自然界中偶数汞同位素非质量分馏的来源和机理研究

大气是汞在大尺度上迁移转化循环的重要媒介。相较于其他金属和非金属元素，汞在对流层中主要以气态形式存在。汞除了独特的半挥发性外，其丰富的氧化-还原性强烈影响了在大气中的传输特性和沉积速率。由于在大气中的浓度很低，汞的大气循环相当复杂的，然而，随着分析技术的发展和改进，高精度汞稳定同位素组成的测定成为现实。在大气相关样品中发现的汞同位素的奇/偶数非质量分馏为其在大气中的研究提供了新的契机。迄今为止，在自然界中观察到的偶数汞同位素非质量分馏依然无法解释。本项目旨在研究平流层中偶数汞同位素非质量分馏的触发机制。我们实验室的初步研究表明，特定波长的紫外线光下可引起空气中汞的氧化并产生大量的偶数汞非质量分馏。然而，汞原子暴露在特定波长的光下被激发的过程，只能发生在平流层上部。所以我们将在实验室系统模拟近似平流层大气环境下的汞同位素的分馏，同时，构建一个汞在平流层/对流层交换模型。

Currently being regulated by the Minamata convention, the element mercury (Hg) is a global pollutant posing severe risks to the health of ecosystems and humans worldwide. The atmosphere serves as an instrumental media for transformation and transport of Hg over large spatial scales. By contrast to other metal and metalloid elements, Hg exists in the troposphere primarily in the gas-phase and to less extent bound to aerosols. Besides a unique semi-volatility among the heavy metals, Hg exhibits a rich redox chemistry, where the transformations between the elemental and divalent states strongly influence the atmospheric transport characteristics and deposition rate of Hg. The atmospheric cycling of Hg is rather complicated and due to ultra-trace concentrations, present atmospheric measurement techniques are not capable of deterministic mercury compound identification but general only admits to determination of bulk fractions that hampers modeling efforts. However, analytical developments and methodological improvements over the last two decades have now expanded the feasibility of high-precision stable isotope analyses of natural samples to heavier elements like Hg. The new scientific field of stable Hg isotope ratio analysis has invigorated atmospheric Hg studies and has revealed a intriguing mass-independent fractionation (MIF) of even Hg isotopes (even-Hg-MIF) associated with atmospheric-related samples. The observations of Hg even-MIF in nature is puzzling, and have hitherto defied a stringent explanation and remains a conundrum. Here, we argue for that stratospheric processes trigger even-Hg-MIF. Pre-study experiments in our laboratory stipulate that UVC-light induced oxidation of Hg0 in air induces massive even-Hg-MIF (∆200Hg in the magnitude of 40‰ observed). The process, which involving electronically excited Hg0, can only take place in the upper stratosphere, where Hg0 become exposed to wavelengths of light that cause photo-excitation. In turn, at these altitudes, a transient pool of the reactant Hg0 exists due to efficient photolytical decomposition of otherwise stratospheric long-lived HgX2. We propose to investigate the Hg isotope composition and fractionation during these processes in systematic laboratory experiments designed to mimic atmospheric conditions and calculate kinetic fractionation factors. The data will be used as a vital input to a model that synthesizes contributions of major processes to isotope fractionation of Hg in the atmosphere.