Global Modeling of Mercury in the Atmosphere using the GEOS-CHEM model Noelle Eckley, Rokjin Park, Daniel Jacob 30 January 2004

Why are we interested in mercury transport? Mercury (Hg) is a global environmental pollutant –Current atmospheric concentrations are 3x higher than in pre-industrial times –Some recent decreases in emissions in Europe, North America, emissions increasing in Asia –Accumulates in food webs as methyl mercury; risk to humans & environment Fish consumption advisories Arctic pollution problem

Regional, national, and international policy interest U.S. state and regional regulations, and some progress from pollution reduction co- benefits – further national action coming soon? UNEP Governing Council (2/2003): agreed that further international policy action needed. UNEP will revisit issue in 2005

Source: UNEP Global Mercury Assessment Hg in the atmosphere: 3 species: elemental Hg (Hg 0 ) divalent Hg (Hg(II)) particulate Hg (HgP). Hg 0 reacts chemically with OH, O 3 to form Hg(II) HgII and HgP undergo wet and dry deposition Measurements: Total Gaseous Mercury (TGM) = Hg 0 +Hg(II)(g) Reactive Gaseous Mercury (RGM) = Hg(II)(g) Particulate mercury (TPM) Typical concentrations: TGM: 1.7 ng m-3 (NH) RGM: pg m-3 HgP: pg m-3

Scientific Questions & Research Methods What are the processes influencing the transport and fate of mercury in the atmosphere? How does mercury reach the Arctic environment? What pathways are important in the Arctic atmosphere? How do pathways and concentrations change over time? Will mercury transport be influenced by global climatic changes? What is the relative importance of natural vs. anthropogenic sources in controlling deposition in different regions? Method: Model global transport and chemistry of mercury species using GEOS-CHEM model

GEOS-CHEM model Atmospheric chemistry and transport model, used extensively at Harvard and elsewhere for oxidant-aerosol chemistry 2x2.5 degree lon/lat resolution, 30 vertical layers, assimilated meteorology from NASA-GMAO

GEOS-CHEM mercury simulation 1995 anthropogenic emissions (GEIA 2002), land and ocean emissions (primary and re-emission) Oxidation reactions in the gas phase: –Hg 0 + OH  Hg(II) k=8.7(+/-2.8) x cm 3 s -1 (Sommar et al. 2001) Realistic results at k=5.9 x cm 3 s -1, lower end of uncertainty –Hg 0 + O 3  Hg(II) k=3(+/-2) x cm 3 s -1 (Hall 1995) Wet and dry deposition of Hg(II), Hg P Not included in model: aqueous chemistry; Hg P chemistry not yet included

GEOS-CHEM mercury simulation results Total Gaseous Mercury (TGM) Results in the range of expected values

Comparing Model with Measurements: Longitudinal Average TGM Basic agreement with hemispheric average concentrations and interhemispheric gradient Lamborg et al GEOS-CHEM

GEOS-CHEM MERCURY SIMULATION: COMPARISON WITH SITE MEASUREMENTS SiteLat.Long.Measured Simulated (GEOS-CHEM) Model Error Alert, Nunavut, Canada82.5 N62.3 W % Zeppelin, Norway78.5 N11.5 E % Pallas, Finland68 N24 E % Rörvik, Sweden57 N25 E % Mace Head, Ireland53.7 N9.6 W % Delta, British Columbia, Canada 49.1 N123.1 W % Cheeka Peak, Washington, USA 48.3 N124.6 W % St. Andrews, New Brunswick, Canada 45.1 N67.0 W % Kejimujik, Nova Scotia, Canada 44.4 N65.2 W % Guiyang, China26 N106 E % Cape Point, South Africa34.4 S18.5 E % Simulated vs. measured TGM at selected sites, annual average ( ng/m 3 )

Comparison with US Mercury Deposition Network (MDN) measurements R 2 =0.77

“Mercury Depletion Events” (MDEs) in the Arctic Episodic depletion of TGM at polar sunrise Correlates with Arctic O 3 depletion events Mechanism: conversion to Hg(II) and subsequent deposition Proposed mechanism: reaction with BrO? AMAP, 2002

Future work: Next Steps Model improvements: Hg P chemistry Arctic behavior – testing the proposed mechanism for mercury depletion events (using GOME BrO columns) Tagged source simulation: how much Hg deposition comes from where? Resolving uncertainties in Hg chemistry (BrO in marine boundary layer?)

Acknowledgments Advisor: Prof. Daniel J. Jacob, Harvard University Collaborators: Dr. Rokjin Park, Bob Yantosca Funding sources: U.S. National Science Foundation Graduate Research Fellowship; Harvard University Committee on the Environment

Anthropogenic Sources Source: Pacyna and Pacyna, 2002

Historical Record of Mercury from Ice Core Pre-industrial concentrations indicate natural source Episodic volcanic input Mining emerges Industrialization, and recent decrease Source: USGS