土壤亚硝酸气体(HONO)排放过程及其驱动机制*

吴电明, 夏玉玲, 侯立军, 刘 敏



土壤亚硝酸气体(HONO)排放过程及其驱动机制*

吴电明1, 夏玉玲1, 侯立军2, 刘 敏1

(1. 华东师范大学地理科学学院/教育部地理信息科学重点实验室 上海 200241; 2. 华东师范大学河口海岸学国家重点实验室 上海 200062)

气态亚硝酸(HONO)是大气中氢氧自由基(OH·)的重要来源, 直接影响到大气氧化能力和空气质量。通过比较外场测定和模型计算的HONO浓度, 发现白天时存在未知的大气HONO来源。研究表明, 土壤可以向大气中排放HONO。其机理可能是土壤亚硝态氮和氢离子的化学平衡作用; 或土壤夜间吸附和白天解吸附的动态物理化学过程; 或氨氧化细菌等微生物的直接排放; 也可能是硝化过程中产生的羟胺, 在土壤颗粒物等表面的化学反应。因此, 土壤HONO排放通量与土壤亚硝态氮浓度、pH、氨氧化细菌丰度、土壤矿物、土壤湿度及C/N值等相关。目前对于土壤HONO排放的研究尚在起步阶段, 国内亦少见相关成果报道。本文综述了土壤HONO排放的研究背景、探讨了土壤HONO排放的机理及影响因素, 以期为减少氮素损失、提高氮肥利用率、评估氮肥的环境效应及城市空气质量等提供理论依据和科学指导。

氮循环; 气态亚硝酸; 土壤; pH; 硝化; 氨氧化细菌; 二氧化氮

HONO是亚硝酸在大气中的气态形式, 是城市污染的一种典型代表物。对于HONO的研究可以追溯到1943年, Jones[1]用红外线光谱检测到了气态HONO的吸收峰。但是, 大气中HONO浓度很低, 并且具有非常高的化学活性, 直到1974年才首次定量了其浓度[2]。一般来说, 晚上至凌晨时, HONO的生成主要通过二氧化氮(NO2)在颗粒物表面的非均相化学反应[3-5], 其浓度可持续累积并在日出前达到最大值。日出后, 在阳光的照射下, HONO可快速光解为氢氧自由基(OH·)和一氧化氮(NO), 其大气寿命约为20~30 min至2 h[6-7]。在空间分布上, 由于机动车燃烧排放、家庭燃煤燃气等造成了城市HONO的浓度可高达2×10-3~10×10-3mg∙m-3[8]; 而郊区、农村或其他偏远地方HONO浓度一般低于2×10-3mg∙m-3[9]。研究表明, HONO是大气OH·的主要来源, 其贡献率最高可达80%[10-13]。而OH·是大气化学研究的核心物质和重要的氧化剂, 它参与了挥发性有机化合物(VOCs)、臭氧(O3)、一氧化碳(CO)和氮氧化物(NO)的循环, 被称为大气中的“清洁剂”[14]。HONO也可能与胺反应形成致癌物质亚硝胺, 被人吸入后直接威胁到人类的健康[15]。因此, 定量大气中HONO的源和汇, 对于大气化学过程、臭氧层空洞、气溶胶生成机制和人体健康等研究具有非常重要的意义。

1 大气中HONO的源和汇

大气HONO的主要来源是NO和OH·的光化学反应(R1), 其反应速率常数为8×10-12cm3molecule-1∙s-1[16]。

NO+OH↔HONO (R1)

激发态的NO2也可以与水蒸气反应, 生成HONO, 其二级速率常数为1.7×10-13cm3molecule-1∙s-1 [17-18]。其他的化学反应, 比如NO2与水蒸气直接反应, 生成HONO和HNO3; NO、NO2和H2O反应, 生成2分子的HONO等。这些化学反应的速率常数都远低于前两者。对流层中HO(OH+HO2)与NO2的反应(R2)被认为也可以生成HONO[19]。但是也有研究表明, 此反应的主要产物是HNO3而不是HONO, 其最大速率常数为5×10-16cm3molecule-1∙s-1[20-21]。

HO2+NO2↔HONO+O2(R2)

大气HONO的汇主要是通过太阳光将其光解为OH·和NO(R1的逆反应), 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与HNO3反应生成NO2和H2O; 以及HONO的自分解反应(R3)[23]。

2HONO→NO+NO2+H2O (R3)

但是这些化学反应的速率很低, 一般认为对HONO汇的贡献不大。但是, 如果这些反应在颗粒物或建筑物等表面进行, 反应速率会增加[24]。也有研究表明, 植物和陆地表面可以从大气中吸收HONO[25-27]。

综合考虑以上的HONO源汇, 通过模型计算HONO浓度, 其结果一般低于野外观测到的HONO浓度, 二者的差值在白天最大, 存在未知的HONO源[28]。因此, 近20年来科学家提出了很多HONO来源的假设, 主要以固体表面的非均相异质反应为主, 例如大气气溶胶、颗粒物等表面的化学反应。其中, 最重要的一个反应为NO2在各种表面的非均相水解反应(R4)。研究表明, HONO的未知源浓度与NO2的光解频率,(HONO), 有很好的相关性。

2NO2+H2O(ads)↔HONO+HNO3(ads)(R4)

此反应被认为是夜晚HONO的主要来源, 与野外观测到的HONO浓度相符合[29-32]。此外, 硝态氮或硝酸光解也可以产生HONO[33]。通过野外测定不同高度的HONO浓度表明, 地表可能是一个潜在的HONO源[34-36], 其中水分、气溶胶以及NO2的浓度是主要影响因素[37]。目前对于大气HONO的源汇平衡尚没有一个统一的结论, 不断有新的机制被提出和质疑, 这个方向也是国际上的一个研究热点。

2 土壤是大气HONO的源还是汇?

早在1985年, 就有文献发表了土壤可以排放亚硝酸的研究[38]。限于当时的试验条件, 作者并没有直接测定HONO的浓度, 而是通过碱液采样收集的方法, 间接证明了土壤HONO的排放。并且, 利用15N同位素的方法证明了HONO的排放主要来自于土壤中的铵态氮[38]。但是, 这篇文章发表以后并没有引起重视, 至今这篇文章的引用率也不超过10次。2011年, Su等[39]利用长光程吸收光谱(LOPAP)第一次直接测到了土壤HONO的排放。作者比较了土壤的排放量与大气中HONO未知源的浓度, 发现二者的值相当。Wong等[40]通过模型计算和野外观测数据, 发现虽然有HONO的沉降, 但地表仍是HONO的净排放源。VandenBoer等[41]的野外结果则证明了地表是HONO的汇。Sörgel等[42]发现森林地表是HONO的汇; 而移去凋落物后, 地表在晚上是HONO的汇, 白天则成了HONO的源。这一结果也与VandenBoer等[27]的另一研究结果相一致。Meusel等[43]比较了实验室测定和野外观测的HONO排放, 前者可解释75%的未知HONO源。Weber等[44]研究表明, 土壤表面的生物结皮可促进HONO和NO的排放。虽然, 目前对土壤HONO的源汇尚无定论, 但一般认为地表土壤可以向大气中排放HONO[45-47]。

3 土壤HONO排放的机理

Su等[39]提出, 土壤HONO通过亚硝态氮(NO2-)和氢离子(H+)的化学平衡产生, 并以气体的形式扩散到大气中。因此, 土壤NO2-浓度和pH是主导HONO排放的重要因素。美国科学家Donaldson等[48]的研究表明, 土壤颗粒表面的pH而不是土壤溶液pH, 主导了HONO的排放。土壤矿物表面, 例如铁氧化物或铝氧化物等, 可以吸附带正电的离子, 形成M-OH2+, 它可以与溶液中的NO2-生成HONO。而亦有研究表明, 白天土壤HONO的排放来自于夜晚HONO的沉降, 土壤对HONO的排放是一个物理化学的吸附解吸的动态过程[27]。土壤氨氧化细菌和表层生物结皮(biological soil crusts)也可以直接排放HONO[44,49], 土壤微生物过程对HONO排放的贡献可能远大于通过化学平衡所产生的HONO[50-51]。Oswald等[49]的研究结果还表明, 土壤HONO的排放量与NO的排放量相当, 在某些土壤中甚至高于NO的排放。Scharko等[50]分析了土壤HONO排放与氨氧化菌的基因丰度相关关系, 发现氨氧化细菌(AOB)的基因丰度大于氨氧化古菌(AOA), 前者可能对HONO排放的贡献更大; 而在酸性土壤中, AOA的贡献可能会大于AOB。Ermel等[52]发现, 硝化细菌在氧化铵的过程中会产生羟胺(NH2OH), 随后在土壤颗粒表面发生化学反应, 生成HONO(R5)。

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

此反应与土壤颗粒的表面积线性相关, 并可以解释低含水量时(<40%最大持水量)土壤HONO的排放。

4 土壤HONO排放的影响因素

pH是影响土壤HONO排放的重要因素。如果土壤HONO的排放是NO2-和H+的化学平衡产生, pH低的土壤HONO排放应该更大。而Oswald等[49]的结果表明, 农田和pH中性或碱性的土壤HONO排放高。Maljanen 等[53]发现, 酸性森林土壤HONO的排放量低于农田土壤的排放量。Scharko等[50]总结了土壤HONO排放通量与pH之间的关系, 发现随着pH的升高, HONO的排放量增加。因此, 主导HONO排放的应该是土壤颗粒表面的pH, 而非土壤总体的pH(bulk pH)[48]。

矿质态氮是影响土壤HONO排放的另一个因素。硝化和反硝化等过程产生的NO2-是土壤HONO排放的一个前体物质, 它的浓度直接决定了HONO排放量的大小。但在好氧条件下, NO2-可以很快被氧化成NO3-, 不易在土壤中累积。Meusel等[43]发现土壤HONO和NO的排放与NO2-和NO3-的含量有很好的相关性; 而Weber等[44]的结果表明, 土壤生物结皮的HONO和NO排放与NO2-和NO3-的含量没有直接的相关关系。一般来说, HONO排放通量与NH4+没有显著的相关关系[50]。但向土壤中添加NH4+会显著增加HONO和NO的排放[51], 而硝化抑制剂则抑制其排放[50]。可见, 土壤HONO和NO的排放主要是通过硝化过程产生的, 施用氮肥会显著促进土壤HONO和NO气体的排放。

Oswald等[49]发现, 土壤氨氧化细菌(AOB,)可以直接排放HONO。因此, 土壤微生物基因丰度、种群结构及相关功能性基因的活性等都会显著影响土壤HONO的排放。Scharko等[50]测定了土壤氨氧化古细菌(AOA)、AOB、亚硝酸盐氧化细菌(NOB)的DNA和RNA丰度, 发现大部分土壤AOB和NOB的丰度高于AOA的丰度。土壤ATP值也可以直接反映微生物活性。Oswald等[49]的结果表明, 灭菌后土壤ATP值比非灭菌土壤显著下降, HONO和NO的排放量也显著下降。AOB的丰度直接影响到土壤硝化速率, 所以后者比NH4+含量能更好地预测土壤HONO的排放[50]。

土壤矿物一方面能够吸附H+, 调节土壤颗粒表面的pH, 从而影响到HONO的排放[48]; 另一方面, 含铁矿物能够与NO2发生化学反应生成HONO[54]。Kebede等[54]研究表明, 土壤pH<5时, 土壤颗粒表面水膜中Fe2+能与NO2发生化学反应生成HONO; pH为5~8, NO2与含铁矿物发生化学反应, 并伴随着NO2-和土壤表面Fe−OH2+的化学反应, 生成HONO。土壤表面的胡敏酸含量也会影响到NO2的转化和HONO的生成[55]。

其他因素, 比如土壤湿度、氧气含量、C/N值、光照等都会影响到HONO的排放。一般认为, 土壤HONO的排放在低含水量(0~40%最大持水量)时产生[49-51]。在此湿度条件下, 土壤氧气含量较高, 有利于硝化作用的进行, 能够产生大量的HONO[50-51]。土壤HONO排放随着C/N值增加而下降, 当C/N值>25时, HONO排放量显著的下降[53]。而光照能够促进土壤颗粒表面的光化学反应, 提高HONO的生成率[55]。

5 结语

作为土壤氮素损失的一个重要途径, 目前对HONO排放的研究尚在起步阶段, 包括土壤排放HONO的机理和影响因素、以及评估土壤HONO排放的大气环境影响都不清楚。土壤HONO的排放受施用氮肥的影响很大, 其增加了HONO排放, 又通过大气化学反应影响到空气质量、臭氧层空洞、气候变化和人体健康等。因此, 迫切需要对土壤HONO排放机理和影响因素进行研究, 尤其是对农田和城市土壤, 二者直接关系到粮食安全和城市大气环境。

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吴电明, 夏玉玲, 侯立军, 刘敏. 土壤亚硝酸气体(HONO)排放过程及其驱动机制[J]. 中国生态农业学报, 2018, 26(2): 190-194

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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).

* 中央高校基本科研业务费专项资金、国家自然科学基金重点项目(41730646)和面上项目(41371451)资助

吴电明, 从事土壤氮循环与全球变化研究。E-mail: dmwu@geo.ecnu.edu.cn