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Development of a mechanistic model of Hg in the terrestrial biosphere

Development of a mechanistic model of Hg in the terrestrial biosphere. Nicole Smith-Downey Harvard University GEOS-Chem Users Meting April 12, 2007. Project Goals. To develop a mechanistic model of mercury in the biosphere and couple this to the GEOS-Chem Hg simulation Specific aims

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Development of a mechanistic model of Hg in the terrestrial biosphere

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  1. Development of a mechanistic model of Hg in the terrestrial biosphere Nicole Smith-Downey Harvard University GEOS-Chem Users Meting April 12, 2007

  2. Project Goals • To develop a mechanistic model of mercury in the biosphere and couple this to the GEOS-Chem Hg simulation • Specific aims • To model the incorporation, storage and emissions of Hg from the terrestrial biosphere • Continuum of timescales (annual to century) • Mechanistic nature will allow us to examine changes in emissions and/or climate • Key is to understand Hg storage in soils

  3. How does Hg enter the soil pool? • There are four forms of Hg deposition to the biosphere • Hg(II) wet • Hg(II) dry • Hg0 dry • Hg(p) oxidation Hg(II) Hg0 reduction wet and dry deposition to canopy Direct deposition to soil surface throughfall

  4. How does Hg enter the soil pool? • Hg0 can be incorporated into leaf tissue (through stomates) then enters the soil pool as litter • Ecosystem dependent - implies large spatial gradient in deposition • Largest source of Hg to some soils oxidation Hg(II) Hg0 reduction wet and dry deposition to canopy and soil surface litterfall throughfall (wet)

  5. What happens to Hg in soils • The oxidation state of Hg in soils determines the rate of re-emission oxidation Hg(II) Hg0 reduction wet and dry deposition to canopy and soil surface litterfall throughfall (wet) Hg0 methylation (anaerobic) Hg(II) Binding to organic ligands, charged soil particles Dissolved Hg(II) in soil water Korg • Korg measured by Lyon et al. 1997 and Khwaja et al. 2006

  6. What happens to Hg in soils oxidation Hg(II) Hg0 reduction wet and dry deposition to canopy and soil surface Immediate re-emission Anderson 1979 litterfall throughfall (wet) Hg0 methylation (anaerobic) Hg(II) Binding to organic ligands, charged soil particles Soils that are organic rich will store Hg effectively (Northern peat soils)

  7. How does Hg leave soils oxidation Hg(II) Hg0 Biomass Burning (including soils) reduction wet and dry deposition to canopy and soil surface Evapotranspiration Volatilization to atmosphere litterfall throughfall (wet) Hg0 methylation (anaerobic) Hg(II) Binding to organic ligands, charged soil particles Decomposition of organic material leads to the removal of Hg(II) from the system (Heyes et al. 1998)

  8. Implies a method to model Hg • Tie the lifetime of Hg in soils to the lifetime of different carbon pools • Base lifetime of Hg(II) against reduction to the lifetime of it’s associated carbon pool • Use existing biogeochemical model (CASA) as the framework for a mercury model • Combines remote sensing and meteorological observations to predict ecosystem productivity • Includes plant growth, decomposition and biomass burning • Tracks soil carbon pools of different types (similar to CENTURY model)

  9. CASA Model • Global 1x1 degree ecosystem model • NPP is calculated as a function of intercepted photosynthetically active radiation and light use efficiency • Using version developed by van der Werf et al. 2001 Potter et al. 1993

  10. CASA Soil Model • At each transfer point, a fraction of the soil pool is respired • Depends on • Litter quality • Temperature • Moisture • Soil Structure

  11. Current Hg Soil Simulation Hg0 incorporated into leaf tissue proportional to Hg0 dry deposition and LAI Hg(II) dry deposition is added to surface litter pools Hg(II) wet deposition is split between the litter pools and the slowpool

  12. Experiment • Use Pre-industrial deposition estimates from Selin et al. in prep • Hg0 dry = 580 Mg/yr • Hg(II) wet = 780 Mg/yr • Hg(II) dry = 1640 Mg/yr • Assume uniform deposition over land areas • Spin up soil pools to equilibrium and examine distribution of Hg storage, emissions and lifetime

  13. Hg storage in soil pools over time

  14. Spatial distribution of Hg storage in soils Armored pool t=2000 yrs Slow pool t=2000 yrs

  15. Respiration driven Hg fluxes out of soils Maximum = 600 g/m2/yr

  16. Hg lifetime in soils against respiration Maximum = 75 years

  17. Current Work - Layered Soil Model • Soils are not at equilibrium with current emissions • Need to track movement of Hg through soil profile • Use approach of Carrasco et al. 2006 for 14C • This will allow us to examine the effects of soil burning and decomposition on the Hg budget

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