2. 华东师范大学河口海岸学国家重点实验室 上海 200062
2. State Key Laboratory of Estuarine and Coastal Research, East China Normal University, Shanghai 200062, China
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2×10-3~10×10-3 mg·m-3[8]; 而郊区、农村或其他偏远地方HONO浓度一般低于2×10-3 mg·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(NOx)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8×10-12 cm3 molecule-1·s-1[16]。
$ {\rm{NO}} + {\rm{OH}} \leftrightarrow {\rm{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1.7×10-13 cm3 molecule-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HOx(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5×10-16 cm3 molecule-1·s-1[20-21]。
$ {\rm{H}}{{\rm{O}}_2} + {\rm{N}}{{\rm{O}}_2} \leftrightarrow {\rm{HONO}} + {{\rm{O}}_2} $ | (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]。
$ 2{\rm{HONO}} \to {\rm{NO}} + {\rm{N}}{{\rm{O}}_2} + {{\rm{H}}_2}{\rm{O}} $ | (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, j(HONO), 有很好的相关性。
$ {\rm{2N}}{{\rm{O}}_2} + {{\rm{H}}_2}{{\rm{O}}_{({\rm{ads}})}} \leftrightarrow {\rm{HONO}} + {\rm{HN}}{{\rm{O}}_{3({\rm{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)。
$ {\rm{N}}{{\rm{H}}_2}{\rm{OH}} + {{\rm{H}}_2}{\rm{O}} + {\rm{surface}} \to {\rm{HONO}} + {\rm{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, Nitrosomonas europaea)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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