樊澤薇,孔少飛*,嚴 沁,鄭淑睿,鄭 煌,姚立全,吳 劍,張 穎,牛真真,吳方琪,程 溢, 曾 昕 ,覃 思,劉 璽,燕瑩瑩,祁士華,2

室內(nèi)木柴燃燒排放水溶性離子粒徑分布特征

樊澤薇1,孔少飛1*,嚴 沁1,鄭淑睿1,鄭 煌1,姚立全1,吳 劍1,張 穎1,牛真真1,吳方琪1,程 溢1, 曾 昕1,覃 思1,劉 璽1,燕瑩瑩1,祁士華1,2

(1.中國地質(zhì)大學(武漢)環(huán)境學院,湖北 武漢 430074；2.中國地質(zhì)大學(武漢),生物地質(zhì)與環(huán)境地質(zhì)國家重點實驗室,湖北 武漢 430074)

基于實驗室模擬燃燒和稀釋通道采樣系統(tǒng),采用荷電低壓撞擊采樣器采集了6種典型木柴燃燒排放的14級粒徑段顆粒物.采用離子色譜分析了8種水溶性離子,獲得水溶性離子的分粒徑排放因子和排放特征.結(jié)果表明,Ca2+的排放因子呈雙峰分布,在0.25～0.38和2.5～3.6μm粒徑段出現(xiàn)峰值,分別為0.14和0.16mg/kg.其余離子的排放因子為單峰分布.NH4+、NO3?和SO42?的排放因子在0.25～0.38μm粒徑段出現(xiàn)峰值,分別為0.41、0.58和0.84mg/kg. K+和Cl?的排放因子在0.15～0.25μm內(nèi)出現(xiàn)峰值,分別為0.89和0.99mg/kg.木柴燃燒排放總水溶性離子的質(zhì)量中值粒徑為(0.30±0.07) μm,各離子的質(zhì)量中值粒徑范圍為0.24～0.44μm.PM0.094、PM0.94、PM2.5和PM10中水溶性離子的排放因子變化范圍分別為1.04～9.33、5.00～48.87、5.46～52.00和6.14～53.68mg/kg.木柴燃燒排放顆粒物中K+/Cl?、K+/NO3?、K+/SO42?和SO42?/NO3?比值隨粒徑變化而變化,其排放初始值在應用于源解析和生物質(zhì)燃燒排放氣溶膠傳輸老化研究時需引起關注.木柴燃燒排放PM10中的陰陽離子當量比值為0.80±0.11,顆粒物的酸度隨顆粒物粒徑而改變,亞微米顆粒物和細顆粒物的酸度高于超細顆粒物和粗顆粒物的酸度.本研究對構(gòu)建生物質(zhì)燃燒排放分粒徑水溶性離子清單,更新和改進相關氣候和空氣質(zhì)量模型的參數(shù)設置,識別煙氣傳輸過程中的老化具有重要意義.

室內(nèi)木柴燃燒；稀釋通道；水溶性離子；粒徑分布；排放因子

生物質(zhì)燃燒是大氣污染物的重要來源之一,其排放的顆粒物對區(qū)域氣候和大氣環(huán)境有重要影響,如增強區(qū)域大氣輻射強迫[1],導致區(qū)域霾事件頻發(fā)[2-3]等.Cheng等[2]通過WRF-CMAQ模擬發(fā)現(xiàn)生物質(zhì)燃燒貢獻了長江三角洲地區(qū)大氣污染期間37%的PM2.5,70%的有機碳和61%的元素碳.Zhang等[4]基于PMF模型解析發(fā)現(xiàn)美國東南部冬季27%的PM2.5源自生物質(zhì)燃燒.在發(fā)展中國家,生物質(zhì)作為家庭能源被廣泛使用[5-6].中國47.6%的農(nóng)村家庭將生物質(zhì)作為日常做飯的主要燃料[6].生物質(zhì)在低效且缺少排放控制措施的爐具中燃燒會導致嚴重的室內(nèi)空氣污染.2016年室內(nèi)空氣污染導致全球380萬人死亡,占全球死亡率的7.7%[7].對生物質(zhì)燃燒排放污染物開展研究是對其進行防控的基礎.

建立準確的一次污染物排放清單是識別相應污染物對環(huán)境和健康影響的基礎.排放因子的缺乏嚴重制約著排放清單的準確性.國內(nèi)外學者開展了生物質(zhì)燃燒水溶性離子排放因子的實測研究.Guo等[19]基于室內(nèi)模擬燃燒實驗,得到6種樹枝明燒和悶燒排放PM2.5中10種水溶性離子的排放因子. Ozgen等[20]模擬兩種木柴在壁爐和木柴爐中的燃燒,得到不同木柴/爐具組合排放超細顆粒物(PM0.1)中水溶性離子的排放因子.劉亞男等[21]研究了秸稈、薪柴及民用煤燃燒排放0～2.5、2.5～10和10～100μm三個粒徑段顆粒物中水溶性離子的排放因子.由于燃料種類、燃燒方式和采樣方法的不同,現(xiàn)有排放因子存在較大差異,需要持續(xù)更新.

氣溶膠化學組分的粒徑分布影響著其環(huán)境、氣候和健康效應.在計算硫酸鹽氣溶膠的直接輻射強迫時,Kiehl等[22]假定硫酸鹽氣溶膠的體積中值粒徑為0.42μm.前人在開展空氣質(zhì)量模擬時,通常默認愛根核模態(tài)和積聚模態(tài)氣溶膠的體積中值粒徑分別為0.03和0.3μm[23].這些參數(shù)值是基于近源觀測得到的大氣氣溶膠粒徑分布的平均值,并非直接的源排放數(shù)據(jù)[24].Elleman等[25]結(jié)合文獻中報道的以交通為主的城市地區(qū)、燃煤電廠和船舶排放的氣溶膠粒徑分布數(shù)據(jù)更新模型的默認粒徑以改進模擬效果,發(fā)現(xiàn)更新參數(shù)前模擬的氣溶膠數(shù)濃度低估1～2個數(shù)量級,而更新后的模擬濃度低估1個數(shù)量級,更接近觀測結(jié)果.另外,顆粒物在人體的沉積位置和其對人體健康的危害與其粒徑大小密切相關.PM10、PM2.5和PM0.1在肺泡區(qū)域的沉積率分別為1%、39%和43%,PM0.1還可擴散到循環(huán)系統(tǒng)并進入肝臟等器官[26-27].因而研究污染源排放顆粒物及其化學組分的粒徑分布,對于更新和改進相應模型的參數(shù)設置,提高模擬結(jié)果的準確性以及評估污染物對人體健康的危害具有重要意義.當前,僅有少數(shù)研究關注了生物質(zhì)燃燒排放水溶性離子的粒徑分布.Zhang等[28]分析了木柴燃燒排放9個粒徑段顆粒物中水溶性離子的分布.Park等[29]研究了秸稈和樹木枝葉燃燒排放K+、Cl?、SO42?和草酸鹽的粒徑分布.Goetz等[30]使用氣溶膠質(zhì)譜研究了牛糞、木柴和農(nóng)作物秸稈在南亞傳統(tǒng)爐具中燃燒排放的有機物、Cl?和SO42?的粒徑分布特征.

本研究基于實驗室模擬燃燒和稀釋通道采樣系統(tǒng),對6種木柴室內(nèi)燃燒排放的14級粒徑顆粒物進行采樣,分析8種水溶性離子,獲得分粒徑水溶性離子排放因子,識別不同粒徑段水溶性離子組成特征,計算得到不同離子的質(zhì)量中值粒徑,為分粒徑組分清單構(gòu)建、相關氣候、空氣質(zhì)量模型參數(shù)設置等提供基礎數(shù)據(jù).

1 材料與方法

1.1 樣品來源

表1 木柴樣品組分分析(%)

注:其余兩種木柴: ZJ:柑橘木,湖北枝江; SY:松木,遼寧沈陽.

本研究從6個不同省市農(nóng)戶家中收集6種木柴,分別為柑橘木(ZJ)、泡桐木(WZ)、樟木(LA)、柏木(JN)、柏木(CZ)和松木(SY).其中4種木柴樣品的組分分析見表1.實驗所用爐具是從市場購買的常見民用爐具,高43cm,外徑30cm,內(nèi)徑12cm.

1.2 燃燒實驗

采用稀釋通道采樣系統(tǒng)采集煙氣,模擬煙氣進入大氣后的稀釋、冷卻和凝結(jié)等過程[31].實驗流程圖見Cheng等[32]的研究.每種木柴分別進行燃燒實驗.每次實驗時,模擬農(nóng)戶炊事過程中木柴的使用量,用電子秤稱取木柴4～5kg,分3次添加.木柴引燃后放入爐具并開始采樣,采樣持續(xù)整個燃燒過程,時長為2h左右.木柴燃燒至無明火時,采樣結(jié)束.燃燒過程中保證充足的空氣使燃料燃燒充分,以模擬農(nóng)戶炊事過程中木柴的燃燒方式.引燃木柴后,風機以恒定流量將排放的煙氣抽入煙道,在距火苗1.5m處使用采樣槍等速采樣,采集部分煙氣進入稀釋通道系統(tǒng).空壓機產(chǎn)生的空氣經(jīng)過濾后進入稀釋艙與煙氣混合稀釋,稀釋倍數(shù)為30倍.待稀釋煙氣在停留艙混合均勻后進行濾膜采集.

1.3 樣品采集

采用荷電低壓撞擊采樣器采集煙氣中的顆粒物,采樣流量為10L/min,采集顆粒物的粒徑分別為0.006～0.016, 0.016～0.030, 0.030～0.054, 0.054～0.094, 0.094～0.15, 0.15～0.25, 0.25～0.38, 0.38～0.60, 0.60～ 0.94, 0.94～1.6, 1.6～2.5, 2.5～3.6, 3.6～5.3和5.3～ 10μm,共14個粒徑段[33].考慮超細顆粒物和亞微米顆粒物對人體健康的危害[34-35],本研究將14級顆粒物分為超細顆粒物、亞微米顆粒物、細顆粒物和粗顆粒物予以討論.本研究中超細顆粒物(PM0.094)包括前4個粒徑段,亞微米顆粒物(PM0.94)包括前9個粒徑段,細顆粒物(PM2.5)包括前11個粒徑段,粗顆粒物(PM2.5～10)包括2.5～3.6、3.6～5.3和5.3～10μm 3個粒徑段.

采樣濾膜為直徑25mm的石英濾膜.采樣前將石英濾膜在馬弗爐中經(jīng)過800℃,0.5h的烘烤,然后在25℃和40%相對濕度的超凈實驗室中平衡48h,稱重后裝入清潔膜盒后放入干燥皿備用.采樣結(jié)束后,將濾膜從采樣器中取出裝入原膜盒,平衡稱重后放入?20℃的冰箱中保存.

1.4 水溶性離子分析與質(zhì)量控制

取整張濾膜剪碎于滅菌離心管中,加入5mL超純水,搖勻后靜置30min,超聲振蕩40min.用孔徑為0.45 μm的水系濾膜過濾上層清液,使用TH-980H型(天虹,武漢)離子色譜儀對8種離子(Na+、NH4+、Mg2+、K+、Ca2+、Cl?、NO3?和SO42?)進行分析.陽離子使用Shodex YS-50分析柱,淋洗液為4.5mmol/L甲烷磺酸,流量為0.9mL/min.陰離子使用Shodex SI-90 4E分析柱,淋洗液為1.8mmol/L的Na2CO3和1.7mmol/L的NaHCO3,流量為1.15mL/ min.8種離子的儀器檢出限均低于0.01 μg/mL.8種離子標準曲線的相關系數(shù)均大于0.999.

引渡本應是反腐敗國際刑事司法協(xié)助最直接、最有效的方式，但由于實際操作上的障礙而難以發(fā)揮作用。我國目前已與多個國家締結(jié)了雙邊引渡條約并且參加了不少含有引渡條款的多邊國際公約，但遺憾的是，由于摻雜著戰(zhàn)略關系和外交關系的考慮，伴隨著司法觀念、人權(quán)理念的沖突，我國與西方發(fā)達國家締結(jié)的引渡條約數(shù)量偏少。此外，很多國家規(guī)定，他國就腐敗犯罪事宜向其提出引渡等請求時，要受到雙重犯罪原則、特定性原則、政治犯罪不引渡原則或死刑不引渡原則的限制，從而為引渡腐敗分子帶來了困難。因此，我國在積極締結(jié)引渡條約的同時，要不斷探索引渡的替代措施，利用多種途徑實現(xiàn)對外逃腐敗犯罪分子的有效懲處:

樣品分析過程執(zhí)行嚴格的質(zhì)量控制.濾膜提取和淋洗液配置均使用電阻率為18.2MΩ·cm的超純水.每天儀器開啟,待基線穩(wěn)定后測樣.每次進樣前先注射一針超純水去除殘留雜質(zhì),每分析10個樣品抽取第1個樣品重復分析,確保前后2次的測量誤差在10%以內(nèi)再進行后續(xù)分析.此外,測定空白濾膜樣,從樣品分析結(jié)果中扣除其平均值.

1.5 排放因子計算

將排放的污染物質(zhì)量除以消耗的燃料質(zhì)量來確定排放因子,表示為每千克消耗的干燃料排放的污染物.排放因子按公式(1)計算[36]:

式中: EF為第種木柴燃燒后類水溶性離子的排放因子,mg/kg;m為第種木柴燃燒后類水溶性離子的質(zhì)量,mg;為煙氣流量,L/min;1為采樣流量, L/min;為稀釋倍數(shù);M為第種木柴的消耗量,kg.

1.6 陰陽離子平衡

陰陽離子平衡用于檢驗生物質(zhì)燃燒排放顆粒物的酸堿性,通過式(2)、(3)計算陰陽離子當量排放因子:

式中:CE為陽離子當量排放因子,AE為陰離子當量排放因子,[]為各水溶性離子排放因子,mg/kg.

2 結(jié)果與討論

2.1 水溶性離子排放因子的粒徑分布

如圖1所示,Ca2+的排放因子呈雙峰分布,粗、細粒子中均有峰值,在粗粒子2.5～3.6μm內(nèi)有主峰,為0.16mg/kg;在細粒子0.25～0.38μm內(nèi)有次峰,為0.14mg/kg.Na+和Mg2+的排放因子較小,在此不作討論.其余離子呈單峰分布,且峰值均出現(xiàn)在亞微米粒徑段內(nèi).NH4+、NO3?和SO42?的排放因子在0.25～ 0.38μm內(nèi)出現(xiàn)峰值,分別為0.41、0.58和0.84mg/kg. K+和Cl?的排放因子在0.15～0.25μm內(nèi)出現(xiàn)峰值,分別為0.89和0.99mg/kg.Park等[29]研究表明,秸稈和樹木枝葉燃燒排放的K+、Cl?和SO42?呈單峰分布,除銀杏葉和楓葉峰值分布在0.55～1.0μm外,其它生物質(zhì)峰值均分布在0.32～0.55μm粒徑段.Goetz等[30]采用AMS測得硬木和樹枝在傳統(tǒng)泥爐中燃燒排放PM1中的Cl?分別在133和123nm處出現(xiàn)峰值,與本研究結(jié)果具有可比性.由此可見,減少木柴燃燒可有效降低大氣亞微米顆粒物中水溶性離子的含量.

圖1 室內(nèi)木柴燃燒水溶性離子排放因子的粒徑分布

EF為各水溶性離子的排放因子,mg/kg;p為各采樣粒徑段上限粒徑和下限粒徑的幾何平均值,mm

2.2 分粒徑顆粒物中水溶性離子的排放因子

如圖2所示,6種木柴燃燒排放88.3%～96.9%的水溶性離子分布在細顆粒物中,79.5%～91.1%分布在亞微米顆粒物中.不同種類木柴的總水溶性離子排放因子相差2～10倍.PM0.094、PM0.94、PM2.5和PM10中水溶性離子的排放因子分別為1.04～ 9.33、5.00～ 48.87、5.46～52.00和6.14～53.68mg/kg.長治柏木中硫、氮和灰分的含量分別是濟南柏木中的1.5、14.6和10.9倍,這可能是造成長治柏木燃燒排放總水溶性離子的排放因子是濟南柏木燃燒排放的4倍的主要原因.長治柏木燃燒排放SO42?、NO3?、NH4+、Na+、Mg2+、K+、Ca2+和Cl?的排放因子分別是濟南柏木燃燒排放的4.0、0.9、8.6、1.7、1.7、4.6、1.3和8.1倍,表明水溶性離子排放因子的差異與木柴的化學組成有關.Ozgen等[20]測得木柴燃燒排放PM0.1中水溶性離子的排放因子為28～ 67mg/kg,是本研究的3～64倍.Calvo等[37]實測得到木柴燃燒排放PM2.5中水溶性離子的排放因子為143.1～300.2mg/kg,是本研究的2.8～55倍.Guo等[19]直接采樣得到PM2.5中水溶性離子的排放因子為425～1546mg/kg,是本研究的8.2～283倍.Alves等[38]的結(jié)果顯示,金荊樹燃燒煙塵中水溶性離子排放因子是本研究的1.5～259倍.

圖2 不同產(chǎn)地木柴室內(nèi)燃燒水溶性離子的排放因子

表2將本研究的排放因子與文獻結(jié)果進行對比,發(fā)現(xiàn)不同研究所得水溶性離子排放因子的差異最大可達80倍.木柴燃燒排放PM10的陽離子中,K+的排放因子最大,為(4.7±3.6) mg/kg,分別為Na+、NH4+、Mg2+和Ca2+排放因子的11.7、2.5、23.5和2.8倍;陰離子中,Cl?和SO42?的排放因子較大,分別為(5.5±5.6)和(4.6±3.8) mg/kg,分別是NO3?排放因子的2.2和1.8倍.Ozgen等[20]的結(jié)果中Ca2+和Cl?的排放因子與本研究結(jié)果相似,K+、NO3?和SO42?排放因子分別比本研究PM0.094結(jié)果高8.9、14.5和36.8倍.Calvo等[37]結(jié)果中Na+的排放因子比本研究PM2.5結(jié)果高100～135倍,櫸木其余離子的排放因子與本研究結(jié)果相似,其它兩種木柴各離子的排放因子比本研究結(jié)果高2～36倍.Guo等[19]采用直接采樣方法所得PM2.5中各離子的排放因子均高于本研究結(jié)果,高7.4～155倍.Sen等[39]的研究結(jié)果中各離子的排放因子高于本研究結(jié)果5～100倍.劉亞男等[21]所得木柴燃燒PM中K+的排放因子比本研究結(jié)果高11～27倍,Ca2+的排放因子比本研究結(jié)果高86～124倍.本研究所得排放因子數(shù)據(jù)偏低,可能是由于采樣儀器和采樣方法的不同導致的.木柴燃燒水溶性離子的排放因子受采樣方法、采樣儀器、燃料類型、燃燒條件和狀態(tài)等影響較大,在應用于排放清單構(gòu)建時,應盡可能涵蓋不同學者的研究結(jié)果.

表2 與文獻中排放因子的對比(mg/kg)

注:–為未分析.除文獻[19]為直接采樣外,其它均為稀釋采樣.文獻[20]中稀釋倍數(shù)為90～150倍,文獻[39]中稀釋倍數(shù)為40～60倍.

2.3 水溶性離子比值的粒徑分布

木柴燃燒排放顆粒物中K+/Cl?、K+/NO3?、K+/SO42?和SO42?/NO3?比值隨顆粒物粒徑的變化見圖3.本研究中K+/Cl?比值在0.054～0.94μm粒徑段表現(xiàn)出高值,變化范圍為0.85～1.32;在<0.054μm和>0.94μm粒徑段表現(xiàn)出低值,變化范圍為0.20～0.66.K+/NO3?比值在0.054～0.94μm粒徑段表現(xiàn)出低值,范圍為2.52～8.10;在<0.054μm和>0.94μm粒徑段表現(xiàn)出高值,范圍為10.85～41.76.K+/SO42?比值在0.38～2.5μm粒徑段表現(xiàn)出低值,范圍為0.46～0.75;在<0.38μm和>2.5μm粒徑段表現(xiàn)出高值,范圍為0.87～3.49,可能是因為木柴燃燒排放的NO3?主要分布在亞微米顆粒物中,排放的SO42?主要分布在亞微米顆粒物和細顆粒物中.SO42?/NO3?比值在0.030～0.94μm粒徑段表現(xiàn)出低值,范圍為2.20～8.85;在<0.030μm和>0.94μm粒徑段表現(xiàn)出高值,范圍為18.57～137.94.

圖3 室內(nèi)木柴燃燒排放分級顆粒物中K+/Cl?、K+/NO3?、K+/SO42?和SO42?/NO3?比值

有研究表明,K+/Cl?和K+/SO42?比值可用于識別不同種類生物質(zhì)燃燒源對大氣顆粒物的貢獻,分粒徑比值還可應用于分粒徑源解析研究中[29,40].K+/ Cl?、K+/NO3?、K+/SO42?和SO42?/NO3?比值還可用于判斷生物質(zhì)燃燒排放氣溶膠在大氣中的傳輸老化程度.在生物質(zhì)燃燒排放氣溶膠傳輸過程中,KCl與SO2和NO發(fā)生非均相反應轉(zhuǎn)化為K2SO4和KNO3,并生成氣態(tài)HCl,導致老化氣溶膠中Cl?含量降低,SO42?和NO3?的含量升高,K+/Cl?比值增加, K+/SO42?和K+/NO3?比值降低[41-42].本研究中木柴燃燒排放的PM0.94中的K+/Cl?比值為1.02,表明煙氣中K+主要與Cl?結(jié)合.而受生物質(zhì)氣溶膠影響的大氣PM1中的K+/Cl?比值為1.60[1],該比值大于1表明Cl?的損失以及SO42?和NO3?的生成.前人研究表明受生物質(zhì)燃燒排放氣溶膠影響的大氣PM1.1、PM1.1～2.1和PM2.1～10中K+/NO3?比值分別為0.50、0.30和0.32, K+/SO42?比值分別為0.28、0.15和0.07[1],各粒徑段比值均低于本研究中的相應初始比值.3個粒徑段的K+/NO3?初始排放比值分別是老化后相應比值的9.1、36.2和51.8倍,K+/SO42?初始排放比值分別是老化后該比值的5.3、3.5和16.2倍.這些結(jié)果均表明生物質(zhì)氣溶膠傳輸過程中發(fā)生了非均相反應導致Cl?、SO42?和NO3?的含量發(fā)生變化.本研究中木柴燃燒排放的PM0.94、PM0.94～2.5和PM2.5～10中SO42?/NO3?比值分別為3.25、41.86和56.19.前人研究表明受生物質(zhì)燃燒排放氣溶膠影響的大氣PM1.1、PM1.1～2.1和PM2.1～10中SO42?/NO3?比值分別為1.78、2.19和4.42[1],遠低于本研究所得的相應初始排放值.3個粒徑段初始排放比值是老化后該比值的1.8、19.1和12.7倍,這可能是由于在生物質(zhì)氣溶膠傳輸過程中NO3?的生成速率大于SO42?的生成速率導致的[43].Akagi等[43]也發(fā)現(xiàn)生物質(zhì)煙氣排放約4.5h后的順風煙羽中的SO42?/NO3?比值降低,由初始值0.037降為0.017.因此,作為研究生物質(zhì)燃燒排放氣溶膠大氣傳輸轉(zhuǎn)化的基礎,這些比值的初始排放值在后續(xù)研究中需要被關注.

2.4 陰陽離子比值的粒徑分布

氣溶膠的酸度是顆粒物的重要化學性質(zhì)之一,可影響二次有機氣溶膠的形成和多種大氣化學反應,對人體健康、環(huán)境和氣候有重要意義[44-45].研究源排放氣溶膠酸度的粒徑分布可以為研究排放源對氣溶膠酸度的貢獻和區(qū)域酸沉降提供參考[46].

圖4 室內(nèi)木柴燃燒排放分級顆物中陰陽離子當量比值

圖4為木柴燃燒排放顆粒物中陰陽離子當量排放因子比值隨粒徑的變化.PM10中的比值為0.80± 0.11,除0.38～0.60μm粒徑段內(nèi)該比值為1.05±0.14外,其它各粒徑段比值變化范圍為0.30～0.92,均小于1,表明陽離子含量高于陰離子含量,可能與陰離子未完全檢測有關,如CO32?、HCO3?以及有機陰離子如草酸根等[47-48].顆粒物中陰陽離子當量排放因子比值在￡0.016μm粒徑段內(nèi)最小,比值為0.30±0.10,隨著顆粒物粒徑的增加,該比值先增大再減小,在0.38～0.6μm粒徑段內(nèi)比值最大,為1.05±0.14,最大值是最小值的3.5倍.PM0.094、PM0.94、PM2.5和PM2.5～10中的陰陽離子當量比值分別為0.64±0.21、0.86± 0.14、0.84±0.12和0.53±0.11,表明亞微米顆粒物和細顆粒物的酸度高于超細顆粒物和粗顆粒物的酸度.PM0.94和PM2.5的酸度較大是因為硫酸鹽和硝酸鹽主要集中在PM0.94和PM2.5中.PM2.5～10的酸度較小可能是粗顆粒物中存在未檢測的CO32?和HCO3?與粗顆粒物中的Ca2+和Mg2+結(jié)合有關.由此可見,木柴燃燒排放不同粒徑段顆粒物的酸度存在差異,其排放后在大氣中的化學過程及潛在環(huán)境效應也會有差異,需進一步深入研究.

2.5 水溶性離子的質(zhì)量中值粒徑

質(zhì)量中值粒徑(MMD)常被用于人體健康風險評估[49]和氣溶膠及其化學組分的氣候、空氣質(zhì)量效應模擬[22-23].本研究得到的木柴燃燒排放水溶性離子的質(zhì)量中值粒徑見表3.木柴燃燒排放總水溶性離子的質(zhì)量中值粒徑為(0.30±0.07)μm,各離子的質(zhì)量中值粒徑范圍為0.24～0.44μm,均在亞微米粒徑段,表明木柴燃燒排放的水溶性離子超過50%可以在人體肺泡區(qū)域沉積,需要提高對木柴燃燒導致的室內(nèi)空氣污染及其帶來的人體健康效應的重視.Kiehl等[22]計算硫酸鹽氣溶膠的輻射強迫時,假定其體積中值粒徑為0.42μm,并根據(jù)Whitby[24]的方法轉(zhuǎn)化成質(zhì)量中值粒徑為0.55μm,高于本研究中SO42?的質(zhì)量中值粒徑((0.38±0.05)μm).Boucher等[50]發(fā)現(xiàn)當假定硫酸鹽體積中值粒徑從0.2μm增加到0.4μm時,模擬的輻射強迫增加約20%.因此,Kiehl等[22]的計算會高估木柴燃燒排放硫酸鹽的輻射強迫.嚴沁等[51]實測得到塊煤明燒、蜂窩煤明燒和悶燒排放SO42?的質(zhì)量中值粒徑分別為0.89、0.79和0.38μm.現(xiàn)有模型中氣溶膠化學組分的質(zhì)量中值粒徑參數(shù)的假設值與實際觀測值存在較大偏差,使得模擬結(jié)果存在較大不確定性.目前此類實測研究仍鮮見報道,需予以重視和持續(xù)更新.

表3 室內(nèi)木柴燃燒排放水溶性離子的質(zhì)量中值粒徑(μm)

3 結(jié)論

3.1 木柴燃燒Ca2+的排放因子呈雙峰分布,在0.25～0.38和2.5～3.6μm粒徑段出現(xiàn)峰值,分別為0.14和0.16mg/kg.其余離子的排放因子呈單峰分布, NH4+、NO3?和SO42?的排放因子在0.25～0.38μm內(nèi)出現(xiàn)峰值,分別為0.41、0.58和0.84mg/kg. K+和Cl?的排放因子在0.15～0.25μm內(nèi)出現(xiàn)峰值,分別為0.89和0.99mg/kg.

3.2 室內(nèi)木柴燃燒排放的PM0.094、PM0.94、PM2.5和PM10中水溶性離子的排放因子分別為1.04～ 9.33、5.00～48.87、5.46～52.00和6.14～53.68mg/kg.

3.3 木柴燃燒排放水溶性離子的K+/Cl?、K+/NO3?、K+/SO42?和SO42?/NO3?比值有明顯的粒徑分布變化,在應用于源解析和研究生物質(zhì)燃燒排放氣溶膠傳輸老化時需注意.

3.4 木柴燃燒排放各粒徑段陰陽離子當量比值變化范圍為0.30～1.05,PM0.094、PM0.94、PM2.5和PM2.5～10中的當量比值分別為0.64±0.21、0.86±0.14、0.84± 0.12和0.53±0.11,亞微米顆粒物和細顆粒物的酸度大于超細顆粒物和粗顆粒物的酸度.

3.5 木柴燃燒排放總水溶性離子的質(zhì)量中值粒徑為(0.30±0.07)μm,各離子的質(zhì)量中值粒徑范圍為0.24～0.44μm,均在亞微米粒徑段.需要通過實測來更新模型的參數(shù)設置以改進模擬結(jié)果.

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Size distribution of water-soluble ions in particles emitted from domestic firewood burning.

FAN Ze-wei1, KONG Shao-fei1*, YAN Qin1, ZHENG Shu-rui1, ZHENG Huang1, YAO Li-quan1, WU Jian1, ZHANG Ying1, NIU Zhen-zhen1, WU Fang-qi1, CHENG Yi1, ZENG Xin1, QIN Si1, LIU Xi1, YAN Ying-ying1, QI Shi-hua1,2

(1.School of Environmental Studies, China University of Geosciences, Wuhan 430074, China；2.State Key Laboratory of Biogeology and Environmental Geology, China University of Geosciences, Wuhan 430074, China)., 2021,41(5)：2064～2072

Six kinds of domestic firewood were burned in a laboratory. The emitted particles in fourteen sizes were collected using a dilution sampling system and an electrical low-pressure impactor. The emission characteristic and size-resolved emission factors (EFs) of eight types of water-soluble ions were identified. Results showed that the EFs of Ca2+exhibited a bimodal size distribution, with peaks of 0.14 and 0.16mg/kg for particles within 0.25～0.38 and 2.5～3.6μm, respectively. The EFs of other ions were characterized by a unimodal size distribution. The EFs of NH4+, NO3?and SO42?peaked at 0.25～0.38μm, with peaks of 0.41, 0.58 and 0.84mg/kg, respectively. The EFs of K+and Cl?exhibited highest values at 0.15～0.25μm of 0.89 and 0.99mg/kg, respectively. The mass median diameters of total water-soluble ions from firewood burning were (0.30±0.07)μm, and those of individual ions ranged in 0.24～0.44μm. The EFs of water-soluble ions in PM0.094, PM0.94, PM2.5and PM10were 1.04～9.33, 5.00～48.87, 5.46～52.00 and 6.14～53.68mg/kg, respectively. The ratios of K+/Cl?, K+/NO3?, K+/SO42?and SO42?/NO3?in particles emitted from firewood burning varied with particle size. Their primary emission values should be emphasized when they were used in source apportionment and smoke aging researches. The anion/cation equivalent ratios of PM10from firewood burning were 0.80±0.11. The acidity of PM0.94and PM2.5were higher than those of PM0.094and PM2.5～10. This study is significant to establish emission inventory of size-resolved water-soluble ions, update and improve the parameter settings of corresponding climate and air quality models, and identify the evolution mechanisms of smokes during transport and aging.

domestic firewood burning；dilution tunnel；water-soluble ions；size distribution；emission factors

X51

A

1000-6923(2021)05-2064-09

樊澤薇(1998-),女,河北邢臺人,中國地質(zhì)大學(武漢)碩士研究生,主要研究方向為民用燃料燃燒排放污染物清單構(gòu)建.

2020-10-09

國家重點研發(fā)計劃(2017YFC0212602;2016YFA0602002);國家自然科學基金資助項目(41830965;42077202)

* 責任作者, 教授, kongshaofei@cug.edu.cn