Emerging evidence of nighttime mercury oxidation driven by nitrate radical-activated heterogeneous halogen chemistry

Nighttime mercury oxidation remains poorly understood, restricting the performance of atmospheric mercury models and limiting current understanding of the global mercury cycle. In this study, one-year speciated mercury observations in Beijing revealed nighttime gaseous oxidized mercury (GOM) enhancements on more than one-third of the monitoring days in each month, usually occurring during early night, with peak values around midnight hours (23:00-01:00), indicating significant mercury oxidation during early night. The enhancements of GOM% (the ratio of GOM to total gaseous mercury) from sunset to the nighttime peak showed monthly variation, ranging from 4.7 ± 3.7 % (January) to 6.1 ± 8.3 % (August). A generalized additive model (R² = 0.70) showed that the nighttime GOM% growth was jointly explained by NO₃· production rate, temperature, and several particle-related parameters (77.0 % in total), with NO₃· emerging as the most influential predictor (35.8 %). We then identified a pathway in which NO₃· reacts with particle-bound halide ions to produce halogen radicals that can cause mercury oxidation. Simplified rate calculations based on this scheme reproduced the magnitude and variability of observed GOM production (R² = 0.62; slope = 1.19). These findings provide plausible evidence for a potential nighttime mercury oxidation pathway, helping to narrow a critical uncertainty in our understanding of atmospheric mercury cycling.