Anthropogenic emissions of the toxic heavy metal mercury (Hg) threaten human health and ecosystems. The elemental form of emitted gaseous mercury, Hg(0), can be transported globally in the atmosphere due to its long lifetime of 4–6 months, but upon oxidation it forms soluble divalent mercury, Hg(II), which is rained out within days. However, there are many uncertainties associated with atmospheric Hg chemistry, leading to uncertain predictions of its fate and ecosystem impacts. Additionally, it is unknown how Hg cycling has been affected by recent, global change-induced trends in atmospheric oxidants (e.g., ozone and halogens). To address these knowledge gaps, the interdisciplinary SUMAC project will train the experienced researcher (ER) to integrate the latest knowledge from laboratory kinetics, computational and isotope chemistry, and field measurements into a global atmospheric Hg model, GEOS-Chem. By applying statistical methods from the field of global sensitivity analysis, the ER will identify the key chemical reaction rates that contribute the most to the uncertainty in the atmospheric Hg lifetime. Using Bayesian inference methods, the ER will develop constraints from field measurements for these reaction rates, establishing a new chemical mechanism for atmospheric Hg models. With the refined Hg chemical mechanism, the ER will conduct historical and future simulations to evaluate temporal trends in the Hg chemical lifetime and resultant impacts on Hg dispersion and deposition. By being the first study to quantify the influence of atmospheric chemistry on observed Hg trends, SUMAC will support the effectiveness evaluation of the Minamata Convention on Mercury, an international treaty aimed at reducing Hg emissions. Aside from the breakthrough research outcomes, SUMAC will strengthen the capacity of Hg modelling within Europe through the ER’s training activities, knowledge transfer to host institutions, open-access model development, and outreach.