Modeling the global atmospheric transport and deposition of mercury to the Great Lakes

Mark Cohen ,Pierrette Blanchard,Paul Kelley,Martin Pilote,David Niemi,Mae Sexauer Gustin,Frits Steenhuisen,Laurier Poissant,Christopher P Loughner,Winston T Luke,Richard S Artz,Alexandra Steffen,Hang Lei,Thomas M Holsen,Daniel A Jaffe,Roland R Draxler,Xinrong Ren,Seth Lyman ,Józef M Pacyna ,Dominique Ratté ,R Tordon ,Young J Han ,Simon Wilson

doi:10.12952/journal.elementa.000118

Abstract

Abstract Mercury contamination in the Great Lakes continues to have important public health and wildlife ecotoxicology impacts, and atmospheric deposition is a significant ongoing loading pathway. The objective of this study was to estimate the amount and source-attribution for atmospheric mercury deposition to each lake, information needed to prioritize amelioration efforts. A new global, Eulerian version of the HYSPLIT-Hg model was used to simulate the 2005 global atmospheric transport and deposition of mercury to the Great Lakes. In addition to the base case, 10 alternative model configurations were used to examine sensitivity to uncertainties in atmospheric mercury chemistry and surface exchange. A novel atmospheric lifetime analysis was used to characterize fate and transport processes within the model. Model-estimated wet deposition and atmospheric concentrations of gaseous elemental mercury (Hg(0)) were generally within ∼10% of measurements in the Great Lakes region. The model overestimated non-Hg(0) concentrations by a factor of 2–3, similar to other modeling studies. Potential reasons for this disagreement include model inaccuracies, differences in atmospheric Hg fractions being compared, and the measurements being biased low. Lake Erie, downwind of significant local/regional emissions sources, was estimated by the model to be the most impacted by direct anthropogenic emissions (58% of the base case total deposition), while Lake Superior, with the fewest upwind local/regional sources, was the least impacted (27%). The U.S. was the largest national contributor, followed by China, contributing 25% and 6%, respectively, on average, for the Great Lakes. The contribution of U.S. direct anthropogenic emissions to total mercury deposition varied between 46% for the base case (with a range of 24–51% over all model configurations) for Lake Erie and 11% (range 6–13%) for Lake Superior. These results illustrate the importance of atmospheric chemistry, as well as emissions strength, speciation, and proximity, to the amount and source-attribution of mercury deposition.

Highlights

There is a pH dependence of several of HOCl and OCl-1 reactions, and for illustrative purposes, the rates of these reactions have been shown for pH = 4 and pH =5; the rates are faster at higher pH. The reason for this dependence is that the rates here have been estimated based on an assumed gas-phase concentration of Cl2, and, the concentrations of dissolved HOCl and OCl-1 in equilibrium with this Cl2 are pH dependent
For the gas-phase oxidation of Hg0 by molecular chlorine (Cl2), we used the rate constant reported by Calhoun and Prestbo (2001) of 4.0 x 10-18 cm3 molecule-1 sec-1
The total atmospheric concentration of Cl2(gas), Cl2(aq), HOCl(aq) and OCl-1(aq) is regarded as a constant, and the equilibrium relationships are used to estimate the relative amounts of each species

Summary

Introduction

The reason for this dependence is that the rates here have been estimated based on an assumed gas-phase concentration of Cl2, and, the concentrations of dissolved HOCl and OCl-1 in equilibrium with this Cl2 are pH dependent. For the gas-phase oxidation of Hg0 by molecular chlorine (Cl2), we used the rate constant reported by Calhoun and Prestbo (2001) of 4.0 x 10-18 cm3 molecule-1 sec-1. Illustrative, Typical Rates of Gas and Aqueous Phase Hg(0) Oxidation Reactions

Results

Conclusion