The Geos-Chem model

Summary

We examine the response of deposition to decreases in anthropogenic emissions using the GEOS-Chem global atmosphere-ocean-land mercury simulation. Total global mercury sources in the model are 9230 Mg yr^-1 (3400 Mg yr^-1 anthropogenic, 650 Mg yr^-1 biomass burning, 2180 Mg yr^-1 land emissions, 3000 Mg yr^-1 ocean emissions). Our atmospheric simulation describes the cycling of mercury through the surface ocean and land reservoirs. The model includes atmospheric oxidation of Hg⁰ by OH and O₃, and in-cloud reduction of Hg^(II). Wet and dry deposition account for 32% and 68% of the global sink, respectively. The lifetime of mercury against deposition is 0.6 years. We conduct four sensitivity simulations where anthropogenic emissions are reduced by 20% over East Asia, Europe, South Asia, and North America, leading to decreases in global deposition of -3.5, -0.9, -0.8, and -0.5% respectively. One third of the deposition decrease occurs in the source regions, and the rest is distributed globally due to decreased long-range transport of Hg⁰ and subsequent oxidation to Hg^(II). Regional decreases in deposition within the source regions range from -12% for East Asia (60% of depositions due to local emissions) to -3% for North America (where only 15% of deposition is due to local emissions). When normalized by total emissions, we find that Hg deposition in the Arctic is more sensitive to decreases in European emissions compared to decreases in East Asian, North American, or South Asian emissions. Our estimates of the distribution of deposition and its response to decreases in anthropogenic emissions are limited by uncertainties in the speciation of anthropogenic emissions, redox chemistry of atmospheric mercury, the role of dry deposition, and the cycling efficiency of mercury in the ocean and land reservoirs.