Mercury Isotopes Reveal Atmospheric Sources and Particle-Mediated Redox Shifts in a Tropical Savanna

The savanna, covering ∼20% of Earth's land surface, plays a crucial yet poorly understood role in the global mercury (Hg) cycle due to its unique climate and susceptibility to anthropogenic influence. This study traces the sources, transport, and transformation of atmospheric Hg in a typical savanna region in Southwest China, using Hg stable isotopes. We hypothesized that unique climate-induced Hg redox in savanna and regional anthropogenic direct emission significantly affected the seasonal variations of Hg concentrations and isotopic signatures. Anthropogenic emissions are the dominate source of total gaseous Hg, particularly during the dry season, accounting for 56.8 ± 11.5% as quantified using an isotopic mixing model. Notably, PM_2.5-Hg exhibits significant mass-independent fractionation, with positive Δ¹⁹⁹Hg and Δ²⁰⁰Hg indicating a substantial influence from secondary processes. Specifically, the secondary Hg formation contributes 72.4 ± 19.7% in the wet season. We also identify complex heterogeneous reactions on PM_2.5 to cause the Δ¹⁹⁹Hg/Δ²⁰¹Hg slope to be 1.28 in the dry season. Finally, our data set provides observational constraints that can improve Hg isotope-enabled modeling in savanna regions, particularly regarding the gas-particle partitioning coefficient for oxidized Hg species, which exerts a strong influence on Δ¹⁹⁹Hg variations in the current model.