1. A Biogeochemical Model for Mercury in GEOS-Chem Noelle Eckley Selin GEOS-Chem 3rd Users’ Meeting April 12, 2007 Hg(0)Hg(II) MeHgHg(II) Why we care about mercury:

2. 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

3. 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]

4. 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

5. 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.

6. 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

7. Hg Deposition Increase since Industrialization Global mean enrichment about a factor of 3 (constrained by sediment cores) Over the US, average of 70% of current deposition Is human-influenced; highest in the Midwest Enrichment [ present/preind ]