Mercury stable isotopes revealing the atmospheric mercury circulation: A review of particulate bound mercury in China

Human activities since industrialization have significantly raised global atmospheric mercury (Hg) levels. China's high Hg emissions with rapid industrial development have a global impact on implementing the Minamata Convention on Mercury due to the dispersion of atmospheric Hg. Here, we comprehensively reviewed the atmospheric particulate bound mercury (PBM) in China, given its highest deposition rate, aiming to elucidate Hg sources, transformation, and geochemical cycles. Firstly, we analyzed its spatiotemporal distribution and long-term trends in China by synthesizing published literature. Most of the studies were conducted in eastern China, where PBM concentrations are higher than in western China due to extensive anthropogenic emissions. Furthermore, meteorological factors and atmospheric transport significantly affect seasonal PBM variations. Local anthropogenic sources, atmospheric transport, and local atmospheric transformation contribute 56.2%, 14.9%, and 29.0% to China's PBM content, respectively. Notably, China's atmospheric PBM concentrations have declined significantly since 2015. Secondly, we compiled a comprehensive global Hg isotope dataset and employed structural equation modeling to quantify Hg geochemical cycling. The isotope ratios of atmospheric Hg primarily overlap with those of Hg raw materials and vegetation. The significant relationship (P < 0.01) between atmospheric Hg and Hg raw materials (0.91) and vegetation (1.78) suggests that they contribute significantly to atmospheric Hg. Besides water and fish, atmospheric Hg also shows a high regression coefficient with human Hg, raising concerns about direct human inhalation of atmospheric Hg. Thirdly, we combined compiled Hg isotopes with machine learning to predict PBM sources in China. Industrial activities, biomass burning, and soil/dust are the primary contributors to PBM in China, accounting for 58.67%, 22.11%, and 17.14%, respectively. Our findings indicate that the contribution of soil/dust to PBM, ranging from 0.51% to 56.42%, has been underrated in previous studies. It is feasible to trace atmospheric Hg transport using Hg isotopes, as PBM mainly undergoes photoreduction reactions, but quantifying regional Hg transport remains challenging.