Formation of HONO from the NH3-promoted hydrolysis of NO2 dimers in the atmosphere
Formation of HONO from the NH3-promoted hydrolysis of NO2 dimers in the atmosphere
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DOI：
10.1073/pnas.1807719115
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发表时间：
2018-06
期刊：
影响因子：
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通讯作者：
Lei Li;Z. Duan;Hui Li;Chongqin Zhu;G. Henkelman;J. S. Francisco;X. Zeng
中科院分区：
文献类型：
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作者：
Lei Li;Z. Duan;Hui Li;Chongqin Zhu;G. Henkelman;J. S. Francisco;X. Zeng
作为“清洁剂”OH自由基的主要来源，亚硝酸（HONO）在大气化学中起着至关重要的作用。尽管进行了广泛的研究，但HONO的来源仍然难以捉摸。虽然最近的研究表明，在气溶胶形成过程中的活性氮化合物的重要性，这些化合物如何反应的机理洞察仍然缺失。在此，基于Born-Oppenheimer分子动力学模拟和自由能采样，我们确定了HONO的形成机制，通过NH3促进水解NO2二聚体（ONONO 2）的水簇/液滴。HONO在水-气界面的近自发形成揭示了水滴在大气化学中的催化作用。这一发现不仅提供了一个缺失的HONO来源，而且还提供了对HONO化学的深入了解。大气化学中的一个具有挑战性的问题是确定亚硝酸（HONO）的来源，它被认为是大气中“清洁剂”OH自由基的主要来源。在这里，我们显示了一个反应路线HONO物种的形成从NH3促进的NO2二聚体（ONONO 2），这需要在室温下的0.5千卡/摩尔的低自由能势垒的水解。本文采用代谢动力学模拟方法和反应路径搜索方法，对NH_3 + ONO_2 + nH_2O体系和水滴体系中HONO的生成进行了系统的研究，揭示了两种不同的机理：（1）在一水合物（n = 1）、四水合物（n = 4）和水滴体系中，只有一个水分子直接参与反应（ii）在二水合物（n = 2）和三水合物（n = 3）中观察到两个相邻水分子的分裂（表示为双水机制）。计算的自由能表面为NH3-自由和NH3-含系统的比较表明，气态NH3可以显着降低HONO形成的自由能垒，同时稳定的产品状态，产生更多的放能反应，相反的吸能反应的NH3-自由系统。更重要的是，水滴将HONO形成的自由能垒降低到0.5 kcal/mol，这在室温下可以忽略不计。我们表明，熵的贡献是重要的NH3促进HONO形成的机制。这项研究提供了深入了解基本HONO化学的重要性及其更广泛的影响气溶胶和云处理化学在空气-水界面。
Significance As the primary source of “detergent” OH radicals, nitrous acid (HONO) plays an essential role in the chemistry of the atmosphere. Despite extensive studies, the source of HONO is still elusive. Although recent studies have shown the importance of reactive nitrogen compounds during aerosol formation, mechanistic insight into how these compounds react is still missing. Herein, based on Born–Oppenheimer molecular-dynamics simulations and free-energy sampling, we identified a formation mechanism for HONO via the NH3-promoted hydrolysis of NO2 dimer (ONONO2) on water clusters/droplets. The near-spontaneous formation of HONO at the water–air interface sheds light on the catalytic role of water droplets in atmospheric chemistry. This finding provides not only a missing HONO source but also insight into HONO chemistry. One challenging issue in atmospheric chemistry is identifying the source of nitrous acid (HONO), which is believed to be a primary source of atmospheric “detergent” OH radicals. Herein, we show a reaction route for the formation of HONO species from the NH3-promoted hydrolysis of a NO2 dimer (ONONO2), which entails a low free-energy barrier of 0.5 kcal/mol at room temperature. Our systematic study of HONO formation based on NH3 + ONONO2 + nH2O and water droplet systems with the metadynamics simulation method and a reaction pathway searching method reveals two distinct mechanisms: (i) In monohydrates (n = 1), tetrahydrates (n = 4), and water droplets, only one water molecule is directly involved in the reaction (denoted the single-water mechanism); and (ii) the splitting of two neighboring water molecules is seen in the dihydrates (n = 2) and trihydrates (n = 3) (denoted the dual-water mechanism). A comparison of the computed free-energy surface for NH3-free and NH3-containing systems indicates that gaseous NH3 can markedly lower the free-energy barrier to HONO formation while stabilizing the product state, producing a more exergonic reaction, in contrast to the endergonic reaction for the NH3-free system. More importantly, the water droplet reduces the free-energy barrier for HONO formation to 0.5 kcal/mol, which is negligible at room temperature. We show that the entropic contribution is important in the mechanism by which NH3 promotes HONO formation. This study provides insight into the importance of fundamental HONO chemistry and its broader implication to aerosol and cloud processing chemistry at the air–water interface.