Experimental study of heterogeneous organic chemistry induced by far ultraviolet light: Implications for growth of organic aerosols by CH 3 addition in the atmospheres of Titan and early Earth
Experimental study of heterogeneous organic chemistry induced by far ultraviolet light: Implications for growth of organic aerosols by CH 3 addition in the atmospheres of Titan and early Earth
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远紫外光诱导的非均相有机化学实验研究：土卫六和早期地球大气中添加 CH 3 对有机气溶胶生长的影响
DOI：
10.1016/j.icarus.2018.02.019
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发表时间：
2018
期刊：
影响因子：
3.2
通讯作者：
Sugita Seiji
中科院分区：
文献类型：
--
作者：
Hong Peng;Sekine Yasuhito;Sasamori Tsutoni;Sugita Seiji
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。我们进行了实验室模拟实验，在CH 4单键CO2气氛下，由莱曼-α线主导的FUV辐射驱动的光化学反应。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/CO2比的减小而减小，当CH 4/CO2< 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，多炔和芳烃等气相碳氢化合物的聚合不能解释有机膜的生长速率，但CH 3自由基对有机膜的加成反应（反应概率约为10 - 2）可以解释生长速率。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会减弱或抑制有机膜的生长。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间大气中的沉淀过程。在土卫六上，有效的CH 3添加会减少大气中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。
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 CH4single bondCO2atmosphere 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/CO2ratio 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 CH3radicals onto the organic film with the reaction probability around 10−2can explain the growth rate. At CH4/CO2< 1, etching by O atom formed by CO2photolysis would reduce or inhibit the growth of the organic film. Our results suggest that organic aerosols would grow through CH3addition onto the surface during the precipitation of aerosol particles in the middle atmosphere of Titan and early Earth. On Titan, effective CH3addition would reduce C2H6production 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.