Oxidation of Carboxylic Acids Regenerates Hydroxyl Radicals in the Unpolluted and Nighttime Troposphere
Oxidation of Carboxylic Acids Regenerates Hydroxyl Radicals in the Unpolluted and Nighttime Troposphere
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DOI：
10.1021/jp101279p
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发表时间：
2010-07-01
影响因子：
2.9
通讯作者：
da Silva, Gabriel
中科院分区：
文献类型：
--
作者：
da Silva, Gabriel
羟基自由基（OH）控制着对流层中有机化合物的去除。大气化学模型显著低估了未污染环境中的OH水平，这意味着它们通过一些未知的机制再生。这项工作使用计算化学证明，烷基羧酸的光化学氧化可以有效地再生羟基自由基通过单分子分解的α-羧基烷基过氧自由基。对于乙酸和丙酸，预测所提出的机制在未污染的对流层下部占主导地位，并且在对流层中上部也可能在一定程度上起作用。烷基羧酸也被预测为整个行星边界层夜间OH的新来源，其中OH水平也被低估。这一类的反应的热力学要求进行了讨论，并确定了一些候选人OH-重整分子特别是相关的芳香族光氧化。采用更广泛的角度来看，α-羧烷基自由基的前体，与O-2反应，形成不稳定的α-羧烷基过氧型自由基，预计也将形成燃烧过程中，在星际介质中，从γ射线照射的甘氨酸和相关的氨基酸，并讨论了这种新的化学在这些环境中的潜在重要性。主方程模拟表明，α-羧烷基+ O-2反应在生物柴油和其他含氧生物燃料的自燃和燃烧过程中提供了迅速的OH源，其中羧酸作为早期氧化产物形成。烯酮燃烧也被认为是通过这些OH-重整α-羧基烷基自由基进行的。在体内形成α-羧烷基过氧自由基，然后氧化为高反应性OH自由基，可能在人体内诱导氧化应激，这是由γ射线引发的过程。最后，烯酮与OH反应形成α-羧基烷基自由基，然后加入NH 2或相关物质，被认为是一种新的外星途径氨基酸。
The hydroxyl radical (OH) controls the removal of organic compounds from the troposphere. Atmospheric chemistry models significantly under-predict OH levels in unpolluted environments, implying that they are regenerated via some unknown mechanism(s). This work uses computational chemistry to demonstrate that the photochemical oxidation of alkyl carboxylic acids can efficiently regenerate the hydroxyl radical via unimolecular decomposition of alpha-carboxyalkylperoxy radicals. For acetic acid and propanoic acid the proposed mechanism is predicted to dominate in the unpolluted lower troposphere, and it may also operate to some extent in the mid to upper troposphere. Alkyl carboxylic acids are also predicted to act as a new source of nighttime OH throughout the planetary boundary layer, where OH levels are also under-predicted. The thermodynamic requirements for reactions of this class are discussed, and some candidate OH-reforming molecules particularly relevant to aromatic photooxidation are identified. Adopting a broader perspective, the alpha-carboxyalkyl radical precursors that react with O-2 to form the unstable alpha-carboxyalkylperoxy type radicals are also expected to form during combustion, in the interstellar medium, and from the gamma-irradiation of glycine and related amino acids, and the potential importance of this new chemistry in these environments is discussed. Master equation simulations suggest that alpha-carboxyalkyl + O-2 reactions provide a prompt OH source during the autoignition and combustion of biodiesel and other oxygenated biofuels, where carboxylic acids are formed as early stage oxidation products. Ketene combustion is also thought to proceed via these OH-reforming alpha-carboxyalkyl radicals. The in vivo formation of alpha-carboxyalkylperoxy radicals followed by oxidation to the highly reactive OH radical may induce oxidative stress in the human body, in a process initiated by gamma-rays. Finally, the reaction of ketenes with OH to form alpha-carboxyalkyl radicals, followed by addition of NH2 or related species, is suggested as a new extraterrestrial pathway to amino acids.