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Multiple observational analyses have identified a decreasing trend over the past decade in surface concentrations of gaseous elemental mercury (GEM), from sites in both the Northern and Southern hemisphere. We use two model-based approaches to identify sets of parameters that could quantitatively explain observed trends in mercury. Using a 12-box model, we quantify which combinations of factors including changes in anthropogenic emissions, oceanic and land evasion, and atmospheric oxidation rates that can explain observed tropospheric mercury decreases. We then use the GEOS-Chem chemical transport model to further examine source attribution at measurement sites where decreasing trends have been observed. We combine these analyses to generate quantitative hypotheses to explain observed trends. We suggest that the tropospheric decline may be best explained by multiple, reinforcing factors.