Constraining speciated mercury dry deposition models with urban observations in eastern China

Abstract. Atmospheric dry deposition constitutes a major pathway of terrestrial mercury (Hg) loading globally, yet estimates of its fluxes remain highly uncertain, particularly in regions with elevated particulate matter (PM) concentrations. This study employed long-term surrogate surface measurements to characterize speciated atmospheric Hg deposition at an urban site in eastern China. Over the period of July 2020 to June 2021, measured total atmospheric mercury (TAM) dry and wet deposition fluxes amounted to 29.19 and 7.29 μg m⁻² yr⁻¹, respectively, corresponding to a dry-to-wet ratio of 4.0. The observed gaseous oxidized mercury (GOM) dry deposition flux over the same period accounted for 17 % of the total dry deposition. These refined flux measurements provided observational constraints for speciated Hg dry deposition modeling. Applying a site-specific correction factor of 2.21 to denuder-based GOM concentrations avoided a 46 % underestimation of the mean GOM dry deposition flux. The explicit inclusion of coarse particulate-bound mercury (PBM) increased the deposition velocity by a factor of 4.5, and revealed that coarse particles accounted for 80 % of total PBM dry deposition. Replacing the reference leaf area index (LAI) with site-specific values reflecting local vegetation phenology better captured bidirectional gaseous elemental mercury (GEM) exchange, including net emission during certain summer periods. The refined simulations further showed that PBM and GEM co-dominated Hg dry deposition at this site, unlike many less PM-polluted regions where GEM plays a more dominant role. This observationally constrained framework improves speciated atmospheric Hg dry deposition modelling and reduces uncertainty in global Hg budget estimates.