安俊嶺,李 穎,湯宇佳,陳 勇,屈 玉 (中國(guó)科學(xué)院大氣物理研究所,大氣邊界層物理與大氣化學(xué)國(guó)家重點(diǎn)實(shí)驗(yàn)室,北京 100029)

HONO來源及其對(duì)空氣質(zhì)量影響研究進(jìn)展

安俊嶺*,李 穎,湯宇佳,陳 勇,屈 玉 (中國(guó)科學(xué)院大氣物理研究所,大氣邊界層物理與大氣化學(xué)國(guó)家重點(diǎn)實(shí)驗(yàn)室,北京 100029)

綜述了HONO來源(源排放、均相反應(yīng)和非均相反應(yīng)生成)、HONO模擬研究以及HONO來源對(duì)空氣質(zhì)量的影響.指出均相反應(yīng)中激發(fā)態(tài) NO2與水汽作用形成 HONO的機(jī)制在高 NOx排放地區(qū)具有重要作用,但反應(yīng)速率需進(jìn)一步證實(shí).非均相反應(yīng)中水解反應(yīng)可能是HONO最主要來源,空氣質(zhì)量模式模擬結(jié)果也支持該觀點(diǎn);soot表面的光照催化反應(yīng)在soot高排放地區(qū)對(duì)HONO貢獻(xiàn)較大,但仍需大量外場(chǎng)實(shí)驗(yàn)證實(shí);土壤排放機(jī)理的外場(chǎng)實(shí)驗(yàn)研究極少,亟待加強(qiáng).

氣態(tài)亞硝酸；OH· ；氮氧化物；氣溶膠；非均相反應(yīng)

OH?是大氣中最重要的氧化劑,對(duì)流層中大多數(shù)痕量氣體主要與 OH?反應(yīng)而被轉(zhuǎn)化或去除,因此,OH?決定著大多數(shù)痕量氣體在大氣中的壽命[1-3].OH?濃度水平可作為大氣氧化能力的指標(biāo),也是局地大氣對(duì)痕量氣體自清潔能力的量度.大氣中 OH?主要來源有 O3的光解,HONO的光解[2,4]、HCHO的光解[2]、H2O2的光解以及 O3與烯烴的反應(yīng)[5].

HONO即氣態(tài)亞硝酸,是城市污染的一種典型代表物,一般在污染嚴(yán)重的城市地區(qū)濃度較高.OH?主要來源于 HONO光解[6-7].近年來大量研究結(jié)果表明,HONO對(duì)OH?的貢獻(xiàn)不僅在清晨,甚至在全天都占有重要地位[6,8-13].據(jù)報(bào)道,白天HONO光解對(duì)OH?的貢獻(xiàn)可達(dá)60%[13].

1979年P(guān)erner和Platt利用長(zhǎng)光程差分吸收光譜儀在大氣中首次觀測(cè)到HONO[14],此后在該方面開展了不少外場(chǎng)實(shí)驗(yàn)研究[4,6-10,13-28].結(jié)果普遍表明,HONO在城市等重污染地區(qū)濃度較高,在鄉(xiāng)村等清潔地區(qū)濃度較低(表 1).在日變化方面,HONO通常在夜間不斷累積,白天光解作用使其濃度在中午或午后達(dá)到最低(量級(jí)為10-12V/V).近幾年的觀測(cè)結(jié)果表明,HONO白天濃度也可達(dá)10-9V/V[11,28],如,智利圣地亞哥春季 HONO白天平均濃度可達(dá)1.90×10-9V/V[10],廣州HONO白天濃度約2.0×10-9V/V[20].如此高的HONO觀測(cè)濃度無法用眾所周知的氣相化學(xué)反應(yīng)生成來解釋(1.2節(jié)),于是HONO來源及其對(duì)空氣質(zhì)量的影響 已成為目前大氣化學(xué)/大氣環(huán)境的研究熱點(diǎn).

表1 各站點(diǎn)HONO、NO2和NOχ觀測(cè)值(×10-9V/V)Table 1 Measurements of HONO, NO2and NOχat the studied sites (×10-9V/V)

1 大氣中HONO來源

1.1 HONO源排放

1.1.1 燃燒過程排放 燃燒過程排放HONO通常根據(jù)機(jī)動(dòng)車尾氣排放的HONO占NOχ的比例來量化. 早期經(jīng)常針對(duì)不同車型進(jìn)行研究,如柴油車排放的HONO約占NOχ排放量的1%,汽油車尾氣排放的HONO不足NOχ排放量的0.01% (燃料充足時(shí))和0.15%(燃料匱乏時(shí))[29].由于排放量依賴于發(fā)動(dòng)機(jī)的類型和運(yùn)行狀況,現(xiàn)在常利用隧道實(shí)驗(yàn)中HONO、NOχ和其他相關(guān)參數(shù)測(cè)量值來估算HONO的排放量,這類實(shí)驗(yàn)HONO/NOχ比值的變化范圍為 0.3%[30]～0.8%[31]. 0.8%該比值是城市交通高密度區(qū)上班期間的典型值,與重污染大氣中監(jiān)測(cè)的比例一致,被許多學(xué)者采納[31-34].

1.1.2 土壤排放 Su等[35]指出土壤中的微生物能夠通過硝化和反硝化作用將含氮養(yǎng)分轉(zhuǎn)化為與 H+結(jié)合生成亞硝酸(R1),亞硝酸通過地氣交換過程以氣態(tài)形式釋放到大氣中.反應(yīng)速率與土壤中亞硝酸鹽含量、pH值、含水量以及溫度有關(guān).該機(jī)制不但會(huì)影響大氣光化學(xué)以及全球陸地生態(tài)系統(tǒng),還會(huì)影響碳氮循環(huán)和氣候變化[36].不過,土壤中亞硝酸鹽濃度通常都很低[37],相關(guān)的外場(chǎng)觀測(cè)實(shí)驗(yàn)極少[38].

1.2 HONO均相反應(yīng)生成

均相反應(yīng)是在單一相態(tài)物質(zhì)中發(fā)生的化學(xué)反應(yīng).HONO最主要的均相反應(yīng)是HONO光解反應(yīng)的逆反應(yīng)R2.該反應(yīng)在白天OH?和NO濃度較高時(shí)重要,夜間OH?濃度一般較低,該反應(yīng)貢獻(xiàn)較小.某些地區(qū)(如珠江三角洲)夜間 OH?濃度較高

[39]時(shí),反應(yīng)R2對(duì)夜間HONO的貢獻(xiàn)不能不考慮.無論如何,反應(yīng)R2無法解釋白天HONO高觀測(cè)濃度.Alicke等[6]指出反應(yīng)R2在中午時(shí)段生成HONO 僅幾×10-12V/V;Su等[22]發(fā)現(xiàn)光化學(xué)穩(wěn)態(tài)平衡法計(jì)算的HONO濃度比觀測(cè)值低一個(gè)數(shù)量級(jí).另外,NO參與的反應(yīng)(如, NO + NO2+ H2O →2HONO)速率很低,在實(shí)際大氣中可以忽略.

實(shí)驗(yàn)室研究發(fā)現(xiàn)鄰硝基酚光解可產(chǎn)生HONO[40].盡管1.0×10-9V/V硝基酚經(jīng)太陽光直射可生成HONO達(dá)100.0×10-12V/V h-1,但相關(guān)機(jī)理仍需外場(chǎng)實(shí)驗(yàn)驗(yàn)證.根據(jù)理論計(jì)算,Zhang等[41]提出氣態(tài)H2O、NO2和NH3經(jīng)均相核化形成HONO的機(jī)制(R3),但反應(yīng) R3既未在實(shí)驗(yàn)室證實(shí),也未在外場(chǎng)實(shí)驗(yàn)觀測(cè)到.

Li等[42]發(fā)現(xiàn)由太陽光激發(fā)的與空氣中水汽反應(yīng)可產(chǎn)生 HONO(以下簡(jiǎn)稱 NO2*機(jī)制),

早期研究認(rèn)為 R4反應(yīng)速率較低,約 1.2× 10-14cm3/(molecule ? s),對(duì)HONO和OH?的貢獻(xiàn)非常有限[43],所以目前大多數(shù)空氣質(zhì)量模式并不包含該機(jī)制.但Li等[42]認(rèn)為該反應(yīng)速率可達(dá)1.7× 10-13cm3/(molecule ? s),比Crowley等[43]結(jié)果高一個(gè)數(shù)量級(jí),該反應(yīng)在NOχ排放量較大的地區(qū)可能具有非常重要的作用.

1.3 HONO非均相反應(yīng)生成

發(fā)生在大氣中顆粒物表面、含表面水層的顆粒物表面、云滴表面的化學(xué)轉(zhuǎn)化和光化學(xué)過程統(tǒng)稱為非均相反應(yīng)[3].HONO的非均相來源大體可分為3類:水解反應(yīng)、還原反應(yīng)和光照催化反應(yīng). 1.3.1 水解反應(yīng) NO2在各種濕潤(rùn)表面發(fā)生的非均相反應(yīng) R5可能是 HONO最主要來源

[26,28,44-46],該反應(yīng)為一級(jí)反應(yīng)[2].R5可能發(fā)生在地表面(包括裸露的土壤、城市建筑物表面、雪地等),也可發(fā)生在氣溶膠表面(包括云滴、霧滴、空氣中的顆粒物等).Finlayson-Pitts等[44]通過實(shí)驗(yàn)室研究給出了反應(yīng) R5在地表面的形成機(jī)理,指出由NO2生成的N2O4是驅(qū)動(dòng)該反應(yīng)發(fā)生的重要物種,產(chǎn)物中HONO有一部分脫離地表面返回大氣,HNO3則留在反應(yīng)表面,該反應(yīng)速率與界面吸附的液態(tài)水含量有關(guān).Wojtal等[47]認(rèn)為反應(yīng)R5可在海洋表面發(fā)生.外場(chǎng)實(shí)驗(yàn)表明大氣中氣溶膠與HONO濃度,或氣溶膠比表面積與HONO濃度具有很好的相關(guān)性[12,20,48],隱含說明氣溶膠表面是 R5發(fā)生反應(yīng)的最主要界面,固定界面(土壤、建筑表面等)可能不重要[38,48].

氣溶膠表面生成的 HNO3可重新參與一系列大氣化學(xué)反應(yīng),使大氣中硝酸鹽濃度增升,氣溶膠可通過長(zhǎng)距離輸送而影響區(qū)域或全球?qū)α鲗踊瘜W(xué)過程.城市地區(qū)大氣氣溶膠組分非常復(fù)雜,可作為云霧形成過程中的凝結(jié)核.大量觀測(cè)表明,在氣溶膠和云滴表面可形成 HONO[49].研究表明,京津冀地區(qū)氣溶膠表面的非均相反應(yīng)對(duì)大氣氧化性、霾和能見度可能具有重要影響[12,38,50-51].

1.3.2 還原反應(yīng) Ammann等[52]首次提出NO2在煙灰(soot)表面可生成HONO(R6).由于soot反應(yīng)后幾分鐘內(nèi)迅速喪失活性,其后研究一般認(rèn)為該反應(yīng)對(duì)大氣中HONO的形成不重要[31].最近研究結(jié)果表明,光照會(huì)保持soot的反應(yīng)活性,該反應(yīng)對(duì)白天HONO形成可能具有重要作用,尤其是在soot排放量較高的地區(qū)[53].

在Ammann等[52]研究基礎(chǔ)上,Gutzwiller等[54]發(fā)現(xiàn)柴油車尾氣排放的半揮發(fā)性有機(jī)物對(duì)HONO形成具有重要影響(R7).柴油車尾氣排放的NOχ約2.3%會(huì)在半揮發(fā)性有機(jī)物表面經(jīng)非均相過程生成HONO. HONO由該途徑的生成量至少是柴油車尾氣排放量的3倍[54].

1.3.3 光照催化反應(yīng) 雖然HONO濃度常在夜間達(dá)到最大,但白天HONO也會(huì)出現(xiàn)意想不到的高值,這種情況常和光照強(qiáng)度密切相關(guān).許多研究均指出光照會(huì)催化某些反應(yīng),有利于白天HONO生成.例如,在可見光照射下,NO2在TiO2表面[55]、腐殖酸表面[56]、固態(tài)有機(jī)化合物表面[57]、酚類物質(zhì)表面以及地表面[58]都會(huì)產(chǎn)生HONO(R8).

Zhou等[59]發(fā)現(xiàn)表面吸附的HNO3或NO3-經(jīng)紫外光解可以產(chǎn)生HONO,HONO通過解吸作用脫離吸附物表面而釋放回大氣(R9).Zhou等[60]在森林冠層上空觀測(cè)到HONO的直接排放,認(rèn)為沉積到森林冠層表面的 HNO3經(jīng)光解可生成HONO,該機(jī)制有利于已沉積到地表的 HNO3再次活化.Li等[28]指出反應(yīng) R9可能是廣州后花園白天 HONO的來源之一.Ziemba等[48]在觀測(cè)實(shí)驗(yàn)中發(fā)現(xiàn) HNO3可在一次有機(jī)氣溶膠表面經(jīng)非均相過程生成HONO.

2 HONO模擬研究

目前區(qū)域空氣質(zhì)量模式,如 CAMχ (http:// www.camx.com/)、CMAQ[61]和 WRF-Chem[62],所采納的氣相化學(xué)反應(yīng)機(jī)制主要包括CBM-IV、CBM-Z、SAPRC、RADM2和RACM等[63].以上這些化學(xué)機(jī)制都將氣相反應(yīng)作為HONO的唯一來源,例如CBM-IV機(jī)制中HONO來源包括OH? + NO + M → HONO + M和NO + NO2+ H2O → 2HONO.前文1.2節(jié)已指出,NO + NO2+ H2O → 2HONO對(duì)HONO的貢獻(xiàn)可忽略.由于模式通常不包括HONO的源排放和非均相反應(yīng)來源,所以HONO模擬值遠(yuǎn)低于觀測(cè)值[33,38,50,64].為改進(jìn)HONO的模擬,Sarwar等[33]將燃燒過程排放HONO、非均相反應(yīng)R5和光照催化反應(yīng)R8引入CMAQ模式,HONO模擬結(jié)果明顯改進(jìn),但模擬值仍低于觀測(cè)值,尤其是在白天時(shí)段.Sarwar等

[33]指出非均相反應(yīng)R5和光照催化反應(yīng)R8是HONO最主要的兩個(gè)來源,對(duì)HONO的平均貢獻(xiàn)分別達(dá)54%和32%,而氣相反應(yīng)R2和燃燒過程排放兩者對(duì) HONO的貢獻(xiàn)僅 14%.非均相反應(yīng)R5對(duì)夜間HONO的貢獻(xiàn)高達(dá)90%,光照催化反應(yīng)R8的貢獻(xiàn)集中在白天.根據(jù)Sarwar等[33]的研究,CMAQv4.7加入了燃燒過程排放HONO、NO2在氣溶膠表面和地表面的非均相過程,但尚未詳細(xì)驗(yàn)證HONO新增來源的影響[65].

Aumont等[66]利用兩層箱模式,加入 NO2在soot表面的氧化還原反應(yīng) R6.模擬結(jié)果表明,若不考慮soot表面的老化過程,HONO模擬值高于觀測(cè)值.Aumont等[32]進(jìn)一步考慮了燃燒過程排放HONO以及NO2在顆粒物表面和地表面的非均相反應(yīng)(R5),發(fā)現(xiàn)城市地區(qū)冬季夜間HONO濃度比夏季夜間高25%;冬夏兩季HONO平均日變化不同,夏季只出現(xiàn)一個(gè)峰值,而冬季有兩個(gè)峰值;污染較重的鄉(xiāng)村地區(qū),冬季夜間HONO濃度比夏季夜間高4倍;燃燒過程排放HONO以及NO2在地表面的非均相反應(yīng)對(duì)HONO貢獻(xiàn)較大,氣溶膠表面的非均相反應(yīng)貢獻(xiàn)較小.

Xu等[67]將氣溶膠表面的 4個(gè)非均相反應(yīng)(R5、R10～R12)引入 CMAQ模式,模擬了 2000年6月26～27日北京HONO濃度,模擬值在觀測(cè)范圍之內(nèi).Xu等[67]沒有考慮氣溶膠吸濕性增長(zhǎng)過程,北京夏季相對(duì)濕度較大,氣溶膠吸濕性增長(zhǎng)對(duì)非均相過程的影響不可忽視(Li等[50]).

Li等[64]將NO2在顆粒物表面和地表面的非均相反應(yīng)(R5)、NO2在 soot表面的非均相反應(yīng)(R6)以及 NO2在半揮發(fā)性有機(jī)物表面的非均相反應(yīng)(R7)引入WRF-Chem模式,模擬了2006年3月24～29日墨西哥城的空氣質(zhì)量.加入HONO新增來源后,模式可以合理反映HONO的實(shí)際日變化;NO2在半揮發(fā)性有機(jī)物表面的非均相反應(yīng)(R7)是HONO最重要的來源,對(duì)白天HONO的貢獻(xiàn)達(dá) 75%.因?yàn)榉磻?yīng) R7只適用于柴油車尾氣排放[54],而Li等[64]將其應(yīng)用于所有NOχ排放源,可能高估了反應(yīng)R7的重要性[38].

Volkamer等[68]利用一個(gè)箱模式開展了敏感性試驗(yàn),結(jié)果表明光照催化產(chǎn)生HONO的反應(yīng),例如NO2在腐殖酸表面[56]的非均相反應(yīng),對(duì)墨西哥城大氣中的HONO并不重要;模式對(duì)HONO的匯考慮不充分,致使9:00的HONO模擬值高于觀測(cè)值70%.S?rgel等[25]模擬結(jié)果說明光照條件下NO2在soot表面的非均相反應(yīng)(R6)[53]對(duì) HONO的貢獻(xiàn)<1%,NO2*機(jī)制(R4)對(duì)HONO的貢獻(xiàn)<10%. Li等[50]將HONO源排放(包括燃燒過程排放和反應(yīng)R7)、NO2*機(jī)制以及氣溶膠表面的非均相反應(yīng)進(jìn)行參數(shù)化,并耦合于WRF-Chem模式,HONO模擬結(jié)果顯著改善.Li等[50]還發(fā)現(xiàn)氣溶膠表面的非均相化學(xué)反應(yīng)是HONO最重要的來源,貢獻(xiàn)約59%;NO2*機(jī)制是HONO第二個(gè)主要來源,貢獻(xiàn)約26%. Czader等[69]指出反應(yīng)R6與R4對(duì)HONO的貢獻(xiàn)較小,而NO2的水解反應(yīng)(R5)可使HONO濃度增大10倍. Gon?alves等[34]在CMAQ模式中加入燃燒過程排放HONO以及NO2在顆粒物表面和地表面的非均相反應(yīng)(R5),雖然HONO的模擬有很大改進(jìn),但仍低于觀測(cè)值,指出 NO2在地表面的非均相反應(yīng)參數(shù)化方案有待改進(jìn).根據(jù)世界各地大型觀測(cè)實(shí)驗(yàn)結(jié)果,在全球尺度上 Elshorbany 等[11]估算HONO/NOχ均值為0.02,并將該比值應(yīng)用于一個(gè)全球化學(xué)輸送模式,模擬結(jié)果在全球范圍內(nèi)較好反映了 HONO的觀測(cè)濃度,在高 NOχ排放地區(qū)HONO來源對(duì)HOχ(≡O(shè)H? + HO2?)、O3以及PAN(過氧乙酰硝酸酯)有顯著影響,并且冬季影響更為顯著,建議大氣化學(xué)模式應(yīng)考慮HONO新來源,改進(jìn)HONO化學(xué)機(jī)制.

3 HONO來源對(duì)空氣質(zhì)量的影響

HONO來源影響空氣質(zhì)量的根本原因有兩方面.其一,HONO光解產(chǎn)生OH?,OH?的增加會(huì)使大氣氧化能力增強(qiáng),引起O3等二次污染物濃度升高;其二,HONO來源包含多種形成機(jī)理,HONO各來源對(duì)O3等污染物的影響不同.

Clapp等[70]指出燃燒過程排放HONO對(duì)Oχ(≡ NO2+ O3)濃度影響較大.若HONO占NOχ排放量的比例從0%增至5%,O3與Ox的濃度則分別增加 1.51×10-9V/V和 1.15×10-9V/V[71].HONO增加1%可使O3增升約0.3×10-9V/V,是NO2增加1%引起O3增加量的3倍[71].An等[38]認(rèn)為京津冀地區(qū)燃燒過程排放 HONO引起該地區(qū)近地面HONO白天和夜間月均濃度最大增幅均超過100%,OH?、HO2?和O3白天月均濃度最大增幅依次為8%、7%和1%.

關(guān)于HONO均相反應(yīng)來源對(duì)O3及相關(guān)污染物的影響,近幾年關(guān)注較多的是 NO2*機(jī)制(R4). Wennberg等[72]利用空氣質(zhì)量模式模擬了1987年8月27～29日美國(guó)加利福尼亞南部地區(qū)O3濃度,結(jié)果表明 NO2*機(jī)制可使 O3增加 55×10-9V/V, PM2.5增加 20μg/m3.O3模擬值高于觀測(cè)值,所以Wennberg等[72]認(rèn)為L(zhǎng)i等[42]建議的反應(yīng)速率常數(shù)可能偏大.Sarwar等[73]開展了類似的工作,指出目前美國(guó)NOχ排放量遠(yuǎn)低于1987年,NO2*機(jī)制對(duì)目前美國(guó)空氣質(zhì)量影響很小,但在NOχ和可揮發(fā)性有機(jī)化合物(VOC)等高排放地區(qū)可能影響很大.Ensberg等[74]分別利用美國(guó)1987年和2005年源排放清單模擬了 NO2*機(jī)制對(duì)加利福尼亞南部地區(qū) O3影響,發(fā)現(xiàn)單獨(dú)削減 NOχ排放量時(shí), NO2*機(jī)制會(huì)促進(jìn)臭氧濃度下降,這對(duì) O3調(diào)控措施有啟示意義.Li等[50]指出 NO2*機(jī)制可使京津冀主要城市 O3時(shí)均峰值最大增量達(dá)(30～50)× 10-9V/V,該地區(qū)HONO白天月均濃度最大增幅達(dá)70%,OH?、HO2?和O3月均濃度最大增幅依次為19%、16%和4%[38],建議NOχ和顆粒物源排放量偏高地區(qū)的空氣質(zhì)量模擬研究應(yīng)考慮燃燒過程排放 HONO、NO2*機(jī)制以及氣溶膠表面的非均相化學(xué)反應(yīng).Jorba等[75]將 NO2*機(jī)制耦合到一個(gè)全球化學(xué)輸送模式,模擬結(jié)果表明在清潔地區(qū)O3濃度可增加(4～6)×10-9V/V,城市地區(qū)O3濃度可增加(6～15)×10-9V/V,O3時(shí)均濃度最大增量在我國(guó)東部可達(dá) 30×10-9V/V;NO2*機(jī)制在海洋上影響較大,尤其是在 NOχ/VOC比值較大的區(qū)域;反應(yīng)速率常數(shù)采用Crowley等[43]的建議值和Li等[42]的建議值對(duì)全球大部分地區(qū)的模擬結(jié)果影響不大,僅對(duì)NOχ排放量較高的美國(guó)加州、美國(guó)東北部、朝鮮半島以及中國(guó)有影響.

關(guān)于HONO非均相反應(yīng)來源對(duì)O3及相關(guān)污染物的影響,Kotamarthi等[76]將NO2在soot表面的非均相反應(yīng)R6引入箱模式,發(fā)現(xiàn)若不考慮soot表面喪失反應(yīng)活性,反應(yīng)R6對(duì)大氣邊界層中O3、OH?和 HO2?影響顯著,O3濃度可增加(8～20)× 10-9V/V;當(dāng)大氣中氣溶膠比表面積較大時(shí),反應(yīng)R6可減少O3夜間損失,使O3夜間濃度升高.由于該箱模式未考慮干沉降和長(zhǎng)距離輸送等物理過程,Kotamarthi等[76]的結(jié)果有待化學(xué)和物理過程相互耦合的三維空氣質(zhì)量模式進(jìn)一步檢驗(yàn)以及外場(chǎng)實(shí)驗(yàn)的驗(yàn)證.Lei等[77]利用一個(gè)化學(xué)輸送模式討論了反應(yīng)R6對(duì)O3的影響.在忽略soot還原表面失去活性條件下,反應(yīng)R6可使O3全天增幅約(4～12)×10-9V/V;O3白天均值增加 7× 10-9V/V,清晨 O3的快速累積過程提早 1小時(shí). Aumont等

圖1 HONO來源對(duì)區(qū)域能見度及霾影響的概念模型Fig.1 A conceptual model for the impacts of HONO sources on regional visibility and haze

[32]指出HONO來源對(duì)O3、NOχ和HOχ的影響在冬季較顯著,在夏季影響一般不大,但夏季光化學(xué)污染過程除外.Li等[64]將 NO2的非均相反應(yīng)(R5～7)引入WRF-Chem模式,結(jié)果表明O3白天均值增加 6×10-9V/V,清晨 O3的快速累積過程提早2小時(shí),上午二次有機(jī)氣溶膠的濃度增加了一倍,硝酸鹽和銨鹽濃度也有所增加,但對(duì)硫酸鹽濃度影響不大.Xu等[67]在CMAQ模式中加入氣溶膠表面的4個(gè)非均相反應(yīng)(R5、R10～12),結(jié)果表明反應(yīng)R5加快了NO向NO2的轉(zhuǎn)化過程,導(dǎo)致O3增加,O3峰值最大增量達(dá)67×10-9V/V;反應(yīng)R5可能是北京城區(qū)白天 O3增加的主要原因之一;北京夏季O3主要受VOC控制,反應(yīng)R5使HONO增多,大氣中 OH?濃度增加,進(jìn)而促使 O3濃度升高.Czader等[69]發(fā)現(xiàn)反應(yīng)R5對(duì)O3的貢獻(xiàn)主要發(fā)生在早晨,NO2*機(jī)制(R4)和NO2在soot表面的非均相反應(yīng)(R6)可使白天 O3濃度都有所增加. Gon?alves等[34]指出燃燒過程排放HONO和反應(yīng)R5可使PM2.5濃度最大增量達(dá)14%,主要原因是HONO來源可使硝酸鹽濃度增加,并建議空氣質(zhì)量模式加入燃燒過程排放HONO和NO2的非均相反應(yīng)(R5).An等[38]指出氣溶膠表面的非均相反應(yīng)使京津冀地區(qū)近地面HONO白天和夜間月均濃度最大增幅均超過2倍,OH?、HO2?和O3白天月均濃度最大增幅依次為56%、60%和8%,NO3-、NH4+白天和夜間月均濃度最大增幅均為48%和35%.

4 結(jié)論

4.1 HONO來源大體可分為HONO源排放(燃燒過程排放和土壤排放)、均相反應(yīng)生成(OH?與NO的氣相反應(yīng)、機(jī)制)和非均相反應(yīng)生成(水解反應(yīng)、還原反應(yīng)和光照催化反應(yīng)).

4.3 水解反應(yīng)可能是 HONO最主要來源,空氣質(zhì)量模式模擬結(jié)果也支持該觀點(diǎn).不過,是否所有土壤和植被、任何建筑物等表面均可發(fā)生該反應(yīng)仍需大量實(shí)驗(yàn)室和外場(chǎng)實(shí)驗(yàn)驗(yàn)證.另外,氣溶膠表面的非均相反應(yīng)(水解反應(yīng))可能與氣溶膠的化學(xué)組分有關(guān),不同氣溶膠組分的表面反應(yīng)機(jī)率(攝取系數(shù))可能不同,亟需外場(chǎng)實(shí)驗(yàn)確定.

4.4 還原反應(yīng)若光照會(huì)保持 soot的反應(yīng)活性,則在soot高排放地區(qū)對(duì)HONO貢獻(xiàn)較大.光照催化反應(yīng)仍需大量外場(chǎng)實(shí)驗(yàn)證實(shí)和定量化.

[1] Levy H II. Normal atmosphere: Large radical and formaldehyde concentrations predicted [J]. Science, 1971,173:141-143.

[2] Finlayson-Pitts B J, Pitts J N. Tropospheric air pollution: Ozone, airborne toxics, polycyclic aromatic hydrocarbons, and particles [J]. Science, 1997,276:1045-1051.

[3] 唐孝炎,張遠(yuǎn)航,邵 敏.大氣環(huán)境化學(xué) [M]. 北京:高等教育出版社, 2006.

[4] Platt U, Perner D, Harris G, et al. Observations of nitrous acid in an urban atmosphere by differential optical absorption [J]. Nature, 1980,285:312-314.

[5] Atkinson R, Aschmann S M, Arey J, et al. Formation of OH· radicals in the gas phase reactions of O3with a series of terpenes [J]. Journal of Geophysical Research, 1992,97:6065-6073.

[6] Alicke B, Platt U, Stutz J. Impact of nitrous acid photolysis on the total hydroxyl radical budget during the limitation of oxidant production/Pianura Padana Produzione di Ozono study in Milan [J]. Journal of Geophysical Research, 2002,107,8196,LOP 9-1-LOP 9-17.

[7] Alicke B, Geyer A, Hofzumahaus A, et al. OH· formation by HONO photolysis during the BERLIOZ experiment [J]. Journal of Geophysical Research, 2003,108,8247,PHO 3-1-PHO 3-17.

[8] Stutz J, Alicke B, Neftel A. Nitrous acid formation in the urban atmosphere: Gradient measurements of NO2and HONO over grass in Milan, Italy [J]. Journal of Geophysical Research, 2002, 107,LOP 5-1-LOP 5-15.

[9] Acker K, Febo A, Trick S, et al. Nitrous acid in the urban area of Rome [J]. Atmospheric Environment, 2006,40:3123-3133.

[10] Elshorbany Y F, Kurtenbach R, Wiesen P, et al. Oxidation capacity of the city air of Santiago, Chile [J]. Atmospheric Chemistry and Physics, 2009,9:2257-2273.

[11] Elshorbany Y F, Steil B, Brühl C, et al. Impact of HONO on global atmospheric chemistry calculated with an empirical parameterization in the EMAC model [J]. Atmospheric Chemistry and Physics, 2012,12(20):9977-10000.

[12] An J, Zhang W, Qu Y. Impacts of a strong cold front on concentrations of HONO, HCHO, O3, and NO2in the heavy traffic urban area of Beijing [J]. Atmospheric Environment, 2009, 43:3454-3459.

[13] Spataro F, Ianniello A, Esposito G, et al. Occurrence of atmospheric nitrous acid in the urban area of Beijing (China) [J]. Science of the Total Environment, 2013,447:210-224.

[14] Perner D, Platt U. Detection of nitrous acid in the atmosphere by differential optical absorption [J]. Geophysical Research Letters, 1979,6:917-920.

[15] Matsumoto M, Okita T. Long-term measurements of atmospheric gaseous and aerosol species using an annular denuder system in Nara, Japan [J]. Atmos. Environ., 1998,32(8):1419- 1425.

[16] Hu M, Zhou F M, Shao K S, et al. Diurnal variations of aerosol chemical components and related gaseous pollutants in Beijing and Guangzhou [J]. Journal of Environmental Science and Health, 2002,A37(4):479-488.

[17] Ren X, Brune W H, Mao J, et al. Behavior of OH?and HO2?in the winter atmosphere in New York City [J]. Atmos. Environ., 2006, 40:252-263.

[18] Hao N, Zhou B, Chen D, et al. Observations of nitrous acid and its relative humidity dependence in Shanghai [J]. Journal of Environmental Sciences, 2006,18:910-915.

[19] Lin Y-C, Cheng M-T, Ting W-Y, et al. Characteristics of gaseous HNO2, HNO3, NH3and particulate ammonium nitrate in an urban city of Central Taiwan [J]. Atmospheric Environment, 2006,40:4725-4733.

[20] Qin M, Xie P, Su H, et al. An observational study of the HONO-NO2coupling at an urban site in Guangzhou City, SouthChina [J]. Atmos. Environ., 2009,43:5731-5742.

[21] Kanaya Y, Cao R, Akimoto H, et al. Urban photochemistry in central Tokyo: 1. Observed and modeled OH· and HO2· radical concentrations during the winter and summer of 2004 [J]. Journal of Geophysical Research, 2007,112,D21312,1-20.

[22] Su H, Cheng Y, Shao M, et al. Nitrous acid (HONO) and its daytime sources at a rural site during the 2004PRIDE-PRD experiment in China [J]. Journal of Geophysical Research, 2008, 113,D14312,1-9.

[23] Yu Y, Galle B, Panday A, et al. Observations of high rates of NO2-HONO conversion in the nocturnal atmospheric boundary layer in Kathmandu, Nepal [J]. Atmospheric Chemistry and Physics, 2009,9:6401-6415.

[24] 朱燕舞,劉文清,謝品華,等.北京夏季大氣 HONO的監(jiān)測(cè)研究[J]. 環(huán)境科學(xué), 2009,30(6):1567-1573.

[25] S?rgel M, Regelin E, Bozem H, et al. Quantification of the unknown HONO daytime source and its relation to NO2[J]. Atmospheric Chemistry and Physics, 2011,11:10433-10447.

[26] Wong K W, Oh H-J, Lefer B L, et al. Vertical profiles of nitrous acid in the nocturnal urban atmosphere of Houston, TX [J]. Atmospheric Chemistry and Physics, 2011,11:3595-3609.

[27] Villena G, Wiesen P, Cantrell C A, et al. Nitrous acid (HONO) during polar spring in Barrow, Alaska: A net source of OH· radicals? [J]. Journal of Geophysical Research, 2011,116,D00R07,1-12.

[28] Li X, Brauers T, H?seler R, et al. Exploring the atmospheric chemistry of nitrous acid (HONO) at a rural site in Southern China [J]. Atmospheric Chemistry and Physics, 2012,12:1497-1513.

[29] Kessler C, Platt U. Nitrous acid in polluted air masses-sources and formation pathways, physico-chemical behaviour of atmospheric pollutants [M]. Reidel, Dordrecht, 1984,412-422.

[30] Kirchstetter T, Harley R, Littlejohn D. Measurement of nitrous acid in motor vehicle exhaust [J]. Environmental Science and Technology, 1996,30:2843-2849.

[31] Kurtenbach R, Becker K H, Gomes J A G, et al. Investigations of emissions and heterogeneous formation of HONO in a road traffic tunnel [J]. Atmos. Environ., 2001,35:3385-3394.

[32] Aumont B, Chervier F, Laval S. Contribution of HONO sources to the NOx/HOx/O3chemistry in the polluted boundary layer [J]. Atmospheric Environment, 2003,37:487-498.

[33] Sarwar G, Roselle S J, Mathur R, et al. A comparison of CMAQ HONO predictions with observations from the Northeast Oxidant and Particle Study [J]. Atmos. Environ., 2008,42:5760-5770.

[34] Gon?alves M, Dabdub D, Chang W L, et al. Impact of HONO sources on the performance of mesoscale air quality models [J]. Atmos. Environ., 2012,54:168-176.

[35] Su H, Cheng Y, Oswald R, et al. Soil nitrite as a source of atmospheric HONO and OH· radicals [J]. Science, 2011,333: 1616-1618.

[36] Kulmala M, Pet?j? T. Soil Nitrites Influence Atmospheric Chemistry [J]. Science, 2011,333:1586-1587.

[37] Cleemput O V, Samater A H. Nitrite in soils: accumulation and role in the formation of gaseous N compounds [J]. Fertilizer Research, 1996,45:81-89.

[38] An J, Li Y, Chen Y, et al. Enhancements of major aerosol components due to additional HONO sources in the North China Plain (NCP) and implications for visibility and haze [J]. Advances in Atmospheric Sciences, 2013,30:57-66.

[39] Lu K D, Rohrer F, Holland F, et al. Observation and modeling of OH?and HO2concentrations in the Pearl River Delta 2006: a missing OH· source in a VOC rich atmosphere [J]. Atmospheric Chemistry and Physics Discussion, 2011,11:11311-11378.

[40] Bejan I, Abd-el-Aal Y, Barnes I, et al. The photolysis of ortho-nitrophenols: A new gas phase source of HONO [J]. Physical Chemistry Chemical Physics, 2006,8:2028-2035.

[41] Zhang B, Tao F M. Direct homogeneous nucleation of NO2, H2O, and NH3for the production of ammonium nitrate particles and HONO gas [J]. Chemical and Physical Letters, 2010,489:143-147.

[42] Li S, Matthews J, Sinha A. Atmospheric hydroxyl radical production from electronically excited NO2and H2O [J]. Science, 2008,319:1657-1660.

[43] Crowley J N, Carl S A. OH· formation in the photoexcitation of NO2beyond the dissociation threshold in the presence of water vapor [J]. Journal of Physical Chemistry, 1997,101A:4178-4184. [44] Finlayson-Pitts B J, Wingen L M, Sumner A L, et al. The heterogeneous hydrolysis of NO2in laboratory systems and in outdoor and indoor atmospheres: integrated mechanism [J]. Physical Chemistry Chemical Physics, 2003,5:223-242.

[45] 丁 杰,朱 彤.大氣中細(xì)顆粒物表面多相化學(xué)反應(yīng)的研究 [J].科學(xué)通報(bào), 2003,48(19):2005-2013.

[46] 葛茂發(fā),劉 澤,王煒罡.二次光化學(xué)氧化劑與氣溶膠間的非均相過程 [J]. 地球科學(xué)進(jìn)展, 2009,24(4):351-362.

[47] Wojtal P, Halla J D, McLaren R. Pseudo steady states of HONO measured in the nocturnal marine boundary layer: A conceptual model for HONO formation on aqueous surfaces [J]. Atmospheric Chemistry and Physics, 2011,11:3243-3261.

[48] Ziemba L D, Dibb J E, Griffin R J, et al. Heterogeneous conversion of nitric acid to nitrous acid on the surface of primary organic aerosol in an urban atmosphere [J]. Atmos. Environ., 2010,44(33):4081-4089.

[49] Notholt J, Hjorth J, Raes F. Formation of HNO2on aerosol surfaces during foggy periods in the presence of NO and NO2[J]. Atmos. Environ., 1992,26(2):211-217.

[50] Li Y, An J, Min M, et al. Impacts of HONO sources on the air quality in Beijing, Tianjin and Hebei Province of China [J].Atmos. Environ., 2011,45:4735-4744.

[51] Zhu T, Shang J, Zhao D F. The roles of heterogeneous chemical processes in the formation of an air pollution complex and gray haze [J]. Science in China, 2011,54B:145-153.

[52] Ammann M, Kalberer M, Jost D T, et al. Heterogeneous production of nitrous acid on soot in polluted air masses [J]. Nature, 1998,395:157-160.

[53] Monge M E, D’Anna B, Mazri L, et al. Light changes the atmospheric reactivity of soot [J]. Proceedings of the National Academy of Sciences, 2010,107:6605-6609.

[54] Gutzwiller L, Arens F, Baltensperger U, et al. Significance of semivolatile diesel exhaust organics for secondary HONO formation [J]. Environ. Sci. Technol., 2002,36: 677-682.

[55] Ndour M, D’Anna B, George C, et al. Photoenhanced uptake of NO2on mineral dust: Laboratory experiments and model simulations [J]. Geophysical Research Letters, 2008,35,L05812, doi:10.1029/2007GL032006.

[56] Stemmler K, Ammann M, Donders C, et al. Photosensitizied reduction of nitrogen dioxide on humic acid as a source of nitrous acid [J]. Nature, 2006,440:195-198.

[57] George C, Sterkowski R S, Kleffmann J, et al. Photoenhanced uptake of gaseous NO2on solid organic compounds: A photochemical source of HONO? [J]. Faraday Discussion, 2005, 130:195-210.

[58] Wong K W, Tsai C, Lefer B, et al. Daytime HONO vertical gradients during SHARP 2009 in Houston, TX [J]. Atmospheric Chemistry and Physics, 2012,12:635-652.

[59] Zhou X, He Y, Huang G, et al. Photochemical production of nitrous acid on glass sample manifold surface [J]. Geophysical Research Letters, 2002,29,1681:261-264.

[60] Zhou X, Zhang N, TerAvest M, et al. Nitric acid photolysis on forest canopy surface as a source for tropospheric nitrous acid [J]. Nature Geoscience, 2011,4:440-443.

[61] Byun D, Schere K L. Review of the governing equations, computational algorithms, and other components of the models-3: Community Multiscale Air Quality (CMAQ) modeling system [J]. Applied Mechanics Reviews, 2006,59:51-77.

[62] Grell G A, Peckham S E, Schmitz R, et al. Fully coupled “online”chemistry within the WRF model [J]. Atmos. Environ., 2005,39: 6957-6975.

[63] 石玉珍,徐永福,賈 龍.大氣化學(xué)機(jī)理的發(fā)展及應(yīng)用 [J]. 氣候與環(huán)境研究, 2012,17(1):112-124.

[64] Li G, Lei W, Zavala M, et al. Impacts of HONO sources on the photochemistry in Mexico City during the MCMA-2006/ MILAGO Campaign [J]. Atmospheric Chemistry and Physics, 2010,10:6551-6567.

[65] Foley K M, Roselle S J, Appel K W, et al. Incremental testing of the Community Multiscale Air Quality (CMAQ) modeling system version 4.7 [J]. Geoscientific Model Development, 2010,3:205-226.

[66] Aumont B, Madronich S, Ammann M, et al. On the NO2+ soot reaction in the atmosphere [J]. Journal of Geophysical Research, 1999,104:1729-1736.

[67] Xu J, Zhang Y, Wang W. Numerical study on the impacts of heterogeneous reactions on ozone formation in the Beijing urban area [J]. Advances in Atmospheric Sciences, 2006,23:605-614.

[68] Volkamer R, Sheehy P, Molina L, et al. Oxidative capacity of the Mexico City atmosphere-Part 1: A radical source perspective [J]. Atmospheric Chemistry and Physics, 2010,10:6969-6991.

[69] Czader B H, Rappenglück B, Percell P, et al. Modeling nitrous acid and its impact on ozone and hydroxyl radical during the Texas Air Quality Study 2006 [J]. Atmospheric Chemistry and Physics, 2012,12:6939-6951.

[70] Clapp L J, Jenkin M E. Analysis of the relationship between ambient levels of O3, NO2and NO as a function of NOxin the UK [J]. Atmos. Environ., 2001,35:6391-6405.

[71] Jenkin M E, Utembe S R, Derwent R G. Modelling the impact of elevated primary NO2and HONO emissions on regional scale oxidant formation in the UK [J]. Atmos. Environ., 2008,42:323-336.

[72] Wennberg P O, Dabdub D. Rethinking ozone production [J]. Science, 2008,319:1624-1625.

[73] Sarwar G, Pinder R W, Appel K W, et al. Examination of the impact of photoexcited NO2chemistry on regional air quality [J]. Atmos. Environ., 2009,43:6383-6387.

[74] Ensberg J J, Carreras-Sospedra M, Dabdub D. Impacts of electronically photo-excited NO2on air pollution in the South Coast Air Basin of California [J]. Atmospheric Chemistry and Physics, 2010,10:1171-1181.

[75] Jorba O, Dabdub D, Blaszczak-Boxe C, et al. Potential significance of photoexcited NO2on global air quality with the NMMB/BSC chemical transport model [J]. Journal of Geophysical Research, 2012,117,doi:10.1029/2012JD017730.

[76] Kotamarthi V R, Gaffney J S, Marley N A, et al. Heterogeneous NOxchemistry in the polluted PBL [J]. Atmos. Environ., 2001, 35:4489-4498.

[77] Lei W, Zhang R, Tie X, et al. Chemical characterization of ozone formation in the Houston-Galveston area: A chemical transport model study [J]. Journal of Geophysical Research, 2004,109, D12301,doi:10.1029/2003JD004219.

Advances in HONO sources, HONO simulations, and the impacts of the HONO sources on regional or global airquality.

AN Jun-ling*, LI Ying, TANG Yu-jia, CHEN Yong, QU Yu (State Key Laboratory of Atmospheric Boundary-layer Physics and Atmospheric Chemistry, Institute of Atmospheric Physics, Chinese Academy of Sciences, Beijing 100029, China). China Environmental Science, 2014,34(2)：273～281

HONO sources (i.e., HONO emissions, homogeneous gas-phase production and heterogeneous reaction production), HONO simulations, and the impacts of the HONO sources on regional or global air quality were reviewed. Reaction of photoexcited NO2with water vapor contributed much to HONO formation in elevated NOχemission areas but the accurate reaction rate needs to be quantified. Heterogeneous formation of HONO on wet surfaces could be the key source of HONO, which was supported by air quality model simulations. The photosensitized NO2reduction on soot is possibly a large contributor to HONO concentrations in high soot emission areas but the related field studies are required. Observations of HONO emissions from soil are very limited and urgently needed.

HONO；hydroxyl radical；NOχ；aerosol；heterogeneous reaction

X131.1

：A

：1000-6923(2014)02-0273-09

安俊嶺(1967-),男,寧夏海原人,研究員,博士,主要從事大氣環(huán)境/大氣化學(xué)研究.發(fā)表論文50余篇.

2013-05-30

國(guó)家自然科學(xué)基金(41175105);中國(guó)科學(xué)院戰(zhàn)略性先導(dǎo)科技專項(xiàng)(B類)課題“大氣灰霾跨界輸送途徑與定量評(píng)估”(XDB05030301)

? 責(zé)任作者, 研究員, anjl@mail.iap.ac.cn