珠江三角洲秋季生物質燃燒對有機氣溶膠的貢獻

楊 磊,袁 斌*,鄭 鍔,葉晨朔,王思行,何賢俊,張瀟瀟,黃 山,胡偉偉,邵 敏

珠江三角洲秋季生物質燃燒對有機氣溶膠的貢獻

楊 磊1,袁 斌1*,鄭 鍔1,葉晨朔2,王思行1,何賢俊1,張瀟瀟1,黃 山1,胡偉偉3,邵 敏1

(1.暨南大學環境與氣候研究院,粵港澳環境質量創新聯合實驗室,廣東 廣州 511443；2.北京大學環境科學與工程學院,北京 100871；3.中國科學院廣州地球化學研究所,有機地球化學國家重點實驗室,廣東 廣州 510640)

本研究基于2018年和2019年秋季在珠江三角洲地區的兩次外場觀測,應用熱脫附-化學電離飛行時間質譜(FIGAERO-ToF-CIMS)獲取了高時間分辨率(每小時)的生物質燃燒示蹤物左旋葡聚糖的濃度數據,并估算出生物質燃燒對有機氣溶膠(OA)的貢獻值.結果表明,秋季珠江三角洲地區城市站點和區域站點的左旋葡聚糖平均濃度分別為(0.07±0.08)和(0.14±0.12)μg/m3,呈現區域站點高于城市站點的空間分布特征以及早晨和夜間出現峰值的日變化特征.觀察到兩個站點的左旋葡聚糖與CO和乙腈之間相關性較低,但與OA之間呈現顯著正相關關系.進一步基于受體示蹤物法估算出生物質燃燒對OA的平均貢獻分別為7.4%(城市站點)和11.4%(區域站點),且兩個站點均顯示出生物質燃燒對OA的貢獻在夜間明顯高于白天.

生物質燃燒；示蹤物；左旋葡聚糖；有機氣溶膠

生物質燃燒是一種重要的空氣污染源,對全球范圍內的空氣質量、氣候變化和人類健康都有著復雜且重要的影響[1-3].特別是農作物秸稈露天焚燒和家庭爐灶燃燒等生物質燃燒過程[4-5],會釋放多種氣體及顆粒物,包括溫室氣體(如二氧化碳)、揮發性有機化合物(VOCs)和由黑碳(BC)、棕碳(BrC)及其他有機物組成的顆粒物[6].這些污染物不僅會對區域大氣產生不利影響,而且還可以通過長距離輸送影響全球空氣質量[7].

珠江三角洲地區(以下簡稱“珠江三角洲”)是中國最發達的地區之一,也是我國重要的農業生產基地,農作物殘余物露天焚燒引起的大氣污染事件經常發生[8-9].有機氣溶膠(OA)作為大氣細顆粒物(PM2.5)的重要組成部分,占PM2.5總質量的20～90%[10-11].而生物質燃燒排放則被認為是大氣顆粒物中OA的重要來源[12],特別是在生物質燃燒影響較大的地區(例如我國關中盆地),甚至可以貢獻31.9%的OA[13].此前研究表明,OA能夠影響大氣顆粒物的物理化學特性,如吸濕性和光學特性等,進而通過改變光的散射和吸收影響大氣能見度和氣候變化.例如,Zong等[14]研究發現OA中的水溶性有機物(WSOM)可以改變氣溶膠吸濕性,從而影響氣溶膠作為云凝結核的能力.Saleh等[15]研究發現BrC作為一類吸光性的OA組分,其在近紫外線和可見光譜區域具有很強的吸光能力,從而影響大氣輻射傳輸過程.大量研究都關注到生物質燃燒對大氣氣溶膠中OA的貢獻及影響[16-18],但目前人們對我國珠江三角洲地區秋季城市站點和區域站點的生物質燃燒貢獻差異了解甚少,在此地區開展研究有利于補充對珠江三角洲不同地區大氣有機氣溶膠受生物質燃燒源影響現狀的認識,為制定城市和區域尺度大氣細顆粒物防治策略提供依據.

左旋葡聚糖(1,6-脫水-β-D-吡喃糖)是纖維素的主要熱解產物,被廣泛作為生物質燃燒的示蹤劑[19-21].左旋葡聚糖的測定通常采用氣相色譜-質譜法(GC-MS)或高效液相色譜法(HPLC),分析方法成熟,但兩種方法都涉及復雜的樣品處理和分析過程,且時間分辨率較低[22-23].近年來,將氣粒雙通道采樣器(FIGAERO)作為進樣口的熱脫附-化學電離飛行時間質譜儀(FIGAERO-ToF-CIMS)測定左旋葡聚糖的方法已被運用[24-25].這種新型的在線檢測技術使用軟電離的化學電離方式以實現待測物離子最小程度的碎片化,并且具有極低的檢測限,可在分子水平上在線檢測和定量含氧化合物,無需復雜的樣品處理過程,同時具有高時間分辨率[26-27].

本文使用FIGAERO-ToF-CIMS在珠江三角洲地區開展秋季外場觀測,獲取高時間分辨率(每小時)的左旋葡聚糖濃度數據,分析其空間分布特征和日變化特征,并與其他生物質燃燒示蹤物進行相關性分析,以了解珠江三角洲地區城市站點和區域站點之間生物質燃燒活動的差異.通過左旋葡聚糖濃度和OA濃度對生物質燃燒貢獻值進行估算以分析大氣氣溶膠中OA受生物質燃燒的影響程度,為我國珠江三角洲地區秋季生物質燃燒引起的大氣細顆粒物污染的防控提供數據支持.

1 材料與方法

1.1 采樣地點

本研究使用2018年和2019年秋季在珠江三角洲地區分別進行的兩次外場觀測數據.珠江三角洲地區包括廣州、佛山、肇慶、深圳、東莞、惠州、珠海、中山和江門9個城市,是我國經濟最活躍、人口最密集的區域之一,工業化、城市化水平較高.城市站點位于廣東省廣州市中國科學院廣州地球化學研究所(23.1°N,113.2°E),周邊主要是居民區和學校,無明顯工業排放源,屬于典型的城市環境[28-29],觀測時間自2018年9月27日～2018年11月12日.區域站點位于廣東省江門市鶴山市廣東省大氣環境超級監測站(22.7°N,112.9°E),距離廣州市市區約80km,周邊無明顯工業源且處于城市下風向,是珠江三角洲地區典型的區域受體站點[30-31],觀測時間自2019年10月2日～2019年11月15日.

1.2 左旋葡聚糖分析方法

觀測期間采用以FIGAERO為進樣口的碘離子化學電離飛行時間質譜(I--ToF-CIMS)測量左旋葡聚糖, I--ToF-CIMS可檢測的質量范圍為1～603Th,質量分辨率為10000～11000[32-33].FIGAERO作為一個多端口進氣裝置,擁有單獨的氣體采樣口和顆粒物采樣口,并通過FIGAERO內可移動滑塊來控制氣體或顆粒物從不同進樣口進入儀器來實現在線測量大氣中微量氣體和氣溶膠顆粒物的化學組分.具體采樣方法為6h的采樣循環,在每1h的采樣周期內FIGAERO以兩種模式進行工作:(1)前24min進行氣態物質實時分析和顆粒物的采集,顆粒物被收集在Teflon膜上;(2)后36min將收集到的顆粒物經高溫氮氣熱解吸進行顆粒物分析.在每36min的顆粒態模式下將收集到的顆粒物熱解吸,以2L/min的氮氣為載氣引入儀器,氮氣在12min內從環境溫度升高到175℃,并保持20min.當經歷5個相同的采樣周期后,通過電磁閥切換進行顆粒物空白采樣作為背景,形成6h的采樣循環,以實現顆粒態數據為1h的高時間分辨率.左旋葡聚糖的標定在實驗室和外場觀測中進行,以保證儀器運行期間數據的準確性.儀器運行期間,人為將不同濃度的左旋葡聚糖標準溶液注射在FIGAERO的Teflon膜上并按顆粒態熱脫附模式升溫以確定左旋葡聚糖的響應因子.

1.3 其他數據分析方法

本次觀測對于乙腈的測量,使用在線質子轉移反應飛行時間質譜(PTR-ToF-MS),因其具有高時間分辨率,高靈敏度,檢測限較低等特點,已廣泛應用于外場觀測[34-35].儀器運行期間,每天利用標準氣體在干燥條件和濕度條件進行多點標定,并根據實驗室濕度實驗以準確定量乙腈濃度[30,36].對于常規痕量氣體CO的測量,使用增強型痕量CO分析儀(型號:48i-TLE)進行了連續在線監測.對于顆粒物中OA濃度的測量,使用Aerodyne公司所研發的高分辨率黑碳飛行時間質譜儀(SP-HR-ToF-AMS)[37-38],可滿足對環境大氣中氣溶膠的化學組分(包括有機物、硫酸鹽、硝酸鹽、銨鹽和氯鹽)的監測.

2 結果與討論

2.1 左旋葡聚糖的濃度水平及空間分布特征

圖1 觀測期間氣象參數和左旋葡聚糖的時間序列

觀測期間各站點氣象參數的時間序列如圖1所示,可以看出城市站點和區域站點的平均溫度分別為(24.2±2.7)和(24.4±3.6)℃,平均濕度分別為(70.1± 17.5)和(66.9±15.4)%,且兩個站點的主導風向為偏北風,平均風速分別為(2.4±1.4)和(1.5±0.8)m/s,較快的風速有利于污染物區域輸送[39].

表1 國內外主要城市秋冬季節大氣顆粒物中左旋葡聚糖濃度水平

通過高時間分辨率測量,觀測期間兩個站點的左旋葡聚糖平均濃度及變化范圍見表1和圖1.由表1可知,珠江三角洲地區城市站點的平均濃度低于區域站點,分別為(0.07±0.08)和(0.14±0.12)μg/m3.在2018和2019年10月中旬前左旋葡聚糖的濃度水平未出現明顯高值,基本處于0.40μg/m3以下的較低水平,于10月中下旬才會在傍晚及夜間出現高值,其原因可能是由于此時珠江三角洲地區已經步入秋收時期,大量農作物殘余物被就地露天焚燒,較高的左旋葡聚糖濃度與周邊及當地的生物質燃燒事件直接相關[40].這與Yuan等[41]以乙腈為生物質燃燒示蹤物,發現珠江三角洲地區10月下旬開始存在普遍焚燒農作物殘余物的現象一致.具體來看,城市站點的最高小時平均濃度出現在10月20日的夜間(0.69μg/m3),區域站點于10月17日開始濃度有所上升,最高小時平均濃度出現在11月18日的傍晚(0.97μg/m3).表明我國珠江三角洲地區秋季左旋葡聚糖濃度呈現區域站點高于城市站點的空間分布特征.

本研究將測量的左旋葡聚糖與其他國內外城市秋冬季節報道的濃度水平進行了比較.從表1可以看出,城市站點測得的左旋葡聚糖平均濃度遠低于秋季成都(0.66μg/m3)和西安(0.58μg/m3)[42-43],與冬季西班牙巴塞羅那(0.06μg/m3)相近[44].區域站點測得的左旋葡聚糖平均濃度與秋季南京(0.18± 0.12)μg/m3相近[45],但遠高于冬季法國多姆山(0.02μg/m3)等海洋地區[46].總體而言,我國珠江三角洲地區秋季左旋葡聚糖濃度低于華北及西南等地區,與歐洲地區觀察到的濃度大致相當.

2.2 左旋葡聚糖的日變化特征

化學示蹤物是目前辨析生物質燃燒事件和量化其對大氣環境影響的常用研究手段[54].作為典型的生物質燃燒示蹤物,觀測期間兩個站點的左旋葡聚糖日變化特征如圖2所示,從圖2反映了各站點左旋葡聚糖濃度隨時間基本呈現早晨和夜間出現峰值的變化趨勢.在早晨,區域站點的濃度持續上升,并于早晨8:00左右出現峰值,而城市站點則一直呈現緩慢下降的變化趨勢,未觀察到明顯的峰值變化,表明區域站點在早晨較為明顯的生物質燃燒活動,城市站點缺乏重要的生物質燃燒排放源.在夜間,兩個站點的左旋葡聚糖濃度均于下午16:00左右開始有明顯上升趨勢,但上升幅度有所不同.城市站點最大上升幅度出現在19:00～20:00左右,從0.09μg/m3上升到0.14μg/m3,上升幅度為55.6%,并在夜間21:00左右達到峰值,隨后濃度開始下降.區域站點最大上升幅度出現在17:00～18:00左右,從0.12μg/m3上升到0.25μg/m3,上升幅度高達108.3%,隨后在夜間19:00左右達到峰值,表明我國珠江三角洲地區秋季區域站點受到了更為嚴重的生物質燃燒污染.相較于區域站點,城市站點的峰值較小且達峰值時間較晚,這種差異可能是因為珠江三角洲地區秋收后農田秸稈露天燃燒活動主要發生在農村地區,城市上風向地區生物質燃燒所排放的污染物可傳輸至城市區域[55].

除了峰值變化外,同時觀察到兩個站點左旋葡聚糖濃度總體上均具有夜間高、日間低的特點,通常在午后達到最低值,隨著光照的減弱和排放量的增加,濃度開始上升并在夜間達到峰值.這不僅因為珠江三角洲地區夜間生物質燃燒活動頻繁以及夜間邊界層高度降低等原因,還因為左旋葡聚糖作為半揮發性有機物,其物理化學性質以及環境條件(溫度、濕度、懸浮顆粒物濃度等)決定了其在氣相和顆粒相中的分配,隨著日間環境溫度的升高,左旋葡聚糖從顆粒相揮發進入氣相[56].同時,目前的研究還發現左旋葡聚糖在大氣環境中并不穩定,會與大氣中OH自由基發生光化學降解,尤其是在高相對濕度條件下[57-58].因此,左旋葡聚糖的濃度變化會受到源排放、光化學和環境條件等作用的影響.

圖2 左旋葡聚糖日變化特征圖

2.3 左旋葡聚糖與其他生物質燃燒示蹤物的相關性

典型的生物質燃燒示蹤物有左旋葡聚糖等有機物以及非海鹽鉀離子等無機物,氣態物質如乙腈和氯甲烷等也被認為是生物質燃燒的示蹤物[59-61]. CO和乙腈因其穩定性較好且壽命相對較長的特點,已被廣泛用于指示生物質燃燒活動[62-63].但研究表明,除了生物質燃燒外,工業排放、汽車排放和燃煤排放也有助于提高它們在大氣中的濃度水平[64-65].

本研究將測量的左旋葡聚糖與一次示蹤物CO和乙腈進行相關性分析,如圖3所示.通過高時間分辨率測量的數據,發現城市和區域站點左旋葡聚糖與CO的具有一定相關性(r=0.43～0.57),在區域站點乙腈的相關性稍高(r=0.59).這表明除了生物質燃燒外,我國珠江三角洲地區秋季CO和乙腈受其他排放源貢獻顯著,如機動車排放以及燃煤排放.同時,城市站點與區域站點獲得的左旋葡聚糖與CO的回歸線斜率基本相同,證實城市和區域影響左旋葡聚糖排放的生物質燃燒類型基本相同.此外,傳統離線采樣提供數據集的時間分辨率一般為1d[16],而高時間分辨率測量則能夠捕捉到明顯的生物質燃燒排放事件.通過小時值和日均值的比較,可以發現兩個站點日均值的相關性要高于小時值,小時值的相關性低,更加凸顯高時間分辨率測量條件下,普通城市排放和生物質燃燒排放的差異.因此,高時間分辨率的測量更易捕捉到生物質燃燒排放事件,也更有助于了解生物質燃燒排放與城市其他排放的差異.

圖3 左旋葡聚糖與一氧化碳和乙腈的散點圖

2.4 生物質燃燒排放對OA的貢獻

在生物質燃燒過程中,植物的纖維素和半纖維素高溫裂解形成脫水糖類物質,其中左旋葡聚糖是氣溶膠中含量最高的脫水糖類化合物,其次是甘露糖和半乳糖,故將左旋葡聚糖作為示蹤物判斷生物質燃燒對OA的貢獻[66-67].珠江三角洲地區城市站點和區域站點的OA平均濃度分別為(19.2±12.6)和(19.2±8.9)μg/m3.從圖4可以看出,兩個站點中測得的左旋葡聚糖與OA之間均呈現顯著的正相關關系,這表明生物質燃燒是我國珠江三角洲地區秋季大氣氣溶膠中OA的重要來源.

左旋葡聚糖與OA的比值(Levoglucosan/OA)已被用于估算生物質燃燒對OA的貢獻,本研究進一步通過受體示蹤物法,利用示蹤物左旋葡聚糖與OA濃度通過下式對生物質燃燒貢獻進行估算:

式中:(Levoglucosan/OA)環境代表觀測期間左旋葡聚糖與OA的平均比值;(Levoglucosan/OA)源代表生物質燃燒源譜中左旋葡聚糖與OA的比值.基于Zhang等[68]和Li等[69]研究得到中國谷類秸稈(水稻、小麥和玉米)燃燒排放的PM2.5中左旋葡聚糖對OC的平均排放因子(8.2%)和OA/OC比值(1.3),本研究估算出兩個站點中生物質燃燒對OA的平均貢獻分別為7.4%(城市站點)和11.4%(區域站點).由圖5可知,生物質燃燒對城市站點OA的貢獻在夜間22:00左右達到峰值(13.3%),對區域站點OA的貢獻在夜間19:00左右達到峰值(18.3%).表明我國珠江三角洲地區秋季生物質燃燒排放對OA的貢獻在夜間明顯高于白天.

同時,Huang等[16]和He等[17]分別于2008年和2009年利用受體模型正矩陣因子分解法(PMF)對高分辨率氣溶膠質譜儀測得的秋季珠江三角洲農村站點和城市站點的OA進行來源識別與定量,解析出生物質燃燒源(BBOA)對OA的貢獻分別為24.5%和24.1%.本研究結果要低于該值,但與Zhu等[70]和Cao等[71]分別于2014年和2015年秋冬季在珠江三角洲城市站點的觀測結果(12.6%和8.9%)相近,說明近年來通過實施嚴格的空氣污染防治措施,使珠江三角洲地區大規模露天焚燒農田秸稈的生物質燃燒活動得到了一定控制[72-73].此外,除了對珠江三角洲地區的研究, Zhang等[74]發現長三角地區夏季小麥收獲和秋季水稻收獲期間的BBOA的貢獻值為36%～39%,以及Xu等[75]發現與以往其他季節相比,生物質燃燒源對北京城市地區OA的貢獻在秋季更加顯著(20%).因此,我國珠江三角洲地區秋季生物質燃燒排放對大氣氣溶膠中OA的貢獻雖然沒有京津冀和長三角地區高,但對珠江三角洲地區大氣細顆粒物污染的影響不可忽視,特別是珠江三角洲農村及周邊地區秋收后大規模的生物質開放式燃燒活動.

圖5 生物質燃燒對OA貢獻的日變化特征

3 結論

3.1 通過高時間分辨率測量,觀測期間城市站點和區域站點的左旋葡聚糖平均濃度分別為(0.07±0.08)和(0.14±0.12)μg/m3,表明我國珠江三角洲地區秋季左旋葡聚糖濃度呈現區域站點高于城市站點的空間分布特征,且與其他國內外城市相比,珠江三角洲地區濃度低于華北及西南等地區,與歐洲地區大致相當.

3.2 兩個站點的左旋葡聚糖濃度隨時間基本呈現早晨及夜間出現峰值的變化趨勢,相較于區域站點,城市站點的峰值較小且達峰值時間較晚,這種差異可能是因為珠江三角洲地區秋收后農田秸稈露天燃燒活動主要發生在農村地區,城市上風向地區生物質燃燒所排放的污染物可傳輸至城市區域.

3.3 兩個站點觀測到左旋葡聚糖與CO和乙腈的相關性較差,表明除了生物質燃燒外,我國珠江三角洲地區秋季CO和乙腈受其他排放源貢獻顯著,如機動車排放以及燃煤排放.通過小時值和日均值的比較,可以發現兩個站點日均值的相關性要高于小時值,小時值的相關性低,更加凸顯高時間分辨率測量條件下,普通城市排放和生物質燃燒排放的差異.

3.4 基于受體示蹤物法估算出生物質燃燒對OA的平均貢獻分別為7.4%(城市站點)和11.4%(區域站點),生物質燃燒對城市站點OA的貢獻在夜間22:00左右達到峰值(13.3%),對區域站點OA的貢獻在夜間19:00左右達到峰值(18.3%).表明我國珠江三角洲地區秋季生物質燃燒排放對OA的貢獻在夜間明顯高于白天.且與其他國內城市相比,我國珠江三角洲地區秋季生物質燃燒排放對大氣氣溶膠中OA的貢獻雖然沒有京津冀和長三角地區高,但對珠江三角洲地區大氣細顆粒物污染的影響不可忽視,特別是珠江三角洲農村及周邊地區秋收后大規模的生物質開放式燃燒活動.

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Contribution of autumn biomass burning to organic aerosol in the Pearl River Delta region.

YANG Lei1, YUAN Bin1*, ZHENG E1, YE Chen-shuo2, WANG Si-hang1, HE Xian-jun1, ZHANG Xiao-xiao1, HUANG Shan1, HU Wei-wei3, SHAO Min1

(1.Institute for Environmental and Climate Research, Guangdong-Hongkong-Macau Joint Laboratory of Collaborative Innovation for Environmental Quality, Jinan University, Guangzhou 511443, China；2.College of Environmental Sciences and Engineering, Peking University, Beijing 100871, China；3.The State Key Laboratory of Organic Geochemistry, Guangzhou Institute of Geochemistry, Chinese Academy of Sciences, Guangzhou 510640, China)., 2023,43(1)：20～28

In this study, we applied thermal desorption chemical ionization Time-of-Flight mass spectrometer (FIGAERO-ToF-CIMS) to obtain high temporal resolution (i.e., hourly) measurements of levoglucosan, which is a widely-used tracer for biomass burning emissions. Using the dataset, we estimate the contributions of biomass burning to organic aerosol (OA) at an urban and a regional site in the autumn of Pearl River Delta (PRD) region. The results demonstrated that the average concentrations of levoglucosan at the urban and regional sites in the autumn PRD region were (0.07 ± 0.08) and (0.14 ± 0.12) μg/m3, respectively. We show that concentrations observed at the regional site were higher than the urban site. The diurnal variation of levoglucosan peaked in the morning and at night at both sites. It was observed that levoglucosan was poorly correlated with CO and acetonitrile at both sites, whereas significantly positive correlation was obtained with OA. The average contributions ofbiomass burning to OA were further estimated based on the tracer method to be 7.4% (urban site) and 11.4% (regional site), respectively,with significantly higher contributions at night than during the day.

biomass burning；tracer；levoglucosan；organic aerosol

X513

A

1000-6923(2023)01-0020-08

楊 磊(1998-),男,安徽銅陵人,暨南大學環境與氣候研究院碩士研究生,主要從事大氣環境化學方面的研究.發表論文1篇.

2022-06-13

國家自然科學基金資助項目(41877302);國家重點研發項目(2019YFE0106300)

* 責任作者, 教授, byuan@jnu.edu.cn