Top-down constraints on atmospheric mercury emissions and implications for global biogeochemical cycling

Abstract. We perform global-scale inverse modeling to constrain present-day atmospheric mercury emissions and relevant physiochemical parameters in the GEOS-Chem chemical transport model. We use Bayesian inversion methods combining simulations with GEOS-Chem and ground-based Hg⁰ observations from regional monitoring networks and individual sites in recent years. Using optimized emissions/parameters, GEOS-Chem better reproduces these ground-based observations and also matches regional over-water Hg⁰ and wet deposition measurements. The optimized global mercury emission to the atmosphere is ~ 5.8 Gg yr⁻¹. The ocean accounts for 3.2 Gg yr⁻¹ (55 % of the total), and the terrestrial ecosystem is neither a net source nor a net sink of Hg⁰. The optimized Asian anthropogenic emission of Hg⁰ (gas elemental mercury) is 650–1770 Mg yr⁻¹, higher than its bottom-up estimates (550–800 Mg yr⁻¹). The ocean parameter inversions suggest that dark oxidation of aqueous elemental mercury is faster, and less mercury is removed from the mixed layer through particle sinking, when compared with current simulations. Parameter changes affect the simulated global ocean mercury budget, particularly mass exchange between the mixed layer and subsurface waters. Based on our inversion results, we re-evaluate the long-term global biogeochemical cycle of mercury, and show that legacy mercury becomes more likely to reside in the terrestrial ecosystem than in the ocean. We estimate that primary anthropogenic mercury contributes up to 23 % of present-day atmospheric deposition.