.但在實(shí)際大氣環(huán)境中,AAE_BC值因各種因素常不為1,將AAE_BC取為1雖然計(jì)算簡便,但會(huì)帶來較大的誤差.之前有研究表明環(huán)境AAE值受OC/EC(有機(jī)碳/元素碳)影響較大,暗示有機(jī)碳在氣溶膠吸光作用中的重要作用[17].

金華市處于浙江省中部,可代表長三角城市大氣情況.望都縣地處北京、天津、石家莊三角地帶,觀測點(diǎn)位于北京的上風(fēng)向,一定程度上可以反映京津冀大氣污染的區(qū)域性特征.本研究采用三波長光聲黑碳光度計(jì)(PASS-3)和氣溶膠質(zhì)譜(AMS),于2011年金華秋季和2014年望都夏季進(jìn)行了大氣細(xì)粒子高時(shí)間分辨率在線測量,采用改進(jìn)的AAE方法,評估我國不同大氣環(huán)境下PM2.5中棕色碳的吸光貢獻(xiàn).

1 儀器和方法

1.1 觀測點(diǎn)設(shè)置

金華觀測點(diǎn)位于金華市東面金東區(qū)環(huán)保大樓(29.1°N, 119.7°E),站點(diǎn)周邊空曠,無顯著的工業(yè)源.距離觀測大樓南面200m左右為區(qū)主干道,測量儀器放置于大樓7樓實(shí)驗(yàn)室,PM2.5采樣頭架設(shè)于樓頂距離地面1.5m左右(距地面25m)處.

望都觀測點(diǎn)位于保定市交通局綠化基地內(nèi)(38.7°N, 115.2°E),周圍是麥地和樹木,無顯著的工業(yè)污染源,PM2.5采樣頭架設(shè)在二層集裝箱頂,距離集裝箱頂部約2m(距離地面約10m).觀測期間,當(dāng)?shù)丶爸苓吙h城有小麥?zhǔn)崭?偶有生物質(zhì)燃燒事件.

1.2 觀測儀器

觀測采用三波長光聲黑碳光譜儀(PASS-3)是由美國DMT公司生產(chǎn)制造的在線原位測量氣溶膠吸收系數(shù)的儀器,測量波長分別為405,532和781nm.PASS-3測量范圍＜8000Mm-1,測量精度＜10%,時(shí)間分辨率為2s,采樣流量為1L/min.三波長下檢測限分別為10.0Mm-1@405nm、10.0Mm-1@532nm和3.0Mm-1@781nm.本研究還利用高分辨率飛行時(shí)間氣溶膠質(zhì)譜AMS,該儀器能實(shí)現(xiàn)測量亞微米級(jí)顆粒物化學(xué)組成和粒徑分布[18].使用該儀器與PASS同步觀測,獲得有機(jī)物的質(zhì)量濃度信息.

2 結(jié)果與討論

2.1 顆粒物吸收系數(shù)

圖1給出了2011年金華秋季和2014年望都夏季觀測期間三波長吸收系數(shù)及有機(jī)物質(zhì)量濃度值時(shí)間序列.金華觀測的時(shí)間為2011年10月30～11月29日,為期30d.整個(gè)觀測期間環(huán)境平均溫度為15.9℃,平均相對濕度為80.8%,降雨少.期間以東南風(fēng)為主.顆粒物在405,532,781nm波段下的平均吸收系數(shù),分別為65.85,42.28,28.48Mm-1.有機(jī)物的質(zhì)量濃度的平均值為14.43μg/m3.

圖1 2011年金華秋季和2014年望都夏季觀測期間三波長吸收系數(shù)及有機(jī)物質(zhì)量濃度值時(shí)間序列Fig.1 The time series of aerosol light absorption and mass concentration of organic aerosol in Jinhua and Wangdu

望都觀測的時(shí)間為2014年6月4～7月4日,為期31d.整個(gè)觀測期間環(huán)境平均溫度為26.1℃,平均相對濕度為59.6%,在后期有幾天連續(xù)的降水,觀測期間主要受來自東南的氣團(tuán)影響.顆粒物在各波段的吸收系數(shù)分別為27.95、19.68和13.10Mm-1.有機(jī)物的質(zhì)量濃度均值為12.02μg/m3.

金華的顆粒物吸收系數(shù)顯著高于望都點(diǎn),約為2倍.金華作為長三角的城市點(diǎn),受本地一次排放與二次生成影響很大,污染物濃度水平較高,化石燃料的燃燒對吸光物質(zhì)黑碳排放的貢獻(xiàn)很大.望都作為區(qū)域背景點(diǎn),除了受周邊生物質(zhì)燃燒和交通排放源影響外,很大程度上受到來自京津冀區(qū)域傳輸?shù)奈廴疚镉绊?

2.2 AAE_BC的確定

吸收Angstrom指數(shù)AAE,是Angstom指數(shù)在光吸收方面的進(jìn)一步應(yīng)用[19],被用來表征吸光物質(zhì)的光譜依賴性,定義如下:

式中:Abs是波長λ下的吸收系數(shù)(Mm-1).傳統(tǒng)的用AAE方法計(jì)算棕色碳貢獻(xiàn)可以被簡單地表述為如下公式[20].

式中:Absλ1和Absλ2分別代表短波段和長波段下的吸收系數(shù)(Mm-1), AAE_BC代表純黑碳?xì)馊苣z吸收的AAE值.BC_Absλ1代表短波段下黑碳的吸收系數(shù)(Mm-1),由AAE_BC和Absλ2推算得到.短波段下總的吸收系數(shù)減去黑碳的吸收系數(shù)即為該波段下棕色碳的吸收系數(shù)BrC_Absλ1(Mm-1).

本研究通過關(guān)注棕色碳與黑碳的相對量對環(huán)境AAE值的影響,利用觀測點(diǎn)位顆粒物的有機(jī)物質(zhì)量濃度和781nm下的光吸收系數(shù),統(tǒng)計(jì)外推符合當(dāng)?shù)禺?dāng)季實(shí)際情況的AAE_BC.公式如下:

式中:Morg表示有機(jī)物的質(zhì)量濃度(μg/m3),由AMS測得; r_org/bc代表有機(jī)物質(zhì)量濃度與黑碳吸光的比值,這一參數(shù)可用來表征OC/EC.

圖2是金華和望都觀測期間405～781nm和532～781nm波段下AAE與r_org/bc的比值的線性相關(guān)分析.R2均在0.90～0.96之間,說明AAE405_781和AAE532_781與r_org/bc線性相關(guān)顯著,AAE與包含棕色碳在內(nèi)的有機(jī)物相對濃度存在正相關(guān).當(dāng)r_org/bc的值取0時(shí),此時(shí)為無棕色碳存在的假設(shè)狀態(tài),吸光物質(zhì)只有純黑碳?xì)馊苣z,擬合直線的截距即為AAE_BC,袁金鳳已采用此方法對珠三角地區(qū)棕色碳的吸光貢獻(xiàn)進(jìn)行了分析研究[22].

金華和望都的AAE_BC在405nm下分別是1.12和0.77,532nm下分別是0.87和0.63,可以看出金華點(diǎn)的AAE_BC值顯著高于望都點(diǎn),暗示金華點(diǎn)黑碳的波譜依賴性較強(qiáng),這與其黑碳?xì)馊苣z的具體來源、粒徑分布特征和混合狀態(tài)有關(guān)[23-25].

圖2 2011年金華秋季和2014望都夏季AAE值隨有機(jī)物/黑碳比值線性相關(guān)Fig.2 The linear relationship between AAE and r_org/bc of organic aerosol in Jinhua and Wangdu

2.3 棕色碳吸光貢獻(xiàn)

依據(jù)公式(4)、(5)計(jì)算得到的AAE_BC和儀器測得的781nm下的光吸收系數(shù),可以通過公式(2)、(3)計(jì)算得出棕色碳在405和532nm下的吸光系數(shù),最終求出棕色碳的吸光貢獻(xiàn).

圖3 2011年金華秋季和2014望都夏季棕色碳吸光系數(shù)的估算Fig.3 Estimation of BrC light absorption in Jinhua and W angdu

圖3直觀地展現(xiàn)了金華秋季和望都夏季觀測期間顆粒物在各波段的平均吸收系數(shù)以及由AAE_BC外推的黑碳吸收系數(shù),兩條線之間的垂直距離就是某波段下棕色碳的吸收系數(shù).金華觀測期間,405nm下棕色碳的吸光貢獻(xiàn)為9.8%, 532nm下棕色碳的吸光貢獻(xiàn)為5.9%.望都夏季觀測期間,405nm下棕色碳的吸光貢獻(xiàn)高達(dá)22.3%, 532nm下棕色碳的吸光貢獻(xiàn)為15.2%.可以看出金華的棕色碳吸光貢獻(xiàn)顯著小于望都,這可能與長三角地區(qū)的金華城市點(diǎn)在該季節(jié)的主要排放源有關(guān),其一次源主要為化石燃料的燃燒,遠(yuǎn)不如生物質(zhì)燃燒過程產(chǎn)生的棕色碳多[8].望都點(diǎn)受到京津冀的區(qū)域傳輸和本地污染的雙重影響,觀測期間正是京津冀地區(qū)夏季的麥?zhǔn)占竟?jié),生物質(zhì)燃燒對PM2.5中有機(jī)碳的貢獻(xiàn)可達(dá)30%以上[26]. Cheng等[27]和Du等[28]指出京津冀地區(qū)的棕色碳主要來源于生物質(zhì)燃燒的一次排放.所以,可推斷望都觀測中的棕色碳受到了生物質(zhì)燃燒一次排放的顯著影響.

3 結(jié)論

3.1 觀測期間,金華城市點(diǎn)的顆粒物在各波段下的平均吸收系數(shù)分別為65.85,42.28, 28.48Mm-1.而望都區(qū)域點(diǎn)的顆粒物在各波段下的平均吸收系數(shù)分別為27.95,19.68,13.10Mm-1.金華城市點(diǎn)受較強(qiáng)的本地污染物排放的影響,顆粒物消光系數(shù)明顯大于望都背景點(diǎn).

3.2 利用觀測點(diǎn)位顆粒物的有機(jī)物質(zhì)量濃度和781nm下的光吸收系數(shù)的比值,經(jīng)統(tǒng)計(jì)外推獲得的金華秋季和望都夏季的AAE_BC(405nm)分別是1.12和0.77,AAE_BC(532nm)分別是0.87和0.63.

3.3 金華和望都PM2.5中的棕色碳在405nm下的吸光貢獻(xiàn)分別為9.8%和22.3%,在532nm下的吸光貢獻(xiàn)分別為5.9%和15.2%.金華的棕色碳吸光貢獻(xiàn)顯著小于望都,望都點(diǎn)受到了周邊生物質(zhì)燃燒產(chǎn)生的棕色碳的影響.

3.4 研究結(jié)果說明,我國大氣中黑碳雖然在顆粒物吸光中發(fā)揮著主導(dǎo)作用,但棕色碳在短波段下的吸光貢獻(xiàn)不容忽視,值得在更多的時(shí)空條件下進(jìn)行分析研究.

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Estimation of light absorption by brown carbon in PM2.5based on on-line measurement.

CUI Jie, HUANG Xiao-feng*, YUAN Jin-feng, CAO Li-ming, WANG Chuan, LAN Zi-juan, HE Ling-yan
(Key Laboratory for Urban Habitat Environmental Science and Technology, School of Environment and Energy, Peking University Shenzhen Graduate School, Shenzhen 518055, China). China Environmental Science, 2017,37(2)：401～406

Two field campaigns were conducted using a three-wavelength photo-acoustic soot spectrometer (PASS-3) and aerosol mass spectrometers (AMS), including urban site Jinhua during autumn 2011 and rural site Wangdu during summer 2014 in this study. Light absorption of brown carbon with improved Absorption AngstromExponent (AAE) method was evaluated by extrapolating the more realistic AAE by “pure” BC (AAE_BC). The AAE_BCwere found to be 1.12, 0.77 between 405nmand 781nm, and 0.87, 0.63 between 532nmand 781nmin the campaign of Jinhua and Wangdu, respectively. The contributions of BrC aerosol light absorption of PM2.5at 405nmwere 9.8% and 22.3%, and 5.9% and 15.2% at 532nmin Jinhua and Wangdu, respectively. The results indicate that the contribution of brown carbon to the aerosol light absorption at shorter wavelength is not negligible in China.

brown carbon (BrC)；black carbon (BC)；light absorption；absorption angstromexponent (AAE)

X513

A

1000-6923(2017)02-0401-06

崔 杰(1990-),男,山東東營人,北京大學(xué)深圳研究生院碩士研究生,主要從事碳質(zhì)氣溶膠吸光性研究.

2016-03-28

國家“973”項(xiàng)目(2013CB228503);國家自然科學(xué)基金項(xiàng)目(21277003)

* 責(zé)任作者, 教授, huangxf@pku.edu.cn