Modeling of Atmospheric mercury

 

The concentration of mercury (Hg) in water has become an important issue since high methylmercury levels have been found in fish. The methylmercury is neurotoxin, which forms from deposited atmospheric mercury and then bioaccumulates in the aquatic food chain. Each year, about 7700 tons of mercury is emitted into the atmosphere. Human activities are responsible for about one third is it, and the remaining emissions come from various natural terrestrial surfaces including soil, water and vegetation.

 

In the atmosphere Hg chemically inter-converts between various forms, namely elemental mercury (GEM), reactive oxidized bivalent mercury (RGM), and particulate mercury (TPM). GEM constitutes more that 90% of total mercury in the atmosphere. The atmospheric chemical processes, which interconvert different mercury species, strongly influence the transport and deposition of the mercury. Modeling efforts to assess global cycling of mercury require an in-depth knowledge of atmospheric mercury chemistry.

 

Mercury is deposited from the atmosphere by wet and dry processes, with lifetime up to 2 years. The mercury states vary in chemical and physical characteristics, and these strongly influence their deposition rates. Deposited Hg undergoes chemical and biochemical processes, which include transformation to methylmercury and reductions to GEM, which is reemitted back to the atmosphere.  Therefore, understanding of exchange processes between atmosphere and Earth surfaces is very important in mercury cycling and transport.

 

The bidirectional mercury surface exchange is based on assumption that the emission of volatile elemental mercury deposited in land and water surfaces follow Fick’s law. The exchange velocity of the mercury between the surface media and the atmosphere are parameterized using a resistance analogy and partitioning coefficients. The change in surface media concentrations is parameterized by a system of ordinary differential equations.

 

Scheme of mercury exchange processes between the atmosphere and Earth surfaces

 

Accurate descriptions of gas phase as well as aqueous phase chemistry of mercury are necessary because there is a complex relationship between a number of species and the aqueous Hg concentrations. A complete model describing the gas and aqueous phase chemistry of mercury contains as large as 40 gas phase species in 80 reactions and 30 aqueous species in 100 reactions. Such model requires special algorithms for solving the corresponding system of differential equations.

 

Scheme of chemical transformations of mercury in the atmosphere

 

 

Quantum-chemical modelling of mercury

Potential energy surface (PES) of the Mercury-Water CS complex structure