Zhen Jiebo, Li Tao, Cai Hongming, Nie Xiaoling, He Sheng, Meng Mei, Wang Yan, Chen Jiubin
School of Environmental Science and Engineering, Shandong University, Qingdao 266237, China.
School of Environmental Science and Engineering, Shandong University, Qingdao 266237, China; Division of Environment and Sustainability, The Hong Kong University of Science and Technology, Kowloon, Hong Kong, China.
J Hazard Mater. 2024 Aug 5;474:134654. doi: 10.1016/j.jhazmat.2024.134654. Epub 2024 May 18.
The photoreduction of mercury (Hg) in clouds is crucial for determining global Hg cycling. The recently-developed isotope approach provides new insight into the fate of atmospheric Hg, however, limited data have been reported on the dynamics of Hg isotopes in clouds. This study presented the isotopic compositions of dissolved mercury (DHg) and particulate mercury (PHg) in cloud water collected at Mt. Tai (1545 m a.s.l.) in eastern China during summer 2021. Both DHg and PHg exhibited positive mass-independent fractionation of odd isotopes (odd-MIF, denoted as ΔHg), with averaged ΔHg values of 0.83 ± 0.34‰ and 0.20 ± 0.11‰, respectively. This high odd-MIF likely resulted from aqueous photoreduction in clouds, with DHg being more susceptible to photolysis than PHg. Our findings indicated that the photoreduction was promoted by sunlight and influenced by the chemical compositions of cloud water that controlled the Hg(II) speciation. The isotope mixing model estimation revealed that particulate-bound Hg and reactive gaseous Hg constituted the principal sources of Hg in cloud water, accounting for 55% to 99% of the total, while gaseous element Hg also made a notable contribution. Additionally, cloud water samples with faster reduction rates of Hg(II) were located outside of the isotope mixing models, which indicated an enhanced photoreduction process in cloud water.
云层中汞（Hg）的光还原对于确定全球汞循环至关重要。最近开发的同位素方法为大气汞的归宿提供了新的见解，然而，关于云层中汞同位素动态的报道数据有限。本研究展示了2021年夏季在中国东部泰山（海拔1545米）采集的云水溶解态汞（DHg）和颗粒态汞（PHg）的同位素组成。DHg和PHg均表现出奇数同位素的正质量非相关分馏（奇数-MIF，记为ΔHg），平均ΔHg值分别为0.83±0.34‰和0.20±0.11‰。这种高奇数-MIF可能源于云层中的水相光还原，其中DHg比PHg更容易受到光解作用的影响。我们的研究结果表明，光还原受阳光促进，并受控制Hg（II）形态的云水化学成分影响。同位素混合模型估计显示，颗粒态汞和活性气态汞构成了云水中汞的主要来源，占总量的55%至99%，而气态单质汞也做出了显著贡献。此外，Hg（II）还原速率较快的云水样品位于同位素混合模型之外，这表明云水中存在增强的光还原过程。
