arXiv 1802.084582018-02-23astro-ph.EP

由远紫外光引起的不均匀有机化学的实验研究：土卫六和早期地球大气中加入CH3对有机气溶胶增长的影响

Experimental study of heterogeneous organic chemistry induced by far ultraviolet light: Implications for growth of organic aerosols by CH3 addition in the atmospheres of Titan and early Earth

Peng K. Hong, Yasuhito Sekine, Tsutoni Sasamori, Seiji Sugita

备注：: 55 pages, 11 figures, 1 table
发表信息：: Icarus Volume 307, 2018, Pages 25-39
DOI：: 10.1016/j.icarus.2018.02.019

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摘要：

在土卫六和早期地球等含甲烷的大气中，已经观察到并提出了由光化学反应驱动的有机气溶胶的形成。然而，由太阳远紫外（FUV）光驱动的有机气溶胶的详细生产和增长机制仍然缺乏约束。我们进行了实验室实验，模拟在CH 4-CO2大气中的光化学反应的FUV辐射为主的莱曼-α线驱动。在实验中，我们分析了在反应池中的光学窗口上形成的固体有机膜的厚度和红外光谱的时间变化。FUV辐射形成的气体物种进行了分析，并与光化学模型计算进行了比较。实验结果表明，有机薄膜的生长速率随反应气体中CH_4/CO_2比值的减小而减小，当CH_4/CO_2 < 1时，这种减小变得非常陡峭。与光化学模型计算的比较表明，气相烃，如多炔和芳烃的聚合，不能占有机膜的生长速率，但CH 3自由基的加成反应到有机膜上的反应概率约为10-2可以解释的生长速率。当CH_4/CO_2 < 1时，CO_2光解生成的O原子对有机膜的刻蚀作用减弱或抑制了有机膜的生长。我们的研究结果表明，有机气溶胶将增长通过CH 3添加到表面上的气溶胶粒子在土卫六和早期地球的中层大气中的沉淀过程。在土卫六上，有效的甲烷添加将减少大气中C2 H6的产生。在早期的地球上，气溶胶粒子的增长效率低于土卫六，可能导致小尺寸单体和影响紫外线屏蔽。

英文摘要：

Formation of organic aerosols driven by photochemical reactions has been observed and suggested in CH4-containing atmospheres, including Titan and early Earth. However the detailed production and growth mechanisms of organic aerosols driven by solar far ultraviolet (FUV) light remain poorly constrained. We conducted laboratory experiments simulating photochemical reactions in a CH4-CO2 atmosphere driven by the FUV radiations dominated by the Lyman-α line. In the experiments, we analyzed time variations in thickness and infrared spectra of solid organic film formed on an optical window in a reaction cell. Gas species formed by FUV irradiation were also analyzed and compared with photochemical model calculations. Our experimental results show that the growth rate of the organic film decreases as the CH4/CO2 ratio of reactant gas mixture decreases, and that the decrease becomes very steep for CH4/CO2 < 1. Comparison with photochemical model calculations suggests that polymerizations of gas-phase hydrocarbons, such as polyynes and aromatics, cannot account for the growth rate of the organic film but that the addition reaction of CH3 radicals onto the organic film with the reaction probability around 10-2 can explain the growth rate. At CH4/CO2 < 1, etching by O atom formed by CO2 photolysis would reduce or inhibit the growth of the organic film. Our results suggest that organic aerosols would grow through CH3 addition onto the surface during the precipitation of aerosol particles in the middle atmosphere of Titan and early Earth. On Titan, effective CH3 addition would reduce C2H6 production in the atmosphere. On early Earth, growth of aerosol particles would be less efficient than those on Titan, possibly resulting in small-sized monomers and influencing UV shielding.

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