天津市大氣顆粒物中有機標(biāo)識物粒徑分布的季節(jié)特征

王曉寧,田瑛澤,薛倩倩

天津市大氣顆粒物中有機標(biāo)識物粒徑分布的季節(jié)特征

王曉寧,田瑛澤*,薛倩倩

(南開大學(xué)環(huán)境科學(xué)與工程學(xué)院,國家環(huán)境保護城市空氣顆粒物污染防治重點實驗室,天津 300350)

為探討采暖季和非采暖季大氣顆粒物中有機標(biāo)識組分的粒徑分布特征,識別其來源,于2018年5月至2019年4月在天津采集分粒徑顆粒物,利用GC-MS對9個粒徑段顆粒物中17種多環(huán)芳烴(PAHs)、20種正構(gòu)烷烴(-Alkanes)和7種藿烷(hopanes)進行分析,并通過有機標(biāo)識物及特征比值的方法探討其主要來源.結(jié)果表明:非采暖季的四環(huán)多環(huán)芳烴Pyr、BaA、Chr和五環(huán)多環(huán)芳烴BbF、BaP呈3峰分布,其余PAHs呈雙峰分布,采暖季的低環(huán)PAHs呈雙峰分布,中高環(huán)PAHs近似單峰分布.根據(jù)PAHs特征比值發(fā)現(xiàn),非采暖季的燃煤源和交通源是PAHs的主要貢獻源,采暖季PAHs受燃煤源的影響更顯著.非采暖季的正構(gòu)烷烴中C29呈單峰分布,C27、C31、C32和C33近似單峰分布,其余正構(gòu)烷烴呈雙峰分布,采暖季的正構(gòu)烷烴均呈雙峰分布.根據(jù)正構(gòu)烷烴碳優(yōu)勢指數(shù)(CPI)和主碳峰數(shù)(max)發(fā)現(xiàn),人為源是正構(gòu)烷烴的主要來源,非采暖季受自然源的影響大于采暖季,自然源排放的正構(gòu)烷烴傾向于富集在粗顆粒物上,人為源排放的正構(gòu)烷烴則更傾向于富集在細(xì)顆粒物上.藿烷在粗粒徑段和細(xì)粒徑段均存在峰值.根據(jù)藿烷特征比值發(fā)現(xiàn),非采暖季的藿烷受交通源的影響較大,采暖季的藿烷受燃煤源的影響更顯著.

大氣顆粒物；有機標(biāo)識組分；粒徑分布；采暖季

有機物是PM的重要組成部分,影響能見度和人體健康[1-2],PM粒徑影響其運輸、轉(zhuǎn)化、降解以及在呼吸道的沉積[3-5].PM上標(biāo)識物的粒徑分布受來源和大氣物理化學(xué)過程影響[6-8],并與物質(zhì)的物理化學(xué)性質(zhì)有關(guān),會隨著季節(jié)和排放源的變化而改變.

目前國內(nèi)外已開展了有關(guān)有機標(biāo)識物粒徑分布特征的研究[9-11],國內(nèi)的研究多集中于PAHs和正構(gòu)烷烴[12-15],如Bi等[3]對廣州冬季PM中PAHs和正構(gòu)烷烴的粒徑分布的研究發(fā)現(xiàn),PAHs和正構(gòu)烷烴主要分布在小于1.5μm的細(xì)顆粒物中,低分子量有機物呈雙峰分布,高分子量有機物呈單峰分布;Xu等[16]對環(huán)巢湖10～12月份PM中正構(gòu)烷烴粒徑分布的研究發(fā)現(xiàn),短鏈正構(gòu)烷烴在粗粒徑段呈單峰分布,長鏈正構(gòu)烷烴呈雙峰分布.上述研究采樣周期較短,多為秋冬季.有機標(biāo)識物粒徑分布的季節(jié)變化方面研究較少,唐小玲[17]對廣州PM中烴類化合物粒徑分布的研究發(fā)現(xiàn),高溫季節(jié)的化合物優(yōu)先富集在細(xì)顆粒物,低溫季節(jié)則向粗顆粒物方向偏移;許紹峰[18]在對北京有機氣溶膠的研究中指出有機物的粒徑分布特征會受季節(jié)和氣象條件的影響.

綜合分析多類有機標(biāo)識物粒徑分布的季節(jié)特征可以為PM來源及污染防治提供更有效的支撐,本研究分析了采樣期為1a包含采暖季和非采暖季的大氣顆粒物上有機標(biāo)識物多環(huán)芳烴、正構(gòu)烷烴和藿烷的粒徑分布特征,利用有機分子標(biāo)識物及其特征比值的方法分析不同時期的主導(dǎo)源,并探討其形成機制,以期為大氣污染防治提供科學(xué)依據(jù).

1 材料與方法

1.1 樣品采集

采樣點設(shè)置在南開大學(xué)津南校區(qū)大氣環(huán)境綜合觀測站樓頂(38°59′40″N、117°20′6″E).采用安德森八級采樣器進行分級采樣,采樣流量為28.3L/ min,采樣濾膜為直徑81mm的石英濾膜.采集的粒徑范圍依次是:>9.0μm,5.8～9.0μm,4.7～5.8μm, 3.3～ 4.7μm, 2.1～3.3μm, 1.1～2.1μm, 0.65～1.1μm, 0.43～ 0.65μm和<0.43μm.采樣時間為2018年5月～2019年4月;春夏季采樣時間為當(dāng)日早上09:00～第3日早上08:00,采樣累積時長47h;秋冬季采樣時間為當(dāng)日早上09:00～次日早上08:00,采樣累積時長23h.依據(jù)采樣條件將樣品分為非采暖季(2018 年5～8月;2019年3月中旬～4月)和采暖季(2018年9月～ 2019年3月中旬).本研究共采集195組分粒徑樣品,共1755個樣品.

1.2 樣品分析

將1/2濾膜剪成碎條放入試管中,加入20mL正已烷和二氯甲烷混合溶液(1:1,:),冰浴超聲提取2次,每次15min并更換提取試劑,將提取液合并后用0.22μm的濾頭過濾,再經(jīng)過預(yù)先活化的固相萃取小柱,經(jīng)正己烷洗脫后轉(zhuǎn)移至圓底燒瓶在旋轉(zhuǎn)蒸發(fā)儀蒸發(fā)濃縮至小于5mL后移至KD瓶進行氮吹,吹至約0.5mL,分別加入50uL 5種氘代PAHs內(nèi)標(biāo)(2μg/mL)、50μL六甲基苯(2μg/mL)和50uL正二十四烷-d50內(nèi)標(biāo)(20μg/mL),用正己烷定容至1mL后,用氣相色譜-質(zhì)譜聯(lián)用儀(7890B/5977B,安捷倫科技有限公司,美國)分析,測定各有機物的濃度.色譜柱為DB-5MS(30m×0.25mm,0.25mm)石英毛細(xì)管柱,載氣為氦氣,恒定流速為1.0mL/min.質(zhì)譜離子源為EI源,電離能為70eV.多環(huán)芳烴和藿烷的入口和傳輸線溫度分別設(shè)置為230℃和280℃,正構(gòu)烷烴的入口和傳輸線溫度為300℃.

1.3 質(zhì)量控制

空白石英膜在采樣前需要在馬弗爐中500℃烘烤2h以去除膜中殘留的有機質(zhì)以減少偏差.所有濾膜在稱重前均需在恒溫恒濕環(huán)境(相對濕度(50±5)%,溫度(20±1)℃)下平衡48h,使用精密度為十萬分之一的電子天平(XPE105型,瑞士METTLER TOLEDO)進行稱重.樣品在-20℃冰箱中避光保存.用全過程空白、基質(zhì)空白和基質(zhì)加標(biāo)平行樣對數(shù)據(jù)進行質(zhì)量控制,以保證實驗結(jié)果的準(zhǔn)確性.分析結(jié)果表明全過程空白和基質(zhì)空白對實驗分析的影響可忽略不計. Nap, Any, Ana的回收率低于50%(由于Nap, Any, Ana的回收率較低,所以在此次研究中不予討論),其余有機化合物的回收率在70 %～130 %.

2 結(jié)果與討論

2.1 多環(huán)芳烴質(zhì)量濃度粒徑分布特征

非采暖季∑14PAHs的平均濃度為20.75ng/m3,細(xì)粒徑段(<2.1μm) ∑14PAHs濃度達12.06ng/m3,占PM10中∑14PAHs的58.12%.采暖季∑14PAHs的平均濃度為38.76ng/m3,細(xì)粒徑段∑14PAHs的濃度為25.89ng/m3,占PM10中∑14PAHs的66.80%,采暖季細(xì)顆粒物中PAHs濃度及占比均有增加, PAHs傾向于富集在細(xì)粒徑段,這與其他研究一致[4,19].

圖1 采暖季和非采暖季多環(huán)芳烴的粒徑分布

由圖1可見,非采暖季的4環(huán)多環(huán)芳烴Pyr、BaA、Chr和5環(huán)多環(huán)芳烴BbF、BaP呈現(xiàn)3峰分布模式,峰值分別在0.43～0.65μm、1.1～2.1μm和4.7～5.8μm處,其余PAHs呈雙峰分布模式,峰值在0.43～0.65μm和4.7～5.8μm處;采暖季低環(huán)(3環(huán)) PAHs呈雙峰分布模式,中高環(huán)(4～6環(huán))PAHs近似單峰分布模式,但中高環(huán)PAHs (BkF和DBA除外)在細(xì)粒徑段的峰值遠高于在粗粒徑段的峰值.總的來說,隨著環(huán)數(shù)的增加,PAHs呈現(xiàn)由粗模態(tài)向細(xì)模態(tài)偏移的趨勢,這主要是因為不同環(huán)數(shù)PAHs的理化性質(zhì)不同[20-21].首先,低環(huán)PAHs有更高的揮發(fā)性,更易從細(xì)顆粒物上揮發(fā)冷凝到粗顆粒物上[22].此外,有研究表明PAHs的辛醇-水分配系數(shù)與分子量呈正相關(guān),低環(huán)PAHs相較于中高環(huán)PAHs更加親水.粗顆粒物含水率較大,對低環(huán)PAHs有更高的親合力[23-24].同時,采暖季的4～5環(huán)多環(huán)芳烴Pyr、BaA、Chr、BbF在細(xì)粒徑段的峰值從非采暖季0.43～0.65μm偏移增長至0.43～1.1μm,這可能是因為采暖季供熱鍋爐和散煤燃燒增加,排放源更為復(fù)雜[25],排放的細(xì)顆粒物粒徑范圍更寬,且采暖季氣溫低、光照弱,細(xì)顆粒物上的PAHs揮發(fā)及光降解作用降低,使PAHs在細(xì)粒徑段聚集.采暖季的四環(huán)PAHs主要分布在0.43～1.1μm粒徑段,它們主要來源于燃煤源排放,五六環(huán)PAHs主要來源于交通源[26],采暖季外界溫度較低,高分子量PAHs排放后難以揮發(fā)而富集于細(xì)顆粒物中[27-28].

多環(huán)芳烴的特征比值如Ant/(Ant+Phe)和IPY/ (IPY+BghiP)經(jīng)常用于辨別PAHs的來源[16].如圖2所示,采暖季及非采暖季各粒徑段的Ant/(Ant+Phe)值均大于0.1,說明PAHs主要來自于高溫燃燒源[29].特征比值IPY/(IPY+BghiP)的范圍0.38～0.74,說明PAHs受到交通源和燃煤源的混合影響[30].采暖季IPY/(IPY+BghiP)比值高于非采暖季,表明燃煤源排放產(chǎn)生的PAHs增加,與上述粒徑分布的結(jié)果一致.因此PAHs受燃煤源和交通源排放的共同影響,而采暖季的PAHs受燃煤源排放的影響更顯著.

圖2 采暖季和非采暖季PAHs的特征比值

2.2 正構(gòu)烷烴質(zhì)量濃度粒徑分布特征

非采暖季∑20-Alkanes 的平均濃度為2009.65ng/m3,細(xì)粒徑段濃度為944.87ng/m3,占PM10中∑20-Alkanes的47.01%.采暖季∑20-Alkanes的平均濃度為3610.17ng/m3,細(xì)粒徑段濃度為1763.43ng/m3,占PM10中∑20-Alkanes的48.84%,濃度和占比均高于非采暖季.由圖3可見,采暖季的正構(gòu)烷烴均呈現(xiàn)明顯的雙峰分布模式,非采暖季的C29呈粗粒徑段單峰分布模式,C27、C31、C32和C33近似粗模態(tài)單峰分布模式,其余的正構(gòu)烷烴均呈現(xiàn)雙峰分布模式,峰值分別為細(xì)粒徑段0.43～ 1.1μm和粗粒徑段3.3～5.8μm.對于低碳數(shù)正構(gòu)烷烴,尤其是C14～C20,在采暖季富集于粗顆粒物上的濃度大于細(xì)顆粒物,非采暖季則不明顯.采暖季燃煤源排放的增加加劇了正構(gòu)烷烴的排放,此時的氣象因素也有利于正構(gòu)烷烴在顆粒物中的富集,采暖季溫度更低,可以使揮發(fā)性高一些的正構(gòu)烷烴揮發(fā)后分配到粗顆粒物上的時間縮短[17,31].非采暖季的高碳數(shù)奇數(shù)正構(gòu)烷烴C27、C29、C31和C33主要來源于高等植物蠟的排放及花粉孢子等植物源[32],在粗粒徑段的濃度遠大于采暖季,說明自然源在非采暖季的排放增強,其排放的正構(gòu)烷烴更傾向于和粗顆粒物結(jié)合[33].

圖4 采暖季和非采暖季正構(gòu)烷烴質(zhì)量濃度分布比較

表1 采暖季和非采暖季不同粒徑段正構(gòu)烷烴的CPI值和主碳峰數(shù)

此外,正構(gòu)烷烴主峰碳數(shù)(max)和碳優(yōu)勢指數(shù)(carbon preference index, CPI)經(jīng)常被用于判定正構(gòu)烷烴的主要來源[34-35],成熟度高的化石燃料具有較低的max和CPI值.由圖4和表1可見,采暖季和非采暖季的正構(gòu)烷烴碳數(shù)分布的峰型均是雙峰型,采暖季的max為C23和C28,說明化石燃料燃燒是正構(gòu)烷烴主要的排放源[15];非采暖季正構(gòu)烷烴的max為C23和C31,分別為交通源和高等植物蠟的主峰碳數(shù)[36],說明非采暖季正構(gòu)烷烴受到人為源和自然源的共同影響.因此,根據(jù)主碳峰數(shù)可以得出,采暖季的正構(gòu)烷烴主要來自于人為源,非采暖季的正構(gòu)烷烴則同時受自然源和人為源的影響.為更好地判斷人為源和自然源的影響,采用正構(gòu)烷烴CPI、CPI1和CPI2來判斷來源.從表1可以看出,采暖季和非采暖季的CPI值均接近于1,表明人為源是正構(gòu)烷烴的主要來源,采暖季的CPI值普遍低于非采暖季,表明采暖季的正構(gòu)烷烴更多地受到人為源的影響,這與其他研究結(jié)果一致[37].細(xì)顆粒物上正構(gòu)烷烴CPI值低于粗顆粒物,說明人為源產(chǎn)生的正構(gòu)烷烴更傾向于和細(xì)顆粒物結(jié)合[18].根據(jù)CPI1和CPI2可以得出,采暖季化石燃料的燃燒是大氣顆粒物中低碳數(shù)正構(gòu)烷烴的主要來源,非采暖季正構(gòu)烷烴受植物源的影響相對較大,采暖季正構(gòu)烷烴受植物源的影響較小.非采暖季顆粒物中正構(gòu)烷烴在粗粒徑段的CPI2值范圍是1.35～1.90,在細(xì)粒徑段CPI2值范圍是1.06～1.20,可見粗粒徑段的CPI2高于細(xì)粒徑段的CPI2,表明粗模態(tài)的顆粒物受植物源排放的影響更大.

2.3 藿烷質(zhì)量濃度粒徑分布特征

圖5 采暖季和非采暖季藿烷的粒徑分布

非采暖季∑7hopanes的平均濃度為11.26ng/m3,細(xì)粒徑段濃度為6.58ng/m3,占PM10中∑7hopanes的58.43%.采暖季的∑7hopanes的平均濃度為45.28ng/m3,細(xì)粒徑段濃度為24.99ng/m3,占PM10中∑7hopanes的55.19%.采暖季的藿烷濃度明顯高于非采暖季,除了采暖季大氣層穩(wěn)定不利于顆粒物擴散外,這主要是因為供暖期燃煤量大大增加所致.由圖5可見,非采暖季的藿烷除C27α在細(xì)模態(tài)有一個峰值、在粗模態(tài)有2個峰值外,其他均呈雙峰分布模式,采暖季的藿烷沒有相似的粒徑分布規(guī)律,其中C29αβ、C30αβ在細(xì)模態(tài)有2個峰值、在粗模態(tài)有一個峰值,C27α、C30βα、C34αβS呈雙峰分布模式,C30ββ和C34αβR則在粗模態(tài)有2個峰值、細(xì)模態(tài)1個峰值.C30ββ和C34αβR被認(rèn)為是成熟度較低的煤的標(biāo)識物,主要來源于褐煤和亞煙煤的燃燒[38],因此采暖季成熟度較低的煤的燃燒增加使得粗模態(tài)顆粒物藿烷濃度有所增加.對于其他藿烷,在粗粒徑段的濃度占比也較高,推斷粗模態(tài)藿烷可能是道路揚塵和交通源排放的混合物[39].

藿烷特征比值C29αβ/C30αβ和異構(gòu)化指標(biāo)C34[/(+)]是判斷其來源的重要參數(shù).研究表明C29αβ/C30αβ的比值是0.4時為柴油車排放,0.6～0.7時為汽油車排放[40],0.6～2.0時為燃煤排放[38].在本研究中,采暖季和非采暖季的C29αβ/C30αβ的比值范圍分別為0.98～1.34和0.66～0.76(表2),這表明采暖季的藿烷主要來源于燃煤源排放,非采暖季的藿烷可能主要來源于汽油車排放.C34[/(+)]隨著燃料成熟度的增加而增加[41],該比值在0.05～0.35時說明顆粒物主要受燃煤源排放影響,大于0.35時標(biāo)識交通排放[38].本研究中的C34[/(+)]在采暖季全粒徑段的比值為0.33,說明藿烷在采暖季受燃煤源排放的影響更為顯著,在非采暖季全粒徑段比值為0.42,說明藿烷在非采暖季可能受機動車排放源的影響較大.

表2 采暖季和非采暖季藿烷的特征比值

3 結(jié)論

3.1 非采暖季的四環(huán)多環(huán)芳烴Pyr、BaA、Chr和五環(huán)多環(huán)芳烴BbF、BaP呈現(xiàn)3峰分布模式,峰值在0.43～0.65μm、1.1～2.1μm和4.7～5.8μm處,其余PAHs呈雙峰分布模式,峰值在0.43～0.65μm和4.7～5.8μm處,采暖季的低環(huán)PAHs呈雙峰分布模式,峰值分別在0.43～1.1μm和4.7～5.8μm,中高環(huán)PAHs近似單峰分布模式,高環(huán)PAHs在細(xì)粒徑段的峰值遠高于在粗粒徑段的峰值.采暖季燃煤源排放增加使得細(xì)模態(tài)四環(huán)PAHs濃度急劇增加.通過PAHs特征比值A(chǔ)nt/(Ant+Phe)和IPY/(IPY+BghiP)得出非采暖季的PAHs主要來源于燃煤源排放和交通源排放,而非采暖季的PAHs受燃煤源的影響較為顯著.

3.2 采暖季的正構(gòu)烷烴呈現(xiàn)穩(wěn)定的雙峰分布模式, 峰值分別在0.43～1.1μm和3.3～5.8μm,非采暖季C29呈單峰分布模式,峰值在3.3～5.8μm,C27、C31、C32和C33近似單峰分布模式,其余的正構(gòu)烷烴均呈現(xiàn)明顯的雙峰分布模式.采暖季的正構(gòu)烷烴在全粒徑段的濃度均高于非采暖季.采暖季和非采暖季正構(gòu)烷烴的CPI值分別為0.75和1.23,均接近1,說明人為源是正構(gòu)烷烴的主要來源,非采暖季受自然源的影響大于采暖季,自然源排放的正構(gòu)烷烴易于富集在粗顆粒物上,人為源排放的正構(gòu)烷烴更傾向于富集在細(xì)顆粒物上.

3.3 藿烷在粗粒徑段和細(xì)粒徑段均有峰.采暖季的藿烷濃度明顯高于非采暖季,受到秋冬季氣溫低、大氣層穩(wěn)定的氣象因素和燃煤源排放增加的共同影響.采暖季的C29αβ/C30αβ和C34[/(+)]分別為1.12和0.33,非采暖季的C29αβ/C30αβ和C34[/(+)]分別為0.70和0.42,說明非采暖季的藿烷受交通源影響較大,采暖季的藿烷受燃煤源排放的影響顯著,采暖季成熟度較低的煤的燃燒增加導(dǎo)致部分藿烷在粗模態(tài)的濃度升高.

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Distribution characteristics of organic markers in particulate matter with different sizes during different seasons in Tianjin.

WANG Xiao-ning, TIAN Ying-ze*, XUE Qian-qian

(State Environmental Protection Key Laboratory of Urban Ambient Air Particulate Matter Pollution Prevention and Control, College of Environmental Science and Engineering, Nankai University, Tianjin 300350, China). China Environmental Science, 2022,42(11)：4974～4982

In order to study the distribution characteristics of organic markers in particulate matter with different sizes during the heating and non-heating period, and identify their pollution sources, the Anderson eight-stage sampler was used to collect one-year samples from May 2018 to April 2019. 17 kinds of polycyclic aromatic hydrocarbons(PAHs), 20kinds of-Alkanes and 7kinds of hopanes in 9particle size segments were analyzed and the main sources of particulate matter were identified by molecular markers and diagnostic ratios. The results showed the 4～5rings PAHs Pyr, BaA, Chr, BbF and BaP showed a three-peak distribution and the rest of the PAHs showed a bimodal distribution during the non-heating period, and 3rings PAHs showed a bimodal distribution and 4～6rings PAHs showed a approximately unimodal distribution during the heating period. The diagnostic ratios of PAHs indicated that vehicle emissions and coal combustion were the main contributors of PAHs during the non-heating period and the coal combustion was the main contributors of PAHs during the heating period. The-Alkanes showed a stable bimodal size distribution during the heating period and C29 showed a unimodal distribution, C27, C31, C32 and C33 showed approximately a unimodal distribution, with the remaining-Alkanes in a bimodal distribution during the non-heating period. According to the CPI andmax, it is found that anthropogenic sources were the main source of-Alkanes during the non-heating period and the heating period. The concentration of high molecular weight-Alkanes was better influenced by natural sources during the non-heating period than during the heating period. In addition,-Alkanes emitted from natural sources were easy to concentrate in coarser particulates, and-Alkanes from anthropogenic sources were more likely to concentrate in the finer particles. The hopanes had peaks in both coarse and fine particle modes during two seasons. Traffic was proposed as the main emission source of hopanes during the non-heating period, and the hopanes mainly originated from coal combustion during the heating period.

atmospheric particulate matter；organic markers；size distribution；the heating period

X511

A

1000-6923(2022)11-4974-09

王曉寧(1997-),女,山東聊城人,南開大學(xué)碩士研究生,研究方向為大氣污染防治.

2022-04-11

國家自然科學(xué)基金(41977181);中國工程院院地合作項目(2020C0-0002)

* 責(zé)任作者, 副研究員,tianyingze@hotmail.com