Heterogeneous Photooxidation of Atmospheric Mercury Enhanced by Mixtures of Metallic Oxides and Sodium Chloride

Heterogeneous processes play an important role in atmospheric mercury (Hg) reaction; however, the Hg uptake induced by mixed components remains unknown. Here, we developed an integrated approach combining a coated-wall flow tube reactor system with theoretical calculations to investigate the gaseous elemental mercury (Hg_(g)⁰) uptake on the surface of metallic oxides and sodium chloride (NaCl). The mixture of titanium dioxide (TiO₂) and NaCl was found to be vital for Hg_(g)⁰ uptake. This uptake can be enhanced significantly under light irradiation across a range of relative humidity (RH) levels, overcoming the inhibitory effect of higher RH that occurs with TiO₂ alone. The uptake coefficients for the mixture of TiO₂ + NaCl range from 2.10 × 10^–5 to 1.21 × 10^–4 over RH values from 10% to 85%, with the maximum value observed at RH = 25%. In contrast, the uptake on TiO₂ alone was only observed at RH ≤ 55%. Integrated with density functional theory calculations, our results reveal that the synergistically enhanced mechanism of Hg_(g)⁰ uptake on TiO₂ + NaCl transitions from air–solid to air–liquid interfacial dominate as humidity increases, a process aided by the formation and migration of chloride radicals. These findings highlight a critical, previously overlooked pathway for atmospheric Hg transformation driven by heterogeneous chemistry. Incorporating these effects into global models is essential for improving the accuracy of Hg deposition and associated risk assessments.