Laboratory measurements of HDO/H2O isotopic fractionation during ice deposition in simulated cirrus clouds
Laboratory measurements of HDO/H2O isotopic fractionation during ice deposition in simulated cirrus clouds
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DOI：
10.1073/pnas.1618374114
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发表时间：
2017-05-30
影响因子：
11.1
通讯作者：
Moyer, Elisabeth J.
中科院分区：
文献类型：
--
作者：
Lamb, Kara D.;Clouser, Benjamin W.;Moyer, Elisabeth J.
水的稳定同位素已在大气和气候研究中使用了50多年，因为它们强烈的温度依赖性优先凝结使它们成为水文循环的有用诊断。然而，在233 K（-40摄氏度）以下的温度下，水蒸气和冰之间的优先凝结程度从未被直接测量过，而233 K（-40摄氏度）以下的温度是在地球大气中形成卷云的必要条件，在极地地区经常观察到，在火星近地表的大气层中也是典型的。模式一般采用Merlivat和Nief的较暖实验的外推[Merlivat L， Nief G (1967) Tellus 19:122-127]。应该改变优先分配的非平衡动力学效应也没有得到很好的实验表征。我们在此介绍了在卷云相关温度下蒸汽和冰之间的HDO/H2O平衡分馏（α (eq)）的直接测量，使用在云室中卷云形成实验中水蒸气同位素组成演变的原位光谱测量。我们排除了最近提出的向上修正的alpha(eq)，并发现值略低于Merlivat和Nief。这些实验还使我们能够对冰-蒸汽系统中过饱和条件下预期发生的动力学修饰进行定量验证。在扩散限制实验的子集中，我们表明动力学同位素效应确实与已发表的模型一致，包括允许小的表面效应。这些结果是根据水同位素测量推断地球和其他行星上的过程的基础。他们还证明了动态原位实验在研究地球化学系统分馏中的效用。
The stable isotopologues of water have been used in atmospheric and climate studies for over 50 years, because their strong temperature-dependent preferential condensation makes them useful diagnostics of the hydrological cycle. However, the degree of preferential condensation between vapor and ice has never been directly measured at temperatures below 233 K (-40 degrees C), conditions necessary to form cirrus clouds in the Earth's atmosphere, routinely observed in polar regions, and typical for the near-surface atmospheric layers of Mars. Models generally assume an extrapolation from the warmer experiments of Merlivat and Nief [Merlivat L, Nief G (1967) Tellus 19:122-127]. Nonequilibrium kinetic effects that should alter preferential partitioning have also not been well characterized experimentally. We present here direct measurements of HDO/H2O equilibrium fractionation between vapor and ice (alpha(eq)) at cirrus-relevant temperatures, using in situ spectroscopic measurements of the evolving isotopic composition of water vapor during cirrus formation experiments in a cloud chamber. We rule out the recent proposed upward modification of alpha(eq), and find values slightly lower than Merlivat and Nief. These experiments also allow us to make a quantitative validation of the kinetic modification expected to occur in supersaturated conditions in the ice-vapor system. In a subset of diffusion-limited experiments, we show that kinetic isotope effects are indeed consistent with published models, including allowing for small surface effects. These results are fundamental for inferring processes on Earth and other planets from water isotopic measurements. They also demonstrate the utility of dynamic in situ experiments for studying fractionation in geochemical systems.