ORCHIDEE-Hg v1.0 (rev. 9721): A new global land-surface model for mercury dynamics – Part 1: Model description and evaluation in temperate regions

Abstract. Mercury (Hg) is a highly toxic global pollutant that is transported throughout the atmosphere and transferred to terrestrial ecosystems through vegetation uptake and atmospheric wet and dry deposition. Owing to its strong association with organic carbon (OC) and long residence time, soils represent the largest Hg reservoirs. Environmental changes can, however, alter Hg stocks and fluxes, potentially enhancing Hg emissions to the atmosphere or export to riverine systems. Existing global models have yet to fully capture the dynamics of dissolved Hg and its transport through terrestrial hydrological pathways, as well as Hg storage and remobilization in deep soil layers, limiting their ability to represent terrestrial Hg responses to environmental change. Here, we present the first version of ORCHIDEE-Hg (ORCHIDEE-Hg v1.0), a global land-surface model of the terrestrial Hg cycle developed within the well-established ORCHIDEE framework. The model uniquely integrates explicit representations of dissolved carbon species and vertical discretization of soil across 15 plant functional types. ORCHIDEE-Hg represents the terrestrial Hg cycle, from atmospheric wet and dry deposition and vegetation uptake to soil storage, dissolved Hg dynamics, and atmospheric re-emissions. We evaluate ORCHIDEE-Hg against recent observations of vegetation Hg concentrations, litterfall Hg fluxes, and soil stocks across two temperate regions. The model successfully reproduces observed median Hg concentrations, stocks, and fluxes within and between key compartments. This evaluation provides a robust foundation for investigating the controls of terrestrial Hg cycling. A companion study will assess model sensitivity to key parameters and processes, providing a detailed understanding of model behavior before using ORCHIDEE-Hg to investigate changes in soil Hg stocks and fluxes under environmental perturbations, including climate change, land-use and land-cover change, and changes in atmospheric deposition.