樊澤薇,孔少飛*,嚴(yán) 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(yán) 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(guó)地質(zhì)大學(xué)(武漢)環(huán)境學(xué)院,湖北 武漢 430074；2.中國(guó)地質(zhì)大學(xué)(武漢),生物地質(zhì)與環(huán)境地質(zhì)國(guó)家重點(diǎn)實(shí)驗(yàn)室,湖北 武漢 430074)

基于實(shí)驗(yàn)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(jí)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(yīng)用于源解析和生物質(zhì)燃燒排放氣溶膠傳輸老化研究時(shí)需引起關(guān)注.木柴燃燒排放PM10中的陰陽(yáng)離子當(dāng)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(xì)顆粒物的酸度高于超細(xì)顆粒物和粗顆粒物的酸度.本研究對(duì)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(jìn)相關(guān)氣候和空氣質(zhì)量模型的參數(shù)設(shè)置,識(shí)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(duì)區(qū)域氣候和大氣環(huán)境有重要影響,如增強(qiáng)區(qū)域大氣輻射強(qiáng)迫[1],導(dǎo)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(xiàn)了長(zhǎng)江三角洲地區(qū)大氣污染期間37%的PM2.5,70%的有機(jī)碳和61%的元素碳.Zhang等[4]基于PMF模型解析發(fā)現(xiàn)美國(guó)東南部冬季27%的PM2.5源自生物質(zhì)燃燒.在發(fā)展中國(guó)家,生物質(zhì)作為家庭能源被廣泛使用[5-6].中國(guó)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(huì)導(dǎo)致嚴(yán)重的室內(nèi)空氣污染.2016年室內(nèi)空氣污染導(dǎo)致全球380萬人死亡,占全球死亡率的7.7%[7].對(duì)生物質(zhì)燃燒排放污染物開展研究是對(duì)其進(jìn)行防控的基礎(chǔ).

建立準(zhǔn)確的一次污染物排放清單是識(shí)別相應(yīng)污染物對(duì)環(huán)境和健康影響的基礎(chǔ).排放因子的缺乏嚴(yán)重制約著排放清單的準(zhǔn)確性.國(guó)內(nèi)外學(xué)者開展了生物質(zhì)燃燒水溶性離子排放因子的實(shí)測(cè)研究.Guo等[19]基于室內(nèi)模擬燃燒實(shí)驗(yàn),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(xì)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(gè)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(xué)組分的粒徑分布影響著其環(huán)境、氣候和健康效應(yīng).在計(jì)算硫酸鹽氣溶膠的直接輻射強(qiáng)迫時(shí),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(shí),通常默認(rèn)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(cè)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(xiàn)中報(bào)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(rèn)粒徑以改進(jìn)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(gè)數(shù)量級(jí),而更新后的模擬濃度低估1個(gè)數(shù)量級(jí),更接近觀測(cè)結(jié)果.另外,顆粒物在人體的沉積位置和其對(duì)人體健康的危害與其粒徑大小密切相關(guān).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(kuò)散到循環(huán)系統(tǒng)并進(jìn)入肝臟等器官[26-27].因而研究污染源排放顆粒物及其化學(xué)組分的粒徑分布,對(duì)于更新和改進(jìn)相應(yīng)模型的參數(shù)設(shè)置,提高模擬結(jié)果的準(zhǔn)確性以及評(píng)估污染物對(duì)人體健康的危害具有重要意義.當(dāng)前,僅有少數(shù)研究關(guān)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(gè)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(jī)物、Cl?和SO42?的粒徑分布特征.

本研究基于實(shí)驗(yàn)室模擬燃燒和稀釋通道采樣系統(tǒng),對(duì)6種木柴室內(nèi)燃燒排放的14級(jí)粒徑顆粒物進(jìn)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(shí)別不同粒徑段水溶性離子組成特征,計(jì)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(guān)氣候、空氣質(zhì)量模型參數(shù)設(shè)置等提供基礎(chǔ)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(yáng).

本研究從6個(gè)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(shí)驗(yàn)所用爐具是從市場(chǎng)購(gòu)買的常見民用爐具,高43cm,外徑30cm,內(nèi)徑12cm.

1.2 燃燒實(shí)驗(yàn)

采用稀釋通道采樣系統(tǒng)采集煙氣,模擬煙氣進(jìn)入大氣后的稀釋、冷卻和凝結(jié)等過程[31].實(shí)驗(yàn)流程圖見Cheng等[32]的研究.每種木柴分別進(jìn)行燃燒實(shí)驗(yàn).每次實(shí)驗(yàn)時(shí),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(gè)燃燒過程,時(shí)長(zhǎng)為2h左右.木柴燃燒至無明火時(shí),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(fēng)機(jī)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(jìn)入稀釋通道系統(tǒng).空壓機(jī)產(chǎn)生的空氣經(jīng)過濾后進(jìn)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(jìn)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(gè)粒徑段[33].考慮超細(xì)顆粒物和亞微米顆粒物對(duì)人體健康的危害[34-35],本研究將14級(jí)顆粒物分為超細(xì)顆粒物、亞微米顆粒物、細(xì)顆粒物和粗顆粒物予以討論.本研究中超細(xì)顆粒物(PM0.094)包括前4個(gè)粒徑段,亞微米顆粒物(PM0.94)包括前9個(gè)粒徑段,細(xì)顆粒物(PM2.5)包括前11個(gè)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(gè)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(duì)濕度的超凈實(shí)驗(yàn)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(duì)8種離子(Na+、NH4+、Mg2+、K+、Ca2+、Cl?、NO3?和SO42?)進(jìn)行分析.陽(yáng)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(biāo)準(zhǔn)曲線的相關(guān)系數(shù)均大于0.999.

引渡本應(yīng)是反腐敗國(guó)際刑事司法協(xié)助最直接、最有效的方式，但由于實(shí)際操作上的障礙而難以發(fā)揮作用。我國(guó)目前已與多個(gè)國(guó)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(guó)際公約，但遺憾的是，由于摻雜著戰(zhàn)略關(guān)系和外交關(guān)系的考慮，伴隨著司法觀念、人權(quán)理念的沖突，我國(guó)與西方發(fā)達(dá)國(guó)家締結(jié)的引渡條約數(shù)量偏少。此外，很多國(guó)家規(guī)定，他國(guó)就腐敗犯罪事宜向其提出引渡等請(qǐng)求時(shí)，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(guó)在積極締結(jié)引渡條約的同時(shí)，要不斷探索引渡的替代措施，利用多種途徑實(shí)現(xiàn)對(duì)外逃腐敗犯罪分子的有效懲處:

樣品分析過程執(zhí)行嚴(yán)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(cè)樣.每次進(jìn)樣前先注射一針超純水去除殘留雜質(zhì),每分析10個(gè)樣品抽取第1個(gè)樣品重復(fù)分析,確保前后2次的測(cè)量誤差在10%以內(nèi)再進(jìn)行后續(xù)分析.此外,測(cè)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(jì)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(jì)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(yáng)離子平衡

陰陽(yáng)離子平衡用于檢驗(yàn)生物質(zhì)燃燒排放顆粒物的酸堿性,通過式(2)、(3)計(jì)算陰陽(yáng)離子當(dāng)量排放因子:

式中:CE為陽(yáng)離子當(dāng)量排放因子,AE為陰離子當(dāng)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(xì)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(xì)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(cè)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(xì)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(zhǎng)治柏木中硫、氮和灰分的含量分別是濟(jì)南柏木中的1.5、14.6和10.9倍,這可能是造成長(zhǎng)治柏木燃燒排放總水溶性離子的排放因子是濟(jì)南柏木燃燒排放的4倍的主要原因.長(zhǎng)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(jì)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(xué)組成有關(guān).Ozgen等[20]測(cè)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(shí)測(cè)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(xiàn)結(jié)果進(jìn)行對(duì)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(dá)80倍.木柴燃燒排放PM10的陽(yáng)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(dǎo)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(yīng)用于排放清單構(gòu)建時(shí),應(yīng)盡可能涵蓋不同學(xué)者的研究結(jié)果.

表2 與文獻(xiàn)中排放因子的對(duì)比(mg/kg)

注:–為未分析.除文獻(xiàn)[19]為直接采樣外,其它均為稀釋采樣.文獻(xiàn)[20]中稀釋倍數(shù)為90～150倍,文獻(xiàn)[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?yàn)槟静袢紵欧诺腘O3?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(xì)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(jí)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(shí)別不同種類生物質(zhì)燃燒源對(duì)大氣顆粒物的貢獻(xiàn),分粒徑比值還可應(yīng)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(yīng)轉(zhuǎn)化為K2SO4和KNO3,并生成氣態(tài)HCl,導(dǎo)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(yīng)初始比值.3個(gè)粒徑段的K+/NO3?初始排放比值分別是老化后相應(yīng)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(yīng)導(dǎo)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(yuǎn)低于本研究所得的相應(yīng)初始排放值.3個(gè)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(dǎo)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(fēng)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(chǔ),這些比值的初始排放值在后續(xù)研究中需要被關(guān)注.

2.4 陰陽(yáng)離子比值的粒徑分布

氣溶膠的酸度是顆粒物的重要化學(xué)性質(zhì)之一,可影響二次有機(jī)氣溶膠的形成和多種大氣化學(xué)反應(yīng),對(duì)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(duì)氣溶膠酸度的貢獻(xiàn)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(jí)顆物中陰陽(yáng)離子當(dāng)量比值

圖4為木柴燃燒排放顆粒物中陰陽(yáng)離子當(dāng)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(yáng)離子含量高于陰離子含量,可能與陰離子未完全檢測(cè)有關(guān),如CO32?、HCO3?以及有機(jī)陰離子如草酸根等[47-48].顆粒物中陰陽(yáng)離子當(dāng)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(yáng)離子當(dāng)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(xì)顆粒物的酸度高于超細(xì)顆粒物和粗顆粒物的酸度.PM0.94和PM2.5的酸度較大是因?yàn)榱蛩猁}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(cè)的CO32?和HCO3?與粗顆粒物中的Ca2+和Mg2+結(jié)合有關(guān).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(xué)過程及潛在環(huán)境效應(yīng)也會(huì)有差異,需進(jìn)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(fēng)險(xiǎn)評(píng)估[49]和氣溶膠及其化學(xué)組分的氣候、空氣質(zhì)量效應(yīng)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(duì)木柴燃燒導(dǎo)致的室內(nèi)空氣污染及其帶來的人體健康效應(yīng)的重視.Kiehl等[22]計(jì)算硫酸鹽氣溶膠的輻射強(qiáng)迫時(shí),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(dāng)假定硫酸鹽體積中值粒徑從0.2μm增加到0.4μm時(shí),模擬的輻射強(qiáng)迫增加約20%.因此,Kiehl等[22]的計(jì)算會(huì)高估木柴燃燒排放硫酸鹽的輻射強(qiáng)迫.嚴(yán)沁等[51]實(shí)測(cè)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(xué)組分的質(zhì)量中值粒徑參數(shù)的假設(shè)值與實(shí)際觀測(cè)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(shí)測(cè)研究仍鮮見報(bào)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(yīng)用于源解析和研究生物質(zhì)燃燒排放氣溶膠傳輸老化時(shí)需注意.

3.4 木柴燃燒排放各粒徑段陰陽(yáng)離子當(dāng)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(dāng)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(xì)顆粒物的酸度大于超細(xì)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(shí)測(cè)來更新模型的參數(shù)設(shè)置以改進(jìn)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(tái)人,中國(guó)地質(zhì)大學(xué)(武漢)碩士研究生,主要研究方向?yàn)槊裼萌剂先紵欧盼廴疚锴鍐螛?gòu)建.

2020-10-09

國(guó)家重點(diǎn)研發(fā)計(jì)劃(2017YFC0212602;2016YFA0602002);國(guó)家自然科學(xué)基金資助項(xiàng)目(41830965;42077202)

* 責(zé)任作者, 教授, kongshaofei@cug.edu.cn