A State-of-the-Science Hg Redox Mechanism for Atmospheric Models: Constraints from Observations and Global Implications

Friday, 18 December 2015
Poster Hall (Moscone South)
Hannah Horowitz1, Daniel J. Jacob2, Helen Marie Amos2, David G Streets3, Yanxu Zhang2,4, Theodore S Dibble5, Franz Slemr6 and Elsie M Sunderland1, (1)Harvard University, Cambridge, MA, United States, (2)Harvard University, School of Engineering and Applied Sciences, Cambridge, MA, United States, (3)Argonne National Laboratory, Argonne, IL, United States, (4)Gradient Corporation, Cambridge, MA, United States, (5)SUNY College of Environmental Science and Forestry, Syracuse, NY, United States, (6)Max Planck Institute for Chemistry, Mainz, Germany
Mercury (Hg) in the atmosphere cycles between two redox forms, Hg0 and HgII. Hg0 has a lifetime of ~1 year allowing near-global transport, while HgII is efficiently removed by deposition within weeks. Understanding atmospheric Hg redox chemistry is critical to determining the patterns of deposition to the surface, where Hg can be transformed to the bioaccumulative neurotoxin, methylmercury. We present a state-of-the-science redox mechanism for use in atmospheric models, with new theoretical data, which we implement in a global 3-D chemical transport model (GEOS-Chem). We evaluate our simulation against atmospheric observations and examine implications for Hg deposition. Modeled HgII wet deposition depends on the oxidation of anthropogenic, ocean, and soil Hg0 emissions and the reduction of emitted anthropogenic HgII. We present a new global anthropogenic atmospheric Hg emissions inventory for 1990 – 2010 with improved speciation of power plant emissions and regional commercial Hg emissions. The seasonal cycle of ocean evasion is also critical to atmospheric Hg variability. We present an advance in our ability to model atmosphere-ocean exchange of Hg, through more realistic ocean circulation from the 3-D MITgcm. Our results suggest Br is the dominant oxidant in the stratosphere, consistent with constraints from aircraft observations of the Hg gradient with depth into the stratosphere. The proposed redox mechanism leads to increased HgII deposition to the Tropics, with implications for tropical surface ocean enrichment, and decreased deposition to the Southern Ocean. Within the uncertainty of Hg0 oxidation rates, we find atmospheric HgII reduction is still needed. We find changes in speciated Hg emissions due to emissions controls can explain recent observed regional trends in atmospheric Hg. These have shifted power plant impacts to relatively more global than local Hg deposition. Coupling to the more realistic 3-D ocean model improves simulated atmospheric Hg seasonal cycles.