Development of a mechanistic model of Hg in the terrestrial

biosphere

Nicole Smith-DowneyHarvard University

GEOS-Chem Users MetingApril 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– 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

How does Hg enter the soil pool?

Hg0Hg(II)oxidation

reductionwet and dry deposition to

canopy

throughfall

Direct deposition to soil surface

There are four forms of Hg deposition to the biosphere•Hg(II) wet•Hg(II) dry•Hg0 dry•Hg(p)

How does Hg enter the soil pool?

Hg0Hg(II)oxidation

reductionwet and dry deposition to canopy and soil surface

litterfallthroughfall(wet)

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

What happens to Hg in soils

Hg0Hg(II)oxidation

reduction

Binding to organic ligands,

charged soil particles

wet and dry deposition to canopy and soil surface

litterfallthroughfall(wet)

methylation (anaerobic)

Hg0

Hg(II)

The oxidation state of Hg in soils determines the rate of re-emission

Dissolved Hg(II) in soil

waterKorgKorg measured by Lyon et al. 1997 and Khwaja et al. 2006

What happens to Hg in soils

Hg0Hg(II)oxidation

reduction

Binding to organic ligands,

charged soil particles

Immediate re-emission

wet and dry deposition to canopy and soil surface

litterfallthroughfall(wet)

methylation (anaerobic)

Hg0

Hg(II)

Soils that are organic rich will store Hg effectively (Northern peat soils)

Anderson 1979

How does Hg leave soils

Hg0Hg(II)oxidation

reduction

Binding to organic ligands,

charged soil particles

Volatilization to atmosphere

wet and dry deposition to canopy and soil surface

litterfallthroughfall(wet)

methylation (anaerobic)

Hg0

Hg(II)

Biomass Burning (including soils)

Decomposition of organic material leads to the removal of Hg(II) from the system (Heyes et al. 1998)

Evapotranspiration

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)

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

CASA Soil Model

At each transfer point, a fraction of the soil pool is respiredDepends on•Litter quality•Temperature•Moisture•Soil Structure

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

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

Hg storage in soil pools over time

Spatial distribution of Hg storage in soils

Armored pool t=2000 yrs

Slow pool t=2000 yrs

Respiration driven Hg fluxes out of soils

Maximum = 600 g/m2/yr

Hg lifetime in soils against respiration

Maximum = 75 years

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