1. 1 Improving the parameterization of land-surface exchange in the GEOS-Chem Hg model Shaojie Song and Noelle Selin Massachusetts Institute of Technology song33@mit.edu The 7 th International GEOS-Chem Meeting 06 May 2015 @ Cambridge, MA

2. 22 Atmosphere-surface exchange fluxes of Hg 0 are still poorly constrained, limiting our ability to fully understand the mercury global biogeochemical cycle. Global mercury cycle and the atmosphere- surface exchange of Hg 0 (GEM) adapted from Driscoll et al. (2013) and Pirrone et al. (2010)

3. 33 Top-down constraints on global Hg 0 emissions and parameters in the GEOS-Chem Hg model Song et al., Atmos. Chem. Phys. Discuss., 2015, doi:10.5194/acpd-15-5269-2015 ground-based observations model-observation comparison ocean and soil emission fluxes Bayesian inversion approach

4. 4 Two-layer ocean model in GEOS-Chem Henry’s law constant Hg 0 Diffusivity Intermediate water Hg 0 concentrations Hg II /Hg P partitioning coefficient (K D ) Biological carbon pump The parameter inversion optimizes two ocean parameters: dark oxidation rate of aqueous Hg 0 (K OX2 ) and aqueous Hg II /Hg P partitioning coefficient (K D ). Reference simulation Parameter inversion Flux in Mmol yr -1 Song et al., Atmos. Chem. Phys. Discuss., 2015, doi:10.5194/acpd-15-5269-2015

5. 55 For NOMADSS, We select background Hg 0 data in the planetary boundary layer. We divide Hg 0 observations into two categories: over-land and over-ocean. Atmosphere-surface exchange of Hg 0 in the Eastern US and Northwest Atlantic

6. 6 GEOS-Chem nested-grid simulations over North America We conduct multiple GEOS-Chem simulations: reference simulation, parameter inversion, and sensitivity simulations. The sensitivity simulation #1 is based on the parameter inversion with elevated springtime soil emission in the mid-latitude region (by a factor of 4) and elevated summertime bromine columns in the tropics and subtropics (by a factor of 3). An increase of springtime soil emission is consistent with results from the emission inversion. Agricultural activities and a larger land surface Hg pool due to deposition in the snow may contribute to such seasonal pattern. Soil emissions from different simulations

7. 7 Model-observation comparison AMNet Hg 0 concentration 1.33 ng m -3 1.30 ng m -3 Among different GEOS-Chem simulations, the sensitivity simulation #1 can best capture the seasonal pattern of AMNet Hg 0 concentration. In 2013 June/July, the sensitivity simulation #1 has a small negative bias (-0.03 ng m -3 ) when compared to the AMNet measurements. The sensitivity simulation #1 gives the closest Hg aq 0 concentration among different simulations, but still 8-40% lower than the ship cruise measurements.

8. 88 Over-landOver-oceanMann–Whitney U test Measurement1.44 ± 0.091.56 ± 0.11p < 0.001 Reference simulation1.40 ± 0.041.32 ± 0.03p < 0.001 Parameter inversion1.30 ± 0.031.28 ± 0.02p < 0.001 Sensitivity simulation #11.29 ± 0.061.26 ± 0.05p < 0.001 Sensitivity simulation #21.30 ± 0.041.29 ± 0.05p = 0.992 N of samples35669 Model-observation comparison Sensitivity simulation #2 increases ocean emission of Hg 0 from the Northwest Atlantic by 80% in summer. The median Hg 0 from the NOMADSS is 1.46±0.10 ng m -3 (N=443), ~ 10% higher than that from the sensitivity simulation #1 (1.29±0.04 ng m -3 ). The overall uncertainty of the NOMADSS Hg 0 measurements is 6~10%. NOMADSS Hg 0 concentration

9. 99 Modeled Hg 0 emission fluxes from the land and ocean during summer (JJA) 0.8 1.3 -0.9 1.5 -0.6 1.9 0.8 Acknowledgments: We thank NSF Atmospheric Chemistry Grant #1053648, and the NOMADSS team, Environment Canada, Ministry of the Environment (Japan), the SEARCH network, X. Feng & X. Fu (IGCAS, China), K. Crist (Ohio University), H. Amos, Y. Zhang, E. Corbitt (Harvard), C. Holmes (FSU), J. Kuss (IOW, Germany). Thanks for your attention! Any questions? Contact: Shaojie Song (song33@mit.edu) Take-home message: GEOS-Chem simulations and observations suggest that the Eastern US terrestrial ecosystem is a net sink of Hg 0 while the Northwestern Atlantic Ocean is a source. Flux: ng m -2 h -1