吳電明, 夏玉玲, 侯立軍, 劉 敏

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土壤亞硝酸氣體(HONO)排放過程及其驅(qū)動(dòng)機(jī)制*

吳電明1, 夏玉玲1, 侯立軍2, 劉 敏1

(1. 華東師范大學(xué)地理科學(xué)學(xué)院/教育部地理信息科學(xué)重點(diǎn)實(shí)驗(yàn)室 上海 200241; 2. 華東師范大學(xué)河口海岸學(xué)國(guó)家重點(diǎn)實(shí)驗(yàn)室 上海 200062)

氣態(tài)亞硝酸(HONO)是大氣中氫氧自由基(OH·)的重要來源, 直接影響到大氣氧化能力和空氣質(zhì)量。通過比較外場(chǎng)測(cè)定和模型計(jì)算的HONO濃度, 發(fā)現(xiàn)白天時(shí)存在未知的大氣HONO來源。研究表明, 土壤可以向大氣中排放HONO。其機(jī)理可能是土壤亞硝態(tài)氮和氫離子的化學(xué)平衡作用; 或土壤夜間吸附和白天解吸附的動(dòng)態(tài)物理化學(xué)過程; 或氨氧化細(xì)菌等微生物的直接排放; 也可能是硝化過程中產(chǎn)生的羥胺, 在土壤顆粒物等表面的化學(xué)反應(yīng)。因此, 土壤HONO排放通量與土壤亞硝態(tài)氮濃度、pH、氨氧化細(xì)菌豐度、土壤礦物、土壤濕度及C/N值等相關(guān)。目前對(duì)于土壤HONO排放的研究尚在起步階段, 國(guó)內(nèi)亦少見相關(guān)成果報(bào)道。本文綜述了土壤HONO排放的研究背景、探討了土壤HONO排放的機(jī)理及影響因素, 以期為減少氮素?fù)p失、提高氮肥利用率、評(píng)估氮肥的環(huán)境效應(yīng)及城市空氣質(zhì)量等提供理論依據(jù)和科學(xué)指導(dǎo)。

氮循環(huán); 氣態(tài)亞硝酸; 土壤; pH; 硝化; 氨氧化細(xì)菌; 二氧化氮

HONO是亞硝酸在大氣中的氣態(tài)形式, 是城市污染的一種典型代表物。對(duì)于HONO的研究可以追溯到1943年, Jones[1]用紅外線光譜檢測(cè)到了氣態(tài)HONO的吸收峰。但是, 大氣中HONO濃度很低, 并且具有非常高的化學(xué)活性, 直到1974年才首次定量了其濃度[2]。一般來說, 晚上至凌晨時(shí), HONO的生成主要通過二氧化氮(NO2)在顆粒物表面的非均相化學(xué)反應(yīng)[3-5], 其濃度可持續(xù)累積并在日出前達(dá)到最大值。日出后, 在陽(yáng)光的照射下, HONO可快速光解為氫氧自由基(OH·)和一氧化氮(NO), 其大氣壽命約為20~30 min至2 h[6-7]。在空間分布上, 由于機(jī)動(dòng)車燃燒排放、家庭燃煤燃?xì)獾仍斐闪顺鞘蠬ONO的濃度可高達(dá)2×10-3~10×10-3mg?m-3[8]; 而郊區(qū)、農(nóng)村或其他偏遠(yuǎn)地方HONO濃度一般低于2×10-3mg?m-3[9]。研究表明, HONO是大氣OH·的主要來源, 其貢獻(xiàn)率最高可達(dá)80%[10-13]。而OH·是大氣化學(xué)研究的核心物質(zhì)和重要的氧化劑, 它參與了揮發(fā)性有機(jī)化合物(VOCs)、臭氧(O3)、一氧化碳(CO)和氮氧化物(NO)的循環(huán), 被稱為大氣中的“清潔劑”[14]。HONO也可能與胺反應(yīng)形成致癌物質(zhì)亞硝胺, 被人吸入后直接威脅到人類的健康[15]。因此, 定量大氣中HONO的源和匯, 對(duì)于大氣化學(xué)過程、臭氧層空洞、氣溶膠生成機(jī)制和人體健康等研究具有非常重要的意義。

1 大氣中HONO的源和匯

大氣HONO的主要來源是NO和OH·的光化學(xué)反應(yīng)(R1), 其反應(yīng)速率常數(shù)為8×10-12cm3molecule-1?s-1[16]。

NO+OH?HONO (R1)

激發(fā)態(tài)的NO2也可以與水蒸氣反應(yīng), 生成HONO, 其二級(jí)速率常數(shù)為1.7×10-13cm3molecule-1?s-1 [17-18]。其他的化學(xué)反應(yīng), 比如NO2與水蒸氣直接反應(yīng), 生成HONO和HNO3; NO、NO2和H2O反應(yīng), 生成2分子的HONO等。這些化學(xué)反應(yīng)的速率常數(shù)都遠(yuǎn)低于前兩者。對(duì)流層中HO(OH+HO2)與NO2的反應(yīng)(R2)被認(rèn)為也可以生成HONO[19]。但是也有研究表明, 此反應(yīng)的主要產(chǎn)物是HNO3而不是HONO, 其最大速率常數(shù)為5×10-16cm3molecule-1?s-1[20-21]。

HO2+NO2?HONO+O2(R2)

大氣HONO的匯主要是通過太陽(yáng)光將其光解為OH·和NO(R1的逆反應(yīng)), 或者H·和NO2, 又或HNO和O(3P)[22]。也有一部分HONO可以被OH·分解生成NO2; HONO與HNO3反應(yīng)生成NO2和H2O; 以及HONO的自分解反應(yīng)(R3)[23]。

2HONO→NO+NO2+H2O (R3)

但是這些化學(xué)反應(yīng)的速率很低, 一般認(rèn)為對(duì)HONO匯的貢獻(xiàn)不大。但是, 如果這些反應(yīng)在顆粒物或建筑物等表面進(jìn)行, 反應(yīng)速率會(huì)增加[24]。也有研究表明, 植物和陸地表面可以從大氣中吸收HONO[25-27]。

綜合考慮以上的HONO源匯, 通過模型計(jì)算HONO濃度, 其結(jié)果一般低于野外觀測(cè)到的HONO濃度, 二者的差值在白天最大, 存在未知的HONO源[28]。因此, 近20年來科學(xué)家提出了很多HONO來源的假設(shè), 主要以固體表面的非均相異質(zhì)反應(yīng)為主, 例如大氣氣溶膠、顆粒物等表面的化學(xué)反應(yīng)。其中, 最重要的一個(gè)反應(yīng)為NO2在各種表面的非均相水解反應(yīng)(R4)。研究表明, HONO的未知源濃度與NO2的光解頻率,(HONO), 有很好的相關(guān)性。

2NO2+H2O(ads)?HONO+HNO3(ads)(R4)

此反應(yīng)被認(rèn)為是夜晚HONO的主要來源, 與野外觀測(cè)到的HONO濃度相符合[29-32]。此外, 硝態(tài)氮或硝酸光解也可以產(chǎn)生HONO[33]。通過野外測(cè)定不同高度的HONO濃度表明, 地表可能是一個(gè)潛在的HONO源[34-36], 其中水分、氣溶膠以及NO2的濃度是主要影響因素[37]。目前對(duì)于大氣HONO的源匯平衡尚沒有一個(gè)統(tǒng)一的結(jié)論, 不斷有新的機(jī)制被提出和質(zhì)疑, 這個(gè)方向也是國(guó)際上的一個(gè)研究熱點(diǎn)。

2 土壤是大氣HONO的源還是匯?

早在1985年, 就有文獻(xiàn)發(fā)表了土壤可以排放亞硝酸的研究[38]。限于當(dāng)時(shí)的試驗(yàn)條件, 作者并沒有直接測(cè)定HONO的濃度, 而是通過堿液采樣收集的方法, 間接證明了土壤HONO的排放。并且, 利用15N同位素的方法證明了HONO的排放主要來自于土壤中的銨態(tài)氮[38]。但是, 這篇文章發(fā)表以后并沒有引起重視, 至今這篇文章的引用率也不超過10次。2011年, Su等[39]利用長(zhǎng)光程吸收光譜(LOPAP)第一次直接測(cè)到了土壤HONO的排放。作者比較了土壤的排放量與大氣中HONO未知源的濃度, 發(fā)現(xiàn)二者的值相當(dāng)。Wong等[40]通過模型計(jì)算和野外觀測(cè)數(shù)據(jù), 發(fā)現(xiàn)雖然有HONO的沉降, 但地表仍是HONO的凈排放源。VandenBoer等[41]的野外結(jié)果則證明了地表是HONO的匯。S?rgel等[42]發(fā)現(xiàn)森林地表是HONO的匯; 而移去凋落物后, 地表在晚上是HONO的匯, 白天則成了HONO的源。這一結(jié)果也與VandenBoer等[27]的另一研究結(jié)果相一致。Meusel等[43]比較了實(shí)驗(yàn)室測(cè)定和野外觀測(cè)的HONO排放, 前者可解釋75%的未知HONO源。Weber等[44]研究表明, 土壤表面的生物結(jié)皮可促進(jìn)HONO和NO的排放。雖然, 目前對(duì)土壤HONO的源匯尚無定論, 但一般認(rèn)為地表土壤可以向大氣中排放HONO[45-47]。

3 土壤HONO排放的機(jī)理

Su等[39]提出, 土壤HONO通過亞硝態(tài)氮(NO2-)和氫離子(H+)的化學(xué)平衡產(chǎn)生, 并以氣體的形式擴(kuò)散到大氣中。因此, 土壤NO2-濃度和pH是主導(dǎo)HONO排放的重要因素。美國(guó)科學(xué)家Donaldson等[48]的研究表明, 土壤顆粒表面的pH而不是土壤溶液pH, 主導(dǎo)了HONO的排放。土壤礦物表面, 例如鐵氧化物或鋁氧化物等, 可以吸附帶正電的離子, 形成M-OH2+, 它可以與溶液中的NO2-生成HONO。而亦有研究表明, 白天土壤HONO的排放來自于夜晚HONO的沉降, 土壤對(duì)HONO的排放是一個(gè)物理化學(xué)的吸附解吸的動(dòng)態(tài)過程[27]。土壤氨氧化細(xì)菌和表層生物結(jié)皮(biological soil crusts)也可以直接排放HONO[44,49], 土壤微生物過程對(duì)HONO排放的貢獻(xiàn)可能遠(yuǎn)大于通過化學(xué)平衡所產(chǎn)生的HONO[50-51]。Oswald等[49]的研究結(jié)果還表明, 土壤HONO的排放量與NO的排放量相當(dāng), 在某些土壤中甚至高于NO的排放。Scharko等[50]分析了土壤HONO排放與氨氧化菌的基因豐度相關(guān)關(guān)系, 發(fā)現(xiàn)氨氧化細(xì)菌(AOB)的基因豐度大于氨氧化古菌(AOA), 前者可能對(duì)HONO排放的貢獻(xiàn)更大; 而在酸性土壤中, AOA的貢獻(xiàn)可能會(huì)大于AOB。Ermel等[52]發(fā)現(xiàn), 硝化細(xì)菌在氧化銨的過程中會(huì)產(chǎn)生羥胺(NH2OH), 隨后在土壤顆粒表面發(fā)生化學(xué)反應(yīng), 生成HONO(R5)。

NH2OH+H2O+surface→HONO+unknown products (R5)

此反應(yīng)與土壤顆粒的表面積線性相關(guān), 并可以解釋低含水量時(shí)(<40%最大持水量)土壤HONO的排放。

4 土壤HONO排放的影響因素

pH是影響土壤HONO排放的重要因素。如果土壤HONO的排放是NO2-和H+的化學(xué)平衡產(chǎn)生, pH低的土壤HONO排放應(yīng)該更大。而Oswald等[49]的結(jié)果表明, 農(nóng)田和pH中性或堿性的土壤HONO排放高。Maljanen 等[53]發(fā)現(xiàn), 酸性森林土壤HONO的排放量低于農(nóng)田土壤的排放量。Scharko等[50]總結(jié)了土壤HONO排放通量與pH之間的關(guān)系, 發(fā)現(xiàn)隨著pH的升高, HONO的排放量增加。因此, 主導(dǎo)HONO排放的應(yīng)該是土壤顆粒表面的pH, 而非土壤總體的pH(bulk pH)[48]。

礦質(zhì)態(tài)氮是影響土壤HONO排放的另一個(gè)因素。硝化和反硝化等過程產(chǎn)生的NO2-是土壤HONO排放的一個(gè)前體物質(zhì), 它的濃度直接決定了HONO排放量的大小。但在好氧條件下, NO2-可以很快被氧化成NO3-, 不易在土壤中累積。Meusel等[43]發(fā)現(xiàn)土壤HONO和NO的排放與NO2-和NO3-的含量有很好的相關(guān)性; 而Weber等[44]的結(jié)果表明, 土壤生物結(jié)皮的HONO和NO排放與NO2-和NO3-的含量沒有直接的相關(guān)關(guān)系。一般來說, HONO排放通量與NH4+沒有顯著的相關(guān)關(guān)系[50]。但向土壤中添加NH4+會(huì)顯著增加HONO和NO的排放[51], 而硝化抑制劑則抑制其排放[50]?？梢? 土壤HONO和NO的排放主要是通過硝化過程產(chǎn)生的, 施用氮肥會(huì)顯著促進(jìn)土壤HONO和NO氣體的排放。

Oswald等[49]發(fā)現(xiàn), 土壤氨氧化細(xì)菌(AOB,)可以直接排放HONO。因此, 土壤微生物基因豐度、種群結(jié)構(gòu)及相關(guān)功能性基因的活性等都會(huì)顯著影響土壤HONO的排放。Scharko等[50]測(cè)定了土壤氨氧化古細(xì)菌(AOA)、AOB、亞硝酸鹽氧化細(xì)菌(NOB)的DNA和RNA豐度, 發(fā)現(xiàn)大部分土壤AOB和NOB的豐度高于AOA的豐度。土壤ATP值也可以直接反映微生物活性。Oswald等[49]的結(jié)果表明, 滅菌后土壤ATP值比非滅菌土壤顯著下降, HONO和NO的排放量也顯著下降。AOB的豐度直接影響到土壤硝化速率, 所以后者比NH4+含量能更好地預(yù)測(cè)土壤HONO的排放[50]。

土壤礦物一方面能夠吸附H+, 調(diào)節(jié)土壤顆粒表面的pH, 從而影響到HONO的排放[48]; 另一方面, 含鐵礦物能夠與NO2發(fā)生化學(xué)反應(yīng)生成HONO[54]。Kebede等[54]研究表明, 土壤pH<5時(shí), 土壤顆粒表面水膜中Fe2+能與NO2發(fā)生化學(xué)反應(yīng)生成HONO; pH為5~8, NO2與含鐵礦物發(fā)生化學(xué)反應(yīng), 并伴隨著NO2-和土壤表面Fe?OH2+的化學(xué)反應(yīng), 生成HONO。土壤表面的胡敏酸含量也會(huì)影響到NO2的轉(zhuǎn)化和HONO的生成[55]。

其他因素, 比如土壤濕度、氧氣含量、C/N值、光照等都會(huì)影響到HONO的排放。一般認(rèn)為, 土壤HONO的排放在低含水量(0~40%最大持水量)時(shí)產(chǎn)生[49-51]。在此濕度條件下, 土壤氧氣含量較高, 有利于硝化作用的進(jìn)行, 能夠產(chǎn)生大量的HONO[50-51]。土壤HONO排放隨著C/N值增加而下降, 當(dāng)C/N值>25時(shí), HONO排放量顯著的下降[53]。而光照能夠促進(jìn)土壤顆粒表面的光化學(xué)反應(yīng), 提高HONO的生成率[55]。

5 結(jié)語(yǔ)

作為土壤氮素?fù)p失的一個(gè)重要途徑, 目前對(duì)HONO排放的研究尚在起步階段, 包括土壤排放HONO的機(jī)理和影響因素、以及評(píng)估土壤HONO排放的大氣環(huán)境影響都不清楚。土壤HONO的排放受施用氮肥的影響很大, 其增加了HONO排放, 又通過大氣化學(xué)反應(yīng)影響到空氣質(zhì)量、臭氧層空洞、氣候變化和人體健康等。因此, 迫切需要對(duì)土壤HONO排放機(jī)理和影響因素進(jìn)行研究, 尤其是對(duì)農(nóng)田和城市土壤, 二者直接關(guān)系到糧食安全和城市大氣環(huán)境。

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吳電明, 夏玉玲, 侯立軍, 劉敏. 土壤亞硝酸氣體(HONO)排放過程及其驅(qū)動(dòng)機(jī)制[J]. 中國(guó)生態(tài)農(nóng)業(yè)學(xué)報(bào), 2018, 26(2): 190-194

WU D M, XIA Y L, HOU L J, LIU M. The mechanisms of HONO emissions from soil: A review[J]. Chinese Journal of Eco-Agriculture, 2018, 26(2): 190-194

The mechanisms of HONO emissions from soil: A review*

WU Dianming1, XIA Yuling1, HOU Lijun2, LIU Min1

(1. School of Geographical Sciences, East China Normal University / Key Laboratory of Geographic Information Sciences, Ministry of Education, Shanghai 200241, China; 2. State Key Laboratory of Estuarine and Coastal Research, East China Normal University, Shanghai 200062, China)

Nitrous acid (HONO) significantly contributes to atmospheric hydroxyl radical (OH·) and also influences atmospheric oxidation capacity and air quality. Comparison of HONO concentrations measured in a field campaign and by modeling showed a large unknown HONO source during daytime. Studies have shown that the unknown HONO source can be attributed to soil emissions, a major source of atmospheric HONO. The mechanisms may be taking the form of chemical equilibrium between soil nitrite and H+, reactive uptake and displacement by soil, emissions by ammonia-oxidizing bacteria (AOB) and other micro-organisms, or surface reaction between hydroxylamine and H2O. Therefore, HONO flux from soils is controlled by soil nitrite concentration, pH, AOB abundance, soil minerals, soil moisture and C/N ratio. The mechanism of HONO emissions from soil has remained a point of hot discussion and few results have been reported from China. Here, we introduced the background of HONO emissions from soil, reviewed studies on the mechanisms of HONO emissions from soil and the related driving factors. This review was a relevant support for research on reducing nitrogen loss, enhancing nitrogen use efficiency, and evaluating the effects of nitrogen fertilization on environmental and urban air quality.

Nitrogen cycle; HONO; Soil; pH; Nitrification; Ammonia-oxidizing bacteria (AOB); Nitrogen dioxide

, WU Dianming, E-mail: dmwu@geo.ecnu.edu.cn

Nov. 17, 2017;

Dec. 4, 2017

10.13930/j.cnki.cjea.171061

X511; S154.1

A

1671-3990(2018)02-0190-05

2017-11-17

2017-12-04

* This study was supported by the Fundamental Research Funds for the Central Universities of China and the National Natural Science Foundation of China (41730646, 41371451).

* 中央高校基本科研業(yè)務(wù)費(fèi)專項(xiàng)資金、國(guó)家自然科學(xué)基金重點(diǎn)項(xiàng)目(41730646)和面上項(xiàng)目(41371451)資助

吳電明, 從事土壤氮循環(huán)與全球變化研究。E-mail: dmwu@geo.ecnu.edu.cn