Biogeochemical cycling of mercury in the atmosphere-ocean-land system: Global and regional modeling
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Mercury (Hg) is a ubiquitous trace metal in the environment originating from both natural and anthropogenic sources. It is a pollutant of concern because of the adverse human health effects caused by the consumption of fish and seafood containing methylmercury, which is a neurotoxin. Through development of two transport and chemistry models in the atmosphere and ocean, this dissertation investigates the regional Hg atmospheric transport and wet deposition over North America, and the global ocean Hg cycle as well its perturbation by anthropogenic Hg emissions. Chapter 2 develops a new nested-grid Hg simulation over North America with a 1/2° latitude by 2/3° longitude horizontal resolution employing the GEOS-Chem global chemical transport model. The nested model shows generally improved skill at capturing the high spatial and temporal variability of a variety of observations including wet deposition fluxes, surface concentrations and aircraft measurements of atmospheric Hg. We find that a hypothesized sub-grid rapid in-plume reduction of reactive to elemental Hg improves the model-observation comparison. The nested model suggests that North American anthropogenic emissions account for 10-22% of Hg wet deposition flux over the U.S., depending on whether the in-plume reduction process is included or not. Chapter 3 examines the trends in Hg precipitation concentrations at 47 Mercury Deposition Network (MDN) sites over the United States during 2004 - 2010. We run the model with constant anthropogenic emissions and subtract the model results from the observations in order to remove the influence of meteorological fluctuations. We find significant decreasing trends in the Northeast U.S. (-4.3±2.2% yr<super>-1</super>) and in the Midwest (-2.5±1.6% yr<super>-1</super>), but weaker trends over the Southeast (-0.63±2.5% yr<super>-1</super>) and West (+0.33±7.7% yr<super>-1</super>). Sensitivity simulation with the nested-grid Hg simulation shows that the combination of domestic emission reductions and decreasing background concentrations explains the observed trends over Northeast and Midwest, with domestic emission reductions accounting for 51-33% of the decreasing trends. Chapter 4 implements Hg biogeochemistry in a global 3D offline ocean tracer model, OFFTRAC, and investigate the natural Hg cycle, prior to any anthropogenic input. This model simulates the transformations among different Hg species, and links them to carbon dynamics in the ocean. In the deep ocean, a region which is not expected to be significantly influenced by anthropogenic emissions, the modeled total Hg concentrations (Hg<super>T</super>, 1.1±0.3 pM) are consistent with observations (1.4±0.9 pM). High concentrations in the mixed layer are modeled at Southern Ocean, coastal regions, closed and shallow water body, western Equatorial Pacific Ocean. The modeled Hg<super>T</super> concentrations in the deep old North Pacific waters are a factor of two higher than in the younger the deep North Atlantic because of the longer time to accumulate Hg sinking from the surface. The modeled fraction of elemental Hg (Hg<super>0</super><sub>aq</sub>) is also higher in aged deep waters because of the slow accumulation of Hg<super>0</super><sub>aq</sub> generated by reduction of oxidized Hg in subsurface waters. Chapter 5 focuses on the perturbation of legacy anthropogenic Hg emissions (1450 - 2008) on oceanic Hg cycle. We couple the OFFTRAC-Hg simulation developed in Chapter 4 with the GEOS-Chem atmospheric Hg simulation. According to the model, the total Hg mass in the global ocean has increased from 1150 Mmol in 1450 to a present-day value of 1640 Mmol. The modeled anthropogenic Hg concentrations peak at a depth of 400-500 m. The model result shows that approximately 43% (210 Mmol) of the anthropogenic Hg resides at depths below the mixed layer and shallower than 1000 m, 55% (270 Mmol) at depths deeper than 1000 m, while only approximately 2% (10 Mmol) in the mixed layer. The model also suggests that sinking with particulate organic carbon is the major pathway for the anthropogenic Hg to penetrate into the deep ocean. The modeled anthropogenic Hg concentrations are higher over the east tropical Pacific Ocean, the east tropical Atlantic Ocean, tropical Indian, west coast of continents, and high-latitude North Pacific and Atlantic Oceans, while being lower over centers of mid-latitude gyres and the Arctic Ocean.
- Atmospheric sciences