Isotope Data Constrains Redox Chemistry of Atmospheric Mercury

The redox chemistry of mercury (Hg) in the atmosphere exerts a significant influence on its global cycle. However, our understanding of this important process remains shrouded in uncertainty. In this study, we utilize three-dimensional atmospheric Hg isotope modeling to evaluate the isotopic composition of particle-bound mercury [HgII(P)] in the global atmosphere. We investigate various chemistry mechanisms and find that they induce remarkably disparate odd-number mass-independent fractionation (odd-MIF) in HgII(P) on a global scale. The observed odd-MIF data identify the essential role of sea salt aerosol debromination in the redox chemistry of atmospheric Hg and underscore the predominant influence of Br oxidation in the marine boundary layer. The odd-MIF signatures significantly narrow the uncertainty range of redox chemistry rates and constrain the photoreduction of HgII(P) at a magnitude of 10–3 JNO2 (local photolysis frequency of NO2) in the global atmosphere. This study advances our understanding of atmospheric Hg chemistry processes and provides insights into the potential impacts of climate change on Hg cycling.大气中汞（Hg）的氧化还原化学过程对其全球循环具有显著影响，但目前学界对这一关键过程的认知仍笼罩在不确定性之中。本研究采用三维大气汞同位素模拟方法，对全球大气中颗粒结合态汞[HgII(P)]的同位素组成展开评估。我们对多种化学机制进行了探究，发现它们在全球尺度下会使颗粒结合态汞产生差异显著的奇数质量独立分馏（odd-MIF）信号。观测到的奇数质量独立分馏数据证实，海盐气溶胶脱溴作用在大气汞氧化还原化学过程中发挥着核心作用，同时也凸显了溴氧化过程在海洋边界层内的主导影响。该奇数质量独立分馏特征显著缩小了氧化还原化学反应速率的不确定性区间，并将全球大气中颗粒结合态汞的光还原速率约束在10⁻³ JNO2（NO₂的局地光解频率）的量级范围内。本研究深化了学界对大气汞化学过程的认知，同时为理解气候变化对汞循环的潜在影响提供了新的视角。