魏林通,曹禮明,魏 靜,何凌燕,黃曉鋒

中國望都地區(qū)夏季大氣氣溶膠揮發(fā)性特征

魏林通,曹禮明,魏 靜,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)

采用熱擴(kuò)散管與氣溶膠質(zhì)譜聯(lián)用系統(tǒng)對(duì)2014年夏季河北望都鄉(xiāng)村點(diǎn)位亞微米級(jí)氣溶膠進(jìn)行在線測量,獲取了兩段污染過程的氣溶膠化學(xué)組成及揮發(fā)性特征:相對(duì)低污染期氣溶膠平均質(zhì)量濃度為(23.3±15.1)μg/m3,有機(jī)物占主導(dǎo),主要受偏北方向氣團(tuán)影響;重污染期平均濃度為(86.6±19.7)μg/m3,硫酸鹽占主導(dǎo),受偏南方向氣團(tuán)影響;主要化學(xué)組分揮發(fā)性順序均為硝酸鹽>氯鹽>銨鹽>有機(jī)物>硫酸鹽;與相對(duì)低污染期相比,重污染期的硫酸鹽對(duì)質(zhì)量濃度貢獻(xiàn)更高且揮發(fā)性降低,而硝酸鹽表現(xiàn)出更高的揮發(fā)性;對(duì)有機(jī)氣溶膠而言,重污染期有機(jī)物氧化態(tài)更高且揮發(fā)性更低,老化特征明顯.氣溶膠半揮發(fā)性特征反映了華北夏季高污染條件下區(qū)域傳輸?shù)闹匾饔?

氣溶膠揮發(fā)性；熱擴(kuò)散管；TD-AMS；有機(jī)氣溶膠

大氣氣溶膠是我國大氣污染的主要因素之一,對(duì)人體健康、大氣能見度、大氣化學(xué)和輻射平衡、全球氣候變化等具有重要影響[1-3].目前國內(nèi)研究多集中在利用離線、在線方法對(duì)氣溶膠理化特性時(shí)空分布等進(jìn)行觀測[4-5].華北地區(qū)大氣氣溶膠污染也是嚴(yán)重的環(huán)境問題之一[4,6-7].半揮發(fā)性是大氣氣溶膠重要的理化性質(zhì)之一,決定氣溶膠中各化學(xué)組分在顆粒相和氣相中的分配,并進(jìn)一步影響氣溶膠在大氣環(huán)境中的濃度、反應(yīng)速率和停留時(shí)間等[8-10],主要取決于顆粒物化學(xué)組成、粒徑、濃度和混合態(tài)特征等[11-13].當(dāng)氣溶膠樣品被加熱或被干凈空氣稀釋,易揮發(fā)性物質(zhì)會(huì)從顆粒相進(jìn)入氣相,低揮發(fā)性物質(zhì)則仍留在顆粒相中[14],導(dǎo)致濃度和粒徑等性質(zhì)發(fā)生變化[15].氣溶膠揮發(fā)性也可反映大氣演化過程,對(duì)研究二次氣溶膠的形成機(jī)制有一定的參考作用[16-18],揮發(fā)特性間接可反映顆粒物對(duì)人體的健康效應(yīng)[3].

近些年,有機(jī)氣溶膠揮發(fā)性的測量得到了重視,展開了很多外場觀測和實(shí)驗(yàn)室測量[19-21]. 目前揮發(fā)性測量常用方法是通過熱擴(kuò)散管對(duì)顆粒物進(jìn)行熱分餾,將其與不同儀器聯(lián)用,如揮發(fā)性串聯(lián)差分電遷移率分析儀(VTDMA),TD-AMS聯(lián)用方法等獲取粒徑組成、化學(xué)組分及揮發(fā)性等信息[2,22-25],探究二次氣溶膠生成和老化過程[16].目前我國相關(guān)觀測結(jié)果較少,對(duì)于不同組分半揮發(fā)性特征尚未有較為清晰的研究結(jié)果[26-27].

本研究利用TD-AMS聯(lián)用系統(tǒng)于2014年對(duì)望都夏季亞微米級(jí)氣溶膠化學(xué)組分進(jìn)行了高分辨率在線測量,對(duì)不同污染水平下化學(xué)組成特征及氣溶膠半揮發(fā)性特征進(jìn)行評(píng)估,結(jié)合氣象條件等因素進(jìn)一步探究我國華北地區(qū)大氣污染水平及成因,為改善華北地區(qū)空氣質(zhì)量提供數(shù)據(jù)支撐,并可以為模型研究提供數(shù)據(jù)參考.

1 儀器與方法

1.1 觀測點(diǎn)位與時(shí)間

望都點(diǎn)位(38.7°N, 115.2°E)設(shè)于中國河北保定望都縣交通局綠化基地內(nèi),處于北京上風(fēng)向,地處京津冀三角地帶,周圍無顯著工業(yè)污染源,屬于典型鄉(xiāng)村點(diǎn)位,可反映華北地區(qū)大氣污染區(qū)域性特征.PM2.5采樣頭距離集裝箱頂部約2m,距離地面約10m.

本研究對(duì)2014年河北望都夏季亞微米級(jí)氣溶膠進(jìn)行了高分辨率在線測量,由于野外電力保障不足等,共獲取了2014年望都夏季觀測期間6月6日~6月9日(相對(duì)低污染期,ep1)和7月3日~ 7月8日(重污染期,ep2)兩段時(shí)期的有效數(shù)據(jù),表1給出了ep1和ep2的氣象參數(shù)、氣態(tài)污染物濃度的平均情況.

表1 觀測期間氣象參數(shù)與氣態(tài)污染物濃度

1.2 TD-AMS聯(lián)用技術(shù)及數(shù)據(jù)分析方法

本研究聯(lián)用美國Aerodyne公司的熱擴(kuò)散管(TD)和高分辨率飛行時(shí)間氣溶膠質(zhì)譜儀(HR-ToF-AMS)獲取亞微米級(jí)顆粒物的化學(xué)組成和揮發(fā)性特征.應(yīng)用Magee公司黑碳儀(Aethalometer,AE-31)獲取黑碳?xì)馊苣z質(zhì)量濃度,黑碳儀采用PM2.5切割頭,根據(jù)Lan等[28]研究PM1中BC質(zhì)量濃度可能偏高約20%.

氣溶膠經(jīng)TD進(jìn)AMS,TD內(nèi)流量0.45L/min,加熱管中停留時(shí)間約為27.9s,HR-ToF-AMS主要用來在線測量真空動(dòng)力學(xué)粒徑小于1μm的非難熔亞微米級(jí)顆粒物(NR- PM1),包括有機(jī)物、SO42-、NO3-、NH4+、Cl-等.在觀測開始時(shí),對(duì)AMS的進(jìn)樣流量、電離效率和粒徑進(jìn)行標(biāo)定.通過SQUIRREL、PIKA軟件分析,并經(jīng)過流量、電離效率、采集效率校正,得到各化學(xué)組分的質(zhì)量濃度,以及有機(jī)物的質(zhì)譜信息[29],其中采集效率根據(jù)文獻(xiàn)[30]中基于化學(xué)組成的方法進(jìn)行計(jì)算.TD由加熱管和擴(kuò)散管組成,切換閥可在加熱氣路(TD)和旁路(BP)中交替,加熱氣路在50,100,150,200℃4個(gè)溫度梯度中循環(huán)變化[27,31-32]. AMS的V模式靈敏度高,用于定量測量;W模式分辨率更高,獲取高分辨率質(zhì)譜數(shù)據(jù),將AMS設(shè)為4個(gè)采樣模式,分別為BPV、TDV、TDW、BPW,經(jīng)傳輸效率校正分析氣溶膠的化學(xué)組分及揮發(fā)性特征[31].揮發(fā)性特征以加熱氣路與旁路質(zhì)量濃度之比的質(zhì)量剩余分?jǐn)?shù)(MFR)進(jìn)行半定量描述[20,22], MFR越高,揮發(fā)性越低.

2 結(jié)果與討論

2.1 環(huán)境氣溶膠濃度水平及化學(xué)組成特征

圖1顯示ep1和ep2的PM1(此處表示NR-PM1與黑碳的總和,下同)平均濃度分別為(23.3±15.1) μg/m3和(86.6± 19.7) μg/m3,各化學(xué)組分占比及質(zhì)量濃度如表2所示,ep1有機(jī)物占主導(dǎo),占重構(gòu)PM1總質(zhì)量濃度的38.5%,ep2硫酸鹽占主導(dǎo),占38.1%.Hu[33]利用AMS觀測北京夏季PM1平均質(zhì)量濃度為(84.0±47.4)μg/m3,與本觀測ep2結(jié)果相近.

圖1 2014年望都觀測期間氣溶膠化學(xué)組分時(shí)間序列

研究表明O/C值是反映有機(jī)物老化程度的重要參數(shù)[16,34].環(huán)境溫度下ep1和ep2的O/C值分別為(0.51 ± 0.09)和(0.63 ± 0.05),與背景點(diǎn)洪澤湖春季結(jié)果0.53及北京夏季觀測結(jié)果(0.56 ± 0.1)接近,高于北京冬季觀測結(jié)果(0.32 ± 0.07)[33,35],說明望都夏季有機(jī)物整體氧化態(tài)較高,主要來自于老化的污染物;在污染程度更高的ep2,O/C值也更高,因此ep2有機(jī)物氧化程度更高,可以說明ep2老化程度較高的有機(jī)氣溶膠占主導(dǎo),體現(xiàn)了一定程度的區(qū)域傳輸特征,ep1受到相對(duì)較輕的影響.

此外,利用Hysplit軟件分別對(duì)兩段時(shí)間內(nèi)的氣團(tuán)軌跡進(jìn)行分析,兩段時(shí)期的反向軌跡如圖2所示,ep1的氣團(tuán)主要來自于望都附近地區(qū)(反向軌跡路徑較短)以及西北方向即河北北部及遠(yuǎn)處的內(nèi)蒙地區(qū),ep2的氣團(tuán)主要來自于望都偏南向的河北南部、山東.結(jié)合化學(xué)組成特征,ep2的二次組分占比更高(ep1和ep2硫酸鹽占PM1分別為17.8%和38.1%),且污染程度顯著加劇,說明來自于南部的區(qū)域傳輸對(duì)ep2的污染過程有決定性影響.

表2 望都夏季觀測期間PM1化學(xué)組分質(zhì)量濃度

2.2 大氣氣溶膠揮發(fā)性特征分析

如圖3所示,ep1和ep2各化學(xué)組分經(jīng)過加熱后都表現(xiàn)出不同程度的揮發(fā)性特征,但MFR隨溫度變化特征卻表現(xiàn)出一定的相似性.其中,NO3-、Cl-揮發(fā)性最高,從環(huán)境溫度加熱至50℃時(shí),MFR值顯著降低,ep1和ep2的NO3-的MFR值分別為0.45和0.27,Cl-的MFR值分別為0.58和0.30,約50%的NO3-、Cl-受熱揮發(fā)進(jìn)入氣相.

圖2 觀測期間兩段污染過程的反向軌跡圖

隨溫度增加,MFR繼續(xù)下降并漸趨平緩,加熱到200℃時(shí),ep1和ep2NO3-的MFR值分別為0.21和0.09,Cl-的MFR值分別為0.19和0.11.SO42-揮發(fā)性最低,加熱到50℃時(shí)MFR仍高于0.8,從100℃升高到150℃后,SO42-的MFR才顯著降低,當(dāng)加熱至200℃時(shí),MFR約為0.1~0.2.由于NH4+在顆粒物中主要是以(NH4)2SO4、NH4NO3和NH4Cl的形式存在,因此其MFR是SO42-和NO3-、Cl-綜合作用的結(jié)果,介于其之間.ep1與ep2不同加熱溫度下SO42-的MFR較為接近,但與ep1相比,ep2的SO42-占比更高,因此在ep2中,NH4+與SO42-揮發(fā)性特征更為相似.此外,有機(jī)物的揮發(fā)性居中,MFR隨溫度升高而平穩(wěn)降低,這是由于有機(jī)物化學(xué)組成較為復(fù)雜,由多種揮發(fā)性不同的有機(jī)物混合而成.這與其他揮發(fā)性觀測結(jié)果相一致[20,31].因此,5種NR-PM1組分的揮發(fā)性順序是硝酸鹽>氯鹽>銨鹽>有機(jī)物>硫酸鹽.較高組分的比例不斷增加,說明具有半揮發(fā)性的有機(jī)物氧化程度較低,間接說明了有機(jī)物的揮發(fā)性隨著老化程度升高而降低,這與VBS模型[36]及以往觀測的結(jié)果相一致[16].

2.3 不同污染程度氣溶膠化學(xué)組分揮發(fā)性特征

2個(gè)觀測時(shí)段的亞微米級(jí)氣溶膠化學(xué)組分的MFR值和質(zhì)量濃度均存在一定相關(guān)性,因此進(jìn)一步探究氣溶膠化學(xué)組分揮發(fā)性特征與自身濃度之間的關(guān)系,可以對(duì)影響揮發(fā)性的相關(guān)因素進(jìn)行判定.圖4是50°C下氣溶膠化學(xué)組分的揮發(fā)性與自身質(zhì)量濃度間的關(guān)系.

ep1和ep2無機(jī)組分濃度有較大差距,尤其SO42-的質(zhì)量濃度存在顯著差異,圖4(b)顯示ep1中SO42-的MFR更低,且較離散,表現(xiàn)出更多新鮮生成的特征,ep2中SO42-揮發(fā)性有所降低,隨著濃度升高M(jìn)FR趨于穩(wěn)定,但ep1與ep2的SO42-濃度水平差距較大,因此本地二次反應(yīng)生成不足以貢獻(xiàn)ep2的高SO42-質(zhì)量濃度水平,反映出望都作為區(qū)域背景點(diǎn),重污染時(shí)期受區(qū)域傳輸影響更為顯著.圖4(c)顯示,隨質(zhì)量濃度增加,NO3-揮發(fā)性先升高后趨于穩(wěn)定,可能是由于高污染時(shí)期,更多新鮮二次NO3-附在氣溶膠表面,在加熱時(shí)更易通過揮發(fā)進(jìn)入氣相中;NH4+的MFR結(jié)果與環(huán)境大氣中的形式有關(guān),Cl-本身濃度較低,因此NH4+、Cl-的結(jié)果較為分散.圖4(a)顯示,對(duì)有機(jī)物而言,ep1和ep2的質(zhì)量濃度水平差異較小,ep1期間隨有機(jī)物質(zhì)量濃度增加,MFR變化較為平穩(wěn),相比之下ep2的MFR相對(duì)更高,在一定程度上說明ep2中老化程度較高的組分占主導(dǎo),更不易揮發(fā),這與SO42-的分析結(jié)果相一致.圖4(e)為NR-PM1的MFR,體現(xiàn)了氣溶膠各化學(xué)組分整體的揮發(fā)性特征,表明ep2顆粒物整體的揮發(fā)性低于ep1,根據(jù)上述分析,NO3-揮發(fā)性最高且隨濃度升高而升高,因而在重污染期,可能是由于顆粒物的有機(jī)物和SO42-組分外層包裹了更多的NO3-,使得NR-PM1整體揮發(fā)性表現(xiàn)出隨濃度升高而降低的特征.

Fig.5Mass-based campaign-averaged size distributions of chemical components of NR-PM1during two episodes

圖5顯示環(huán)境溫度與50℃加熱條件下各組分粒徑分布形狀相近且峰值粒徑基本一致,說明不同粒徑段顆粒物半揮發(fā)性差異不大.與之前研究結(jié)果相同[20,32].ep2各化學(xué)組分峰值粒徑偏大,有機(jī)物和硫酸鹽尤為明顯,從粒徑角度驗(yàn)證兩段污染過程可能具有不同來源,且ep2顆粒物老化程度相對(duì)更高.

2.4 大氣亞微米級(jí)有機(jī)氣溶膠揮發(fā)性水平分析

不同有機(jī)氣溶膠揮發(fā)性也存在差異.由于本次觀測獲取的數(shù)據(jù)量有限無法進(jìn)行氣溶膠質(zhì)譜正矩陣因子(PMF)解析,選取具有代表性的碎片進(jìn)行討論./43(主要離子為C3H7+和C2H3O+)與/44(主要離子為CO2+)可作為氧化態(tài)有機(jī)氣溶膠的特征離子碎片,如圖6所示,/44揮發(fā)性最低,/43揮發(fā)性居中;/57(C4H9+和C3H5O+,以C4H9+為主)和/60(主要離子為C2H4O2+)分別是還原態(tài)有機(jī)氣溶膠和生物質(zhì)燃燒特征碎片,表現(xiàn)出相對(duì)較高揮發(fā)性.有機(jī)物整體揮發(fā)性居中,與前人研究結(jié)果相似[20].污染程度更重的ep2/44的MFR更低,且有機(jī)物半揮發(fā)性更接近于/44,說明與ep1相比,ep2期間有機(jī)物中更多高氧化態(tài)、老化的有機(jī)氣溶膠占主導(dǎo),與2.2中對(duì)O/C值的分析結(jié)果一致,與香河夏季觀測結(jié)果相似[32].表3表明,與ep1相比,ep2的/44增加倍數(shù)遠(yuǎn)高于/57,說明在有機(jī)物中,二次有機(jī)組分增加顯著,而對(duì)無機(jī)組分而言,SNA(SO42-、NO3-和NH4+)增加近5倍,也顯著高于代表一次污染的BC.總體而言,ep2二次有機(jī)和無機(jī)組分增加倍數(shù)均高于ep1,驗(yàn)證了二次污染在重污染期中占主導(dǎo)地位.

表3 兩段污染過程部分有機(jī)碎片及各組分質(zhì)量濃度

3 結(jié)論

3.1 觀測期間,相對(duì)低污染期氣溶膠平均濃度為(23.3±15.1) μg/m3,有機(jī)物占主導(dǎo),反向軌跡表明主要受偏北方向氣團(tuán)影響,重污染期平均濃度為(86.6± 19.7)μg/m3,硫酸鹽占主導(dǎo),受偏南方向氣團(tuán)影響.

3.2 亞微米級(jí)氣溶膠主要化學(xué)組分中,揮發(fā)性順序?yàn)橄跛猁}>氯鹽>銨鹽>有機(jī)物>硫酸鹽.與相對(duì)低污染期相比,重污染期的硫酸鹽對(duì)PM1質(zhì)量濃度的貢獻(xiàn)高且具有更低的揮發(fā)性,而硝酸鹽在高污染下表現(xiàn)出更高的揮發(fā)性和更為新鮮的生成特征.

3.3 對(duì)有機(jī)氣溶膠而言,隨溫度增加,O/C升高,意味著大部分具有半揮發(fā)性的有機(jī)物氧化態(tài)較低.而在重污染期,有機(jī)物氧化態(tài)更高,且揮發(fā)性更低,表明重污染期區(qū)域傳輸?shù)挠绊戯@著加大.

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Characterization of atmospheric aerosol volatility in North China during summertime.

WEI Lin-tong, CAO Li-ming, WEI Jing, HE Ling-yan,HUANG Xiao-feng*

(Key Laboratory of Urban Human Residential Environmental Science and Technology, Shenzhen Graduate School, Peking University, Shenzhen 518055, China)., 2019,39(9)：3647~3654

Thermal denuder-aerosol mass spectrometer (TD-AMS) system was utilized to measure the aerosol chemical composition and volatility in the summer of 2014 in Wangdu, a rural site in North China Plain, and data of two different episodes were collected: average PM1mass concentration of relatively low pollution episodes (ep1) was (23.3 ± 15.1) μg/m3, dominated by sulfate, mainly influenced by air masses from the north; while (86.6 ± 19.7) μg/m3during high pollution episodes (ep2), dominated by organics aerosol (OA), influenced by air masses from the south; the volatility sequence of chemical species was nitrate>chloride>ammonia> organic matter>sulfate; when compared with ep1, during ep2, sulfate had a higher contribution for PM1and was less volatile, and nitrate was more volatile; OA during ep2 was less oxidized and showed lower volatility, indicating it was more aged. The semi-volatility of atmospheric aerosol suggested that regional transport played a key role under high pollution conditions in North China during summer.

aerosol volatility；thermal denuder；TD-AMS；organic aerosol

X513

A

1000-6923(2019)09-3647-08

魏林通(1995-),女,江西吉安人,碩士,主要從事大氣氣溶膠揮發(fā)性特征研究.發(fā)表論文1篇.

2019-01-28

國家重點(diǎn)研發(fā)計(jì)劃項(xiàng)目(2017YFC0210004);國家自然科學(xué)基金資助項(xiàng)目(41622304;91544215)

* 責(zé)任作者, 教授, huangxf@pku.edu.cn