我國大氣環(huán)境中汞污染現(xiàn)狀

吳曉云,鄭有飛*,林克思(1.南京信息工程大學(xué)中國氣象局氣溶膠與云降水重點(diǎn)開放實(shí)驗(yàn)室,江蘇 南京210044；2.南京信息工程大學(xué)大氣物理學(xué)院,江蘇 南京 210044；.南京三和環(huán)保技術(shù)有限責(zé)任公司,江蘇 南京210048)

我國大氣環(huán)境中汞污染現(xiàn)狀

吳曉云1,2,鄭有飛1,2*,林克思3(1.南京信息工程大學(xué)中國氣象局氣溶膠與云降水重點(diǎn)開放實(shí)驗(yàn)室,江蘇 南京210044；2.南京信息工程大學(xué)大氣物理學(xué)院,江蘇 南京 210044；3.南京三和環(huán)保技術(shù)有限責(zé)任公司,江蘇 南京210048)

綜述了最近幾十年中國大氣汞的排放、分布、傳輸和沉降方面的研究.中國人為源每年汞排放量為世界最高,達(dá)到500～700t左右,超過全球人為排放量的25%～30%,并以每年4.2%的速度增加.通過已有觀測和模型估算,中國來自自然地表過程(包括裸露地、地表水和森林土壤等)的汞排放量與人為源汞排放量相當(dāng),不容忽視.中國城市、農(nóng)村和偏遠(yuǎn)地區(qū)大氣汞濃度分布和變化范圍很大,城市地區(qū)的總氣態(tài)汞(TGM)濃度是北美和歐洲地區(qū)相似城市類型TGM的1.5～5倍左右;中國城市顆粒態(tài)汞(PHg)濃度比北美和歐洲地區(qū)高出2個(gè)數(shù)量級;中國沿海地區(qū)和偏遠(yuǎn)的背景區(qū)TGM、PHg和活性汞(RGM)低于中國內(nèi)地城市地區(qū),但是也明顯高于背景值和北美和歐洲地區(qū).相應(yīng)地,高含量的大氣汞濃度導(dǎo)致大量的大氣汞沉降到地表,城市地區(qū)和背景區(qū)大氣汞沉降分別比北美地區(qū)高出1～2個(gè)數(shù)量級和1～2倍.

大氣汞；排放；分布；沉降

China Environmental Science, 2015,35(9)：2623～2635

大氣中汞主要分為3類,即氣態(tài)單質(zhì)汞(Hg0),活性氣態(tài)汞(RGM)和顆粒態(tài)汞(PHg)[1].Hg0是大氣汞的主要賦存形態(tài),占大氣總汞的 95%以上,Hg0在大氣中的居留時(shí)間很長(0.5～2a),能遷移數(shù)萬公里[2].活性氣態(tài)汞在水中的溶解度比 Hg0要高,它們很容易通過濕沉降和干濕降過程帶到表生生態(tài)系統(tǒng)中,大氣中的居留時(shí)間最短(數(shù)小時(shí)到幾天),只能被遷移數(shù)十到幾百公里.顆粒態(tài)汞在大氣中的居留時(shí)間取決于顆粒物的大小,一般在數(shù)天到幾個(gè)月,在大氣中的遷移距離為幾百至上千公里.不同形態(tài)汞在大氣中的遷移距離不同,對生態(tài)系統(tǒng)影響各異,只有氣態(tài)單質(zhì)汞才是全球性污染物.聯(lián)合國環(huán)境規(guī)劃署(UNEP)已正式將汞納入環(huán)境外交.在2013年,全球140個(gè)國家聯(lián)合簽署了第一份具有法律約束力的國際條約-《水俁汞防治公約》,旨在減少全球汞排放.未來幾年,我國將要面對來自國際環(huán)境外交談判和國內(nèi)經(jīng)濟(jì)發(fā)展的雙重削減汞排放的壓力.

人為活動(dòng)和自然過程都會(huì)向大氣排放汞.人為向大氣排放汞的活動(dòng)主要包括:燃煤、垃圾焚燒、氯堿生產(chǎn)、金屬冶煉與加工等[5].自然排汞的過程包括自然源排汞過程和先前排放的汞沉降到地表后的再排放過程[6],主要的自然源排汞過程有:土壤、水體、植被、火山活動(dòng)、森林火災(zāi)和地殼去氣作用向大氣的排汞[7].

大氣汞源和遷移轉(zhuǎn)化已成為全球汞循環(huán)的重要主題,中國是最大的發(fā)展中國家,高速度的經(jīng)濟(jì)發(fā)展使中國成為全球第一大能源消耗國,再加上水泥生產(chǎn),金屬冶煉等使中國同時(shí)也是最大的人為汞排放國.通過當(dāng)前的研究和模型計(jì)算,中國大氣汞排放占全球汞排放的 25%～ 40%左右[3],因此,中國汞排放對于全球汞循環(huán)起到至關(guān)重要的作用.隨著中國對全球環(huán)境的關(guān)注和對Hg的毒性及危害的深入認(rèn)識,有關(guān)大氣汞的研究日益加強(qiáng).本文總結(jié)了近些年來中國大氣汞研究的概況,旨在更好的提供對中國大氣汞研究的認(rèn)識.

1　大氣汞來源

1.1人為源汞排放

Streets[4]2005首次對中國大氣汞來源清單進(jìn)行估算,該研究認(rèn)為,來自人為活動(dòng)向大氣排放量約為(536±36)t,有色金屬冶煉占向大氣排放量的45%,煤炭燃燒占38%,電池、日光燈以及水泥生產(chǎn)占17%,各省汞排放清單見圖1.隨著中國經(jīng)濟(jì)在過去幾十年的快速發(fā)展,人為汞排放量也在急速增加,Wu[5]研究認(rèn)為,在1995年人為排汞量在 514t,并以每年 3.0%的速度增加,到2003年增加到695t.Pacyna[8]研究2000年全球人為源向大氣排放清單表明,中國每年大約排放605t汞進(jìn)入大氣,占全球排放量的28%,其中化石燃料燃燒和工業(yè)生產(chǎn)各占 78%和 22%.根據(jù)Pacyna[8]估算全球汞排放清單,在 2005年全球人為汞排放量為825t,中國占43%,其中化石燃料燃燒占中國汞排放清單的 47%.不同研究對中國不同年份大氣汞排放量見圖2.Zhang等[9]更新了中國人為源大氣汞排放清單,2000年中國大氣人為源汞排放為356t,并以每年4.2%的速度持續(xù)增加,在2010年達(dá)到538t,這一研究結(jié)果要比之前研究估算[4-6]的大氣汞排放都要低, Zhang等[9]研究結(jié)果的不確定性(-20% ～ +23%)要明顯的低于之前的研究,而造成這一原因主要由于對于燃煤和生產(chǎn)過程中脫汞設(shè)施對汞去除效率評價(jià)存在較大差異,使用其研究的不確定度的研究方法分別測定了煤炭燃燒、有色金屬冶煉和水泥生產(chǎn)過程中汞的釋放的不確定性,基于概率技術(shù)為基礎(chǔ)的排放因子模型和汞排放因子中參數(shù)的詳細(xì)數(shù)據(jù),很大程度上降低了主要點(diǎn)源汞排放的不確定性,其中包括燃煤火電廠(-35%,+45%),工業(yè)燃煤(-45%,+47%),居民燃煤(-48%,+50%),鋅冶煉((-59%,+72%),鉛冶煉(-65%,+84%),銅冶煉(-59%,+72%)和水泥生產(chǎn)(-66%,+72%);之前研究[4-6]采用的是國際經(jīng)驗(yàn)的單一排放因素來估算有色金屬冶煉和水泥生產(chǎn)過程中汞的釋放, 而 Zhang等[9]采用的是現(xiàn)場實(shí)測值和結(jié)合采用基于排放因子模型來估算,改善了汞排放清單的準(zhǔn)確性.

這些估算存在著巨大的不確定性,Wu等[5]對中國在1995年和2003年人為汞排放清單的估算不確定性分別達(dá)到±78%和±44%,這些不確定性主要來自對基礎(chǔ)數(shù)據(jù)的收集和排放因子的估算.根據(jù)之前的研究結(jié)果,煤炭燃燒、有色金屬冶煉、水泥生產(chǎn)和鋼鐵生產(chǎn)被認(rèn)為是中國大氣汞排放的主要來源.

圖1　中國各省份人為源排放汞量[4]Fig.4　Anthropogenic emissions of mercury by province in China

圖2　中國人為源向大氣汞排放量[3-8]Fig.4　Anthropogenic mercury emissions into the atmosphere in China

1.1.1煤炭燃燒 Feng等[10]估算在1994年,來自人為源的燃煤汞排放量大約為296t;Wang等[11]認(rèn)為,中國在1995年通過煤炭燃燒向大氣排汞量為 214t,并以每年 4.8%的速度增加; Wu等[5]和Pirrone等[12]的研究表明,在 2003年煤炭燃燒向大氣排放汞量大約為 256～268t,占中國總?cè)藶樵磁欧诺?0%左右.這些估算存在著較大差異,原因主要是煤炭的汞濃度估計(jì)造成的,煤炭汞濃度對估算燃煤汞排放量具有重要影響.Wang等[11]和Zhang等[13]使用0.22mg/kg作為全國煤炭汞平均濃度;其他的一些研究分別以 0.15mg/kg[14]和0.16mg/kg[15]作為平均濃度;美國地質(zhì)調(diào)查局[16]分析了全國所有省市的305個(gè)樣品,得到的中國平均汞濃度為 0.16mg/kg;Zheng 等[17]總結(jié)了前人研究的 1699個(gè)樣品的研究結(jié)果,并以0.19mg/kg作為全國煤炭汞平均濃度;Tian等[18]也通過總結(jié)之前的研究認(rèn)為全國平均煤炭汞濃度范圍在 0.18～0.20mg/kg;最近一次的對全國煤炭汞濃度的估算為0.17mg/kg[19].

另外,影響燃煤汞排放的還有煤炭燃燒過程中的空氣污染控制裝置,Wang等[20]研究了中國6個(gè)典型火電廠,發(fā)現(xiàn)空氣污染控制裝置對汞的去除效率達(dá)到73%,Zhou等[21]研究表明燃煤小鍋爐在按照脫硫裝置后,汞的排放量也可以降低23%～86%.賦存在燃煤中的汞經(jīng)過燃燒后幾乎所有的汞會(huì)轉(zhuǎn)變?yōu)?Hg0進(jìn)入高溫的煙氣,經(jīng)過各污染控制設(shè)備過程中,由于溫度、煙氣成分及飛灰等的影響,Hg0會(huì)發(fā)生復(fù)雜的物理化學(xué)反應(yīng)而轉(zhuǎn)化為不同的形態(tài):顆粒態(tài)汞(pHg)、氧化態(tài)汞(Hg2+)以及元素態(tài)汞(Hg0).一般顆粒態(tài)汞易于被除塵器收集,Hg2+易溶于水,易于被濕法煙氣脫硫裝置去除,而Hg0揮發(fā)性高、不溶于水,不溶于酸,很難被除塵器去除.因此,汞的排放形態(tài)直接影響汞的脫除效率.在燃煤過程中,煙氣中不同形態(tài)的汞的含量及比例主要受到煤種、鍋爐的燃燒方式及燃燒溫度、煙氣成分、煙氣中的 HCl和飛灰等因素綜合作用[22].John等[23]研究表明,煙煤燃燒產(chǎn)生的煙氣中的汞是以Hg2+為主的,亞煙煤燃燒后,煙氣中的Hg2+含量與Hg0含量相當(dāng),褐煤燃燒后煙氣中以Hg0為主,而且當(dāng)爐膛溫度較高時(shí),煙氣中Hg0含量較大,大多數(shù)的 Hg2+形成的氧化物不穩(wěn)定,會(huì)發(fā)生分解生成單質(zhì)汞,當(dāng)煙氣溫度降低于750K時(shí),煙氣中汞主要以Hg2+形式存在;另外,煙氣成分也會(huì)影響 Hg0的氧化作用,由于煙氣成分的復(fù)雜性,煙氣中可能含有促進(jìn)煙氣中的零價(jià)汞氧化的物質(zhì)存在,使汞形態(tài)發(fā)生變化;同時(shí)研究還表明,含氯物質(zhì)有利于Hg2+的形成,這可能是煙氣中的氯將 Hg0氧化的結(jié)果[22].因此,將煙氣中的Hg0轉(zhuǎn)化為Hg2+和PHg更有利于煙氣中汞的去除,實(shí)施除塵器平均除汞效率為23%,靜電除塵器效率為 29%,靜電除塵和濕式煙氣脫硫裝置連用去除效率為 60%～70%,如果在靜電除塵和濕式煙氣脫硫裝置連用基礎(chǔ)上再加上選擇性催化還原裝置將煙氣中的Hg0轉(zhuǎn)化為Hg2+,去除效率可以到達(dá) 69%;中國燃煤電廠普遍使用的袋式除塵器裝置加上濕式煙氣脫硫裝置對汞的去除效率可以達(dá)到最大的89%[9].

1.1.2有色金屬冶煉 有色金屬冶煉同樣是人為源排放汞的重要組成部分,Wu等[5]估算在1995年中國有色金屬冶煉向大氣排汞量大約為230t,而這一數(shù)字到2003年增加到320t;Hylander 等[24]推算來自鋅、鉛、銅冶煉排放的汞量約為83.2t;Zhang等[9]研究表明有色金屬冶煉在2000～2010年期間排放量在96.4～146.4t,與Wu[5]等在同年(2003年)研究結(jié)果相差47%,主要因?yàn)閷σ睙掃^程中脫汞設(shè)施對汞的去除效率估算存在較大差異.在有色重金屬中,鋅冶煉過程中汞排放量最大[4-5],Li等[25]通過野外調(diào)查推算在2002～ 2006期間中國鋅冶煉排汞量在 80.7～104.2t之間;Yin等[26]通過同位素質(zhì)量分餾估算2006年中國鋅冶煉排汞量與 Li等[25]研究相似,為 107.7t; 而Zhang等[9]通過現(xiàn)場測定估算2006年中國鋅冶煉排汞量 82.3t,比之前研究結(jié)果要低[4-5],因?yàn)橹把芯客ㄟ^采用國際經(jīng)驗(yàn)的單一排放因素模型來估算鋅冶煉過程中汞排放,現(xiàn)場實(shí)測值要低于這一模型計(jì)算值.

與全球人為汞排放來源來看,中國有色金屬冶煉所占比例明顯高于世界上其他國家,但由于不同的冶煉方法和尾氣的處置方式導(dǎo)致這一過程中汞的排放存在很大的不確定性,Wang等[20]對濕法鋅冶煉過程中的煙氣進(jìn)行觀測,發(fā)現(xiàn)大部分汞可以通過煙氣處理裝置而被去除,每噸鋅冶煉汞排放量僅為 0.5g.另一個(gè)影響有色重金屬冶煉過程中汞排放的重要因素是礦石的含汞量,與煤炭相比,礦石中汞的含量要明顯高很多.Streets 等[4]指出中國鋅中的汞濃度范圍在1～1000mg/kg左右;Song 等[27]采集和分析了中國 208個(gè)鋅礦石樣品,得到中國鋅中汞的幾何平均濃度為9.45mg/kg;Yin等[26]發(fā)現(xiàn)礦石中的汞濃度取決于礦石類型和來源,并通過分析82個(gè)礦石樣品得到鋅礦石的幾何平均濃度為7.34mg/kg;Wu 等[5]研究認(rèn)為中國鋅、鉛、銅礦石中汞的濃度分別為9.7,10.3,2.9mg/kg.

1.1.3其他生產(chǎn)活動(dòng) 在 1995～2003期間,黃金冶煉向大氣排汞量從96t降低到45t[5],2003年大規(guī)模的黃金生產(chǎn)和手工與小規(guī)模的汞礦活動(dòng)分別占其中的 36%和 64%.水泥生產(chǎn)也是大氣汞的重要來源,中國水泥產(chǎn)量在2011年達(dá)到200億t,占世界產(chǎn)量的一半,Pirrone等[12]估算這一排汞量大約為236t,要高于Wu 等[5]對2003年的估算值,這一差異的主要是因?yàn)閷λ嗌a(chǎn)過程中原材料(包括石灰?guī)r、煤炭、黏土、礦渣等)汞濃度的估算造成的,例如,石灰?guī)r中汞的濃度范圍在0.5～ 2000μg/kg,煤炭中汞的濃度范圍在5～1000μg/kg[5,12].

而世界其他國家的人為大氣汞釋放要遠(yuǎn)低于中國,美國在 1994～1995年期間年平均排汞量為158t[28],大約有87%的大氣汞釋放來自能源的燃燒(垃圾和化石燃料的燃燒),而到2000年大氣汞排放總量為109.2t,有著明顯的下降趨勢.這種明顯的下降趨勢是由于在20世紀(jì)90年代美國就開始對不同的類型的火力發(fā)電廠的燃煤、煙氣、燃燒過程中逐步去除汞而降低大氣汞的排放,而這些脫汞設(shè)施可以大大降低燃煤過程中汞的排放;因此使得北美人為源汞釋放占全球汞的3%,而美國火電廠燃煤汞排放僅占總排放量的 1%[29],因此燃煤電廠安裝脫汞設(shè)施是降低大氣汞的釋放的重要途徑.由于中國基礎(chǔ)建設(shè)的快速發(fā)展,區(qū)別于其他國家汞排放的另一個(gè)重要特點(diǎn)就是水泥生產(chǎn)和有色金屬冶煉在中國汞排放中占有很大比例[9],所以降低中國大氣汞排放的另一個(gè)重點(diǎn)在于降低水泥生產(chǎn)和有色金屬冶煉過程中大氣汞的釋放.

1.2自然源汞釋放

自然源汞釋放在全球汞的生物地球化學(xué)循環(huán)過程中貢獻(xiàn)較大.Pirrone等[12]通過模型計(jì)算得出全球自然源汞年釋放量在 5207t,遠(yuǎn)大于認(rèn)為源汞排放源 2320t.火山爆發(fā),森林火災(zāi)等是主要的自然汞釋放來源.然而地表/大氣汞交換和之前通過沉降到地表后在經(jīng)過地表釋放到大氣中的汞是自然汞排放源的重要途徑,而這一過程汞的排放過程非常復(fù)雜,通常受到氣象條件(溫度、輻射、風(fēng)速、濕度等),土壤自身?xiàng)l件(溫濕度、汞濃度、土壤類型、pH、有機(jī)質(zhì)等),大氣汞濃度等諸多條件影響.

表1　中國自然地表/大氣汞通量研究匯總Table 1　Statistic summary of mercury fluxes from natural surfaces in China

表1總結(jié)了中國不同類型地表/大氣汞交換的研究結(jié)果.通過這些研究,可以得到以下研究結(jié)論:(1)地表和大氣之間汞交換是雙向的,但是總的來說土壤主要向大氣釋放,大氣向土壤沉降往往在溫度和輻射較低的冬季發(fā)生或大氣汞濃度升高抑制了土壤的釋放[30-37];(2)中國地表/大氣汞交換要遠(yuǎn)比歐洲和北美大,在北美背景區(qū)裸露地表/大氣汞交換通量在0.1～2.7ng/(m2?h),加拿大水體和森林土壤汞的釋放通量在0～5ng/(m2?h)和-0.4～2.2ng/(m2?h)[32],導(dǎo)致這一原因可能是中國土壤汞濃度較高和大氣汞沉降較大,沉降到地表后的汞又被重新釋放到大氣中.(3)不同地表汞釋放通量差別較大,城市和裸露地汞釋放通量較大,而偏遠(yuǎn)地區(qū)和被植被覆蓋的草地、森林以及水體汞釋放通量較小.(4)具有明顯的季節(jié)性變化,夏季地表/大氣汞交換通量較大,冬季較小,主要是由大氣溫度和太陽輻射的影響.

一些通過野外和模型估算地表向大氣排放汞通量研究指出,地表汞釋放占全球汞排放相當(dāng)大的比例,Shetty等[53]指出中國通過地表向大氣排放的汞在2001年達(dá)到462t,占人為源的80%左右(575t).Pan等[54]估算中國每年向大氣排放1140t汞,其中565t是來自自然源.因此這些研究基本上都認(rèn)為,中國人為源汞排放與自然源汞排放水平相當(dāng),但是自然源汞排放受到的影響條件較多,當(dāng)前對自然源汞排放往往是通過經(jīng)驗(yàn)主義模型估算的,不確定性遠(yuǎn)遠(yuǎn)大于人為源汞排放.

通常情況下,中國巨大的自然源汞排放要遠(yuǎn)遠(yuǎn)高于北美和歐洲等國家的自然源的汞釋放,當(dāng)前報(bào)道的美國背景區(qū)土壤/大氣汞平均交換通量在 0.1～2.7ng/(m2?h),加拿大水體環(huán)境和森林土壤汞的交換通量分別～5,-0.4～2.2ng/(m2?h)[55].中國自然源的土壤和水體的較高的汞排放通量可能是由以下原因造成的,(1)首先中國的通量研究通常是在污染區(qū)進(jìn)行測定的,較高的土壤汞濃度使得排放通量的增加;(2)中國較高的大氣汞沉降通量使得沉降到地表的大氣汞重新釋放到大氣中;(3)不同的通量實(shí)驗(yàn)測定方法和實(shí)驗(yàn)參數(shù)造成地表較高的大氣汞排放.

2　大氣汞形態(tài)和濃度分布

2.1城市地區(qū)和工業(yè)區(qū)大氣汞分布

中國大氣汞濃度分布有著明顯的空間分布特征,在城市和工業(yè)區(qū)總氣態(tài)汞(TGM)濃度變化范圍在 2.7～35ng/m3,是北美和歐洲所報(bào)道的1.5～7倍左右[56-57].在中國沿海地區(qū),例如上海、寧波和廈門 TGM 濃度較低,分別為(2.7±1.7), (3.79±1.29)和(3.50±1.21)ng/m3(圖3),可能是相對清潔的海洋氣流影響.與之相比,內(nèi)地城市觀測到較高的 TGM濃度,可能原因是這些城市有較多的工業(yè)區(qū)和火電廠等人為汞排放源.與中國緊鄰的韓國和日本相比,中國內(nèi)地 TGM濃度都相對較高,例如Kim等[58]報(bào)道首爾的TGM、PHg和RGM濃 度 分 布 為 (3.22±2.10)ng/m3,(23.9± 19.6)pg/m3和(27.2±19.3)pg/m3;Osawa等[59]觀測了日本內(nèi)地城市的TGM濃度為3.78ng/m3;而在美國密爾沃基觀測到的大氣汞濃度更低,GEM、PHg和 RGM 濃度分布為(2.48±0.02)ng/m3, (11.8±0.31)pg/m3和(10.3±0.2)pg/m3[60].

圖3　中國不同區(qū)域大氣TGM濃度分布[40,61-77]Fig.4　TGM concentrations distribution in the atmosphere in different regions of China

中國城市PHg濃度范圍在109～1180pg/m3,占城市大氣汞濃度的10%左右.Valente等[78]總結(jié)了世界上所報(bào)道的城市,農(nóng)村和偏遠(yuǎn)地區(qū)大氣汞形態(tài)濃度的分布發(fā)現(xiàn),北美和歐洲大氣PHg汞濃度一般低于100pg/m3.中國城市地區(qū)的RGM研究較少,貴陽和廈門 RGM 濃度分別為(35.7± 43.9)pg/m3和(61.05±69.41)pg/m3(表2).中國城市和工業(yè)地區(qū)高濃度的大氣形態(tài)汞說明該區(qū)域是大氣汞的重要來源,城市地區(qū)是中國人口的集中地,大量的能源消耗和工業(yè)生產(chǎn)成為重要的大氣汞源.而這些區(qū)域大氣汞釋放也成為了周邊地區(qū)大區(qū)汞污染的重要源,城市周邊地區(qū)也觀測到較高的TGM濃度(圖3).

2.2偏遠(yuǎn)地區(qū)大氣汞分布

在中國偏遠(yuǎn)地區(qū)觀測到的大氣TGM濃度一般都比北半球大氣 TGM 背景值要高(1.5～ 1.7ng/m3)[12],除Fu等[64]在長白山觀測到的TGM(1.60±0.51ng/m3),其中貢嘎山觀測到的大氣汞濃度最高,為(3.98±1.62)ng/m3(圖 3),主要來自當(dāng)?shù)鼐幼〉娜藶榈拿禾咳紵驮摰貐^(qū)的有色金屬冶煉導(dǎo)致的.而這一原因?qū)е碌拇髿夤瘽舛壬咴谥袊渌貐^(qū)普遍發(fā)生,因?yàn)闆]有有效政策和法律約束這一行為,而且分散的家庭煤炭和生物質(zhì)燃燒汞的釋放因子遠(yuǎn)比火電廠和鍋爐燃煤要高[4,79].Wan等[63,80]對中國北方溫度森林長白山地區(qū)大氣形態(tài)汞為期一年的觀測發(fā)現(xiàn),大氣的 TGM,PHg和 RGM 濃度分別為(3.58± 1.78)ng/m3,(77±136)pg/m3,(65±84)pg/m3,濃度都高于北半球的背景值[12].由于RGM和PHg不能長距離遷移,所以可能主要由當(dāng)?shù)氐墓廴舅斐傻?由Fu等[56]在該區(qū)域不同點(diǎn)連續(xù)觀測TGM濃度的研究可以證明這一點(diǎn),該研究連續(xù)兩年觀測發(fā)現(xiàn) TGM 濃度只有(1.60±0.51)ng/m3,與北半球TGM濃度相當(dāng).

Fu等[65]在雷公山觀測得到的TGM濃度為(2.80±1.51)ng/m3,也高于背景值,可能由于西南地區(qū)較高的土壤汞背景濃度和加上貴州人為源的排放造成的.在我國西北地區(qū)的瓦里關(guān)全球大氣觀測基準(zhǔn)站觀測到的TGM,PHg和RGM濃度分 別 為 (1.98±0.98)ng/m3,(19.4±18.1)pg/m3和(7.4±4.8)pg/m3[67],比高于歐洲和北美地區(qū)的背景值[81-82],周邊的一些大城市,比如西寧、西安和蘭州等可能是其主要的汞來源.

2.3大氣汞濃度的地理分布

通過已有的模型和觀測可以看出,中國大氣汞濃度分布有著明顯的地理空間分布特征[83-88], Lin等[88]通過模型模擬了東亞地區(qū)大氣汞的濃度分布,發(fā)現(xiàn)該區(qū)域內(nèi)大氣汞濃度分布有 2個(gè)主要的特征,一是大氣汞濃度分布的空間格局類似天然來源的排放量[53],但是不同的是大的汞釋放源附近有著較高的大氣汞濃度,在中國大的汞釋放源占人為源汞排放的 45%[88],大的汞釋放源點(diǎn)主要來自煤炭和和鋅和鉛的冶煉,因此像遼寧、河北、廣東、貴州、甘肅等人為源排放較大的省份相應(yīng)的也有較高的大氣汞濃度,與自然源和區(qū)域源相反的是,大的汞釋放源因?yàn)榫哂休^高的溫度,所以可以進(jìn)入高層大氣和長距離傳輸,同時(shí)也造成周邊大氣汞污染.另一個(gè)特點(diǎn)是由于人為源造成較高大氣汞濃度的內(nèi)地省份,包括遼寧、北京、河北、山西、安徽、湖北、江西、湖南、貴州、廣西、廣東等地,向太平洋和中國的西部地區(qū)有著明顯的濃度梯度,表明中國大陸在冬季的西北季風(fēng)和夏季的東南季風(fēng)將人為源排放的汞向太平洋和中國的西部地區(qū)有著明顯的傳輸作用.

所要指出的是通過以上模型估算我國中部和西南的大氣GEM濃度一般在2.0～2.5ng/m3,要比實(shí)際觀測值低很多,PHg也要實(shí)際觀測值要低, 而 RGM濃度比實(shí)測值要高,造成這種的原因有很多,首先由于地理區(qū)域的汞背景值有較大差異,例如在貴州省,較高的土壤汞背景值使貴陽市大氣汞濃度明顯高于其他相似人為源排放的地區(qū)[74];另外當(dāng)前觀測受到當(dāng)?shù)睾蛥^(qū)域汞源釋放的影響,使得實(shí)測值高估了區(qū)域的大氣形態(tài)汞的背景值;在現(xiàn)有的估算清單和排放模型中具有很大的不確定性,有可能低估了或漏估了某些汞釋放源[85,88-92].

3　大氣汞沉降

3.1城市地區(qū)大氣汞沉降

中國城市和城市郊區(qū)降雨濃度和大氣汞濕沉降量分別在55.3～354ng/L和77.6～152 μg/(m2?a)之間(表2),明顯比美國城市和工業(yè)地區(qū)觀測到的降雨濃度和濕沉降量大很多[80-82].中國東北地區(qū)長春市降水汞濃度特別高,這與該城市嚴(yán)重的汞污染有關(guān),降水中的汞濃度主要來自大氣中RGM和PHg[1],從表2可以看出長春市的PHg濃度很高,造成了雨水中汞濃度也很高.

表2　中國大氣PHg和RGM濃度和汞沉降研究統(tǒng)計(jì)匯總Table 1　Statistic summary of atmospheric Hg deposition fluxes in China

3.2偏遠(yuǎn)地區(qū)大氣汞沉降

與城市地區(qū)的降水汞濃度和濕沉降量相比,中國半偏遠(yuǎn)地區(qū)和偏遠(yuǎn)地區(qū)的降水濃度和沉降量明顯降低,分別在 2.98～34ng/L和 5.4～34.7μg/ (m2?a)之間(表 2).Zhou等[1]在云南哀牢山研究發(fā)現(xiàn)降水汞濃度年平均值為 2.98ng/L,為觀測到的降水濃度汞的最低值,原因是該地區(qū)遠(yuǎn)離人為汞的污染源,相對清潔.但總體來說,中國半偏遠(yuǎn)地區(qū)和偏遠(yuǎn)地區(qū)的降水濃度和沉降量要比北美類似地區(qū)的要高,北美的降水濃度一般小于10ng/L,濕沉降量小于 10μg/(m2?a)[101-103],主要原因是中國大量的人為源汞排放到大氣中,不僅造成污染附近大量的沉降(城市地區(qū)),而且這些汞通過長距離遷移到達(dá)偏遠(yuǎn)地區(qū),造成我國偏遠(yuǎn)地區(qū)的污染,而在北美和歐洲較低的人為源汞排放使得偏遠(yuǎn)地區(qū)大氣汞沉降量也較低[104-106].

3.3大氣汞干沉降

除去大氣汞濕沉降以外的沉降為干沉降,在林冠層覆蓋區(qū)一般包括穿透水沉降部分減去濕沉降部分,和凋落物汞沉降.Wang等[98]和 Fu 等[65,97]收集了鐵山坪、貢嘎山和雷公山地區(qū)的降水、穿透水和凋落物,通過計(jì)算得出三個(gè)區(qū)域的干沉降通量分別為256, 66.4和44μg?m-2?yr-1,在森林覆蓋地區(qū)大氣汞干沉降是濕沉降的 2.54～ 7.3倍.同時(shí)與北美和歐洲相比中國大氣汞干沉降明顯較高[104-106].這要?dú)w因于中國大氣較高的TGM,PHg和RGM濃度,而這些高濃度的污染物主要來自人為源的排放和沉降到地表后又經(jīng)過地表排放進(jìn)入到大氣[107].

4　結(jié)論

4.1中國來自人為活動(dòng)排放的汞量大約為500～700t/a,占世界人為源排汞量的25%～30%.然而,自然源排放的不確定性更大,自然源汞排放是中國向大氣排放的重要組成部分,占人為源的80%左右.通常情況下,中國裸露地、植被覆蓋地和水面汞釋放要比北美和歐洲這些自然源汞排放量要大很多,這要部分的歸因于之前大量的汞沉降到地表后,在次釋放到大氣中.

4.2與國外相比,中國不管是城市、郊區(qū)、農(nóng)村和偏遠(yuǎn)地區(qū)大氣中TGM和PHg濃度都要明顯高于北美和歐洲地區(qū),尤其是城市地區(qū),而這些污染的來源主要是來自當(dāng)?shù)厝祟惿a(chǎn)和生活對能源和自然資源的消耗,并通過大氣的長距離遷移傳送到偏遠(yuǎn)地區(qū).中國背景區(qū)的大氣汞沉降與發(fā)達(dá)國家相比要高1～2倍左右,而在城市地區(qū)大氣汞沉降是北美和歐洲的1～100倍左右.

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《中國環(huán)境科學(xué)》獲評“2014中國最具國際影響力學(xué)術(shù)期刊”

2014年12月,中國環(huán)境科學(xué)學(xué)會(huì)主辦的《中國環(huán)境科學(xué)》被評為“2014中國最具國際影響力學(xué)術(shù)期刊”.

“中國最具國際影響力學(xué)術(shù)期刊”是《中國學(xué)術(shù)期刊(光盤版)》電子雜志社有限公司、清華大學(xué)圖書館、中國學(xué)術(shù)國際評價(jià)研究中心對我國5600余種中外文學(xué)術(shù)期刊，根據(jù)總被引頻次、影響因子、被引半衰期等計(jì)算出的國際影響力綜合評價(jià)指標(biāo)CI進(jìn)行排序,遴選出的排名前5%的期刊.獲評“中國最具國際影響力學(xué)術(shù)期刊”的科技類期刊共175種.

自2012年開始此項(xiàng)評選以來，《中國環(huán)境科學(xué)》已連續(xù)3年獲此殊榮.

《中國環(huán)境科學(xué)》編輯部

Chinese atmospheric mercury pollution status.

WU Xiao-yun1,2, ZHENG You-fei1,2*, LIN Ke-si3(1.Key Laboratory for Aerosol-Cloud-Precipitation of China Meteorological Administration, Nanjing University of Information Science and Technology, Nanjing 210044, China；2.School of Atmospheric Physics, Nanjing University of Information Science and Technology, Nanjing 210044, China；3.Nanjing Sanhe Environmental Protection Technology Co., LTD, Nanjing 210048, China).

China's economy has developed rapidly in recent decades, but it also brought the persistent pollutants of mercury emissions to the atmosphere rapidly. Mercury as a global pollutant, Chinese mercury emission has received a considerable attention internationally. Thus, this paper reviews the current understanding of and knowledge on atmospheric Hg emissions, distribution transport and deposition in China. Anthropogenic mercury annual emissions from China was the highest in the word and up to about 500～700t, which was more than 25～30% of the global anthropogenic emissions and with an annual increase rate of 4.6%. Emissions of Hg from natural surfaces (including bare soil, water, and vegetation covered soil) was comparable to anthropogenic emissions by the estimation from existing model. Atmospheric Hg exhibits a significant concentration variability among urban, semi-remote, and remote areas. Total Gaseous Mercury (TGM) concentrations in urban areas of China were often 1.5～5 folds higher compared to the corresponding settings in North America and Europe. In turn, particulate mercury (PHg) concentrations in urban areas of China were up to two orders of magnitude higher compared to North America and Europe. TGM, PHg and reactive gaseous mercury (RGM) in coastal areas and remote areas of China were lower than that in central cities, but they were still higher than the corresponding settings in North America and Europe. Accordingly, Hg deposition fluxes tended to be higher in China, with urban areas and remote areas being 1～2 magnitude and1～2 times higher than those in North America and Europe, respectively.

atmospheric mercury；emissions；distribution；deposition

X511,P421

A

1000-6923(2015)09-2623-13

2015-01-08

國家自然科學(xué)基金項(xiàng)目(41075114)

*責(zé)任作者, 教授, myheartwillgoonk@163.com

吳曉云(1984-),女,安徽蕪湖人,講師,博士,主要從事大氣物理與大氣環(huán)境方面的研究.發(fā)表論文10余篇.