PM2.5中類腐殖質表面活性測定方法與實例分析

白 瑤,吳志軍,劉玥晨,王玉玨,郭 松,胡 敏

PM2.5中類腐殖質表面活性測定方法與實例分析

白 瑤,吳志軍*,劉玥晨,王玉玨,郭 松,胡 敏

(北京大學環境科學與工程學院,環境模擬與污染控制國家重點聯合實驗室,北京 100871)

建立了大氣細粒子中類腐殖質(HULIS)表面活性的動態表征方法,并以華北平原鄉村站點冬季大氣PM2.5樣品為例,對PM2.5中HULIS的表面活性進行表征.HULIS碳質組分(HULIS-C)濃度為2.0~4.6μg C/m3,占水溶性有機碳和總有機碳的比例分別為31%~40%和20%~26%.濃度為88~200mg C/L的HULIS水溶液,其表面張力相對于純水降低了18%~22%.HULIS-C濃度在低于70mg C/L時表面張力降低顯著,在88~320mg C/L之間降低相對緩慢.動態表面張力隨著時間變化逐漸降低,在液滴形成后200s以內表面張力下降迅速,之后趨于平緩,說明表面活性分子在液滴中擴散趨于穩定需要一定的時間,該特征時間可能影響表面活性物質在云凝結核活化時的作用.證實了在污染地區的大氣PM2.5中含有一定量的表面活性物質,這些物質可能對顆粒物活化為云滴、霧滴過程產生顯著影響;表面活性物質的存在可能在外界濕度變化過程中導致顆粒物發生液-液相分離現象,在顆粒物表面形成有機膜,影響活性分子攝取以及半揮發性物質的氣-粒分配過程,從而影響大氣非均相反應過程.

表面活性物質；表面張力；類腐殖質；大氣細粒子

目前,人們對氣溶膠與云相互作用的認識有限,不能精確預測全球和區域尺度的氣候變化[1].大氣氣溶膠中可溶性無機組分如硫酸鹽、硝酸鹽、銨鹽等對云凝結核(CCN)活化成為云滴的影響得到了廣泛的研究[2-3],但有機物質與水相互作用的機制還不清楚[4].有機物成分復雜,占顆粒物總質量的20%~ 90%[5],需要進一步研究有機物的理化性質以厘清它們在云滴形成過程中的角色.

描述云滴的形成和生長的寇拉理論考慮到顆粒物的化學組分和粒徑對顆粒物吸濕活化的影響,分為溶質效應和開爾文效應.表面張力是開爾文效應中的一個重要參數,表面張力的降低可能會降低液滴活化的臨界過飽和度,形成更多的云滴[6],這可能導致云的生命周期延長,從而增強云的反照率[7-8].為了簡化寇拉理論的應用,基于寇拉理論發展的к-K?hler理論考慮化學組分對水活度的影響并忽略表面張力效應[9-11].然而最近的研究[12-13]提出表面活性物質除了作為溶質影響水活度外,其表面活性也會影響CCN活性,這意味著目前的計算方法可能還不夠準確.進一步的優化模型可通過測定不同濃度的表面張力獲得經驗公式,結合計算[14-15]或測量[9]的水活度輸入寇拉方程模擬顆粒的吸濕活化.

表面活性物質含有親水基團和疏水碳鏈,傾向于分布在液滴表面.一方面降低液滴的表面張力,影響成云活性.另一方面阻礙痕量氣體和水的吸收,影響氣溶膠的非均相反應過程[16-17].大氣顆粒物中的表面活性物質包括有機酸、二元酸、蛋白質和類腐殖質(HULIS)[18-19]等.HULIS是一類多官能團的大分子有機物[20],從城市農村、森林、海洋采集的顆粒物中都檢測到HULIS[21-22].目前針對HULIS的研究主要集中在化學組分、濃度水平及其來源解析上[23-25],對HULIS的表面活性以及對顆粒物成云活性影響的研究十分有限.另外,部分研究選用模型化合物替代實際環境HULIS進行分析[9,26],這些物質可能也無法完全代表實際環境下HULIS的性質,不足以作為HULIS的表面張力參數輸入寇拉方程.因此,進一步研究實際大氣環境下HULIS的表面活性并優化現有模型具有重要價值.尤其是我國霧霾的發生與顆粒物吸濕增長消光和活化成為霧滴的過程密切相關,對HULIS的表面活性研究有助于揭示霧霾形成機制.

為表征我國實際大氣顆粒物中HULIS的表面活性,本研究建立了系統的HULIS水溶液表面張力測定方法,以一次冬季觀測大氣PM2.5樣品為例表征大氣顆粒物中HULIS的表面活性.

1 HULIS表面活性測定方法

HULIS表面活性的測定方法可以分為大氣顆粒物采集、顆粒物質量濃度分析、超聲提取以及固相萃取HULIS、碳質組分分析、表面張力測定.采集后的PM2.5樣品進行顆粒物質量濃度分析和碳質組分分析.其后樣品進行超聲水提測定總水溶性有機碳(WSOC)濃度,使用固相萃取法提取樣品中的HULIS,并測定其中HULIS-C濃度,得到HULIS-C占總水溶性有機碳和有機碳的比例,最后測定HULIS水溶液的表面張力.具體操作步驟如下.

1.1 PM2.5樣品采集

本研究使用流量為1050L/min的大流量采樣器(TH-1000,武漢天虹公司)和流量為16.7L/min的TH-16A大氣顆粒物智能采樣器(武漢天虹公司)采集PM2.5樣品,分別配備石英纖維濾膜(Whatman Inc., 203mm×254mm)和Teflon膜(Whatman Inc,USA, 47mm).其中Teflon膜采集的樣品用于膜稱重,石英纖維濾膜采集的樣品用于元素碳、有機碳以及HULIS組分分析.石英纖維濾膜在采樣前置于潔凈鋁箔中,在馬弗爐(550℃)中灼燒5.5h去除其中的有機物.采樣用的鑷子和棉花在使用前用二氯甲烷超聲清洗3次.采樣器和切割頭也在采樣前完成流量標定和清洗,采樣后的樣品置于冷藏柜冷凍保存(?20℃),采樣期間記錄采樣時間、采樣流量和天氣等參數.

1.2 顆粒物質量濃度分析

將Teflon膜置于超凈實驗室恒溫恒濕(溫度:(20±1)℃,相對濕度:(40±5)%)平衡24h后,用十萬分之一天平(AX105DR型,瑞士Mettler Toledo公司)稱重,并根據采樣前后樣品的質量差除以采樣體積計算顆粒物的質量濃度.

1.3 HULIS提取

本研究采用固相萃取法提取HULIS,主要包括3個步驟:(1)超純水超聲提取;(2)固相萃取分離和濃縮HULIS組分;(3)洗脫目標物.

在大流量采樣膜上切一個Φ47mm的圓形樣品放于提取罐中,用20mL去離子水(Milli-Q Gradient, Millipore Company,美國)超聲提取40min(防止水溫過高,每20min向超聲儀中換冰水).其后用0.45μm PTFE微孔濾膜過濾提取液,除去不溶性顆粒和雜物,收集濾液;取出2mL稀釋至10mL用于測定水溶性有機碳濃度;用2.4mol/L鹽酸將剩余提取液酸化至pH=2.使用固相萃取柱(DSC-18,Sigma-Aldrich,USA)分離和富集HULIS,用3mL甲醇活化柱子、3mL去離子水和1mL 0.01mol/L鹽酸分別對萃取柱活化、清洗、平衡;將酸化后的提取液過柱,經固相萃取,水溶液中的無機組分、極性較強的低分子量有機酸和糖類未被保留[27],極性相對較弱的有機組分被填充物保留.樣品加完后用2mL去離子水清洗柱子2次,以除去填充柱中的殘留提取液;用1.5mL甲醇洗脫下被SPE小柱保留的組分,即為HULIS.將洗脫液用高純氮氣吹干,用1.5mL去離子水再溶解HULIS,取0.5mL稀釋至10mL用于測定HULIS-C的濃度,另外1mL用于表面張力測定.Lin等[28]選用HULIS的替代物SRFA和NAFA測定提取方法的回收率分別為94%和92%.關于SPE步驟的具體討論詳見文獻[29].

1.4 PM2.5中含碳組分測定

使用美國Sunset Laboratory 的碳分析儀分析石英纖維濾膜上采集的有機碳(OC)和元素碳(EC),分析方法為熱光透射法(NIOSH5040),儀器最低檢出限為0.2μg/m2,儀器精密度為±5%,具體儀器分析原理、分析方法和質量控制詳見文獻[30].

采用日本島津公司總有機碳分析儀(TOC- LCPH)測定樣品溶液總水溶性有機碳(WSOC)濃度和HULIS-C濃度.由于樣品量有限(10mL),本研究采用NPOC法測定,樣品先經酸化曝氣除去無機碳,注入燃燒管在高溫條件下氧化為CO2,再用檢測器測量CO2濃度.每個樣品測定2次,差值大于3%進行第3次測定.測量樣品前先測定配制好的標液,當測量結果與實際濃度差值在5%以內可進行樣品水溶性有機碳測定.

1.5 表面張力測定

本研究選用懸滴法測量HULIS溶液的動態表面張力,懸滴法的優點是用量少,適合測定含量較低的實際大氣樣品[14].此外,該方法不僅可以獲得表面張力的平衡值,還可以捕捉到表面活性物質分布到液滴表面的變化過程[26].

本研究測定儀器(Krüss DSA30)主要包括液滴形成系統、照明系統、溫度控制系統、成像系統以及計算機計算系統.采用1mL一次性注射器和(1.812±0.02)mm的針頭,測量時將注射器安裝在進樣器上使之插入一個封閉溫控腔內,在注射器針頭形成液滴直到增大到快要掉落達到最大穩定體積.照明系統在溫控箱的一側產生均勻照明的背景.溫度控制系統主要由控溫箱,溫度傳感器和恒溫水槽組成,本研究測定溫度為(20±0.3)℃.懸掛的液滴圖像由一個配置了顯微鏡鏡頭的相機記錄.實驗過程中保留2~3mm長度的針頭,使針頭與液滴在同一畫面內以便對液滴圖像放大倍率進行校準,懸滴連同針頭的圖像被采集到同一畫面內(見圖1),在穩定的光源下呈現出清晰的圖像,計算機將圖像數字化得到液滴的形狀因子和半徑,使用公式1獲得液滴表面張力值,計算方法為:

式中:為液滴的表面張力(mN/m);Δ為液滴與周圍氣相相差的密度(g/cm3);為重力加速度(N/kg);d為液滴半徑(mm);為液滴形狀因子.溶液的密度用分析天平測定(AX105DR型,瑞士Mettler Toledo公司).液滴形成穩定后,每隔5s拍一張照,記錄15~ 20min,之前有研究報道動態測定時長為10min[4],本研究在此基礎上多測量5~10min.

使用超純水對儀器進行校準,測定9次,每次20min,液滴形成相同時間下9個測量值的相對標準偏差為0.61%~0.88%,測定過程中液滴的體積變化<5%.每次測定樣品表面張力前先測定超純水的表面張力,與理論值差值小于5%,進行樣品表面張力測定,每個樣品測定3次.

圖1 懸滴示意

2 實例分析

本研究采樣點位于山東德州平原縣氣象局(37.09°N,116.26°E).于2017年12月13日~12月20日共采集了7套大氣環境PM2.5樣品,采樣時間為早晨8:30~次日8:00.12月21號采集空白樣品,將石英膜放置在采樣器上不開泵采集30min.

2.1 顆粒物質量濃度和碳質組分濃度

表1 HULIS-C?WSOC和OC的濃度

碳質組分濃度如表1所示,HULIS占 WSOC的31%~40%,可見HULIS在水溶性有機碳組分中占重要比例.

表2為研究報道的生物質燃燒排放顆粒物、城市和鄉村大氣顆粒物中HULIS的濃度及其在WSOC和OC中的占比.根據換算系數HULIS/ HULIS-C=1.94[28],本研究HULIS的濃度為3.9~ 8.9μg/m3,略高于歐洲地區和西藏地區,與珠江三角洲和上海相近,低于巴西地區.HULIS-C對WSOC的貢獻在15%~74%之間,對OC的貢獻范圍在5%~51%之間,測定結果在報道研究的范圍內,較寬的變化范圍可能由于HULIS的來源、形成途徑和老化程度不同造成.采樣期間PM2.5濃度為66.5~125.0μg/m3, HULIS占PM2.5的6%~10%,該比例與我國廣東玉米秸稈(11.2±7.5)%和松樹枝(11.4±3.8)%燃燒排放顆粒物的測量結果[31]及珠江三角洲地區環境大氣顆粒的測量結果(11.7±2.1)%相近[28].

表2 不同環境大氣顆粒物中HULIS濃度及其對WSOC?OC的貢獻

2.2 HULIS溶液表面張力

本研究測定20℃下超純水的表面張力均值為72.3mN/m,空白樣品HULIS碳濃度為2mg C/L,表面張力均值為71.9mN/m,兩者都與理論值72.8mN/m相差很小,說明測定的表面張力結果可信且提取過程未影響目標物的表面張力.各采樣日HULIS溶液的表面張力如圖2所示,濃度為88~200mg C/L的HULIS溶液的表面張力在液滴形成20min后為56.7~59.4mN/m,與純水的表面張力相比降低了18%~22%.液滴表面張力的降低程度與HULIS含量并未呈現單調的遞增或遞減,不同天樣品的HULIS溶液表面張力有細微差別.

液滴形成后,表面張力呈現出隨著時間變化逐漸降低接近平衡值,前200s表面張力值降低迅速,之后趨于平緩,說明表面活性分子在液滴中擴散趨于穩定需要一定的時間.Nozièr等[32]測定大氣PM10中的表面活性物質的動態表面張力達到平衡時為30~50mN/m,表面張力快速下降的時間范圍為8~110s,達到平衡的時間為36~495s,本研究的樣品表面張力變化的時間尺度與該報道相似.研究還指出表面活性劑對低云或層云等垂直上升氣流緩慢的云的形成影響較大,測量結果顯示表面活性物質的表面張力達到平衡的時間在這類云的形成時間范圍內(1~30min),這將有助于云滴的形成.

不同濃度的HULIS溶液其液滴在形成15min后的表面張力變化如圖3所示.隨著HULIS溶液濃度增加,其表面張力降低,HULIS-C濃度在低于70mg C/L時表面張力降低迅速,88~320mg C/L之間表面張力降低相對緩慢.

結果表明華北平原污染環境大氣顆粒物中含有表面活性物質.表面活性物質的存在可能影響顆粒物的吸濕增長,降低其活化的飽和蒸汽壓,進而直接影響大氣顆粒物成為霧滴和云滴的活化過程.在未來研究中可將測定的HULIS溶液的表面張力輸入寇拉方程,量化HULIS的表面活性對液滴吸濕活化的影響.其次,在環境大氣濕度變化過程中,表面活性物質的存在可能使得顆粒物出現液-液相分離,形成核殼結構,影響對活性分子的攝取,從而影響大氣多相化學過程,未來研究可測定HULIS顆粒在不同相對濕度下的形貌,在高相對濕度下是否發生液-液相分離現象.

圖2 HULIS液滴動態表面張力

圖3 不同HULIS-C溶液表面張力

2.3 不同環境含碳物質表面活性比較

目前,關于實際大氣顆粒物中表面活性物質表面張力的研究十分有限,表3為不同環境大氣樣品中表面活性物質表征的結果.與純水相比實際大氣樣品中表面張力降低范圍在15%~42%,表面張力受有機物濃度、液滴酸度、溫度、金屬離子等因素影響[42].Kiss等[43]研究發現鄉村環境大氣顆粒物中HULIS在溶液濃度為1g/L時,與純水相比其表面張力降低25%~ 42%,濃度為0.2g/L時,其表面張力降低13%~28%.樣品的表面活性呈現季節性的變化特征,不同季節樣品的元素組成相似時,表面張力變化是由官能團的位置、鏈長度、芳香性等引起的.Salma等[44]測量顯示城市環境大氣(布達佩斯)HULIS液滴在1g/L和44mg/L濃度下表面張力與純水表面張力相比分別降低32%和18%,動態表面張力結果顯示高濃度的HULIS溶液的表面張力達到平衡值所需的時間更短.Asa-Awuku等[45]報道了濃度0.85g C/L的生物質燃燒顆粒物的水提液與純水相比表面張力降低18%.Facchini等[46]測定意大利霧水樣品中的WSOC濃度為0.1g C/L時表面張力降低15%~20%.將水溶性有機物分成中性化合物?單羧酸和多元羧酸3類,其中多元羧酸(類腐殖質)的濃度最低,但是表面活性最強.實際測量的云水和霧滴的表面張力與純水相比,分別降低了16%和30%,計算表明表面張力降低30%將導致云滴數量增加20%[8,47].標準腐殖酸對液滴表面張力的降低程度比大氣顆粒物中HULIS低7%~23%[43].腐殖酸濃度為1g/L時,表面張力僅降低12%[48].本研究HULIS溶液濃度為0.09~ 0.20g C/L時與純水相比表面張力降低18%~22%,與匈牙利K-puszta、布達佩斯和意大利等地測量結果接近,比美國哥倫布生物質燃燒顆粒的表面活性更顯著.

表3 實際大氣樣品表面張力測量結果

3 結論

3.1 建立了大氣顆粒物中HULIS動態表面活性的測定流程與方法.以華北地區鄉村站點冬季大氣PM2.5樣品為例,基于上述方法對PM2.5中HULIS含量和表面活性進行表征.測量結果顯示HULIS-C的濃度為2.0~4.6μg C/m3,分別占WSOC和OC的31%~40%和20%~26%.濃度在88~200mg C/L的HULIS水溶液的表面張力在液滴形成20min后數值范圍為56.7~59.4mN/m,與純水的表面張力相比降低了18%~22%.

3.2 動態表面測量顯示HULIS液滴表面張力隨著時間變化逐漸降低,前200s下降迅速,之后趨于平緩.

3.3 HULIS-C濃度低于70mg C/L時表面張力降低顯著,88~320mg C/L之間表面張力降低相對緩慢.

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The surface activity of humic-like substances: methodology and case study.

BAI Yao, WU Zhi-jun*, LIU Yue-chen, WANG Yu-jue, GUO Song, HU Min

(State Key Joint Laboratory of Environmental Simulation and Pollution Control, College of Environmental Sciences and Engineering, Peking University, Beijing 100871, China)., 2019,39(8)：3137~3143

A dynamic methodology was developed to characterize the surface activity of water soluble humic-like substances (HULIS) in PM2.5.As a case study, HULIS in PM2.5samples collected from a rural site during wintertime in North China Plain region were investigated. The HULIS carbon content varied from 2.0~4.6μg C/m3, accounting for 31%~40% of water-soluble organic carbon and 20%~26% of organic carbon, respectively. While the carbon content of HULIS aqueous solution ranged from 88~200mg C/L, correspondingly, the surface tension reduced by 18%~22% compared with that of pure water. The surface tension decreased significantly when the concentration was lower than 70mg C/L, while it decreased slowly between 88~320mg C/L. The dynamic measurements showed that surface tension gradually decreased with time consuming. Surface tension decreased rapidly within 200 seconds after droplet formation and then tended to be stable, indicating that the distribution of surface-active organic molecules onto the droplet surface was not instantaneous. Such process may affect droplet activation. There were substantial surface-active substances in atmospheric particulate matters in the polluted atmosphere. These surfactants may have significant impacts on the activation of aerosol particles into cloud or fog droplets. In addition, the HULIS may lead to liquid-liquid phase separation and form the core-shell structure of particles when ambient relative humidly fluctuates. Such structure could influence the uptake of reactive molecules and the gas-particle partitioning, therefore, affecting heterogeneous chemistry.

surfactants；surface tension；humic-like substances；PM2.5

X513

A

1000-6923(2019)08-3137-07

白 瑤(1993-),女,云南紅河人,北京大學碩士研究生,主要從事大氣顆粒物中類腐殖質的表面活性研究.

2019-01-02

國家自然科學基金國際(地區)合作與交流項目(41571130021, 41875149) ;國家重點研發計劃(2016YFC0202801)

* 責任作者, 研究員, zhijunwu@pku.edu.cn