Particle bound mercury (PBM) deposition on the Earth's surface threatens biota and humans. The photoreduction of PBM competes with deposition and thereby modifies global mercury cycling, yet its pathway and mechanism remain poorly understood. Herein, we reveal the photoreduction process of PBM by comprehensively using field observation, mercury stable isotope analysis and controlled experiment. We found the Δ¹⁹⁹Hg values in wet haze episodes (0.34‰±0.30‰) were significantly higher than those in clean periods (0.14‰±0.19‰), majorly attributed to the elevated aerosol water content (AWC) which shifts aerosol phase from solid state to liquid state, promoting soluble HgCl₂ and HgBr₂ photoreduction reactions. The carboxyl functional groups of water-soluble organic carbon (WSOC) was further identified as the crucial compound that induces PBM photoreduction, whose reaction rates were ~2 times higher than phenol and ketone ligands and 3–6 times higher than those observed in other atmospheric aqueous phases. Considering the ubiquitously distributed carboxyl ligands and significant positive Δ¹⁹⁹Hg signals in the atmospheric aqueous phases, the PBM photoreduction mediated by carboxyl ligands is highlighted to significantly influence global mercury transformations, regional depositions and isotopic compositions of atmospheric mercury pools.

大气,气溶胶,汞,光还原