深圳市冬季PM2.5中水溶性有機(jī)物的來源特征

高茂尚,李慧穎,李嫣婷,魏 靜,孫逸飛,何凌燕,黃曉鋒

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深圳市冬季PM2.5中水溶性有機(jī)物的來源特征

高茂尚,李慧穎,李嫣婷,魏 靜,孫逸飛,何凌燕,黃曉鋒*

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

利用連續(xù)液化采樣器(PILS)-超聲霧化器-氣溶膠化學(xué)組分檢測儀(ACSM)聯(lián)用系統(tǒng),對深圳市冬季PM2.5中水溶性組分進(jìn)行在線連續(xù)觀測,獲取高時(shí)間分辨率的水溶性有機(jī)物(WSOM),SO42-,NO3-,NH4+和Cl-濃度信息以及WSOM的質(zhì)譜結(jié)構(gòu)信息.分析結(jié)果表明:PM2.5中水溶性組分的總質(zhì)量濃度變化范圍為4.0~117μg/m3,平均濃度為20.1μg/m3,其中WSOM(25.2%)和SO42-(22.4%)是最主要的貢獻(xiàn)組分.ACSM質(zhì)譜顯示W(wǎng)SOM具有氧化態(tài)有機(jī)氣溶膠(OOA)的質(zhì)譜特征,其氧碳比(O/C)的平均值達(dá)到(0.60±0.09),且WSOM與二次無機(jī)離子(SO42-+ NO3-)和鉀(K)有強(qiáng)相關(guān)性,與黑碳(BC)的相關(guān)性較弱,表明了觀測期間WSOM主要來源于二次反應(yīng)產(chǎn)生的二次有機(jī)氣溶膠(SOA)和生物質(zhì)燃燒,與機(jī)動(dòng)車等一次排放沒有明顯關(guān)系.

水溶性有機(jī)物(WSOM)；連續(xù)液化采樣器(PILS)；氣溶膠化學(xué)組分檢測儀(ACSM)；在線分析；二次有機(jī)氣溶膠

水溶性有機(jī)物(WSOM)通常指顆粒物中能夠以水為溶劑提取下來的有機(jī)化合物.WSOM是顆粒物中的重要組成成分,約占細(xì)顆粒物質(zhì)量的10%~ 70%[1].WSOM作為大氣顆粒物的主要組分之一,能夠產(chǎn)生多種顯著的環(huán)境和氣候效應(yīng):它具有較強(qiáng)的吸濕性,可以降低大氣能見度,改變云凝結(jié)核(CCN)的特性;此外,WSOM還可以廣泛地參與含水氣溶膠和云中復(fù)雜甚至未知的液相化學(xué)反應(yīng),從而可能對區(qū)域環(huán)境質(zhì)量和全球大氣化學(xué)循環(huán)轉(zhuǎn)化產(chǎn)生影響[2-3],因此WSOM受到越來越廣泛的關(guān)注.WSOM不是一種或一類化合物,而是由一系列官能團(tuán)不同的復(fù)雜化合物組成.目前,已被解析出來的WSOM組分大致分為5個(gè)類別,分別是多羥基化合物和聚醚,單羧酸與二羧酸,氨基酸,多羧酸和腐殖質(zhì)[4].其測量常采用離線測量方法,將其采集到特氟龍膜上,提取至溶液后,采用不同的手段檢測混合物的具體組分信息.

相比離線分析方法,在線原位測量方法可為研究有機(jī)氣溶膠在大氣中的來源及轉(zhuǎn)化過程提供高時(shí)間分辨率的數(shù)據(jù)信息.Sullivan等[5]通過將連續(xù)液化氣溶膠采樣器(PILS)與總有機(jī)碳分析儀(TOC)聯(lián)用,開發(fā)了第一套PILS-TOC系統(tǒng),并成功應(yīng)用于2003年圣路易斯地區(qū)水溶性有機(jī)碳(WSOC)的測量,得到了WSOC與OC的比值及其變化規(guī)律,并解析了WSOC的來源.Xu等[6]在2013年美國的城市和鄉(xiāng)村點(diǎn)的觀測中,在HR-ToF-AMS的采樣路徑前加了PILS裝置,實(shí)現(xiàn)了基于氣溶膠質(zhì)譜技術(shù)的細(xì)顆粒物中的WSOM在線測量,并利用正交矩陣因子分析模型(PMF)解析WSOM的來源,結(jié)果表明WSOM主要來自于生物質(zhì)燃燒氣溶膠(BBOA)和二次有機(jī)氣溶膠(SOA).Li等[7]在2016年搭建了PILS-超聲霧化器-ACSM(氣溶膠化學(xué)組分檢測儀)聯(lián)用系統(tǒng),ACSM是AMS的一種簡化版本,與AMS原理類似[8],具有運(yùn)行穩(wěn)定、操作簡便的優(yōu)點(diǎn),PILS-超聲霧化器-ACSM實(shí)現(xiàn)了在線連續(xù)測量2016年深圳春季PM2.5中水溶性組分,并提供高時(shí)間分辨率的WSOM,SO42-,NO3-,NH4+及Cl-的定量信息和WSOM的質(zhì)譜特征信息,為研究WSOM的來源與轉(zhuǎn)化機(jī)制提供了更全面的信息支持.

本研究應(yīng)用PILS-超聲霧化器-ACSM聯(lián)用的分析系統(tǒng),對深圳冬季PM2.5中的水溶性組分進(jìn)行在線測量,進(jìn)一步識別珠江三角洲地區(qū)WSOM來源,完善對其轉(zhuǎn)化機(jī)制和影響因素的認(rèn)識.

1 儀器與方法

1.1 觀測點(diǎn)位

本研究觀測點(diǎn)位于深圳大學(xué)城北京大學(xué)深圳研究生院內(nèi)(22°35′28″N,113°58′30″E)(圖1),該點(diǎn)位處于深圳市市區(qū)西部,屬于中度污染水平,能代表一定的城市區(qū)域污染特征.儀器的采樣口架設(shè)于教學(xué)樓四樓樓頂,距地面約25m,周圍主要是水庫和療養(yǎng)基地,植被覆蓋率高,除了距離采樣點(diǎn)約100m的距離內(nèi)有一條柏油馬路,周圍無明顯污染源.觀測時(shí)間為2016年12月9日~2017年1月19日,此時(shí)深圳正值冬季,平均氣溫為18.6℃,平均相對濕度為73%,主導(dǎo)風(fēng)為偏北風(fēng),平均風(fēng)速較小(0.8m/s).

圖1 觀測點(diǎn)位地理位置

1.2 分析方法

本研究利用PILS-超聲霧化器-ACSM聯(lián)用的分析系統(tǒng),在連續(xù)液化氣溶膠采樣器中(PILS),氣溶膠中的水溶性組分被不間斷地提取出來形成連續(xù)的樣品溶液.使用CETAC公司生產(chǎn)的U5000AT+型超聲霧化器(UN),將水溶性提取液通過蠕動(dòng)泵的輸送作用以0.25mL/min的流速送入霧化腔,在超聲震蕩的作用下樣品液滴被碎裂為細(xì)小濃密的含水氣溶膠,使用合成空氣作為系統(tǒng)載氣,將這些含水氣溶膠通過硅膠干燥管和Nafion Dryer后送入ACSM系統(tǒng)進(jìn)行檢測.整個(gè)氣溶膠的霧化和干燥過程均在常溫下進(jìn)行.PILS-超聲霧化器-ACSM系統(tǒng)的樣品采集時(shí)間分辨率為15min,因此能夠得到高時(shí)間分辨率的WSOM,SO42-,NO3-,NH4+及Cl-濃度信息和WSOM的質(zhì)譜特征信息.具體分析方法詳見Li[7].

本研究除了同步應(yīng)用微振蕩天平細(xì)顆粒物測量儀(Taper Element Oscillating Microbalance, TEOM, 1400a, R&P Co., Inc., USA)在線測量大氣PM2.5的濃度,黑碳儀(Aethalometer, AE-31, Magee Sci., USA)在線測量PM2.5中黑碳(BC)的濃度之外,還利用一臺自動(dòng)多金屬檢測儀(Xact 625,先河環(huán)保, 中國)在線監(jiān)測氣溶膠中的生物質(zhì)燃燒示蹤物-鉀的濃度.其中,測量大氣PM2.5濃度的TEOM會定期進(jìn)行流量校準(zhǔn)并與ACSM和黑碳儀的測量結(jié)果進(jìn)行比對,比對結(jié)果具有較高的相關(guān)性(2>0.85). Xact 625通過卷輪式濾紙帶采集樣品,并通過X射線熒光光譜(XRF)技術(shù)對沉積在紙帶濾膜的顆粒物進(jìn)行無損分析.Xact 625完成采樣后,控制器會記錄金屬的濃度數(shù)據(jù)(ng/m3),可在控制器上直接查看濃度數(shù)據(jù).

2 結(jié)果與討論

2.1 水溶性組分的組成與變化趨勢

圖2展示了觀測期間PM2.5及各化學(xué)組分的時(shí)間變化趨勢.可以看出,1月1日~12日,出現(xiàn)了一次長時(shí)間的顯著污染過程;隨后受到了強(qiáng)冷空氣影響, PM2.5中各化學(xué)組分質(zhì)量濃度均呈現(xiàn)顯著下降趨勢.總體來說,深圳冬季溫濕度較低,風(fēng)速較小的氣象條件不利于污染物的擴(kuò)散與清除,容易造成較高的污染.

圖2 深圳市冬季觀測期間水溶性組分的組成與變化趨勢

(a)觀測期間的氣象條件;(b)觀測期間PM2.5的時(shí)間序列;(c)觀測期間水溶性組分和BC的時(shí)間序列;(d)整個(gè)冬季觀測時(shí)期,重污染時(shí)期和強(qiáng)冷空氣時(shí)期污染物各組分平均質(zhì)量濃度組成

整個(gè)觀測期間,水溶性組分的總質(zhì)量濃度變化范圍為4.0~117μg/m3,平均濃度為(20.1±16.7)μg/m3,其中WSOM和SO42-是最主要的貢獻(xiàn)組分,分別占總組分質(zhì)量的25.2%和22.4%,之后依次是NO3-(13.8%),NH4+(14.3%),和Cl-(1.7%).重污染期間水溶性組分的平均質(zhì)量濃度達(dá)到(24.9±21.3)μg/m3,相對整個(gè)觀測期間平均值上升了19.3%.冬季重污染期間NO3-的平均濃度升高達(dá)到50%,增長最為顯著,說明了氮氧化物轉(zhuǎn)化的光化學(xué)反應(yīng)對于冬季顆粒物高污染形成的重要性.深圳受強(qiáng)冷空氣影響期間,大氣的擴(kuò)散條件較好,各組分質(zhì)量濃度均有明顯地下降,水溶性組分的平均質(zhì)量濃度僅(16.9± 8.0)μg/m3,對深圳市冬季的重污染起到良好的緩解作用.

2.2 WSOM的日變化趨勢

圖3展示了各水溶性組分及O/C的日變化趨勢.觀測期間WSOM的日變化趨勢相對平穩(wěn),與SO42-的日變化趨勢相似,體現(xiàn)了區(qū)域傳輸?shù)奶卣鱗10].另外,WSOM的日變化趨勢與以往研究中二次有機(jī)物(SOA)的日變化趨勢也相符合[11-13],說明可能主要來自于二次生成.WSOM的O/C值的日變化趨勢,基本呈現(xiàn)“白天高,夜晚低”的特征.O/C值的日變化趨勢在16:00時(shí)左右達(dá)到峰值,與WSOM質(zhì)量濃度在下午達(dá)到峰值的時(shí)間一致,說明光化學(xué)二次生成對于WSOM的貢獻(xiàn)較大.另外WSOM在早上9:00(早高峰之后1h),出現(xiàn)了一個(gè)較小的峰值, WSOM兩次峰值時(shí)間均與NO3-的峰值時(shí)間相同,說明冬季W(wǎng)SOM中部分是由機(jī)動(dòng)車排放的氣態(tài)前體物經(jīng)大氣光化學(xué)反應(yīng)產(chǎn)生的.黃曉鋒等[14]根據(jù)PMF模型解析結(jié)果得出在深圳2009年秋、冬季采集的PM2.5中SOA的主要來源是機(jī)動(dòng)車排放的氣態(tài)前體物.

2.3 WSOM的平均質(zhì)譜特征

冬季觀測期間WSOM的平均質(zhì)譜如圖4所示.有機(jī)氣溶膠中氧元素和碳元素的物質(zhì)的量比(O/C)是表征有機(jī)氣溶膠老化程度的重要參數(shù),是能夠直接代表大氣有機(jī)氣溶膠氧化態(tài)的表征參數(shù). Canagaratna等[29]通過對AMS高質(zhì)量分辨率質(zhì)譜的識別,得出計(jì)算O/C的擬合公式.以往AMS的研究結(jié)果顯示,不同來源或不同性質(zhì)的有機(jī)氣溶膠的O/C不同.一次有機(jī)氣溶膠的O/C值較低,機(jī)動(dòng)車排放、餐飲源和生物質(zhì)燃燒產(chǎn)生的有機(jī)物的O/C值分別為0.03~ 0.04[30],0.11~0.14[30],0.3~0.45[27-31].Ng等[15]基于全球AMS的監(jiān)測數(shù)據(jù)總結(jié)了OOA的O/C值的范圍(0.3~0.9),平均值為(0.62 ± 0.15).本研究中WSOM的O/C值范圍為0.27~0.92,平均O/C值為(0.60±0.09)屬于OOA的O/C值范圍,說明深圳冬季觀測期間WSOM主要為OOA.

離子碎片/60(C2H4O+)主要來源于生物質(zhì)燃燒產(chǎn)生的左旋葡聚糖等多醇化合物的分子斷裂,是生物質(zhì)燃燒氣溶膠(BBOA)的特征離子碎片[20-27].當(dāng)城市地區(qū)不受生物質(zhì)燃燒影響時(shí),離子碎片/60的60低于0.3%;當(dāng)受生物質(zhì)燃燒影響時(shí),60大于0.3%[23,27-28].而本研究中觀測得到的60均在0.3%左右./60主要由生物質(zhì)燃燒中纖維素分解形成的左旋葡聚糖等糖類水溶性物質(zhì)產(chǎn)生,因此BBOA水溶性的不確定性對質(zhì)譜中60的影響并不明顯.以往研究表明,在氣相和水相中,左旋葡聚糖的氧化較為迅速,其快速氧化導(dǎo)致/60的信號快速衰減,使BBOA的質(zhì)譜失去其特征標(biāo)記[32-34].Bougiatioti等[35]在地中海的觀測結(jié)果表明, BBOA能夠在不到一天的時(shí)間內(nèi)迅速轉(zhuǎn)換為OOA,導(dǎo)致60不斷下降至可忽略水平.本研究觀測得到的60均在0.3%左右,這一結(jié)果也與采樣點(diǎn)所處的城市特征一致,采樣點(diǎn)周邊沒有生物質(zhì)燃燒活動(dòng),生物質(zhì)燃燒主要來自采樣點(diǎn)北方農(nóng)村地區(qū)的污染傳輸.

圖4 深圳冬季觀測期間WSOM平均質(zhì)譜

2.4 相關(guān)性分析

通過對WSOM與源示蹤物進(jìn)行相關(guān)性分析可以進(jìn)一步確定WSOM的來源屬性.圖5展示了觀測期間WSOM與各組分的相關(guān)性比對和時(shí)間序列比對.BC是化石燃料和生物質(zhì)不完全燃燒的產(chǎn)物,是機(jī)動(dòng)車等一次排放產(chǎn)物的示蹤物.冬季W(wǎng)SOM與BC相關(guān)性較弱(2=0.27),但與無機(jī)二次離子組分具有很高的相關(guān)性(2=0.76),說明在冬季采集得到的WSOM受大氣二次反應(yīng)作用較大,而與機(jī)動(dòng)車等一次排放無明顯相關(guān)性.K+是生物質(zhì)燃燒的指示物[36],可用于大氣顆粒物中的生物質(zhì)燃燒排放貢獻(xiàn)的識別.觀測期間,并沒有K+同步在線測量數(shù)據(jù),但自動(dòng)多金屬檢測儀在線測得的K元素濃度與通過離子色譜測量同一采樣點(diǎn)的24h膜樣品K+的濃度高度相關(guān)(2=0.82)(圖6),結(jié)合考慮鉀鹽易溶,因此可以用K元素作為生物質(zhì)燃燒產(chǎn)生氣溶膠的替代示蹤物.冬季采集的WSOM與K具有較好的相關(guān)性(2=0.54),說明在冬季采集得到的WSOM同樣受到生物質(zhì)燃燒的影響,這也與黃曉鋒等[14]關(guān)于珠江三角洲地區(qū)生物質(zhì)燃燒是細(xì)粒子中有機(jī)物或WSOM的一個(gè)重要來源的研究結(jié)果一致.

圖5 WSOM與源示蹤物的時(shí)間序列和相關(guān)性分析

(a)WSOM與BC的時(shí)間序列及其相關(guān)性比對;(b)WSOM與鉀的時(shí)間序列及其相關(guān)性比對;(c)WSOM與二次無機(jī)離子的時(shí)間序列及其相關(guān)性比對

圖6 深圳冬季觀測期間K+與K的相關(guān)性比對

3 結(jié)論

3.1 觀測得到水溶性組分的總質(zhì)量濃度變化范圍為4.0~117μg/m3,平均濃度為(20.1±16.7)μg/m3,其中WSOM和SO42-是最主要的貢獻(xiàn)組分,分別占總組分質(zhì)量的25.2%和22.4%.重污染期間NO3-的平均濃度增長最為顯著,體現(xiàn)了氮氧化物的光化學(xué)反應(yīng)對形成高污染的重要性.

3.2 觀測期間WSOM的日變化趨勢相對平穩(wěn),總體體現(xiàn)了區(qū)域傳輸?shù)奶卣?而WSOM的峰值時(shí)間說明冬季機(jī)動(dòng)車排放的氣態(tài)前體物大氣光化學(xué)反應(yīng)對WSOM具有顯著影響.

3.3 觀測期間WSOM的平均O/C值為(0.60 ± 0.09),屬于OOA的O/C值范圍,說明深圳冬季觀測期間采集得到的WSOM主要為OOA.

3.4 相關(guān)性分析表明,WSOM與二次無機(jī)離子(SO42-+ NO3-)和生物質(zhì)氣溶膠的指示物-鉀(K)有強(qiáng)相關(guān)性,而與BC的相關(guān)性很弱,表明了觀測期間WSOM主要來源于二次有機(jī)氣溶膠和生物質(zhì)燃燒.

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Source characteristics of water-soluble organic matters in PM2.5in the winter of Shenzhen.

GAO Mao-shang, LI Hui-ying, LI Yan-ting, WEI Jing, SUN Yi-fei, HE Ling-yan, HUANG Xiao-feng*

(Key Laboratory of Urban Human Residential Environmental Science and Technology, Shenzhen Graduate School, Peking University, Shenzhen 518055, China)., 2018,38(11)：4017~4022

In this study, a combined system of particle-into-liquid sampler (PILS)-Nebulizer-aerosol chemical speciation monitor (ACSM) was applied in the on-line measurement of water-soluble components in PM2.5in winter in Shenzhen. High temporal resolution concentrations of WSOM, sulfate, nitrate, ammonium and chloride and their mass spectra were simultaneously obtained. The mass concentration of the water-soluble species in PM2.5ranged from 4.0~117μg/m3, with an average of 20.1μg/m3, and sulfate and WSOM were the most abundant components. The characteristics of the mass spectrum of WSOM was similar to that of oxygenated organic aerosols, and the average O/C was 0.60±0.09. We found that WSOM strongly correlated with secondary inorganic ions (SO42-+NO3-) and potassium (K), while the correlation with black carbon (BC) weakened. All evidence showed that the contribution of secondary organic aerosol (SOA) and biomass burning to WSOM was much greater than local primary emission in winter of Shenzhen.

water-soluble components (WSOM)；particle-into-liquid sampler (PILS)；aerosol chemical speciation monitor (ACSM)；on-line measurement；SOA

X513

A

1000-6923(2018)11-4017-06

高茂尚(1993-),男,安徽宿州人,碩士,主要從事PM2.5中水溶性有機(jī)物的研究.

2018-04-22

國家自然科學(xué)基金資助(91744202);深圳市科技計(jì)劃(JCYJ20170412150626172,JCYJ20160122105855253)

* 責(zé)任作者, 教授, huangxf@pku.eud.cn