Revealing coastal atmospheric mercury redox processes with a photochemical box model MECCA-Hgv1.0.

Atmospheric reactions and source-sink pathways of atmospheric mercury (Hg) in coastal regions remain complex. Here we conducted field observations of atmospheric Hg in coast Qingdao in northern China, and developed a photochemical box model (MECCA-Hgv1.0) to investigate its redox processes. Our model successfully reproduced observed concentrations and diurnal variations of gaseous oxidized mercury (GOM), revealing its main species as HgCl2 (∼80%) and brominated speciation. Bromine radicals initiate 85% of Hg(0) oxidation fluxes, but the brominated products are readily photolyzed, ultimately making HgCl2 as the stable GOM species through the hydroxyl-initiated oxidation followed by HCl substitution. The GOM removal process is highly determined by BrHgOH photolysis and gas-liquid partitioning. Sensitivity analysis reveals that either higher humidity or increased temperatures can significantly reduce GOM concentrations via enhanced aqueous uptake and thermal decomposition of Hg(I) intermediates, respectively. Unexpectedly, GOM production reversely responds to anthropogenic volatile organic compounds (VOCs) that consume reactive halogen species in coastal atmosphere. Future emission control of anthropogenic VOCs likely leaves more available reactive halogens, thereby enhancing the potential for coastal Hg oxidation. This study provides critical mechanistic insights into the coastal atmospheric mercury cycle driven by interactions between anthropogenic pollution and marine halogen chemistry.