李博偉，黃 宇，何世恒，薛永剛，劉隨心，程 燕，王麗琴，曹軍驥

我國(guó)大氣中揮發(fā)性有機(jī)物的分布特征

李博偉1,2，黃 宇2，何世恒2，薛永剛2，劉隨心2，程 燕1，王麗琴2，曹軍驥2

1.西安交通大學(xué) 人居環(huán)境與建筑工程學(xué)院 地球環(huán)境科學(xué)系，西安 710049
2.中國(guó)科學(xué)院地球環(huán)境研究所 中國(guó)科學(xué)院氣溶膠化學(xué)與物理重點(diǎn)實(shí)驗(yàn)室，西安 710061

大氣中揮發(fā)性有機(jī)物（VOCs）是臭氧和二次有機(jī)氣溶膠形成的關(guān)鍵前體物之一，研究表明烷烴、烯烴、芳香烴是我國(guó)大氣VOCs的重要組分。在不同區(qū)域，城市地區(qū)烷烴含量最高，而偏遠(yuǎn)地區(qū)芳香烴為含量最豐富的VOCs。VOCs濃度日間變化多呈雙峰分布趨勢(shì)，峰值多出現(xiàn)在早晨與傍晚的上下班高峰期。目前對(duì)我國(guó)臭氧污染事件的研究均表明芳香烴和烯烴是對(duì)臭氧生成貢獻(xiàn)最大的化合物。VOCs源解析中廣泛運(yùn)用的模型包括CMB、PMF和PCA/APCS，各模型均存在優(yōu)點(diǎn)和局限性。比較各地VOCs源解析結(jié)果，發(fā)現(xiàn)交通排放源和工業(yè)排放源為我國(guó)VOCs的主要人為來(lái)源。VOCs的跨區(qū)域傳輸決定與周邊地區(qū)的合作將是未來(lái)空氣治理中的發(fā)展方向。

揮發(fā)性有機(jī)物（VOCs）；源解析；分布特征

揮發(fā)性有機(jī)物（volatile organic compounds，VOCs）是一類重要的空氣污染物，主要包括烴類、醛酮類、酯類、醇類等。機(jī)動(dòng)車尾氣、溶劑使用、化石燃料的燃燒、工業(yè)生產(chǎn)過(guò)程為其主要人為來(lái)源（Mo et al，2014；王麗琴等，2016）。VOCs不僅對(duì)人體健康有明顯的毒副作用，且是地表臭氧形成的重要前體物之一（Ait-Helal et al，2014）。此外，VOCs經(jīng)過(guò)光化學(xué)反應(yīng)可生成二次有機(jī)氣溶膠（secondary organic aerosol，SOA）（曹軍驥和李建軍，2016）。Volkamer et al（2006）根據(jù)墨西哥市的研究結(jié)果推算出人為源VOCs每年可產(chǎn)生3 — 25 Tg SOA，占全球SOA總量的1/3。

高濃度的臭氧污染事件與人為源VOCs的大量排放密切相關(guān)。據(jù)統(tǒng)計(jì)，1990 — 2010年，隨著歐美國(guó)家空氣污染控制政策的有效實(shí)行，歐洲和美國(guó)VOCs濃度水平分別以每年3.3%、3.3%的速率下降，臭氧濃度水平分別以每年0.54%、0.66%的速率下降。受經(jīng)濟(jì)快速發(fā)展、汽車保有量急劇增加的影響，東亞國(guó)家VOCs和臭氧濃度水平近年來(lái)分別以每年2.3%和1.49%的速率上升（Xing et al，2015），在我國(guó)北京鄉(xiāng)村地區(qū)夏季地表臭氧小時(shí)平均濃度最高可達(dá)286 ppbv（文中ppbv代表nL · L?1）（Xue et al，2014a）。高濃度的臭氧暴露可誘發(fā)人體呼吸系統(tǒng)、皮膚等機(jī)體的病變，從而引起住院率和死亡率增加，同時(shí)可導(dǎo)致農(nóng)作物減產(chǎn)，研究表明美國(guó)小麥和大豆在高濃度臭氧環(huán)境中產(chǎn)量分別降低4.9%和6.7%（Wei et al，2014；Lapina et al，2016）。

光化學(xué)反應(yīng)是近地表臭氧重要來(lái)源，臭氧的產(chǎn)率與VOCs和氮氧化物（nitrogen oxides，NOx）具有非線性關(guān)系（Perring et al，2013；Tie and Dai，2016）。當(dāng)NOx濃度較低時(shí)，臭氧的產(chǎn)率隨NOx濃度增加，稱為NOx控制區(qū)；隨著NOx濃度的增加，近地表臭氧的產(chǎn)率隨NOx濃度增加的趨勢(shì)減弱，臭氧產(chǎn)率隨VOCs濃度升高而迅速升高，稱為VOCs控制區(qū)（Sillman，1999）。我國(guó)多數(shù)城市臭氧的生成過(guò)程處于VOCs控制區(qū)，因此研究我國(guó)VOCs的排放特征和采取有效的控制措施對(duì)降低臭氧的生成量至關(guān)重要（Geng et al，2008；Shao et al，2009； Chen et al，2013；Tie et al，2013）。

在臭氧形成轉(zhuǎn)換機(jī)制研究中，VOCs與NOx間比值等參數(shù)多是參考洛杉磯研究所確定的比值（Seinfeld，1989）。而中西方國(guó)家能源結(jié)構(gòu)、植被覆蓋率等因素的差異使臭氧形成機(jī)制的研究受到參考數(shù)據(jù)的限制。如我國(guó)汽油中烯烴含量遠(yuǎn)高于西方國(guó)家，機(jī)動(dòng)車尾氣產(chǎn)生的乙烯和丙烯對(duì)近地表臭氧具有重要貢獻(xiàn)（Mo et al，2014）；而美國(guó)機(jī)動(dòng)車尾氣中烯烴含量顯著低于中國(guó)，生物源異戊二烯為含量最高的不飽和烯烴（Baker et al，2008）。此外，我國(guó)大氣組成（顆粒物，NOx，VOCs等）也與西方國(guó)家存在較大差異，因此國(guó)外標(biāo)準(zhǔn)不一定適用于我國(guó)污染情況，建立適用于我國(guó)的檢測(cè)方法，結(jié)合WRF-Chem、CMAQ等數(shù)值模式對(duì)臭氧生成狀況進(jìn)行實(shí)際模擬是今后研究中亟待解決的問(wèn)題。

研究我國(guó)大氣VOCs的分布特征對(duì)采取有效的空氣污染控制措施至關(guān)重要。本文綜述了目前我國(guó)大氣VOCs的時(shí)空分布特征，概述了高濃度臭氧事件與VOCs的關(guān)系，介紹了常用的VOCs源解析方法，包括正定矩陣因子分解法（positive matrix factorization，PMF）、化學(xué)質(zhì)量平衡模型法（chemical mass balance，CMB），及主成分分析/絕對(duì)主成分得分法（principal component analysis/absolute principal component scores，PCA/ APCS），并展望了我國(guó)VOCs研究的發(fā)展趨勢(shì)。

1 我國(guó)VOCs時(shí)空分布

VOCs的組成和分布隨時(shí)間和空間而變化，了解我國(guó)不同區(qū)域VOCs的排放特征和分布狀況，識(shí)別主要排放源和主要活性化合物，有利于確定應(yīng)優(yōu)先控制的污染物種類，提高污染物控制效率。

1.1 區(qū)域分布

我國(guó)幅員遼闊，南北方產(chǎn)業(yè)結(jié)構(gòu)、氣候條件和經(jīng)濟(jì)發(fā)展水平的差異導(dǎo)致排放到環(huán)境大氣中的VOCs呈現(xiàn)出不同的理化特征。污染源強(qiáng)度是不同區(qū)域VOCs濃度與組成差異的重要原因。在城市地區(qū)，人為源是大氣中VOCs的最主要來(lái)源，常見(jiàn)的VOCs人為源可分為六類：工業(yè)、居民煤燃燒，機(jī)動(dòng)車尾氣，燃料揮發(fā)，試劑揮發(fā)，石油化工，生物質(zhì)燃燒等（Liu et al，2008b；Yuan et al，2010；Zhang et al，2013a；Zheng et al，2013； Wang et al，2014b；Wei et al，2014）。其中燃料揮發(fā)是大氣中烷烴類VOCs的主要來(lái)源，異戊烷是其標(biāo)志VOCs。而不飽和烴類和苯主要產(chǎn)生于燃料的不完全燃燒過(guò)程，常來(lái)源于汽車尾氣排放。醇、醚、酯和苯系物等物質(zhì)主要來(lái)自溶劑揮發(fā)，而工業(yè)過(guò)程易產(chǎn)生鹵代烴（Wei et al，2012）。通過(guò)比較我國(guó)幾個(gè)大城市地區(qū)的VOCs濃度水平（表1），發(fā)現(xiàn)廣州和上海地區(qū)甲苯含量較高，可能與珠江三角洲地區(qū)存在較多的印刷、建筑裝飾裝修、電子和機(jī)械設(shè)備制造業(yè)大量的溶劑使用有關(guān)（Li and Wang，2012；Zou et al，2015）；而上海地區(qū)高濃度水平的甲苯（4.7 ppbv）主要與煉鋼和燃煤電廠有關(guān)（Cai et al，2010）。不同于南方城市，北京地區(qū)機(jī)動(dòng)車排放為VOCs的最大排放源，乙烷（6.38 ppbv）、丙烷（10.8 ppbv）較其他化合物含量高（Li et al，2015a）；與北京、上海、廣州等地相比，蘭州地區(qū)苯與甲苯比值最高，可能與存在不同于其他城市的一次排放源有關(guān)（Jia et al，2016）。

表1 我國(guó)主要城市VOCs平均濃度水平（ppbv）Tab.1 The average concentrations of VOCs in different cities in China （ppbv）

（待續(xù) To be continued）

（續(xù)表1 Continued Tab.1）

城市地區(qū)能源消耗總量較高，燃料揮發(fā)、不完全燃燒及工業(yè)活動(dòng)會(huì)排放大量的VOCs，與人類活動(dòng)息息相關(guān)。如京津冀地區(qū)VOCs中濃度較高的是含碳數(shù)為4以下的烷烴，其次為烯烴和苯系物。Wang et al（2014b）估算了北京市VOCs的排放量，發(fā)現(xiàn)北京市TVOCs（total volatile organic compounds，TVOCs）排放量為（419 ± 201）Gg · a?1，其中非甲烷烴（non-methane hydrocarbons，NMHC）的排放量約為其他城市的兩倍（表1）；而珠三角地區(qū)丙烷與苯系物的濃度較高，TVOCs的濃度（約20 — 40 ppbv）亦高于京津冀地區(qū)（約24 ppbv），與廣州、深圳等地區(qū)制造業(yè)較多且珠三角地區(qū)出租車多使用液化石油氣（liquefi ed petroleum gas，LPG）有關(guān)（Li et al，2015）。不同城市之間的VOCs排放特征差異顯著。例如自1997年香港環(huán)保署推行清潔能源 —— 液化石油氣的使用以來(lái)，截止到2010年底約有99%的出租車和51%的私家車以LPG為燃料，使香港地區(qū)大氣中含量較高的VOCs由此前的甲苯轉(zhuǎn)變?yōu)楸楹投⊥椋↙ing and Guo，2014；Lyu et al，2016）；丙烯和乙炔在廣州大氣環(huán)境中含量較高（Li and Wang，2012）；北京地區(qū)機(jī)動(dòng)車多以汽油為燃料，乙烯、乙炔和含碳數(shù)2 — 5的烷烴含量相對(duì)較高（Li et al，2015b）。

偏遠(yuǎn)地區(qū)大氣中VOCs的組成與濃度主要受自然源排放及大氣遠(yuǎn)距離傳輸?shù)挠绊憽Ｑ芯匡@示，位于我國(guó)西南部的貢嘎山與山東中部泰山大氣中主要產(chǎn)生自生物排放源的烯烴1-丁烯、異戊二烯等的排放比例（7% — 8%、4% — 7%）顯著高于北京、上海、香港等城市（0.3% — 1%、0.2% — 2%），且由于芳烴較強(qiáng)的遠(yuǎn)距離傳輸能力，貢嘎山大氣中芳烴類化合物排放比例相對(duì)較高，如貢嘎山和泰山大氣中苯的比例分別為13.4%和12.8%，高于北京、上海、香港等城市地區(qū)（3% — 7%）（圖1）（Mao et al，2009；Zhang et al，2014a）。Wei et al（2012）通過(guò)計(jì)算得出我國(guó)貴州、廣西、四川等地大氣中不飽和烴和苯比例高于東部發(fā)達(dá)城市（北京、天津、上海、浙江等），這與固定燃燒源和道路機(jī)動(dòng)車排放源在各省市間的分布差異有關(guān)。

圖1 不同地區(qū)1-丁烯、異戊二烯及苯的排放比例（單位：%）Fig.1 Emission ratios (Unit: %) of 1-butene, isoprene and benzene in different regions

1.2 垂直分布

VOCs的垂直分布受大氣邊界層結(jié)構(gòu)、大氣穩(wěn)定性、風(fēng)速風(fēng)向和排放源等因素的影響，且不同高度處VOCs和NOx間存在不同的反應(yīng)機(jī)制。飛機(jī)、熱氣球和高層建筑物是測(cè)定VOCs垂直濃度變化的主要輔助工具。VOCs濃度的垂直分布變化在霧霾天氣或存在逆溫層時(shí)更加明顯，濃度峰值往往出現(xiàn)在近地面處。了解VOCs的垂直分布有助于對(duì)污染物垂直輸送及化學(xué)傳輸模式的正確評(píng)估。

早在1996年，Kofl mann et al（1996）研究表明，隨高度上升，高反應(yīng)活性VOCs濃度比低反應(yīng)活性VOCs濃度降低速度更快。Xue et al（2011）對(duì)我國(guó)東北地區(qū)NMHC的垂直分布進(jìn)行了研究，發(fā)現(xiàn)化學(xué)壽命較長(zhǎng)的物種，如乙烷和乙炔（壽命為數(shù)周至數(shù)月）在大氣邊界層及自由對(duì)流層中的混合比分別為地面處混合比的30% — 50%和12% — 34%，而對(duì)于化學(xué)壽命以時(shí)或天計(jì)的物種，如丁烷、戊烷和烯烴等，此值則分別為12% — 40%和3% — 23%。VOCs在垂直梯度上的變化亦可導(dǎo)致臭氧形成機(jī)制發(fā)生變化。Chen et al（2013）在2007 — 2010年對(duì)北京市航測(cè)實(shí)驗(yàn)的分析發(fā)現(xiàn)，在北京市上空約1 km處，臭氧生成從VOCs控制區(qū)轉(zhuǎn)變?yōu)镹Ox控制區(qū)。不同大氣污染條件下，VOCs的垂直分布存在較大差異。Mao et al（2008）在北京氣象塔上布置VOCs的垂直梯度采樣，發(fā)現(xiàn)晴天VOCs的垂直分布復(fù)雜，大多數(shù)VOCs的濃度與高度成反比，而霧霾天氣下VOCs濃度在距地面8 — 140 m呈下降趨勢(shì)，之后又開(kāi)始上升直至280 m處，其中，乙酸乙酯及甲基異丁基酮僅存在于140 m以上，通過(guò)PCA及聚類分析證明不同高度處VOCs具有不同的來(lái)源。Lin et al（2011）研究發(fā)現(xiàn)，高雄市午夜23:00 — 1:00，13 m處的VOCs濃度水平高于地面，而早晨7:00 — 9:00，距地面32 m處濃度值高于13 m處，認(rèn)為是受上風(fēng)向工業(yè)污染源的影響或13 — 32 m有逆溫層存在。

1.3 VOCs的時(shí)間變化

隨著污染源強(qiáng)度和污染物歸趨強(qiáng)度的變化，大氣中VOCs的濃度與組成呈現(xiàn)顯著的季節(jié)和晝夜差異。

1.3.1 季節(jié)變化趨勢(shì)

驅(qū)動(dòng)大氣中VOCs濃度季節(jié)間變化的主要因素有以下幾種：（1）光化學(xué)反應(yīng)去除效應(yīng)（主要是與OH自由基作用）。VOCs與OH自由基反應(yīng)在溫暖季節(jié)較快；（2）大氣層混合稀釋效應(yīng)。溫暖季節(jié)混合層較高，有利于污染物稀釋擴(kuò)散；（3）排放源的季節(jié)變化。

Guo et al（2004a）研究發(fā)現(xiàn)香港大氣中VOCs的濃度與組成存在顯著季節(jié)差異，其中城區(qū)大氣中VOCs濃度的季節(jié)變化受一次排放源的控制，二氯甲烷、二甲苯和三甲苯在夏季時(shí)濃度略高于冬季，而氯甲烷、苯和四氯乙烯大氣濃度峰值出現(xiàn)于冬季；北京地區(qū)由于夏季光化學(xué)反應(yīng)較強(qiáng)，烷烴和烯烴濃度通常在11月份較高，在7月份濃度最低，而夏季芳香烴（甲苯、乙苯、二甲苯等）濃度與冬季相當(dāng)且高于春季，可能與揮發(fā)作用的季節(jié)性變化有關(guān)（Liu et al，2005）；與城區(qū)不同，郊區(qū)大氣VOCs濃度受大氣邊界層擴(kuò)散速率的影響，從而郊區(qū)大氣中多數(shù)VOCs化合物冬季的濃度高于夏季。Li and Wang（2012）研究中也發(fā)現(xiàn)類似的結(jié)果，由于廣州屬典型的亞熱帶氣候，且受亞洲季風(fēng)影響顯著，大氣VOCs濃度最高值往往出現(xiàn)在秋季，低值出現(xiàn)在春季。

在背景區(qū)，由于人為源影響減弱，大氣中VOCs主要來(lái)源于大氣遠(yuǎn)距離傳輸以及生物源排放（Tang et al，2007；Zhang et al，2014a），此外，人類的旅游活動(dòng)也可能影響背景區(qū)大氣VOCs的濃度與組成。由于春季旅游活動(dòng)、較弱的光降解反應(yīng)及大氣混合效應(yīng)，貢嘎山大氣VOCs的濃度水平在春季（12.9 ppbv）高于秋季（6.44 ppbv）（Zhang et al，2014a）。對(duì)青藏高原高海拔地區(qū)的研究顯示，夏季時(shí)大氣中顆粒物主要由生物源VOCs氧化生成的氣溶膠組成，而冬季則主要由遠(yuǎn)距離傳輸多環(huán)芳烴組成，間接說(shuō)明偏遠(yuǎn)地區(qū)生物源VOCs排放主要集中于夏季（Shen et al，2015）。

1.3.2 晝夜變化趨勢(shì)

VOCs濃度的晝夜變化主要受排放源、氣象條件和光化學(xué)活性等因素的影響。在城市地區(qū)，VOCs濃度的日間變化多呈雙峰變化（北京、上海、廣州、南京等地）（Wang et al，2012），峰值多出現(xiàn)于早晨（8:00 — 10:00）及傍晚（18:00 — 20:00），最低值出現(xiàn)在14:00左右。早晨的VOCs濃度峰值多源于上班高峰期時(shí)較大的車流量，大量污染物難以在短時(shí)間內(nèi)擴(kuò)散而累積，之后隨著太陽(yáng)輻射增強(qiáng)，地面溫度逐漸升高，誘導(dǎo)上層包含污染物的冷空氣下移與近地面空氣混合，從而使VOCs在早間出現(xiàn)濃度峰值（Xiong et al，2013）。與之相似，傍晚的濃度峰值可歸結(jié)為以下兩個(gè)原因：傍晚的下班高峰期及近地面空氣隨著陽(yáng)光照射強(qiáng)度減弱而快速降溫形成的逆溫層阻止了污染物的擴(kuò)散。午后較低的VOCs濃度則源于較強(qiáng)的太陽(yáng)輻射和大氣擴(kuò)散能力。研究表明在中午日光照射最強(qiáng)且城區(qū)邊界層高度達(dá)3000 m以上，有利于光化學(xué)反應(yīng)的進(jìn)行和污染物的擴(kuò)散，從而使VOCs濃度在這一時(shí)間段內(nèi)出現(xiàn)最低值（Guinot et al，2006）。大氣生物源VOCs（biogenic volatile organic compounds，BVOCs）的濃度主要受植被影響，BVOCs的排放與植物生長(zhǎng)密切相關(guān)。與人為污染源產(chǎn)生的VOCs不同，主要受天然排放源影響的異戊二烯的日間變化呈單峰變化，其峰值多出現(xiàn)在上午或者午后（Tang et al，2007；Wang et al，2013）。

2 大氣VOCs的轉(zhuǎn)化及其產(chǎn)物

2.1 含氧揮發(fā)性有機(jī)物（oxygenated volatile organic compounds，OVOCs）

OVOCs不僅可由人為源直接排放，也可由VOCs經(jīng)大氣自由基的氧化作用產(chǎn)生，圖2以活性較強(qiáng)的異戊二烯為例展示了其在O3的強(qiáng)氧化作用下產(chǎn)生OVOCs的過(guò)程。OVOCs是VOCs氧化產(chǎn)生臭氧和SOA的重要中間產(chǎn)物，醛酮類為其主要組分。不同地區(qū)醛酮化合物含量差異顯著，研究表明，北京市醛酮類化合物水平約為香港的3 — 5倍，為墨西哥城的35%（Liu et al，2015）。

圖2 異戊二烯臭氧化的中間產(chǎn)物及反應(yīng)產(chǎn)物Fig.2 Possible intermediates and reaction products generated in the ozonolysis of isoprene

目前用于定量醛酮化合物來(lái)源的常用方法有多元線性回歸法及基于光化學(xué)反應(yīng)強(qiáng)度的參數(shù)化方法（de Gouw et al，2005），后者考慮了醛酮化合物傳輸過(guò)程中的化學(xué)損失和二次生成。各計(jì)算過(guò)程中存在的諸多假設(shè)使研究者所得結(jié)果不一。Li et al（2010）用多元線性回歸法得出北京市大氣中的甲醛約76%來(lái)自人為源直接排放；而Yuan et al（2012）用參數(shù)化方法所得結(jié)果僅為22%，與Liu et al（2009）對(duì)北京市夏季的研究結(jié)果相似。諸多研究表明甲醛、乙醛和丙酮是我國(guó)各城市中檢出的含量最豐富的OVOCs（Ho et al，2002；Mu et al，2007；Lü et al，2010；Dai et al，2012；Yuan et al，2012）。圖3為各地檢測(cè)出的甲醛、乙醛和丙酮的濃度水平，其中香港地區(qū)含量較低，而北京市丙酮含量顯著高于其他城市。Louie et al（2013）研究發(fā)現(xiàn)，珠江三角洲地區(qū)大氣中甲醛、乙醛和丙酮的濃度分別為（2530 ± 370）pptv（pptv=10?3ppbv）、（1110 ± 140）pptv、（2470 ± 370）pptv，約占OVOCs總量的80%。OVOCs的分布易受溫度、光照強(qiáng)弱、相對(duì)濕度（relative humidity，RH）植被覆蓋率等因素的影響。嚴(yán)重污染事件時(shí)，上海市醛酮化合物濃度水平與北京相當(dāng)，略高于廣州（圖3）。Ho et al（2015）分析了我國(guó)九個(gè)城市中OVOCs的濃度水平，發(fā)現(xiàn)醛酮化合物含量最高的兩個(gè)城市為武漢和成都，可能是由于武漢和成都光化學(xué)反應(yīng)較強(qiáng)且存在大量的工業(yè)排放源；濃度最低值則出現(xiàn)在受海風(fēng)影響較大的廈門和人為排放源較少的青海湖。

甲醛/乙醛（C1/C2）和乙醛/丙醛（C2/C3）的摩爾比常被用于判定甲醛的來(lái)源，城市地區(qū)C1/C2的值一般在1 — 2，鄉(xiāng)村地區(qū)此值可達(dá)10（Shepson et al，1991；Possanzini et al，1996）（圖3）。Ho et al（2015）分析了我國(guó)九個(gè)城市C1/C2值，其值為0.15 — 15.7，較高的比值說(shuō)明受自然源影響嚴(yán)重。相比于其他醛酮類化合物，丙醛僅受人為源影響，因此C2/C3的值常表現(xiàn)為鄉(xiāng)村高、城區(qū)低。

圖3 我國(guó)各地甲醛、乙醛、丙酮含量（單位：μg · m?3）及C1/C2、C2/C3（據(jù)Ho et al（2014，2015）、 Cheng et al（2014）、Wang et al（2010b）修改））Fig.3 Concentration level (Unit: μg · m?3) of formaldehyde, acetaldehyde and acetone plus the molar ratio of C1/C2 and C2/C3 in different (Modifi ed from Ho et al (2014, 2015), Cheng et al (2014), Wang et al (2010b))

2.2 VOCs與地表臭氧污染

臭氧是大氣的重要組成部分，平流層臭氧通過(guò)阻擋紫外線直接進(jìn)入地表保護(hù)人類居住環(huán)境，而近地表臭氧由于具有較強(qiáng)的氧化能力，可危害人體健康。北半球觀測(cè)到近地面臭氧濃度呈逐漸上升趨勢(shì)，其中中緯度地表臭氧濃度在20 —45 ppbv，與一個(gè)世紀(jì)之前相比約增加了兩倍，背景區(qū)臭氧濃度目前仍在上升（Vingarzan，2004）。在我國(guó)，華東地區(qū)的臭氧濃度在1990 — 2010年以每年1.49%的速率增長(zhǎng)，北京市臭氧濃度以每年2%的速率增加（Xing et al，2015）。

地表臭氧主要來(lái)源于大氣光化學(xué)反應(yīng)和高層大氣臭氧入侵（Vingarzan，2004），在臭氧形成的光化學(xué)反應(yīng)中，VOCs與NOx是形成地表臭氧的前體物，目前研究中多認(rèn)為地表臭氧濃度的升高趨勢(shì)主要源于人類向環(huán)境大量排放的VOCs及NOx。Wu et al（2016）指出我國(guó)TVOCs年排放量在2008 — 2012年以每年7.38%的速率上升，而同時(shí)期內(nèi)Ma et al（2016）發(fā)現(xiàn)我國(guó)東北部一鄉(xiāng)村地區(qū)臭氧濃度水平亦呈上升趨勢(shì)（圖4）。由于各地區(qū)天氣狀況、盛行風(fēng)向和污染源等因素不同，臭氧形成機(jī)制及其前體物來(lái)源存在差異。其中，Xue et al（2014a）研究發(fā)現(xiàn)廣州、上海和蘭州等地臭氧前體物主要來(lái)自本地源的貢獻(xiàn)，而北京、香港近地面高濃度臭氧的生成則與周邊地區(qū)VOCs污染物傳輸有關(guān)。Li et al（2015b）研究了京津冀地區(qū)近地面臭氧形成機(jī)理及VOCs在此過(guò)程中的作用，發(fā)現(xiàn)北京市35% — 60%的臭氧前體物來(lái)自天津、河北地區(qū)；2005 — 2011年，北京市夏季TVOCs濃度以每年6%的速率下降，而臭氧濃度則以每年5.3%的速率增長(zhǎng)。此外，2002 — 2013年，香港地區(qū)活性芳香烴排放量逐漸下降，而臭氧濃度仍呈上升趨勢(shì)，說(shuō)明VOCs的區(qū)域傳輸可影響周邊地區(qū)大氣中的臭氧濃度（Xue et al，2014b；Zhang et al，2014b）。因此，若要有效治理VOCs和臭氧的污染，應(yīng)加強(qiáng)同周邊地區(qū)的合作。

圖4 2008 — 2012年中國(guó)東北一鄉(xiāng)村地區(qū)O3和全國(guó)TVOCs排放量變化趨勢(shì)Fig.4 Temporal trend of O3in a rural area of northeastern China and TVOCs in China between 2008 — 2012

不同VOCs化合物因其活性不一，在臭氧生成中的貢獻(xiàn)也不同。分析與甄別各地的活性VOCs及其來(lái)源對(duì)控制臭氧污染具有重要意義。通常采用最大增量反應(yīng)活性法（maximum incremental reactivity，MIR）及等效丙烯濃度法評(píng)價(jià)不同反應(yīng)活性的VOCs物種在臭氧生成中的貢獻(xiàn)（Chameides et al，1992；Carter，1994）。研究表明，珠江三角洲地區(qū)臭氧形成中的關(guān)鍵活性物種為芳香烴（Shao et al，2009；Cheng et al，2010；Zhang et al，2012）。An et al（2014）用MIR法估算了南京地區(qū)VOCs的光化學(xué)反應(yīng)活性，研究表明，烯烴對(duì)臭氧生成潛勢(shì)（ozone formation potential，OFP）的貢獻(xiàn)最大，說(shuō)明該地影響臭氧生成的關(guān)鍵活性物質(zhì)為烯烴。Wei et al（2014）在北京市煉油廠區(qū)的研究中發(fā)現(xiàn)烯烴亦是對(duì)OFP貢獻(xiàn)最大的物種，占44.3%，其次為烷烴（29.6%）和芳香烴（26.1%）。其他城市的關(guān)鍵活性物種見(jiàn)表2。

基于光化學(xué)模式對(duì)我國(guó)臭氧的生成機(jī)制的研究表明，大城市中臭氧生成多處于VOCs控制區(qū)，而偏遠(yuǎn)地區(qū)多處于NOx控制區(qū)。Pan et al（2015）用區(qū)域大氣化學(xué)機(jī)制模式（regional atmospheric chemistry mechanism，RACM），對(duì)長(zhǎng)江三角洲鄉(xiāng)村地區(qū)秋收期（露天秸稈燃燒頻發(fā)）臭氧的生成進(jìn)行了研究，結(jié)果表明臭氧生成在早晨受控于VOCs，下午由于光化學(xué)反應(yīng)對(duì)NOx的快速去除而受NOx控制。NOx/VOCs排放比的變化對(duì)區(qū)域范圍內(nèi)臭氧生成具有重要影響，Tie et al（2013）用WRF-Chem模式對(duì)上海市臭氧形成的研究表明，當(dāng)NOx/VOCs值約為0.4時(shí)，上海地區(qū)處于VOCs控制區(qū)，當(dāng)NOx/VOCs值為0.1左右時(shí)則處于NOx控制區(qū)。

3 我國(guó)VOCs源解析

我國(guó) VOCs 排放源種類多、排放成分復(fù)雜。了解各地主要VOCs排放源及各排放源所占比例，對(duì)我國(guó)大氣復(fù)合污染研究的開(kāi)展及污染控制策略的制定具有重要意義。

3.1 常用源解析方法

VOCs源解析受體模型包括多種多元統(tǒng)計(jì)方法，因其不受排放源的排放條件、地形和氣象數(shù)據(jù)等因素的限制，而常被用于識(shí)別空氣污染物的來(lái)源及其貢獻(xiàn)。常見(jiàn)污染源有特征示蹤物（表3），如乙烯和乙炔為燃燒源示蹤物；汽油揮發(fā)源常以異戊烷、正丁烷和異丁烯作示蹤化合物；工業(yè)排放源中鹵代烴含量較高；甲苯、乙苯和二甲苯等芳香烴則常來(lái)自于溶劑使用源。目前我國(guó)常用的受體模型有CMB、PCA/APCS和PMF模型等，其中PMF模型和CMB模型是美國(guó)環(huán)保署（United States Environmental Protection Agency，USEPA）推薦的源解析技術(shù)（http://www3.epa.gov/ttn/scram/ receptorindex.htm）。CMB、PCA/ APCS和PMF各有其優(yōu)缺點(diǎn)（詳見(jiàn)表4），如CMB雖然原理清楚、容易操作，但需要預(yù)先了解詳細(xì)的VOCs源成分譜；而PMF雖不需預(yù)先知道污染源成分譜，卻難以解析活性較強(qiáng)的VOCs來(lái)源。生物源排放的VOCs多為活性較強(qiáng)的物質(zhì)，受體模型對(duì)該來(lái)源進(jìn)行解析時(shí)往往只選取異戊二烯作為示蹤物質(zhì)，從而所得生物源占比往往較低（Fujita et al，1995）。因此本文只重點(diǎn)討論我國(guó)大氣中VOCs的主要人為來(lái)源。

表2 我國(guó)城市地區(qū)對(duì)臭氧生成貢獻(xiàn)最大的10種VOCsTab.2 The top 10 VOCs contribute to ozone formation in urban areas

表3 常見(jiàn)排放源的相應(yīng)示蹤物Tab.3 Tracers of common emission sources

表4 主要源解析方法比較Tab.4 Comparisons between different source apportionment methods

3.2 我國(guó)VOCs的主要人為源

國(guó)外對(duì)大氣中VOCs的源解析研究起步較早，20世紀(jì)70年代USEPA就發(fā)行了應(yīng)用軟件CMB1.0，經(jīng)不斷完善，目前已發(fā)展至CMB8.0版本（Fujita et al，1995；Scheff et al，1996；Na and Pyo Kim，2007；Zielinska et al，2014）。我國(guó)VOCs源解析的工作于20世紀(jì)80年代末才逐漸開(kāi)展，由于我國(guó)相應(yīng)的源譜信息匱乏，所以CMB模型應(yīng)用不如PMF應(yīng)用廣泛。Wang et al（2010a）運(yùn)用CMB解析了北京地區(qū)55種VOCs的來(lái)源，CMB模型運(yùn)行參數(shù)R2、χ2變化范圍分別為0.83 — 0.90和2.74 — 4.08，結(jié)果表明該地區(qū)VOCs主要來(lái)源為機(jī)動(dòng)車尾氣和汽油揮發(fā)。

PMF能夠同時(shí)確定污染源個(gè)數(shù)及其所占百分比，但需要的樣品量較大，Huang et al（2015）用PMF解析出LPG為香港路邊大氣環(huán)境中VOCs的最大來(lái)源；相比于其他源解析方法，PCA對(duì)活性較強(qiáng)的化合物具有較好的解析結(jié)果，但該源解析模型只能識(shí)別5 — 8個(gè)排放源且要求用于分析的樣品數(shù)量不低于50個(gè)，解析結(jié)果不及PMF細(xì)致，如在PMF解析結(jié)果中汽車尾氣和汽油揮發(fā)為兩個(gè)源，而PCA中不能將兩個(gè)源分開(kāi)；An et al（2014）運(yùn)用PCA/APCS模型解析南京工業(yè)區(qū)環(huán)境中VOCs來(lái)源，結(jié)果表明工業(yè)生產(chǎn)和機(jī)動(dòng)車排放為該地區(qū)VOCs的主要來(lái)源。

眾多研究表明，我國(guó)城市地區(qū)VOCs的主要來(lái)源為交通排放源和工業(yè)排放源（圖5）。研究表明，北京市加強(qiáng)空氣質(zhì)量管理期間，環(huán)境中VOCs水平降低主要由對(duì)機(jī)動(dòng)車與工業(yè)源排放的控制所引起，兩者在VOCs減排中的貢獻(xiàn)分別為50%和27%，源解析結(jié)果亦表明北京市VOCs主要來(lái)源于機(jī)動(dòng)車（46%）和工業(yè)源排放（24%）。與北京、南京、香港等城市相比，上海市溶劑使用源貢獻(xiàn)較高（19.4%），或與上海市存在較多的印刷制造業(yè)有關(guān)。香港地區(qū)機(jī)動(dòng)車多以LPG為燃料且家庭烹飪中LPG的運(yùn)用也日益增多，使LPG對(duì)香港大氣中VOCs的貢獻(xiàn)（41.3%）顯著高于我國(guó)其他地區(qū)（Lau et al，2010）。汽油車排放、工業(yè)排放和LPG與助燃劑排放為珠江三角洲地區(qū)VOCs最主要來(lái)源，三者貢獻(xiàn)率分別為23%、16%、13%，其中交通排放源的貢獻(xiàn)率與上海地區(qū)（25%）相近而顯著低于北京地區(qū)（46%），可能與北京市較大的機(jī)動(dòng)車保有量相關(guān)（Cai et al，2010；Yuan et al，2013；Li et al，2015a）。而且歸類方法（交通排放源是否包括汽油車尾氣和汽油蒸發(fā)等）、源解析方法和采樣時(shí)間段的差異也可能是部分原因。天津城區(qū)所植綠化樹(shù)種多為釋放VOCs較多的闊葉樹(shù)種，使該地區(qū)植被排放源貢獻(xiàn)（14%）顯著，甚至高于其在植被覆蓋率較高的貢嘎山VOCs來(lái)源（5%）中所占比例。隨著產(chǎn)業(yè)結(jié)構(gòu)及相關(guān)政策的轉(zhuǎn)變，Wei et al（2012）研究表明，我國(guó)溶劑使用源和工業(yè)過(guò)程源正在逐漸代替道路交通源，成為VOCs的最大排放源。

圖5 我國(guó)各地區(qū)排放源所占百分比Fig.5 Contribution percentages for major emission sources in different areas

4 結(jié)論與展望

目前，國(guó)內(nèi)學(xué)者對(duì)大氣中VOCs的研究主要集中在其濃度組成的時(shí)空變化特征、來(lái)源，及對(duì)臭氧生成的貢獻(xiàn)等方面，多數(shù)研究表明，烷烴、烯烴、芳香烴為我國(guó)各大城市中VOCs的主要組分，其中城市地區(qū)烷烴含量最高，而泰山、貢嘎山等背景地區(qū)，芳香烴為含量最豐富的VOCs。VOCs濃度日間變化多呈雙峰分布趨勢(shì)，峰值常在早晨及傍晚的上下班高峰期出現(xiàn)。各地區(qū)臭氧污染事件不僅與本地排放源有關(guān)，外地污染氣團(tuán)的跨界傳輸也不容忽視。

隨著國(guó)家對(duì)環(huán)境保護(hù)的重視，VOCs被納入主要污染物總量控制范圍，目前北京、上海、江蘇、安徽、湖南5個(gè)地區(qū)已出臺(tái)VOCs排污費(fèi)征收辦法，明確指出對(duì)石油煉制、油品儲(chǔ)運(yùn)及銷售、汽車船舶制造等企業(yè)征收VOCs排污費(fèi)。在現(xiàn)有法規(guī)和政策逐步出臺(tái)及強(qiáng)化的現(xiàn)狀下，根據(jù)以往研究中所得結(jié)論與經(jīng)驗(yàn)，建立各地排放清單、完善源解析手段及監(jiān)測(cè)方法、確定各地臭氧形成中的關(guān)鍵活性化合物是我國(guó)今后VOCs研究的主要內(nèi)容。

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Characteristics of volatile organic compounds in China

LI Bowei1,2, HUANG Yu2, HO Steven Sai Hang2, XUE Yonggang2, LIU Suixin2, CHENG Yan1, WANG Liqin2, CAO Junji2
1. Department of Environmental Science and Technology, School of Human Settlements and Civil Engineering, Xi’an Jiaotong University, Xi’an 710049, China
2. Key Laboratory of Aerosol Chemistry and Physics, Institute of Earth Environment, Chinese Academy of Sciences, Xi’an 710061, China

Background, aim, and scope Volatile organic compounds (VOCs) are a series of atmospheric pollutants that are characterized as high volatility, and strong environmental impact. Some of VOCs are smelly, toxic and carcinogenic, which do harm to human health. In addition, Most of VOCs are key precursors of tropospheric ozone and secondary organic aerosol. In recent years, the concentrations of ground level ozone are increasing in many megacities of China, especially in Beijing, Pearl River Delta (PRD) and Yangtze River Delta (YRD) region. Consequently, identifying the distributions and profiles of VOCs is in urgent need for environmental management. In contrast to our country, USA and Europe have conducted more comprehensive studies on VOCs, and most information we use are referenced from these foreign countries, thus it’s necessary to establish our own test method and database. Materials and methods This paper reviews studies about VOCs in China, of which thedistribution, sources apportionment, and effects on ozone and secondary organic aerosol (SOA) formation were discussed in detail. Results In China, the concentrations of VOCs are mainly affected by anthropogenic sources, such as combustion processes (utilizing fossil fuels, petroleum refi ning, and storage), distribution of petroleum products, solvent use and other industrial processes. Alkanes, alkenes and aromatics are the most abundant VOCs in China, and concentrations and composition of VOCs vary with sites. In China, ground-level ozone production was limited by the concentrations of VOCs in most eastern urban areas, and limited by NOxin western areas. Industry and traffi c emissions are the top two emission sources of VOCs in China. Discussion In general, concentrations of VOCs is higher in megacities, and it is found that VOCs level in Beijing, Shanghai, Guangzhou is higher than other cities, while composition of VOCs in remote sites is mainly infl uenced by long range atmospheric transport and biogenic emission. The concentration of atmospheric VOCs generally presented signifi cant temporal variations because of emission strength and photochemical reactions. Levels of VOCs in urban areas were mainly controlled by emission sources, while in suburban the diffusion rate of atmospheric boundary layer is the main factor, which results in the VOCs level higher in winter than that in summer in most cases. In the remote sites, VOCs mainly come from biogenic source and air pollutant transportation. For diurnal variations, it’s common to see bimodal distributions of atmospheric concentrations, concentration peaks are highly correlated with the traffi c fl ow, and concentrations mainly peak at 8:00 LST (local standard time, LST) and a second one from 16:00 to 19:00 LST. Under the effect of free radical, ozone, atmospheric VOCs could be oxidized to carbonyls, and carbonyls are one of the most important radical sources, especially for the wintertime in polluted urban environments. Formaldehyde, acetaldehyde and acetone were found to be the most abundant OVOCs in China, and the lowest concentration levels were found in Xiamen and Qinghai. It was reported that formaldehyde/acetaldehyde (C1/C2) ratio usually varied from 1 to 2 at urban areas and was about 10 at forest areas; therefore, the C1/C2 ratio could be used as a measure of a biogenic source of formaldehyde, while acetaldehyde/propionaldehyde (C2/C3) ratio was often used as effective indicators of anthropogenic carbonyls. In recent years, increasing ground-level ozone concentrations were observed in both the background and urban sites. As key ozone precursors, VOCs are the most important chemicals contributing to high ozone production rates in Pearl River Delta and Beijing-Tianjin-Hebei region, where ozone formation is sensitive to VOCs. It is found that ozone in the lower troposphere over Beijing had a strong positive trend (2% per year) during the period 1995 to 2005. As VOCs-sensitive chemistry has been found to be most likely to occur in urban sites of China, it is critical to distinguish the contribution of individual VOCs to ambient ozone formation for efficient emission control. The ozone formation potential (OFP) is a widely used method for evaluating the maximum ozone formation capacity, and aromatics and alkenes take the most part of OFP in China. Principal component analysis (PCA), chemical mass balance (CMB), and positive matrix factorization (PMF) are widely used source apportionment tools in the world, each of them have advantages and shortages. PMF results are not affected by uncertainties in emission profiles, making it the most popular method among these three tools. Conclusions Nowadays, most studies on VOCs are concentrated in megacities such as Beijing, Shanghai and Guangzhou in China, and most of researches were focused on factors like wise source apportionment, VOCs compositions, spatialtemporal variation etc. Recommendations and perspectives Although studies on OVOCs were rarely today, there will be more related studies coming up because of their important role in ozone formation. By targeting the high OFP-contributing species rather than the high emission-contributing species, reactivity-based control was more effi cient than the emission-based approach in alleviating ozone pollution.

volatile organic compounds (VOCs); sources apportionment; distribution characteristics

Date: 2016-12-29; Accepted Date: 2017-03-24

National Natural Science Foundation of China (41401567, 41573138)

HUANG Yu, E-mail: huangyu@ieecas.cn

2016-12-29；錄用日期：2017-03-24

國(guó)家自然科學(xué)基金項(xiàng)目（41401567，41573138）

黃 宇，E-mail: huangyu@ieecas.cn

李博偉，黃 宇，何世恒, 等. 2017. 我國(guó)大氣中揮發(fā)性有機(jī)物的分布特征[J]. 地球環(huán)境學(xué)報(bào), 8(3): 225 – 242.

: Li B W, Huang Y, Ho S S H, et al. 2017. Characteristics of volatile organic compounds in China [J]. Journal of Earth Environment, 8(3): 225 – 242.