A Biogeochemical Model for Mercury in GEOS-Chem

Noelle Eckley SelinGEOS-Chem 3rd Users’ Meeting

April 12, 2007

Hg(0) Hg(II)

MeHgHg(II)

Why we care about mercury:

Hg in GEOS-Chem Standard Code

• 3 species: Hg(0), Hg(II), Hg(P) (chemically inert)• Main scientific issues:

• Oxidation and Reduction (N.E. Selin, C. Holmes)• Ocean-atmosphere coupling (S. Strode and E. Sunderland)• Emissions (E. Sunderland)• Long-range transport (S. Strode)• Soil biogeochemistry (N.S. Downey)• Land-atmos. coupling, global biogeochemistry (N.E. Selin)

Hg(0): volatile, insoluble, predominant form in atmosphere

Hg(II) (also, “reactive mercury” or RGM in measurements): soluble, main

depositing form

GEOS-Chem Hg vs. Observations

• We agree well with: average TGM, northern midlatitudes seasonal variation; interhemispheric gradient; wet deposition measurements, day-to-day variation at Okinawa [Jaffe et al. 2005]

• We have problems with cruise data

TGM=Hg(0)+Hg(II)(g)

For More Info:[Selin et al. 2007]

Standard code

For the ocean: [Strode et al. 2007]

Moving beyond the standard code -- we want:

• A biogeochemically consistent representation of land-atmosphere interactions

• A better constraint on the terrestrial source in the global budget

• Ability to “track” mercury through the land reservoir, where timescales are relevant

• An estimate of the “natural” vs. “anthropogenic” contribution to deposition

Constructing a coupled land-atmosphere simulation Replaces “re-emissions”

Soil lifetime:about 1000 yrs

Soil emission:F(solar rad, T, soil conc)

Veg emission:F(soil conc, transpiration)

20% of Hg is “promptly recycled” back to the atmosphere (based on isotope studies); the rest enters the long-lived soil reservoir.

Pre-Industrial Simulation • Constrains magnitude, distribution of Hg evasion

from land

GC Preindustrial budget, Mg (fluxes in Mg y-1)

Deposition (thus Hg in atmosphere) is 1/3 of present-day value, constrained by sediment cores

Steady state assumption:-Soil Hg comes from the atmosphere (for about 90% of land area)-What goes down, must come up…

GEOS-Chem (4x5) grid box Runoff:

negligible

Deposition = Evasion

Hg Deposition Increase since IndustrializationGlobal meanenrichment about a factor of 3 (constrained by sediment cores)

Over the US, average of70% of current depositionIs human-influenced;highest in the Midwest

Enrichment [present/preind]