单质汞被卤素原子（Cl，Br）在气相氧化过程产生的汞同位素分馏：大气汞循环研究的新方法

The element mercury (Hg) is a global pollutant, for which the atmosphere serves as an instrumental media for transport and transformation. Contrary to other metal and metalloid elements, Hg exists in the troposphere primarily in the gas-phase and to less extent bound to aerosols. The main species of atmospheric mercury is elemental vapor (Hg0), making up at least 95% of the total concentration as interpreted from ambient air ground-based measurements. [Munthe and Sommar, 2003]. The main atmospheric sink of Hg0 is via oxidation to divalent molecular species (e.g. HgCl2 etc.), which in-turn are highly soluble in water (Henry's Law coefficient > 106 M atm-1) and therefore have a lifetime against deposition of days compared to ～1 year for Hg0 in the lower troposphere. From field, laboratory and modeling studies, it has been inferred that reactive halogen species (Br atoms and to less extent Cl atoms) are mainly responsible for the oxidation of Hg0. During the last decade, developments in analytical technique have opened the possibility to detect small variations in isotopic composition of heaviest elements i.e. Hg, Tl and U. Mercury stable isotope geochemistry has become a rapidly growing field used to examine Hg biogeochemical cycling. However, up to now, related studies to understand processes of Hg with relationship to the atmosphere are very scarce and none of them has direct bearing on oxidation processes. Here, we propose to investigate the Hg isotope composition and fractionation during gas-phase oxidation of Hg0 by Cl and Br atoms in systematic laboratory experiments designed to mimic atmospheric conditions to the utmost possible extent. Pre-study simulations have been performed in order to stipulate optimal experimental conditions for the study of the net process Hg0(g) ? HgX2(g) (X=Cl, Br) following pseudo-first order kinetics. The critical steps of separating samples of the reactant and products from the reaction mixture and the transfer to aliquots suitable for isotope analysis will be approached by a state-of-the-art speciation technique followed by a purge-trapping method. Using multi-collector inductively coupled plasma mass spectrometry, will allow us to address potential anomalous isotope fractionation of Hg as well as conventional mass dependent fractionation during the oxidation and to calculate kinetic fractionation factors. The data expected will be a vital input to a model that synthesizes contributions of major processes to isotope fractionation of Hg in the atmosphere.

汞（Hg）是通过大气传输和转化的全球性污染物。汞在大气中主要以单质形式存在，占气态总汞95%以上，而与气溶胶结合的汞占的比例很低。气态单质汞的主要汇是通过氧化反应转化为易溶于水的二价汞化合物（HgCl2等），缩短其在大气中的居留时间。野外观测、实验室和模型研究均表明卤素原子（Br，Cl）诱导并参与了汞的氧化反应。近年来，随着同位素测试技术的发展，Hg同位素作为研究地球化学循环的新手段得到了快速发展。然而，关于大气汞同位素的研究极端缺乏，更未见关于大气汞氧化过程汞同位素分馏研究的报道。本项目拟研究大气卤素原子氧化气态单质汞的反应过程汞的同位素分馏。该氧化过程服从假一阶反应动力学，本研究中将设计反应器以采用吹脱-吸附法分离反应物与产物供测定汞同位素组成，探索这些反应过程可能产生的汞同位素质量分馏及可能存在的非质量分馏，并计算动力学分馏因子。研究结果将提高我们对大气汞转化过程汞的同位素分馏机理。

汞（ Hg）是通过大气传输和转化的全球性污染物。汞在大气中主要以单质形式存在，占气态总汞 95%以上，而与气溶胶结合的汞占的比例很低。气态单质汞的主要汇是通过氧化反应转化为易溶于水的二价汞化合物（ HgCl2 等），缩短其在大气中的居留时间。野外观测、实验室和模型研究均表明卤素原子（ Br， Cl）诱导并参与了汞的氧化反应。近年来，随着同位素测试技术的发展， Hg 同位素作为研究地球化学循环的新手段得到了快速发展。然而，关于大气汞同位素的研究极端缺乏，更未见关于大气汞氧化过程汞同位素分馏研究的报道。.本项目通过构建烟雾箱模拟自然界中卤素原子（ Br， Cl）与汞的氧化反应，得到了一系列重要的成果。首先获得了单质汞被卤素Br/Cl原子氧化的动力学反应速率k(Cl+Hg)和K(Br+Hg)分别是(1.8±0.5) × 10-11 和 (1.6±0.8) × 10-12cm3 molecule-1 s-1。单质汞与氯原子和溴原子的氧化过程中，汞同位素质量分馏因子 分别为0.99941± 0.00006（2σ）和1.00074±0.00014（2σ）。同时，我们发现反应物气态Hg0中，均表现为正的奇数汞同位素非质量分馏。其中，单质汞与氯原子的反应， 可数汞同位素的非质量分馏∆199Hg 为~1.2‰ 和∆201Hg 为~0.6‰；单质汞与溴原子的反应中，奇数汞同位素分馏相对较小，∆199Hg 最大达 ~0.4‰。首次在氧化过程中，观测到偶数汞同位素Hg200的非质量分馏，这一发现印证和解释了大气降水和花鸟岛颗粒物中观测到的Hg200的非质量分馏。这些研究结果将提高我们对大气汞转化过程的同位素分馏机理的理解和应用汞同位素示踪污染物来源。