Global model of atmosphere-terrestrial mercury exchange and accumulation of historical mercury in organic soils

Terrestrial soils represent the largest surface reservoir in the global biogeochemical cycling of mercury, with an estimated 10⁵ Mg incorporated in organic soils. Mercury binds to organic material in soils and is released back to the atmosphere through respiration on timescales of years to centuries. Deposition of mercury to land has increased by approximately a factor of three due to anthropogenic activities over the past two centuries. We are interested in the fate of historically emitted mercury in terrestrial soils and the response of atmosphere-terrestrial mercury exchange to changing emissions and climate in the future. To address these questions, we build on the Global Terrestrial Mercury Model (GTMM), which tracks mercury deposited from the atmosphere through cycling in soil pools based on its association with organic matter (Smith-Downey et al. 2010).

We use historical anthropogenic emissions since 1840 to drive the accumulation of mercury in soils (Streets et al. 2011). Most emissions over that time period come from European and North American industry and mining, but the relatively long lifetime of Hg⁰ in the atmosphere results in a more hemispherically distributed accumulation in soils. The greatest magnitude of anthropogenic mercury accumulation occurs in the slowly overturning soil pools, while the greatest relative enhancement is in the most labile soil pools. We find that soil respiration more than doubles from the preindustrial period to the present due to the accumulation of anthropogenic mercury.

To examine atmosphere-terrestrial feedbacks in the biogeochemical cycling of mercury, we couple the GEOS-Chem atmosphere-ocean model with the GTMM. We update the model's air-land interface by including more mechanistically based dry deposition and surface photoreduction and revolatilization processes. We test the sensitivity of model results to uncertainties in the affinity of mercury to different soil pools and the fraction of mercury released upon soil respiration. Finally, we evaluate the model with comparison to field observations of mercury and carbon concentrations in vegetation, leaf litter, and soils across a range of ecosystems in the United States. Where observations are available, we compare modeled surface fluxes to measured dry deposition, throughfall, and washoff. We examine the seasonal and interrannual variability of modelled atmosphere-terrestrial mercury exchange and comment on the implications for long-term climate change.